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4600 Series IP Telephone LAN Administrator’s Guide
Overview of Voice over IP
2-2
■ Routing is related to addressing and allows connections to be established between
endpoints.
Though these functions are common to data and voice networks, the implementations differ.
Delay and Jitter 2
Data traffic is generally short and comes in bursts. Data networks like the Internet were designed
to manage these bursts of traffic from many sources on a first-come, first-served basis. Data
packets are sent to multiple destinations, often without any attempt to keep them in a particular
order.
Voice networks are designed for continuous transmission during a call. The traffic is not bursty,
and the conversation uses a specific amount of bandwidth between the two ends for the duration
of the call.
Several features of data networks are unsuitable for voice telephony:
■ Data networks are designed to deliver data at the destination, but not necessarily within a
certain time. This produces delay (latency). In data networks, delay tends to be variable. For
voice messages, variable delay results in jitter, an audible chopiness in conversations.
■ Variable routing also can result in loss of timing synchronization, so that packets are not
received at the destination in the proper order.
■ Data networks have a strong emphasis on error correction, resulting in repeated
transmissions.
While data network concepts include prioritization of traffic types to give some forms of traffic
greater reliability (for example, for interactive transactions), data requirements tend to be not as
strict as most voice requirements.
Release 1.1 of the 4600 Series IP Telephones includes a dynamic jitter buffer. This feature
automatically smooths jitter to improve audio quality.
Tandem Coding 2
Tandem coding (also called transcoding) refers to the conversion of a voice signal from analog to
digital and back again. When calls are routed over multiple IP facilities, they may be subject to
multiple transcodings. The multiple conversions between analog and digital coding result in a
deterioration in the voice quality. Tandem coding should be avoided wherever possible in any
compressed voice system (for example, minimizing analog trunking on the PBX).