Fundamentals
158
Voice over IP (VoIP)
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 overloaded 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 propagation 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 Forum 5004/5008/5012 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
starting on page 190). Multiple TCP
connections are made between an IP telephone and Forum 5004/5008/
5012 for the telephone’s start procedure, registration and signalling.
All voice data are directly exchanged between IP telephones using the RTP
(“Realtime Transport Protocol”) protocol.
Propagation
delay
200-300 ms
3 level
depreciation
Loss 4-6 %
3 level
depreciation
Propagation
delay
> 300 ms
4 level
depreciation
Loss > 6 %
4 level
depreciation
Packet propagation delay and packet loss
Value
Quality level
Value
Quality level
