atcomsystems.ca/forum Forums TDM Business Telephone Systems Inter-Tel Telephones & Systems IP phone echo

Echo...ahh
Echo can happen when VoIP is used mainly anytime that there is a hybrid in use. Let me explain.
Reflection plus delay = echo. What is reflection, some refer to it as talk back or side tone. On a normal phone it is where you can hear yourself talk through the receiver. Reflection is present on any analog phone so it is considered normal audio characteristic, if you ever picked up a phone and couldnt hear yourself in the receiver it would seem weird. So now we try to have reflection in delay intensive networks (at times) and we now induce this delay to the reflection and this equates to echo to the human ear. It is the same talkback, it is that it is delayed thus the echo sound. An analog handset actually has 4 wires, one pair for Tx and one pair for Rx. This is converted to a two wire virutally combining tx an rx onto two wires (tip and ring), this is known as a hybrid. Anytime a hybrid is involved, reflection is created. Most times the delay is very minimal so the reflection is acceptable.
2 Wire: Loop Start Trunk, Ground Start Trunk, Analog PBX port, IPSLA, etc
4 Wire: any digital station port, T1/PRI, IPRC, etc
Hopefully that helps a bit to explain where echo comes from. Sometimes you can fix it sometimes you cannot. In my experience, elimiate hybrids and reduce delay as much as possible and you should be able to provide toll quality voice via VoIP without a problem everytime.