Aastra Telecom 800 manual Voice Quality

Voice over IP (VoIP) Fundamentals

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.

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

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.

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