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Circuit-Switching Services

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Circuit switching, as opposed to packet switching, sets up a dedicated communication channel between two end systems. Voice calls on the telephone networks are the best example. A phone connects to a local telephone switching center via twisted-pair cables. If the connection is between two phones in the same area, the local switch creates a connection between the circuits from each phone. This is pictured as connection A1-A2 in Figure 1.

Figure 1

If the connection is between phones in two different areas, a circuit is set up through an intermediate exchange, as shown by circuit C1-C2. Long-distance circuits are made through remote switching offices, as shown by circuit B1-B2. When telephone circuits are used for data, they provide private links with guaranteed bandwidth.

A dedicated circuit is a circuit that is switched on once and stays on. You lease the line from the local carrier or combination of local and long-distance carriers. Circuit services are often called TDM services because they use TDM (time division multiplexing). Data is transmitted in time slots that can guarantee bandwidth for time sensitive voice and video, assuming enough bandwidth is allocated in the circuit.

The advantage of switched circuit is that you can connect to a variety of preconfigured locations rather than a single location. This type of service is good for backing up and replicating data between remote systems, or for videoconferences. ISDN is an example of a circuit-switching service that is practical for home and small office users.

An alternative to circuit-switched services is packet-switching services. The main characteristic of packet-switching services is that you share a network with many other users and organization and connections can be made to a variety of endpoints. ATM (called cell switching), frame relay, and the Internet use this model. Packet switching is ideal for long-distance any-to-any connectivity. See "Packets and Packet-Switching Networks."

A virtual circuit is a dedicated end-to-end link over a packet- or cell-switched network. A dedicated path is selected through a mesh network for the transmission of a data. There are PVCs (permanent virtual circuits) and SVCs (switched virtual circuits). The former are preprogrammed and the latter are set up "on the fly."

While everyone has been advocating the advantages of packet-switched networks such as the Internet for metro and WAN connections, interesting developments are taking place in optical networking. As the bandwidth and capabilities of fiber-optic networks improve with DWDM (dense wavelength division multiplexing) and optical switching, the concept of "switched lambda circuits" has emerged. A lambda is an individual wavelength of light, and hundreds or thousands of lambdas can occupy a single fiber cable. DWDM makes so many wavelengths available that service providers can lease private lambda circuits to organizations on a switched or permanent basis. This circuit can stretch across the service provider's entire network and provide an economical way to link remote offices with very high bandwidth links. See "Optical Networks."

According to George Gilder (Gilder Technology Report, April 2000), "In a world of bandwidth abundance, bandwidth-wasting circuits become ideal once again. Rather than economizing on bandwidth by chopping everything into packets and multiplexing them into time slots, the mandate is to waste bandwidth. As in the old telephone system, the best approach is circuits."

DTM (Dynamic Synchronous Transfer Mode) is a form of circuit switching for fiber-optic networks that employs TDM (time division multiplexing) in a new way that dynamically reallocates available bandwidth to users that need it. DTM was designed to remove the bottleneck at fiber network access points. These bottlenecks are typically caused by the need to process and buffer data large amounts packet-based data. DTM seeks to limit complexity and use transmission capacity more efficiently. In particular, DTM can fully support high bit-rate real-time traffic and multicasting, and when used as a link layer for IP networks, can support strict QoS. See "DTM (Dynamic Synchronous Transfer Mode)."

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.