 a circuit is not immediately available, and the length of time it has to wait to be processed. This queuing process however happens at every single node through which the packet passes (e.g. Figure 8.2). The total delay is the sum of the delay experienced by the packet as it traverses the packet-switched network. The metrics of interest are therefore the overall delay, and the percentage of packets which will wait more than a certain length of time. As a packet traverses through network nodes, each nodes takes a finite length of time to 'process' the packet. This processing time is mainly for a switch or a router to open the appro- priate address field and determine to which port the packet is to be sent. A received packet at a router for example is stored in a buffer, the IP header is read and then the packet is copied to a port buffer as determined by the routing table. The outgoing buffer is then emptied gradually depending on the speed of the outgoing link. For example a 1 megabit per second (mbps) link Broadband Telecommunications Technologies and Management requires 10 ms to transmit a 10 000 bit (1250 bytes) packet. If 1 ms is necessary for the router to read the IP header and copy the packet form input to the output buffer, then a total of 11 ms is necessary to process this packet. The processing time can be reduced if more servers and more/faster outgoing links can be installed. If a new packet arrives while an earlier packet is being processed, it is stored in the input buffer until the router is free. If more servers are available, then the average 'waiting time' for packet processing can be decreased. The overall service time is therefore a function of servers' processing time and outgoing links. The probability that a packet may be queued is calculated from the Erlang-C formula: Ex! n!n-a where a is the offered traffic in erlangs and n is the number of servers. From this formula several parameters such as average waiting time, required length of buffer, probability of buffer overflow and SO on can b