Voice over IP (VoIP) Fundamentals11010.2 FundamentalsVoIP makes the transmission of voice and telephony signalling via IP (“InternetProtocol”) possible. After a connection is established, the terminal collects voicedata (PCM data), which is then sent to the receiver using an IP packet. PCM datacan also be compressed to save bandwidth.10.2.1 Propagation Delay and BandwidthIP-based data networks are generally not able to guarantee a specific minimumbandwidth and defined propagation delay. A synchronised 64 kilobit/s ISDN lineguarantees a fixed data rate as long as the connection exists. In an IP-based datanetwork, the data rate and propagation delay can vary. Short-term bottlenecks orretransmission due to errors may be the cause. A data flow interruption of a fewseconds is barely noticeable when fetching a Web page, but it can be seriouslyinterfere with a telephone call.A modern Intranet normally offers enough performance reserves and reliability tomake good-quality VoIP telephony possible. Specific components can also beoptimised; for example by using a modern switch which evaluates the TOS byte ofIP packets or by replacing unreliable connections.10.2.2 Latency and Packet LengthFor technical reasons, there is always a delay (“latency”) between the recording ofvoice data via the microphone and playback via the receiver. Voice data isrecorded for a short period so that it can be sent in an IP packet. The IP packet alsohas a signal-propagation delay before the receiver can begin playback. For thesereasons, the extra time required for voice-data encoding and decoding may beneglected.An IP packet consists of protocol data and user data. Sending shorter voice-datapackets causes the ratio between the user data and the protocol data to becomeunfavourable and increases the bandwidth required. Sending longer voice-datapackets increases latency.The length of the voice-data packets must therefore be adjusted to the require-ments of the transmission medium. Shorter voice-data packets can be sent if adirect ethernet connection exists. If an 64 kilobit/s ISDN line is to be used for trans-mission, then longer voice-data packets should be used.
