. A packet is, in essence, a store-and-forward mechanism for transmitting infor- mation. Packets are forwarded through a series of packet switches, also known as routers, that ultimately lead to the destination. A packet header contains two very important pieces of information: the destination address and the sequence number. The original forms of packet switching (developed in the late 1960s and early 1970s) were connectionless infrastructures. In a connectionless environment, each packet is routed individually, and the packets might not all take the same path to the destination point, and hence they may arrive out of sequence. Therefore, the Chapter 3 Establishing Communications Channels sequence number is very important; the terminating point needs it to be able to reassemble the message in its proper order. Generally, in packet switching, packets from many different sources are statisti- cally multiplexed and sent on to their destinations over virtual circuits. Multiple connections share transmission lines, which means the packet switches or routers must do many more routing calculations. Figure 3.5 illustrates a packet-switched network that uses virtual circuits. Packets are queued up at the various nodes, based on availability of the virtual circuits, and this queuing can impose delays. The first generation of packet-switched networks could support only data, not voice or video, because there was so much delay associated with those networks. As packet-switched environments evolve, we are developing techniques for sepa- rating and prioritizing the various traffic types. (Chapter 10 talks about these issues in depth.) Computer Packet Switch Packet Switch Virtual Circuit Packet Switch Input Ports Output Ports Packet switches examine packet header and check destination Retransmission against a routing table requested (e.g., TCP) Packets queued up Packets can be for subsequent transmission; lost due to congestion; introduces latency causes packet loss Figure 3.5 A packet-switched network A Transportation Analogy