Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

In the world of computer networking, the term "circuit" is used in many different contexts. A circuit is a communication channel between two devices.

• Physical circuit    A wire that carries a single channel such as a telephone call, or multiplexed circuits as described here.


• FDM (frequency division multiplexing)    In this scheme, circuits are created by dividing available bandwidth into separate carrier frequencies and then modulating data onto the carriers.


• WDM (wave division multiplexing)    This is basically frequency division multiplexing with light, done on fiber-optic cables and open-air laser transmissions. Think of a "color" of light as being a circuit. See "WDM (Wavelength Division Multiplexing)" for more information.


• TDM (time division multiplexing)    In this scheme, circuits are created by dividing a signal stream into multiple time slots and allocating a regular repeating time slot to each user's transmission.


• CDM (code division multiplexing)    In this scheme, all transmissions are sent across one large channel, spread out across the bandwidth using spread spectrum technology. Each user's transmission is broken up into small segments that are coded, transmitted, and then reassembled at the receiving end. Refer to "CDMA (Code Division Multiple Access)" and "Spread Spectrum Signaling."


• Virtual circuits    This type of circuit exists in a mesh network in which cells, frames, or packets are transmitted among nodes in a network. A virtual circuit is a path between end systems through the mesh network.

The analog cellular telephone network is a circuit-switched FDM cellular system. When you make a call, you get a dedicated frequency for transmitting and another dedicated frequency for receiving. These basically act like hardwired dedicated circuits between caller and callee. Digital cellular systems extend the analog system with TDM or CDM technologies and digitizing voice.

Virtual circuits are common in WANs (wide area networks) and internal networks that use switches in the workgroup area and the backbone. Basically, a virtual circuit appears to the end systems as a dedicated wire for transmitting information across a communication system. However, the underlying network consists of a path through one or more switches that is either permanent or switched, as described under "Virtual Circuit."

Circuits are also defined in higher layers of the protocol stack. The virtual circuit just described is a connection for transferring a variety of traffic between two points, usually over a layer 2 switched network. In contrast, a TCP connection-oriented circuit is established between software applications across a packet switched network. This connection allows both systems to manage the exchange of packets, recover lost packets, and actively control the packet exchange rate so that one system does not overwhelm the other or the network. See "Transport Protocols and Services" and "Reliable Data Delivery Services."

Higher up in the protocol stack, client/server applications may use RPC request/response mechanisms to exchange information as if there were a "communication circuit" between the systems. The opposite approach is to use a message passing, store-and-forward approach in which two systems exchange information by putting it in a message and forwarding the message to the other system. This is like two people exchanging e-mail.

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