Frame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. Frame Relay originally was designed for use across Integrated Services Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well. Frame Relay is an example of a packet-switched technology. Packet-switched networks enable end stations to dynamically share the network medium and the available bandwidth.  The following two techniques are used in packet switching technology:
Variable length packets
Variable-length packets are used for more efficient and flexible data transfers. These packets are switched between the various segments in the network until the destination is reached.
Statistical multiplexing techniques control network access in a packet-switched network. The advantage of this technique is that it accommodates more flexibility and more efficient use of bandwidth. Most of today’s popular LANs, such as Ethernet and Token Ring, are packet-switched networks. Frame Relay often is described as a streamlined version of X.25, offering fewer of the robust capabilities, such as windowing and retransmission of last data that are offered in X.25. This is because Frame Relay typically operates over WAN facilities that offer more reliable connection services and a higher degree of reliability than the facilities available during the late 1970s and early 1980s that served as the common platforms for X.25 WANs. As mentioned earlier, Frame Relay is strictly a Layer 2 protocol suite, whereas X.25 provides services at Layer 3 (the network layer) as well. This enables Frame Relay to offer higher performance and greater transmission efficiency than X.25, and makes Frame Relay suitable for current WAN applications, such as LAN interconnection. 
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History of Frame Relay
Over the last decade, packet switching technology has been dominated by X.25, one of the oldest and most widely used communication transports in the world. Many sources describe frame relay as the next generation of packet switching. Frame relay derives its origins from the ISDN (Integrated Services Digital Network) specifications developed in the 1980s. The first contributions to the standards communities on the frame relay protocol appeared in late 1984. However, it was not until 1988 that the American National Standards Institute (ANSI) Accredited Technical Committee T1 approved the initial frame relay specification. Frame relay services started to become generally available in late 1993.With the rapid evolution of reliable data communications equipment and transmission facilities, frame relay has become more and more popular as the next step in packet technology transport. 
What is X.25 Protocol
X.25 is an International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) protocol standard for WAN communications that defines how connections between user devices and network devices are established and maintained.
X.25 network devices fall into three general categories: data terminal equipment (DTE), data circuit-terminating equipment (DCE), and packet-switching exchange (PSE). Data terminal equipment devices are end systems that communicate across the X.25 network. DCE devices are communications devices, such as modems and packet switches, which provide the interface between DTE devices and a PSE. PSEs are switches that compose the bulk of the carrier’s network. They transfer data from one DTE device to another through the X.25 PSN. The figure above illustrates the relationships among the three types of X.25 network devices. 
X.25 versus Frame Relay
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 (retransmission of data) up to the end-points, which speeds up overall data transmission. 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.
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 transmits packets at the data link layer of the Open Systems Interconnection (OSI) model rather than at the Network layer. A frame can incorporate packets from different protocols such as Ethernet and X.25. It is variable in size and can be as large as a thousand bytes or more. Frame relay relies on the customer equipment to perform end to end error correction. Each switch inside a frame relay network just relays the data (frame) to the next switch. X.25, in contrast, performs error correction from switch to switch. The networks of today are sufficiently error free to move the burden of error correction to the end points. Most modern protocols such as SDLC, HDLC, TCP/IP, stat mux protocols do that anyway. 
How Frame Relay Works
When looking into frame relay, most people raise the following question: How can one router with a single direct link into a frame relay network establish connection with multiple routers or CPEs? To answer this question, let’s first define some terms. The discussion following these definitions will give you a better understanding of how PVCs, DLCIs and LMI function together to enable and manage frame relay links to other routers. PVC Permanent Virtual Circuits are one example of connection-oriented service. Most protocols operate in connection-oriented mode. This makes more efficient use of the circuit by bringing down the link when not in use. DLCI the Data Link Connection Identifier distinguishes separate virtual circuits across each access connection. It allows the frame (packet) to be routed to the correct destination within a frame relay network. This is similar to X.25 implementation of the LAP-D core protocol functions.
Frame Relay Packet Format
Like other bit-synchronous protocols, frame relay uses a frame or packet structure as the basis for transmission. The frame format used by frame relay is based on Link Access Protocol for ISDN-D channels, which defines the functions for the OSI Data-link layer. (The frame structure for frame relay is derived from the high-level data link control or HDLC procedure.) Frame relay was originally defined by the CCITT as a network service within the framework of ISDN. Because hardware already provided support of ISDN, using the derivative of the LAP-D protocol cuts down on protocol implementation and the need to change hardware.
Structure of a frame relay Packet.
Explanation of Packet.
The fields in the frame relay packet are as follows: The Flag fields delimit where the data frame begins and ends.The Frame Relay Header contains the DLCI, the FECN and BECN bits, and other information (see the “Operation” section for a description of how the header is used).The Information field holds the actual data being transmitted (the “payload”). It can hold from 262 to 1600 or more octets (equivalent to a byte). The FCS (Frame Check Sequence) is an error checking field. Frame relay uses a Cyclic Redundancy Check (CRC). If Frame Relay detects an error here, it drops the frame. The Network-layer protocol must request a retransmission.
The DLCI fields in the frame relay. The fields in the frame relay address header contain the Data Link Connection Identifier, described earlier. These fields can store two octets containing a 10-bit DLCI.The EA (Extended Address) bits make it possible to extend the header field to support DLCI addresses of more than 10 bits. The FECN (Forward Explicit Congestion Notification) bit may be used to notify the user that congestion was experienced in the direction of the frame carrying the FECN indication. The BECN (Backward Explicit Congestion Notification) bit may be used to notify the user that congestion was experienced in the opposite direction of the frame carrying the FECN indication. The C/R field in the header contains Command/Response information. These bits relate to congestion information stored if the network is experiencing congestion because several data sources are contending for the same bandwidth. The DE (Discard Eligibility) bit allows the network to determine which frames may be discarded under congestion situations.
Example of how DLCI addresses are used in sending packets across a frame relay network.
When the network becomes congested to the point that it cannot process new data transmissions, it begins to discard frames. These discarded frames are retransmitted, thus causing more congestion. In an effort to prevent this situation, several mechanisms have been developed to notify user devices at the onset of congestion, so that the offered load may be reduced. Two bits in the Frame Relay header are used to signal the user device that congestion is occurring on the line: They are the Forward Explicit Congestion Notification (FECN) bit and the Backward Explicit Congestion Notification (BECN) bit. The FECN is changed to 1 as a frame is sent downstream toward the destination location when congestion occurs during data transmission. In this way, all downstream nodes and the attached user device learn about congestion on the line. The BECN is changed to 1 in a frame traveling back toward the source of data transmission on a path where congestion is occurring. Thus the source node is notified to slow down transmission until congestion subsided.
Frame relay places the responsibility of ensuring data delivery on the end-point devices that are operating with multi-level protocols. End-points can be devices such as networks, workstations, and hosts. To ensure that all packets have been received, the Transport layer (layer 4) of the OSI model places a sequence number on the frames that are sent. As with X.25, this functionality is performed in the Data-link layer. Special management frames, with a unique DLCI address, can be passed between the network and the access device. These frames monitor the status of the link and indicate whether the link is active or inactive. They can also pass information regarding status of the PVC and DLCI changes. This frame relay management protocol is referred to as the Local Management Interface (LMI). Its function is to provide information about PVC status. Originally, the frame relay specification did not provide for this kind of status. Since then, a method for LMI has been developed and has been incorporated into the ANSI and CCITT standards.
Advantages of Frame Relay
The main advantage of Frame Relay over point-to-point leased lines is cost. Frame Relay can provide performance similar to that of a leased line, but with significantly less cost over long distances. The reason is the customer only has to make a dedicated point-to-point connection to the provider’s nearest frame switch. From there the data travels over the provider’s shared network. The price of leased lines generally increases based on distance. So, this short-haul point-to-point connection is significantly less expensive than making a dedicated point-to-point connection over a long distance.
The three main areas in which frame relay demonstrates significant advantages over other WAN protocols are:
Reduced internetworking costs (in both hardware and carrier tariffs)
Increased performance with reduced network complexity
Increased interoperability via international standards
Increased Performance with Reduced Network Complexity. Frame relay reduces the complexity of the physical network without disrupting higher-level network functions. Frame Relay functions using only the bottom two layers of the OSI model, as compared to X.25 which includes the Network layer. By reducing the amount of processing required, and by efficiently using high-speed digital transmission lines, frame relay can improve performance and response times for most applications.
Disadvantages of Frame Relay
Although frame relay has many advantages, there are two areas within frame relay that can promote potential problems: congestion control and frame discard.
Congestion Control. As with most WAN services, without careful design, a frame relay network can quickly become congested. When frames are being sent beyond the agreed CIR,(Committed Information Rate) there is eligibility for discarding frames due to congestion.
Frame Discard. When a problem is experienced with a single frame, frame relay simply ignores the problem and discards the frame. If a large number of problems occur, a significant number of frames are discarded and the end user system must recover from the situation. These errors cause retransmissions, thus placing additional bandwidth demands on the frame relay network.
ANSI applied specifications for Congestion Notification Mechanisms to allow frame relay devices to indicate the existence of congestion in the network. In the frame relay packet header, two bits are used for explicit congestion notification:
Forward explicit congestion notification (FECN)
Backward explicit congestion notification (BECN)
When a node on the network approaches a congestion condition caused by a temporary peak in traffic, the node detects the onset of congestion and signals all the downstream nodes. All attached devices learn that congestion has occurred and minimize until the network traffic subsides, as shown in the Figure below.
The FECN and BECN bits can be used for congestion control in a frame relay network.
In the case of traffic going in one direction (that is, from Florida to California), frame relay standards prohibit the network from generating any frames with the DLCI (Data Link Control Identifier) of a particular virtual circuit causing the traffic. Therefore, the congestion notification must wait for traffic in the reverse direction.
Frame Relay Applications
The most popular frame relay application provides companies with local area network (LAN) to LAN communication. This allows companies to integrate their information systems in order to have employees throughout the enterprise to access specific information residing on a LAN somewhere in the enterprise. The devices on the LANs can communicate over the frame relay network regardless of their native protocol. For example, native protocols that can traverse frame relay networks include SNA, DECnet, IPX, TCP/IP, and AppleTalk. Therefore, frame relay has the ability to make the users perceive that the entire company is on one large LAN. Application software such as groupware, e-mail, document sharing, database and many other LAN applications can utilize frame relay technology.
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Companies are also integrating communication for legacy systems, such as SNA, onto frame relay networks (Thyfault, 1995B). This allows companies to connect devices such as cluster controllers and front-end processors directly to FRADs in order to use the frame relay network for communications. Frame relay’s ability to support both the legacy applications and LAN applications provides an excellent backbone for those companies that are in the process of migrating their information systems from centralized mainframe processing to distributed client/server systems. Companies can turn up legacy applications on the frame relay network and slowly migrate the LAN applications as they are developed.
Frame relay is a simplified form of packet-mode switching, optimized for transporting today’s protocol-oriented data. The result of this simplification is that frame relay offers higher throughput, while still retaining the bandwidth and equipment efficiencies that come from having multiple virtual circuits share a single port and transmission facility. Thus, the use of frame relay can:
Reduce the cost of transmission facilities and equipment
Provide increased performance, reliability, and application response time
Increase interoperability through well-defined international standards
A major reason for the high level of interest in frame relay is that it is a technology that has been developed in response to a clear market need. With the proliferation of powerful end-point devices (such as PCS and workstations) operating with intelligent protocols (such a TCP/IP, XNS and DECnet), users are seeking WAN communication methods that offer higher throughput and more cost-effective use of digital transmission lines. With that need in mind, frame relay has been developed and standardized to have precisely the combination of characteristics needed by today’s corporate networks.
Coupled with the NetWare MultiProtocol Router, frame relay provides customers a flexible, highly manageable solution at a reasonable cost. Frame relay is just one of many WAN alternatives available. Given the right planning, it will provide users with efficient high-bandwidth connectivity now and into the future.
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