|  | 6.1 | Frame Relay Technology
| 6.1.2 | Local access rate, DLCI, LMI, CIR, committed burst, excess burst, FECN, BECN, and DE

Following are some terms that are used in this chapter to discuss Frame Relay: 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. Data-link connection identifier (DLCI) -- As shown in Figure 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. 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. LMIs can include support for a keepalive mechanism, which verifies that data is flowing; a multicast mechanism, which can provide the network server with its local DLCI; multicast addressing, providing a few DLCIs to be used as multicast (multiple destination) addresses and the ability to give DLCIs global (whole Frame Relay network) significance, rather than just local significance (DLCIs used only to the local switch); and a status mechanism, which provides an ongoing status on the DLCIs known to the switch. There are several LMI types, and routers need to be told which LMI type is being used. Three types of LMIs are supported: cisco, ansi, and q933a. Committed information rate (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. Backward explicit congestion notification (BECN) -- 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).
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