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Issue 6 January 2008 251
Echo and Signal Levels
As mentioned before, in circuit-switched telephony, echo may be caused by acoustic reflection
in the remote party's environment, or by electrical reflection from 2-to-4 wire analog hybrid
impedance mismatches. Impedance mismatch can occur in analog telephones and analog line/
trunk cards, electrical cross-talk in circuitry, or in telephony wiring (particularly in low-cost
headsets). For this reason, in circuit-switched analog and digital phones, a relatively large
transmit loss is implemented in order to minimize the perceived echo due to electrical reflection
and cross-talk effects. In principle, the transmit loss of telephones could be made very large
followed by signal amplification in the receiving telephone. In practice however, the transmit loss
should be limited to prevent the electrical voice signal from dropping below electrical
background noise. This has resulted in the adoption of transmit loss and receive loss values
around 8 dB and 2 dB, respectively, although country-specific values may actually deviate quite
significantly from these values.
The loss plan administration provided by Avaya Communication Manager software is primarily
intended to control signal losses in telephones and gateways, and not intended to control echo.
However, in case of severe echo, the administered loss can be changed to a different plan. In
general, an increase of the loss between two endpoints by a certain amount will decrease the
echo level by twice this amount. It is not advised to use loss plan administration in this way
without consultation with Avaya Services personnel. It is better to reduce the echo by using
Avaya products with echo cancellers to minimize echo.
Tone Levels
The level of call progress and DTMF tones played out through telephones must adhere to
specified levels in order to be satisfying for the average user. Again, respective standards are
country specific and can be set in Communication Manager by administration. The volume of
received call progress tones can be adjusted by the telephone volume control.
Codecs
Codecs (Coder-Decoders) convert between analog voice signals and digital signals. Avaya
supports several different codecs offering varying bandwidth usage and voice quality, including
the following codecs:
● G.711 produces uncompressed audio at 64 kbps
● G.729 produces compressed audio at 8 kbps
● G.723.1 produces compressed audio at 5.3 or 6.3 kbps
● G.722 produces compressed audio at 64, 56, or 48 kbps