Echo is an amusing phenomenon to experience while visiting the Grand Canyon, but echo on a phone conversation can range from slightly annoying to unbearable, making conversation unintelligible.
Hearing your own voice in the receiver while you are talking is common and reassuring to the speaker. Hearing your own voice in the receiver after a delay of more than about 25 ms, however, can cause interruptions and can break the cadence in a conversation.
In a traditional toll network, echo is normally caused by a mismatch in impedance from the four-wire network switch conversion to the two-wire local loop (as shown in Figure 7-3). Echo, in the standard Public Switched Telephone Network (PSTN), is regulated with echo cancellers and a tight control on impedance mismatches at the common reflection points, as depicted in Figure 7-3.
Figure 7-3 Echo Caused by Impedance Mismatch
Echo has two drawbacks: It can be loud, and it can be long. The louder and longer the echo, of course, the more annoying the echo becomes.
Telephony networks in those parts of the world where analog voice is primarily used employ echo suppressors, which remove echo by capping the impedance on a circuit. This is not the best mechanism to use to remove echo and, in fact, causes other problems. You cannot use Integrated Services Digital Network (ISDN) on a line that has an echo suppressor, for instance, because the echo suppressor cuts off the frequency range that ISDN uses.
In today's packet-based networks, you can build echo cancellers into low-bit-rate codecs and operate them on each DSP. In some manufacturers' implementations, echo cancellation is done in software; this practice drastically reduces the benefits of echo cancellation. Cisco VoIP, however, does all its echo cancellation on its DSP.
To understand how echo cancellers work, it is best to first understand where the echo comes from.
In this example, assume that user A is talking to user B. The speech of user A to user B is called G. When G hits an impedance mismatch or other echo-causing environments, it bounces back to user A. User A can then hear the delay several milliseconds after user A actually speaks.
To remove the echo from the line, the device user A is talking through (router A) keeps an inverse image of user A's speech for a certain amount of time. This is called inverse speech (–G). This echo canceller listens for the sound coming from user B and subtracts the –G to remove any echo.
Echo cancellers are limited by the total amount of time they wait for the reflected speech to be received, a phenomenon known as echo tail. Cisco has configurable echo tails of 16, 24, 32, 64, and 128 ms.
It is important to configure the appropriate amount of echo cancellation when initially installing VoIP equipment. If you don't configure enough echo cancellation, callers will hear echo during the phone call. If you configure too much echo cancellation, it will take longer for the echo canceller to converge and eliminate the echo.