Frame relay basics

Please describe the basic features of frame relay, and how this technology is best used.

Frame relay is a quickly expanding technology that is helping the internetworking industry leap into the 21st century. It is now being offered by numerous service providers nationwide. In some cases, analysts have said that the frame relay industry in the United States is growing at 50 percent annually. In other parts of the world, it’s even higher.

Frame relay is technology that is part of the “fast packet” concept. In fact, frame relay can be defined as one of the two implementations of the fast-packet methodology, the other being “cell-relay” technlogy, which is used in Asynchronous Transfer Mode (ATM). Fast packet technology helps bring communications networking services, i.e. data, voice and video, together. It also helps improve performance, reliability and the management of these connectivity schemes.

Let us now examine how frame relay works. The frame relay network is accessed via frame relay access devices (FRADs). The telephone company’s frame relay switches or (FRSes) provide the frame relay service to users, much like telephone switches provide telephone services to consumers.

Now we’ll define the concept of the user network interface (UNI), which is comprised of the physical interface between the user and the FRS, the frame relay link layer protocol and the signalling procedures within the channel. The UNI specification itself is administered by bodies such as the frame relay forum, typcially called frame relay user to network interface spec FRF-1.

The link between the FRAD and the provider’s FRS is called a permanent virtual circuit (PVC). The size of this link can range from 56 to 2048 Kbps.

The customer’s local area network is connected via a FRAD such as a frame relay enabled router to a DSU/CSU. Typically, this connection is via a V.35 connector. The connector interface between the CSU and the PVC is typically a RJ-48C or Bantam. The PVC’s bandwidth can range from 56Kbps to E1 speed.

At the telco, the FRS receives the frames from the customer’s site.

High-level datalink control (HDLC) is the defining protocol for frame relay’s link layer. What does this mean? Frames are separated by idle flags or the information contained between idle flags comprises a frame. The user’s data is contained as payload in these frames. In addition to the encapsulated user data, the frame also contains such entities as the frame relay header and a frame check sequence. The frame is transported through the network via HDLC zero insertion methodology. Frame order is preserved as the frames move through the network, i.e. frames follow each other.

An important fact to note is frame relay’s no-guarantee policy on delivery of data. This is the responsibility of higher layers. and results in a simple, fast and efficient wide-area delivery system. There is no error-checking overhead or need to send acknowledgements or retransmissions.

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