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108
Fundamentals
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.
The following table provides an overview of the required band-
width for a telephone connection with various parameter settings.
The values apply to half-duplex ethernet; for full-duplex the val-
ues can be halved.
Required bandwidth (kbit/s) with respect to Packet Length
and Codec
Packet
Length
(ms)
20
30
40
50
60
70
80
Note: To ensure SIP compatibility, the older system telephones
Forum IPhone 520 and Forum IPhone 530 do not support the
G.723 codec any more.
Voice Quality
The achievable voice quality depends on various factors. It is pos-
sible to optimise voice-data transmission on an existing network
using the available configuration settings. Measuring the network
quality may also help.
Voice over IP (VoIP)
G.711
G.729A
(not com-
approx.
pressed)
8 kbit/s
180.8
68.8
51.2
42.4
37.12
33.6
31.09
29.2
G.723.1
G.723.1
6.3 kbit/s
5.3 kbit/s
48.0
45.9
30.4
28.3
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