Echo canceller control is intended to improve voice quality on a call by call basis.

The Branch Gateway has multiple echo cancellers of various capabilities. For best echo cancellation performance, the general rule is to enable only one echo canceller in any direction -- the one with the greater capacity in terms of echo tail control in the steady state. Tandeming echo cancellers in the same direction in a media path results in poorer performance in terms of echo control, double-talk performance, noise, etc. In addition, if a smaller tail echo canceller is in the echo path of a longer tail canceller, audible echo can result when echo exists partly in one canceler's window and partly in the other.

For cases where there is no echo to cancel, it is usually best to disable any echo canceller in the path. Echo cancellers are not totally transparent and sometimes introduce undesirable artifacts.

However, the best echo cancellation policy varies depending on each specific call configuration. The Branch Gateway has an internal table for determining which VoIP engine and analog card echo cancellers to enable on a case-by-case basis. This table is consulted when the default auto mode is specified in the echo cancellation CLI commands. The CLI commands also offer the option of overriding the default automatic mode, but those alternative modes are intended for debugging and diagnostics purposes only.