Delay and Jitter

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 not be as strict as most voice requirements.

Starting with Release 1.1, the 4600 Series IP Telephones include a dynamic jitter buffer. This feature automatically smooths jitter to improve audio quality.