Frame Relay was originally designed for use across Integrated Services Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well.
Frame Relay Features :
Frames Relay provides a data communications interface between user devices and network devices. This interface forms the basis for communication between user devices across a WAN.
Typical communication speeds for Frame Relay are between 56 Kbps and 2 Mbps (although lower and higher speeds are supported).
Frame Relay is considerably more efficient than X.25, the protocol for which it is often considered a replacement. Because it supports technological advances such as fibre-optic cabling and digital transmission, Frame Relay can eliminate time-consuming processes (such as error correction and flow control) that are necessary when using older, less reliable WAN media and protocols.
Frame Relay Standardisation
Internationally, Frame Relay was standardised by the International Telecommunications Union - Telecommunications Sector (ITU-T). In the United States, Frame Relay is an American National Standards Institute (ANSI) standard.
The Local Management Interface (LMI) specification, developed in 1990, further extends the functionality of Frame Relay.
Frame Relay Devices
Devices attached to a Frame Relay WAN fall into two general categories:
Data terminal equipment (DTE) -- DTE are customer-owned end node and internetworking devices. Examples of DTE devices are terminals, personal computers, routers, and bridges.
Data circuit-terminating equipment (DCE) -- DCE are carrier-owned internetworking devices. In most cases, these are packet switches (although routers or other devices can be configured as DCE as well).
DTE and DCE devices are logical entities. That is, DTE devices initiate a communications exchange, and DCE devices respond.
The following figure shows the relationship between the two categories of devices:
Frame Relay Virtual Circuits
Frame Relay provides connection-oriented data link layer communication. This service is implemented using virtual circuits.
A Frame Relay virtual circuit is a logical connection created between two data terminal equipment (DTE) devices across a Frame Relay packet-switched network (PSN).
Virtual circuits provide a bi-directional communications path from one DTE device to another. They are uniquely identified by a data link connection identifier (DLCI).
A virtual circuit can pass through any number of intermediate data circuit-terminating equipment (DCE) devices (switches) located within the Frame Relay PSN. A number of virtual circuits can be multiplexed into a single physical circuit for transmission across the network.
Frame Relay virtual circuits fall into two categories:
• Switched virtual circuit (SVC)
• Permanent virtual circuit (PVC)
Frame Relay Switched Virtual Circuits (SVCs)
A switched virtual circuit (SVC) is one of the two types of virtual circuits used in Frame Relay implementations. SVCs are temporary connections that are used when there is 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 -- In this state, the virtual circuit between two Frame Relay DTE devices are established.
Data transfer -- In this state, data is being transmitted between the DTE devices over the virtual circuit.
Idle -- In this state, the connection between DTE devices is still active, but no data is being transferred.
Call termination -- In this state, 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.
Frame Relay Permanent Virtual Circuits (PVCs)
A permanent virtual circuit (PVC) is one of two types of virtual circuits used in Frame Relay implementations. PVCs are permanently established connections that are used when there is frequent and consistent data transfer 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 are always in one of the following two operational states:
Data transfer -- In this state, data is being transmitted between the DTE devices over the virtual circuit.
Idle -- In this state, the connection between DTE devices is active, but no data is being transferred.
DTE devices can begin transferring data whenever they are ready because the circuit is permanently established.
Frame Relay Data Link Connection Identifier (DLCI)
Frame Relay virtual circuits are identified by data link connection identifiers (DLCIs). DLCI values are typically assigned by the Frame Relay service provider (for example, the Telephone Company).
Frame Relay DLCIs have local significance. That is, the values themselves are not unique in the Frame Relay WAN. Two DTE devices connected by a virtual circuit might use a different DLCI value to refer to the same connection.
The following figure shows how a single virtual circuit might be assigned a different DLCI value on each end of the connection:
