Echo Cancelling

                                                                     Echo Cancelling

Echo is an effect of hearing your own voice echoed back to you when speaking into a handset.
It is generated as a result of the delays incurred within the PLMN due to speech coding
decoding and signal processing.
This is rarely a problem when communicating between two MSs. However, when connecting
to a PSTN telephone, the signal must pass through a 4-wire to 2-wire hybrid transformer. The
function of this transformer is such that some of the energy at the 4-wire receive side from the
mobile is coupled back to the 4-wire transmit side and thus speech is retransmitted back to
the mobile.
This effect is not noticeable to the PSTN subscriber but introduces an irritating delayed echo
of about 180mS to the mobile subscriber.
To counter this effect, an Echo Canceller (EC) is inserted at the interface between the MSC and
the PSTN. A standard EC reduces this delay by about 80mS, making it generally transparent
to the mobile subscriber.

Summary
This section has covered the following topics:
• Mobile Originated Calls
• Mobile Terminated Calls
• Intra-PLMN Call Routing
• Inter-PLMN Call Routing
• Echo Cancelling
PLMN
HLR
HMS
C VVLR
VMSC