Fundamentals
Voice over IP (VoIP)
95
Optimisation
If you detect a large fluctuation in the propagation delay during meas-
urement, 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 523/524 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
100-150 ms
1 level depreci-
ation
Loss 2-3 %
1 level depreci-
ation
Propagation
delay
150-200 ms
2 level depreci-
ation
Loss 3-4 %
2 level depreci-
ation
Propagation
delay
200-300 ms
3 level depreci-
ation
Loss 4-6 %
3 level depreci-
ation
Propagation
delay
> 300 ms
4 level depreci-
ation
Loss > 6 %
4 level depreci-
ation
Packet Propagation Delay and Packet Loss
Value
Quality Level
Value
Quality Level
Summary of Contents for forum 523
Page 1: ...Forum 523 524 Mounting and Commissioning Manual ...
Page 10: ...8 ...
