Friday, April 13, 2007

Circuit Switching: What is it? Why do we use it? What is it used for?

Circuit switching is the most common method used to build communication networks in the world. In telecommunications, a circuit-switching network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate. A physical point-to-point path is obtained and dedicated to a single connection between two end-points in the network for the duration of the connection.

Early telephone exchanges are a good example of circuit switching. A caller would have to ask the operator to connect them to the person the caller wanted to reach. This was then done on the same exchange or via an inter-exchange link and another operator. The two parties in the phone call would then be in a physical electrical connection through their telephones for the duration of the call. During that time, no one else can use the physical lines involved, even if no actual communication is taking place in the dedicated circuit, that channel still remains unavailable to other users. Channels that are available for new calls to be set up are said to be idle.

In modern circuit-switched networks, electronic signals pass through several switches before a connection is established. And during a call, no other network traffic can use those switches.

Switched circuits allow data connections that can be initiated when needed and terminated when communication is complete. This works much like a normal telephone line works for voice communication. Integrated Services Digital Network (ISDN) is another good example of circuit switching. When a router has data for a remote site, the switched circuit is initiated with the circuit number of the remote network. In the case of ISDN circuits, the device actually places a call to the telephone number of the remote ISDN circuit. When the two networks are connected and authenticated, they can transfer data.

Circuit switching technology became a necessity for communications equipment that required high quality, real-time data transmission. Circuit switch technology allowed high-speed, low latency, simultaneous connections between mainframes, workstations, servers, and data storage systems.

Since the first days of the telegraph it is possible to multiplex multiple connections over the same physical conductor, Regardless, though, each channel on the multiplexed link was either dedicated to one call at a time, or it was idle between calls. Circuit switching can be relatively inefficient because capacity is wasted on connections, which are set up but are not in continuous use (however momentarily). On the other hand, the connection is immediately available and capacity is guaranteed until the call is disconnected.

I’ll talk about packet switching in the next article, and how it’s seen as a better alternative to circuit switching.