Network Sales and Services Handbook (Cisco Press Networking Technology)

Frame Relay was intended for LAN-to-LAN internetworking over the WAN. Frame Relay WANs are deployed as a cost-effective replacement for point-to-point leased line services. Frame Relay WANs are made up of four functional components:

• Access Router or Frame Relay Access Device (FRAD)

• Local access loop to the service provider network

• Frame Relay network switch access port

• LMI parameters are defined here

Frame Relay devices fall into the following two general categories:

• DTEs, which include terminals, personal computers, routers, and bridges

• DCEs, which transmit the data through the network and often are carrier-owned devices (although, increasingly, enterprises are buying their own DCEs and implementing them in their networks)

Frame Relay networks transfer data using one of the following two connection types:

• PVCs are permanent connections between two Frame Relay sites. PVCs use DLCIs for addressing.

• SVCs are temporary connections created for each data transfer and are terminated when the data transfer is complete. SVCs use X.121 addresses for connection addressing.

The DLCI is a value assigned to each virtual circuit and DTE device connection point in the Frame Relay WAN. Two different connections can be assigned the same value within the same Frame Relay WAN, one on each side of the virtual connection.

A set of Frame Relay enhancements called LMI offer a number of features (referred to as extensions) for managing complex internetworks, including the following:

• Global addressing

• Virtual circuit status messages

• Multicasting