Aastra OpenCom 100 series, Mounting And Commissioning User Manual

Page: 116 / 256

Voice over IP (VoIP)
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Optimisation
If you detect a large fluctuation in the propagation delay during measurement, this
may also cause the voice quality to deteriorate. This may indicate a defective or over-
loaded line caused by bit-error or collision correction resulting from retransmission by
the transmission procedure.
An existing star-topology ethernet-network may uses a Hub as the central distributor
of ethernet packets. A Hub repeats all ethernet packets received on all connected
lines. This can cause substantial collisions and result in a high fluctuation in the propa-
gation delay.
If this is the case, use a modern switch component. Selective forwarding of ethernet
packets (“Layer 2 switching”) avoids collisions. Modern switch components also
evaluate the TOS byte of IP packets, thereby providing the optimal prerequisites for
VoIP telephony.
Note
The OpenCom 100 uses a TOS byte (“Type of Service”) value of 0xB8 for IP packets
with VoIP data. This requests “Minimise Delay” and “Maximise Throughput” for IP
packets marked with this value.
Call Set-up
Various IP-based protocols are used for system telephony via the Internet protocol
(“IP”) (see also Start Procedure starting on page 134). Multiple TCP connections are
made between an IP telephone and OpenCom 100 for the telephone’s start pro-
cedure, registration and signalling.
All voice data are directly exchanged between IP telephones using the RTP (“Realtime
Transport Protocol”) protocol.
Channels on a Media Gateway (MGW) are allocated for making a telephone con-
nection with an ordinary terminal or for dial tones. The MGW converts IP voice data
Propagation
delay > 300 ms 4 level
depreciation Loss > 6 % 4 level depreciation
Packet Propagation Delay and Packet Loss
Value Quality Level Value Quality Level