An EC is used on the PSTN side of the MSC for all voice circuits. Echo control is required at the switch because the inherent GSM system delay can cause an unacceptable echo condition, even on short distance PSTN circuit connections.
The total round trip delay introduced by the GSM system (the cumulative delay caused by call processing, speech encoding and decoding etc) is approximately 180 ms. this would not be apparent to the MS subscriber, but for the inclusion of a 2-wire to 4-wire hybrid transformer in the circuit. This is required at the land party’s local switch because the standard telephone connection is 2-wire.
The transformer causes the echo. This does not affect the land subscriber.
During a normal PSTN land to land call, no echo is apparent because the delay is too short and the user is unable to distinguish between the echo and the normal telephone “side tone”. However, without the EC and with the GSM round trip delay added, the effect would be very irritating to the MS subscriber, disrupting speech and concentration.
The standard EC will provide cancellation of up to 68 milliseconds on the “tail circuit” .The tail circuit is the connection between the output of the EC and the land telephone.