Frame Relay Protocol: Structure, Protocol, Benefits, and Resources

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What is Frame Relay Protocol

Frame relay is a protocol standard for local area networks (LANs), which provides a fast and efficient method of transmitting information from one user device to another across multiple switches and routers. It is based on packet-switching technologies, by which the end points of a network connection dynamically share the network medium and the available bandwidth. Frame relay is considered more efficient and reliable than alternative methods of interconnecting LANS, such as dedicated lines and X.25 networks.

How does Frame Relay Work

In frame relay, data are sent in High-Level Data Link Control (HDLC) packets, referred to as “frames.” Each frame is transmitted across the network via virtual circuits that create bidirectional communication paths between two terminal devices and are uniquely identified by a Data-Link Connection Identifier (DLCI). They are called “virtual circuits” because they do not consist of direct electrical connections from one end of the connection to the other but rather “logical” connections through which data are transmitted. A number of virtual circuits can be combined through a process called “multiplexing” into a single physical circuit. Multiplexing reduces the equipment and network complexity required to connect multiple terminals. A virtual circuit can pass through any number of intermediate switches within the network.

Benefits of Frame Relay Protocol

Frame Relay offers an attractive alternative to both dedicated lines and X.25 networks for connecting LANs to bridges and routers.

1. Because virtual circuits consume bandwidth only when they transport data, many virtual circuits can exist simultaneously across a given transmission line. In addition, each device can use more of the bandwidth as necessary, and thus operate at higher speeds.

2. The improved reliability of communication lines and increased error-handling sophistication at end stations allows the Frame Relay protocol to discard erroneous frames and thus eliminate time-consuming error-handling processing.

3. These two factors make Frame Relay a desirable choice for data transmission; however, they also necessitate testing to determine that the system works properly and that data is not lost.

Types of Virtual Circuits

There are two types of virtual circuit: Permanent Virtual Circuits (PVCs), which are defined by the network administrator for dedicated point-to-point connections, and Switched Virtual Circuits (SVCs), which are set up on a call-by-call basis using Integrated Services Digital Network (ISDN) signaling standards. In frame relay, as in X.25 networks, permanent virtual circuits are set up during the installation process and maintained 24 hours a day, seven days a week; they are not created and ended by users at their terminals. Users can run applications across frame relay circuits that allow them to initiate and end sessions from their terminals, but this is a function of the application, not the underlying network.

Why use Frame Relay Protocol

1. Users have more data to communicate, and they’d like that data to travel faster and in larger chunks than current technology has allowed.

2. Physical transmission gets faster every year and introduces fewer and fewer errors into the data.

3. Computers and workstations with the intelligence to handle high-level protocols have replaced dumb terminals as the instruments of choice.

Frame relay can also be used on “meshed” networks. Since each endpoint of the connection can have one or more DLCI addresses, each user location can be connected to any or all of the other locations. A network is said to be “fully meshed” when every user location is connected to every other location in the network. The frame relay switch uses a cross-referencing mapping table to route the data packets from one internal port of the switch to another.

Frame Structure

Frame structure is based on the LAPD protocol, with a two-byte frame header altered to contain the DLCI and Explicit Congestion Notification (ECN) bits in place of the normal address and control fields. Congestion bits are used to prevent the loss of frames as a result of network congestion. When a network becomes congested to the point that it cannot process new data transmissions, it begins to discard frames; these discarded frames are then retransmitted, which adds to the network congestion. The ECN bits notify the user terminals at the onset of congestion, enabling them to reduce the load of packets offered for transmission. The Forward Explicit Congestion Notification (FECN) bit is changed from 0 to 1 when a frame sent downstream toward the destination encounters network congestion. In this way, all downstream nodes and the destination device are apprised of excess traffic. The Backward Explicit Congestion Notification (BECN) bit is changed from 0 to 1 when a frame traveling back toward the source of transmission encounters congestion, thus notifying the source device to slow down transmission until the congestion subsides.

DLCI
10-bit DLCI field represents the address of the frame and corresponds to a PVC.

C/R
Designates whether the frame is a command or response.

EA
Extended Address field signifies up to two additional bytes in the Frame Relay header, thus greatly expanding the number of possible addresses.

FECN
Forward Explicit Congestion Notification.

BECN
Backward Explicit Congestion Notification.

DE
Discard Eligibility.

Information
The Information field may include other protocols within it, such as an X.25, IP or SDLC (SNA) packet.

Frame Relay Protocol Standards

ANSI T1.618
ANSI T1.617
ANSI LMI
Manufacturers’ LMI
Frame Relay NNI PVC (FRF.2)

In contrast to the X.25 protocol, which performs error correction from switch to switch, frame relay relies on the user terminals to perform end-to-end error correction. This is possible because current network technology is sufficiently error proof to relegate this process to the end points. Because frame relay passes data packets from switch to switch without error correction, transmission between terminals through the network is extremely fast.

Resources about Frame Relay Protocol

Protocols.com information about FRP

Cisco Internetworking Technology Handbook

Resource by E.Trio

I am an online writer and soon to be published author. I grew up in Southern California, but I currently live in the Bay Area while I work on my books. I enjoy camping and hiking with my dog Max or just relaxing on the back porch while I write. Stay tuned for my first children’s book to be released about Max.

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