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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 circuit terminating 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.

Circuit Switching

Circuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session. Circuit switching accommodates two types of transmissions: datagram transmissions and data-stream transmissions. Used extensively in telephone company networks, circuit switching operates much like a normal telephone call. Integrated Services Digital Network (ISDN) is an example of a circuit-switched WAN technology.

Packet Switching

Packet switching is a WAN switching method in which network devices share a single point-to-point link to transport packets from a source to a destination across a carrier network. Statistical multiplexing is used to enable devices to share these circuits. Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Service (SMDS), and X.25 are examples of packet-switched WAN technologies.

Virtual Circuit

A virtual circuit is a logical circuit created to ensure reliable communication between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete. SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but decrease the cost associated with constant virtual circuit availability.

A PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but increase costs due to constant virtual circuit availability.

WAN Dialup Services

Dialup services offer cost-effective methods for connectivity across WANs. Two popular dialup implementations are dial-on-demand routing (DDR) and dial backup.

DDR is a technique whereby a router can dynamically initiate and close a circuit-switched session as transmitting end station demand. A router is configured to consider certain traffic interesting (such as traffic from a particular protocol) and other traffic uninteresting. DDR can be used to replace point-to-point links and switched multiaccess WAN services.

Dial backup is a service that activates a backup serial line under certain conditions. The secondary serial line can act as a backup link that is used when the primary link fails or as a source of additional bandwidth when the load on the primary link reaches a certain threshold.

WAN Devices

WAN Switch
A WAN switch is a multiport internetworking device used in carrier networks. These devices typically switch such traffic as Frame Relay, X.25, and SMDS and operate at the data link layer of the OSI reference model.

Access Server
An access server acts as a concentration point for dial-in and dial-out connections.

Modem
A modem is a device that interprets digital and analog signals, enabling data to be transmitted over voice-grade telephone lines.

CSU/DSU
A CSU/DSU (channel service unit/digital service unit) is a digital-interface device that adapts the physical interface on a DTE (Data Terminal Equipment) device to the interface of a DCE (Data Circuit-Terminating) device in a switched-carrier network. The CSU/DSU also provides signal timing for communication between these devices.

ISDN Terminal Adapter
An ISDN terminal adapter is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces, such as EIA/TIA-232. A terminal adapter is essentially an ISDN modem.

High-Speed Serial Interface
The High-Speed Serial Interface (HSSI) is a DTE/DCE interface developed by Cisco Systems to address the need for high-speed communication over WAN links. HSSI defines both the electrical and the physical DTE/DCE interfaces. It therefore corresponds to the physical layer of the OSI reference model. The flexibility of the HSSI clock and data-signaling protocol makes user (or vendor) bandwidth allocation possible. The DCE controls the clock by changing its speed or by deleting clock pulses. In this way, the DCE can allocate bandwidth between applications. HSSI assumes a peer-to-peer intelligence in the DCE and DTE. The control protocol is simplified, with just two control signals required ("DTE available" and "DCE available"). Both signals must be asserted before the data circuit is valid. The DCE and DTE are expected to be able to manage the networks behind their interfaces. HSSI technical characteristics are summarized as follows: