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Voice quality network requirements
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 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 sound, listeners may adjust relative to
the 10 dB loss value by changing the volume control on their telephone.
The IP Telephony loss values are globally identical and specified in ITU-T Recommendations
and other regional local standards. Note that in principle, the telephone transmit and receive
loss values could have been implemented either as all transmit loss or as all receive loss. The
chosen implementation, where loss is split between speaker and listener, originates from the
circuit-switching history of telephony systems. See the subsection on Echo and Signal Levels
for more details.
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 received voice signal 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 control values. In order to ensure that the signal levels are
controlled properly within the scope of the voice network consisting of Avaya systems, the
appropriate country-dependent loss plan should be administered.
Besides administering loss for two-party calls, Communication Manager also allows
country-dependent conference call loss administration. Loss is applied depending on the
number of parties involved in the conference.
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
210 Avaya Application Solutions IP Telephony Deployment Guide
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