Internet Standards/CD2.mdf/ccitt/1992/g/g824.asc

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9.    Recommendation G.824



THE CONTROL OF JITTER AND WANDER WITHIN DIGITAL
NETWORKS

WHICH ARE BASED ON THE 1544 kbit/s HIERARCHY





    The CCITT,



considering



    (a)    that timing jitter and alignment jitter can arise in digital networks;



    (b)    that, if proper control is not exercised, then under certain circum-
stances jitter can accumulate to such an extent that the following impair-
ments can arise;



    i)    an increase in the probability of introducing errors into digital sig-
nals at points of signal regeneration as a result of timing signals being
displaced from their optimum position in time;



        ii)    the introduction of uncontrolled slips into digital signals resulting
from either data overflow or depletion in digital equipment
incorporating buffer stores and phase comparators, such as jit-
ter reducers and certain digital multiplex equipment;



       iii)    a degradation of digitally encoded analogue information as a
result of phase modulation of the reconstructed samples in the
digital-to-analogue conversion device at the end of the con-
nection, which may have significant impact on digitally
encoded video signals;



    (c)    that unlike some other network impairments, jitter can be reduced in
magnitude by the use of jitter reducers, and in complex networks, it may
be necessary to employ such devices;



    (d)    that wander can arise due to variations in transmission characteris-
tics of the media and equipment, including disruptions in synchronization
reference distribution;



    (e)    that it is necessary to accommodate wander at the input ports of dig-
ital equipment if controlled or uncontrolled slips are to be minimized;



recommends



    that the following guidelines and limits be applied in the planning of net-
works and in the design of equipment.



1.    Basic jitter and wander control philosophy



    The goal of the strategy outlined in this Recommendation is to minimize
impairments due to jitter and wander in digital networks. The strategy
provides the following elements:



    a)    specification of network limits not to be exceeded at any hierarchi-
cal interface;



    b)    a consistent framework for the specification of individual digital
equipment;



    c)    information and guidelines to predict and analyze jitter and wander
accumulation in any network configuration, facilitate satisfactory
control of the impairments due to this accumulation, and to provide
insight into the jitter and wander performance of individual digital
equipments;



    d)    measurement methodology to facilitate accurate and repeatable jitter
and wander measurements.



    Suggestions for measurement of parameters recommended below can be
found in Supplement No.3.8 of the O-Series (for jitter) and Supplement
No.35 (for wander).



2.    Network limits for maximum output at hierarchical interfaces and wan-
der at synchronous network nodes



2.1    Network limits for jitter



    Specification of maximum permissible values of output jitter at hierarchi-
cal network interfaces is necessary to enable the interconnection of digi-
tal network components (line section, multiplex equipment, exchanges)
to form a digital path or connection. These limits should be met regard-
less of the number of interconnected network components preceding the
interface. The limits are intended to be compatible with the minimum jit-
ter tolerance of all equipment operating at the same hierarchical level.



    The limits given in Table 1/G.824 represent maximum permissible output
jitter limits at hierarchical interfaces of a digital network. In circum-
stances where the maximum permissible jitter amplitude occurs at an
interface between two countries, it is left to the discretion of national
administrations to take the appropriate remedial action. This situation is
unlikely to occur very often.





TABLE 1/G.824



+ûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûûû
ûûûûûûûûûûûûûûûûûû+

_                                                                           _

_           Maximum permissible output jitter at hierarchical interfaces    _

+ûûûûûûûûûûûû+ûûûûûûûûûûûûûûûûûûû+ûûûûûûûûûûûûûûûûûûûûûûûû
ûûûûûûûûûûûûûûûûûû+

_            _ Network Limit     _                                          _

_            _                   _  Bandpass filter having a lower cut-off  _

_            _ (UI peak-to-peak) _  frequency f1 or f3 and a minimum upper
_

_            _                   _  cut-off frequency f4                    _

+ûûûûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûûûû+ûûûûûûûûûûû
ûû+ûûûûûûûûûûûûûûûû+

_            _         _         _           _             _                _

_Digital rate_  B1     _  B2     _    f1     _      f3     _      f4        _

_  (kbit/s)  _         _         _   (Hz)    _    (kHz)    _     (kHz)      _

+ûûûûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûûûû+ûûûûûûûûûûû
ûû+ûûûûûûûûûûûûûûûû+

_            _         _         _           _             _                _

_  1544      _  5.0    _ 0.1a)   _    10     _      8      _      40        _

+ûûûûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûûûû+ûûûûûûûûûûû
ûû+ûûûûûûûûûûûûûûûû+

_            _         _         _           _             _                _

_  6312      _  3.0    _ 0.1a)   _    10     _      3      _      60        _

+ûûûûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûûûû+ûûûûûûûûûûû
ûû+ûûûûûûûûûûûûûûûû+

_            _         _         _           _             _                _

_ 32064      _  2.0    _ 0.1a)   _    10     _      8      _     400        _

+ûûûûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûûûû+ûûûûûûûûûûû
ûû+ûûûûûûûûûûûûûûûû+

_            _         _         _           _             _                _

_ 44736      _  5.0    _ 0.1     _    10     _     30      _     400        _

+ûûûûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûûûû+ûûûûûûûûûûû
ûû+ûûûûûûûûûûûûûûûû+

_            _         _         _           _             _                _

_ 97728      _  1.0    _ 0.05    _    10     _    240      _    1000        _

+ûûûûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûû+ûûûûûûûûûûû+ûûûûûûûûûûû
ûû+ûûûûûûûûûûûûûûûû+





UI    Unit interval



a)    This value requires further study.



    For systems in which the output signal is controlled by an autonomous
clock (e.g., quartz oscillator) more stringent output jitter values may be
defined in the relevant equipment specifications (e.g., for the muldex in
Recommendation G.743, output jitter should not exceed 0.01 UI r.m.s).



    The arrangements for measuring output jitter at a digital interface are
illustrated in Figure 1/G.824. The specific jitter limits and values of filter
cut-off frequencies are given in Table 1/G.824.



FIGURE 1/G.824



Measurement arrangements for output jitter from

a hierarchical interface or an equipment output port





2.2    Network limits for wander



    Network output wander specifications at synchronous network nodes are
necessary to ensure satisfactory network performance (e.g. slips, error
bursts). For network nodes the following limits are specified, based on
the assumption of a non-ideal synchronizing signal (containing jitter,
wander, frequency departure, and other impairments) on the line deliver-
ing timing information. The maximum time interval error (MTIE) (see
Recommendation G.811) over a period of S seconds shall not exceed the
following:



    (10-2S + 10,000) ns; applicable to values of S greater than 104.



Note - The resultant overall specification is illustrated in Figure 2/G.824.



3.    Framework for the specification of individual digital equipments



3.1    Basic specification philosophy



    Jitter and wander control inherently depends on both network and equip-
ment design. Network considerations are discussed in º2. The principal
parameters of importance when considering the jitter and wander perfor-
mance of digital equipment are:



    i)    the amount of jitter and wander that can be tolerated at the input;



    ii)    the proportion of this input jitter and wander which filters through to
the output; and



    iii)    the amount of jitter and wander generated by the equipment.



    The intention of this section is to provide a foundation for the develop-
ment of equipment requirements which will ensure that the various net-
work equipments are compatible from the standpoint of jitter and wander
performance.



FIGURE 2/G.824



Permissible maximum time interval error (MTIE)

versus observation period S for the output of a network node



    Further study is required to quantify the difference in limits for transit
and local nodes. In addition, wander accumulation in networks is closely
tied to the stability specifications contained in RecommendationsG.811,
G.81x, Q.511.



3.1.1    Jitter and wander tolerance of input ports



    In order to ensure that any equipment will operate satisfactorily when
connected to a hierarchical interface within the network, it is necessary
that the equipment input ports be capable of accommodating levels of
network output jitter up to the maximum network limits specified in
Table1/G.824.  Specification of input jitter tolerance in terms of a single
Recommendation applicable to all categories of digital equipment
ensures that a certain minimum jitter tolerance is satisfied by all network
elements.  Most specifications of equipment input tolerance are in terms
of the amplitude of sinusoidal jitter that can be applied at various fre-
quencies without causing a designated degradation of error performance.
The simplicity of this form of specification has great appeal, since it is
easily verified with conventional test equipment. However, it is important
to recognize that the test condition is not, in itself, intended to be repre-
sentative of the type of jitter to be found in practice in a network. For
some equipment, therefore, it may be necessary to specify supplemental
jitter tolerance tests, and reference to the individual equipment Recom-
mendation should always be made.



    As a minimum guideline for equipment tolerance, it is recommended that
all digital input ports of equipments be able to tolerate the sinusoidal jit-
ter and wander defined by Figure 3/G.824 and Table 2/G.824. The limits
are to be met in an operating environment.



    In deriving the specifications contained in Table 2/G.824 for frequencies
above f3, the effects of the amount of alignment jitter of the equipment
clock decision circuit are considered to be predominant. Measurements
carried out to verify compliance with these specifications may provide
environment dependent results, hence allowing some ambiguity in their
interpretation. Account should be taken of the requirement at the design
stage of the equipment; Supplement No.3.8 (O-Series) provides guid-
ance regarding environment independent measurements.



    In deriving these specifications, the wander effects are considered to be
predominant at frequencies below f1, and many transmission equip-
ments, such as digital line systems and asynchronous muldexes using jus-
tification techniques, are effectively transparent to these very low
frequency changes in phase. However, such phase variation does need to
be accommodated at the input of certain equipments (e.g. digital
exchanges and synchronous muldexes). The requirement contained in
Table 2/G.824 for frequencies below f1 is not amenable to simple practi-
cal evaluation, but account should be taken of the requirement at the
design stage of the equipment.



    Equipment wander tolerance must be compatible with network output
wander limits specified in Figure2/G.824. Insufficient wander tolerance
at synchronous equipment input ports may result in controlled or uncon-
trolled slips, depending on the specific slip control strategy employed.





TABLE 2/G.824



Jitter and wander tolerance of input ports

(Provisional values)  (Note 1)



+ûûûûûûûû+ûûûûûûûûûûûûûûûûûûûûûûûû+ûûûûûûûûûûûûûûûûûûûûûûû
ûûûûûûûûû+ûûûûûûûûûû+

_  Bit   _    Jitter amplitude    _                                _   Test   _

_ Rates  _     (peak-to-peak)     _           Frequency            _  Signal  _

_(kbit/s)+ûûûûûûûû+ûûûûûû+ûûûûûûûû+ûûûûûûûû+ûûûû+ûûûûûû+ûûû
ûû+ûûûûû+ûûûûûûûûûû_

_        _  A0    _  A1  _   A2   _  f0    _ f1 _ f2   _ f3  _ f4  _          _

_        _ (╡s)   _ (UI) _  (UI)  _ (Hz)   _(Hz)_(Hz)  _(kHz)_(kHz)_          _

+ûûûûûûûû+ûûûûûûûû+ûûûûûû+ûûûûûûûû+ûûûûûûûû+ûûûû+ûûûûûû+û
ûûûû+ûûûûû+ûûûûûûûûûû+

_        _        _      _        _        _    _      _     _     _          _

_   1544 _  18    _  5.0 _   0.1  _1.2x10-5_ 10 _  120 _   6 _   40_ 220-1    _

_        _(Note 2)_      _(Note 2)_        _    _      _     _     _(Note 3)  _

_   6312 _  18    _  5.0 _   0.1  _1.2x10-5_ 10 _   50 _ 2.5 _   60_ 220-1    _

_        _(Note 2)_      _        _        _    _      _     _     _(Note 2)  _

_  32064 _  18    _  2.0 _   0.1  _1.2x10-5_ 10 _  400 _   8 _  400_ 220-1
_

_        _(Note 2)_      _        _        _    _      _     _     _(Note 2)  _

_  44736 _  18    _  5.0 _   0.1  _1.2x10-5_ 10 _  600 _  30 _  400_ 220-1
_

_        _(Note 2)_      _(Note 2)_        _    _      _     _     _(Note 2)  _

_  97728 _  18    _  2.0 _   0.1  _1.2x10-5_ 10 _12 000_ 240 _1 000_ 223-
1    _

_        _(Note 2)_      _        _        _    _      _     _     _(Note 2)  _

+ûûûûûûûû+ûûûûûûûû+ûûûûûû+ûûûûûûûû+ûûûûûûûû+ûûûû+ûûûûûû+û
ûûûû+ûûûûû+ûûûûûûûûûû+



                                                                               Note 1 - Reference to
individual equipment specifications should always be made to check if
supplementary input jitter tolerance requirements are necessary.



Note 2 - This value requires further study.



Note 3 - It is necessary to suppress long zero strings in the test sequence
in networks not supporting 64kbit/s transparency.



Note 4 - The value A0 (18 us) represents a relative phase deviation
between the incoming signal and the internal local timing signal derived
from the reference clock.













FIGURE 3/G.824



Mask of peak-to-peak jitter and wander which must be accommodated

at the input of a node in a digital network

(Measurement method - refer to Supplement No. 3.8 (O-Series)

and Supplement No. 35)



3.1.2    Jitter and wander transfer characteristics



    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 inherently attenuate the higher fre-
quency jitter components present at the input. CCITT Recommendations
dealing with particular equipment will ultimately define limiting values
for its particular jitter transfer characteristics. To control jitter in cascaded
homogeneous digital equipment situations, it is important to restrict the
value of jitter gain.



    Because the bandwidth of phase smoothing circuits in asynchronous dig-
ital equipment is generally above 10Hz, wander on the input signal may
appear virtually unattenuated on the output. However, in certain particu-
lar 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 val-
ues for particular wander transfer characteristics.



3.1.3    Intrinsic jitter and wander generation



    Intrinsic jitter and wander generation is defined as output jitter and wan-
der in the absence of input jitter and wander. It is necessary to restrict the
amount of intrinsic jitter and wander generated within individual digital
equipments to provide control over network jitter and wander accumula-
tion from cascaded network elements. Limits for output jitter and wander
for individual digital equipments are defined in the specific CCITT
equipment Recommendations. The actual limits applied depend upon the
type of equipment.



3.2    Digital line sections



    To ensure that the maximum network limit (º2.1) is not exceeded within
a digital network, it is necessary to control the jitter and wander contrib-
uted by transmission systems.



    The jitter specifications for digital line sections will ultimately be found
in Recommendations G.911 to G.915.



3.3    Digital muldexes



    To ensure that the maximum network limit (º2.1) is not exceeded within
a digital network, it is necessary to control the jitter and wander contrib-
uted by transmission systems.



    The jitter specifications for digital muldexes using positive justification
are found in Recommendations G.743 and G.752.



3.4    Digital exchanges



    To ensure that the maximum network limit (to be specified in º2.2) is not
exceeded within a digital network, it is necessary to control jitter and
wander transfer and generation, as appropriate, for digital exchanges.



    Output wander specifications for primary reference clocks are addressed
in Recommendation G.811. The jitter and wander specifications for digi-
tal transit exchanges and digital local exchanges are found in
RecommendationQ.511.



4.    Jitter and wander accumulation in digital networks



    The variability of network configurations presents a multitude of connec-
tion possibilities. To analyze a particular network configuration, it is nec-
essary to use the information about the jitter characteristics of individual
equipments in conjunction with appropriate jitter accumulation models.
Supplement No.36 provides information to aid organizations in carrying
out such evaluations.





     The term digital section is used in these Recommendations as a gen-
eral term to include digital line section and digital radio section. This
term is defined in Recommendation G.701 (see also Figure 1/G.701 and
Figure 2/G.960).  Digital sections are defined as component parts of digi-
tal links operating at particular bit rates and may be regarded as "black
boxes". For digital sections used in digital hierarchy (network) applica-
tions the inputs and outputs are recommended in the form of"equipment
interfaces" (i.e. in RecommendationG.703 for hierarchical bit rates or in
the Recommendation G.931 for non-hierarchical bit rates). For digital
sections used for ISDN customer access the "section boundaries" are at
the T reference point and the appropriate V reference point. User-net-
work interfaces which may be used at the T reference point are recom-
mended in the I.400-Series of Recommendations and the exchange
interfaces which may be used at the V reference points are recommended
in the           Q.500-Series of Recommendations. Digital section Recom-
mendations contain the common network-related requirements applica-
ble to digital radio, metallic and optical transmission systems. The
performance requirements relate to network performance objectives.