that services in the future may expect to be based on the concept of an
Integrated services digital network (ISDN);
.PP
(b)
that errors are a major source of degradation in that they affect voice
services in terms of distortion of voice, and data type services in terms
of lost or inaccurate information or reduced throughout;
.bp
.PP
(c)
that while voice services are likely to predominate, the
ISDN is required to transport a wide range of service types and it is therefore
desirable to have a unified specification;
.PP
(d)
that an explanation of network performance objectives and their relationship
with design objectives is given in Recommendation G.102,
.sp 1P
.LP
\fIrecommends\fR
.sp 9p
.RT
.PP
that within the following scope and definitions the requirements set out
in Table 1/G.821 and subsequent paragraphs should be met.
.sp 2P
.LP
\fB1\fR \fBScope and definitions\fR
.sp 1P
.RT
.PP
1.1
The
performance objectives
are stated for each direction of a 64\ kbit/s circuit\(hyswitched connection
used for voice traffic or as a
\*QBearer Channel\*U for data\(hytype services.
.sp 9p
.RT
.PP
1.2
Recommendation I.325 gives reference configurations for the
ISDN connection types listed in Recommendation\ I.340. In the context of
error performance of 64\ kbit/s circuit\(hyswitched connection types and the
allocation of performance to the connection elements, an all digital
hypothetical reference configuration\ (HRX) is given in Figure\ 1/G.821. It
encompasses a total length of 27 | 00\ km and is a derivative of the
standard hypothetical reference configuration given in Figure\ 1/G.801 and
of the reference configuration given in Figure\ 3/I.325.
.sp 9p
.RT
.PP
1.3
The performance objective is stated in terms of \fBerror
performance parameters\fR each of which is defined as follows:
.sp 9p
.RT
.LP
\*QThe percentage of averaging periods each of time interval \fIT\fR\d0\uduring
which the
bit error ratio
(BER) exceeds a threshold value. The percentage is assessed over a much
longer time interval \fIT\fR\d\fIL\fR\u\*U (see Note\ 3
to Table\ 1/G.821).
.PP
It should be noted that total time (\fIT\fR\d\fIL\fR\u) is split into two
parts, namely, time for which the connection is deemed to be available
and that time when it is unavailable (see Annex\ A).
.PP
Requirements relating to the permissible percentage of unavailable
time will be the subject of a separate Recommendation.
.RT
.PP
1.4
The following BERs and intervals are used in the statement of
objectives:
.sp 9p
.RT
.LP
a)
a BER of less than 1 | (mu | 0\uD\dlF261\u6\d for \fIT\fR\d0\u\ =\ 1\ minute;
.LP
b)
a BER of less than 1 | (mu | 0\uD\dlF261\u3\d for \fIT\fR\d0\u\ =\ 1\ second;
.LP
c)
zero errors for \fIT\fR\d0\u\ =\ 1\ second (equivalent to the concept
of error free seconds EFS).
.PP
These categories equate to those of Table\ 1/G.821. In assessing
these objectives, periods of unavailability are excluded (see Annexes\ A
and\ B).
.PP
1.5
The performance objectives aim to serve two main functions:
.sp 9p
.RT
.LP
a)
to give the user of future national and international digital
networks an indication as to the expected error performance under real
operating conditions, thus facilitating service planning and terminal
equipment design;
.LP
b)
to form the basis upon which performance standards are derived for transmission
equipment and systems in an ISDN connection.
.PP
1.6
The performance objectives represent a compromise between a
desire to meet service needs and a need to realize transmission systems
taking into account economic and technical constraints. The performance
objectives,
although expressed to suit the needs of different services are intended to
represent a single level of transmission quality.
.bp
.sp 9p
.RT
.LP
.PP
The performance objective for degraded minutes [Table\ 1/G.821\ (a)] as
stated, is based on an averaging period of one minute. This averaging
period and the exclusion of errors occurring within severely errored seconds
which occur during this one minute period (see Table\ 1/G.821, Note\ 2), may
allow
connections with frequent burst errors to meet this particular part of the
overall objective, but such events will be controlled to a certain extent by
the severely errored seconds objective [Table\ 1/G.821\ (b)]. However,
there is some doubt as to whether the objectives are adequate for proper
operation of
real\(hytime video services with relatively long holding times, and this is the
subject of further study.
.PP
1.7
Since the performance objectives are intended to satisfy the
needs
of the future digital network it must be recognized that such objectives
cannot be readily achieved by all of today's digital equipment and systems.
The
intent, however, is to establish equipment design objectives that are
compatible with the objectives in this Recommendation. These aspects are
currently the subject of discussion within the CCITT and CCIR.
.sp 9p
.RT
.PP
It is further urged that all technologies, wherever they appear in the
network, should preferably be designed to better standards than those
indicated here in order to minimize the possibility of exceeding the end\(hyto\(hyend
objectives on significant numbers of real connections.
.PP
1.8
The objectives relate to a very long connection and recognizing that a
large proportion of real international connections will be shorter, it
is expected that a significant proportion of real connections will offer
a
better performance than the limiting value given in \(sc\ 2. On the other
hand, a small percentage of the connections will be longer and in this
case may exceed the allowances outlined in this Recommendation.
.sp 9p
.RT
.PP
\fINote\fR \ \(em\ Controlled slips, which may be perceived as short bursts
of errors, are not included in the calculations of the error performance
objectives in this Recommendation. Therefore, users should be aware that
error performance measurements which include controlled slip effects may
produce
poorer performance than would be indicated by this Recommendation. Users are
directed to Recommendation\ G.822, which specifies the controlled slip rate
objectives, for guidance in estimating the possible effects on their
applications.
.PP
1.9
The error performance objectives detailed in \(sc\(sc\ 2 and\ 3 of this
Recommendation apply to a 64\ kbit/s circuit switched connection (as defined
in \(sc\ 1.2). However, it is recognized that in practical situations these
objectives will need to be evaluated from measurements made at higher bit
rates.
.sp 9p
.RT
.PP
Therefore, Annex D defines preliminary guidelines for estimating 64\ kbit/s
error performance parameter information from measurements made at the primary
and higher bit rates.
.sp 2P
.LP
\fB2\fR \fBPerformance objectives\fR
.sp 1P
.RT
.PP
The performance objectives for an international ISDN connection as
identified in \(sc\(sc\ 1.1 and 1.2 are shown in Table\ 1/G.821. It is
intended that
international ISDN connections should meet all of the requirements of
Table\ 1/G.821 concurrently. The connection fails to satisfy the objective if
any of the requirements is not met.
.RT
.sp 2P
.LP
\fB3\fR \fBAllocation of overall objectives\fR
.sp 1P
.RT
.PP
\fB
Since the objectives given in \(sc\ 2 relate to an overall connection it
is necessary to sub\(hydivide this to constituent parts. This paragraph
outlines the basic principles and strategy for apportioning the performance
objectives.
.PP
The overall apportionment philosophy involves the use of two slightly different
strategies, one applicable to the degraded minutes requirement and
the errored seconds requirement [see classifications\ a), c)] and the other
applicable to the severely errored seconds requirement [see
classification\ b)].
.bp
.RT
.ce
\fBH.T. [T1.821]\fR
.ce
TABLE\ 1/G.821
.ce
\fBError performance objectives for\fR
.ce
\fBinternational ISDN connections\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(120p) .
Performance classification Objective (Notes 3, 5)
_
.T&
cw(60p) | lw(120p) .
{
(a)
(Degraded minutes)
(Notes 1, 2)
} {
Fewer than 10% of one\(hyminute intervals to have a bit error ratio worse
than 1 | (mu | 0\uD\dlF261\u6\d (Note 4)
}
_
.T&
cw(60p) | lw(120p) .
{
(b)
(Severely errored seconds)
(Note 1)
} {
Fewer than 0.2% of one\(hysecond intervals to have a bit error ratio worse
than 1 | (mu | 0\uD\dlF261\u3\d
}
_
.T&
cw(60p) | lw(120p) .
{
(c)
(Errored seconds)
(Note 1)
} {
Fewer than 8% of one\(hysecond intervals to have any errors
(equivalent to 92% error\(hyfree seconds)
}
.TE
.LP
\fINote\ 1\fR
\ \(em\ The terms \*Qdegraded minutes\*U, \*Qseverely errored seconds\*U and
\*Qerrored seconds\*U are used as a convenient and concise performance objective
\*Qidentifier\*U. Their usage is not intended to imply the acceptability, or
otherwise, of this level of performance.
.LP
\fINote\ 2\fR
\ \(em\ The one\(hyminute intervals mentioned in Table 1/G.821 and in the notes (i.e. the periods for M | | in Annex\ B) are derived by removing unavailable
time and severely errored seconds from the
total time and then consecutively grouping the remaining seconds into blocks
of\ 60. The basic one\(hysecond intervals are derived from a fixed period.
.LP
\fINote\ 3\fR
\ \(em\ The time interval \fIT\fI
, over which the percentages are to be assessed has not been specified since the period may depend upon the
application. A period of the order of any one month is suggested as a
reference.
.LP
\fINote\ 4\fR
\ \(em\ For practical reasons, at 64 kbit/s, a minute containing four
errors (equivalent to an error ratio of 1.04 \(mu 10\uD\dlF261\u6\d) is not
considered degraded. However, this does not imply relaxation of the error
ratio objective of 1 | (mu | 0\uD\dlF261\u6\d.
.LP
\fINote\ 5\fR
\ \(em\ Annex B illustrates how the overall performance should be
assessed.
.nr PS 9
.RT
.ad r
\fBTABLEAU 1/G.821 [T1.821], p.1\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 20
.bp
.sp 1P
.LP
3.1
\fIBasic apportionment principles\fR
.sp 9p
.RT
.PP
Apportionment is based on the assumed use of transmission systems having
qualities falling into one of a limited number of different
classifications.
.PP
Three distinct quality classifications have been identified
representative of practical digital transmission circuits and are independent
of the transmission systems used. These classifications are termed local
grade, medium grade and high grade and their usage generally tends to be
dependent on their location within a network (see Figure\ 1/G.821).
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure 1/G.821,\fR
.sp 1P
.RT
.ad b
.RT
.PP
The following general assumptions apply to the apportionment
strategy that follows:
.LP
\(em
in apportioning the objectives to the constituent elements of a connection
it is the \*Q% of time\*U that is subdivided;
.LP
\(em
an equal apportionment of the objectives applies for both the degraded
minutes and errored seconds requirements [classifications\ a), c)];
.LP
\(em
the error ratio threshold is not sub\(hydivided. The rationale for this
is based on the assumption that the performance of real circuits
forming the parts of the HRX (Figure\ 1/G.821) will normally be significantly
better than the degraded minute threshold (see Note to \(sc\ 3.1);
.LP
\(em
no account is taken of the error contribution from either
digital switching elements or digital multiplex equipments on the basis
that it is negligible in comparison with the contribution from transmission
systems.
.PP
These quality classifications for different parts of the
connection are considered to represent the situation for a large proportion
of real international connections. Administrations are free to use whatever
transmission systems they wish within their own networks and these other
arrangements are considered as being completely acceptable provided that the
overall performance of the national portion is no worse than it would have
been if the standard CCITT arrangements had been employed.
.PP
It should be noted that a small percentage of connections will be
longer than the 27 | 00\ km HRX. By definition the extra connection length
will be carried over high\(hygrade circuits and hence the amount by which
such
connections exceed the total allowance envisaged in this Recommendation
will be proportional to the amount by which the 25 | 00\ km section is
exceeded.
Administrations should note that if the performance limits in the various
classifications could be improved in practical implementations, the occurrence
of these situations could be significantly reduced.
.PP
\fINote\fR \ \(em\ For terrestrial systems the apportionment of the
\*Qdegraded minute\*U performance
classification to smaller entities
(e.g.\ hypothetical reference digital section) may require sub\(hydivision
of the error ratio objective, as well as the sub\(hydivision of \*Q% of
time\*U, with
distance. This is the subject of further study.
.RT
.sp 1P
.LP
3.2
\fIApportionment strategy for the\fR
\fIdegraded minutes\fR and
\fIerrored seconds\fR \fIrequirements\fR
.sp 9p
.RT
.PP
The apportionment of the permitted degradation, i.e.\ 10% degraded minutes
and 8% errored seconds, is given in Table\ 2/G.821. The derived network
performance objectives are given in Annex\ C.
.bp
.RT
.ce
\fBH.T. [T2.821]\fR
.ce
TABLE\ 2/G.821
.ce
\fBAllocation of the degraded minutes and errored\fR
.ce
\fBseconds objectives for the three circuit classifications\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(120p) .
Circuit classification {
Allocation of the degraded minutes and
errored seconds objectives given in Table 1/G.821
}
_
.T&
lw(60p) | lw(120p) .
Local grade (2 ends) {
15% block allowance to each end
(Notes 1, 4 and 5)
}
_
.T&
lw(60p) | lw(120p) .
Medium grade (2 ends) {
15% block allowance to each end
(Notes 2, 4 and 5)
}
_
.T&
lw(60p) | lw(120p) .
High grade {
40% (equivalent to conceptual quality of 0.0016% per km for
25 | 00\ km, but see Note to \(sc\ 3.1)
(Notes 3, 6 and 7)
}
.TE
.LP
\fINote\ 1\fR
\ \(em\ The local grade apportionment is considered to be a block allowance, i.e. an allowance to that part of the connection regardless of length.
.LP
\fINote\ 2\fR
\ \(em\ The medium grade apportionment is considered to be a block
allowance, i.e. an allowance to that part of the connection regardless of
length. The actual length covered by the medium grade part of the connection
will vary considerably from one country to another. Transmission systems in
this classification exhibit a variation in quality falling between the other
classifications.
.LP
\fINote\ 3\fR
\ \(em\ The high grade apportionment is divided on the basis of length
resulting in a conceptual per kilometre allocation which can be used to derive a block allowance for a defined network model (e.g. Hypothetical Reference
Digital Link). For practical planning purposes of links in network models,
link allowances based on the number of 280\ km sections nominally 280\ km (as
specified in Table 2/G.921) can be used in place of the per kilometre
allocation specified in this Recommendation. For longer sections which are not an exact integer multiple of 280\ km, the next highest integer multiple is
used.
.LP
\fINote\ 4\fR
\ \(em\ The local grade and medium grade portions are permitted to cover up the first 1250 km of the circuit from the T\(hyreference point (see
Figure\ 1/G.821) extending into the network. For example, in large countries
this portion of the circuit may only reach the Primary Centre whilst in small countries it may go as far as the Secondary Centre, Tertiary Centre or the
International Switching Centre (see Figure\ 1/G.821).
.LP
\fINote\ 5\fR
\ \(em\ Administrations may allocate the block allowances for the local and medium grade portions of the connection as necessary within the total allowance of 30% for any one end of the connection. This philosophy also applies to the objectives given for local and medium grades in Table\ 3/G.821.
.LP
\fINote\ 6\fR
\ \(em\ Based on the understanding that satellite error performance is
largely independent of distance, a block allowance of 20% of the permitted
degraded minutes and errored second objectives is allocated to a single
satellite\ HRDP employed in the high\(hygrade portion of the\ HRX.
.LP
\fINote\ 7\fR
\ \(em\ If the high\(hygrade portion of a connection includes a satellite
system and the remaining distance included in this category exceeds
12 | 00\ km or if the high\(hygrade portion of a non\(hysatellite connection exceeds
25 | 00\ km, then the objectives of this Recommendation may be exceeded. The
occurrence of such connections is thought to be relatively rare and studies
are continuing in order to investigate this. The concept of satellite
equivalent distance (the length of an equivalent terrestrial path) is useful in this respect and it has been noted that a value in the range 10 | 00 to
13 | 00\ km might be expected.
.LP
\fINote\ 8\fR
\ \(em\ For subscriber premises installation, between the T\(hyreference point and terminal equipment, no specific requirements are given. However careful
attention should be paid to the choice of the subscriber equipment since the
overall performance of the connection depends heavily, not only on the network performance, but also on the quality of the terminal installation.
.nr PS 9
.RT
.ad r
\fBTable 2/G.821 [T2.821], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
3.3
\fIApportionment strategy for\fR
\fIseverely errored seconds\fR
.sp 9p
.RT
.PP
The total allocation of 0.2% severely errored seconds is subdivided into
each circuit classification (i.e.\ local, medium, high grades) in the
following manner:
.RT
.LP
a)
0.1% is divided between the three circuit classifications in the same
proportions as adopted for the other two objectives. This results in the
allocation as shown in Table\ 3/G.821.
.ce
\fBH.T. [T3.821]\fR
.ce
TABLE\ 3/G.821
.ce
\fBAllocation of severely errored seconds\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(120p) .
Circuit classification {
Allocation of severely
errored seconds objectives
}
_
.T&
lw(60p) | lw(120p) .
Local grade {
0.015% block allowance to each end
(Note 5 to Table 2/G.821)
}
_
.T&
lw(60p) | lw(120p) .
Medium grade {
0.015% block allowance to each end
(Note 5 to Table 2/G.821)
}
_
.T&
lw(60p) | lw(120p) .
High grade 0.04% (Notes 1, 2)
.TE
.LP
\fINote\ 1\fR
\ \(em\ For transmission systems covered by the high grade classification
each 2500\ km portion may contribute not more than\ 0.004%.
.LP
\fINote\ 2\fR
\ \(em\ For a satellite HRDP operating in the high grade portion there is a block allowance of\ 0.02% severely errored seconds (see also Note\ 6 to
Table\ 2/G.821).
.nr PS 9
.RT
.ad r
\fBTable 3/G.821 [T3.821], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
b)
The remaining 0.1% is a block allowance to the medium and
high grade classifications to accommodate the occurrence of adverse network
conditions occasionally experienced (intended to mean the worst month of the
year) on transmission systems. Because of the statistical nature of the
occurrence of worst month effects in a world\(hywide connection, it is
considered that the following allowances are consistent with the total
0.1%
figure:
.LP
\(em
0.05% to a 2500 km HRDP for radio relay systems which can be used in
the high grade and the medium grade portion of the connection;
.LP
\(em
0.01% to a satellite HRDP (the CCIR are continuing studies on severely
errored seconds performance for satellites systems and this value may eventually
need to be increased).
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation G.821)
.sp 9p
.RT
.ce 0
.ce 1000
\fBAvailable and unavailable time\fR
.sp 1P
.RT
.ce 0
.PP
A period of unavailable time begins when the bit error ratio (BER) in each
second is worse than 1 | (mu | 0\uD\dlF261\u3\d for a period of ten
consecutive seconds. These ten seconds are considered to be unavailable
time. A new period of available time begins with the first second of a
period of ten consecutive seconds each of which has a BER better than 10\uD\dlF261\u3\d.
.sp 1P
.RT
.PP
Definitions concerning availability can be found in
Recommendation\ E.800\(hyseries.
.bp
.ce 1000
ANNEX\ B
.ce 0
.ce 1000
(to Recommendation G.821)
.sp 9p
.RT
.ce 0
.ce 1000
\fBGuidelines concerning the interpretation of Table 1/G.821\fR
.sp 1P
.RT
.ce 0
.LP
.rs
.sp 49P
.ad r
\fBFigura, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce 1000
ANNEX\ C
.ce 0
.ce 1000
(to Recommendation G.821)
.sp 9p
.RT
.ce 0
.ce 1000
\fBAllocation of objectives to constituent parts\fR
.sp 1P
.RT
.ce 0
.ce
\fBH.T. [T4.821]\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(144p) .
TABLE\ C\(hy1/G.821
.T&
cw(144p) .
{
\fBAllocation of % degraded minute intervals and\fR
\fBerrored seconds objectives\fR
}
.T&
cw(60p) | cw(42p) sw(42p) , ^ | c | c.
{
Circuit classification
(see Figure 1/G.821)
} {
Network performance objectives at 64\ kbit/s
}
% degraded minutes % errored seconds
_
.T&
lw(60p) | cw(42p) | cw(42p) .
Local grade 1.5 1.2
_
.T&
lw(60p) | cw(42p) | cw(42p) .
Medium grade 1.5 1.2
_
.T&
lw(60p) | cw(42p) | cw(42p) .
High grade 4.0 3.2
_
.TE
.nr PS 9
.RT
.ad r
\fBTable C\(hy1/G.821 [T4.821], p.\fR
.sp 1P
.RT
.ad b
.RT
.ce 1000
ANNEX\ D
.ce 0
.ce 1000
(to Recommendation G.821)
.sp 9p
.RT
.ce 0
.ce 1000
\fBPreliminary guidelines for the\fR
\fBassessment of the\fR
.sp 1P
.RT
.ce 0
.ce 1000
\fBperformance of higher bit rate systems\fR
.ce 0
.LP
D.1
\fIInterim guidelines\fR
.sp 1P
.RT
.PP
Recognizing the need for interim guidance, the formulas below are offered
prior to the results of further study. They may be used to provide a
normalized estimate (to the 64\ kbit/s parameters cited in this Recommendation)
of the error performance. It should be noted that the measurement may only
be valid at the bit rate at which the measurement was made; this concern
applies especially for certain types of bursty error distribution. Hence
an assessment of system error performance by means of these formulas does
not assure
\fIcompliance\fR with this Recommendation.
.PP
In order to estimate error performance normalized to 64\ kbit/s in
terms of:
.RT
.LP
\(em
% errored seconds;
.LP
\(em
% degraded minutes; and
.LP
\(em
% severely errored seconds,
.LP
from error performance measurements at primary bit rates and above, the
following provisional formulas are provided.
.sp 1P
.LP
D.1.1
\fIErrored seconds\fR
.sp 9p
.RT
.PP
The percentage errored seconds normalized to 64 kbit/s is given
by:
\v'6p'
.RT
.sp 1P
.ce 1
\fBFormula F1.821 to be inserted here.\fR
.sp 2P
.ad r
.ad b
.RT
.LP
where:
.LP
i)
\fIn\fR | is the number of errors in the \fIi\fR \ut\d\uh\d second
at the measurement bit rate;
.LP
ii)
\fIN\fR | is the higher bit rate divided by 64 kbit/s;
.bp
.LP
iii)
\fIj\fR | is the integer number of one second periods
(excluding unavailable time) which comprises the total measurement
period;
.LP
iv)
the ratio
@ left ( { fIn\fR } over { fIN\fR } right ) @
\fIi\fR
for the \fIi\fR \ut\d\uh\d seconds is
\v'6p'
.sp 1P
.ce 1000
@ { fIn\fR } over { fIN\fR } @
, if 0 < \fIn\fR < \fIN\fR , or
.ce 0
.sp 1P
.ce 1000
1, if \fIn\fR \(>=" \fIN\fR
.ce 0
.sp 1P
.LP
.sp 1
D.1.2
\fIDegraded minutes\fR \fI(see Note\ 1)\fR
.sp 9p
.RT
.PP
The percentage of degraded minutes normalized to 64 kbit/s can be taken
directly from measurements at primary bit rates and above,
i.e.\ \fIX\fR %
degraded minutes at the primary rate or above yields \fIX\fR % degraded
minutes at 64\ kbit/s.
.RT
.sp 1P
.LP
D.1.3
\fISeverly errored seconds\fR \fI(see Note\ 1)\fR
.sp 9p
.RT
.PP
The percentage of severly errored seconds normalized to 64\ kbit/s that
can be assessed from measurements made at primary bit rates and above is
given by:
\v'6p'
.RT
.sp 1P
.ce 1000
\fIY\fR % + \fIZ\fR %
.ce 0
.sp 1P
.LP
.sp 1
.LP
where:
.LP
\fIY\fR percentage severly errored seconds at the measurement bit rate; and
.LP
\fIZ\fR percentage of non severely errored seconds at the
measurement
bit rate containing one or more loss of frame alignment at the measurement
bit rate.
.PP
\fINote\ 1\fR \ \(em\ The calculation of the bit error ratio at the
measurement bit rate (e.g.\ 10\uD\dlF261\u6\d for degraded minutes) will
sometimes
result in non\(hyintegral values of errors over the integration period. For
clarification purposes, the next integer number of errors above the calculated
value is considered to exceed the threshold of the performance objective
(e.g.\ 123\ errors over a minute for a bit rate of 2048\ kbit/s, resulting in a
BER worse than 10\uD\dlF261\u6\d, is considered as a degraded minute).
.PP
\fINote\ 2\fR \ \(em\ In order to assure the proper operation of:
.RT
.LP
\(em
higher bit rate services (e.g.\ TV);
.LP
\(em
64 kbit/s services,
.LP
it is necessary to determine performance requirements for higher bit rate
systems (i.e. above 64\ kbit/s). While it is not clear which of these services
has the most demanding requirements, in both cases it appears to be necessary
to determine performance requirements for the higher bit rate systems either
by using integration period much shorter than one second or by applying
more
stringent limits for severely errored seconds.
.PP
For 64 kbit/s services, the need for shorter integration periods or more
stringent limits arises from the operation of the de\(hymultiplexing
equipment and in particular from the operation of the justification control
and re\(hyframing processes in the presence of error bursts much shorter
than one
second. For example, errors which do not result in severely errored seconds
at the 64\ kbit/s level as a result of loss of frame alignment in higher
\ \(em\ For interfaces within national networks the frequency values
(
\fIf\fR
2 and \fIf\fR
3) shown in parenthesis may be used.
.LP
\fINote\ 3\fR
\ \(em\ UI\ =\ Unit Interval:
.LP
For 64 kbit/s
1UI\ =\ 15.6 \(*ms
.LP
For 2048 kbit/s
1UI\ =\ 488 ns
.LP
For 8448 kbit/s
1UI\ =\ 118 ns
.LP
For 34 | 68 kbit/s
1UI\ =\ 29.1 ns
.LP
For 139 | 64 kbit/s
1UI\ =\ 7.18 ns
.LP
\fINote\ 4\fR
\ \(em\ The value for \fIA\fR
0 (18 \(*ms) represents a relative phase deviation between the incoming signal and the internal timing local signal derived
from the reference clock. This value for \fIA\fR
0 corresponds to an absolute
value of 21\ \(*ms at the input to a node (i.e. equipment input port) and assumes a maximum wander of the transmission link between two nodes of 11\ \(*ms. The
difference of 3\ \(*ms corresponds to the 3\ \(*ms allowed for long\(hyterm phase deviation in the national reference clock [Recommendation G.811, \(sc\ 3c].
.nr PS 9
.RT
.ad r
\fBTableau 2/G.823 [T2.823], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 12
.bp
.sp 1P
.LP
3.1.2
\fIMaximum output jitter in the absence of input jitter\fR
.sp 9p
.RT
.PP
It is necessary to restrict the amount of jitter generated within individual
equipments. Recommendations dealing with specific systems define the maximum
levels of jitter that may be generated in the absence of input jitter.
The actual limits applied depend upon the type of equipment. They should
be met regardless of the information content of the digital signal. In
all cases the limits never exceed the maximum permitted network limit.
The arrangement for
measuring output jitter is illustrated in Figure\ 1/G.823.
.RT
.sp 1P
.LP
3.1.3
\fIJitter and wander transfer characteristics\fR
.sp 9p
.RT
.PP
Jitter transfer characteristics define the ratio of output jitter to input
jitter amplitude versus jitter frequency for a given bit rate. When
jitter is present at the digital input port of digital equipment, in many
cases some portion of the jitter is transmitted to the corresponding digital
output port. Many types of digital equipment inherent attenuate the higher
frequency jitter components present at the input. To control jitter in
cascaded
homogeneous digital equipment, it is important to restrict the value of
jitter gain. The jitter transfer for a particular digital equipment can
be measured
using a digital signal modulated by sinusoidal jitter.
.PP
Figure 4/G.823 indicates the general shape of a typical jitter
transfer characteristics. The appropriate values for the levels \fIx\fR
and \(em\fIy\fR \ dB and the frequencies\ \fIf\fR , \fIf\fR\d5\u, \fIf\fR\d6\uand
\fIf\fR\d7\ucan be obtained from the relevant Recommendation.
.PP
Because the bandwidth of phase smoothing circuits in asynchronous
digital equipment is generally above 10\ Hz, wander on the input signal may
appear virtually unattenuated on the output. However, in certain particular
digital equipments (e.g. nodal clocks) it is necessary that wander be
sufficiently attenuated from input to output. CCITT Recommendations dealing
with synchronous equipment will ultimately define limiting values for
particular wander transfer characteristics.
.RT
.LP
.rs
.sp 12P
.ad r
\fBFigure 4/G.823, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
3.2
\fIDigital sections\fR
.sp 9p
.RT
.PP
To ensure that the maximum network limit (\(sc\ 2) is not exceeded
within a digital network, it is necessary to control the jitter contributed
by transmission systems.
.PP
The jitter limits for digital sections are found in
Recommendation\ G.921.
.RT
.sp 1P
.LP
3.3
\fIDigital muldexes\fR
.sp 9p
.RT
.PP
The jitter limits for digital multiplexers and demultiplexers are found
in the appropriate equipment Recommendations.
.bp
.RT
.sp 2P
.LP
\fB4\fR \fBGuidelines concerning the\fR
\fBmeasurement of jitter\fR
.sp 1P
.RT
.PP
There are two clearly identifiable categories under which jitter
measurement may be classified:
.RT
.LP
\(em
measurements using an undefined traffic signal which may
generally be considered as quasi\(hyrandom (generally applicable
under operational circumstances);
.LP
\(em
measurements using specific test sequencies (generally
applicable during laboratory, factory and commissioning
circumstances).
.sp 1P
.LP
4.1
\fIMeasurements using an undefined traffic signal\fR
.sp 9p
.RT
.PP
Because of the quasi\(hyrandom nature of jitter and its possible
dependency on traffic loading, accurate peak\(hyto\(hypeak measurements in
operational networks need to be made over long periods of time. In practice
it is expected that, with experience of particular systems, it will be
possible to identify abnormalities measured over a shorter measurement
period which would indicate that the maximum permissible limit might be
exceeded over a longer
measurement interval.
.PP
The network limits recommended in \(sc\ 2 are so derived that the
probability of exceeding such levels is very small. The practical observation
of such a magnitude with a high degree of confidence requires an unacceptable
measurement interval. To take account of such an effect it may be necessary
to introduce a smaller, but related, limit which has a greater probability
of
occurrence, facilitating its measurement over a reasonably short measurement
interval. These aspects are the subject of further study.
.RT
.sp 1P
.LP
4.2
\fIMeasurements using a specific test sequence\fR
.sp 9p
.RT
.PP
Given that it is advantageous to assess the jitter performance of digital
line equipment using a specific pseudo\(hyrandom binary sequence (PRBS),
it is necessary to derive limits appropriate to this unique test condition.
Although the use of such deterministic test signals is extremely useful for
factory acceptance tests and commissioning tests, the results need to relate
to an operational situation in which the information content of the signal
is
likely to be more random (e.g.\ a telephony type signal). Based on practical
experience, it is usually possible to relate a traffic\(hybased measurement
to a PRBS\(hybased measurement by the application of an appropriate correction
factor (Annex\ A).
.PP
The use of a PRBS in the measurement of jitter may have shortcomings in
that for the measurement to be valid the PRBS must have adequate spectral
content within the jitter bandwidth of the system being measured. In
circumstances where the spectral content is insufficient, a suitable correction
must be applied if a measured value is to be meaningfully compared with
specified limits. This aspect is the subject of further study (Annex\ A).
.RT
.sp 1P
.LP
4.3
\fITest signal interaction with signal processing devices integral\fR
\fIto transmission systems\fR
.sp 9p
.RT
.PP
The inclusion of additional signal processing devices integral to a transmission
system often influences the observed jitter performance. Studies have shown
that the transmitted signal, particularly if it is pseudo\(hyrandom
or highly structured, interacts with digital scramblers and line code
converters to produce interesting effects which are observed as changes
in the performance of such equipments. All interaction effects result in
a
modification to the statistics of the transmitted signal causing a
consequential change in the pattern\(hysensitive jitter generated within each
repeater. A typical manifestation is that successive measurements on a
transmission system incorporating these devices, using an identical test
signal on each occasion, yield a widely varying range of peak\(hyto\(hypeak
and r.m.s.
jitter amplitudes.
.PP
Studies have shown that the following factors influence the observed jitter
performance:
.RT
.LP
\(em
the feedback connections on both the PRBS test signal
generator and the transmission system's scrambler;
.LP
\(em
the number of stages on the PRBS test signal generator and
the transmission system's scrambler;
.LP
\(em
the presence of a code converter in the transmission
system.
.PP
Consequently, considerations concerning the choice of test signal for equipment
validation purposes should take account of the following
points:
.LP
a)
It is inadvisable to use a PRBS test signal generator with a
cycle length that has common factors with the scrambler
incorporated in the transmission system.
.LP
b)
The equal configuration of the PRBS test signal generator
and the transmission system's scrambler should be avoided if a
random signal is required.
.bp
.sp 2P
.LP
\fB5\fR \fBJitter accumulation in digital networks\fR
.sp 1P
.RT
.PP
The variability of network configurations prevents the
consideration of every possible case. To analyse a particular network
configuration, it is necessary to use the information about the jitter
characteristics of individual equipments in conjunction with appropriate
jitter accumulation models. Annex\ B aims to provide sufficient information
to enable organizations to carry out such evaluations.
\ \(em\ The 2048 kbit/s digital sections may be operating synchronously or
plesiochronously within the same environment.
}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 1/G.921 [T1.921], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
1.1.2
\fISpecial properties\fR
.sp 9p
.RT
.PP
The digital sections based on the 2048 kbit/s hierarchy should be bit sequence
independent.
.RT
.sp 1P
.LP
1.2\fR \fICharacteristics of interfaces\fR
.sp 9p
.RT
.PP
The digital interfaces should comply with
Recommendation\ G.703.
.RT
.sp 1P
.LP
1.3
\fIPerformance standards\fR
.sp 9p
.RT
.PP
The performance requirements (e.g. errors, jitter and availability) are
specified in terms of a Hypothetical Reference Digital Section (HRDS).
Such a model is defined in Recommendation\ G.801.
.RT
.sp 1P
.LP
1.3.1
\fIError performance\fR
.sp 9p
.RT
.PP
Depending on the various applications in the differenct parts of a connection
as specified in Recommendation\ G.821, different section quality
classes have been defined in Table\ 2/G.921.
.RT
.sp 1P
.LP
1.3.2
\fIJitter\fR
.sp 9p
.RT
.PP
To ensure that the maximum network limit of jitter (see \(sc\ 2 of
Recommendation\ G.823) is not exceeded within a digital network it is necessary
to control the jitter contributed by transmission systems.
.RT
.sp 1P
.LP
1.3.2.1
\fIIntroduction\fR
.sp 9p
.RT
.PP
The jitter specifications relate to hypothetical reference digital sections
(HRDS) defined in Table\ 2/G.921.
.PP
The limits given below have been derived on the basis that only a
few digital sections will be connected in cascade and, moreover, no account
has been taken of jitter originating from asynchronous multiplexing equipment.
However, in certain real network configurations some Administrations may
find it necessary to have more sections in cascade along with many asynchronous
digital multiplex. For effective jitter control in these situations it
might be necessary to satisfy more demanding limits and/or to use other
means of jitter minimization.
.PP
All the limits given below for digital sections are to be satisfied
for all sections regardless of length and the number of repeaters.
.PP
It is important to note that the limits must be met regardless of the transmitted
signal. In such circumstances the choice for a test sequence is
left to the discretion of national Administrations. The measurement guidelines
given in \(sc\ 4 of Recommendation\ G.823 should be taken into account.
.RT
.sp 1P
.LP
1.3.2.2
\fILower limit of tolerable input jitter\fR
.sp 9p
.RT
.PP
The requirements given in Figure 2/G.823 and Table 2/G.823 should be met.
.PP
\fINote\fR \ \(em\ It is recognized that for 2048 kbit/s line sections
and under practical conditions of interference the permissible maximum
input jitter may have to be reduced in the frequency range \fIf\fR\d3\uto
\fIf\fR\d4\u(but retaining the existing 20\ dB/decade slope below the frequency
\fIf\fR\d3\uwhich would result in a slightly lower value for frequency\
\fIf\fR\d2\u). Considering that these
sections are used in the lowest levels of the network and that actual
2048\ kbit/s sources have very low output jitter in the high frequency range
(cf.\ Recommendations\ G.732, G.742 and\ Q.551), the resulting performance
will be entirely satisfactory.
.RT
.sp 1P
.LP
1.3.2.3
\fIJitter transfer characteristics\fR
.sp 9p
.RT
.PP
The maximum gain of the jitter transfer function should not exceed the
value of 1\ dB.
.PP
\fINote\ 1\fR \ \(em\ The low frequency limit should be as low as possible
taking into account the limitations of measuring equipment. A value in
the order of
5\ Hz is considered acceptable.
.PP
\fINote\ 2\fR \ \(em\ For line sections at 2048 kbit/s complying with the
alternative national interface option (Note\ 2 to Table\ 2/G.823), a jitter
gain of 3\ dB is permitted.
.bp
.RT
.ce
\fBH.T. [T2.921]\fR
.ce
TABLE\ 2/G.921
.ce
\fBDigital section quality classifications for error
.ce
performance\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(42p) | cw(60p) | cw(30p) | cw(72p) .
{
Section quality
classification
} {
HRDS length (km)
(see Figure 4/G.801) (Note\ 2)
} Allocation (Notes\ 3, 4) {
To be used in circuit classification
(see Figure
1/G.821)
(Notes 5 and 6)
}
_
.T&
cw(42p) | cw(60p) | cw(30p) | cw(72p) .
1 280 0.45%\fR High grade
_
.T&
cw(42p) | cw(60p) | cw(30p) | cw(72p) .
2 280 2% Medium grade
_
.T&
cw(42p) | cw(60p) | cw(30p) | cw(72p) .
3 \ 50 2% Medium grade
_
.T&
cw(42p) | cw(60p) | cw(30p) | cw(72p) .
4 \ 50 5% Medium grade
.TE
.LP
\fINote\ 1\fR
\ \(em\ There is no intention to confine any quality classification to any specific bit rate. The possibility of introducing additional options (for
instance concerning length) requires further study.
.LP
\fINote\ 2\fR
\ \(em\ The indicated values of length are those identified in
Recommendation\ G.801. They should be understood to correspond to maximum
lengths of real digital sections. If a real digital section is shorter, there will be no reduction of the bit error allocation (i.e. percentage value in the third column). This takes into account that:
.LP
\fR
\(em
in many line systems (especially on copper wire pairs) most bit errors
occur at the ends of the system;
.LP
\(em
in the interest of econmy, short\(hyhaul systems may be designed with
greater per\(hykilometre error ratio than long\(hyhaul systems.
.LP
If a real digital section is longer (e.g. 450\ km), its overall allocation
should correspond to that of an integer number of HRDSs (of the same quality
classification) the combined lengths of which are at least as long as the real section length (e.g. 2 \(mu 280\ km).
.LP
\fINote\ 3\fR
\ \(em\ The values in this column are percentages of the overall
degradation (at 64\ kbit/s) specified in Recommendation\ G.821; i.e. of the 8%
errored seconds, of the 10% degraded minutes and of the 0.1% severely errored seconds which are allocated according to the same rules as the two other
parameters.
.LP
\fINote\ 4\fR
\ \(em\ To obtain 64\ kbit/s error performance data from error measurement
at primary bit rates and above, the method described in
Recommendation\ G.821, Annex\ D, should be used.
.LP
\fINote\ 5\fR
\ \(em\ May also be used within a lower grade portion of the connection
as defined per Figure\ 1/G.821.
.LP
\fINote\ 6\fR
\ \(em\ To take account of adverse propagation conditions on radio systems as detailed in Recommendation\ G.821, an additional percentage of 0.05% of
severely errored seconds has been allocated to a 2500\ km radio\(hyrelay HRDP for systems operating in the high and medium grade quality part of the HRX. This
corresponds for a 280\ km section to a value of 0.0055% to be added to section quality classification 1 and 2 allocation when applied to severely errored
seconds.
.LP
This would result in an additional allowance of 0.025% of severely errored
seconds available for the medium grade part of the connection if it is realized entirely with class 1 radio sections. Where the medium grade portion of the
network is realized with a mixture of different classifications, part of this additional allowance may be allocated to classes 3 and 4 at the discretion of Administrations.
.LP
To be consistent with the statistical assumptions made in G.821 \(sc\ 3.3 | )
regarding the munber of radio sections in the HRX, and the occurrence of worst month effects it may be necessary to take into account the probability of worst month effects occurring simultaneously for all radio sections in a connection. A statistical model to be used for network planning and performance evaluation to assess the consistency of a given connection to the overall objective of
G.821 is under study.
.nr PS 9
.RT
.ad r
\fBTableau 2/G.921 [T2.921], p.24\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
1.3.2.4
\fIOutput jitter in the absence of input jitter\fR
.sp 9p
.RT
.PP
The maximum peak\(hyto\(hypeak jitter in the absence of input jitter, for
any valid signal condition, should not exceed the limit given in
Table\ 3/G.921.
.RT
.ce
\fBH.T. [T3.921]\fR
.ce
TABLE\ 3/G.921
.ce
\fBThe maximum output jitter in the absence of input jitter for a
.ce
digital section\fR
.ce
(Measurements are made in accordance with the method shown in
.ce
Figure 1/G.823)
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(36p) | cw(30p) | cw(30p) sw(36p) | cw(30p) sw(36p) sw(30p) , ^ | ^ | c | c | c s s
The digital interface of the DOV system should be as specified in Recommendation\
G.703, \(sc\ 6.
.RT
.sp 1P
.LP
3.1.2
\fIDisturbances of the analogue signal by the DOV signal\fR
.sp 9p
.RT
.PP
The increase to the total distributed noise due to the DOV signal measured
in any 4\ kHz bandwidth should be less than 750\ pW0p for a reference
length of 2500\ km (less than 0.3\ pW0p/km).
.PP
\fINote\fR \ \(em\ The total distributed noise of the line when analogue
and DOV signals are present should be below 7500\ pW0p for a reference
length of 2500\ km (less than 3\ pW0p/km).
.PP
The level of single tone interference should be less than
\(em70\ dBm0.
.RT
.sp 1P
.LP
3.1.3
\fIDOV system performance\fR
.sp 9p
.RT
.PP
The performance relating to error rate, jitter and availability
should be in accordance with Recommendation\ G.921.
.bp
.RT
.sp 1P
.LP
3.2
\fICharacteristics of the FDM line systems used to carry the DOV\fR
\fIsignal\fR
.sp 9p
.RT
.PP
To allow the through\(hyconnection of DOV signals on FDM line systems,
spurious analogue signals within the frequency band of the DOV signal should
be suppressed before the coupling point up to a power level of \(em60\
dBm0 within
4\ kHz bandwidth.
.RT
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation G.941)
.sp 9p
.RT
.ce 0
.ce 1000
\fBExamples of hierarchical DIV systems\fR
.sp 1P
.RT
.ce 0
.ce
\fBH.T. [T1.941]\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(42p) | cw(54p) | cw(54p) | cw(54p) .
Administration Digital interface Analogue interface DIV system performance
_
.T&
cw(42p) | cw(54p) | cw(54p) | cw(54p) .
NTT {
1544 kbit/s
Rec. G.703, \(sc\ 2
} Mastergroup (812\(hy2044 kHz) Rec. G.911
_
.T&
cw(42p) | cw(54p) | cw(54p) | cw(54p) .
FRG {
2048 kbit/s
Rec. G.703, \(sc\ 6
} Mastergroup (812\(hy2044 kHz) Rec. G.921
_
.T&
cw(42p) | cw(54p) | cw(54p) | cw(54p) .
NTT {
6312 kbit/s
Rec. G.703, \(sc\ 3
} Mastergroup (812\(hy2044 kHz) Rec. G.912
_
.T&
cw(42p) | cw(54p) | cw(54p) | cw(54p) .
FRG {
8448 kbit/s
Rec. G.703, \(sc\ 7
} {
Supermastergroup
(8516\(hy12 | 88 kHz)
} Rec. G.921
_
.T&
cw(42p) | cw(54p) | cw(54p) | cw(54p) .
Italy {
8448 kbit/s
Rec. G.703, \(sc\ 7
} {
15 supergroup assem.
(312\(hy4028 kHz)
} Rec. G.921
_
.TE
.nr PS 9
.RT
.ad r
\fBTable [T1.941], p.\fR
.sp 1P
.RT
.ad b
.RT
.ce 1000
ANNEX\ B
.ce 0
.ce 1000
(to Recommendation G.941)
.sp 9p
.RT
.ce 0
.ce 1000
\fBExamples of systems other than those recommended\fR
.sp 1P
.RT
.ce 0
.ce 1000
\fBin Recommendation G.941\fR
.ce 0
.ce 1000
(see Note 1)
.ce 0
.ce
\fBH.T. [T2.941]\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(42p) | cw(42p) | cw(54p) | cw(54p) .
Administration Bit rate (kbit/s) Analogue interface {
Design bit error ratio for regeneration section
}
_
.T&
cw(42p) | cw(42p) | cw(54p) | cw(54p) .
France (see Note 2) \ 704 Supergroup (312\(hy552 kHz) 10\uD\dlF261\u8\d
_
.T&
cw(42p) | cw(42p) | cw(54p) | cw(54p) .
Netherlands 2048 2 supergroups 10\uD\dlF261\u8\d
.TE
.LP
\fINote\ 1\fR
\ \(em\ Modems for the transmission of digital signals at 48\(hy72\ kbit/s or
twice these bit rates are covered in Recommendations V.36 and V.37.
.LP
\fINote\ 2\fR
\ \(em\ The digital interface of this DIV equipment is at 2048\ kbit/s
according to Recommendation G.703 \(sc\ 6, and with a frame structure according to Recommendation G.704 \(sc\ 3.3.1. Only 11 (including TS0) among the 32 time slots are effectively used: the useful bit rate is then equal to 10\ times 64\ kbit/s. The other characteristics of the DIV system satisfy to \(sc\ 2 of this
Recommendation.
}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable [T2.941], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
\fBReferences\fR
.sp 1P
.RT
.LP
[1]
CCITT Recommendation \fIMake\(hyup of a carrier link\fR , Vol.\ III,
Rec.\ G.211.
.LP
[2]
CCITT Recommendation \fI12\(hyMHz systems on standardized 2.6/9.5\(hymm\fR
\fIcoaxial cable pairs\fR , Vol.\ III, Rec.\ G.332.
.LP
[3]
CCITT Recommendation \fI18\(hyMHz systems on standardized 2.6/9.5\(hymm\fR
\fIcoaxial pairs\fR , Vol.\ III, Rec.\ G.334.
.LP
[4]
CCITT Recommendation \fI6\(hyMHz systems on standardized 1.2/4.4\(hymm\fR
\fIcoaxial cable pairs\fR , Vol.\ III, Rec.\ G.344.
.LP
[5]
CCITT Recommendation \fI12\(hyMHz systems on standardized 1.2/4.4\(hymm\fR
\fIcoaxial cable pairs\fR , Vol.\ III, Rec.\ G.345.
.LP
[6]
CCITT Recommendation \fI18\(hyMHz systems on standardized 1.2/4.4\(hymm\fR
\fIcoaxial cable pairs\fR , Vol.\ III, Rec.\ G.346.
.LP
.rs
.sp 40P
.LP
\fBMONTAGE:\ \fR REC.\ G.950 A LA FIN DE CETTE PAGE
