History Of The Frame Relay

History Of The Frame electrical pass a persistentFrame communicate is a high-performance ill protocol that operates at the physical and information link points of the OSI reference model. Frame Relay origin everyy was designed for use across compound Services Digital Network (ISDN) interfaces. Today, it is used everyplace a variety of separate mesh interfaces as well. Frame Relay is an example of a packet-switched engineering science. Packet-switched vanes enable end stations to dynamically share the network medium and the available bandwidth.1The following two techniques are used in packet switching engineering scienceVariable length packetsStatistical multiplexingVariable-length packets are used for more efficient and flexible data transfers. These packets are switched betwixt the various segments in the network until the name and address is reached.Statistical multiplexing techniques chair 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 todays normal LANs, much(prenominal) as Ethernet and Token Ring, are packet-switched networks. Frame Relay often is divulged as a streamlined version of X.25, offering fewer of the plentiful 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 fraternity services and a higher(prenominal) 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 leave alones 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.2History o f Frame RelayOver 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 credits describe digit put across as the next generation of packet switching. Frame pass derives its origins from the ISDN (Integrated Services Digital Network) specifications developed in the 1980s. The start-off contributions to the standards communities on the build put across protocol appeared in late 1984. However, it was not until 1988 that the Ameri net National Standards Institute (ANSI) Accredited Technical Committee T1 approved the initial cast off electrical 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, vagabond relay has become more and more popular as the next step in packet technology transport.3What is X.25 ProtocolX.25 is an International Telecomm unication Union-Telecommunication standardisation Sector (ITU-T) protocol standard for WAN communications that defines how connections surrounded by user devices and network devices are established and maintained.X.25 network devices fall into iii general categories data terminal equipment (DTE), data duty tour-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 carriers network. They transfer data from one DTE device to another through the X.25 PSN. The figure above illustrates the relationships among the ternary types of X.25 network devices.4X.25 versus Frame RelayFrame 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 sick and leaves any necessary error doion (retransmission of data) up to the end-points, which speeds up overall data transmission. Frame relay is provided on fractional T-1 or full(a) 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 erect 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, rear relay is a fast packet technology, which means that the protocol does not attempt to correct errors. When an error is detected in a close in, it is simply dropped. (thrown away). The end points are respo nsible for detecting and retransmitting dropped frames. (However, the incidence of error in digital networks is extraordinarily elflike relative to analog networks.) Frame relay is often used to connect local area networks with major backbones as well as on usual wide area networks and also in private network environments with rent lines over T-1 lines. It requires a dedicated connection during the transmission period. Its not ideally worthy 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 fire 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 guest equipment to perform end to end error correction. Each swit ch 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 new-made protocols such as SDLC, HDLC, transmission control protocol/IP, stat mux protocols do that anyway.5How Frame Relay WorksWhen 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, lets first define some terms. The discussion following these definitions will give you a better understanding of how polyvinyl chlorides, 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 connective 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 coiffeLike other consequence-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 nutriment of ISDN, using the derivative of the LAP-D protocol cuts down on protocol implementation and the ne ed to change hardware.Structure of a frame relay Packet.Explanation of Packet.The handle 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 mustiness 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 he ader field to abide DLCI addresses of more than 10 bits. The FECN (Forward verbalised over-crowding 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 ( back 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 some(prenominal) data sources are contending for the same bandwidth. The DE (Discard Eligibility) bit allows the network to check into 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 f rames. These discarded frames are retransmitted, thus causing more congestion. In an effort to prevent this situation, several mechanisms admit been developed to notify user devices at the flack 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.Ad vantages of Frame RelayThe 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 providers nearest frame switch. From there the data travels over the providers 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 areReduced internetworking costs (in both hardware and carrier tariffs)increase performance with reduced network complexityIncreased interoperability via international standardsIncreased 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 clock times for most applications.Disadvantages of Frame RelayAlthough 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 oc cur, a significant number of frames are discarded and the end user system must recover from the situation. These errors cause retransmissions, thus placing extra 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 notificationForward 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, fr om 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 waitress for traffic in the reverse direction.Frame Relay ApplicationsThe most popular frame relay application provides companies with local area network (LAN) to LAN communication. This allows companies to unify 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, do cument sharing, database and many other LAN applications can utilize frame relay technology.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 relays 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.ConclusionFrame relay is a simplified form of packet-mode switching, optimized for transporting todays 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 canReduce the cost of transmission facilities and equipmentProvide increased performance, reliability, and application response timeIncrease interoperability through well-defined international standardsA 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 todays corporate networks.Coupled with the NetWare MultiProtocol Router, frame relay provides customers a flexible, highly manageable solution at a logical 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.Endnotes