Aastra Telecom 800 - Manual (Page 64)

Voice over IP (VoIP) Fundamentals
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5. You can only conduct a certain number of telephone calls simultaneously with
internal SIP telephones. The number licen sed can be viewed on the System:
Licences page. The number of SIP telephones currently licen sed 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.
7.2 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.
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.