voice traffic. Provided the overall traffic volume is within the design conditions, no significant queuing will exist in network buffers. Delay will then be kept within reasonable values, and voice quality will be good. See Figure 7.9. Nevertheless when the total amount of traffic starts to rise, there will be queuing in the network buffers (router buffers in IP) and then the long packets of IP data (more than 15 times longer than voice packets) will be included in the queue causing long delays for the voice traffic. The amount of the overall delay in buffers might then become high enough to be annoying for the people involved in a voice conversation. The main advantage ATM has in terms of QoS compared with IP voice in data networks is derived from the fact that all the packets (cells) in ATM are short (53 bytes) and have the same size. Therefore there are no chances that long packets introduce unacceptable delays in the VoATM traffic. Switch/Router Processor Output transmission queue queue Frame arrivals Data frame Data frame Data frame Data frame LAN/WAN connection WAN connection In this example there are 2 single To analyse the queuing delays and server queues in the switch/router queue sizes it is necessary to describe: the arrival process The processor is the server for the the serving process processor queue - the serialization process at the output. The transmission line connecting the switch/router to the WAN is the server for the output transmission queue Figure 7.8 Critical processes in a packet-handling device Quality of Service Sent signal Continuous data stream = 125 us Cell 1 Cell 2 Cell 3 transmission Cell 1 Cell 2 Cell 3 Buffer Cell 1 Cell 2 Cell 3 Attot = 125 us Received signal (Total delay) Figure 7.9 Packetization delay when converting PCM to ATM Coder Queuing Dejitter buffer delay delay Sydney Stockholm Propagation delay Leased transmission Serialization delay Coaxial cable about 0.005 ms/km Fibre cable about 0.005 ms/km Satellite hop about 240 ms Dejitter buffer could be around 50 ms for a lo