BELGACOM Forum 523, Mounting And Commissioning Manual

Page: 112 / 244

Fundamentals 110
Voice over IP (VoIP)
Note: Measurements made with “ping” are round-trip propaga-
tion delays. Divide the maximum value displayed by two.
Optimisation
If you detect a large fluctuation in the propagation delay during
measurement, this may also cause the voice quality to deterio-
rate. 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 eth-
ernet packets received on all connected lines. This can cause sub-
stantial collisions and result in a high fluctuation in the propaga-
tion delay.
If this is the case, use a modern switch component. Selective for-
warding of ethernet packets (“Layer 2 switching”) avoids colli-
sions. 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 “Mini-
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