Allworx 24X Manual de usuario descargar pdf (Pagina 9)

VoIP White Paper

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Revised: February 8, 2007

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5 Echo in VoIP Networks

From time to time, echo can be a problem during telephone calls. Certainly everyone has experienced echo at

one time or another, even for ordinary, non-VoIP calls. Echo is most commonly experienced in international

calls or long-distance calls to rural areas, and of course in calls on cell phones. Unfortunately, the

characteristics of VoIP telephony connections increase the opportunity for echo problems to occur. This is due

to the introduction of additional latency (delay) of the voice as it travels from source to destination over the

packetized data network.

To the human observer, echo of his or her own voice is only noticeable when it is heard back with some

amount of delay. Echo without delay simply sounds like side-tone. Side-tone is the sound of your talking fed

back directly between microphone and speaker without any delay. It is normally introduced purposely on

phones so that the phone does not sound dead when you are talking. Therefore, when echo exists in the

analog phone network, especially in local calls, it is completely covered up by the existing side-tone. However,

when a VoIP system is attached to that very same analog phone line, the additional latency of the data network

now carrying the voice to/from the IP phone now makes that echo noticeable to the user – since the echo now

arrives back well after the speaker has finished each sentence.

Echo round-trip delays of only a few milliseconds (ms) are not really noticeable, but as the delay accumulates

into the area of 10ms, the system will start sounding hollow and eventually will start sounding like the reverb of

a large stadium echo. As echo latencies run into the area of 50ms and beyond, the speaker’s own speech will

be followed by a very distinct echo of the same words back in his or her ear. At this point, unless the echo is

very quiet, it starts to become annoying.

Generally speaking, the loudness and the latency impact how objectionable the echo sounds to the talker. As

the echo gets quieter and/or less delayed, it becomes less objectionable. Alternatively, we can say that the

more delayed the echo is, the quieter it needs to be in order to be acceptable. For echo to be acceptable in a

VoIP system, it typically needs to be quieter than it was to start with in the analog-only part of the network. This

is the role of the echo canceller in a VoIP system, which we will cover later.

5.1 Where does the echo come from?

Echo results in the phone network when two-wire phone lines carrying voice in both directions on the same

wire pair are converted into four-wire circuits (where a separate wire pair carries the audio in each direction).

The analog device that does these conversions is called a hybrid – and its job is to convert back and forth

between the two-wire analog loop world and the four-wire Central Office (CO) switch world. If hybrids were

perfectly matched to the particular phone and phone lines installed at every site, there would be no echo.

However, in the real world, hybrids are not installed with perfect impedance matches, and therefore echo

results when sounds bounce off the hybrids.

In a typical analog phone call, there will be at least two hybrids involved: one at the CO for the calling party

(near-end echo) and one at the CO for the called party (far-end echo). Beyond the hybrid electrical echo, there

can be other sources of echo. The most common is acoustical echo at the far end – when you call someone

who has a low-quality phone or is using a desktop-type speakerphone. The near-end echo is determined by

the local phone line loop on which you are making the call, while the far-end echo depends on the party being

called. For this reason, near-end echo is generally referred to as “line echo” and the other sources of echo are

collectively referred to as network echo.

Delay times for line echo are not a concern for ordinary analog phone calls because the delay path is short

enough that the echo sounds like side-tone. This means that the phone company can ignore the effects of line

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