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Voice quality network requirements
250 Avaya Application Solutions IP Telephony Deployment Guide
Resource 320, the Avaya IP SoftPhone, and the Avaya 4600 Series IP Telephone all
incorporate echo cancellation designed for IP Telephony to improve voice quality.
Signal levels
In order to provide more natural-sounding conversations, voice communication systems attempt
to emulate a typical communication scenario where the two parties are speaking directly and
are separated by one meter. To achieve these conditions, an acoustic loss of 10dB is added
between speaker and listener. Any significant differences from this loss level will be audible as a
signal level that is either too soft or too loud and thus may result in some degree of listener
discomfort.
In IP Telephony networks, the end-to-end loss of 10 dB is implemented as 8 dB in the speaker's
telephone, 0 dB in the IP network, and another 2 dB loss in the listener's telephone. To account
for personal preferences or the presence of background distortions, listeners may adjust
relative to the 10 dB loss value by adjusting the volume control of their telephone. The IP
Telephony loss values are globally identical and specified in ITU-T Recommendations.
In traditional circuit-switched networks the telephone transmit, receive, and inter-port line/trunk
losses are country-dependent. The end-to-end country-specified losses often also differs
somewhat from the 10dB loss value for historical reasons. The country-dependency of loss
values makes it more challenging to guarantee a proper listener signal level when the PSTN is
involved or when country borders are traversed.
IP Telephony gateways should provide proper signal level adjustments in the direction from the
IP network to the circuit-switched network and in the reverse direction, and also between
circuit-switched ports.
To allow for multi-country deployment of Avaya telephones and gateways, these devices
facilitate programmable loss values. In order to ensure that the signal levels are controlled
properly within the scope of a voice network consisting of Avaya systems, the appropriate
country-dependent loss plan should be administered.
In addition to administering loss for two-party calls, Communication Manager allows
country-dependent conference call loss administration. Loss is applied depending on the
number of parties involved in the conference.