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InfoMagic Standards 1994 January
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InfoMagic Standards - January 1994.iso
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ccitt
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1988
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6_3_03.tro
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1991-12-13
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.rs
.\" Troff code generated by TPS Convert from ITU Original Files
.\" Not Copyright ( c) 1991
.\"
.\" Assumes tbl, eqn, MS macros, and lots of luck.
.TA 1c 2c 3c 4c 5c 6c 7c 8c
.ds CH
.ds CF
.EQ
delim @@
.EN
.nr LL 40.5P
.nr ll 40.5P
.nr HM 3P
.nr FM 6P
.nr PO 4P
.nr PD 9p
.po 4P
.rs
\v | 5i'
.sp 1P
.ce 1000
\v'3P'
SECTION\ 5
.ce 0
.sp 1P
.ce 1000
\fBCONTINUITY\ CHECK\ OF\ THE\ SPEECH\ PATH\fR
.ce 0
.sp 1P
.sp 2P
.LP
\fBRecommendation\ Q.271\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB5.1\ GENERAL\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.271''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.271 %'
.ce 0
.sp 1P
.PP
Because the signalling in System\ No.\ 6 does not pass over the speech
path, facilities should be provided for making a continuity check of the
speech path prior to the commencement of conversation. The check is not
.sp 1P
.RT
.LP
intended to eliminate the need for routine testing of the transmission path.
.PP
This specification relates only to that part of an international connection
served by Signalling System\ No.\ 6. The part of the speech path to be
checked may include a\ TASI circuit.
.PP
As the presence of active echo suppressors
in the circuit would interfere with the continuity check, it is necessary to
disable the suppressors during the check and to re\(hyenable them, if required,
after the check has been completed.
.RT
.sp 1P
.ce 1000
\fB5.2\ RELIABILITY\ OF\ THE\ SPEECH\ PATH\ ACROSS\ THE\ EXCHANGE\fR
.ce 0
.sp 1P
.PP
Administrations shall ensure the reliability of a connection through a
switching machine (cross\(hyoffice check) either on a per call basis or
by a statistical method. With either method, the probability of the connection
being established with an unacceptable speech path, transmission quality
should not exceed 10\uD\dlF261\u5\d as the long\(hyterm average.
.sp 1P
.RT
.sp 1P
.ce 1000
\fB5.3\ CONTINUITY\ CHECK\ OF\ THE\ SPEECH\ CIRCUIT\ BETWEEN\ EXCHANGES\fR
.ce 0
.sp 1P
.PP
The
continuity check of the speech
circuit will be
done, link\(hyby\(hylink, on a per call basis prior to the commencement of
conversation. The loop checking method used is specified in the following
sections.
.sp 1P
.RT
.sp 1P
.ce 1000
\fB5.4\ LOOP\ CHECKING\ METHOD\fR
.ce 0
.sp 1P
.PP
The
continuity check transceiver
(check\(hytone
transmitter and receiver) is connected to the GO and RETURN paths of the
outgoing circuit at the first and each succeeding exchange, excluding the
last exchange, in that part of the international connection served by Signalling
System\ No.\ 6. The check loop should be connected to the GO and RETURN
paths of the incoming circuit at each exchange except the first in that
part of the
international connection served by Signalling System\ No.\ 6. A continuity
check is considered successful when a tone is sent on the GO path and is
received on the RETURN path within acceptable transmission and timing limits.
.sp 1P
.RT
.sp 1P
.ce 1000
\fB5.5\ TRANSMISSION\ REQUIREMENTS\ FOR\ THE\ CONTINUITY\ CHECK\fR
.ce 0
.sp 1P
.sp 2P
.LP
5.5.1
\fITransmitting equipment\fR
.sp 1P
.RT
.PP
The check\(hytone frequency will be 2000\ \(+-\ 20\ Hz.
.PP
The sending level of the check tone will be \(em12\ \(+-\ 1\ dBm0.
.bp
.RT
.sp 1P
.LP
5.5.2
\fICheck loop\fR
.sp 9p
.RT
.PP
The check loop will have a loss of\ 0\ dB, taking into account any
.PP
difference between the relative levels of the two paths at the point of
attachment.
.RT
.sp 1P
.LP
5.5.3
\fIReceiving equipment\fR
.sp 9p
.RT
.PP
The check\(hytone receiver will have the following
characteristics:
.RT
.sp 1P
.LP
5.5.3.1
\fIOperating requirements\fR \v'3p'
.sp 9p
.RT
.LP
Signal\ frequency:
2000\ \(+-\ 30\ Hz
.LP
Signal\ level\ range:
The absolute power level\ \fIN\fR \ of the check
tone shall be within the limits
.LP
(\(em18\ +\ \fIn\fR ) \(= \fIN\fR \(= (\(em6\ +\ \fIn\fR )\ dBm
.LP
where\ \fIn\fR \ is the relative power level at the
receiver input
.LP
Recognition\ time:
30\ to\ 60\ ms
.PP
The frequency and level range tolerances allow for variations at the sending
end and for variations in line transmission that are considered
acceptable.
.LP
.sp 1P
.LP
5.5.3.2
\fINon\(hyoperating requirements\fR \v'3p'
.sp 9p
.RT
.LP
Signal\ frequency:
outside the frequency band 2000\ \(+-\ 200\ Hz
.LP
Signal\ level:
below or equal to\ \(em22\ +\ \fIn\fR \ dBm0
.PP
The limit is\ 10\ dB below the nominal absolute level of the check
tone at the input of the receiver. If the level falls below this point,
transmission is considered unacceptable.
.LP
Signal\ duration:
shorter than 30\ ms
.PP
The level range of\ (\(em18\ +\ \fIn\fR ) \(= \fIN\fR \(= (\(em6\ +\ \fIn\fR
)\ dBm
will serve as a GO/NO\(hyGO check on the links in that part of the international
connection served by Signalling System\ No.\ 6.
.sp 1P
.LP
5.5.3.3
\fIRelease requirements\fR
.sp 9p
.RT
.PP
If the receiver is used to test for the removal of check tone (see
Recommendation\ Q.261, \(sc\ 4.1.4):
.RT
.LP
\(em
after recognition of tone, interruptions of up to 15\ ms shall be ignored;
this will prevent switching through the speech path
prematurely;
.LP
\(em
the indication of tone removal should not be delayed more than
40\ ms; and
.LP
\(em
the release level of the receiver should be lower than
\(em27\ +\ \fIn\fR \ dBm.
.sp 1P
.ce 1000
\fB5.6\ CONTINUITY\ SIGNAL\fR
.ce 0
.sp 1P
.PP
The procedure for sending the continuity signal is given in
Recommendation\ Q.261, \(sc\ 4.1.4.
.sp 1P
.RT
.sp 1P
.ce 1000
\fB5.7\ TIMING\ CONSIDERATIONS\ FOR\ THE\ CONTINUITY\ CHECK\fR
.ce 0
.sp 1P
.sp 2P
.LP
5.7.1
\fITime\(hyout period of the continuity check\fR
.sp 1P
.RT
.PP
The continuity check is considered to have failed if the receiver has not
responded within a period determined by the Administration concerned. This
period should not exceed 2\ seconds.
.PP
The time\(hyout period of the continuity check should always exceed the
continuity recognition time, \fIT\fR\d\fIC\fR\\d\fIR\fR\u, given by:
.RT
.LP
.sp 1P
.ce 1000
\fIT\fR\d\fIC\fR\\d\fIR\fR\u\ =\ 2\fIT\fR\d\fIp\fR\u\ +\ \fIT\fR \s6\fIIAM\fR
.PS 10
\ +\ \fIT\fR\d\fIT\fR\\d\fIC\fR\u\ +\ \fIT\fR\d\fIL\fR\u\ +\ \fIT\fR\d\fIR\fR\u\
\(em\ \fIT\fR\d\fIT\fR\u
.RT
.ce 0
.sp 1P
.LP
where:
\fIT\fR\d\fIp\fR\u =
one\(hyway propagation time of the
speech
circuit
and the
signalling link
(where they are the same),
.LP
\fIT\fR\d\fIT\fR\\d\fIC\fR\u =
TASI clip time
for two TASI systems in series (for connections not using TASI, \fIT\fR\d\fIT\fR\\d\fIC\fR\u\
=\ \fB0\fR ),
.bp
.LP
\fIT\fR\d\fIR\fR\u =
receiver response time
,
.LP
\fIT\fR\d\fIL\fR\u =
loop connecting time
(maximum),
.LP
\fIT\fR\d\fIT\fR\u =
transceiver connecting time
(minimum),
.LP
\fIT\fR \s6\fIIAM\fR .PS 10
=
emission time of the longest initial address message
.
.RT
.PP
If retransmission of an IAM is to be included in \fIT\fR\d\fIC\fR\\d\fIR\fR\u,
the following formula may be used:
.LP
.sp 1P
.ce 1000
\fIT\fR\d\fIC\fR\\d\fIR\fR\u= 4\fIT\fR\d\fIp\fR\u+ 2\fIT\fR \s6\fIIAM\fR .PS 10
+
\fIT\fR \s6\fIACU\fR .PS 10
+ \fIT\fR\d\fIx\fR\u+ \fIT\fR\d\fIy\fR\u+ \fIT\fR\d\fIL\fR\u+
\fIT\fR\d\fIR\fR\u\(em \fIT\fR\d\fIT\fR\u
.RT
.ce 0
.sp 1P
.LP
where:
\fIT\fR \s6\fIACU\fR .PS 10
=
emission time of an\ ACU
(length of an\ ACU),
.RT
.LP
\fIT\fR\d\fIx\fR\u =
time between receiving an\ IAM and emitting
an\ ACU
,
.LP
\fIT\fR\d\fIy\fR\u =
time between receiving an\ ACU and emitting an
initial address message
.
.sp 1P
.LP
5.7.2
\fISwitching times of continuity check equipment\fR
.sp 9p
.RT
.PP
The connection and disconnection of the equipment used for the
continuity check and also the disabling and subsequent enabling of echo
suppressors should be related to the following stages of progress in the
establishment of the connection:
.RT
.LP
.PP
a)
\fIPreparation at System No.\ 6 exchange applying the\fR
\fItransceiver\fR .\ \(em\ Action should be initiated at the termination
of the handling time \fIT\fR\d\fIh\fR\uof the initial address message,
i.e.\ when it is inserted
in the output buffer and is available for emission.
.PP
b)
\fIPreparation at System No.\ 6 exchange connecting the check\fR \fIloop\fR
.\ \(em\ Action should be initiated at the moment of recognition of the
initial address message received.
.PP
c)
\fIDisconnection at System No.\ 6 exchange connecting the\fR
\fIcheck loop\fR .\ \(em\ Action follows the receipt of the continuity
signal or the
clear\(hyforward signal, or the emission of signals indicating that the call
cannot be established, e.g.\ circuit\(hygroup congestion signal.
.PP
d)
\fIDisconnection at System No.\ 6 exchange applying the\fR
transceiver.\ \(em\ Action should be initiated on the successful completion
or the failure
of the continuity check. Exceptionally, if disconnection has not previously
occurred action should be initiated at the moment of recognition of the
address\(hycomplete signals, the answer signals, signals indicating that
the call cannot be established, or on the emission of a clear\(hyforward
signal.
.PP
It is recommended that the mean time, both for the connection and
for the disconnection, be less than 100\ ms. A mean time of 200\ ms should
not be exceeded. See Recommendation\ Q.261.
.LP
.rs
.sp 20P
.ad r
BLANC
.ad b
.RT
.LP
.bp
.LP
\fBMONTAGE:\fR PAGE 60 = PAGE BLANCHE
.sp 1P
.RT
.LP
.bp
.sp 1P
.ce 1000
\v'3P'
SECTION\ 6
.ce 0
.sp 1P
.ce 1000
\fBSIGNALLING\ LINK\fR
.ce 0
.sp 1P
.sp 2P
.LP
\fBRecommendation\ Q.272\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB6.1\ REQUIREMENTS\ FOR\ THE\ SIGNALLING\ DATA\ LINK\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.272''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.272 %'
.ce 0
.sp 1P
.sp 2P
.LP
6.1.1
\fIGeneral\fR \v'3p'
.sp 1P
.RT
.PP
a)
The
signalling data link
may be either an analogue signalling data link (\(sc\ 6.1.1.1 below) or
a digital signalling data link
(\(sc\ 6.1.1.2 below).
.LP
.PP
b)
System No.\ 6 is capable of operating over signalling data links
with the longest loop propagation time visualized (see also \(sc\ 6.7.3 below).
.PP
c)
To reduce the possibility of the called party being distorted
or clipped, the propagation time of the signalling data link should be
as low as possible and should not be significantly greater than that of
any speech
circuits with which it is associated.
.LP
.PP
d)
The signalling data link shall be dedicated to the use of a
System\ No.\ 6 signalling link between two points, the only switching to be
provided being that required for the security arrangements (see
Recommendation\ Q.292).
.PP
e)
A means must be furnished for disabling the echo suppressors
which might be associated with the circuits used for the signalling data
links. Disabling must be accomplished by local action by the processor
at each
terminal.
.sp 1P
.LP
6.1.1.1
\fIAnalogue signalling data link\fR
.sp 9p
.RT
.PP
The
analogue signalling data link
shall be made up of
standard
international voice\(hyfrequency channels, either 3\(hykHz or 4\(hykHz
spaced, and
associated modems. The overall transmission characteristics of the
voice\(hyfrequency channels must be equalized if necessary to meet the
recommendations of \(sc\ 6.1.3 below.
.RT
.sp 1P
.LP
6.1.1.2
\fIDigital signalling data link\fR
.sp 9p
.RT
.PP
The
digital signalling data link
shall be derived from the 1544 kbit/s (Recommendation\ Q.47) or
2048\ kbit/s\ (Recommendation\ Q.46)
primary multiplex equipment and includes the appropriate digital interface
adaptor.
.RT
.LP
.sp 2P
.LP
6.1.2
\fIError rate characteristics of the data channel\fR
.sp 1P
.RT
.sp 1P
.LP
6.1.2.1
\fIAnalogue data channel\fR
.sp 9p
.RT
.PP
The data transmitted at 2400 bits per second with four\(hyphase PSK
(\fIphase shift keying\fR ) modulation over
a\ data channel as specified should
meet a long\(hyterm bit error rate of less than 1\ in\ 10\u5\d in normal
operation
(see\ Recommendation\ Q.295, \(sc\ 9.2.7). This figure excludes interruptions
exceeding 350\ ms in length.
.bp
.RT
.sp 1P
.LP
6.1.2.2
\fIDigital data channel\fR
.sp 9p
.RT
.PP
The data transmitted at permitted data rates over digital data
channels as specified should meet a long term bit error rate of less than
1 in 10\u6\d in normal operation (see Recommendation\ Q.295, \(sc\ 9.2.7).
This figure
excludes interruptions exceeding 350\ ms in length.
.RT
.LP
.sp 1P
.LP
6.1.3
\fITransmission characteristics of the voice\(hyfrequency channel\fR
.sp 9p
.RT
.PP
The transmission characteristics of the voice\(hyfrequency channels
used in the signalling data link are based on those in Recommendation\ M.761.
.PP
However, for the System No.\ 6 data rate and modulation method,
Recommendation\ M.761 offers some latitude in the selection of channels. The
equalization for attenuation distortion and delay distortion of the channels
can be restricted to the frequency band\ 1000 to\ 2600\ Hz (see Figures\
15/Q.272 and\ 16/Q.272).
.RT
.PP
a)
\fIOverall loss at 800\ Hz\fR .\ \(em\ The overall loss at 800\ Hz
of the channels of a transfer link is not specified.
.PP
The channels of a transfer link should be set up so that when a test
signal at a level of \(em10\ dBm0 is connected to the input of the transfer
channel, the level received at the output of the transfer channel at the
distant end is as close as possible to \(em10\ dBm0.
.PP
b)
\fIVariation of overall loss at 800\ Hz\fR .\ \(em\ The variation with
time overall loss at 800\ Hz should be as small as possible but should not
exceed the following limits:
.ad r
Short\(hyterm variation (over a period of a few seconds)
\(+- | \ dB
.ad b
.RT
.LP
Long\(hyterm variation (over long periods including
.ad r
daily and seasonal variations)
\(+- | \ dB
.ad b
.RT
.LP
.PP
c)
\fIAttenuation/frequency distortion\fR .\ \(em\ The variation of the
overall loss of the channel with frequency over the range of\ 1000 to\ 2600\ Hz
relative to the attenuation at 800\ Hz should not exceed the limits shown in
Figure\ 15/Q.272.
.LP
.rs
.sp 26P
.ad r
\fBFIGURE 15/Q.272 p.1\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
d)
\fIDelay/frequency distortion\fR .\ \(em\ The delay/frequency
distortion in the band of frequencies from\ 1000 to\ 2600\ Hz relative in that
band should not exceed the limits given in Figure\ 16/Q.272. It may be
necessary to select channels and/or provide suitable delay distortion equalizers
to
ensure that these limits are not exceeded.
.PP
Where the full\(hytime reserved link is a
TASI speech circuit
taken into use for signalling purposes, this characteristic may not be met
unless all TASI channels in the route meet the transmission requirements
specified above. In addition, it may be necessary to restrict the number of
3\(hykHz\(hyspaced channels used in a signalling data link.
.LP
.rs
.sp 36P
.ad r
\fBFIGURE\ 16/Q.272 p.2\fR
.sp 1P
.RT
.ad b
.RT
.PP
e)
\fIUniform spectrum random circuit noise\fR .\ \(em\ See
Recommendation\ M.761, partifularly the note to Recommendation\ M.761,
\(sc\ 2.6.
.PP
f
)
\fIImpulse noise\fR .\ \(em\ Impulsive noise on the
voice\(hyfrequency channel should not exceed 18\ peaks in 15\ minutes,
greater than \(em21\ dBm0. Measurements should be made during peak hours.
.PP
According to Recommendation\ M.761, impulsive noise should be measured
with an instrument complying with Recommendation\ O.71. The value given
above
is a provisional limit for maintenance purposes; final values are still
under study.
.bp
.LP
.sp 1P
.LP
6.1.4
\fINominal data carrier power level\fR
.sp 9p
.RT
.PP
The nominal data carrier power level is \(em15\ dBm0 (see
Recommendation\ Q.15).
.PP
Recommendations\ H.41 and V.2 allow a power level of \(em10\ dBm0
when no more than\ 5% of the channels in a multichannel system are used for
non\(hyspeech applications simultaneously in both directions. If the percentage
of channels in this type of service is considerably more than\ 5%, the
power
should be reduced. Recommendation\ Q.15 allows a mean absolute power level
of\ \(em15\ dBm0.
.RT
.sp 1P
.LP
6.1.5
\fISlip characteristics of the digital data channel\fR
.sp 9p
.RT
.PP
The occurrence of slips adversely affects the service dependability of
the signalling system. Means must be provided for
.RT
.LP
a)
preventing slips from occurring, e.g., by use of
synchronization or by use of a contra\(hydirectional interface, or
.LP
b)
detecting slips, or
.LP
c)
providing accurate clocks to reduce the occurrence of
undetected slips.
.LP
.PP
Although a means can be provided to detect slips, in general each
slip
that occurs will cause a signal unit to be received in error. When using
a slip detecting mechanism, the slip rate must be such that the dependability
requirements of Recommendation\ Q.276, \(sc\ 6.6.1 are still met (see also
Recommendation\ Q.276, \(sc\ 6.8.3).
.sp 1P
.LP
6.1.5.1
\fIThe\fR
\fI1544 kbit/s primary multiplex\fR
.sp 9p
.RT
.PP
Provisionally, the need for a slip requirement is not foreseen.
.RT
.sp 1P
.LP
6.1.5.2
\fIThe\fR
\fI2048 kbit/s primary multiplex\fR \v'3p'
.sp 9p
.RT
.PP
a)
4 kbit/s signalling rate
.PP
The coding for deriving the 4 kbit/s channel from the 64 kbit/s
bearer is designed so that slips are always detected and the true data
recovered.
.PP
b)
56 kbit/s signalling rate
.PP
The coding for deriving the 56 kbit/s channel from the 64 kbit/s
bearer may be used to detect slips. Provisionally an undetected slip rate
not exceeding once in 16\ days is required.
.LP
.rs
.sp 18P
.ad r
BLANC
.ad b
.RT
.LP
.bp
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation Q.272)
.sp 9p
.RT
.ce 0
.ce 1000
\fBProposed changes presently under study\fR
.sp 1P
.RT
.ce 0
.LP
.rs
.sp 34P
.ad r
\fBFigura\ A\(hy15/Q.272 p. 3\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 14P
.ad r
BLANC
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 33P
.ad r
\fBFigura\ A\(hy16/Q.272 p. 4\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
\fBRecommendation\ Q.273\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB6.2\ DATA\ TRANSMISSION\ RATE\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.273''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.273 %'
.ce 0
.sp 1P
.sp 2P
.LP
6.2.1
\fIAnalogue data channel rate\fR
.sp 1P
.RT
.PP
The preferred data transmission rate on analogue channels is
2400\ bit/s.
.RT
.sp 1P
.LP
6.2.2
\fIDigital data channel rates\fR
.sp 9p
.RT
.PP
The preferred data transmission rate on digital channels is 4\ kbit/s for
both the 1544 kbit/s and 2048 kbit/s international digital multiplexes.
In addition, the rate of 56\ kbit/s may be used with the 2048 kbit/s international
digital multiplex.
.bp
.RT
.LP
.sp 2P
.LP
\fBRecommendation\ Q.274\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB6.3\ TRANSMISSION\ METHODS\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.274''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.274 %'
.ce 0
.sp 1P
.sp 2P
.LP
6.3.1
\fIAnalogue modulation methods\fR
.sp 1P
.RT
.PP
The modulation technique described in this Recommendation uses
\fIphase shift keying\fR to transmit serial binary data over analogue telephone\fI\fR
\fI\fR channels. The binary data signal is encoded by first grouping it
into
bit pairs (dibits). Each dibit is represented by one of four possible carrier
phase shifts. Thus, the output from the phase modulator consists of a serial
train of phase\(hyshifted carrier pulses at half the data bit rate. The phase
shift between two consecutive modulation elements contains the information
to be transmitted.
.PP
The data receiver uses differentially coherent detection to recover the
sense of the binary data from the line signal. This type of detection has
proven to be relatively insensitive to the types of distortions and
interference encountered on telephone\(hytype transmission media. It also
allows rapid recovery from such catastrophic impairments as drop\(hyouts
and large phase hits.
.PP
Receiver timing recovery can be accomplished in several ways. A very
rapid timing recovery scheme can be provided using certain properties of the
transmitted spectrum.
.PP
Receiver timing information can also be extracted from the zero
crossings, on a dibit basis, of the received baseband data signals. The
latter method is capable of providing synchronization holdover through
extended
drop\(hyouts and periods of high noise.
.RT
.sp 1P
.LP
6.3.2
\fIDigital transmission methods\fR
.sp 9p
.RT
.PP
The methods used to derive the 4 and 56 kbit/s digital channels from the
1544 and 2048 kbit/s primary multiplexes are described below.
.RT
.sp 1P
.LP
6.3.2.1
\fIDerivation from the 1544 kbit/s primary multiplex\fR
.sp 9p
.RT
.PP
The binary data from the signalling terminal is transferred serially at
the data transmission rate of 4\ kbit/s to the 1544\ kbit/s primary multiplex.
At the primary multiplex each bit of the data stream is successively inserted
into the S bit position (see Recommendation\ Q.47, \(sc\ 4.1).
.PP
In the receive direction the primary multiplex extracts the bits from
the S\(hybit position and transfers them serially to the signalling
terminal.
.RT
.sp 1P
.LP
6.3.2.2
\fIDerivation from the 2048 kbit/s primary multiplex\fR \v'3p'
.sp 9p
.RT
.PP
a)
\fIData transmission at 4 kbit/s rate\fR .\ \(em\ The binary data from
the signalling terminal is transferred serially to the digital interface
adaptor. At the digital interface adaptor the 4 kbit/s data stream is modulated
on a 64 kbit/s bearer channel such that 16\ bits of the bearer channel
correspond to one bit of the 4 kbit/s channel. The 64 kbit/s data stream is
transferred serially to the 2048 kbit/s primary multiplex in alignment
with an 8 kHz clock (byte timing). At the primary multiplex the 16\ bits
corresponding to one signalling information bit are inserted into the designated
channel time slot of two successive frames.
.PP
In the receive direction the primary multiplex extracts the bits from the
designated channel time slot and transfers them serially at 64\ kbit/s
in
alignment with an 8 kHz clock to the digital interface adaptor. At the
digital interface adaptor the 16\ bits corresponding to one signalling
information bit are detected and the binary data is transferred serially
to the signalling
terminal at the data transmission rate of 4\ kbit/s.
.LP
.PP
b)
\fIData transmission at 56 kbit/s rate\fR .\ \(em\ The binary data from
the signalling terminal is transferred serially to the digital interface
adaptor. At the digital interface adaptor, the 28\ bits of a signal unit are
placed in bit positions 1\ to\ 7 of four 8 bit bytes [see also \(sc\ 6.4.2.4 | )
below].
These four bytes are transferred serially at the data transmission rate of
64\ kbit/s to the 2048 kbit/s primary multiplex in alignment with an 8
kHz clock (byte timing). At the primary multiplex, the four bytes are inserted
into the designated channel time slot of four successive frames.
.PP
In the receive direction the primary multiplex extracts the bits from the
designated channel time slot and transfers them serially at the data
transmission rate of 64\ kbit/s to the digital interface adaptor in alignment
with an 8 kHz clock. In the digital interface adaptor the bits 1\ to\ 7
of each 8 bit byte are transferred serially to the signalling terminal
at the data
transmission rate of 56\ kbit/s.
.bp
.sp 1P
.ce 1000
\fB6.4\ MODEM\ AND\ INTERFACE\ REQUIREMENTS\fR
.ce 0
.sp 1P
.sp 2P
.LP
6.4.1
\fIAnalogue modem requirements\fR
.sp 1P
.RT
.PP
The requirements for a 2400\ bits per second modem are given
below.
.RT
.LP
.sp 1P
.LP
6.4.1.1
\fIPrincipal requirements\fR
.sp 9p
.RT
.PP
The principal requirements of a modem used for System\ No.\ 6 are as
follows:
.RT
.LP
a)
Use of differential four\(hyphase modulation (see
Recommendation\ V.26, alternative\ B);
.LP
b)
Use of differential coherent 4\(hyphase demodulation;
.LP
c)
Full duplex operation over a 4\(hywire data link;
.LP
d)
A modulation rate of 1200\ bauds;
.LP
e)
A bit rate of 2400\ bits per second.
.sp 1P
.LP
6.4.1.2
\fIFrequency requirements\fR \v'3p'
.sp 9p
.RT
.LP
a)
The basic timing frequency shall be 2400\ Hz (one cycle per
bit);
.LP
b)
The carrier frequency shall be 1800\ Hz;
.LP
c)
The carrier envelope frequency shall be 600\ Hz
(see \(sc\ 6.4.1.4\ below);
.LP
d)
All frequencies generated in the modem shall be stable to
within \(+- | .005% of the nominal value. They must have a constant phase
relationship with respect to one another. This implies that all
frequencies should be derived from a basic clock or that they be
phase\(hylocked.
.sp 1P
.LP
6.4.1.3
\fIEncoding phase relationships\fR
.sp 9p
.RT
.PP
The encoding phase relationship must be as follows:
.RT
.LP
Dibit
Phase\ change
.LP
\fB\ 0 | \fR \ \ +\ 45\(de
.LP
\fB\ 0 | \fR \ \ +135\(de
.LP
\fB\ 1 | \fR \ \ +225\(de
.LP
\fB\ 1 | \fR \ \ +315\(de
.PP
The phase change is the actual on\(hyline phase shift in the transition
region from the end of one signalling element to the beginning of the following
signalling element.
.LP
.sp 1P
.LP
6.4.1.4
\fILine signal envelope\fR
.sp 9p
.RT
.PP
The data carrier pulse shape can be closely approximated by the
following expression for a signal element centred at \fIt\fR \ =\ 0 (see
Figure\ 17/Q.274):
\v'6p'
.RT
.LP
and\ \
Envelope (\fIt\fR ) =
$$4ocos
[Unable to Convert Formula]
[Unable to Convert Formula]
$$3u1 \(em cos
[Unable to Convert Formula]
$$3e
for \(em \(34 \fIT\fR \(= \fIt\fR \(= \(34 \fIT\fR and\ \ Envelope (\fIt\fR
) = 0 for \(em \fIT\fR \(= \fIt\fR \(= \(em \(34 \fIT\fR and \(34 \fIT\fR
\(=
\fIt\fR \(= \fIT\fR
.LP
.sp 1
.LP
where
\fIf\fR\d\fId\fR\u =\ the dibit rate of 1200\ Hz,
.LP
and
\fIT\fR =\ the dibit period of 1/1200\ s.
.bp
.LP
.rs
.sp 27P
.ad r
\fBFIGURE 17/Q.274 p.5\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
6.4.1.5
\fILine power spectrum\fR
.sp 9p
.RT
.PP
The line power spectrum produced by the transmission of random data is
shown in Figure\ 18/Q.274. The spectral lines produced by the transmission
of repeated dibits (using the encoding phase relationship of \(sc\ 6.4.1.3\
above) are also shown.
.RT
.sp 1P
.LP
6.4.1.6
\fITransmitter requirements\fR \v'3p'
.sp 9p
.RT
.PP
a)
The transmitter output level shall be \(em15\ \(+-\ 1\ dBm0 (see
also Recommendation\ Q.272, \(sc\ 6.1.4).
.PP
b)
In the data transmitter, the bit timing and carrier
frequency are derived from the same source to facilitate receiver timing
recovery.
.sp 1P
.LP
6.4.1.7
\fIReceiver requirements\fR \v'3p'
.sp 9p
.RT
.PP
a)
The receiver sensitivity range shall be \(em15\ \(+-\ 8\ dBm0 [see
\(sc\ 6.4.1.6 above and Recommendation\ Q.272, \(sc\ 6.1.3 | )].
.PP
b)
The modem receiver shall be capable of establishing bit
synchronization as fast as possible, but in any case within 150\ milliseconds
while receiving synchronization signal units.
.PP
c)
The receiver shall maintain bit synchronization with the
distant transmitter for at least 500\ milli
seconds\ during a loss of data
carrier after initial bit synchronization has been established.
.bp
.LP
.rs
.sp 27P
.ad r
\fBFIGURE 18/Q.274 p.6\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
6.4.1.8
\fIInterface requirements\fR
.FS
The interface requirements for the digital version can be followed for
the analogue version. This admits the use of a universal signalling terminal.
.FE
.sp 9p
.RT
.PP
Each Administration may at its discretion integrate the modem into
the signalling terminal equipment or use a separate modem. If the modem is a
separate unit, then the interface requirements of Recommendations\ V.24/V.28
should be followed as far as possible. Alternatively the interface requirements
of \(sc\ 6.4.2.3 below may be followed.
.PP
The transmitting and receiving signalling terminals derive timing from
the timing frequency of the modem transmitter and receiver respectively.
.RT
.sp 2P
.LP
6.4.2
\fIDigital interface requirements\fR
.sp 1P
.RT
.sp 1P
.LP
6.4.2.1
\fIGeneral\fR \v'3p'
.sp 9p
.RT
.PP
a)
The interface between the signalling terminal and primary multiplex equipment
can be functionally represented as shown in
Figures\ 19/Q.274, 20/Q.274 and\ 21/Q.274. See also Recommendation\ G.703.
.PP
b)
The
interface adaptor
functions are rate conversion of data where required, rate and/or direction
conversion of clocks where required, generation of a receive holdover clock
and transfer of a loss of frame
alignment indication.
.PP
c)
The receive holdover clock must maintain bit synchronism for at least 500\
ms during data channel failure at all data rates after initial bit synchronization
has been established.
.PP
d)
The transmit and receive clock signals shall be in phase with the respective
data signals.
.bp
.sp 1P
.LP
6.4.2.2
\fIInterface and adaptor requirements\fR \v'3p'
.sp 9p
.RT
.PP
a)
\fIThe 4 kbit/s data transmission rate, 1544 kbit/s primary\fR
\fImultiplex.\fR \(em The interface and
adaptor functions for the 4 kbit/s data transmission rate over a 1544 kbit/s
primary multiplex are shown
in
Figure\ 19/Q.274. The diagram is intended to show functions and should
not be construed as depicting equipment.
.LP
.rs
.sp 19P
.ad r
\fBFIGURE 19/Q.274 p.7\fR
.sp 1P
.RT
.ad b
.RT
.PP
The interface adaptor is transparent to the send and receive data
and to a loss of frame alignment indication. Data channel failure is covered
in \(sc\ 6.5.
.PP
A holdover function on the 4 kHz receive clock to the signalling
terminal is provided to maintain bit synchronism for a minimum interval
during which the receive clock is not present.
.RT
.PP
b)
\fIThe 4 kbit/s data transmission rate, 2048 kbit/s primary\fR \fImultiplex.\fR
\(em The interface and
adaptor functions for the 4 kbit/s data transmission rate
over a 2048 kbit/s primary multiplex are shown in
Figure\ 20/Q.274. The
diagram is intended to show functions and should not be construed as depicting
equipment.
.LP
.PP
\fR The receive\(hyrate converter converts the receive data on the
64 kbit/s bearer channel to receive data at 4\ kbit/s using the 8 kHz and
64 kHz receive clocks. The 4 kHz receive clock is derived in the receive
clock
converter.
.PP
This material is subject to revision pending results of further
work by Study Group\ XVIII.
.FE
The send rate converter converts the send data at 4\ kbit/s to send
data on the 64 kbit/s digital bearer channel using the 8 kHz and 64 kHz
send clocks. The 4 kHz send clock is derived in the send clock
converter.
.PP
The interface adaptor is transparent to a loss of frame alignment
information. A holdover function on the 4 kHz receive clock to the signalling
terminal is provided to maintain bit synchronism for a minimum interval
during which the receive clock is not present. Data channel failure is
covered in
\(sc\ 6.5 below.
.RT
.PP
c)
\fIThe 56 kbit/s data transmission rate, 2048 kbit/s primary\fR
\fImultiplex\fR . \(em The interface and
adaptor functions for the
56\ kbit/s data transmission rate over a 2048 kbit/s primary multiplex
are shown in
Figure\ 21/Q.274. The diagram is intended to show functions and should
not be construed as depicting equipment.
.bp
.LP
.rs
.sp 26P
.ad r
\fBFIGURE 20/Q.274 p. 8\fR
.sp 1P
.RT
.ad b
.RT
.PP
The interface adaptor is transparent to the send and receive data
and to a loss of frame alignment indication. Data channel failure is covered
in \(sc\ 6.5 below.
.PP
The send data at 56 and 64\ kbit/s is aligned with the 8 kHz send clock.
Similarly, the receive data is aligned with the 8 kHz receive clock.
.PP
A holdover function on the receive clock to the signalling terminal is
provided to maintain bit synchronism for a minimum interval during which
the
receive clock is not present.
.RT
.sp 1P
.LP
6.4.2.3
\fIInterface electrical\fR \fIrequirements\fR
.sp 9p
.RT
.LP
.PP
Interface electrical requirements are given in Recommendation\ G.732 and
Recommendation\ G.733, for the interface between the primary multiplex
and the interface adaptor. Arrangements for the interface between the interface
adaptor and the signalling terminal are left to the discretion of
Administrations.
.PP
Each Administration may at its discretion integrate the interface
adaptor into the signalling terminal or the primary multiplex equipment
or may use a separate interface adaptor. If the interface adaptor is a
separate unit then the interface electrical requirements above must be
met. If it is
integrated into either the signalling terminal equipment or the multiplex
equipment the remaining interface must meet the interface electrical
requirements.
.bp
.RT
.LP
.rs
.sp 32P
.ad r
\fBFIGURE 21/Q.274 p.9\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
6.4.2.4
\fIInterface adaptor electrical requirements\fR \v'3p'
.sp 9p
.RT
.LP
a)
\fIThe 1544 kbit/s primary multiplex, 4 kbit/s channel\fR
.PP
The send and receive data and the send clock signals traverse the
interface adaptor
without modification.
.PP
The receive clock and the data channel failure information are separated
in the interface adaptor. The receive clock from the primary multiplex
synchronizes the receive holdover clock. The holdover clock provides the
receive clock to the signalling terminal. The interface adaptor recognizes
data channel failure by the absence of the receive clock from the primary
multiplex. This information is separately transferred to the signalling
terminal.
.PP
The receive holdover clock should:
.RT
.LP
\(em
maintain bit synchronism for at least 500\ ms after initial bit
synchronism is established, and
.LP
\(em
have a tolerance of \(+- | 0\ parts per million
when the receive clock is not present.
\v'3p'
.LP
b)
\fIThe 2048 kbit/s primary multiplex, 4 kbit/s channel\fR
.PP
Each bit of the 4 kbit/s data is represented by two
channel\(hytime\(hyslots
in the transmitted 64 kbit/s stream. These sixteen bits are encoded by the
send\(hyrate converter according to Table\ 4/Q.274\fR . The 8 bit bytes
are aligned
with the 8 kHz clock.
.bp
.LP
.rs
.sp 10P
.ad r
\fBTABLE [4/Q.274] p.10\fR
.sp 1P
.RT
.ad b
.RT
.PP
\fR Transmission of the data in this form makes it possible to detect
and correct for single, channel\(hytime\(hyslot slip avoiding the loss
of signalling data. This is achieved in the receiver\(hyrate converter
as follows. The 64\ kbit/s data stream is collected into 8 bit bytes using
the 8 kHz clock, and each byte is decoded. The reception of three consecutive
bytes of the same code indicates that channel\(hytime\(hyslot duplication
has occurred, and that a half cycle delay
must be introduced into the 4 kHz receive clock, whereas reception of a
single byte with a given code followed by a byte with a code signifying
a different
bit position, indicates that omission of a channel\(hytime\(hyslot has
occurred and that the 4 kHz clock must be advanced by half a cycle.
.PP
The send clock at 4\ kHz is derived directly from the 64 kHz and 8 kHz
send clocks. The 4 kHz receive clock is derived from the 64 kHz and 8 kHz
receive clocks, but it must be adjustable to take account of channel\(hytime\(hyslot
slip detected in the receive rate converter. The receive holdover clock
.PP
provides the receive clock to the signalling terminal. The interface adaptor
recognizes loss of frame alignment by the absence of the 8 kHz clock from
the primary multiplex or by an indication transmitted from the primary
multiplex
over a separate connection.
This information is separately
transferred to the signalling terminal.
.PP
The receive holdover clock should:
.RT
.LP
\(em
maintain bit synchronism for at least 500\ ms after initial bit
synchronism is established, and
.LP
\(em
have a tolerance of \(+- | 0\ parts per million
when the receive clocks are not present.
\v'3p'
.LP
c)
\fIThe 2048 kbit/s primary multiplex, 56 kbit/s channel\fR
.PP
The send and receive data and the send clock signals traverse the
interface adaptor without modification.
.PP
The 28\ bits of a signal unit are represented by bit positions 1\ to\ 7
of four consecutive channel time slots in the 64 kbit/s stream transmitted
from or received at the interface adaptor. Bit position\ 8 of consecutive
octets is coded \fB0\fR , | fB0\fR , | fB1\fR , | fB1\fR , | fB0\fR , | fB0\fR
, | fB1\fR , | fB1\fR ,\ . | | \fR in a continuing sequence. This pattern
is not suitable for direct transmission to the 1544 kbit/s
multiplex.
.PP
The receive holdover clock should:
.RT
.LP
\(em
maintain channel\(hytime\(hyslot synchronism for at least 500\ ms
after initial synchronism is established, and
.LP
\(em
have a tolerance of \(+- | 0\ parts per million
when the receive clocks are not present.
.LP
.sp 5
.bp
.sp 2P
.LP
\fBRecommendation\ Q.275\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB6.5\ DATA\ CHANNEL\ FAILURE\ DETECTION\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.275''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.275 %'
.ce 0
.sp 1P
.sp 2P
.LP
6.5.1
\fIGeneral\fR
.sp 1P
.RT
.PP
Detection of data channel failure is required to supplement the
eight\(hybit cyclic code. In case of unsatisfactory data transmission conditions
a data channel failure indication should be given to the terminal for use
in the error control equipment (see Recommendation\ Q.277, \(sc\ 6.7.2).
.RT
.sp 2P
.LP
6.5.2
\fIDetector requirements\fR
.sp 1P
.RT
.sp 1P
.LP
6.5.2.1
\fIData channel failure detector\ \(em\ analogue version\fR
.sp 9p
.RT
.PP
In this case the
data channel failure detector
is known as
the
\fIdata carrier failure detector\fR .
.RT
.LP
.PP
\fR a)
The data carrier failure detector is required to indicate
failure when transmission becomes unsatisfactory because of decreased carrier
level. A failure should be indicated when the received carrier is below
the
minimum sensitivity of the modem used, and should indicate no failure when
the level is above \(em23\ dBm0.
.PP
b)
The detector is required to detect the loss of carrier even
though the decrease in carrier power may be accompanied by an increase
in noise power. If a signal guard technique is used to distinguish carrier
power from
noise power, the received spectrum from 300\ Hz to 500\ Hz should be used to
detect the amount of noise power.
.PP
c)
The indication of failure or re\(hyestablishment of carrier should have
a nominal delay of 5\ ms with limits of 4\ ms minimum and 8\ ms
maximum.
.sp 1P
.LP
6.5.2.2
\fIData channel failure detector\ \(em\ digital version\fR
.sp 9p
.RT
.PP
In the case of both the 1544\ kbit/s and the 2048\ kbit/s primary
multiplexes, the data channel failure detector is known as the
\fIloss of frame alignment detector\fR .
.RT
.LP
.PP
a)
The loss of frame alignment detector is required to indicate
when the digital multiplex has lost frame alignment.
.PP
b)
The indication of loss or re\(hyestablishment of frame alignment should
have a mean delay of 2\ ms or less after the PCM\ equipment has detected
the loss or re\(hyestablishment of frame alignment.
.sp 1P
.LP
6.5.3
\fIInterface\fR
.sp 9p
.RT
.PP
In the case of the 1544\ kbit/s primary multiplex, data\(hychannel
failure is electrically indicated by inhibiting the 4 kHz receive clock.
.PP
In the case of the 2048\ kbit/s primary multiplex, loss of frame
alignment is electrically indicated by inhibiting the 8 kHz receive clock or
by an indication transmitted from the primary multiplex over a separate
connection.
\v'6p'
.RT
.LP
.sp 2P
.LP
\fBRecommendation\ Q.276\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB6.6\ SERVICE\ DEPENDABILITY\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.276''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.276 %'
.ce 0
.sp 1P
.sp 2P
.LP
6.6.1
\fIDependability requirements\fR
.sp 1P
.RT
.PP
The following dependability requirements should be obtained with
signalling links having the error rate characteristics as described in
Recommendation\ Q.272, \(sc\ 6.1.2. These requirements refer to each signalling
link.
.bp
.RT
.PP
a)
Signal units which carry telephone signal information and which are delayed
as a consequence of correction by retransmission:
.LP
\fInot more than one in 10\fR \fI\fI
\u\fI4\fR\dsuch signal units to be delayed as a long\(hyterm average.
.PP
b)
Signal units of any type which give rise to wrongly\(hyaccepted signals
due to undetected errors and causing false operation (e.g.,\ false
clear\(hyback signal):
.LP
\fInot more than one error in 10\fR \fI\fI
\u\fI8\fR\dof all signal units
trasmitted.
.PP
c)
As in item\ b) but causing serious false operation (e.g.,\ false metering
or false clearing of connection):
.LP
\fInot more than one error in 10\fI
\u\fI0\fR\dof all signal units
transmitted.
.PP
d)
Interruption to the signalling service (including both normal and reserve
links):
.LP
\(em
interruption of duration between 2\ seconds and 2\ minutes\ \(em\ not
more than once a year;
.LP
\(em
interruption of duration exceeding 2\ minutes\ \(em\ not more than
once in 10\ years.
.PP
Items a), b) and c) assume one telephone signal per signal unit. Results
for a multi\(hyunit message will be at least comparable to those for
one\(hyunit messages transmitting the same information.
.sp 1P
.LP
6.6.2
\fIRetransmission considerations\fR
.sp 9p
.RT
.PP
The requirement of \(sc\ 6.6.1 | ) above is inserted to limit the
.PP
percentage of the answer signals which are delayed through the retransmission
process. The amount of retransmission depends on the number of bits in
the
signal units and on interferences such as those caused by short interruptions
and intermittent bursts of noise up to the point at which changeover to
the
reserve link occurs.
.RT
.sp 1P
.LP
6.6.3
\fIService interruption considerations\fR
.sp 9p
.RT
.PP
The requirement of \(sc\ 6.6.1 | ) depends largely on the performance of
the voice frequency links or digital links assigned for signalling. Therefore
precautions should be taken in the design stage of the terminal equipment
to
ensure that the contribution to the total is relatively small.
\v'1P'
.RT
.LP
.sp 2P
.LP
\fBRecommendation\ Q.277\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB6.7\ ERROR\ CONTROL\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.277''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.277 %'
.ce 0
.sp 1P
.sp 2P
.LP
6.7.1
\fIError detection\fR \fIby the use of check bits\fR
.sp 1P
.RT
.PP
The disturbance of a signal unit during transmission will be detected by
the use of coders and decoders, connected at the transmitting and receiving
terminals respectively. The coder will generate 8\ check bits based on
the
polynomial \fBX\fR\(ua\fB8\fR \ +\ \fBX\fR\(ua\fB2\fR \ +\ \fBX\fR \
+\ \fB1\fR (see Table\ 5/Q.277 for the matrix and for a typical implementation).
.PP
These check bits will constitute bits 21\(hy28 of each signal unit and
are inverted before transmission to provide protection against a single
bit\(hyslip of synchronization.
.PP
When the decoder at the receiving terminal has received all 28\ bits of
a signal unit after the check bits have been reinverted, it will indicate
whether or not the signal unit has been checked correctly. This information
will be
stored for inclusion in the acknowledgement field of an\ ACU to be emitted in
the return direction. An ACU will be transmitted after each 11\ signal
units to form a block (see Recommendation\ Q.251, \(sc\ 1.1.2).
.bp
.RT
.LP
.rs
.sp 32P
.ad r
\fBTABLE [5/Q.277] p.11\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
6.7.2
\fIError detection by data channel failure detection\fR
.sp 9p
.RT
.PP
The data carrier failure detector or loss of frame alignment detector will
supplement the error detection by use of check bits. Indication of data
channel failure at any time during the process of reception will cause the
rejection of signal units in the process of reception. Regardless of the
result of decoding, the\ ACU should acknowledge the signal unit as received
incorrectly.
.RT
.LP
.sp 1P
.LP
6.7.3
\fIError correction\fR
.sp 9p
.RT
.PP
Correction is achieved by retransmission of the messages which are
not acknowledged to have been received correctly. The \fIblock structure\fR
and the contents of the\ ACU have been described in Recommendations\ Q.251,
\(sc\ 1.1.2,
and\ Q.259, \(sc\ 3.3.1. The acknowledgement indicators should be transmitted
in the same sequence as the signal unit to which they refer.
.PP
A retransmission to comply with the information in the ACU will be made
possible by storing at the transmitting terminal the signal units with
their
block reference numbers at the time of emission. This record must be maintained
until the receipt of the associated ACU, when the record of messages which
are acknowledged to have been correctly received should be eliminated.
In the case of multi\(hyunit messages, the complete message should be retransmitted
if any of its constituent signal units fail to check correctly. A multi\(hyunit
message may contain signal units which are transmitted in two adjacent
blocks, but it must be ensured that the records of the constituent signal
units of the multi\(hyunit message remain until the acknowledgement indicators
show that the complete
multi\(hyunit message has been received correctly.
.bp
.PP
In the unlikely event that a terminal is unable to accept a
correctly\(hyreceived signal unit, e.g.\ due to input buffer congestion, the
appropriate acknowledgement indicator bit in the outgoing ACU is marked
as if the signal unit were received in error.
.PP
The maximum permitted delay between the emission of a signal unit and
the subsequent reception of the ACU containing the acknowledgement of this
signal unit is as follows:
.RT
.PP
a)
\fIWhere the multi\(hyblock monitoring procedure is not used\fR , the
maximum permitted delay between the emission of a signal unit and the
subsequent handling of the received ACU containing the acknowledgement
of that signal unit must not exceed the time taken to send 8\ blocks (96\
signal units). Of this time (96\ signal units), the time for 64\ signal
units (maximum) is
available for the loop propagation time of the data link (see Note\ 1). At a
data rate of 2400\ bit/s this caters for a loop propagation time of up
to 740\ ms (see Note\ 2).
.LP
.PP
b)
\fIWhere the\fR
\fImulti\(hyblock monitoring procedure\fR \fIis\fR
\fIused\fR , the maximum
permitted delay between the emission of a signal unit and the subsequent
handling of the received ACU containing the acknowledgement of that signal
unit must not exceed the time taken to send 256\ blocks (see Note\ 3).
Of this time
(up to 3072\ signal units), all but about 32\ signal units are available
for the loop propagation time of the data link. At a data rate of 56\ kbit/s,
this
caters for a loop propagation time of up to 1520\ ms.
.PP
\fINote\ 1\fR \ \(em\ The number, 64 signal units, is based on the consideration
that out of the total number of 96\ signal units, 32\ signal units are
allocated as follows:
.PP
At the exchange emitting signal units:
.RT
.LP
emission\ of\ SU
.LP
reception\ of\ ACU
not more than the time for
sending 3\ signal units
.LP
processing
.PP
At the exchange receiving signal units:
.LP
reception\ of\ SU
.LP
generation\ of\ ACU
.LP
time\ in\ ACU\ queue
.LP
not more than the time for
sending 29\ signal units
.LP
emission\ of\ ACU
.LP
time\ for\ drift\ compensation
.LP
processing
.PP
\fINote\ 2\fR \ \(em\ The time for sending 64\ signal units is also equivalent
to
.LP
448\ ms at \ 4\ kbit/s
.LP
\ 32\ ms at 56\ kbit/s.
.PP
\fINote\ 3\fR \ \(em\ The full 256\ blocks need not be handled in all designs,
e.g. block memory may be limited to that required for the expected range
of
loop propagation delays and data rates at which the terminal will be applied.
If the error control loop cannot exceed 8\ blocks, multi\(hyblock monitoring
equipment need not be provided.
.PP
The messages, which are not acknowledged to have been correctly received
should be presented for retransmission, at which time the record of their
previous transmission should be eliminated. The exception to the general
rule is that the following signalling system control units should never
be
retransmitted: acknowledgement, synchronization, multi\(hyblock monitoring,
multi\(hyblock acknowledgement, and changeover.
.PP
All signal units in a block except the SYU, ACU, multi\(hyblock monitoring,
multi\(hyblock acknowledgement, and changeover system control signal units
must be retransmitted if the\ ACU, referring to that block, is not received
correctly. This may arise owing to the fact that the\ ACU fails to check
correctly on
account of errors during transmission or owing to drift between the data
streams in the two directions (see Recommendation\ Q.279).
.PP
The first three bits of the ACU (i.e. the heading code) may be used for
identification purposes (see Recommendation\ Q.\ 259, \(sc\ 3.3.2.2). If
the\ ACU
checks to be error\(hyfree and the heading is correct the probability of an
undetected error is extremely small.
.bp
.RT
.LP
.sp 2P
.LP
\fBRecommendation\ Q.278\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB6.8\ SYNCHRONIZATION\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.278''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.278 %'
.ce 0
.sp 1P
.sp 2P
.LP
6.8.1
\fIGeneral\fR
.sp 1P
.RT
.PP
The SYU will contain, in addition to the 8 check bits, a 16 bit
pattern for
bit and signal unit synchronization
and a 4 bit number for
block synchronization
. The same 16\(hybit pattern will appear in
every\ SYU. The 4\(hybit number will describe the position of the SYU within
its
block (see Recommendation\ Q.259, \(sc\ 3.3.3.2).
.PP
Each signalling terminal requires 2 counters of up to 8\ bits capacity,
to keep a record of the blocks completed and acknowledged.
.PP
The block\(hycompleted counter (BCC) indicates the sequence number of the
last block transmitted by the terminal. The last 3\ bits of this number
are also sent to the\ ACU of the block and occupy the bit positions reserved
for the block\(hycompleted sequence number\ (BCSN).
.PP
The block\(hyacknowledged counter (BAC) is up\(hydated using the
block\(hyacknowledged sequence number (BASN) in the incoming ACUs and therefore
indicates the sequence number of the block being acknowledged by the last
received\ ACU. In order to keep it up to date even when ACUs are detected in
error, the block\(hyacknow
ledged\ counter is incremented whenever the twelfth
signal unit of a block is received in error. In the case where the
block\(hyacknowledged sequence number does not have the expected value, the
block\(hyacknowledged counter will be up\(hydated as follows:
.RT
.LP
\(em
If the BASN has the same value as in the previous ACU, the
BAC will not be incremented;
.LP
\(em
If the BASN has an unexpected value greater than the previous BASN, then
the least significant three bits of the BAC are replaced by the
latest BASN;
.LP
\(em
If the BASN has a value less than the previous BASN, then the BAC is
incremented by eight and the least significant three bits replaced by
the latest BASN.
.PP
If the terminal is in multiblock synchronization, and if a jump in the
BASN of greater than 2 or equal to or less than \(em1 occurs, then multiblock
synchronization must be checked immediately.
.PP
The counters are set to zero during normal synchronization and are
checked periodically by using the multiblock monitoring procedure.
.PP
If the capacity of the counters is exceeded by the number of blocks in
the error control loop, the signalling link is not capable of being used.
.PP
Some variations of the synchronization procedures in the specification
may be incompatible with \fIGreen\ Book\fR synchronization procedures.
.RT
.sp 1P
.LP
6.8.2
\fINormal synchronization\fR
.sp 9p
.RT
.PP
This synchronization procedure is used whenever a signalling link is brought
into service, either initially or after a total loss of synchronism.
.PP
Normal synchronism will be established in the following manner. Each
terminal will emit either:
.RT
.LP
\(em
a series of blocks containing eleven SYUs, plus one ACU, or
.LP
\(em
a series of blocks of
\fIfaulty\(hylink information\fR as
covered in Recommendation\ Q.293, \(sc\ 8.6.1, when changeover has been
requested.
.PP
In both cases ACUs are transmitted initially with the
acknowledgement indicators set to \fB1\fR and the block\(hycompleted and
block\(hyacknowledged sequence number set to\ \fB0\fR .
.PP
The instant of commencement of emission at the terminals is immaterial.
.PP
After bit synchronism has been established in the demodulator, the
incoming bit stream will be monitored to find the\ SYU pattern. Once this
pattern is found and verified the sequence number can be determined and
the\ ACU position located.
.PP
In due course, three consecutive ACUs should be correctly received with
block\(hyacknowledged sequence number set to\ \fB0\fR .
.PP
At this time the acknowledgement indicators in the next outgoing ACU are
set to reflect the detected errors in the signal units of the associated
received block. Both sequence numbers in the\ ACU remain at\ \fB0\fR .
.bp
.PP
The reception of at least two consecutive ACUs with block\(hyacknowledged
sequence numbers set to\ \fB0\fR which check correctly and acknowledge
one or more signal units as correct indicates that both terminals are in
bit, signal unit and block synchronism.
.PP
At this time the one\(hyminute proving period is started and block sequence
numbering is initiated as follows:
.RT
.LP
the block\(hycompleted counter and block\(hycompleted sequence number in
the next outgoing ACU are set to\ \fB1\fR . Thereafter the counter and the
block\(hycompleted sequence number in the ACU are incremented by 1\ each
time an\ ACU is transmitted. The block\(hyacknowledged sequence number in
outgoing ACUs is now up\(hydated from the block\(hycompleted sequence number
of the appropriate received ACU.
.PP
When the terminal receives an ACU with a block\(hyacknowledged sequence
number other than \fB0\fR , the block\(hyacknowledged counter is set to
this number.
Thereafter, the counter is up\(hydated by the appropriate block\(hyacknowledged
sequence number each time an ACU is received.
.PP
When the block\(hyacknowledged counter is incremented for the first time
the number of blocks in the error control loop may be determined by subtracting
the contents of the block\(hyacknowledged counter from the contents of
the
block\(hycompleted counter. Should the result be negative, then the counters
should be reset and block sequence numbering should be restarted.
.PP
The counter capacity is insufficient if the block\(hycompleted counter
recycles before the block\(hyacknowledged counter advances.
.PP
If, and only if, the initial synchronization procedure has indicated
more than eight blocks in the error control loop, should the
multi\(hyblock\(hymonitoring procedure be used once every cycle of the
block\(hycompleted counter. In this case the multi\(hyblock monitoring
procedure
should also be used for block resynchronization (see \(sc\ 6.8.4\ below).
.PP
Whenever a multi\(hyblock\(hymonitoring signal is received it must be
acknowledged by a multi\(hyblock\(hyacknow
ledgement\ signal within the
time required to send 40\ signal units.
.PP
When the multi\(hyblock\(hyacknowledgement signal is received, the multi\(hyblock
and block numbers are compared with the contents of the block\(hyacknowledged
counter. If the received number is within minus four to plus three of the
contents of the block\(hyacknowledged counter then it is assumed that multi\(hyblock
synchronism exists.
.PP
When a multi\(hyblock\(hyacknowledgement signal is not received in response
to a multi\(hyblock\(hymonitoring signal transmitted no action need be
taken. However, if a multi\(hyblock\(hymonitoring signal is acknowledged
as being received in error or if the\ ACU is in error then the multi\(hyblock
monitoring procedure may be
restarted.
.PP
If the signal unit error rate is acceptable at the end of the one\(hyminute
proving period, two load\(hytransfer signals are emitted in the case of
a regular link, or two standby\(hyready signals for synchronized reserve
links.
Acknowledgement of these signals by the other terminal is as covered in
Recommendation\ Q.293, \(sc\(sc\ 8.6.2 and\ 8.8. Signalling traffic may then be
offered to regular links, while synchronized reserve links may be marked as
ready for service.
.PP
The one\(hyminute and the emergency proving periods and the load\(hytransfer
signalling sequence are omitted for non\(hysynchronized reserve links when
a
changeover is made from the regular link as covered in
Recommendation\ Q.293, \(sc\ 8.6.1.
.PP
Bit synchronism
is maintained by the transition between dibits for the analogue modem or
by the receive clock on digital links; loss of
synchronism will result in signal units failing to check correctly; however,
incorrect
.PP
signal units are more likely to result from line interference than loss of
synchronism. Monitoring of the bit stream should result in the recognition
of the 16 bit pattern of an SYU and enable synchronism to be restored if
it had
been lost.
.RT
.sp 1P
.LP
6.8.3
\fISignal unit resynchronization\fR
.sp 9p
.RT
.PP
Loss of signal unit synchronism
will result in continuous failure of signal units to check. When the signalling
terminal receives
consecutive signal units in error, it may take unilateral action to
resynchronize to the incoming bit stream. In any ACUs transmitted during
this procedure, all the indicator bits must be set to\ \fB1\fR and the
block\(hyacknowledged number and the block\(hycompleted number must be
incremented as in normal
operation. When synchronism is re\(hyestablished on the incoming channel the
indicators are set according to the incoming signal units, i.e.\ normal
operation is resumed. The
signal unit error rate monitor
must continue to count signal units in error throughout this procedure.
.bp
.PP
During unilateral resynchronization, means must be provided to ensure that
false resynchronization is kept to a level that is compatible with the
dependability requirements (Recommendation\ Q.276). For this reason signal
units should be checked to see that synchronization is valid.
.RT
.LP
.sp 1P
.LP
6.8.4
\fIBlock resynchronization\fR
.sp 9p
.RT
.PP
Equipment must be provided to detect loss of block synchronism.
.PP
Loss of block synchronism will be recognized when a valid signal unit,
which is not an\ ACU, is received in the 12th\ position in a block.
.PP
Loss of block synchronism may also be recognized by any of the
following:
.RT
.LP
a)
an\ ACU is received in other than the 12th position in a
block;
.LP
b)
the block\(hycompleted sequence number is not the one expected
(see Note\ 2);
.LP
c)
an\ SYU sequence number is not the one expected.
.PP
Loss of block synchronism will not be recognized prior to the
initial incrementing of the block\(hyacknow
ledged\ counter, during either
initial
synchronization or after a total loss of synchronism as specified in \(sc\
6.8.2.
.PP
When
loss of block synchronism
has been recognized \(em by any of the four events described above\ \(em
the terminal will stop sending telephone
signals and send only SYUs and repeated ACUs (see Recommendation\ Q.279).
.PP
When the terminal has identified the signal unit position in a block
either by recognizing the SYU\ number or by identifying an\ ACU, and has
subsequently recognized two consecutive ACUs with correctly advancing
block\(hycompleted sequence numbers, synchronism is deemed to have been
regained.
.PP
After successful block\(hysynchronization, the block being transmitted
is completed with SYUs and an\ ACU. At least one complete block of 11\
SYUs shall be sent before resuming normal traffic.
.PP
The first\ ACU sent after synchronization has been regained will have the
following characteristics:
.RT
.LP
a)
the indicator bits are all set to\ \fB1\fR ;
.LP
b)
the blocked\(hycompleted sequence number is set to the next in
sequence;
.LP
c)
the block\(hyacknowledged sequence number will correspond to
the latest received ACU.
.PP
Upon resynchronization, a terminal may receive an ACU with an
acknowledged block number which differs from that expected. All messages
sent in unacknowledged blocks should be retransmitted.
.PP
After the completion of block resynchronization, multi\(hyblock synchronism
should be checked, if applicable.
.PP
When block synchronism cannot be regained within 350\ ms, the link is
considered to have failed and resynchronization according to \(sc\ 6.8.2 is
commenced. The relevant link security procedures of Recommendation\ Q.293
will be initiated where appropriate (e.g.\ changeover, emergency restart,\
etc.). In the case of a link that is not carrying signalling traffic, resynchronization
should commence without waiting for 350\ ms, that is, unilateral block
resynchronization should be dispensed with.
.PP
\fINote\ 1\fR \ \(em\ An all\(hyzero signal, i.e. a signal unit consisting of
20\ zeros with the correct check bits, may cause a discontinuity in the
transmitted signal unit sequence.
.PP
A receiving terminal which can recognize such a signal may, optionally,
take steps to ensure that synchronism is not lost. In this case, the zero
signal units should be treated as if they were in error, causing the error
counter to be stepped, but no request for retransmission should be sent.
Thus if zero signal units are received too frequently, a changeover or
emergency
restart will be initiated.
.PP
\fINote\ 2\fR \ \(em\ If an unexpected ACU with both the BASN and BCSN equal to
zero is received, reset the block counters, restart the block sequence
numbering as in \(sc\ 6.8.2 and count the ACU as being in error.
.bp
.RT
.sp 1P
.LP
6.8.5
\fIMulti\(hyblock resynchronization\fR
.sp 9p
.RT
.PP
If the multi\(hyblock and block numbers in a multi\(hyblock\(hyacknowledgement
signal unit are not within minus four to plus three of the contents of
the
block\(hyacknowledged counter a new multi\(hyblock\(hymonitoring signal
is sent. If the result of the second measurement is not within the above
limit, multi\(hyblock
synchronism has been lost. However, if the results of the measurements
are the same, multi\(hyblock synchronism can be regained by up\(hydating
the contents of the block\(hyacknowledged counter to the obtained result.
.PP
When the second multi\(hyblock\(hymonitoring signal is sent the terminal
will send only SYUs and ACUs for three blocks. Normal traffic is then resumed
and
all messages transmitted in the interval between the two multi\(hyblock\(hymonitoring
signals are retransmitted.
.PP
If multiblock synchronism cannot be regained, the link is considered to
have failed and resynchronization according to \(sc\ 6.8.2 is commenced.
The block counters will be reset and the block sequence numbering restarted.
The relevant link security procedures of Recommendation\ Q.293 will be
initiated where
appropriate (e.g.\ changeover, emergency restart.\ etc.).
\v'6p'
.RT
.LP
.sp 2P
.LP
\fBRecommendation\ Q.279\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB6.9\ \fR \fBDRIFT\ COMPENSATION\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.279''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.279 %'
.ce 0
.sp 1P
.sp 2P
.LP
6.9.1
\fIGeneral\fR
.sp 1P
.RT
.PP
The difference in clock rates at the two terminations of a signalling link
will result in a drift between the bit streams transmitted in the two
directions.
.PP
The slower terminal will find at some stage that it has two blocks
awaiting acknowledgement. When this occurs, only the second (later block)
should be acknowledged (
\fIskipping\fR of an\ ACU
). On receipt of the
acknowledgement of the second block, the sending terminal will initiate the
transmission of all messages in the first block as if they were received in
error before proceeding with any necessary retransmission relating to the
second block.
.PP
Moreover, the faster terminal will find at some stage that it has no complete
new block to acknowledge in the\ ACU it is about to transmit. In this case,
the acknowledgement fields for the indicators and block number (bits\ 4
to\ 17) from the previous block are repeated (
\fIrepeating\fR of an\ ACU
).
This\ ACU will be recognized to be a repetition by the cyclic number (bits\ 15
to\ 17) and should be ignored by the slow terminal (see Recommendation\ Q.259,
\(sc\ 3.3.2).
.RT
.sp 1P
.LP
6.9.2
\fIDrift compensation hysteresis\fR
.sp 9p
.RT
.PP
When the time difference between the moment at which the second block is
received and the moment at which the acknowledgement should be sent is
very small (e.g.\ less than one signal unit), drift compensation may be
required at frequent intervals. In order to avoid alternative skipping
and repeating ACUs too frequently, it is recommended that a certain interval
elapses between the opposite decisions \fIto skip\fR and \fIto repeat\fR
ACUs (drift compensation
hysteresis). This interval must be sufficiently long to avoid unnecessary
drift compensations, but short enough that acknowledging of the concerned
block is
not delayed too much.
.RT
.LP
.rs
.sp 6P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.sp 1P
.ce 1000
\v'3P'
SECTION\ 7
.ce 0
.sp 1P
.ce 1000
\fBSIGNAL\ TRAFFIC\ CHARACTERISTICS\fR \v'6p'
.ce 0
.sp 1P
.sp 2P
.LP
\fBRecommendation\ Q.285\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB7.1\ SIGNAL\ PRIORITY\ CATEGORIES\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.285''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.285 %'
.ce 0
.sp 1P
.sp 2P
.LP
7.1.1
\fIRules for signal priority\fR
.sp 1P
.RT
.PP
The following rules for establishing priority categories must be
.PP
followed in normal operation; within any of the priority categories, signals
are transmitted in order of their arrival at the output buffer (see
Recommendation\ Q.251, \(sc\ 1.1.1):
.RT
.LP
a)
Acknowledgement signal units (12th signal unit of each
block) have absolute priority for emission at their fixed
predetermined position;
.LP
b)
Faulty link information (Recommendation\ Q.293, \(sc\ 8.6.1) has
priority over all other signals;
.LP
c)
The answer signal, charge, the answer signal, no charge and
the multi\(hyblock\(hymonitoring and multi\(hyblock\(hyacknowledgement signals
have priority over other waiting telephone signals and
signalling\(hysystem\(hycontrol signals except those cited in a) and b)\fR
above;
.LP
d)
All other telephone signals, one\(hyunit or multi\(hyunit
messages, and all other signalling\(hysystem\(hycontrol signals, except
synchronization signal units, have priority over management or other
signals concerned with the bulk handling of traffic;
.LP
\fINote\fR \ \(em\ In the event that a management signal concerns the bulk
restoration of service, e.g.\ RSB, RBA, TFA, TAA, this signal may
take priority over other telephone or signalling system control
signals.
.LP
e)
Any signal which is to be retransmitted will take precedence
over other waiting signals in the same priority category;
.LP
f
)
Management signals have priority over
synchronization signal units;
.LP
g)
Synchronization signal units have no priority.
.sp 1P
.LP
7.1.2
\fIBreak\(hyin\fR \v'3p'
.sp 9p
.RT
.PP
a)
Potential for a priority one\(hyunit message to break into a multi\(hyunit
message is provided in the design of the format, but initially this feature
will not apply except for\ ACU;
.PP
b)
If a multi\(hyunit message is used for a management signal,
potential for break\(hyin by a lone signal unit should be retained as a future
option. However, there is no intent to provide means for a multi\(hyunit
message to apply break\(hyin to another multi\(hyunit message.
.PP
c)
In the rare event that a SYU breaks into a multi\(hyunit message (e.g.
owing to severe processor overload), the multi\(hyunit message may be
accepted as valid.
.bp
.LP
.sp 2P
.LP
\fBRecommendation\ Q.286\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB7.2\ SIGNALLING\ CHANNEL\ LOADING\ AND\ QUEUEING\ DELAYS\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.286''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.286 %'
.ce 0
.sp 1P
.sp 2P
.LP
7.2.1
\fILoading potential\fR
.sp 1P
.RT
.PP
According to Recommendation\ Q.257, \(sc\ 3.1.3.3, the System No.\ 6
design provides the potential in circuit labels to identify 2048\ telephone
circuits. Considering that the load per signalling system will vary according
to the traffic characteristics of the circuits served and the number of
signals in use, it is not practicable to specify a general maximum limit
of circuits
that a system can handle. The maximum number of circuits to be served must
be determined for each situation, taking into account the traffic characteristics
which apply, so that the total signalling load is held to a level which
will
maintain an acceptable signalling delay value resulting from queueing.
.RT
.LP
.sp 1P
.LP
7.2.2
\fIQueueing delays\fR
.sp 9p
.RT
.PP
Common channel signalling systems handle the required signals for
many circuits on a time\(hyshared basis. With time\(hysharing, signalling delay
occurs when it is necessary to process more than one signal in a given
interval of time. When this occurs, a queue is built up from which signals
are
transmitted in order of their time of arrival and of their priority. Formulae,
which are in close agreement with computer simulation tests and are recommended
for calculating average queueing delays for the signals listed and the
variables noted, are given in the Annex\ A to this Recommendation.
\v'6p'
.RT
.LP
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation Q.286)
.sp 9p
.RT
.ce 0
.ce 1000
\fBQueueing delay formulae for telephone signals\fR
.sp 1P
.RT
.ce 0
.sp 2P
.LP
\fIAnswer signal:\fR \ One\(hyunit message with priority
\v'6p'
.sp 1P
.RT
.ce 1000
\fIQ
\dw\u\fR =
@ { ~+ (\fID\fR~\(em~1) \fIa~\dd~\u\fR } over { 1~\(em~\fIa~\dc\u\fR ) (1~\(em~\fIa~\dc\u\fR~\(em~~\fIa~\dwM~\u\fR ) } @ \(mu
[Unable to Convert Formula]
.ce 0
.ad r
(1)
\v'1p'
.ad b
.RT
.sp 1P
.LP
.sp 1
\fIOther telephone signals:\fR \ One\(hyunit message without priority
\v'6p'
.sp 9p
.RT
.ce 1000
\fIQ
\do\u\fR =
@ { ~+ (\fID\fR~\(em~1) \fIa~\dd~\u\fR } over { 1~\(em~\fIa~\dc\u\fR~\(em~\fIa~\dpM~\u\fR ) (1~\(em~\fIa~\dc\u\fR~\(em~\fIa~\dwM~\u\fR ) } @ \(mu
[Unable to Convert Formula]
.ce 0
.ad r
(2)
.ad b
.RT
.sp 1P
.LP
.sp 1
\fIAddress signal:\fR \ Multi\(hyunit message without priority
\v'6p'
.sp 9p
.RT
.ce 1000
\fIQ
\dd
\u\fR = \fIQ
\do\u\fR +
@ { \fID\fR~\(em~1) \fIa~\dc\u\fR } over { ~\(em~\fIa~\dc\u\fR } @ \(mu
\fIT
\de\u\fR
.ce 0
.ad r
(3)
\v'1p'
.ad b
.RT
.LP
.sp 2
.LP
where
\fIQ\fR\d\fIw\fR\u\fI,\fR \ \fIQ\fR\d\fIo\fR\u\fI,\fR \ \fIQ\fR\d\fId\fR\u =
average queueing delay,
.LP
\fIa\fR\d\fIw\fR\u =
traffic of answer signals if multi\(hyblock
synchronization signal units are not used,
.LP
\fR \fIa\fR\d\fIw\fR\\d\fIM\fR\u =
traffic of answer signals, multi\(hyblock
monitoring and multi\(hyblock acknowledgement
signals if multi\(hyblock synchronization signal
units are used,
.LP
\fIa\fR\d\fId\fR\u =
traffic of multi\(hyunit address messages,
.bp
.LP
\fR \fR \fIa\fR\d\fIp\fR\u =
traffic of all telephone signals, if
multi\(hyblock synchronization signal units
are not used,
.LP
\fIa\fR\d\fIp\fR\\d\fIM\fR\u =
traffic of all telephone signals,
multi\(hyblock monitoring and multi\(hyblock
acknowledgment signals if multi\(hyblock
synchronization signal units are used,
.LP
\fIa\fR \fI\d\fIc\fR\u =
traffic of acknowledgement signal
units,
.LP
\fIT\fR\d\fIe\fR\u =
emission time of a signal unit,
.LP
\fID\fR =
number of SUs composing a multi\(hyunit address
message.
.PP
When multi\(hyunit address messages are of different length the average
queueing delay for the messages composed of \fID\fR\d\fIi\fR\u\ SUs is
given by formula\ (3) using \fID\fR\d\fIi\fR\ufor\ \fID\fR . In formulae\
(1) and\ (2), the following values should be used:
\v'6p'
.sp 1P
.ce 1000
\fID\fR =
@ { pile { fBi\fR above sum above \fBi\fR } fID~\di\u~a~\ddi~\u\fR } over { fIa~\dd~\u\fR } @ and
\fIa
\dd
\u\fR =
@ pile { fBi\fR above sum above \fBi\fR } @\fIa
\ddi
\u\fR $$Be
.ce 0
.sp 1P
.LP
.sp 1
where \fIa\fR\d\fId\fR\\d\fIi\fR\uis the traffic of the messages composed
of \fID\fR\d\fIi\fR\u\ SUs.
.PP
\fINote\ 1\fR \ \(em\ The unit of traffic is the erlang. The traffic \fIa\fR\d\fIp\fR\uincludes
\fIa\fR\d\fIw\fR\u\fI, a\fI\d\fId\fR\uand the traffic of other one\(hyunit
messages, but excludes\ \fIa\fR\d\fIc\fR\u.
.PP
\fINote\ 2\fR \ \(em\ These formulae include the effects of systematic
delay (due to synchronous operation and block composition) and of traffic
delay, but do
not include the emission time of the signal message and the delay resulting
from eventual retransmission of signal messages.
.PP
\fINote\ 3\fR \ \(em\ In addition, formula\ (3) includes the effect of
break\(hyin by acknowledgement signal units.
.PP
\fINote\ 4\fR \ \(em\ Signal units of lower priority, e.g. management signal
units and synchronization signal units, have no influence on the delay of
telephone signals.
.RT
.LP
.rs
.sp 27P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.sp 1P
.ce 1000
\fBExample of queueing delays\fR
.sp 1P
.RT
.ce 0
.sp 1P
.PP
The traffic model assumed is given in Table\ 6/Q.286, from which the proportion
of signal traffic may be obtained as shown in
Table 7/Q.286. Using Table 7/Q.286, queueing delays are calculated as shown
in Figure\ 22/Q.286.
.sp 1P
.RT
.LP
.rs
.sp 47P
.ad r
\fBFigure 22/Q.286 p. 12\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 22P
.ad r
\fBTableau [6/Q.286] p. 13\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 20P
.ad r
\fBTableau [7/Q.286] p. 14\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
\fBRecommendation\ Q.287\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB7.3\ SIGNAL\ TRANSFER\ TIME\ REQUIREMENTS\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.287''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.287 %'
.ce 0
.sp 1P
.PP
The cross\(hyoffice signal transfer should be fast so as not to
lose the
advantage of the fast signalling capability of the System\ No.\ 6. While
no firm time requirements in regard to the various components of signal
transfer time have been established, Annex\ A to this Recommendation contains
design
objectives in terms of average and 95%\ level time values for \fIT\fR\d\fIh\fR\uand
\fIT\fR\d\fIc\fR\ufor the answer signal, other one\(hyunit messages and
the initial address message at the specified data rates. These figures
have to be viewed as
reasonable design requirements.
.sp 1P
.RT
.LP
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation Q.287)
.sp 9p
.RT
.ce 0
.ce 1000
\fBEstimates for transfer times\fR
.sp 1P
.RT
.ce 0
.LP
1.
\fIDesign objectives\fR
.sp 1P
.RT
.PP
The design objectives for the handling time \fIT\fR\d\fIh\fR\uand the
cross\(hyoffice transfer time \fIT\fR\d\fIc\fR\uare shown in Table\ 8/Q.287.
.RT
.LP
.rs
.sp 18P
.ad r
\fBTABLE [8/Q.287] p.15\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
2.
\fICalculation for\fR
\fIcross\(hyoffice transfer time\fR
.sp 1P
.RT
.sp 1P
.LP
\fIAverage value:\fR
.sp 9p
.RT
.PP
The average value of the cross\(hyoffice transfer time,
\fIT
\dcAV
\u\fR , is calculated by the following formula:
.RT
.ad r
\fIT
\dcAV
\u\fR = \fIT
\dr\u\fR + \fIT
\dhAV
\u\fR +
\fIT
\dsAV
\u\fR .
(1)
.ad b
.RT
.PP
The average value of the sender transfer time,
\fIT
\dsAV
\u\fR , is approximated as follows:
.LP
\fIT
\dsAV
\u\fR = \fIT
\dqAV
\u\fR + \fIT
\dm\u\fR + \fIT
\de\u\fR ,
for one\(hyunit messages
(2a)
.bp
.ad r
.ad b
.RT
.ad r
\fIT
\dsAV
\u\fR = \fIT
\dqAV
\u\fR + \fIT
\dm\u\fR + (\fID\fR \(mu
\fIT
\de\u\fR ),
for multi\(hyunit messages
\fI\fR
(2b)
.ad b
.RT
.LP
.LP
where
\fIT\fR\d\fIe\fR\u =
emission time of a signal unit,
.LP
\fIT\fR\d\fIm\fR\u
=
time for encoding and modulation and, where
present, parallel to serial conversion,
.LP
\fIT\fR\d\fIr\fR\u =
receiver transfer time,
.LP
\fID\fR =
number of SUs composing a multi\(hyunit message.
.PP
The average queueing delay,
\fIT
\dqAV
\u\fR , is equivalent to
\fIQ\fR\d\fIw\fR\u, \fIQ\fR\d\fIo\fR\uor\ \fIQ\fR\d\fId\fR\uwhich is calculated
by the formula in
Annex\ A to Recommendation\ Q.286.
.sp 1P
.LP
\fI95% level value:\fR
.sp 9p
.RT
.PP
The 95% level value of the cross\(hyoffice transfer time,
\fIT
\dc | fR 95%\fI
\u\fR , is approximated by the
following formula:
\v'6p'
.RT
.sp 1P
.ce 1000
\fIT
\dc | fR 95%\fI
\u\fR = \fIT
\dcAV
\u\fR
$$0v(\(*D\fIT
\dh\u\fR )
\u2\d +
(\(*D\fIT
\dq\u\fR )
\u2\d
$$ve
\v'6p'
.ce 0
.sp 1P
.ce 1000
.sp 1
where
(3)
\(*D\fIT
\dh\u\fR = \fIT
\dh | fR 95%\fI
\u\fR \(em \fIT
\dhAV
\u\fR \v'6p'
.ce 0
.sp 1P
.ce 1000
\(*D\fIT
\dq\u\fR = \fIT
\dq | fR 95%\fI
\u\fR \(em \fIT
\dqAV
\u\fR
.ce 0
.sp 1P
.LP
.sp 1
.PP
The 95% level value of the queueing delay,
\fIT
\dq\fR 95%\fI
\u\fR , may
be determined by simulation.
.sp 1P
.LP
\fIExample\ 1:\fR
.sp 9p
.RT
.PP
Table 9/Q.287 shows a calculated example at 2.4\ kbit/s of
\fIT
\dcAV
\u\fR and \fIT
\dc\fR 95%\fI
\u\fR for
\fIa\fR\d\fIp\fR\u\ =\ 0.4\ erlang with
the\ traffic model of Table\ 6/Q.286. As a result of simulation for this
model, it has been determined that
\fIT
\dq\fR 95%\fI
\u\fR \ =\ 3.5\ \(mu\ \fIT
\dqAV
\u\fR . The values
of
\fIT
\dhAV
\u\fR and \fIT
\dh | fR 95%\fI
\u\fR are those assumed
for Table\ 8/Q.287 and \fIT\fR\d\fIr\fR\u\ =\ \fIT\fR\d\fIm\fR\u\ =\ 2\
ms is assumed.
.RT
.LP
.rs
.sp 11P
.ad r
\fBTABLE [9/Q.287] p.16\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
\fIExample\ 2:\fR
.sp 9p
.RT
.PP
Figure 23/Q.287 and Table 10/Q.287 show a calculated example of the average
\fIT\fR\d\fIc\fR\ufor traffic of 2000\ circuits served by systems of different
data transmission rates with 10\ calls per speech circuit per hour, with the
traffic model of Table\ 6/Q.286. Answer message average handling time
\fIT\fR\d\fIh\fR\u\ =\ 10\ ms (other message average handling time \fIT\fR\d\fIh\fR\u\
=\ 20\ ms) and
\fIT\fR\d\fIr\fR\u\ =\ \fIT\fR\d\fIm\fR\u\ =\ 2\ ms are assumed. The number
of blocks in the error control loop is assumed not to exceed eight.
.bp
.RT
.LP
.rs
.sp 16P
.ad r
\fBTable [10/Q.287] p.17\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 28P
.ad r
\fBFIGURE 23/Q.287 p.18\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.ce 1000
\v'3P'
SECTION\ 8
.ce 0
.sp 1P
.ce 1000
\fBSECURITY\ ARRANGEMENTS\fR
.ce 0
.sp 1P
.sp 2P
.LP
\fBRecommendation\ Q.291\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB8.1\ GENERAL\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.291''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.291 %'
.ce 0
.sp 1P
.PP
Since a common signalling link carries the signals for many
speech
circuits, a failure of this link will affect all the speech circuits served.
Therefore, arrangements must be made to ensure continuity of service for the
circuits.
.sp 1P
.RT
.LP
.PP
The security arrangements involve the provision of reserve
facilities, that may be one or more of the following:
.RT
.LP
\(em
another signalling link, used in the quasi\(hyassociated or
load\(hysharing mode,
.LP
\(em
a dedicated reserve signalling link,
.LP
\(em
a dedicated reserve transfer link, or
.LP
\(em
a circuit, normally used for speech (or other service
purposes), to be withdrawn when required for use as a transfer
link.
.PP
In the last two cases the transfer links must be equipped with
signalling terminals and modems and interface adaptors to form signalling
links.
.PP
Other than possible signalling traffic carrying considerations, there
are no restrictions in using a digital reserve signalling link for an analogue
regular signalling link and vice versa.
.PP
When the regular signalling link fails, all waiting messages marked for
retransmission as well as all unacknowledged signal units should be
retransmitted over the reserve facility. Subsequent signalling traffic
destined for the failed link should then be transferred to the reserve
facility.
.PP
Signalling traffic should be directed to the reserve facility only after the
proper preparations have been made [see \(sc\ 8.6.1 | ) below].
.PP
When no signalling link is available for carrying signalling traffic
during the period of changeover to a non\(hysynchronized reserve or a nominated
speech circuit, or during an emergency restart condition, measures must be
taken to prevent the storage capacity of the failed signalling system from
being exceeded so as to prevent messages from being lost. It is recommended
that all free speech circuits should be removed from service during this
period (by local busying at each end), to permit traffic to overflow to
other routes which are serviceable. When there is no overflow facility,
appropriate
circuit\(hygroup\(hycongestion signals should be returned.
.RT
.sp 1P
.ce 1000
\fB8.2\ BASIC\ SECURITY\ ARRANGEMENTS\fR
.ce 0
.sp 1P
.PP
The basic security requirement is taken from the dependability requirements
for continuity of signalling service
[Recommendation\ Q.276, \(sc\ 6.6.1 | )].
.sp 1P
.RT
.PP
Steps should be taken to open up a reserve facility as soon as
possible after detection of a fault.
.bp
.PP
Once the reserve facility has been taken into service, the regular
signalling link should not be brought back into service for signalling
traffic until it has been checked to be giving satisfactory performance
for 1\ minute.
.PP
Should it happen that the reserve signalling link also fails, another
reserve facility should be opened up. When there is no other reserve facility
available, an attempt to transfer to any suitable signalling link, using
the
emergency restart procedure described in Recommendation\ Q.293,\ 8.7, must
take place.
.RT
.ce 1000
\fB8.3\ TYPES\ OF\ FAILURE,\ RECOGNITION\ OF\ FAILURE\fR
.ce 0
.sp 1P
.ce 1000
\fBAND\ ABNORMAL\ ERROR\ RATES\fR
.ce 0
.sp 1P
.sp 2P
.LP
8.3.1 \fI
\fITypes of failure\fR
.sp 1P
.RT
.PP
The interruption of signalling service may be caused by several types of
faults affecting the transfer channels, the modems or interface adaptors
or the signalling terminal equipment.
.PP
The failure may be indicated as follows:
.RT
.LP
a)
loss of the analogue data carrier
or
loss of
the digital frame alignment
,
.LP
b)
continuous
failure of signal units
to check
correctly,
.LP
c)
unacceptable
intermittent failure of signal units
to check correctly, or
.LP
d)
loss of block or multi\(hyblock synchronism.
.sp 1P
.LP
8.3.2 \fI
\fIRecognition of failure\fR
.sp 9p
.RT
.PP
Monitoring equipment is provided to recognize all types of signalling channel
failures.
.PP
At each terminal, the monitoring will be performed on the incoming
signalling channel by:
.RT
.LP
a)
monitoring the signal unit error rate, and
.LP
b)
detection of loss of block or multi\(hyblock
synchronism.
.PP
The \fIsignal unit error rate monitor\fR | recognizes unacceptably high
percentages of signal units received incorrectly. A signal unit is recognized
as being received incorrectly as a result of an indication from the check
bit decoder or the data channel failure detector (see Recommendation\ Q.277,
\(sc\(sc\ 6.7.1 and\ 6.7.2). The signal unit error rate monitor should have the
hyperbolic error rate time characteristic shown in Figure\ 24/Q.291. The
signal unit error rate monitor shall be reset to zero whenever:
.LP
\(em
the monitor output has been recognized, indicating that the
signal unit error rate, as detected by the decoder or the data
channel failure detector, has become unacceptable, or
.LP
\(em
synchronism of the signalling link has been achieved, or
.LP
\(em
after signalling link failure.
.PP
\fILoss of block\fR | or multi\(hyblock synchronism is detected as described
in Recommendation\ Q.278.
.sp 1P
.LP
8.3.3 \fI
\fIRecognition of end of failure\fR \v'3p'
.sp 9p
.RT
.PP
a)
\fIOne\(hyminute proving period\fR
.PP
End\(hyof\(hyfailure monitoring equipment is provided at each terminal to
recognize satisfactory performance of the signalling link after initial
synchronization or after a link failure. The signalling link shall not be
placed into service until a signal unit error rate of\ 0.2% or less has been
achieved in a proving period of one minute. The end\(hyof\(hyfailure monitor
will
indicate that this error rate has been achieved when it recognizes that
no more than:
.PP
\ 10\ signal\ units at\ 2400\ bit/s, or
.PP
\ 16\ signal\ units at\ 4\ kbit/s, or
.PP
240\ signal\ units at\ 56\ kbit/s
.RT
.LP
have been received in error in a proving period of one minute.
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure 24/Q.291 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
In the event that the end\(hyof\(hyfailure monitor indicates that more
than
the appropriate number of signal units in error have been received before
the one\(hyminute proving period has elapsed, then the end\(hyof\(hyfailure
monitor shall be reset and the one\(hyminute proving period recommenced.
\v'3p'
.LP
b)
\fIEmergency proving period\fR
.PP
An emergency proving period is used in conjunction with the emergency
restart procedure (see Recommendation\ Q.293, \(sc\ 8.7). The emergency proving
period is a 2\ to 3\ second period during which the error rate on the link is
such that the end\(hyof\(hyfailure monitor does not give an output. The
emergency
proving period begins when a regular or reserve link achieves synchronism.
In the event that the monitor gives an output before the emergency proving
period has elapsed, the end\(hyof\(hyfailure monitor shall be reset and
the emergency
proving period recommenced.
\v'3p'
.LP
c)
\fINo proving period\fR
.PP
No proving period is required when
.LP
\(em
changeover to a reserve link is caused by failure of a
signalling link (as specified in Recommendation\ Q.293, \(sc\ 8.6.1),
or when
.LP
\(em
block and multi\(hyblock synchronism is regained (as specified
in Recommendation\ Q.278, \(sc\(sc\ 6.8.4 and\ 6.8.5).
\v'1P'
.LP
.sp 2P
.LP
\fBRecommendation\ Q.292\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB8.4\ RESERVE\ FACILITIES\ PROVIDED\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.292''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.292 %'
.ce 0
.sp 1P
.PP
The reserve facilities provided can be subdivided into three
groups following below in the order of ready availability:
.sp 1P
.RT
.LP
a)
quasi\(hyassociated reserve signalling links
,
.LP
b)
full\(hytime reserved transfer links
,
.LP
c)
nominated direct circuits
.
.PP
Within each group, one or more arrangements can be distinguished which
differ in the preparatory actions to be taken to bring the reserve
facility into active service.
.PP
The choice of the particular facilities to be used can be governed by several
factors, e.g.\ the possibility of using quasi\(hyassociated signalling
links, the number of circuits served, the geographical distance between the
System\ No.\ 6 exchanges,\ etc. The choice of method(s), therefore, will
be made by the Administrations involved according to the circumstances
which apply.
.PP
As a matter of principle, the reserve facility to be used should follow
a route different from the route of the regular signalling link.
.RT
.sp 1P
.LP
8.4.1
\fIQuasi\(hyassociated reserve signalling links\fR
.sp 9p
.RT
.PP
The method of using a quasi\(hyassociated signalling link as a reserve
facility is directly derived from the principles accepted for System No.\
6
(Recommendation\ Q.253).
.PP
This method assumes an adequate signalling network and requires prior
agreements on its adoption between the Administration(s) through whose
signal transfer point(s) the signalling traffic may overflow.
.PP
Methods of controlling quasi\(hyassociated signalling are described in
Recommendation\ Q.266, \(sc\ 4.6.2.
.RT
.sp 1P
.LP
8.4.2
\fIFull\(hytime reserved transfer links\fR
.sp 9p
.RT
.PP
A transfer link is permanently assigned to provide the reserve
signalling link.
.bp
.PP
The following arrangements can be distinguished:
\v'3p'
.RT
.LP
a)
\fILoad sharing\fR
.LP
.PP
Both transfer links are equipped with modems or interface adaptors and
signalling terminals and are in use on the basis of duplicate working with
load sharing. Each link in this method is the reserve for the signal load
on the
other link.(See also Recommendation\ Q.293, \(sc\ 8.9.)
.PP
The circuits shall be assigned identical labels on both links and each
circuit shall be assigned to one of the parallel signalling links as its
regular link. The exchange must be capable of accepting signalling traffic
for the labels over either link at any time.(See also Recommendation\ Q.293,
\(sc\ 8.9.)
\v'3p'
.RT
.LP
b)
\fISynchronized reserve\fR
.PP
The transfer link is equipped with modems or interface adaptors and
signalling terminals, thus forming a reserve signalling link.
.PP
The link is not in use, but its channels are
synchronized.
\v'3p'
.RT
.LP
c)
\fINon\(hysynchronized reserve\fR
.PP
The transfer link is not equipped with modems or interface adaptors and
signalling terminals. A switching operation is thus required to convert
the
transfer link into a signalling link, before synchronizing of the signalling
channels can start.
.PP
Arrangements a) and b) are considered to be more usual than c) and will
no doubt be the general rule in the case of a full\(hytime reservation
of a
transfer link. However, for international exchanges at which very many
signalling links terminate, Administrations may prefer not to use the
arrangements\ a) and\ b) above but to pool available modems, interface
adaptors and signalling terminals for common use to a number of reserve
transfer
links.
.RT
.sp 1P
.LP
8.4.3
\fINominated direct circuits\fR
.sp 9p
.RT
.PP
A nominated direct circuit is permanently assigned to be converted
into a signalling link, when required. The following arrangements can be
distinguished:
\v'3p'
.RT
.LP
a)
\fISpeech circuit reserve\fR
.PP
The nominated circuit is normally in speech (or other service)
condition. Switching action and synchronizing must be performed when the
transfer link of the circuit is required for the reserve signalling link.
The switching action is allowed only when the transfer link is not in use.
For this reason, Administrations should ensure that the nominated speech
circuit has a high probability of being free (for example, by using a last\(hychoice
circuit).
.PP
The available modems and signalling terminals may be pooled for common
use to a number of speech circuit groups.
\v'3p'
.RT
.LP
b)
\fITASI\(hythrough reserve\fR \fI(analogue only)\fR
.PP
The nominated circuit is a TASI\(hythrough circuit. The circuit is not to
be used for speech. When it is required to open up a reserve signalling
link, data are applied in the normal way. These data will be sufficient
to operate
the speech detector at each end and cause TASI\ channels to be associated
with the circuit for as long as the data are applied.
.PP
Arrangement b) cannot be ranked as a general solution since it depends
on having a TASI\ system between the two international exchanges
involved.
.RT
.LP
.sp 1P
.LP
8.4.4
\fILink\(hysets, signalling routes, signalling route sets\fR
\fIand opposite signalling route sets\fR \v'3p'
.sp 9p
.RT
.LP
a)
\fILink sets, signalling routes, and signalling route sets\fR
.PP
A regular link and reserve links directly connecting two System No.\ 6
exchanges, a System No.\ 6 exchange and an STP or two STPs, and which provide
signalling for the same 2048\ circuit labels are termed a
link set
.
Where quasi\(hyassociated signalling facilities are provided, the security
arrangements for a band of speech circuits will consist of one or more link
sets. The different signalling paths so formed are known as
signalling
routes
. The list of signalling routes in their priority order is known
as a
signalling route set
. One signalling route set is allocated for
all bands which have the same security arrangements.
\v'3p'
.bp
.LP
b)
\fIOpposite signalling rout sets\fR
.PP
At an STP traffic passes from the originating to destination exchange and
vice versa using a signalling route set in each direction. Each of these
signalling route sets, which form a complementary pair, is termed an opposite
signalling route set with respect to the other.
.LP
.sp 1P
.LP
8.4.5
\fIChoice of reserve facility\fR
.sp 9p
.RT
.PP
When the regular link in a link set has failed and where more than one
type of service is provided, signalling should first be restored on a
synchronized reserve, such as a load shared or full\(hytime synchronized
reserve transfer link in the same link set. If these are not provided or
are not
available, signalling should then be restored via one or more link sets
using quasi\(hyassociated signalling. If this option is not provided or
is not
available, then an attempt should be made to restore signalling on a
nonsynchronized reserve such as a full\(hytime nonsynchronized reserve transfer
link or nominated direct circuit, within the original link set. Should the
failed link be a reserve link, then changeover follows the same priority
order as above except that the search should commence on the link below
the one that has just failed. Changeover to a proved reserved link of a
higher priority is only possible by using the emergency restart procedure.
See
Recommendation\ Q.293, \(sc\ 8.7.
.PP
For each band or group of bands the provision of the different types of
reserve, the search order to be applied where a number of one type are
provided, and the selection order between link sets should be specified
by the Administrations concerned.
\v'3P'
.RT
.LP
.sp 2P
.LP
\fBRecommendation\ Q.293\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fB8.5\ INTERVALS\ AT\ WHICH\ SECURITY\ MEASURES\ ARE\ TO\ BE\ INVOKED\fR
.EF '% Fascicle\ VI.3\ \(em\ Rec.\ Q.293''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.293 %'
.ce 0
.sp 1P
.PP
The following action points are defined:
.sp 1P
.RT
.LP
\fIT\fR\d\fI0\fR\u =
time when signalling fault indication starts,
.LP
\fIT\fR\d\fIw\fR\u =
time when warning of failure is issued (for
example, to busy a nominated speech circuit reserve),
.LP
\fIT\fR\d\fId\fR\u =
time when decision to change over is made,
.LP
\fIT\fR\d\fIu\fR\u =
time when signalling traffic is offered to the
reserve link.
.LP
.PP
The intervals \fIT\fR\d\fIw\fR\u\ \(em\ \fIT\fR\d0\uand \fIT\fR\d\fIu\fR\u\
\(em\ \fIT\fR\d\fId\fR\uare not
specified. It
is recognized that these intervals will vary from one method or arrangement
to another.
.PP
The interval \fIT\fR\d\fId\fR\u\ \(em\ \fIT\fR\d0\udoes not include the
time for the
processor to react. Its value is determined in the case of:
.RT
.LP
\(em
continuous failure, by all signal units being in error for
350\ milliseconds;
.LP
\(em
intermittent failure, by the instant the signal unit error rate
monitor gives an output indicating that the signalling error rate has
become unacceptable; or
.LP
\(em
loss of block or multi\(hyblock synchronism, by the failure to
achieve block resynchronization within about 350\ ms.
.LP
.rs
.sp 2P
.ad r
\fBBLANC\fR
.ad b
.RT
.LP
.bp
.sp 1P
.ce 1000
\fB8.6\ \fR \fBCHANGEOVER\ AND\ CHANGEBACK\ PROCEDURES\fR
.ce 0
.sp 1P
.sp 2P
.LP
8.6.1
\fIChangeover from faulty signalling links\fR \v'3p'
.sp 1P
.RT
.PP
a)
Consider two exchanges\ A and\ B with a fault in signalling
link\ AB, \fIaffecting both directions\fR .
.PP
Each exchange at time \fIT\fR\d\fId\fR\uinitiates the synchronization procedure
(Recommendation\ Q.278), where applicable, on the reserve signalling link.
When both ends are in synchronism over the reserve link, the processors
switch over without any proving period and use this link.
.PP
On detection of failure of a working link at time \fIT\fR\d0\u, each
terminal \fIstarts sending faulty\(hylink information\fR | on the link
just failed.
This information consists of a number of changeover signals (completing the
block being sent) plus\ ACU, followed by a continuous stream of alternating
blocks of changeover signals and of\ SYUs (11\ changeover signals\ +\ ACU,
11\ SYUs\ +\ ACU, 11\ changeover signals\ +\ ACU,\ etc.).
.PP
When a terminal is unable to accept a correctly received signal unit, the
relevant bit in the\ ACU acknowledging the signal unit shall be set to\
\fB1\fR . If the terminal has lost synchronism, then the normal synchronization
procedure
is started (Recommendation\ Q.278, \(sc\ 6.8.2).
.PP
With the reserve facility properly prepared, each exchange retransmits
on the reserve facility all waiting signals marked for retransmission and
all signals not acknowledged by the other exchange, followed by new signalling
traffic from the failed link as specified in Recommendation\ Q.291,
\(sc\ 8.1.
.RT
.PP
b)
Consider a fault affecting \fIonly one direction\fR | for example A\ to\
B. The fault will be detected at terminal\ B and at a time \fIT\fR\d\fId\fR\uthis
terminal will act as under \(sc\ 8.6.1 | ) above.
.PP
Upon receipt of two changeover signals on the working signalling
channel within a period of 3\ seconds, exchange\ A commences the synchronization
procedure, when applicable, on the reserve signalling link. On the failed
channel, exchange\ A will commence the resynchronization procedure as in
Recommendation\ Q.278, \(sc\ 6.8.2, allowing the block numbering sequence to be
re\(hyestablished. If exchange\ A has not itself lost synchronism on the failed
channel, it may skip over superfluous actions within the synchronization
procedure, i.e.\ faulty\(hylink information, the sending of all\ \fB1\fR s
acknowledgement indicators, SYU search, and proving period. The detection
and timing of the loss of block synchronization should be cancelled at
this time. Exchange\ A will proceed to retransmit all the failed link messages
as
described in Recommendation\ Q.291, \(sc\ 8.1, and transfer all subsequent
signalling traffic destined for the failed link to the reserve link for the
duration of the failure.
.LP
.PP
c)
If more than one type of reserve is provided, the choice of
reserve facility should be in accordance with Recommendation\ Q.292, \(sc\
8.4.5.
Nominated speech circuits will be made
busy to outgoing traffic at each end immediately, or as soon as free, until
transfer to a nominated reserve signalling link has been accomplished.
At time \fIT\fR\d\fId\fR\u, an operable reserve will be selected, by hunting
through the
available choices in a fixed predetermined order as specified by the
Administrations concerned. Nominated direct circuits in use for speech are
skipped over in the selection process.
.PP
If a synchronized reserve or quasi\(hyassociated route is selected, a
later transfer to a non\(hysynchronized full\(hytime reserve link or a
nominated
direct circuit may be effected as described in \(sc\ 8.6.3.2 below.
.PP
When a failure is encountered on a reserve signalling link, faulty\(hylink
information is sent in the same manner as on a regular signalling link
encountering a failure. If the reserve link is carrying signalling traffic,
the procedure as covered in Recommendation\ Q.291, \(sc\ 8.2, will be initiated.
.RT
.PP
d)
When a changeover is to another link in the same linkset,
Signalling System Control Signals (SCUs) waiting on the faulty link are
not retransmitted on the new link. When a changeover is to one or more
quasi\(hyassociated routings, the telephone signal units, network maintenance
signals and network management signals are retransmitted on their respective
routings after band translation where necessary. SCUs and signalling network
management signals are not retransmitted on quasi\(hyassociated
routings.
.bp
.PP
When a link becomes faulty and no reserve facility is available for
some or all bands on the link, then any waiting signal units for such bands
will not be able to be retransmitted as described above. Where such signals
refer to remote routes and are using the System\ No.\ 6 exchange as a signal
transfer point, these signals should be deleted and a message\(hyrefusal signal
returned for each telephone signal and a transfer\(hyprohibited signal returned
for each network maintenance signal (see Recommendation\ Q.266, \(sc\(sc\
4.6.2.1
and\ 4.6.2.3).
.LP
.sp 1P
.LP
8.6.2
\fIChangeback to the regular link\fR
.sp 9p
.RT
.PP
When either terminal has regained synchronism on the failed regular link,
it will begin both its one\(hyminute and emergency proving periods. However,
if synchronism had been continuously maintained at one end during the
failure, this exchange need not start a new proving period. When
the received signal unit error rate has remained acceptable for the one\(hyminute
proving period, the exchange will \fIcease sending faulty\(hylink information\fR
by
replacing the changeover signals (if it is sending changeover signals) with
SYUs (plus\ ACUs).
.PP
To return to the regular link, the exchange A initiating the changeback
sends two load\(hytransfer signals on the regular link. From this time
until
changeback is either completed or abandoned, exchange\ A must be in a position
to receive and process all signals on both the regular link and the reserve
in use. When exchange\ B receives a load\(hytransfer signal and knows the
regular link is operational, it responds with a load\(hytransfer acknowledgement
signal on the regular link, then immediately transfers its signalling traffic
from the
reserve to the regular link. When exchange\ A receives one
load\(hytransfer\(hyacknowledgement signal, it transfers its signalling
traffic from the reserve to the regular link. Should an exchange receive
a load\(hytransfer
signal on the link that is carrying traffic, then this signal shall be
acknowledged.
.PP
Until the load\(hytransfer and acknowledgement signal sequence has been
satisfactorily completed as described above, signalling continues over the
reserve link. After this signal sequence is completed, exchanges\ A and\ B
continue to monitor the reserve link until all signals initially transmitted
on the reserve link have had the opportunity to be acknowledged. Signals
sent on the reserve link acknowledged as having been received in error
are
retransmitted on the reserve link. After 5\ \(+-\ 1\ seconds, when all
signals have had the opportunity to be acknowledged as correctly received,
each end will
return reserve voice\(hyfrequency links with switched terminals and modems to
their original status. A nominated speech circuit must be returned to service
for outgoing traffic without delay by completing an unblocking sequence
even
though blocking signals have not previously been exchanged for the circuit.
This unblocking sequence will remove any previous circuit state at both ends
and return the circuit to the idle state. Any resultant failure indications
occuring on the reserve link during the 5\ \(+-\ 1 second time interval may be
ignored. (See also \(sc\ 8.9.)
.PP
In the event exchange B decides not to change back when it receives a
load\(hytransfer signal, it withholds the load\(hytransfer\(hyacknowledgement
signal.
Exchange\ A must therefore time for an interval of approximately 2\ minutes
for the receipt of a load\(hytransfer\(hyacknowledgement signal. If the
time interval
elapses without receiving a load\(hytransfer\(hyacknowledgment signal,
exchange\ A
will transmit two more load\(hytransfer signals and recycle the timing.
.PP
If exchange A decides to terminate the changeback procedure at any time
before the process is completed, it will interrupt the changeback procedure
and transmit faulty link information as for a normal changeover. Exchange\
B will
respond to the faulty\(hylink information even though it has agreed to
changeback and has started sending messages on the regular link. In the
event of
changeover before the load\(hytransfer signalling sequence is completed, both
exchanges will remain on the reserve link which the changeback commenced.
.PP
If the changeback procedure is interrupted or terminated as above before
the procedure is completed, the regular link should continue to meet the
one\(hyminute proving period requirement.
.PP
In the event that both exchanges\ A and\ B start changeback procedures
at about the same time, either exchange, having transmitted two load\(hytransfer
signals, shall respond to a received load\(hytransfer signal with a
load\(hytransfer\(hyacknowledgement signal and shall transfer signalling
traffic to the regular link on the receipt of either a load\(hytransfer
signal or a
load\(hytransfer\(hyacknowledgement signal.
.RT
.sp 2P
.LP
8.6.3
\fIChangeover from working signalling links\fR
.sp 1P
.RT
.sp 1P
.LP
8.6.3.1
\fIManual changeover procedure\fR \v'3p'
.sp 9p
.RT
.PP
a)
In the event that it is desired to change over to a reserve
link for rearrangements, changes, maintenance,\ etc., on a link currently
carrying the signalling traffic for the link set, the exchange\ A desiring
the changeover will send a manual\(hychangeover signal on the working link.
.bp
.LP
This
working link may be the regular link, a full\(hytime synchronized reserve
link, or one link of a load shared pair. When exchange\ B receives this
signal, the
selection of a reserve link is initiated by both exchanges. The
selection order
for this reserve differs from that for the normal changeover (described in
\(sc\ 8.4.5 above) in that quasi\(hyassociated routings are excluded from
the search if one or more non\(hysynchronized reserve links are provided
in the link
set. This is specified in order to transfer the signalling load directly
to a non\(hysynchronized reserved link, thus preventing a possible double
load transfer due to the load transfer procedure (automatic) as specified
in \(sc\ 8.6.3.2 being initiated on a quasi\(hyassociated routing subsequent
to the manual changeover.
When a transfer to a non\(hysynchronized reserve link is indicated, the cyclic
procedure described in \(sc\ 8.6.3.2 below will be used as appropriate. When
exchange\ B has selected a quasi\(hyassociated route or other synchronized
reserve link or has gained synchronism on a non\(hysynchronized link, a
manual\(hychangeover\(hyacknowledgement signal is sent back on the original
working link.
.LP
.PP
Exchange A must not send a manual\(hychangeover signal or exchange B send
a manual\(hychangeover\(hyacknowledgement signal if the desired changeover
would
cause the complete failure of a signalling route set. That is, the signalling
for a group of bands would be lost. However, these signals will not be
inhibited if the affected signalling route set is for bands for which the
exchange is acting as a signal transfer point.
.PP
If a quasi\(hyassociated route or other synchronized reserve link is
selected for the changeover, the exchanges\ A and B\ transfer their signalling
traffic subsequent to the exchange of the manual\(hychangeover\(hyacknow
ledgement signal.
.PP
If a non\(hysynchronized reserve signalling link is selected and the
manual\(hychangeover\(hyacknowledgement signal has been received, two load
transfer signals are sent by exchange\ A on this link when the link is
in synchronism and has passed the one\(hyminute proving period. On receipt
of one
load\(hytransfer\(hyacknowledgement signal, exchange\ A will transfer its
signalling traffic.
.PP
For all cases, both exchanges A and B continue to monitor the original
working link for 5\ \(+-\ 1\ seconds, until all signals initiated on this
link have the opportunity of being acknowledged as correctly received.
Signals
acknowledged as having been received incorrectly are retransmitted on the
orignal working link. Subsequent to this timing period, the exchange initiating
the manual changeover may continue to transmit SYUs\ +\ ACUs in the normal
manner or may remove the link from service. The exchange acknowledging
the manual
changover should maintain synchronism and, should the link be removed,
detect loss of synchronization.
.RT
.LP
.PP
b)
If exchanges A and B simultaneously send manual\(hychangeover
signals, both exchanges must send manual\(hychangeover\(hyacknowledgement
signals. In the quasi\(hyassociated route or other synchronized reserved
link case,
exchanges\ A and\ B transfer their signalling traffic subsequent to the
receipt of the manual\(hychangeover\(hyacknowledgement signal. For all
other cases, each end, subsequent to receipt of a manual\(hychangeover\(hyacknow
ledgement signal on the original working link, will transmit two load\(hytransfer
signals on the selected reserve which will be acknowledged by the other
end.
.PP
When either end receives a load\(hytransfer signal, while expecting a
load\(hytransfer\(hyacknowledgement signal from the other end after sending two
load\(hytransfer signals, it may transfer its signalling traffic from the
original working link to the reserve link after sending a
load\(hytransfer\(hyacknowledgement signal.
.PP
c)
In the event that a manual\(hychangeover signal is not
acknowledged by the other exchange, a suitable interval shall elapse (e.g.\
one minute), before the request is repeated. If the second manual\(hychangeover
signal is not acknowledged, the maintenance staff at the exchange requesting
changeover should be alerted.
.PP
d)
Changeback from the reserve link will always be to the
regular link and is initiated by the end which previously initiated the
manual changeover. The procedure used is the same as the normal changeback
as
described in \(sc\ 8.6.2 above. In the event of simultaneous manual changeover,
or in the case where the regular link is not the link from which manual
changeover had originally taken place, either end can initiate the changeback
to the
regular link.
.PP
If the link from which manual changeover originally took place is not the
regular link but is a synchronized reserve, the end initiating the manual
changeover will initiate the restoration of the link to the standby ready\(hystate
as described in \(sc\ 8.8\ c) below. This will commence when the link is
considered serviceable again and may occur independently of the load transfer
to the
regular link.
.bp
.LP
.sp 1P
.LP
8.6.3.2
\fILoad\(hytransfer procedure (automatic)\fR \v'3p'
.sp 9p
.RT
.PP
a)
An automatic load\(hytransfer from a quasi\(hyassociated routing
or other synchronized reserve to a prepared non\(hysynchronized reserve may be
provided by agreement if desired by the Administrations concerned. This
procedure may be used to limit the signalling traffic load at the signal
transfer point or to maintain two synchronized links within the link set.
Three types of automatic load\(hytransfer are possible. In the first type,
the
signalling traffic for a group of bands using a signal transfer point is
transferred back to the associated link set. In the second type, the signalling
traffic in a link set is transferred from a synchronized reserve to a prepared
non\(hysynchronized reserve allowing the synchronized reserve to remain
as a
standby link. In the third type the signalling traffic from a failed load
sharing link in a link set is transferred from the other load sharing link
to a prepared non\(hysynchronized reserve allowing the working load sharing
link and
the prepared reserve to remain as mutual reserves.
.LP
.PP
b)
Subsequent to the initial transfer of signalling traffic to a synchronized
reserve, both exchanges attempt to achieve synchronization on a
secondary reserve facility. If more than one facility is provided, the two
exchanges use the following selection procedure to establish synchronization
on a secondary facility.
.PP
Each exchange will select the first choice non\(hysynchronized reserve
and will attempt to synchronize for a prearranged time interval of 5\ +/\(em
0.25\ seconds at one exchange and 7.5\ +/\(em 0.25\ seconds at the ohter. The
selection sequence and the time interval will be fixed by bilateral agreement.
If synchronization is not accomplished within the specified time interval,
an attempt is made to synchronize on each of the available reserves in
turn. If
unsuccessful on the last choice non\(hysynchronized reserve, the selection
cycle is repeated unless the regular link has become operative. The difference
in
timing at the two exchanges ensures that even in the event the exchanges
do not attempt synchronization on the same reserve initially, both exchanges
will
ultimatley meet on the reserve for a minimum interval of 2\ seconds.
.PP
When synchronism is established on the reserve and the error rate has been
acceptable during the one minute proving period, load\(hytransfer and
load\(hytransfer\(hyacknowledgement signals are interchanged on the selected
reserve prior to transfer of the traffic as described in \(sc\ 8.6.3.1
above. Signal units originally transmitted on the synchronized reserve
are retransmitted as
necessary on the same reserve.
.RT
.LP
.sp 1P
.ce 1000
\fB8.7\ EMERGENCY\ RESTART\ PROCEDURE\fR
.ce 0
.sp 1P
.PP
a)
The
emergency restart procedure
is intended to
re\(hyestablish signalling communication on a link set between two exchanges
without waiting for
the one\(hyminute proving period, whenever the regular, and all synchronized
links in the link set of lower priority than the last working links,
have failed, or non\(hysynchronized reserve links cannot be synchronized
within 2\ to 3\ seconds of failure of the working link. Any link between
the two exchanges which has achieved synchronism and has passed the emergency
proving period (see Recommendation\ Q.291, \(sc\ 8.3.3) will be selected
to re\(hyestablish
signalling communication. Maintenance personnel are alerted whenever an
emergency restart condition exists. Either exchange may unilaterally commence
the emergency restart procedure and the other exchange must respond even
though it is unaware of an emergency signalling situation. The emergency
restart
procedure will be initiated on a link set even though all the signalling
traffic may have successfully transferred to quasi\(hyassociated reserves.
However, the emergency restart procedure will not be initiated on a link set,
.sp 1P
.RT
.LP
if after termination of link set signalling a manually changed\(hyover link
remains in the link set. In this case, the link set carries out the emergency
restart procedure only if the subsequent failure of a signalling route
set
occurs [except STP signalling route sets, see \(sc\ 8.6.3.1 | )]. This
failure would be for signalling traffic transferred from the link set to
a quasi\(hyassociated routing at the manual changeover. Therefore, the
manually changed\(hyover link can be included in the emergency restart
procedure if it is capable of being
synchronized and emergency proved.
.bp
.PP
b)
If faulty\(hylink information is being sent on a previously
failed link, it will continue to be sent until that link has passed its
emergency proving period.
.PP
If at any time after the emergency proving period the signal unit error
rate monitor indicates an unsatisfactory performance of the link, faulty\(hylink
information is again sent on the link and the change\(hyover or emergency
restart procedure is begun.
.PP
To minimize the number of calls affected by the emergency restart
condition, Recommendation\ Q.291, \(sc\ 8.1 should be followed,
particulary the recommendation to remove free speech circuits from service.
However, this will only be necessary when the link\(hyset failure has caused
the failure of an entire signalling route set, and hence no quasi\(hyassociated
routings are available.
.PP
The following procedure is designed to attempt emergency restart on as
many signalling links as possible at the same time. Both exchanges will
simultaneously connect terminals to as many voice\(hyfrequency links as
possible between the two exchanges. Quasi\(hyassociated signalling routes
are excluded from this procedure. The regular link and all synchronized
reserve links have
terminals permanently assigned to them. Terminals for non\(hysynchronized
reserve links will be assigned from a pool of reserve terminals. Assume
that the total number of links is\ \fIn\fR and the available number of
reserve terminals is\ \fIT\fR .
If\ \fIT\fR \ \(>="\ \fIn\fR , then a reserve terminal is assigned to each
of the\ \fIn\fR
non\(hysynchronized reserve links and synchronization is simultaneously
attempted on all links. If\ \fIT\fR \ <\ \fIn\fR , then\ \fIT\fR \ \(em\
1 reserve terminals are assigned to as many non\(hysynchronized reserve
links, and one terminal will be cycled through
the remaining non\(hysynchronized reserve links following the procedure
described in \(sc\ 8.6.3.2 | ) above.
.PP
Idle status of previously engaged nominated speech circuits at each
exchange during the emergency restart procedure is recognized either by
reception of a clear\(hyforward signal from a preceding exchange or by
reception of a clear\(hyback signal from a succeeding exchange.
.RT
.PP
c)
When one or more links have passed the emergency proving
period, two emergency\(hyload\(hytransfer signals are sent periodically (at
2\(hy3\ seconds intervals) over each link. Each exchange may receive signals
on the links during the emergency restart procedure and must take steps
either to
process these signals or deliberately reject them by setting the relevant
ACU indicators to\ \fB1\fR . However, after sending ELT signals on any
link, all signals received on the link must be processed. Although both
exchanges may send
emergency\(hyload\(hytransfer signals, only one exchange (designated the
emergency
restart control exchange by mutual agreement of the two Administrations)
will acknowledge these signals. The non\(hycontrol exchange must respond
by sending
emergency\(hy load\(hytransfer signals over the same signalling link, whenever
it
receives these signals and the link has passed the emergency proving period.
.PP
Both exchanges continue sending pairs of emergency\(hyload\(hytransfer
signals at 2\(hy3\ seconds intervals over links which have passed the emergency
proving period until the control exchange has sent two
load\(hytransfer\(hyacknowledgement signals and one has been received by the
non\(hycontrol exchange.
.PP
Upon receiving two emergency\(hyload\(hytransfer signals within 3\ seconds
on one or more links, the control exchange will select one of these links
which
has passed the emergency proving period and respond with two
load\(hytransfer\(hyacknowledgement signals. The control exchange may now start
sending signalling traffic over this link. The non\(hycontrol exchange may also
commence signalling traffic when it receives a load\(hytransfer\(hyacknowledgement
signal. The signalling traffic that is restarted (or allowed for STP traffic)
will be for bands where no working signalling path is at that time available
via this exchange. Other signalling traffic may only be transferred from
working links after the one\(hyminute proving period using the normal changeback
or automatic load transfer procedures.
.PP
This interchange of signals will take place even if the selected link had
previously been manually changed\(hyover, and irrespective of whether or
not the control exchange had initiated the manual change\(hyover. Once
the link is
selected the manual change\(hyover condition will be removed at both ends.
.PP
A guard period of 5\ \(+-\ 1\ seconds shall be commenced on transfer of
traffic to the selected link. During this guard period, any emergency\(hyload\(hy
transfer signals, received at the control exchange on the link on which
traffic has been resumed, shall be acknowledged. Emergency\(hyload\(hytransfer
signals
received on any other link, between the two exchanges, or received by the
non\(hycontrol exchange on any link,
.bp
.PP
shall be ignored. If, during the
guard period, the signal unit error rate monitor indicates an unsatisfactory
performance of the link carrying traffic or if faulty\(hylink information is
received on that link, then the guard period is terminated and \(sc\ 8.7 | ),
second paragraph applies.
.PP
After the emergency restart procedure has been terminated, subsequent
failures are treated in the normal manner. The load\(hytransfer or standby\(hyready
signalling sequences are not initiated on the selected link during the
emergency restart procedure, although they shall be sent after the one\(hyminute
proving period in order to carry out the normal changeback and automatic
load transfer procedures or to confirm the proving status of the link for
subsequent link security procedures.
.PP
If an exchange receives two emergency\(hyload\(hytransfer signals, it must
respond in the manner described and transfer signalling traffic to the
indicated signalling link, even though it may not be in the emergency restart
state.
.RT
.sp 1P
.ce 1000
\fB8.8\ FULL\(hyTIME\ SYNCHRONIZED\ RESERVE\ LINKS\fR
.ce 0
.sp 1P
.sp 2P
.LP
8.8.1
\fIFailure of a synchronized reserve link\fR
.sp 1P
.RT
.PP
On detection of failure of a synchronized reserve link, the
terminal starts sending faulty\(hylink information as described in \(sc\
8.6.1 | )
above. Receipt of faulty\(hylink information indicates that the link is not
suitable for use as a reserve.
.RT
.sp 1P
.LP
8.8.2
\fIRemoval of a full\(hytime synchronized reserve link from service\fR
\fIavailability\fR
.sp 9p
.RT
.PP
It may be necessary, for reasons of rearrangements, changes,
maintenance,\ etc., to remove a full\(hytime synchronized reserve link,
which is
not currently carrying the signalling traffic for the link set, from service
availability.
.PP
In this case the Exchange\ A desiring the removal will send a
manual\(hychangeover signal on the reserve link. On receipt of this signal
Exchange\ B will mark the reserve as unavailable for service and respond
with a manual changeover\(hyacknowledgement signal. Exchange\ A, on receipt
of the
acknowledgement signal, will also mark the reserve as unavailable for service
and may then continue to transmit SYUs or ACUs in the normal manner or
may
remove the link from service. Exchange\ B acknowledging the removal should
maintain synchronism and, should the link be removed, detect loss of
synchronization. Subsequent to these actions the provisions for inclusion of
the reserve link in an emergency restart procedure apply as specified in
\(sc\ 8.7\ a).
.PP
In the event that the manual\(hychangeover signal is not acknowledged by
the other exchange, a suitable interval shall elapse (e.g.\ one minute),
before the request is repeated. If the second manual\(hychangeover is not
acknowledged
the exchange initiating the removal may unilaterally remove the link from
service (provided that it is still acting as a reserve and is not carrying
signal traffic) by sending faulty link information or disconnection of the
carrier but may not mark the link as unavailable and continue to transmit
SYUs + ACUs in the normal manner.
.PP
Restoration of the reserve link from unavailable to available (standby
ready) status will be initiated by the end which previously initiated the
removal using the procedure described in \(sc\ 8.8.3) below.
.RT
.sp 1P
.LP
8.8.3
\fIRestoration of a synchronized reserve link\fR
.sp 9p
.RT
.PP
When both terminals are again in synchronism over the reserve link and
the error rate has met the requirement for the one\(hyminute proving period
(see Recommendation\ Q.291, \(sc\ 8.3.3), the faulty\(hylink information
will be
replaced with blocks of\ SYU (plus\ ACU) to indicate that the proving period
has been completed.
.PP
To confirm that the proving period has been completed at both exchanges,
the exchange\ A finishing the proving period sends two standby\(hyready
signals on the reserve link.
.bp
.PP
When exchange\ B receives
a standby\(hyready signal and knows the reserve link is usable, it responds
with a
standby\(hyready\(hyacknowledgement
signal
on the reserve link. When exchange\ A receives one
standby\(hyready\(hyacknow
ledgement\ signal, it has confirmation that the reserve link is available
for use.
.RT
.sp 1P
.ce 1000
\fB8.9\ LOAD\ SHARING\ METHOD\fR
.ce 0
.sp 1P
.PP
The
load sharing method
is described in \(sc\ 8.4.2 | ).
The method implies that the total signalling load on the link set is shared
between two working links. Steps should be taken to ensure that the load is
approximately equalized between the two links. This will normally be done by
allocating each circuit to one of the signalling links as its regular link,
and arranging for half of the total number of circuits to be allocated
to each
link. Although not mentioned in \(sc\ 8.4.2 | ), other allocation methods are
possible such as allocating each circuit to one of the signalling links on a
per\(hycall basis. This follows from the fact that under failure conditions on
one link the signalling traffic will be transferred to the remaining link
and hence each exchange must be capable of accepting signalling traffic
for all
labels on either link. It is therefore unnecessary for both exchanges to use
the same allocation method for their outgoing signalling traffic and each
Administration will decide on a suitable method. (For example, free option
for each label, an odd\(hyeven label basis, a per\(hyband basis or a per\(hycall
basis.)
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It must be ensured that one signalling link can handle all the
signalling traffic without unacceptable queueing delays. Load sharing should
not, therefore, be used to increase the signalling capacity of a link set.
If extra capacity is required, then a second link set with separate links
should be provided.
.PP
When a faulty link within a load\(hyshared pair becomes workable again,
the procedure used is the changeback procedure of \(sc\ 8.6.2 (and not the
procedure of \(sc\ 8.8). The signals standby\(hyready and
standby\(hyready\(hyacknowledgement are not used. As both links remain
in use, the
5\ \(+-\ 1 second guard timing is not used.
.PP
In general, any link set will probably contain a maximum of two
synchronized links, although more may be provided by agreement between
Administrations. Normally there will be no mixing between different security
arrangements (i.e.,\ a load shared pair with full\(hytime synchronized
reserves,\ etc.) although it may be provided by agreement between
Administrations.
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