Voice over IP (VoIP) Fundamentals
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9.2.1 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.
9.2.2 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.
An IP packet consists of protocol data and user data. Sending shorter voice-data
packets causes the ratio between the user data and the protocol data to become
unfavourable and increases the bandwidth required. Sending longer voice-data
packets 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 a
direct ethernet connection exists. If an 64 kbit/s ISDN line is to be used for trans-
mission, then longer voice-data packets should be used.
Longer voice data packages are generally used for SIP telephony over the Internet.
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