n address of the packet and one or more forwarding rules. These rules are local: the packet forwarding may not take into consideration what the optimal global forwarding route might be. It only follows a set of rules set by the local administrator on which node a packet may be forwarded to, based on the final address. In general, the rules are sum- marised in a routing table, which lists all addresses and the corresponding set of possible next nodes. A simple packet-switched communication between two computers is illustrated in Figure 8.2. Packets are forwarded at each router and may take different paths before they arrive at the final destination. Broadband Telecommunications Technologies and Management Figure 8.2 Packet-switched data transfer The packet-switched system can be likened to send- ing letters through a postal network where a letter is processed at each distribution office.3 No single post office determines the end-to-end route which a letter should take. Instead each distribution office deter- mines based on a set of local rules to which centre the letter should be forwarded in order that the letter is finally delivered. Packet switching can flexibly assign different forwarding routes based on the traffic situation of the local and regional network. Should a forwarding path be costly, over- loaded or temporarily unavailable alternative routes may be selected, again based on a set of local rules. In the example of a post office, a letter may travel part of its way using a track and part of its way using an aeroplane with a much faster speed. This architecture provides for significant redundancy and robustness in the presence of accidental/planned link disruptions. Packet switching provides significant advantages over circuit switching for intermittent information transmissions. One advantage is a more efficient usage of transmission medium as short and long messages from multiple transmitters are time-multiplexed together on one link. Another is the possibility of using links with different transmiss