Aastra Telecom 800 User Manual

Voice over IP (VoIP)

Fundamentals

65

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.

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