One of the major technological innovations initiated by pioneers of the Internet was the development of alternatives to the traditional switched network model first used by telephone switchboard operators at the turn of the 19th century. Even though some communications networks, including many of the cell phone networks, still follow the switched model, computer networks such as the Internet are based on a more complex but bandwidth-sparing packet model. The major disadvantage of the much simpler switching model is that it can't provide more communications channels than there are switches.
As shown in Figure 3-5, in a switched communications network, once a connection is established, it monopolizes the circuit until the switch is released, even though the data has been transferred from source to destination and the connection may remain idle. As such, the switching circuit in the figure is capable of supporting only six simultaneous bi-directional communications channels. That is, the communications path through the network is identical, regardless of which party originates the message.
Figure 3-5 Switched Communications. In switched communications networks, a fixed, continuous bi-directional connection is established between the message source and recipient.
Packet communications makes use of the pauses and breaks in typical communications allowing a single physical communications circuit to establish multiple, virtual channels. In the packet paradigm, messages are parsed into small segments and packaged into labeled packets by the message disassembler and packet generator (see Figure 3-6). These packets travel via various routes through the network to the destination,
Figure 3-6 Packet Communications. Multiple, virtual communications channels are established by breaking up messages into small packets and reassembling them at the destination.
At the destination, the data packets are captured by the packet organizer and re-assembled in their original order by the message assembler. Even though the recipient is capable of receiving information at any time, the communications channel is capable of carrying packets to and from other subscribers. That is, the recipient can receive messages at any time without blocking the use of the communications channel for other communications. This is in contrast with switched communications, which holds the channel captive until the subscriber releases it, even if no data are being transferred. Even though packet communications uses separate channels for two-way communications, it is nonetheless several orders of magnitude more bandwidth-efficient than switched communications. This is especially true with intermittent communications with relatively long periods of idle time, such as sending and receiving e-mail or reading content on the Web.