Aastra Telecom 800 - Manual (Page 65)

Voice over IP (VoIP) Fundamentals
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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.
The following table provides an overview of the required bandwidth for a tele-
phone connection with various parameter settings. The values apply to half-
duplex ethernet; for full-duplex the values can be halved.
7.2.3 Voice Quality
The achievable voice quality depends on various factors. It is possible to optimise
voice-data transmission on an existing network using the available configuration
settings. Measuring the network quality may also help.
Required bandwidth (kbit/s) with respect to Packet Length and Codec
Packet Length (ms) G.711 (not compressed) G.729A approx. 8 kbit/s
20 180.8 68.8
30 51.2
40 42.4
50 37.12
60 33.6
70 31.09
80 29.2