Frame Relay Virtual Circuits

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Frame Relay provides connection-oriented data link layer communication. This means that a defined communication exists between each pair of devices and that these connections are associated with a connection identifier. This service is implemented by using a Frame Relay virtual circuit, which is a logical connection created between two data terminal equipment (DTE) devices across a Frame Relay packet-switched network (PSN).

Virtual circuits provide a bidirectional communications path from one DTE device to another and are uniquely identified by a data-link connection identifier (DLCI). A number of virtual circuits can be multiplexed into a single physical circuit for transmission across the network. This capability often can reduce the equipment and network complexity required to connect multiple DTE devices.

A virtual circuit can pass through any number of intermediate DCE devices (switches) located within the Frame Relay PSN.

Frame Relay virtual circuits fall into two categories:

• Switched virtual circuits (SVCs)

• Permanent virtual circuits (PVCs)

Switched Virtual Circuits

Switched virtual circuits (SVCs) are temporary connections used in situations requiring only sporadic data transfer between DTE devices across the Frame Relay network. A communication session across an SVC consists of four operational states:

• Call setup The virtual circuit between two Frame Relay DTE devices is established.

• Data transfer Data is transmitted between the DTE devices over the virtual circuit.

• Idle The connection between DTE devices is still active, but no data is transferred. If an SVC remains in an idle state for a defined period of time, the call can be terminated .

• Call termination The virtual circuit between DTE devices is terminated.

After the virtual circuit is terminated, the DTE devices must establish a new SVC if there is additional data to be exchanged. It is expected that SVCs will be established, maintained , and terminated using the same signaling protocols used in ISDN. Few manufacturers of Frame Relay DCE equipment, however, support SVCs. Therefore, their actual deployment is minimal in today's Frame Relay networks.

Permanent Virtual Circuits

Permanent virtual circuits (PVCs) are permanently established connections that are used for frequent and consistent data transfers between DTE devices across the Frame Relay network. Communication across a PVC does not require the call setup and termination states that are used with SVCs. PVCs always operate in one of the following two operational states:

• Data transfer Data is transmitted between the DTE devices over the virtual circuit.

• Idle The connection between DTE devices is active, but no data is transferred. Unlike SVCs, PVCs are not terminated under any circumstances if they are in an idle state. DTE devices can begin transferring data whenever they are ready because the circuit is permanently established.

Data-Link Connection Identifier

Frame Relay virtual circuits are identified by data-link connection identifiers (DLCIs). The Frame Relay service provider (for example, the telephone company) typically assigns DLCI values. Frame Relay DLCIs have local significance, which means that the values themselves are not unique in the Frame Relay WAN. Two DTE devices connected by a virtual circuit, for example, may use a different DLCI value to refer to the same connection. Figure B-2 illustrates how a single virtual circuit can be assigned a different DLCI value on each end of the connection.

Figure B-2. A Single Frame Relay Virtual Circuit Can Be Assigned Different DLCIs on Each End of a VC

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