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The TCP/IP Guide  9  Networking Fundamentals       9  Fundamental Network Characteristics

Circuit Switching and Packet Switching Networks 1 2 3 Messages: Packets, Frames, Datagrams and Cells

The Relationship Between Connection Orientation and Circuits

You can probably immediately see the relationship between the concepts of circuits and connections. Obviously, in order to establish a circuit between two devices, they must also be connected. For this reason, circuit-switched networks are inherently based on connections. This has led to the terms “circuit-switched” and “connection-oriented” being used interchangeably.

However, this is an oversimplification that results due to a common logical fallacy—people make the mistake of thinking that if A implies B, then B implies A, which is like saying that since all apples are fruit, then all fruit are apples. A connection is needed for a circuit, but a circuit is not a prerequisite for a connection. There are, therefore, protocols that are connection-oriented, while not being predicated on the use of circuit-based networks at all.

These connection-oriented protocols are important because they enable the implementation of applications that require connections, over packet-switched networks that have no inherent sense of a connection. For example, to use the TCP/IP File Transfer Protocol, you want to be able to connect to a server, enter a login and password, and then execute commands to change directories, send or retrieve files, and so on. This requires the establishment of a connection over which commands, replies and data can be passed. Similarly, the Telnet Protocol obviously involves establishing a connection—it lets you remotely use another machine. Yet, both of these work (indirectly) over the IP protocol, which is based on the use of packets, through the principle of layering.

To comprehend the way this works, one must have a basic understanding of the layered nature of modern networking architecture (as I discuss in some detail in the chapter that talks about the OSI Reference Model). Even though packets may be used at lower layers for the mechanics of sending data, a higher-layer protocol can create logical connections through the use of messages sent in those packets.

Circuit Switching and Packet Switching Networks 1 2 3 Messages: Packets, Frames, Datagrams and Cells