Voice over IP (VoIP) Fundamentals113Note: Measurements made with “ping” are round-trip prop-agation delays. Divide the maximum value displayed by two.10.2.4 OptimisationIf you detect a large fluctuation in the propagation delay during measurement,this may also cause the voice quality to deteriorate. This may indicate a defectiveor overloaded line caused by bit-error or collision correction resulting fromretransmission 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 allconnected 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 componentsalso evaluate the TOS byte of IP packets, thereby providing the optimal prerequi-sites for VoIP telephony.Note: The OpenCom 510 uses a TOS byte (“Type of Service”)value of 0xB8 for IP packets with VoIP data. This requestsPacket Propagation Delay and Packet LossValue Quality Level Value Quality LevelPropagation delay< 50 msOptimal Loss < 1 % OptimalPropagation delay50-100 ms0.5 leveldepreciationLoss 1-2 % 0.5 leveldepreciationPropagation delay100-150 ms1 leveldepreciationLoss 2-3 % 1 leveldepreciationPropagation delay150-200 ms2 level deprecia-tionLoss 3-4 % 2 leveldepreciationPropagation delay200-300 ms3 level deprecia-tionLoss 4-6 % 3 leveldepreciationPropagation delay> 300 ms4 level deprecia-tionLoss > 6 % 4 leveldepreciation
