Question 27/XII - Talker echo, propagation time and stability in
telephone networks, ISDN and interconnection with ISDN
(merger of Questions 6/XII and 27/XII studied in 1985-1988; new wording)

How should Recommendations be revised to take account of transmission
planning aspects of talker echo, propagation time, and stability in
telephone networks?

Specific points for particular study are:

a) the effect of propagation delay on transmission performance under
varying echo conditions;

b) the allocation of the maximum values of mean one-way propagation
times specified in Recommendation G.114 among international circuits and
national extensions;

c) the updating of Table 1/G.114 to include delay values for new devices
or systems;

d) echo control in ISDN and PSTN connections and in ISDN-PSTN
interconnections;

e) clarification of the stability clauses in G.122;

f) echo control in private network-PSTN interconnections;

g) echo control for packetized speech;

h) echo control for connections with non-linear or non-stationary
conditions in the echo path, e.g. low rate encoding, adaptive balancing,
etc.;

i) the adequacy of the values in Table 3, Supplement No. 2, for circuits
with short propagation times;

j) the effect of irregular sensitivity/frequency characteristics on the
calculation of echo loss according to G.122.

Note 1 - Recommendation G.114 deals with mean one-way propagation time.

Recommendation G.122 deals with the influence of national networks on
stability and echo in international connections. Recommendation G.131
deals with stability and echo in an international connection.

Recommendations G.161, G.164 and G.165 deal with echo control devices.

Supplement No. 2 gives the basis for talker echo control on
international connections.

Note 2 - Echo control may be required in wholly 4-wire connections (e.g.
ISDN) due to echo through acoustical feedback paths. If control is
required, it is necessary to determine whether echo cancellers
(conforming to Recommendation G.165) are effective in controlling the
echo caused by acoustical feedback in the handset of digital telephones.

Note 3 - It will be necessary to define the test conditions of any
experimental investigations of the effect of echo and/or delay on
transmission performance. Annex 1 provides details. It would be
desirable that subjective tests be conducted in a manner to facilitate
the extension of the existing opinion models.

Note 4 - Study Group XI should be kept informed of progress in
clarification of the stability clauses of Recommendation G.122.

Note 5 - Study Group XV is developing Recommendations for acoustic echo
controllers and should be kept informed of the progress on this
Question.

Note 6 - Study Group XVIII should be kept informed of progress in echo
control for ISDN connections, ISDN-PSTN interconnections and for
packetized speech.

Note 7 - Annex 2 provides information on the ISDN-PSTN interworking echo
control issues that are of concern to Study Group XVIII. COM XII-204 is
also relevant. (Note to Secretariat - For Annex 2 use COM XII-203).

Note 8 - Useful information on the concerns of Study Group XI, and on
the technical issues involved in clarification of the stability clauses
in Recommendation G.122 is contained in COM XII-211, and COM XII-215).

Note 9 - Other relevant information to the study of the Question can be
found in COM XII-R 5, R 12, R 17, R 29.
ANNEX 1

(to Question 27/XII)

Annex 2 to Question 6/XII in COM XII-1, 1985-1988 study period


ANNEX 2

(to Question 27/XII)

COM XII-203, 1984-1988 study period