2.4.5. Echo Cancellation 
The HG 35xx gateways support on board echo cancellation (performed
on the on-board DSP modules) compliant with the ITU-T G.168 standard.
More detailed information on echo cancellation
Echo arises at all analog 2-wire/4-wire hybrids, e.g. in the hybrid
coil of an analog telephone, as well as through acoustic over-coupling
of speaker and microphone in the recipient’s handset.
If the runtime between the generation of an audible signal and the
manifestation of the echo at the speaker (Round Trip Delay) is very slight,
the echo has no interfering effect. However, the greater this runtime,
the greater the disturbance of the echo. The runtimes in the case of
IP transmission are very high. Thus, echoes are always perceived as extremely
disturbing. Therefore, it is important that all signals are free of echo
prior to being transmitted in the IP network. The resulting effects of
delay and echo are explained in detail in Section 1.6.2,
“Delay and Echo”.
Echo cancellation suppresses the echo by filtering it out of the data
stream. Echo cancellation should always be used as close to the source
of the echo as possible.
The following example is intended to clarify this:
- Subscriber A is connected to Subscriber B via an IP route.
- The terminal device of Subscriber B generates an echo of the signal of Subscriber A.
- The echo is to be suppressed near the source, i.e. in the vicinity of Subscriber B, so that the signal from B to A is free of echo before being transmitted across the IP route.
Echo cancellation (echo suppression) is a very complex operation in digital
signal processing, which only has a minimal effect on the transmitted
wanted signal (without echo).
Echo cancellation is configured automatically for all circuits, but
can be deactivated.
Analog modems and fax machines use complicated phase modulation procedures
for data transfer which can be affected by interference caused by echo
cancellation.