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Issue 6 January 2008 245
Voice quality network requirements
In addition to the influence of the telephony terminals at either end of a connection, there are
several network parameters that can affect voice quality. This chapter lists some of the more
important ones. The concept of voice quality has different aspects that need to be properly
understood and considered. IP Telephony quality can be engineered and administered to
several different levels to accommodate differing business needs and budget. Avaya therefore
provides network requirements options to allow the customer to choose which "voice quality"
level best suits their specific business needs.
Before implementing IP Telephony, Avaya recommends a network assessment to measure
latency, jitter, and packet loss to ensure that all values are within bounds.
This section covers the topics:
● Network delay
● Jitter
● Packet loss
● Echo
● Signal levels
● Codecs
● Silence suppression/VAD
● Transcoding/tandeming
Network delay
In IP networks, packet delay (latency) is the length of time for a packet to traverse the network.
Each element of the network, including switches, routers, WAN circuits, firewalls, and jitter
buffers, adds to packet delay.
Delay can have a noticeable effect on voice quality but can be controlled somewhat in a private
environment (LAN/WAN). For example, delay can be reduced by managing the network
infrastructure or by agreeing on a Service Level Agreement (SLA) with a network provider. An
enterprise has less control over the delay when using the public Internet for VoIP.
Previously, the ITU-T recommended 150 ms one-way delay (including endpoints) as a limit for
conversations. However, this value was largely misinterpreted as the limit to calculate a network
delay budget for connections. Depending on the desired voice quality, network designers might
choose to exceed this number for their network.