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Voice over IP (VoIP) Fundamentals
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7.2.4 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 dis-
tributor of ethernet packets. A Hub repeats all ethernet packets received on all
connected lines. This can cause substantial collisions and result in a high fluctu-
ation in the propagation delay.
If this is the case, use a modern switch component. Selective forwarding of eth-
ernet packets (“Layer 2 switching”) avoids collisions. Modern switch components
also evaluate the TOS byte of IP packets, thereby providing the optimal prerequi-
sites for VoIP telephony.
Note: The Aastra 800 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.
Packet Propagation Delay and Packet Loss
Value Quality Level Value Quality Level
Propagation delay
< 50 ms
Optimal Loss < 1 % Optimal
Propagation delay
50-100 ms
0.5 level
depreciation
Loss 1-2 % 0.5 level
depreciation
Propagation delay
100-150 ms
1 level
depreciation
Loss 2-3 % 1 level
depreciation
Propagation delay
150-200 ms
2 level
depreciation
Loss 3-4 % 2 level
depreciation
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