In addition to the common LMI features, several optional LMI extensions are extremely useful in an internetworking environment. The first important optional LMI extension is global addressing. With this extension, the values inserted in the DLCI field of a frame are globally significant addresses of individual end-user devices (for example, routers).

The main functions of LMI process are:
To determine the operational status of the various PVCs that the router knows about
To transmit keepalive packets to ensure that the PVC stays up and does not shut down due to inactivity
To tell the router what PVCs are available

In addition to the basic Frame Relay protocol functions for transferring data, the Frame Relay specification includes LMI extensions that make supporting large, complex internetworks easier. Some LMI extensions are referred to as common and are expected to be implemented by everyone who adopts the specification. Other LMI functions are referred to as optional.
LMI extensions
Virtual circuit status messages (common)
Multicasting (optional)
Global addressing (optional)
Simple flow control (optional)
In addition to the common LMI features, several optional LMI extensions are extremely useful in an internetworking environment. The first important optional LMI extension is global addressing. With this extension, the values inserted in the DLCI field of a frame are globally significant addresses of individual end-user devices (for example, routers).

FRAME FORMAT

The Frame Relay frame format is shown below. The flag fields indicate the beginning and end of the frame. Following the leading flag field are 2 bytes of address information. 10 bits of these 2 bytes make up the actual circuit ID (that is, the DLCI).

Backward Explicit Congestion Notification (FECN)

A bit set in a frame that notifies a DTE that congestion avoidance procedures should be initiated by the receiving device. As shown in Figure when a Frame Relay switch recognizes congestion in the network, it sends a BECN packet to the source router, instructing the router to reduce the rate at which it is sending packets. If the router receives any BECNs during the current time interval, it decreases the transmit rate by 25%.
Discard Eligibility (DE) Indicator
A set bit that indicates the frame may be discarded in preference to other frames if congestion occurs. When the router detects network congestion, the Frame Relay switch will drop packets with the DE bit set first. The DE bit is set on the oversubscribed traffic (that is, the traffic that was received after the CIR was met).

Frame Relay can be used as an interface to either a publicly available carrier-provided service or to a network of privately owned equipment. You deploy a public Frame Relay service by putting Frame Relay switching equipment in the central office of a telecommunications carrier. In this case, users get economic benefits from traffic-sensitive charging rates, and don't have to spend the time and effort to administer and maintain the network equipment and service.

CIR

The CIR is the guaranteed rate, in bits per second, that the service provider commits to providing.
Committed Burst
The maximum number of bits that the switch agrees to transfer during a time interval. (It is noted Bc)
Excess Burst
The maximum number of uncommitted bits that the Frame Relay switch attempts to transfer beyond the CIR. Excess burst is dependent on the service offerings available by the vendor, but is typically limited to the port speed of the local access loop.
Forward Explicit Congestion Notification (FECN)
A bit set in a frame that notifies a DTE that congestion avoidance procedures should be initiated by the receiving device. When a Frame Relay switch recognizes congestion in the network, it sends a FECN packet to the destination device, indicating that congestion has occurred.

access rate

The clock speed (port speed) of the connection (local loop) to the Frame Relay cloud. It is the rate at which data travels into or out of the network.

A DLCI is a number that identifies the end point in a Frame Relay network. This number has significance only to the local network. The Frame Relay switch maps the DLCIs between a pair of routers to create a permanent virtual circuit. It is used to identify logical connections.
Local Management Interface (LMI)
A signaling standard between the customer premises equipment (CPE) device and the Frame Relay switch that is responsible for managing the connection and maintaining status between the devices.

Terms that are used in this chapter to discuss Frame Relay:
Access rate
Data-link connection identifier (DLCI)
Local management interface (LMI)
Committed information rate (CIR)
Committed burst
Excess burst
Forward explicit congestion notification (FECN)
Backward explicit congestion notification (BECN)
Discard eligibility (DE) indicator