Fundamentals
Voice over IP (VoIP)
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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 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.
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
requirements 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 transmission, then longer voice-data packets should be used.