Frame Relay Basics
Frame relay is defined as a communications protocol as well as a packet data service. This paper will describe frame relay as a packet data service because of its ability to create a Wide Area Network (WAN) that can integrate an enterprise infrastructure.
The frame relay service is a high speed packetized data service that consists of physical and logical components. The physical components include Frame Relay Assemblers Disassemblers (FRADs), access circuits, and frame relay ports. The logical components consist of permanent virtual circuits (PVCs). Frame Relay Assemblers Disassemblers (FRADs) are devices, such as routers, that assemble data into frame relay packets and that transmit the packets through the local access circuits to which they are connected. The local access circuits, whose bandwidths can range from 56 Kbps to 1544 Kbps, are digital circuits that connect the FRADs to the frame relay network provider's site. A frame relay port, whose bandwidth equals that of the local access circuit, physically connects the local access circuit to the frame relay network.
Frame Relay's logical component differentiates it from the traditional high speed data services that are physical in nature. The logical component of frame relay consists of Permanent Virtual Circuits (PVCs). PVCs are the logical connections that allow the various sites, having physical network access, to communicate among one and other. Each logical connection is defined in the frame relay provider's network and in the FRAD equipment. Logical connections in the frame relay providers network are software defined connections that can create many types of network topologies (i.e., hub- and-spoke, full mesh, partial mesh). Because the connections are defined using software, companies may provision logical connections in a few hours as opposed to the few weeks that are necessary with traditional high speed data services like private lines. In addition to provisioning the logical connections within the frame relay provider's network, the FRADs must be programmed to send the data to the correct permanent virtual circuit. Every PVC provisioned in the frame relay network must also be programmed into the FRAD. FRADs are programmed using data link connection identifiers (DLCIs) that associate an identification number to each PVC. Each PVC has a Committed Information Rate (CIR) that is the bandwidth associated to the logical connection. Frame relay networks have the ability to transmit data at a rate higher than that of the CIR. This allows a "bandwidth-on-demand" feature that improves the performance of bandwidth intensive applications.
Frame Relay History
Frame relay is a Data Link Layer protocol that is built on the existing CCITT X.25 and ISDN standards (Miller, 1991). Frame relay is often thought of as the next generation packet network succeeding X.25. X.25 was developed about 20 years ago and was designed for use with "noisy" analog lines. To compensate for the errors caused by these "noisy" data links, X.25 checks and corrects the data traversing every data path. This extensive error checking adversely effects the network throughput as compared to frame relay. Sometimes, depending on the packet size, 50% of the X.25 packets that traverse a data link are associated with the X.25's error checking rather than with the application's data. Frame relay, on the other hand, realizes its efficiencies by only making error calculations at the source and destination devices rather than making them at each switching node, as does X.25. Error corrections at each switching node are not necessary because today's digital networks have very few errors compared to those on analog networks. (Digital error rates are less than one error in every 1,000,000 bits transmitted.) Since the error verification and correction burden is removed from the network, network response times are greatly improved. These improved network response times improve the response times of applications that traverse the wide area network.
Frame Relay State-Of-The-Art
This author believes that frame relay's state-of-the-art incorporates the use of multiprotocol routing, Integrated Services Digital Networks (ISDN), dial access, and/or voice networking within the frame relay network. The incorporation of these additional technologies improves the reliability, performance, and flexibility of the frame relay infrastructure.
Multiprotocol routing necessitates that the FRAD equipment adhere to RFC 1490 which allows multiple protocols to traverse a single PVC. Multiprotocol routing also makes it easier to increase the capacity utilization of the frame relay links (PVCs) by allowing many different devices, that communicate using different protocols, to use the same PVC.
Companies are also using ISDN to enhance the performance and reliability of their wide area network. ISDN is a high speed data network that allows devices to dial one or more 56 Kbps data channels. It operates similarly to the standard telephone service, but multiple connections can be made and those connections are at a data rate of 56 Kbps or 64 Kbps. Wide area network reliability can be increased by having an alternate network (ISDN) on standby in the event that the frame relay network or access circuits fail. Today's FRAD equipment will automatically switch the connections from the frame relay to ISDN in the event of a frame relay failure (Greenfield, 1995). Additionally, improved performance can also be realized by increasing the bandwidth on the physical components of the network. FRADs can dial-up additional bandwidth when pre- programmed thresholds are met. For example, FRAD's can enable additional data channels when line utilization exceeds a pre- defined percentage utilization.
Today's frame relay networks are also accessible by remote users with "dial-up to frame relay" data connections (Greenfield, 1995). Since employees spend approximately 20% of their day away from their desks, remote access to information is becoming more important within many companies (DeBelina, 1995). "Dial-up to frame relay" capability is usually cost effective because companies neither have to purchase expensive equipment such as terminal servers and authentication servers nor supply administrative support for user IDs, passwords, or security administration.
Frame relay is designed as a data only service, but many companies are using specially designed FRADs to deploy voice applications (Rendleman, 1995; Sweeney, 1995). The specially designed FRADs packetize voice into frame relay packets and transmit them across the network. This allows companies to use their existing data infrastructure to transport voice calls. Voice applications over the frame relay network are not very popular because today's voice networks are very robust from a price, feature and performance standpoint. However, there are some companies that are transmitting voice over frame relay networks in order to reduce telephone expenditures.
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.
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.
The Future of Frame Relay: Interoperability with Asynchronous Transfer Mode
The future of frame relay is its interoperability with the next generation of high speed packetized data service - Asynchronous Transfer Mode (ATM). To this end, the standards bodies have ratified a new standard that establishes interoperability issues between frame relay and ATM networks (Thyfault, 1995). ATM is a cell-based high speed network designed to transport voice, data, image, and video information. Rather than variable length frames like frame relay, ATM has fixed 53 byte cells. Because the "packets" have a fixed size, their delay time through a network is fixed. This predictable time delay is ideal for isochronous traffic such as voice and video.
Many users prefer to connect their systems to ATM networks from frame relay networks rather than from native ATM networks (ATM equipment on location) because of the bandwidth constraints on low speed native ATM connections. Since ATM uses a 53 byte cell with 5 bytes of header information, the native ATM packets contain about 10% overhead due to the protocol. Frame relay, on the other hand, has a packet size ranging from 500 bytes to 4000 bytes and is less susceptible to protocol overhead from consuming bandwidth. Frame relay access to ATM networks will not append ATM's protocol overhead until the data is in the provider's network making the added overhead invisible to the users.
Conclusions
Frame relay is a technology that can create a robust wide area networking fabric that integrates information systems together to form an enterprise network. It is an affordable and capable service for supporting today's bandwidth intensive applications as well as those residing on legacy systems. Because logical connections are defined in software, it is easy to manage moves, changes, additions, and deletions of logical connections. Systems analysts should consider the use of frame relay for corporate applications that incorporate information systems at more than one location because of the technologies many technical, financial, logistical advantages.
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