T-Comfort Comfort Pro S, Mounting And Commissioning Manual

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Fundamentals 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 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.
Longer voice data packages are generally used for SIP telephony over the Internet.