Data Link Connection Identifier DLCI

A data-link connection identifier (DLCI) identifies the Frame Relay PVC. Frames are routed through one or more virtual circuits identified by DLCIs. Each DLCI has a permanently configured switching path to a certain destination. Thus, by having a system with several DLCIs configured, you can communicate simultaneously with several different sites. The User-Network Interface (UNI) provides the demarcation between the FRAD and the Frame Relay network. The combination of the UNI and the DLCI specifies the endpoint for a particular virtual circuit. The DLCI has local significance and the numbering is usually decided by the user and assigned by the Frame Relay Service Provider. The customer assigned DLCI numbers are usually in the range of 1 <= DLCI <= 1022.

Frame Relay PVCs are extremely popular. Most enterprise circuit migrations usually take place from leased lines to Frame Relay PVCs. Other forms of Frame Relay virtual circuits provisioned by carriers include SVCs and soft PVCs. Refer to Figure 2-6. The bandwidth of the local loop access line, which connects the FRAD to the Frame Relay network, is also called port speed. Frame Relay services can be offered from subrate fractional T1 up to port speeds of n x DS1. The carrier's choice of Frame Relay point of presence (PoP) equipment usually influences the maximum port speed that can be offered to the customer. The Cisco MGX 8220 concentrator can support Frame Relay up to 16 Mbps using an HSSI-based port.

Figure 2-6. Frame Relay Virtual Circuits from a Customer Perspective

Figure 2-6. Frame Relay Virtual Circuits from a Customer Perspective

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