Internet Standards/CD2.mdf/ccitt/1992/g/g823a.asc

Text File | 1993-08-15 | 6KB | 191 lines

8.    Amendments to Recommendation G.823 (Fascicle III.3 of the Red Book)



8.1    º2 of G.823 should be amended as shown below:



"2.    Network limits for the maximum output jitter at any hierarchical interface



2.1    Network limits for jitter



    The limits given in Table 1/G.823 represent the maximum permissible levels of jitter
at hierarchical interfaces within a digital network. The limits should be met for all
operating conditions and regardless of the amount of equipment preceding the inter-
face. These network limits are compatible with the minimum tolerance to jitter that
all equipment input ports are required to provide."



8.2    At the end of º2, new text should be added as shown below:































FIGURE 1/G.823



Measurement arrangements for output jitter

from an hierarchical interface or an equipment output port





    It is assumed that, within a synchronized network, digital equipment provided at
nodes will accommodate permitted phase deviations on the

incoming signal, together with jitter and wander from the transmission plant, i.e.
under normal synchronized conditions, slip will not occur. However, it should be
recognized that, as a result of some performance degradations, failure conditions,
maintenance actions and other events, the relative time interval error (TIE) between
the incoming signal, and the internal timing signal of the terminating equipment may
exceed the wander and jitter tolerance of the equipment which will result in a con-
trolled slip.



    At nodes terminating links interconnecting independently synchronized networks (or
where plesiochronous operation is used in national networks), the relative TIE
between the incoming signal and the internal timing signal of the terminating equip-
ment may eventually exceed the wander and jitter tolerance of the equipment in
which case slip will occur. The maximum permissible long-term mean controlled
slip rate resulting from this mechanism is given by RecommendationG.811, i.e. one
slip in 70 days.



    At nodes terminating links interconnecting independently synchronized networks (or
where plesiochronous operation is used in national networks), the relative TIE
between the incoming signal and the internal timing signal of the terminating equip-
ment may eventually exceed the wander and jitter tolerance of the equipment in
which case slip will occur. The maximum permissible long-term mean controlled
slip rate resulting from this mechanism is given by Recommendation G.811, i.e. one
slip in 70 days.



    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 RecommendationG.735, the maxi-
mum peak-to-peak output jitter is 0.05 UI).



8.3    Modifications to º2, Recommendation G.823 (Fascicle III.3 of the Red Book)



2.2    Network limits for wander



    A maximum network limit for wander at all hierarchical interfaces has not been
defined. Actual magnitudes of wander, being largely dependent on the fundamental
propagation characteristics of transmission media and the ageing of clock circuitry
(see Recommendation G.811, º 3), can be predicted. Studies have shown that, pro-
vided input ports can tolerate wander in accordance with the input tolerance require-
ments of º 3.1.1, then slips introduced as a result of exceeding the input tolerance
will be rare. For interfaces to network nodes the following limits apply:



    The MTIE (see Recommendation G.811) over a period of S seconds shall not exceed
the following:



    1)    S < 104; this region requires further study;



    2)    (10-2 S + 10000) ns: applicable to values of S greater than 104



Note - The resultant overall specification is illustrated in Figure2/G.823.





FIGURE 2/G.823



Permissible maximum time interval error (MTIE)

vs. observation period S for the ouput of a network node



8.4    Paragraph 3.1.3 should be amended as shown below:



3.1.3    Jitter and wander transfer characteristics



    Jitter transfer characteristics define the ration 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 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.



    Figure 3/G.823 indicates the general shape of a typical jitter transfer characteristic.
The appropriate values for the levels x and -ydB and the frequencies f, f5, f6 and f7
can be obtained from the relevant Recommendation.



    Because the bandwidth of phase smoothing circuits in asynchronous digital equip-
ment is generally above 10Hz, 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 out-
put. CCITT Recommendations dealing with synchronous equipment will ultimately-
define limiting values for particular wander transfer characteristics.