Avaya BCM 2.0 IP Telephony Manual de usuario descargar pdf (Pagina 40)

40 Engineering guidelines

Enterprise Edge 2.0 IP Telephony Configuration Guide P0911590 Issue 02

Echo cancellation

When a two-wire telephone cable connects to a four-wire PBX interface or a central

office (CO) interface, the system uses hybrid circuits to convert between two wires

and four wires. Although hybrid circuits are very efficient in their conversion

ability, a small percentage of telephony energy is not converted but instead is

reflected back to the user. This is called echo.

If the user is near the PBX or CO switch, the echo comes back so quickly it cannot

be detected. However, if the delay is more than about 10 ms, the user can hear an

echo. To prevent this occurrence, gateway vendors include special code in the DSPs

that listens for the echo signal and subtracts it from the listener’s audio signal.

Echo cancellation is important for gateway vendors because the IP network delay

can be 40−50 ms, so the echo from the far-end hybrid can be important at the near

end. Far-end echo cancellation removes this.

Echo cancellation can cause broken speech in conversations in a low audio

conversation. Although echo cancellation can be disabled, it is not recommended.

Non-linear processing

Non-linear processing (NLP) is part of echo cancellation. It improves echo

cancellation by reducing remaining echo. NLP mutes background noise during

periods of far-end silence and prevents additional comfort noise from occurring.

Some listeners find muted background noise a problem. NLP can be disabled to

prevent this, but with the trade-off of increased heard echo.

Jitter buffer

A major cause to reduced voice quality is IP network packet delay and network

jitter. Network delay represents the average length of time for a packet to move

across a network. Network jitter represents the differences in arrival time of a

packet. Both important in determining voice quality, delay is like the average, jitter

is like the standard deviation.

To allow for differences in arrival time of a packet and continue to produce a steady

out-going stream of speech, the far-end gateway does not play out the speech when

the first packet arrives. Instead, it holds it for a some time in part of its memory

called the jitter buffer, and then plays it out. The amount of this hold time is the

measure of the jitter buffer, for example, a 50 ms hold time indicates a 50 ms jitter

buffer.

As the network delay (total time, including codec processing time) exceeds about

200 ms, the two speakers increasingly use a half-duplex communications mode,

where one speaks, the other listens and pauses to make sure the speaker is done. If

the pauses are ill timed, they end up “stepping” on each other’s speech. This is the

problem that occurs when two persons speak over a satellite telephony connection.

The result is a reduction in voice quality.

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Avaya BCM 2.0 IP Telephony Manual de usuario Pagina 40