BELGACOM Forum 523, Mounting And Commissioning Manual

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Fundamentals 107
Voice over IP (VoIP)
Fundamentals
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.
Propagation Delay and Bandwidth
IP-based data networks are generally not able to guarantee a spe-
cific minimum bandwidth and defined propagation delay. A syn-
chronised 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 inter-
ruption 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. Spe-
cific 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.
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-prop-
agation 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.