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Frame Relay

Frame Relay

Frame Relay is one of the predominate WAN transport methods for connecting remote sites because its cost is lower and its scaling more effective than those of leased lines. Whenever two or more locations must have data connectivity, Frame Relay is an option. It offers known performance and manageability and is the most common mode of private network connectivity.

Frame Relay is a connection-oriented Layer 2 protocol that lets you multiplex several data connections (referred to as virtual circuits) onto a single physical link. The higher-layer protocols often perform error correction and flow control, so the primary consideration for Frame Relay is establishing connections between customer equipment.

A connection identifier maps packets to outbound ports on the service provider’s switch. When the switch receives a frame, it uses a lookup table to map the frame to the correct outbound port. The switch determines the entire path to the destination before it sends the frame.

Frame Relay only specifies the connection between a router and a service provider’s local access switching equipment. It does not specify the data transmission within the provider’s Frame Relay cloud.

Frame Relay Equipment
The two general categories of Frame Relay equipment are data terminal equipment (DTE) and data communications equipment (DCE).

DTE is the terminating equipment used by companies or organizations using Frame Relay connections. Typically located on the customer’s premises, DTE can be owned by the customer or rented by the Frame Relay provider. Examples of DTE devices are terminals, personal computers, routers, and bridges.

DCEs are carrier-owned internetworking devices. DCEs provide clocking and switching services in a network.

Virtual Circuits
Logical connections called virtual circuits establish Frame Relay connections. Virtual circuits can pass through several DCE devices throughout the Frame Relay packet-switched network (PSN). You can multiplex several virtual circuits into a single physical circuit for transmission across the network. The two types of virtual circuits are switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

Switched Virtual Circuit
A switched virtual circuit (SVC) is a temporary connection for sporadic data transfer between DTE devices across the Frame Relay network. SVC sessions have four distinct operational states: call setup, data transfer, idle, and call termination. If the connection is idle for some predetermined amount of time, the network terminates the connection. After the connection is terminated, the network must establish a new call for data to flow again.

PVC
A permanent virtual circuit (PVC) is an established connection that remains up at all times. You should use PVCs for frequent and consistent data transfer between DTE devices. PVCs require no call setups or termination procedures. The operational states are data transfer and idle.

Local Management Interface
Local Management Interface (LMI) is a signaling standard for managing a connection between a router and a Frame Relay switch. LMIs track and manage keepalive mechanisms, multicast messages, and status.

Data-Link Connection Identifier (DLCI)
Frame Relay virtual circuits are identified by data-link connection identifiers (DLCIs). Typically, the Frame Relay service provider assigns DLCI values. Frame Relay DLCIs are of local significance only. In other words, the DLCI values are only unique at the endpoints, not over the WAN. Therefore, two DTE devices connected by a virtual circuit might use different DLCI values to refer to the same connection. In addition, two DTEs can connect on the same virtual circuit but still have different DLCIs. This figure shows how a single virtual circuit might be assigned a different DLCI value on each end of the connection.

Congestion Notification
Frame Relay reduces network overhead by implementing simple congestion-notification mechanisms. Frame Relay uses two methods of congestion notification, forward explicit congestion notification (FECN) and backward explicit congestion notification (BECN).

FECN sends a message to the destination device when a Frame Relay switch senses congestion in the network. A DTE device receiving this message can relay this information to a higher-layer protocol for processing, which in turn can initiate flow control or simply ignore the message.

BECN sends a message to the source router when a Frame Relay switch senses congestion in the network. A BECN message requests a reduced data-transmission rate. BECN messages are also relayed to the higher-layer protocols, which can initiate some form of flow control or traffic shaping. In some cases, the higher-layer protocols ignore BECN messages.




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