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All drawings appearing in this Fascicle have been done in Autocad.
Recommendation Q.703
SIGNALLING LINK
1 General
1.1 Introduction
1.1.1 This Recommendation describes the functions and procedures for and
relating to the transfer of signalling messages over one signalling data link.
The signalling link functions, together with a signalling data link as bearer,
provide a signalling link for reliable transfer of signalling messages between
two directly connected signalling points.
Signalling messages delivered by superior hierarchical levels are
transferred over the signalling link in variable length signal units. The signal
units include transfer control information for proper operation of the signalling
link in addition to the signalling information.
1.1.2 The signalling link functions comprise:
a) signal unit delimitation,
b) signal unit alignment,
c) error detection,
d) error correction,
e) initial alignment,
f) signalling link error monitoring,
g) flow control.
All these functions are coordinated by the link state control (see Figure
1/Q.703).
Figure 1/Q.703 - CCITT 35002
Fascicle VI.7 - Rec. Q.703 PAGE1
1.2 Signal unit delimitation and alignment
The beginning and end of a signal unit are indicated by a unique 8-bit
pattern, called the flag. Measures are taken to ensure that the pattern cannot be
imitated elsewhere in the unit.
Loss of alignment occurs when a bit pattern disallowed by the delimitation
procedure (more than six consecutive 1s) is received, or when a certain maximum
length of signal unit is exceeded.
Loss of alignment will cause a change in the mode of operation of the
signal unit error rate monitor.
1.3 Error detection
The error detection function is performed by means of 16 check bits
provided at the end of each signal unit. The check bits are generated by the
transmitting signalling link terminal by operating on the preceding bits of the
signal unit following a specified algorithm. At the receiving signalling link
terminal1), the received check bits are operated on using specified rules which
correspond to that algorithm.
If consistency is not found between the received check bits and the
preceding bits of the signal unit, according to the algorithm, then the presence
of errors is indicated and the signal unit is discarded.
1.4 Error correction
1.4.1 Two forms of error correction are provided, the basic method and the
preventive cyclic retransmission method. The following criteria should be used
for determining the international fields of application for the two methods:
a) the basic method applies for signalling links using
non-intercontinental terrestrial transmission means and for
intercontinental signalling links where the one-way propagation delay
is less than 15 ms;
b) the preventive cyclic retransmission method applies for
intercontinental signalling links where the one-way propagation delay
is greater than or equal to 15 ms and for all signalling links
established via satellite.
In cases where one signalling link within an international link set is
established via satellite, the preventive cyclic retransmission method should be
used for all signalling links of that link set.
1.4.2 The basic method is a non-compelled, positive/negative acknowledgement,
retransmission error correction system. A signal unit which has been transmitted
is retained at the transmitting signalling link terminal until a positive
acknowledgement for that signal unit is received. If a negative acknowledgement
is received, then the transmission of new signal units is interrupted and those
signal units which have been transmitted but not yet positively acknowledged
starting with that indicated by the negative acknowledgement will be
retransmitted once, in the order in which they were first transmitted.
1.4.3 The preventive cyclic retransmission method is a non-compelled, positive
acknowledgement, cyclic retransmission, forward error correction system. A signal
unit which has been transmitted is retained at the transmitting signalling link
terminal until a positive acknowledgement for that signal unit is received.
During the period when there are no new signal units to be transmitted all the
signal units which have not yet been positively acknowledged are retransmitted
cyclically.
The forced retransmission procedure is defined to ensure that forward
error correction occurs in adverse conditions (e.g. high error rate and/or high
traffic loading).
When a predetermined number of retained, unacknowledged signal units
exists, the transmission of new signal units is interrupted and the retained
signal units are retransmitted cyclically until the number of unacknowledged
signal units is reduced.
1) A signalling link terminal refers to the means of performing all of the functions
defined at level 2 regardless of their implementation.
PAGE18 Fascicle VI.7 - Rec. Q.703
1.5 Initial alignment
The initial alignment procedure is appropriate to both first time
initialization (e.g., after "switch-on") and alignment in association with
restoration after a link failure. The procedure is based on the compelled
exchange of status information between the two signalling points concerned and
the provision of a proving period. No other signalling link is involved in the
initial alignment of any particular link, the exchange occurs only on the link to
be aligned.
1.6 Signalling link error monitoring
Two signalling link error rate monitor functions are provided; one which
is employed whilst a signalling link is in service and which provides one of the
criteria for taking the link out of service, and one which is employed whilst a
link is in the proving state of the initial alignment procedure. These are called
the signal unit error rate monitor and the alignment error rate monitor
respectively. The characteristics of the signal unit error rate monitor are based
on a signal unit error count, incremented and decremented using the "leaky
bucket" principle whilst the alignment error rate monitor is a linear count of
signal unit errors. During loss of alignment, the signal unit error rate monitor
error count is incremented in proportion to the period of the loss of alignment.
1.7 Link state control functions
Link state control is a function of the signalling link which provides
directives to the other signalling link functions. The interfaces with link state
control are shown in Figure 1/Q.703 and Figure 7/Q.703. The split into the
functional blocks shown in the figures is made to facilitate description of the
signalling link procedures and should not be taken to imply any particular
implementation.
The link state control function is shown in the overview diagram, Figure
2/Q.703, and the detailed state transition diagram, Figure 8/Q.703.
1.8 Flow control
Flow control is initiated when congestion is detected at the receiving end
of the signalling link. The congested receiving end of the link notifies the
remote transmitting end of the condition by means of an appropriate link status
signal unit and it withholds acknowledgements of all incoming message signal
units. When congestion abates acknowledgements of all incoming message signal
units is resumed. While congestion exists, the remote transmitting end is
periodically notified of this condition. The remote transmitting end will
indicate the link as failed if the congestion continues too long.
2 Basic signal unit format
2.1 General
Signalling and other information originating from a User Part is
transferred over the signalling link by means of signal units.
A signal unit is constituted of a variable length signalling information
field which carries the information generated by a User Part and a number of
fixed length fields which carry information required for message transfer
control. In the case of link status signal units, the signalling information
field and the service information octet is replaced by a status field which is
generated by the signalling link terminal.
2.2 Signal unit format
Three types of signal unit are differentiated by means of the length
indicator contained in all signal units, i.e., message signal units, link status
signal units and fill-in signal units. Message signal units are retransmitted in
case of error, link status signal unit and fill-in signal units are not. The
basic formats of the signal units are shown in Figure 3/Q.703.
Fascicle VI.7 - Rec. Q.703 PAGE1
2.3 Function and codes of the signal unit fields
2.3.1 General
The message transfer control information encompasses 8 fixed length fields
in the signal unit which contain information required for error control and
message alignment.
Figure 2/Q.703 - CCITT 40321
Figure 3/Q.703 - CCITT 35611
2.3.2 Flag
The opening flag indicates the start of a signal unit. The opening flag of
one signal unit is normally the closing flag of the preceding signal unit. The
closing flag indicates the end of a signal unit. The bit pattern for the flag is
01111110.
2.3.3 Length indicator
The length indicator is used to indicate the number of octets following
the length indicator octet and preceding the check bits and is a number in binary
code in the range 0-63. The length indicator differentiates between the three
types of signal units as follows:
Length indicator = 0: fill in signal unit
Length indicator = 1 or 2: link status signal unit
Length indicator > 2: message signal unit
In the case that the signalling information field of a message signal unit
is spanning 62 octets or more, the length indicator is set to 63.
It is mandatory that LI is set by the transmitting end to its correct
value as specified above.
2.3.4 Service information octet
The service information octet is divided into the service indicator and
the subservice field. The service indicator is used to associate signalling
information with a particular user part and is present only in message signal
units.
The content of the subservice field is described in Recommendation Q.704,
S 14.2.2.
Note - The Message Transfer Part may handle messages for different users
(i.e., messages with different service indicators) with different priorities.
These priorities are for further study.
2.3.5 Sequence numbering
The forward sequence number is the sequence number of the signal unit in
which it is carried.
The backward sequence number is the sequence number of a signal unit being
acknowledged.
The forward sequence number and backward sequence number are numbers in
binary code from a cyclic sequence ranging from 0 to 127 (see SS 5 and 6).
2.3.6 Indicator bits
The forward indicator bit and backward indicator bit together with the
forward sequence number and backward sequence number are used in the basic error
control method to perform the signal unit sequence control and acknowledgement
functions. (See SS 5.2 and 6.)
2.3.7 Check bits
Every signal unit has 16 check bits for error detection. (See S 4.)
2.3.8 Signalling information field
The signalling information field consists of an integral number of octets,
greater than or equal to 2 and less than or equal to 272.
The value 272 allows a single message signal unit to accommodate
information blocks of up to 268 octets in length accompanied by a routing label.
The format and codes of the signalling information field are defined for
each user part.
2.3.9 Status field
The formats and codes of the status field are described in S 11.
2.3.10 Spare fields
Spare fields are coded 0, unless otherwise indicated (see Figures 3/Q.703
and 6/Q.703).
2.4 Order of bit transmission
Each of the fields mentioned in S 2.3 will be transmitted in the order
indicated in Figure 3/Q.703.
PAGE18 Fascicle VI.7 - Rec. Q.703
Within each field or subfield the bits will be transmitted with the least
significant bit first. The 16 check bits are transmitted in the order generated
(see S 4).
3 Signal unit delimitation
3.1 Flags
A signal unit includes an opening flag (see S 2.2). The opening flag of a
signal unit is normally considered to be the closing flag of the preceding signal
unit (however, see Note to S 5). In certain conditions (e.g., signalling link
overload) a limited number of flags may be generated between two consecutive
signal units. However, a signalling link terminal always should be able to
receive consecutive signal units with one or more multiple flags inserted between
them.
3.2 Zero insertion and deletion
To ensure that the flag code is not imitated by any other part of the
signal unit the transmitting signalling link terminal inserts a 0 after every
sequence of five consecutive 1s before the flags are attached and the signal unit
is transmitted. At the receiving signalling link terminal, after flag detection
and removal, each 0 which directly follows a sequence of five consecutive 1s is
deleted.
Fascicle VI.7 - Rec. Q.703 PAGE1
4 Acceptance procedure
4.1 Acceptance of alignment
4.1.1 A flag which is not followed immediately by another flag is considered an
opening flag. Whenever an opening flag is received, the beginning of a signal
unit is assumed. When the next flag (a closing flag) is received it is assumed to
be the termination of the signal unit.
4.1.2 If seven or more consecutive 1s are received, the signal unit error rate
monitor or alignment error rate monitor enters the "octet counting" mode (see S
4.1.4) and the next valid flag is searched for.
4.1.3 After deletion of the 0s inserted for transparency, the received signal
unit length is checked for being a multiple of 8 bits and at least 6 octets,
including opening flag. If it is not, then the signal unit is discarded and the
signal unit error rate monitor or alignment error rate monitor is incremented. If
more than m + 7 octets are received before a closing flag, the "octet counting"
mode is entered (see Figure 11/Q.703) and the signal unit is discarded. m is the
maximum length of the signalling information field (in octets) allowed on a
signalling link. m takes the value 272. In the case of the basic error control
method a negative acknowledgement will be sent, if required, according to the
rules set out in S 5.2.
4.1.4 When the "octet counting" mode is entered all the bits received after the
last flag and before the next flag are discarded. The "octet counting" mode is
left when the next correctly-checking signal unit is received, and this signal
unit is accepted.
4.2 Error detection
The error detection function is performed by means of 16 check bits
provided at the end of each signal unit.
The check bits are generated by the transmitting signalling link terminal.
They are the ones complement of the sum (modulo 2) of:
i) the remainder of xk (x15 + x14 + x13 + x12 . . . + x2 + x + 1) divide
(modulo 2) by the generator polynomial x16 + x12 + x5 + 1, where k is
the number of bits in the signal unit existing between, but not
including, the final bit of the opening flag and the first bit of the
check bits, excluding bits inserted for transparency; and
ii) the remainder after multiplication by x16 and then division (modulo 2)
by the generator polynomial x16 + x12 + x5 + 1 of the content of the
signal unit existing between, but not including, the final bit of the
opening flag and the first bit of the check bits, excluding bits
inserted for transparency.
As a typical implementation, at the transmitting signalling link terminal,
the initial remainder of the division is preset to all 1s and is then modified by
division by the generator polynomial (as described above) on all the fields of
the signal unit; the 1s complement of the resulting remainder is transmitted as
the 16 check bits.
At the receiving signalling link terminal, the correspondence between the
check bits and the remaining part of the signal unit is checked; if a complete
correspondence is not found the signal unit is discarded.
As a typical implementation at the receiving signalling link terminal, the
initial remainder is preset to all 1s, and the serial incoming protected bits
including the check bits (after the bits inserted for transparency are removed)
when divided by the generator polynomial will result in a remainder of
0001110100001111 (x15 through x0, respectively) in t e absence of transmission
errors.
5 Basic error correction method
5.1 General
The basic error correction method is a noncompelled method in which
correction is performed by retransmission. In normal operation, the method
ensures correct transfer of message signal units over the signalling link, in
sequence and with no double delivery. As a consequence, no resequencing or
eliminating of the received information is required within the user parts.
PAGE18 Fascicle VI.7 - Rec. Q.703
Positive acknowledgements are used to indicate correct transfer of message
signal units. Negative acknowledgements are used as explicit requests for
retransmission of signal units received in a corrupt form.
To minimize the number of retransmissions and the resulting message signal
unit delay, a request for retransmission is made only when a message signal unit
(not another signal unit) has been lost because of, for example, transmission
errors or disturbances.
The method requires that transmitted but not yet positively acknowledged
message signal units remain available for retransmission. To maintain the correct
message signal unit sequence when a retransmission is made, the message signal
unit, the retransmission of which has been requested, and any subsequently
transmitted message signal units are retransmitted in the order in which they
were originally transmitted.
As part of the error correction method, each signal unit carries a forward
sequence number, a forward indicator bit, a backward sequence number and a
backward indicator bit. The error correction procedure operates independently in
the two transmission directions. The forward sequence number and forward
indicator bit in one direction together with the backward sequence number and
backward indicator bit in the other direction are associated with the message
signal unit flow in the first direction. They function independently of the
message signal unit flow in the other direction and its associated forward
sequence number, forward indicator bit, backward sequence number and backward
indicator bit.
The transmission of new message signal units is temporarily stopped during
retransmissions or when no forward sequence number values are available to be
assigned to new message signal units (due to a high momentary load or corruption
of positive acknowledgements) (see S 5.2.2).
Under normal conditions, when no message signal units are to be
transmitted or retransmitted, fill-in signal units are sent continuously. In some
particular cases link status signal units, continuous fill-in signal units or
flags may be sent as described in SS 7, 8 and 11.
5.2 Acknowledgements (positive acknowledgement and negative acknowledgement)
5.2.1 Sequence numbering
For the purposes of acknowledgement and signal unit sequence control, each
signal unit carries two sequence numbers. The signal unit sequence control is
performed by means of the forward sequence number. The acknowledgement function
is performed by means of the backward sequence number.
The value of the forward sequence number of a message signal unit is
obtained by incrementing (modulo 128, see S 2.3.5) the last assigned value by 1.
This forward sequence number value uniquely identifies the message signal
unit until its delivery is accepted without errors, and in correct sequence, by
the receiving terminal. The forward sequence number of a signal unit other than a
message signal unit assumes the value of the forward sequence number of the last
transmitted message signal unit.
5.2.2 Signal unit sequence control
Information regarding the service information octet, signalling
information field, forward sequence number and the length of each message signal
unit is retained at the transmitting signalling link terminal until a positive
acknowledgement for that signal unit is received (see S 5.2.3). In the meantime
the same forward sequence number cannot be used for another message signal unit
(see S 5.2.3).
A forward sequence number value can be assigned to a new message signal
unit when a positive acknowledgement concerning that value incremented by at
least 1 (modulo 128) is received (see S 5.2.3).
This means that not more than 127 message signal units may be available
for retransmission.
The action to be taken at the receiving signalling link terminal upon
receipt of a correctly checking signal unit is determined by comparison of the
received forward sequence number with the forward sequence number of the last
previously accepted signal unit, and on comparison of the received forward
indicator bit with the latest sent backward indicator bit. In addition, as the
appropriate action differs for a message signal unit and another signal unit, the
length indicator of the received signal unit must be examined.
Fascicle VI.7 - Rec. Q.703 PAGE1
a) If the signal unit is a fill-in signal unit then:
i) if the forward sequence number value equals the forward sequence
number value of the last accepted message signal unit, the signal
unit is processed within the message transfer part;
ii) if the forward sequence number value is different from the forward
sequence number of the last accepted message signal unit, the
signal unit is processed within the message transfer part. If the
received forward indicator bit is in the same state as the last
sent backward indicator bit, a negative acknowledgement is sent.
b) If the signal unit is a link status signal unit then it is processed
within the message transfer part.
c) If the signal unit is a message signal unit then:
i) if the forward sequence number value is the same as that of the
last accepted signal unit, the signal unit is discarded, regardless
of the state of the indicator bits;
ii) if the forward sequence number value is one more (modulo 128, see S
2.3.5) than that of the last accepted signal unit and if the
received forward indicator bit is in the same state as the last
sent backward indicator bit, the signal unit is accepted and
delivered to level 3.
Explicit positive acknowledgements to the accepted signal units are
sent as specified in S 5.2.3.
If the forward sequence number is one more than that of the last
accepted signal unit and if the received forward indicator bit is
not in the same state as the last sent backward indicator bit, then
the signal unit is discarded;
iii) if the forward sequence number value is different from those
values mentioned in (i) and (ii) above, the signal unit is
discarded. If the received forward indicator bit is in the same
state as the last sent backward indicator bit, a negative
acknowledgement is sent.
Processing of the backward sequence number value and backward
indicator bit value as described in S 5.3 is performed for message
signal units and fill in signal units except when unreasonable
backward sequence number value or unreasonable forward indicator
bit value is received. Discarding a signal unit means that if it is
a message signal unit, it is not delivered to level 3.
5.2.3 Positive acknowledgement
The receiving signalling link terminal acknowledges the acceptance of one
or more message signal units by assigning the forward sequence number value of
the latest accepted message signal unit to the backward sequence number of the
next signal unit sent in the opposite direction. The backward sequence numbers of
subsequent signal units retain this value until a further message signal unit is
acknowledged, which will cause a change of the backward sequence number sent.
The acknowledgement to an accepted message signal unit also represents an
acknowledgement to all, if any, previously accepted, though not yet acknowledged,
message signal units.
5.2.4 Negative acknowledgement
If a negative acknowledgement is to be sent (see S 5.2.2), then the
backward indicator bit value of the signal units transmitted is inverted. The new
backward indicator bit value is maintained in subsequently sent signal units
until a new negative acknowledgement is to be sent. The backward sequence number
assumes the value of the forward sequence number of the last accepted message
signal unit.
PAGE18 Fascicle VI.7 - Rec. Q.703
5.3 Retransmission
5.3.1 Response to a positive acknowledgement
The transmitting signalling link terminal examines the backward sequence
number value of the received message signal units and fill-in signal units that
have satisfied the polynomial error check. The previously sent message signal
unit, which has a forward sequence number value identical to the received
backward sequence number value, will no longer be available for transmission.
When an acknowledgement of a message signal unit having a given forward
sequence number value is received, all other message signal units which preceded
that message signal unit are considered to be acknowledged even though the
corresponding backward sequence numbers have not been received.
In the case that the same positive acknowledgement is consecutively
received a number of times, no further action is taken.
In the case that a message signal unit or fill-in signal unit is received
having a backward sequence number value which is not the same as the previous one
or one of the forward sequence number values of the signal units available for
retransmission, the signal unit is discarded. The following message signal unit
or fill-in signal unit is discarded.
If any two backward sequence number values in three consecutively received
message signal units or fill-in signal units are not the same as the previous one
or any of the forward sequence number values of the signal units in the
retransmission buffer at the time that they are received, then level 3 is
informed that the link is faulty.
A timing mechanism, timer T72), shall be provided which generates an
indication of excessive delay of acknowledgement if, assuming that there are at
least one outstanding MSU in the retransmission buffer, no new-acknowledgement
has been received within a time-out T7 (see S 12.3). In the case of excessive
delay in the reception of acknowledgements a link failure indication is given to
level 3.
5.3.2 Response to a negative acknowledgement
When the received backward indicator bit is not in the same state as the
last sent forward indicator bit, all the message signal units available for
retransmission are transmitted in correct sequence starting with the signal unit
which has a forward sequence number value of one more (modulo 128, see S 2.3.5)
than the backward sequence number associated with the received backward indicator
bit.
New message signal units can only be sent when the last message signal
unit available for retransmission has been transmitted.
At the start of a retransmission the forward indicator bit is inverted, it
thus becomes equal to the backward indicator bit value of the received signal
units. The new forward indicator bit value is maintained in subsequently
transmitted signal units until a new retransmission is started. Thus, under
normal conditions the forward indicator bit included in the transmitted signal
units is equal to the backward indicator bit value of the received signal units.
If a retransmitted message signal unit is lost, then this is detected by a check
on the forward sequence number and forward indicator bit (see S 5.2.2) and a new
retransmission request is made.
In the case that a message signal unit or a fill-in signal unit is
received having a forward indicator bit value indicating the start of a
retransmission when no negative acknowledgement has been sent, then that signal
unit is discarded. The following message signal unit or fill-in signal unit is
discarded.
If any two forward indicator bit values in three consecutively received
message signal units or fill-in signal units indicate the start of a
retransmission when no negative acknowledgement has been sent at the time that
they are received, then level 3 is informed that the link is faulty.
2) Timers defined in Recommendation Q.703 are absolute time values; this means that, due
to the possibility to insert multiple flags between signal units (see S 3.1), there may
be no fixed relation between the time-out values and the number of signal units
transmitted/received during the time-out periods.
Fascicle VI.7 - Rec. Q.703 PAGE1
5.3.3 Repetition of message signal units
The signal unit sequence control makes it possible to repeat a message
signal unit which has not yet been acknowledged without affecting the basic error
correction procedure. Thus a form of forward error correction by means of
repetition of message signal units is possible as a national option (e.g., to
reduce the effective signalling link speed in special national applications, and
in long loop delay applications to lower the retransmission rate and thus reduce
the average message delay). In the case of repetition, each signal unit should be
defined by its own opening and closing flags (i.e., there should be at least two
flags between signal units) to ensure that the repeated signal unit is not lost
by the corruption of only a single flag.
6 Error correction by preventive cyclic retransmission
6.1 General
The preventive cyclic retransmission method is essentially a noncompelled
forward error correction method, whereby positive acknowledgements are needed to
support the forward error correction.
Each message signal unit must be retained at the transmitting signalling
link terminal until a positive acknowledgement arrives from the receiving
signalling link terminal.
Error correction is effected by preventive cyclic retransmission of the
message signal units already sent, though not yet acknowledged. Preventive cyclic
retransmission takes place whenever there are no new message signal units or link
status signal units available to be sent.
To complement preventive cyclic retransmission, the message signal units
available for retransmission are retransmitted with priority when a limit of the
number of message signal units or a limit of the number of message signal unit
octets available for retransmission has been reached.
Under normal conditions, when no message signal units are to be
transmitted or cyclically retransmitted, fill-in signal units are sent. In some
particular cases link status signal units, continuous fill-in signal units or
flags may be sent as described in SS 7, 8 and 11.
6.2 Acknowledgements
6.2.1 Sequence numbering
For the purposes of acknowledgement and signal unit sequence control, each
signal unit carries 2 sequence numbers. The signal unit sequence control is
performed by means of the forward sequence number. The acknowledgement function
is performed by means of the backward sequence number.
The value of the forward sequence number of a message signal unit is
obtained by incrementing (modulo 128, see S 2.3.5) the last assigned value by 1.
This forward sequence number value uniquely identifies the message signal unit
until its delivery is accepted without errors and in correct sequence, by the
receiving signalling link terminal. The forward sequence number of a signal unit
other than a message signal unit assumes the value of the forward sequence number
of the last transmitted message signal unit.
6.2.2 Signal unit sequence control
Information regarding the service information octet, signalling
information field, forward sequence number and the length of each message signal
unit is retained at the transmitting signal link terminal until the related
acknowledgement for that signal unit is received (see S 6.2.3). In the meantime
the same forward sequence number value cannot be used for another message signal
unit (see S 6.2.3).
A forward sequence number value can be assigned to a new message signal
unit to be sent when a positive acknowledgement concerning that value incremented
by at least 1 (modulo 128) is received (see S 6.2.3).
The action to be taken at the receiving signalling link terminal upon
receipt of a correctly checking signal unit is determined by comparison of the
received forward sequence number with the forward sequence number of the last
previously accepted signal unit.
PAGE18 Fascicle VI.7 - Rec. Q.703
In addition, as the appropriate action differs for a message signal unit
and another signal unit, the length indicator of the received signal unit must be
examined. The forward indicator bit and the backward indicator bit are not used
and are set to 1.
a) If the signal unit is not a message signal unit, then the signal unit
is processed within the message transfer part.
b) If the signal unit is a message signal unit then:
i) if the forward sequence number value is the same as that of the
last accepted signal unit, the signal unit is discarded;
ii) if the forward sequence number value is one more (modulo 128, see S
2.3.5) than that of the last accepted signal unit, the signal unit
is accepted and delivered to level 3. Explicit positive
acknowledgements for the accepted signal units are sent as
specified in S 6.2.3;
iii) if the forward sequence number value is different from the
values mentioned in i) and ii) above, the signal unit is discarded.
Processing of the backward sequence number value as described in
Section 6.3 is performed for message signal units and fill-in
signal units except when unreasonable backward sequence number
value is received. Discarding a signal unit means that if it is a
message signal unit, it is not delivered to level 3.
6.2.3 Positive acknowledgement
The receiving signalling link terminal acknowledges the acceptance of one
or more message signal units by assigning the forward sequence number value of
the latest accepted message signal unit to the backward sequence number of the
next signal unit sent. The backward sequence numbers of subsequent signal units
retain this value until a further message signal unit is acknowledged, which will
cause a change of the backward sequence number sent. The acknowledgement to an
accepted message signal unit also represents an acknowledgement to all, if any,
previously accepted though not yet acknowledged signal units.
6.3 Preventive cyclic retransmission
6.3.1 Response to a positive acknowledgement
All message signal units sent for the first time are retained until they
have been positively acknowledged.
The transmitting signalling link terminal examines the backward sequence
number value of the received message signal units and fill-in signal units that
have satisfied the polynomial error check. The previously sent message signal
unit, the forward sequence number value of which is the same as the backward
sequence number value, will no longer be available for retransmission.
When an acknowledgement for a message signal unit having a given forward
sequence number value is received, all other message signal units, if any, having
forward sequence number values preceding that value (modulo 128) are considered
to be acknowledged, even though the corresponding backward sequence number has
not been received.
In the case that the same positive acknowledgement is consecutively
received a number of times, no further action is taken.
In the case that a message signal unit or fill-in signal unit is received
having a backward sequence number value which is not the same as the previous one
or one of the forward sequence number values of the signal units in the
retransmission buffer, the signal unit is discarded. The following message signal
unit or fill-in signal unit is discarded.
If any two backward sequence number values in three consecutively received
message signal units or fill-in signal units are not the same as the previous one
or any of the forward sequence number values of the signal units in the
retransmission buffer at the time that they are received, then level 3 is
informed that the link is faulty.
A timing mechanism, timer T7, shall be provided which generates an
indication of excessive delay of acknowledgement if, assuming that there is at
least one outstanding MSU in the retransmission buffer, no new acknowledgement
has been received within a time-out T7 (see S 12.3). In the case of excessive
delay in the reception of acknowledgements a link failure indications is given to
level 3.
6.3.2 Preventive cyclic retransmission procedure
i) If no new signal units are available to be sent, the message signal
units available for retransmission are retransmitted cyclically.
Fascicle VI.7 - Rec. Q.703 PAGE1
ii) If new signal units are available, the retransmission cycle, if any,
must be interrupted and the signal units be sent with priority.
iii) Under normal conditions, when no message signal units are to be
transmitted or cyclically retransmitted, fill-in signal units are sent
continuously. In some particular cases link status signal units,
continuous fill-in signal units or flags may be sent as described in SS
7, 8 and 10.
6.4 Forced retransmission
To maintain the efficiency of error correction in those cases where
automatic error correction by preventive cyclic retransmission alone is made
impossible (by, for example, high signalling load), the preventive cyclic
retransmission procedures must be complemented by the forced retransmission
procedure.
6.4.1 Forced retransmission procedure
Both the number of message signal units available for retransmission (N1)
and the number of message signal unit octets available for retransmission (N2)
are monitored continuously.
If one of them reaches its set limit, no new message signal units or
fill-in signal units are sent and the retransmission cycle is continued up to the
last message signal unit entered into retransmission buffer with priority, in the
order in which they were originally transmitted. If all those message signal
units have been sent once and neither N1 nor N2 is at its limit value, the normal
preventive cyclic retransmission procedure can be resumed. If not, all the
message signal units available for retransmission are sent again with priority.
6.4.2 Limitation of the values N1 and N2
N1 is limited by the maximum numbering capacity of the forward sequence
number range which dictates that not more than 127 message signal units can be
available for retransmission.
In the absence of errors, N2 is limited by the signalling link loop delay
TL. It must be ensured that not more than TL/Teb + 1 message signal unit octets
are available for retransmission,
where
TL is the signalling link loop delay, i.e., the time between the sending
of a message signal unit and the reception of the acknowledgement for
this message signal unit in undisturbed operation; and
Teb is the emission time of one octet.
When some signalling data links of different loop delays are alternated
for application to that signalling link, the longest possible signalling link
delay may be used to calculate the value of TL.
7 Initial alignment procedure
7.1 General
The procedure is applicable to activation and to restoration of the link.
The procedure provides a "normal" proving period for "normal" initial alignment
and an "emergency" proving period for "emergency" initial alignment. The decision
to apply either the "normal" or the "emergency" procedures is made unilaterally
at level 3 (see Recommendation Q.704). Only the signalling link to be aligned is
involved in the initial alignment procedure (i.e., no transfer of alignment
information over other signalling links is required).
7.2 Initial alignment status indications
The initial alignment procedure employs four different alignment status
indications:
- status indication "O": out of alignment;
- status indication "N": "normal" alignment status;
- status indication "E": "emergency" alignment status;
- status indication "OS": out of service.
These indications are carried in the status field of the link status
signal units (see S 2.2).
Status indication "O" is transmitted when initial alignment has been
started and none of the status indications "O", "N" or "E" are received from the
link. Status indication "N" is transmitted when, after having started initial
alignment, status indication "O", "N" or "E" is received and the terminal is in
the "normal" alignment status. Status indication "E" is transmitted when, after
having started initial alignment, status indication "O", "N" or "E" is received
and the terminal is in the "emergency" alignment status, i.e., it must employ the
short "emergency" proving period.
PAGE18 Fascicle VI.7 - Rec. Q.703
Status indications "N" and "E" indicate the status of the transmitting
signalling link terminal; this is not changed by reception of status indications
indicating a different status at the remote signalling link terminal. Hence, if a
signalling link terminal with a "normal" alignment status receives a status
indication "E" it continues to send status indication "N" but initiates the short
"emergency" proving period.
Status indication "OS" informs the remote signalling link terminal that
for reasons other than processor outage (e.g., link failure) the signalling link
terminal can neither receive nor transmit message signal units. Status indication
OS is sent on completion of "power on" (see Figures 2/Q.703 and 8/Q.703) until
initial alignment is started.
7.3 Initial alignment procedure
The alignment procedure passes through a number of states during the
initial alignment:
- State Idle: the procedure is suspended.
- State "not aligned": the signalling link is not aligned and the
terminal is sending status indication "O". Time-out T2 is started on
3) Timers defined in Recommendation Q.703 are absolute time values; this means that, due
to the possibility to insert multiple flags between signal units (see S 3.1), there may
be no fixed relation between the time-out values and the number of signal units
transmitted/received during the time-out periods.
4) "If automatic allocation of signalling terminals or signalling data links is applied at
both ends of a signalling link, it must be ensured that the values of this time-out are
different at each end of a signalling link (see Recommendation Q.704, S 12). In this
case T2 los (see S 12.3) is allocated to the signalling point with the lower point code
and T2 high to the signalling point with the higher point code. In all other cases, the
value of time-out T2 can be the same at both ends of the link.
Fascicle VI.7 - Rec. Q.703 PAGE1
entry to State and stopped when State is left4 ).
- State "aligned": the signalling link is aligned and the terminal is
sending status indication "N" or "E", status indications "N", "E" or
SOS" are not received. Time-out T33) is started on entry to State and
stopped when State is left.
- State 03, "proving"; the signalling link terminal is sending status
indication "N" or "E", status indication "O" or "OS" are not received,
proving has been started.
Proving is the means by which the signalling link terminal validates
the link's ability to carry signal units correctly by inspecting the
signal units. ½Proving╗ must last for a period of T4 before the link
can enter the ½aligned ready╗ link state. Expiry of timer T4 (see S
12.3) indicates a successful proving period unless the proving period
has been previously aborted up to four times.
- Following successful alignment and proving procedure, the signalling
terminal enters Aligned Ready state and the aligned ready time-out T1
is stopped on entry in the In service state and the duration of
time-out T1 should be chosen such that the remote end can perform four
additional proving attempts.
The procedure itself is described in the overview diagram, Figure 4/Q.703,
and in state transition diagram, Figure 9/Q.703.
7.4 Proving periods
The nominal values of the proving periods are:
Pn = 216 octets transmission time
Pe = 212 octets transmission time
for both 64 kbit/s and lower bit rates. For the corresponding timer T4 values
(proving periods), see S 12.3.
Figure 4/Q.703 - CCITT 40332
PAGE18 Fascicle VI.7 - Rec. Q.703
8 Processor outage
The procedure for dealing with local and/or remote processor outage is
described in Figure 10/Q.703.
A processor outage situation occurs when, due to factors at a functional
level higher than level 2, use of the link is precluded.
In this context, processor outage refers to a situation when signalling
messages cannot be transferred to functional levels 3 and/or 4. This may be
because of, for example, a central processor failure. A processor outage
condition may not necessarily affect all signalling links in a signalling point,
nor does it exclude the possibility that level 3 is able to control the operation
of the signalling link.
When level 2 identifies a local processor outage condition, either by
receiving an explicit indication from level 3, (i.e., local signalling link
blocking, see Recommendation Q.704, S 3.2.6), or by recognizing a failure of
level 3, it transmits link status signal units indicating processor outage and
discards message signal units received. Provided that the level 2 function at the
far end of the signalling link is in its normal operating stage (i.e.,
transmitting message signal units or fill-in signal units), upon receiving link
status signal units indicating processor outage, it notifies level 3 and begins
to continuously transmit fill-in signal units.
When the local processor outage condition ceases, normal transmission of
message signal units and fill-in signal units is resumed (provided that no local
processor outage condition has arisen also at the remote end); as soon as the
level 2 function at the remote end correctly receives a message signal unit or
fill-in signal unit, it notifies level 3 and returns to normal operation.
Format and code of link status signal units indicating processor outage
(status indication "PO") appear in S 11.
9 Level 2 flow control
9.1 General
The procedure is used to handle a level 2 congestion situation. After the
congestion is detected at the receiving end of the signalling link, both positive
and negative acknowledgements to message units are withheld and a status
indication "B" (Busy) is sent from the receiving end of the link to the remote
end in order to enable the remote transmitting end to distinguish between
congestion and failure situations.
This indication is carried in the status field of a link status signal
unit.
Note - The receiving end continues to process BSN and BIB carried in
signal units received in order to avoid, as far as possible, disturbance of the
message flow in the opposite direction and in addition may continue to accept
message signal units.
9.2 Detection of congestion
The mechanism for detecting congestion at the receiving end of a
signalling link is implementation dependent and not to be specified.
9.3 Procedure in the congestion situation
The receiving end of a signalling link which detected a congestion
situation, periodically returns a link status signal unit containing a status
indication "B" to the remote transmitting end of the link at interval T5 (see S
12.3).
The receiving level 2 also withholds acknowledgement of the message signal
unit, which triggered off the congestion detection, and of message signal units
received during the congestion situation; that is fill-in signal units or message
signal units are sent as usual, but with the backward sequence number and
backward indicator bit assigned the values which are contained in the last
transmitted signal unit before the congestion is recognized.
At the remote end of the signalling link, every reception of a link status
signal unit containing indication "B" causes the excessive delay of
acknowledgement timer T7 to be restarted. In addition first reception of the link
status signal unit containing a status indication "B" starts a longer supervision
timer T6 (see S 12.3). Should timer T6 expire, link failure indication is
generated.
Fascicle VI.7 - Rec. Q.703 PAGE1
9.4 Congestion abatement procedure
When congestion abates at the receiving end of the signalling link,
transmission of link status signal unit containing a status indication "B" is
stopped and normal operation resumed.
At the remote end, the supervision timer T6 is stopped when a negative or
positive acknowledgement whose backward sequence number acknowledges a message
signal unit in the retransmission buffer is received in case of the basic error
correction method, or a positive acknowledgement in case of the PCR method.
Note - Congestion onset and abatement detection is an implementation
dependent function. Sufficient hysteresis should be provided in the
implementation to prevent excessive oscillation between congested and
non-congested states.
10 Signalling link error monitoring
10.1 General
Two link error rate monitor functions are provided; one which is employed
whilst a signalling link is in service and which provides one of the criteria for
taking the link out of service, and one which is employed whilst a link is in the
proving state of the initial alignment procedure (see S 7.3). These are called
the signal unit error rate monitor and the alignment error rate monitor
respectively.
10.2 Signal unit error rate monitor
10.2.1 The signal unit error rate monitor has as its function the estimation of
the signal unit error rate in order to decide about the signalling link fault
condition. The signal units in error are those rejected by the acceptance
procedure (see S 4). The three parameters which determine the signal unit error
rate monitor are: the number T (signal units), of consecutive signal units
received in error that will cause an error rate high indication to level 3, the
lowest signal unit error rate 1/D (signal unit errors/signal unit) which will
ultimately cause an error rate high indication to level 3, and the number N
(octets) of octets that causes an increment of the counter while in the "octet
counting" mode. See Figure 5/Q.703.
10.2.2 The signal unit error rate monitor may be implemented in the form of an
up/down counter decremented at a fixed rate (for every D received signal units or
signal unit errors indicated by the acceptance procedure), but not below zero,
and incremented every time a signal unit error is detected by the signal unit
acceptance procedure (see S 4), but not above the threshold [T (signal units)].
An excessive error rate will be indicated whenever the threshold T is reached.
Figure 5/Q.703 - T1114950-88
PAGE18 Fascicle VI.7 - Rec. Q.703
10.2.3 In the "octet counting" mode (see S 4.1) the counter is incremented for
every N octets received until a correctly-checking signal unit is detected
(causing the "octet counting" mode to be left).
10.2.4 When the link is brought into service the monitor count should start from
zero.
10.2.5 The values of the three parameters are:
T = 64 signal units ü
D = 256 signal units/signal unit error ìFor 64 kbit/s
N = 16 octets Φ
T = 32 signal units ü
D = 256 signal units/signal unit error ìFor lower bit rates
N = 16 octets Φ
In the case of loss of alignment, these figures will give times of
approximately 128 ms and 854 ms to initiate changeover for 64 kbit/s and 4.8
kbit/s respectively.
10.2.6 In the case where only random signal unit errors occur over the signalling
link, the relationship between the expected number of signal units until
threshold of T (signal units) is reached and the signal unit errors rate (signal
unit errors/signal units) can be established. This relationship may be expressed
by an orthogonal hyperbola which has parameters (T, 1/D). See Figure 5/Q.703.
10.3 Alignment error rate monitor
10.3.1 The alignment error rate monitor is a linear counter which is operated
during normal and emergency proving periods.
10.3.2 The counter is started from zero whenever the proving state (Figure
9/Q.703) of the alignment procedure is entered and is then incremented for every
signal unit error detected, if not in the octet counting mode. It is also
incremented for every N octets received while in the octet counting mode, as
described in S 9.2.3.
10.3.3 When the counter reaches a threshold Ti, that particular proving period is
aborted; on receipt of a correct signal unit or the expiry of the aborted proving
period the proving state is reentered. If proving is aborted M times, the link is
returned to the out-of-service state. A threshold is defined for each of the two
types of proving period (normal and emergency, see S 7). These are Tin and Tie
and apply to the normal proving period and the emergency proving period
respectively.
Proving is successfully completed when a proving period expires without an
excessive error rate being detected and without the receipt of status indication
"O" or "OS".
10.3.4 The values of the four parameters for both 64 kbit/s and lower bit rates
are:
Tin = 4
Tie = 1
M = 5
N = 16
Note - It is noted that the emergency proving period may be successfully
completed with some probability with a marginal and degraded bit error rate,
i.e., around one error in 104 bits - subsequently, the SUERM will quickly
indicate an excessive error rate. However, short term operation on a degraded
link may be acceptable (e.g., to send management messages).
11 Level 2 codes and priorities
11.1 Link status signal unit
11.1.1 The link status signal unit is identified by a length indicator value
equal to 1 or 2. If the length indicator has a value of 1 then the status field
consists of one octet; if the length indicator has a value of 2 then the status
field consists of two octets.
Fascicle VI.7 - Rec. Q.703 PAGE1
11.1.2 The format of the one octet status field is shown in Figure 6/Q.703.
Figure 6/Q.703 - CCITT 35621
When a terminal, which is able to process only a one octet status field,
receives a link status signal unit with a two octet status field, the terminal
shall ignore the second octet for compatibility reasons but process the first
octet as specified.
11.1.3 The use of the link status indications is described in S 7; they are coded
as follows:
C B A
0 0 0 - Status indication "O"
0 0 1 - Status indication "N"
0 1 0 - Status indication "E"
0 1 1 - Status indication "OS"
1 0 0 - Status indication "PO"
1 0 1 - Status indication "B"
The spare bits should be ignored at the receiving side.
Note - For the use of spare bit D in the national option for a SIF
compatibility mechanism, see Recommendation Q.701, S 7.2.6.
11.2 Transmission priorities within level 2
11.2.1 Five different items can be transmitted:
i) new message signal units;
ii) message signal units which have not yet been acknowledged;
iii) link status signal units;
iv) fill-in signal units;
v) flags.
In certain failure conditions, it may only be possible to send flags or
nothing at all.
11.2.2 For the basic error control method the priorities are:
Highest 1. Link status signal units.
2. Message signal units which have not yet been acknowledged and
for which a negative acknowledgement has been received.
3. New message signal units.
4. Fill-in signal units.
Lowest 5. Flags.
PAGE18 Fascicle VI.7 - Rec. Q.703
11.2.3 For the preventive cyclic retransmission method, the priorities are:
Highest 1. Link status signal units.
2. Message signal units which have not yet been acknowledged and
which are stored in a retransmission buffer and exceed one of
the parameters N1 and N2.
3. New message signal units.
4. Message signal units which have not yet been acknowledged.
5. Fill-in signal units.
Lowest 6. Flags.
Note - In the basic error control method, where the repetition of message
signal units is employed as a national option, the repeated message signal unit
will have a priority immediately below that of link status signal units.
12 State transition diagrams and timers
12.1 Section 12 contains the description of the signalling link control
functions, described in this Recommendation, in the form of state transition
diagrams according to the CCITT Specification and Description Language (SDL). The
following list summarizes these diagrams:
- 2 - Functional block diagram:
Figure 7/Q.703.
- Link state control: Figure 8/Q.703.
- Initial alignment control: Figure 9/Q.703.
- Processor outage control: Figure 10/Q.703.
- Delimitation, alignment and error detection (receiving):
Figure 11/Q.703.
- Delimitation, alignment and error detection (transmitting):
Figure 12/Q.703.
- Basic transmission control: Figure 13/Q.703.
- Basic reception control: Figure 14/Q.703.
- Preventive cyclic retransmission transmission control:
Figure 15/Q.703.
- Preventive cyclic retransmission reception control: Figure
16/Q.703.
- Alignment error rate monitor: Figure 17/Q.703.
- Signal unit error rate monitor: Figure 18/Q.703.
- Congestion control part: Figure 19/Q.703.
The detailed functional breakdown shown in the following diagrams is
intended to illustrate a reference model and to assist interpretation of the text
in the earlier sections. The state transition diagrams are intended to show
precisely the behaviour of the signalling system under normal and abnormal
conditions as viewed from a remote location. It must be emphasized that the
functional partitioning shown in the following diagrams is used only to
facilitate understanding of the system behaviour and is not intended to specify
the functional partitioning to be adopted in a practical implementation of the
signalling system.
In the following figures the term signal unit refers to units which do not
contain all error control information.
12.2 Abbreviations
AERM Alignment error rate monitor
BIB Backward indicator bit
BIBR BIB received
BIBT BIB to be transmitted
Fascicle VI.7 - Rec. Q.703 PAGE1
BIBX BIB expected
BSN Backward sequence number
BSNR BSN received
BSNT BSN to be transmitted
Cp Count of aborted proving attempts [Figure 9/Q.703 (sheets 2 of 3
and 3 of 3)]
Cm Counter of MSU in TD [Figure 13/Q.703 (sheet 1 of 2) and Figure
15/Q.703 (sheet 1 of 3)]
Ca AERM count (Figure 17/Q.703)
Cs SUERM count (Figure 18/Q.703)
CC Congestion control
DAEDR Delimitation, alignment and error detection (receiving)
DAEDT Delimitation, alignment and error detection (transmitting)
FIB Forward indicator bit
FIBR FIB received
FIBT FIB transmitted
FIBX FIB expected
FISU Fill-in signal unit
FSN
PAGE18 Fascicle VI.7 - Rec. Q.703
Forward sequence number
FSNC Forward sequence number of last message signal unit accepted by
remote level 2
FSNF FSN of the oldest MSU in the RTB
FSNL FSN of the last MSU in the RTB
FSNR FSN received
FSNT FSN of the last MSU transmitted
FSNX FSN expected
IAC Initial alignment control
L2 Level 2
L3 Level 3
LSC Link state control
LSSU Link status signal unit
MGMT Management system - Unspecified implementation dependent
management function
MSU Message signal unit
M Maximum length of the SiF in the MSU
NSU Correct SU count
NACK Negative acknowledgement
Fascicle VI.7 - Rec. Q.703 PAGE1
N1 Maximum number of MSU which are available for retransmission
(fixed by the numbering capacity of the FSN)
N2 Maximum number of MSU octets which are available for
retransmission (fixed by the common channel loop delay time)
POC Processor outage control
RC Reception control
RTB Retransmission buffer
RTR If = 1 means retransmission expected
SIB Status indication "B" ("Busy")
SIE Status indication "E" ("emergency alignment")
SIN Status indication "N" ("normal alignment")
SIO Status indication "O" ("out of alignment")
SIOS Status indication "OS" ("out of service")
SIPO Status indication "PO" ("processor outage")
SU Signal unit
SUERM Signal unit error rate monitor
T SUERM threshold
TB Transmission buffer
TXC
PAGE18 Fascicle VI.7 - Rec. Q.703
Transmission control
UNB Counter of unreasonable BSN
UNF Counter of unreasonable FIB
Ti AERM threshold
Tie Emergency AERM threshold
Tin Normal AERM threshold
Z Pointer to sequence number of next MSU to be retransmitted in
transmission code
12.3 Timers
T1 Timer "alignment ready"
T1 (64) = 40-50 s bit rate of 64 kbit/s
T1 (4.8) = 500-600 bit rate of 4.8 kbit/s
s
T2 = 5-150 s Timer "not aligned"
T2 low = 5-50 s only for automatic allocation of
T2 high = 70-150 s signalling data links and terminals
T3 = 1-1.5 s Timer "aligned"
T4 Proving period timer = 216 or 212 octet
T4n (64) = 7.5-9.5 transmission time
s normal proving period at 64 kbit/s
nominal value 8.2 s (corresponding to Pn = 216)
T4n (4.8) = 100-120 nominal proving period at 4.8 kbit/s
s (corresponding to Pn = 216)
Nominal value 110 s emergency proving period at 64 kbit/s
T4e (64) = 400-600 (corresponding to Pe = 1212)
ms emergency proving period at 4.8 kbit/s
Nominal value 500 (corresponding to Pe = 212)
ms
T4e (4.8) = 6-8 s
Nominal value 7 s
T5 = 80-120 ms Timer "sending SIB"
T6 Timer "remote congestion"
T6 (64) = 3-6 s bit rate of 64 kbit/s
T6 (4.8) = 8-12 s bit rate of 4.8 kbit/s
T7 Timer "excessive delay of acknowledgement"
T7 (64) = 0.5-2 s bit rate of 64 kbit/s
For PCR method, values less than 0.8s should not be used
T7 (4.8) = 4-6 s bit rate of 4.8 kbit/s
Pe Emergency proving period
Pn Normal proving period
Fascicle VI.7 - Rec. Q.703 PAGE1
