|  | 2.5 | WAN Link Options
| 2.5.5 | Frame Relay

Frame Relay was designed to be used over high-speed, high quality digital facilities. As a result, Frame Relay does not offer much error checking or reliability, but expects upper-layer protocols to attend to these issues. Frame Relay is a packet-switching data communications technology that can connect multiple network devices on a multipoint WAN, as shown in the Figure . The design of Frame Relay WANs can affect certain aspects (such as split horizon) of higher-layer protocols such as IP, IPX, and Apple-Talk. Frame Relay is called a non-broadcast multi-access technology because it has no broadcast channel. Broadcasts are transmitted through Frame Relay by sending packets to all network destinations Frame Relay defines the connection between a customer DTE and a carrier DCE. The DTE is typically a router, and the DCE is a Frame Relay switch. (In this case, DTE and DCE refer to the data link layer, not the physical layer.) Frame Relay access is typically at 56 kbps, 64 kbps, or 1.544 Mbps. Frame Relay is a cost-effective alternative to point-to-point WAN designs. Each site can be connected to every other by a virtual circuit. Each router needs only one physical interface to the carrier. Frame Relay is implemented mostly as a carrier-provided service but can also be used for private networks. Frame Relay service is offered through a PVC. A PVC is an unreliable data link. A data-link connection identifier (DLCI) identifies a PVC. The DLCI number is a local identifier between the DTE and the DCE that identifies the logical circuit between the source and destination devices. The Service Level Agreement (SLA) specifies the committed information rate (CIR) provided by the carrier, which is the rate, in bits per second, at which the Frame Relay switch agrees to transfer data. (These topics are covered in depth in the, "Frame Relay" chapter) Two common topologies can be used in a Frame Relay solution: Fully meshed topology -- Every Frame Relay network device has a PVC to every other device on the multipoint WAN. Any update sent by one device is seen by every other. If this design is used, the entire Frame Relay WAN can be treated as one data link. Partially meshed topology --This is also often called a star topology or hub-and-spokes topology. In a partially meshed topology, not every device on the Frame Relay cloud has a PVC to every other device.