Multiplexing in Circuit-Switched Networks

Multiplexing in Circuit-Switched Networks

A circuit in a link is implemented with either frequency-division multiplexing (FDM) or time-division multiplexing (TDM). With FDM, the frequency spectrum of a link is divided up among the connections established across the link. Particularly, the link dedicates a frequency band to each connection for the duration of the connection. In telephone networks, this frequency band normally has a width of 4 kHz (that is, 4,000 hertz or 4,000 cycles per second).

A simple circuit-switched network consisting of four switches and four links

The width of the band is called, not surprisingly, the bandwidth. FM radio stations also use FDM to share the frequency spectrum between 88 MHz and 108 MHz, with each station being assigned a specific frequency band.

For a TDM link, time is divided into frames of fixed duration, and each frame is divided into a fixed number of time slots. When the network establishes a connection across a link, the network dedicates one time slot in every frame to this connection. These slots are dedicated for the sole use of that connection, with one time slot available for use (in every frame) to broadcast the connection's data.

The following figure demonstrates FDM and TDM for a particular network link supporting up to four circuits. For FDM, the frequency domain is segmented into four bands, each of bandwidth 4 kHz. For TDM, the time domain is segmented into frames, with four time slots in each frame; each circuit is assigned the same dedicated slot in the revolving TDM frames. For TDM, the transmission rate of a circuit is equal to the frame rate multiplied by the number of bits in a slot. For instance, if the link transmits 8,000 frames per second and each slot consists of 8 bits, then the transmission rate of a circuit is 64 kbps.

Advocates of packet switching have always argued that circuit switching is wasteful because the dedicated circuits are idle during silent periods. For instance, when one person in a telephone call stops talking, the idle network resources (frequency bands or time slots in the links along the connection's route) cannot be used by other ongoing connections. As another example of how these resources can be underutilized, think about a radiologist who uses a circuit-switched network to remotely access a series of x-rays.

With FDM, each circuit continuously gets a fraction of the bandwidth.

The radiologist establishes a connection, requests an image, studies the image, and then requests a new image. Network resources are distributed to the connection but are not used (i.e., are wasted) during the radiologist's inspection periods. Supporters of packet switching also enjoy pointing out that establishing end-to-end circuits and reserving end-to-end bandwidth is complex and requires complicated signaling software to coordinate the operation of the switches along the end-to-end path.

Before we finish our discussion of circuit switching, let's work through a numerical example that should shed further insight on the topic. Let us suppose how long it takes to send a file of 640,000 bits from Host A to Host B over a circuit-switched network. Suppose that all links in the network use TDM with 24 slots and have a bit rate of 1.536 Mbps. Also suppose that it takes 500 msec to set up an end-to-end circuit before Host A can begin to transmit the file. How long does it take to send the file? Each circuit has a transmission rate of (1.536 Mbps)/24 = 64 kbps, so it takes (640,000 bits)/(64 kbps) = 10 seconds to transmit the file. To this 10 seconds we add the circuit establishment time, giving 10.5 seconds to send the file. Note that the transmission time is independent of the number of links: The transmission time would be 10 seconds if the end-to-end circuit passed through one link or a hundred links. (The actual end-to-end delay also includes a propagation delay: see "Delay, Loss and Throughput in Packet-Switched Networks")


transmission rate, packet switching, circuit switching

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