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Dedicated, Switched, and Virtual Circuits

Circuits can be provisioned on a dedicated, switched or virtual basis, depending on the nature of the application and the requirements of the user organization. Ultimately, issues of availability and cost-effectiveness determine the specific selection.

Dedicated Circuits

Dedicated circuits involve dedicated physical circuits that directly connect devices (e.g., PBXs and host computers) across a network (Figure 2.1). Such circuits are reserved for the use of a single user organization rather than being available to serve multiple users. Dedicated circuits offer the user the advantage of a high degree of availability and specified levels of capacity and quality; dedicated circuits can be specially conditioned to deliver specific levels of performance, whereas switched circuits cannot. Additionally, dedicated circuit costs are not usage-sensitive; that is, they can be used continuously and to their full capacity without additional costs. However, the reservation of a circuit for a specific customer has a deleterious effect on the network provider (carrier), as it is no longer available for use in support of the traffic of other users. Therefore, dedicated circuits tend to be rather expensive, with cost being sensitive to distance and capacity. Additionally, the process of determining the correct number, capacity, and points of termination of such circuits can be a difficult and lengthy design and configuration process; in response to the customer request, long lead times are often required for the carrier(s) to provision such a circuit. Finally, as dedicated circuits are susceptible to disruption, backup circuits are often required to ensure effective communications in the event of catastrophic failure or performance degradation. Traditionally, dedicated digital circuits have been used to connect large data centers that communicate intensively; similarly, many large end user organizations with multiple locations have used dedicated circuits to tie together multiple PBXs. In both cases, the advantages of assured availability, capacity and quality often outweigh considerations of configuration difficulty and risk of circuit failure.


Figure 2.1  Dedicated circuits between New York host computer and Seattle terminals.

Switched Circuits

Switched circuits are connected on a flexible basis through a circuit switch, such as a customer-owned PBX or a telephone company Central Office exchange, as illustrated in Figure 2.2. The switch serves as a concentration and contention device; therefore switched circuits are available to be shared, on demand, among multiple subscribers and applications, as required and as available. As a result, the network providers clearly realize significant operational efficiencies. The end users realize the advantages of flexibility and redundancy because the network can generally provide a connection between any two physical locations through multiple alternate routes.


Figure 2.2  Switched circuits between single-line sets through a Central Office switch.

In the voice world, all local, regional, and national networks are interconnected; however, a similar high level of interconnection typically is not provided in the data world. The costs of using switched circuits is sensitive to factors such as the distance between originating and terminating locations, duration of the communication, time of day (prime time vs. nonprime time), and day of the year (business day vs. weekend or holiday). Yet switched circuits offer great advantage where calls are of short duration, where specific locations communicate relatively infrequently, where network redundancy is important, and where network traffic cannot support the cost of dedicated circuits. By way of example, most voice calls are carried over switched circuits.

Virtual Circuits

Virtual circuits are logical, as opposed to physical, circuits, with virtual circuit connectivity being provided over high capacity, multichannel physical circuits, such as fiber optic cables. Virtual circuits are defined in software and made available as required and as available, with the physical path or circuit being defined and effectively guaranteed, perhaps on a priority basis. A virtual circuit transparently provides the same level of connectivity as a physical circuit; in other words, a virtual circuit provides connectivity as though it were a physical circuit. Such a physical circuit can often support many logical circuits, or logical connections. In the high-capacity, fiber optic backbone carrier networks, dedicated circuits are provided to users on a virtual basis; the capacity and other performance characteristics of the circuit behave as though the circuit were dedicated.

At this point, it is worth pausing to further define and contrast the terms transparent and virtual. Transparent means that a network element (e.g., hardware or software) exists but it appears to the user as though it does not. Virtual means that the network element does not exist but it appears to the user as though it does. In this context, a user would be provided access to a virtual circuit on a transparent basis.

It also is necessary to further define a logical circuit or channel, as opposed to one that is physical. A logical circuit refers to the entire range of network elements (e.g., physical circuits, buffers, switches, and control software) that support or manage a communication between a transmitter and receiver. A physical path may be in the form of copper wire (e.g., twisted pair or coaxial cable), radio (e.g., microwave or satellite), or glass or plastic fiber (fiber optic). In order to establish connectivity between transmitter and receiver, a physical path must be selected for the information transfer [2-1].

Two-Wire versus Four-Wire Circuits

Early telegraphy and telephony communication circuits were metallic one-wire, which proved satisfactory, even for two-way communications. Soon after the invention of the telephone, however, two-wire circuits were found to offer much better performance characteristics, due largely to their improved immunity from electrical interference. Four-wire offers still better performance, but at higher cost. Both two- and four-wire circuits are widely used, although the trend is clearly toward four-wire.


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