In order to achieve transmission objectives on long automatic and semi-
automatic telephone connections, it is necessary to take into account the
effects of echo. A general discussion of echo considerations is given in Recommendation
Q.42 which is an extract of Recommendation G.131. The characteristics of terminal
half-echo control devices are given in Recommendations G.161 [1] and G.164 [2].
The characteristics of echo cancellers are given in Recommendation G.165 [3].
In order to achieve optimum echo control for each call, it is necessary
to control both types of echo control devices.
This can be carried out at switching centres only if sufficient
information is available to coordinate an overall control action.
Logical means to obtain pertinent information and the switching
considerations governing its practicable use are detailed below. Control based on the
transfer of signals between switching centres is given particular attention. Self-contained control action such as tone disablement of echo suppressors and echo
cancellers for data transmission is not within the scope of this section.
In the cases to be discussed, control methods will be applied at
international exchanges (CTs), but it is recognized that in some countries covering large
geographic areas it may be appropriate to extend the control methods into
national networks.
The actions described in 3.5 to 3.8 about the analysis of information
and the decision to be taken in an outgoing transit or incoming international
exchange are summarized in the SDL flowchart of Annex A.
Annex A does not cover the handling of echo control devices in the case
of different bearer services and CCITT Signalling System No. 7.
3.2 Terminology
a) Subsequent discussion of control measures will refer to the standard
terminal half-echo suppressor specified in Recommendation G.164 [2] and the echo
cancellers specified in Recommendation G.165 [2]. The terms echo suppressor and echo
canceller will be used to denote these devices. The term echo control device will
comprise both, echo suppressors and echo cancellers.
b) Two means of introducing echo control devices are considered as
acceptable, these are, the use of permanently associated echo control devices and the use
of echo control devices inserted from a common pool of echo control devices.
c) With respect to control of permanently associated echo control devices
control actions are said to enable or disable.
d) With respect to echo control devices provided from pools, control actions
are concerned with inserting or not inserting.
e) The signals assigned in Systems R2, No. 6 and No. 7 (and reserved in
System No. 4) for echo control equipment control are in most cases a means to guide
subsequent exchanges in taking necessary action with respect to possible
introduction of an incoming echo control device. Thus the descriptive phrases associated
with the various signalling systems, as given below, convey comparable meaning in
the control plan.
Systems No. 4 and R2: incoming half-echo suppressor (half-echo
canceller) required;
Systems No. 6 and No. 7: outgoing half-echo suppressor (half-echo
canceller) included in the connection.
f) A secondary signalling function related to echo control device control
provides for the possibility that echo control device may not be available at
an originating CT. In this case responsibility for both outgoing and incoming echo
control device may be delegated by signal.
g) A long circuit is considered as one which, if used by itself, would require
echo control.
h) A short circuit is considered as one which, if used by itself, would not
require echo control.
3.3 Compatibility of echo control devices and signalling equipment
a) Arrangements should be incorporated in the switching equipment to
prevent echo suppressor and echo canceller action from disturbing simultaneous forward
and backward signalling via the speech paths.
For this case typical arrangements are:
i) locating the echo control device on the switching side of the
signalling equipment;
ii) inhibiting the action of echo control device located on the line
side of the signalling equipment by means of an
appropriate condition extended from the signalling
equipment to the echo control device while signalling is in
progress.
Note 1 - The standard half-echo suppressor (Recommendations G.161 [1] and
G.164 [2]) if located on the line side of line signalling equipment may
adversely affect signalling. This difficulty is possible because with the new
standard half-echo suppressor normal operation will at times cause 6 dB
additional loss to appear in the path to a line signalling receiver. Operating
margins are correspondingly reduced. For example, with signalling receivers
for System No. 5 as specified in Recommendation Q.112, signalling reliability could
be impaired. Accordingly, adequate operating margins should be assured or the echo
suppressor should not be located on the line side of line signalling receivers.
With regard to inter-register signalling which requires simultaneous transmission in
both directions, similar considerations call for disabling the echo suppressors
while inter-register signalling is in progress in order to prevent the 6 dB loss.
Note 2 - Echo cancellers will not introduce any fixed loss during in-band
signalling. But they can cause a problem during the continuity check used in CCITT
Signalling Systems No. 6 (Recommendation Q.271) and No.7
(Recommendation Q.724), or with compelled signals having the same frequency(ies) on both directions
of transmission in Signalling System No. 5 (Recommendation Q.112)
where the received signal is processed through the existing echo path model and
produces an interfering signal in the return path.
Note 3 - Some echo control devices are capable of internally providing either
signalling bypass or an appropriate internal function which permits transparent
operation to in-band signalling or other in-band tones.
b) Arrangements should be incorporated in the Systems No. 6 and No. 7
equipment to prevent echo suppressor action from disturbing the procedure for making the
continuity check of the speech path. Echo suppressors and echo cancellers must be
permanently disabled, if a circuit is used for common channel signalling.
3.4 Operation without signals
In Signalling Systems No. 5 and R1, signals are not available for echo
control information. In System No. 4 a signal may be applied only if multilateral or
bilateral agreements authorize its use. Accordingly the recommended control plan
relies on means other than signals in cases where it has not been found practicable to
provide signals. In the case of System No. 5, the normal field of application to
long circuits typically indicates the presence of echo control device. In the case of
System R1, regional control procedures not requiring signals are applicable.
3.5 Analysis of information at an outgoing international exchange
The outgoing international exchange, hereafter designated "A", must make a
decision with respect to its echo control requirements at the time an outgoing
circuit is selected. Unless echo control devices are not available, one or more of the
following items of information should influence this decision:
i) country code of destination and possibly some additional address
digits;
ii) information about the actual routing of the call;
iii) nature of outgoing international circuit at A (e.g. satellite
circuit);
iv) nature of incoming national circuit at A;
v) signals received over the incoming national circuit at A;
vi) requested bearer service (see Recommendation I.231[4]).
With respect to iii) and iv), the characteristic of primary interest is
propagation time. Two general categories, long and short, are the basis of
control action. See 3.3 g) and h) above, for definition of terminology.
3.6 Decision to be taken at the outgoing international exchange
If the factors i) to vi) in 3.5 above indicate that there is no need to
provide echo control devices on a particular connection, the outgoing exchange
should act accordingly and advise subsequent exchanges by signal or other
appropriate means, of its decision.
If the information available indicates that the connection to be
established will require echo control and if it is known that an outgoing echo control
device is not already provided in the national network, then the outgoing exchange
should provide for the outgoing echo control device. The outgoing exchange should
also, if signals are available, indicate by signal to subsequent exchanges as
appropriate what action it has taken.
In the event that an outgoing exchange is unable to provide an outgoing
echo control device when a need is known, it may call for cooperative action.
(Signal I-11 in System R2 is specifically assigned to make possible a cooperative
transfer of responsibility for echo control device control from an originating CT to
a transit CT. The signal outgoing half-echo suppressor not included could be used
with Systems No. 6 and No. 7, but such an application would in effect assume that
a modern exchange found sufficient reason to displace an outgoing echo control
device from its preferred location.)
3.7 Decision to be taken at an international transit exchange
The decision at an international transit exchange depends on an
assessment of switching and signalling information available after the transit CT has
selected an outgoing circuit. Information similar to that listed in 3.5 i) to vi)
above is of interest.
a) When the first transit CT knows that an outgoing echo control device has
not yet been provided closer to the call source by a signal of CCITT Systems
Nos. 6, 7 and R2, or by bilateral agreements for specific exceptions, the transit CT
should consider the outgoing circuit selected, the ultimate call destination and
such other information as indicated above. If a connection requiring echo control
may result, an outgoing echo control device should be enabled or inserted at the
first transit CT.
b) When the transit CT concerned knows that an outgoing echo control device
is located closer to the call source, the question to be decided is the location
of the incoming echo control device. The incoming echo control device is located
at the transit CT only when a location nearer to the called party is not
practicable. Specifically, an exception may result when the transit CT selects a short
terminal circuit equipped with CCITT Signalling Systems No. 4, 5 or R1. In this
case, an incoming echo control device should be enabled or inserted at the transit
CT.
c) It follows from the above that in every case where an international
transit centre interconnects two circuits and knows that echo control device will be
provided at a preceding location and also at a more distant location, the transit
centre should disable or not insert its own echo control device. (Full echo
control device is not covered in the control plan and should not be affected by the
procedures described in this section.)
d) It is, of course, commonly the case that an outgoing echo control device
has not been introduced at the outgoing exchange because none is required. When
the transit exchange has reason to know of such a situation it should not
introduce an echo control device and should advise the subsequent exchange when possible
that an incoming echo control device is not required (or equivalently, that an
outgoing echo control device has not been introduced).
e) In the case of a routing where both an incoming and outgoing echo control
device has already been inserted at earlier points, the transit exchange should
advise the subsequent exchange, where possible, that an incoming half echo control
device is not required.
3.8 Decision to be taken at the incoming international exchange
Short circuits equipped with CCITT Systems No. 5, R1 and No. 4 (unless
bilateral agreements are reached), provide no signals at the incoming CT for
selective use of echo control devices. As a result, in the absence of separate circuit
groups on the same route or other alternatives, the economic choice is to omit
echo control devices. In the case of a call that has passed through a transit
exchange en route to the incoming exchange, the requirement for an incoming echo
control device should then be met at the preceding CT as covered in 3.7 b) above.
With CCITT Systems Nos. 6, 7, R2 and No. 4 (assuming multilateral or
bilateral agreement) selective use of echo control devices on short terminal links is
a basic option. Therefore, the terminal CT acts in accordance with the control
signal received. When an outgoing echo control device has been included at a
preceding CT, the incoming CT should enable or insert an incoming echo control device.
When no echo control device has yet appeared elsewhere in the connection,
none should be enabled or inserted at the incoming CT.
3.9 Other considerations
It is recognized that when echo control devices are inserted from pools,
there is a small probability that no echo control device will be available when
needed. In this case an (equipment) congestion signal should be given to the
calling subscriber.
Nothing in this Recommendation should be construed as discouraging
control measures which may supplement the plan described and lead to improved results
in specific situations. For example, regional procedures which introduce loss to
control echo may be arranged to satisfy both regional and international needs on a
selective basis. In addition for multiple ISC in one country the procedure of
Annex B may be applied. It is recognized that possibilities for echo control have
not been exhausted. If switching and signalling equipment have a changed role in
the application of future procedures, this Recommendation will be subject to
revision.
Annex A
(to Recommendation Q.115)
Call processing logic - Echo suppressor control
FIGURE B-1/Q.115
Echo control on multiple ISCs in a country
REFERENCES
[1] CCITT Recommendation Echo suppressors suitable for circuits having either
short or long propagation times, Volume III of Orange Book,
Recommendation G.161.
[2] CCITT Recommendation Echo Suppressors, Volume III, Recommendation G.164.
[3] CCITT Recommendation Echo Cancellers, Volume III, Recommendation G.165.
[4] CCITT Recommendation Bearer services supported by an ISDN, Volume III,
