Frame Relay -- Summary

Network Consultants Handbook - Frame Relay
by Matthew Castelli

Frame Relay is a Layer 2 (data link) wide-area network (WAN) protocol that works at both Layer 1 (physical) and Layer 2 (data link) of the OSI model. Although Frame Relay services were initially designed to operate over ISDN service, the more common deployment today involves dedicated access to WAN resources.

Frame Relay networks are typically deployed as a cost-effective replacement for point-to-point private line, or leased line, services. Whereas point-to-point customers incur a monthly fee for local access and long-haul connections, Frame Relay customers incur the same monthly fee for local access, but only a fraction of the long-haul connection fee associated with point-to-point private line services.

Frame Relay was standardized by two standards bodies -- internationally by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and domestically by ANSI (American National Standards Institute).

Frame Relay is a packet-switched technology, meaning that each network end user, or end node, will share backbone network resources, such as bandwidth. Connectivity between these end nodes is accomplished with the use of Frame Relay virtual circuits (VCs).

Frame Relay WAN service primarily comprises four functional components:

• Customer premise Frame Relay access device (FRAD).
• Local access loop to the service provider network.
• Frame Relay switch access port. Link Management Interface parameters are defined here.
• Frame Relay VC parameters to each end site.

Frame Relay is a connection-oriented service, operating the data link layer (Layer 2) of the OSI model. A data-link connection identifier (DLCI) is used to identify this dedicated communication path between two end nodes. This path, or VC, is a bidirectional logical connection across the WAN between two end node DTE devices.

DLCIs are of local significance, unless an agreement has been made with the network service provider to deploy global DLCIs. Local significance means that DLCIs are of use only to the local Frame Relay network device. Frame Relay DLCIs are analogous to an organizations telephone network utilizing speed-dial functions.

Two types of Frame Relay VCs exist:

• Permanent virtual circuits (PVCs) -- These are permanently established, requiring no call setup, and utilize DLCIs for endpoint addressing.

• Switched virtual circuits (SVCs) -- These are established as needed, requiring call setup procedures and utilizing X.121 or E.164 addresses for endpoint addressing.