T-Comfort Comfort Pro S, Mounting And Commissioning Manual

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Voice over IP (VoIP) Fundamentals
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.
N
Note: The Comfort Pro S 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 Start
Procedure starting on page 112). Multiple TCP connections are made between an IP telephone and
Comfort Pro S 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
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
Packet Propagation Delay and Packet Loss
Value Quality Level Value Quality Level