Thursday, October 21, 2010

Improving VoIP quality of service and packet loss

It is therefore necessary not only for QoS in the network, but also in the terminals, and in the processes that develop in them, hence it is necessary also to say that sensitivity to packet loss, delays and fluctuations, experienced by voice over IP services depend largely on the mechanisms implemented in the terminals.

The preparation of media in the terminals to be sent and transferred over an IP network involves several processes: scanning, compression and packaging at the sending end, and the inverse processes at the receiving end. All this is accomplished through a complex algorithm processing that is determined, which in turn develops in some time interval, ie delay involves delay processing and packaging:
o Processing delay: delay for implementing the encryption algorithm, which delivers a stream of bytes ready to be packaged;
o packetization delay is the time required to form a voice packet from the encoded bytes.

It should be noted that the outcome of this consolidation - packetization directly affects the QoS, and also how they perform. Thus, when encoding speed reduces the bandwidth requirements are also reduced, allowing the network towards being able to handle more concurrent connections, but increases the delay and distortion of voice signals. The opposite occurs when increasing the speed of encoding.

Another aspect to consider is the tradeoff between packetization delay and channel utilization (the ratio between bytes and bytes of header information in each voice packet), ie, the search for greater use of the channel leading to further delay of packetization for a coding standard. Of course, as the encoding standard used shall be the resulting delay in connection with the use of the channel, which widened when the use of the channel is above 50% with a growth delay in exponentially If low speed codecs such as G.723.1. The packetization delay can also be reduced by multiplexing several voice connections on the same IP packet.

A delay of processing and packaging also adds the delay introduced by the process of buffering in the terminals, and the delay of "glue" in the network. This gives an end-to-end delay perceived by the end user a greater or lesser extent. End to end delays below 400 milliseconds do not compromise the interactivity in the conversation, but above 150 milliseconds, echo control is required.

The delays discussed above are logical result of the characteristics and mode of operation of IP networks, as well as the nature of voice signals.

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