Frame Relay
Frame Relay is a connection-oriented, Layer 2 WAN connection technology. It operates at speeds from 56 kpbs to 45 Mbps. It is very flexible and offers a wide array of deployment options. Frame Relay operates by statistically multiplexing multiple data streams over a single physical link. Each data stream is known as a virtual circuit (VC). Frame Relay VCs come in two types: Permanent Virtual Circuits (PVCs) and Switched Virtual Circuits (SVCs).
Each VC is tagged with an identifier to keep it unique. The identifier, known as a Data Link Connection Identifier (DLCI), is determined on a per-leg basis during the transmission. It must be unique and agreed upon by two adjacent Frame Relay devices. As long as the two agree, the value can be any valid number, and the number does not have to be the same end to end. Valid DLCI numbers are 16-1007. For DLCI purposes, 0-15 and 1008-1023 are reserved. The DLCI also defines the logical connection between the Frame Relay (FR) switch and the customer premises equipment (CPE).
Frame Relay devices fall into two possible roles, data terminal equipment (DTE) or data circuitterminating equipment (DCE). The DTE/DCE relationship is a Layer 2 (data link) layer relationship. DTE and DCE relationships are normally electrical. The DTE/DCE relationship at Layer 1 is independent of that at Layer 2.
• DTEs are generally considered to be terminating equipment for a specific network and are located at the customer premises.
• DCEs are carrier-owned internetworking devices. DCE equipment provides clocking and switching services in a network; they are the devices that actually transmit data through the WAN. In most cases, the devices are packet switches.
Local Management Interface (LMI) is the means by which Frame Relay edge devices maintain keepalive messages. The Frame Relay switch is responsible for maintaining the status of the CPE device(s) to which it is attached. LMI is the communication by which the switch monitors status. LMI implements a keepalive mechanism that verifies connectivity between DCE and DTE and the fact that data can flow. A LMI multicast capability, in conjunction with an LMI multicast addressing mechanism, enables attached devices to learn local DLCIs as well as provide global, rather than local, significance to those DLCIs. Finally, LMI provides a status indicator that is constantly exchanged between router and switch. The LMI setting is configurable.
Frame Relay networks provide more features and benefits than simple point-to-point WAN links, but to do that, Frame Relay protocols are more detailed. Frame Relay networks are multiaccess networks, which means that more than two devices can attach to the network, similar to LANs. However, unlike LANs, you cannot send a data link layer broadcast over Frame Relay. Therefore, Frame Relay networks are called nonbroadcast multi-access (NBMA) networks. Also, because Frame Relay is multiaccess, it requires the use of an address that identifies to which remote router each frame is addressed.