Packet-switched networks fundamentally differ from circuit-switched networks. As shown in Figure 3-2, a connection must first be established between two end hosts in order for them to communicate in a circuit-switched network. This can be achieved by i n-band signaling (call_setup) within a circuit—in other words, the end host transmits a set of signals that allows the circuit to be extended, hop-by-hop, through the network. Alternatively, as in the case of the Integrated Services Digital Network (ISDN), the circuits can be established with the assistance of a second "control-plane" network, which is usually a lower-bandwidth, packet-switched network, and carries only the call setup packets. This requirement for a pre-established circuit prior to communication is in contrast to IP's "connectionless" paradigm, in which a host can begin transmitting to any other host on the network at any time.
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