Connection Analysis

MVLS03: VoIP packets are arriving out of order.

Unlike TCP (Transmission Control Protocol) traffic VoIP uses UDP. TCP is designed to ensure data arrives and is processed in order. To that end the TCP protocol requires data to move in both directions, the return data being the confirmation process. TCP can and will recover lost data, after all it would be hard to do online banking if packets got lost or discarded. The decimal point might get lost and then major problems will likely ensue. TCP does not get used for VoIP because the TCP protocol is simply too expensive in time consumed to be workable for what is synchronised speech between two parties. UDP, which is the protocol used for VoIP, is one-way only, i.e. data only travels only in one direction and there is no concept of ‘safe arrival’ and ‘recovery’. If data gets lost in transit or arrives too early or late then the data is discarded as unusable.

Because VoIP represents two or more parties in conversation using a telephone or mobile handset, it must deliver a fluid data flow to support conversation. The very nature of voice communication therefore requires precise timing of packets in both directions. However because packets can travel at different speeds across different network segments and can even take different routes to reach the same destination then it is likely that packets will arrive at varying times, some in order some well out of order. Unfortunately this is counter to the way a voice communication normally works. To combat these delays both the sending end and the receiving end will introduce a timing delay buffer to overcome the network extended delays to ensure the audio is uniform and continuous. That said there will always be some packets that arrive out-of-order either ‘too late’ or #too early’ for acceptable use and quality of the speech is lost as a result.

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