Frame Relay

Chapter 11: Frame Relay


A frame relay is a telecommunication service designed for cost-efficient data transmission for intermittent traffic between local area networks(LANs) and between end-points in a wide area network(WAN). Frame relay puts data in a variable-size unit called a frame and leaves any necessary error correction (re-transmission of data) up to the end-points, which speeds up overall data transmission. For most services, the network provides a permanent virtual circuit (PVC), which means that the customer sees a continuous, dedicated connection without having to pay for a full-time leased line, while the service provider figures out the route each frame travels to its destination and can charge based on usage. An enterprise can select a level of service quality - prioritizing some frames and making others less important. Frame relay is offered by a number of service providers, including AT&T. Frame relay is provided on fractional T-1 or full T-carrier system carriers. Frame relay complements and provides a mid-range service between ISDN, which offers bandwidth at 128 Kbps, and Asynchronous Transfer Mode(ATM), which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbps or 622.080 Mbps.

• Virtual circuits consume bandwidth only when they transport data. Consequently, many virtual circuits can exist across a given transmission line, which is an improvement compared to dedicated leased lines. In addition, each device can use more than the allowed bandwidth as necessary, and thus operate at higher speeds.
• The increased error-handling sophistication at end stations and the improved reliability of communication lines allows the Frame Relay protocol to discard bad frames and thus eliminate time-consuming error-handling processing.

Frame relay is based on the older X.25 packet-switching technology which was designed for transmitting analog data such as voice conversations. Unlike X.25 which was designed for analog signals, frame relay is a fast packet technology, which means that the protocol does not attempt to correct errors. When an error is detected in a frame, it is simply "dropped." (thrown away). The end points are responsible for detecting and retransmitting dropped frames. (However, the incidence of error in digital networks is extraordinarily small relative to analog networks.)

Frame relay is often used to connect local area networks with major backbones as well as on public wide area networks and also in private network environments with leased lines over T-1 lines. It requires a dedicated connection during the transmission period. It's not ideally suited for voice or video transmission, which requires a steady flow of transmissions. However, under certain circumstances, it is used for voice and video transmission.

Frame Relay Structure

Standards for the Frame Relay protocol have been developed by ANSI and CCITT simultaneously. The separate LMI specification has basically been incorporated into the ANSI specification. The following discussion of the protocol structure includes the major points from these specifications.

The Frame Relay frame structure is based on the LAPD protocol. In the Frame Relay structure, the frame header is altered slightly to contain the Data Link Connection Identifier (DLCI) and congestion bits, in place of the normal address and control fields. This new Frame Relay header is 2 bytes in length and has the following format:

 How does Frame Relay differ from other Techniques?


The biggest difference in frame Relay from other techniques is the use of Virtual Connections rather than Static Connections. As shown before, each location can have one port into a Frame Relay Network. From this port, it can have multiple Virtual Connections to various locations. It can make multiple redundant connections possible through the use of PVC's between various routers, without having to use multiple physical links.
Also, since Frame relay is not media specific, and offers a way to buffer speed differences, it can make a good interconnect medium between various devices that run at various speeds.
The multiplexed nature of Frame Relay facilitates especially the transmission of Bursty Traffic. In a traditional fixed-bandwidth multiple connection scenario, a lot of bandwidth will be wasted at most times, since it is not actually being used. The bandwidth on frame is shared, allowing for multiple bursts to be handled sequentially, therefor allowing better utilization of bandwidth. The chance of congestion is however also greater, since the bandwidth in the Frame Relay Port may become a bottleneck.
frame Relay pricing is where things really get different: From the above discussion we know there are two elements to Frame Relay, the Access Port and the Private Virtual Circuits. Per port, we can feed as many circuits as we please (up to the equipment's limit..). The price of the port depends on it's bandwidth. A bigger port costs more money. The price of a PVC is fixed, and not dependent on bandwidth or usage. Data sent over the Frame Connection is not subject to additional charges. The interesting part about this pricing is that there is no distance charge involved. A Frame Relay Access Port is expensive. Two frame ports between two cities are usually also more expensive than a direct T-1 as long as there is only a one or two LATA Hop, say for example Knoxville to Nashville. However, when you feed multiple cities and Hop multiple Lata's, a Frame solution is an attractive one. In local traffic, Frame becomes attractive when you feed a large number of connections, at Fractional T-1 bandwidth generally 8 or more.

What is frame relay useful for?

Frame Relay can be used for various types of connections. It should be seen as a flexible protocol that lies on the Data Level of the Connections between routers. It can currently be used effectively for carrying all sorts of data, up to speeds of about 4 megabits per second.

Due to the implementation and cost of frame relay, it is most suitable for permanent or semi-permanent connections. It is not desirable as of yet to access frame relay on a dialup basis for economical purposes.

For a number of U.S. Internet clients, use of Frame Relay may allow for the connection of various remote sites using one technology. It is possible to connect the clients router to Frame Relay and access Internet Services over this circuit. At the same time, if the customer has other Frame Relay ports available in different locations, the same port may be used to connect to Wide Area offices the client has. This integrated approach can be attractive for both parties; from one Frame port, U.S. Internet can connect multiple customers. At the same time, the client can connect to multiple remote locations using the one port. Although there is a greater chance of connection then when dedicated lines are used, the economical advantage will in most cases compensate for this.

Typical Topology for Frame Relay:








souces link:

en.wikipedia.org/wiki/Frame_Relay 
www.protocols.com/pbook/frame.htm
www.alliancedatacom.com/fra... - Estados Unidos
www.arcelect.com/frame_relay-56kbps_ft1-t1...