Question 25/XII - Transmission impairments in the evolving mixed
analogue/digital and ISDN networks
(continuation of Question 25/XII studied in 1985-1988; revised wording)

Considering

(a) that the telephone network will be a mixture of analogue and digital
circuits and exchanges for a significantly long period before it becomes
wholly digital;

(b) that ISDN and mixed ISDN-PSTN connections are expected to become
available in the next few years;

(c) that increasingly sophisticated digital processes (codecs, DCME, and
packet voice, etc.) are expected to be used in the mixed
analogue/digital, ISDN and ISDN-PSTN connections;

(d) that these processes will introduce impairments (e.g., quantizing
distortion, speech clipping and delay) that will impact on the quality
of the services carried on these connections;

(e) that an increase can be expected in the number of sophistication of
non-speech services over the switched telephone network in advance of
the introduction of extensive integrated services digital network, and
that such non-speech services could be affected by the increased
impairments,

the CCITT has decided to study:

1. What changes are necessary to the hypothetical reference connections
to model the changes and effects foreseen in the consideranda?

2. What Recommendations can be made to control the effect these changes
could have on telephone services including non-speech services carried
by the switched telephone network?

Specific points for particular study are:

a) the effect of introducing digital processing techniques for telephony
other than those described in Recommendations G.711, G.721, G.722 and
G.72Z. For example:

-16 kbit/s codecs;

-non waveform codecs;

- DCME (Digital Circuit Multiplication Equipment);

- packetized voice systems.

3. What impairments are introduced by the new digital processes and what
can be recommended concerning allowable level of these impairments on
connections conveying speech, voice band data and other non-speech
signals?

b) the use of circuits for non-telephony applications including
alternative use of an established connection for various non-speech
services. For example:

- voice band data transmission;

- Telefax;

- Videotex.

c) Recommendation G.113 provides for speech services information on the
number of units of quantizing distortion produced by various digital
processes. Can additional processes be identified and, if so, can the
appropriate number of units of quantizing distortion be specified?

d) can a planning rule (similar but not equal to a qdu) be identified
which indicates the impact of a digital process on voice band data and
other non-speech signals. Note in G.113, section 4, these important
points concerned with this aspect of the Question:

"From the point of view of developing a simple planning rule which can
be used to assess the effects of digital processes on voice band data
performance, several points are important:

1) Impairments (especially transients) other than those customarily
measured for speech performance are important for measuring voice band
data performance.

2) A simple measure of the steady-state impairments (e.g., signal-to-
total noise ratio) may not prove to be a satisfactory basis for a voice
band data planning rule. A planning rule may have to take the transient
impairments into account.

3) Modem type and speed must be taken into account. Thus, unlike the
planning rules for speech, rules for voice band data may turn out to be
modem-specific.

4) The type of data service may influence the extent to which certain
kinds of data errors and thus certain impairments are important. Thus
the planning rules may be service- specific.

5) Only an objective measurement method taking these first four points
into account is likely to provide a successful basis for deriving useful
planning rules.

6) Such a measurement method does not exist at present."

e) Can the planning rule in G.113, which is now applicable only to mixed
analogue/digital connections, be extended to cover ISDN and mixed ISDN-
PSTN connections?

f) Can the planning rule in G.113 be revised to incorporate the effects
of bit errors on the qdu assignment made to digital processes and of all
noise type impairments (total noise) (e.g., quantizing, circuit, inter-
modulation, impulse, etc.) and, if so, what additivity laws for the
impairments apply, how is the total noise allocated to portions of the
connections, and what are the end-to-end objectives for the total noise?
(Note that relevant information on this part of the Question is
contained in COM XII-221, COM XIIR 29 and in COM XII-185. Note also
Recommendations Q.551 and Q.553 from Study Group XI may contain helpful
information concerning this study and that Study Group IV is studying
similar questions and should be kept advised of our work.) (Note that
the effect of delay and delay allocation rules are being studied in
Question 27/XII.)

g) What is the effect of frame slips and random bit errors on speech and
voice band data signals?

h) What Recommendations can be made with regard to planning rules for
the introduction of DCME and packetized voice systems in the world-wide
networks.