Physical circuits can be classified into two broad categories:

An exclusive use physical circuit is one where a computer and attached devices have the exclusive use of the connection media. This is shown on the Exclusive Use Physical Circuit Diagram.

A shared use physical circuit is one where multiple devices share the same physical media. Access to the shared media is dependent on the media access protocol that is used (such as Token Ring or Ethernet). The Shared Use Physical Circuit Diagram illustrates this arrangement, also referred to as a bus.

A virtual circuit is a communications path that appears to be a single circuit even though the data often travels along various physical circuits between the source and destination nodes. This concept has its roots in X.25 packet switching. The Virtual Circuit Diagram illustrates this concept.

There are two types of virtual circuits, PVCs and SVCs:

What is the relationship between virtual circuits and physical circuits?

   PVCs and SVCs

The choice of whether to use a PVC or a SVC varies between networks, depending on traffic volumes, traffic patterns, degree of connectivity, types of applications, and other parameters. Customers can mix PVCs and SVCs to meet their changing requirements.

The primary difference between a PVC and SVC is the length of time when the connections are defined and the resources allocated. PVCs are typically provisioned (configured) by the network operator, whether the operator be the carrier (public services) or a management information systems (MIS) staff member (private networks). Once the PVC is provisioned, the connection is available for use at all times unless there is a change in the service or in the event of a service outage. An SVC is established at the time the connection is required because there is data to transfer. Prior to each use, an SVC is established to the destination end user. The connection is terminated after each use.

SVC applications are ideal for networks that have the following characteristics:

Highly meshed connectivity refers to large networks that need any-to-any connectivity. In a nonhierarchical networking environment where there is a need to communicate with many locations, SVCs can offer a viable solution. The advantages of SVCs are magnified as the number of locations and degree of connectivity requirements increase. Highly meshed networks are becoming more common as more companies deploy intranets. It is conceivable that many users will have their own World Wide Web (Web) pages within the organization. This will increase the amount of peer-to-peer intracompany traffic. Additionally, it can offer a cost-effective solution for occasional intercompany connections to suppliers, partners, and even
customers--a private extranet.

A network that has highly intermittent applications most often translates to traffic that is unpredictable and short in duration. Because SVCs only consume network bandwidth when there is information to send, it is a good solution for short-duration applications.

Some small office and telecommuter locations may not be able to initially cost justify PVCs to all locations to which they need connectivity because of low traffic volumes and intermittent usage. These locations can start with SVCs and gradually migrate to PVCs as traffic volumes increase. A hybrid PVC/SVC solution can be implemented as well. PVCs are often established between locations that require frequent exchange of information, whereas SVCs are established to locations that only need occasional interaction.

SVC Information Transfer

The transfer of information, across an SVC consists of three primary events:

The SVC Sequence Diagram illustrates an SVC operation.

When information needs to be sent across an SVC, a setup message is sent to the destination network. Once the setup phase is complete and the connection has been established, information can be sent back and forth across the network. The call release phase consists of a disconnect message being sent from the source to the destination. The connection to the network is released at both networks, and a circuit no longer exists between the two networks.

There are two terms that are commonly used with public SVC offerings, User-to-Network Interface (UNI) and the Network-to-Network Interface (NNI). UNI and NNI are specific to a technology, such as frame relay or ATM. The UNI/NNI Diagram below shows the relationship between the network and these interface points. The SVC UNI details how an SVC can be established and released from an end user device. The SVC NNI details how an SVC can be established and released between two or more independent networks. The interconnected networks can be private, public, or both.

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