We saw in Figure 1.6 how a circuit switch reserves a path for a connection when a call is set up and releases that bandwidth and switch capacity when the call clears. Each call requires a specific amount of each resource. It is relatively easy to calculate the fixed number of calls that can exist on each part of the network at one time. That is, a transmission link contains a fixed number if DS-0 channels. The number of simultaneous backplane paths in a voice switch may limit the number of calls it can set up to far fewer than half the ports on the switch. When a switch reaches its capacity, the next call must get a busy signal.
Packet-switched networks operate very differently. Resources are shared across all users. Some users, or rather their packets, may get higher priority or be allowed to occupy more resources than others, but bandwidth, memory, and CPU power are shared.
Rather than dedicate resources like bandwidth to a path or connection, a packet switch creates routing and forwarding tables that control where to send packets after they arrive (Figure 4.1). Regardless of the packet format, each switch or router answers the same questions about every packet it receives: