Frame Relay Concept

The Frame Relay WAN
In the late 1970s and into the early 1990s, the WAN technology joining the end sites was typically using the X.25 protocol. Now considered a legacy protocol, X.25 was a very popular packet switching technology because it provided a very reliable connection over unreliable cabling infrastructures. It did so by including additional error control and flow control. However, these additional features added overhead to the protocol. Its major application was for processing credit card authorization and for automatic teller machines. This course mentions X.25 only for historical purposes.
When you build a WAN, regardless of the transport you choose, there is always a minimum of three basic components, or groups of components, connecting any two sites. Each site needs its own equipment (DTE) to access the telephone company's CO serving the area (DCE). The third component sits in the middle, joining the two access points. In the figure, this is the portion supplied by the Frame Relay backbone.
Frame Relay has lower overhead than X.25 because it has fewer capabilities. For example, Frame Relay does not provide error correction, modern WAN facilities offer more reliable connection services and a higher degree of reliability than older facilities. The Frame Relay node simply drops packets without notification when it detects errors. Any necessary error correction, such as retransmission of data, is left to the endpoints. This makes propagation from customer end to customer end through the network very fast.
Frame Relay handles volume and speed efficiently by combining the necessary functions of the data link and network layers into one simple protocol. As a data link protocol, Frame Relay provides access to a network, delimits and delivers frames in proper order, and recognizes transmission errors through a standard Cyclic Redundancy Check. As a network protocol, Frame Relay provides multiple logical connections over a single physical circuit and allows the network to route data over those connections to its intended destinations.
Frame Relay operates between an end-user device, such as a LAN bridge or router, and a network. The network itself can use any transmission method that is compatible with the speed and efficiency that Frame Relay applications require. Some networks use Frame Relay itself, but others use digital circuit switching or ATM cell relay systems. The figure shows a circuit-switching backbone as indicated by the Class 4/5 switches. The remaining graphics in this section show more contemporary packet-switching Frame Relay backbones.
Frame Relay Operation
The connection between a DTE device and a DCE device consists of both a physical layer component and a link layer component:
The physical component defines the mechanical, electrical, functional, and procedural specifications for the connection between the devices. One of the most commonly used physical layer interface specifications is the RS-232 specification.
The link layer component defines the protocol that establishes the connection between the DTE device, such as a router, and the DCE device, such as a switch.
When carriers use Frame Relay to interconnect LANs, a router on each LAN is the DTE. A serial connection, such as a T1/E1 leased line, connects the router to the Frame Relay switch of the carrier at the nearest point-of-presence (POP) for the carrier. The Frame Relay switch is a DCE device. Network switches move frames from one DTE across the network and deliver frames to other DTEs by way of DCEs. Computing equipment that is not on a LAN may also send data across a Frame Relay network. The computing equipment uses a Frame Relay access device (FRAD) as the DTE. The FRAD is sometimes referred to as a Frame Relay assembler/dissembler and is a dedicated appliance or a router configured to support Frame Relay. It is located on the customer's premises and connects to a switch port on the service provider's network. In turn, the service provider interconnects the Frame Relay switches.
Virtual Circuits
The connection through a Frame Relay network between two DTEs is called a virtual circuit (VC). The circuits are virtual because there is no direct electrical connection from end to end. The connection is logical, and data moves from end to end, without a direct electrical circuit. With VCs, Frame Relay shares the bandwidth among multiple users and any single site can communicate with any other single site without using multiple dedicated physical lines.
There are two ways to establish VCs:
SVCs, switched virtual circuits, are established dynamically by sending signaling messages to the network (CALL SETUP, DATA TRANSFER, IDLE, CALL TERMINATION).
PVCs, permanent virtual circuits, are preconfigured by the carrier, and after they are set up, only operate in DATA TRANSFER and IDLE modes. Note that some publications refer to PVCs as private VCs.
Multiple VCs
Frame Relay is statistically multiplexed, meaning that it transmits only one frame at a time, but that many logical connections can co-exist on a single physical line. The Frame Relay Access Device (FRAD) or router connected to the Frame Relay network may have multiple VCs connecting it to various endpoints. Multiple VCs on a single physical line are distinguished because each VC has its own DLCI. Remember that the DLCI has only local significance and may be different at each end of a VC.
This capability often reduces the equipment and network complexity required to connect multiple devices, making it a very cost-effective replacement for a mesh of access lines. With this configuration, each endpoint needs only a single access line and interface. More savings arise as the capacity of the access line is based on the average bandwidth requirement of the VCs, rather than on the maximum bandwidth requirement.


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