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Data packets travel independently of one another, and are subject to various delays depending on the exact route they take. Out-of-sequence packets are not considered a problem for data transfers, as data transfer protocols can re-order packets and reconstruct data without corruption. Due to the time-sensitive nature of voice communications, VoIP systems are required to handle out-of-sequence packets in quite a different manner.
Some VoIP systems discard packets received out of order, while other systems discard out-of-order packets if they exceed the size of the internal buffer, which in turn causes jitter as described in the previous chapter. Sequence errors cause significant degradation of call quality.
Sequence errors may occur because of the way packets are routed. Packets may travel different paths through different IP networks, causing different delivery times. As a result, lower-numbered packets may arrive at the endpoint later than higher-numbered ones. The packets are usually received in the buffer, allowing the endpoint to rearrange out-of-order frames and reconstruct the original signal. However, the size of internal buffer is limited to control jitter, and significant variance in the orderly delivery of packets may cause the endpoints to discard frames, resulting in both jitter and dropped packet issues.
Routing VoIP calls through consistent routes to avoid spreading packets from the same call over different paths allows for significant reduction in sequencing errors.
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