Aastra Telecom 800 manual Fundamentals, Propagation Delay and Bandwidth, Latency and Packet Length

Voice over IP (VoIP) Fundamentals

5.You can only conduct a certain number of telephone calls simultaneously with internal SIP telephones. The number licensed can be viewed on the System: Licences page. The number of SIP telephones currently licensed can be deter- mined on the System Info: Telephony: SIP phones page. If you click on Reset licences, the available licenses will be reassigned with the next incoming or outgoing calls.

7.2Fundamentals

VoIP makes the transmission of voice and telephony signalling via IP (“Internet Protocol”) possible. After a connection is established, the terminal collects voice data (PCM data), which is then sent to the receiver using an IP packet. PCM data can also be compressed to save bandwidth.

7.2.1 Propagation Delay and Bandwidth

IP-based data networks are generally not able to guarantee a specific minimum bandwidth and defined propagation delay. A synchronised 64 kbit/s ISDN line guarantees a fixed data rate as long as the connection exists. In an IP-based data network, the data rate and propagation delay can vary. Short-term bottlenecks or retransmission due to errors may be the cause. A data flow interruption of a few seconds is barely noticeable when fetching a Web page, but it can be seriously interfere with a telephone call.

A modern Intranet normally offers enough performance reserves and reliability to make good-quality VoIP telephony possible. Specific components can also be optimised; for example by using a modern switch which evaluates the TOS byte of IP packets, by replacing unreliable connections, or by using a separated VLAN for VoIP.

7.2.2 Latency and Packet Length

For technical reasons, there is always a delay (“latency”) between the recording of voice data via the microphone and playback via the receiver. Voice data is recorded for a short period so that it can be sent in an IP packet. The IP packet also has a signal-propagation delay before the receiver can begin playback. For these reasons, the extra time required for voice-data encoding and decoding may be neglected.

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