ANNEX 2

                (to draft Recommendation G.136)


SOURCE: THE USSR TELECOMMUNICATION ADMINISTRATION

TITLE:  INTERRELATION BETWEEN ERRORS IN A DIGITAL CIRCUIT AND IMPULSE NOISE IN
        VOICE-FREQUENCY CHANNELS OF THE PCM SYSTEM

                                   ŽŽŽŽŽŽŽŽŽŽ


1.     Introduction

        Voice-frequency channels of PCM as well as FDM systems should be fit for
transmitting  various types of signals. It is well known that  the  transmission
quality  of discrete signals in voice-frequency channels is affected by  impulse
noise.  At present, Recommendation G.712 has no requirements to voice- frequency
PCM-channels regarding impulse noise. However, under real-life conditions  in  a
v.-f.  PCM channel impulse noise contributes to the error-rate of digital links.
The present contribution gives the investigation results of impulse noise in v.-
f. PCM-channels.

2.     Influence of digital circuit errors on impulse noise in an idle v.-f. _
       PCM-channel

        Evaluation of error influence on digital links on the value of  impulse
noise  in  v.-f.  channels was conducted experimentally on a channel  equipment
(satisfying  Recommendation G.712) of a PCM transmission system (2048  kbit/s).
With  the  help of an error simulator, errors had been introduced into  one  or
several  bits  corresponding to a chosen idle v.-f. channel of a  digital  link
(Figure 1). In the v.-f. channel impulse noise could be observed with the  help
of  an  oscillograph. The shape of the pulse response in the v.-f.  channel  is
presented in Figure 2.

        The  parameters of pulse response are given in Table 1 (the values  are
chosen  for the point of the relative zero level at a resistance of  600  ohm).
These data allow us to formulate the following conclusions:

       -     the  pulse amplitude of the response depends on the bit number
           which  contains  the error; the errors in the  more  significant
           bits cause a greater amplitude of the response;

       -     with single errors the maximum value of the pulse peak A1  (in
           case of an error in the second bit) is -22.1 dBmO;

       -     with  burst-building and with an increase in the number of errored
           bits  in the code word of the prime digital path (2048 kbit/s)   the
           response  amplitude values A1, A2, A3, ... grow, but their duration,
           as  determined  by  the  response of  the  channel's  low  frequency
           receiving filter, remains unchanged. This applies to the cases where
           in  a prime digital path, the error bursts affect the digital stream
           for  not more than one discretization period, i.e. the number of the
           errors  in  a burst does not exceed 256. With errors in  code  words
           occurring every 125 s the superposition of responses takes place  as
           a  result  of the receiving filter reaction on the error  pulses  in
           each following
            discretization period.


                            TABLE 1


  ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ
                                                                  Duration     of
Amplitude of pulse response         pulse response
     Errored bits      ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ
     in a frame of        A1         A2         A3        t1       t2     t3
   primary multiplex   ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ
                         dBmO       dBmO       dBmO       s       s     s
  ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ


   2                    -22.1      -28.2      -33.8       320      160    130
   3                    -34.1      -40.2      -45.8       320      160    130
   2 and 3              -10.1      -16.2      -21.8       320      160    130
   2 and 3 and 4         -4.1      -10.2      -15.8       320      160    130
   from 2 to 8,
   2 discretization
   periods               +4.3       -6.7      -14.8       440      180    100
   from 2 to 8,
   3 discretization
   periods               +4.3       -4.9      -14.8       600      200    100
   from 2 to 8,
   4 discretization
   periods               +4.3       -4.7      -14.8       680      180    120
   from 2 to 8,
   5 discretization
   periods               +4.3       -6.7      -14.8       840      200    120
   from 2 to 8,
   6 discretization
   periods               +3.8       -4.3      -14.8       930      200    100
   from 2 to 8,
   7 discretization
    periods               +5.25       -8.7      -14.8      1100       180     140
ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ

        Thus,  when  errors  on a 2048 kbit/s digital path grow  into  bursts  of
2  errors  and more there is a certain probability that the value of the  impulse
noise  in  a  PCM v.-f. channel exceeds -21 dBmO given in Recommendation  M.1020,
 2.6.

        With error bursts of 256 and more bits, the above-mentioned impulse noise
will always be present.

        The quantitative relationship between the number of bursts, the number of
errors  in  them within a definite time interval and the number of impulse  noise
interferences and the BER in a v.-f. channel is under study at present.