 a suitable direction for forwarding a packet. A packet sent from Sydney to Los Angeles may be sent in a different direction compared with a packet destined for London. The routing table maintains the number of nodes a packet needs to pass through before it reaches a certain IP address. To summarise, a receiving node examines a packet's MAC layer header information to determine who the intended recipient is, and therefore to where the packet should be sent. If Packetised Data Communications Server Router Router Router Figure 8.16 A simple routing network the link layer header indicates that the packet is intended for the local area network, a 'switching' function is carried out. If the MAC layer indicates an external gateway address, the packet is forwarded to the gateway. At the gateway, the IP layer header is examined to find the destina- tion IP address. A routing table is then consulted to find to which direction the packet is to be sent. This process is known as 'routing'. Packet Switching Performance Metrics As discussed in Chapter 3, circuit-switched voice communications systems performance is measured using the grade of service metric, which shows the probability that a call may not find a free circuit and may therefore be blocked. This is because the number of concurrent circuit-switched calls is limited by the number of circuits. When all circuits are busy any new incoming call is dropped as no end-to-end physical link can be established. Because packet-switched data communications systems are (relatively) delay insensitive, incoming packets may be stored in a local buffer if all links are in use. The packet may then be placed in a queue and transmitted when the link becomes free. The only restriction is the size of the memory buffer: if an infinite buffer can be installed, no packet may ever be lost. The circuit switching grade of service metric is therefore not applicable to packet-switched systems. The metrics used for packet-switched systems are based on probabilities a packet may be queued, that is