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Voice quality network requirements
In addition to the influence of the telephone terminals at either end of a connection, there are a
number of network parameters that have the potential to 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. Voice quality issues are a key element in the Avaya
commitment to international standardization. IP Telephony quality can be engineered to several
different levels to accommodate differing business needs and budget. Avaya therefore presents
options in network requirements to allow the customer to choose which "voice quality" level best
suits their specific business needs.
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 it takes 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 but can be controlled somewhat in a private environment
(LAN/WAN). The company or enterprise can manage the network infrastructure to minimize
delay or can have a Service Level Agreement (SLA) with its provider. An enterprise has less
control over the delay when using the public network.
Previously, the ITU-T was recommending 150 ms one-way delay (including endpoints) as a limit
for conversations. However, this value has been largely misinterpreted as the only range to
calculate a network delay budget for connections.
One-way delays in excess of 250 ms can cause the well-known problem of "talk-over," when
each person starts to talk because the delay prevents them from realizing that the other person
has already started talking. End-to-end one-way delays of 200 to 250 ms may not be noticed.
However, in some applications, delays less than 150 ms can impact the perceived quality,
Issue 3.4.1 June 2005
205
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