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| Monitoring and Troubleshooting VoIP Networks with a Network Analyzer |
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Packet loss occurs in every kind of network. All network protocols are designed to cope with the loss of packets in one way or another. TCP protocol, for example, guarantees packet delivery by sending re-delivery requests for the lost packets. RTP employed by the VoIP protocol does not provide delivery guarantee, and VoIP must implement the handling of lost packets. While a data transfer protocol can simply request re-delivery of a lost packet, VoIP has no time to wait for the packet to arrive. In order to maintain call quality, lost packets are substituted with interpolated data. A technique called Packet Loss Concealment (PLC) is used in VoIP communications to mask the effect of dropped packets. There are several techniques that may be used by different implementations. Zero substitution is the simplest PLC technique that requires the least computational resources. These simple algorithms generally provide the lowest-quality sound when a significant number of packets are discarded. Waveform substitution is used in older protocols, and works by substituting the lost frames with artificially generated substitute sound. The simplest form of substitution simply repeats the last received packet. Unfortunately, waveform substitution often results in unnatural, "robotic" sound when a long burst of packets is lost. The more advanced algorithms interpolate the gaps, producing the best sound quality at the cost of using extra computational resources. The best implementation can tolerate up to 20% of packets lost without significant degradation of voice quality. While some PLC techniques work better than others, no masking technique can compensate for a significant loss of packets. When bursts of packets are lost due to network congestion, noticeable degradation of call quality occurs. In VoIP, packets can be discarded for a number of reasons, including network congestion, line errors, and late arrival. A network analyzer displays the number of lost packets. Better network analyzers display real-time charts that allow visualizing the number of dropped packets as well as detect quality-degrading bursts. Seeing the exact shape of packet-loss graphs allows network administrators to choose a Packet Loss Concealment technique that best matches the characteristics of a particular environment, as well as to implement measures to reduce packet loss on the network. 
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