ITU-T G.168 PDF

Echo cancellers are voice operated devices placed in the 4-wire portion of a circuit (which may be an individual circuit path or a path carrying a multiplexed signal) and are used for reducing the echo by subtracting an estimated echo from the circuit echo (see Figure 1).

NOTE – Functionally, a digital echo canceller (DEC) interfaces at 64 kbit/s. However, 24 or 30 digital echo cancellers, for example, may be combined corresponding to the primary digital hierarchy levels of 1544 kbit/s or 2048 kbit/s, respectively.

This Recommendation is applicable to the design of echo cancellers using digital techniques, and intended for use in circuits where the delay exceeds the limits specified by [ITU-T G.114] and [ITU-T G.131]. It is necessary for all echo control devices used on international connections to be compatible with each other. Echo cancellers designed to this Recommendation will be compatible with each other, with echo cancellers designed in accordance with [ITU-T G.165], and with echo suppressors designed in accordance with [ITU-T G.164]. Compatibility is defined as follows:

Given:

1) that a particular type of echo control device (say Type I) has been designed so that satisfactory performance is achieved when any practical connections is equipped with a pair of such devices; and

2) that another particular type of echo control device (say Type II) has been likewise designed, then Type II is said to be compatible with Type I if it is possible to replace an echo control device of one type with one of the other type, without degrading the performance of the connection to an unsatisfactory level. In this sense, compatibility does not imply that the same test apparatus or methods can necessarily be used to test both Type I and Type II echo control devices.

Freedom is permitted in design details not covered by the requirements. This Recommendation is for the design of digital echo cancellers and defines tests that ensure that echo canceller performance is adequate under wider network conditions than specified in [ITU-T G.165], such as performance on voice, fax, residual acoustic echo signals and mobile networks.

This Recommendation does not apply to echo cancellation through active 2-wire/4-wire hybrids or 2-wire repeaters. This Recommendation does not cover acoustic echo cancellation as per [ITU-T P.340].

This Recommendation defines objective tests that if passed will ensure (but will not guarantee) a minimum level of performance when installed in the network. An echo canceller which passes these tests should not harm equipment nor degrade transmission performance of voiceband signals and services below acceptable limits. These tests are lab-type tests and are not designed to be run in-service. Also, these tests are objective tests and do not replace or eliminate the need for subjective tests to measure the perceived quality of echo cancellers. Echo cancellers are complex devices with multiple parameters, and the correlation of these parameters and their interactions to the subjective quality of an echo canceller is difficult to specify. Clause I.7.5 gives some guidelines on how subjective test results were used in order to develop objective tests. ITU-T P.851 describes methods and procedures for conducting subjective performance evaluation of network echo cancellers. Thus, this Recommendation does not specify nor imply selection criteria; however, guidelines are provided herein, and Administrations have the freedom to specify criteria in their selection process. This set of criteria may include some or all of the thresholds and/or tests in this Recommendation.