Frame relay
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Frame relay, also found written as frame-relay, is an efficient data transmission technique used to send digital information quickly and cheaply to one or many destinations from one or many end-points. Commonly implemented for voice and data as an encapsulation technique, used between local area networks (LANs) over a wide area network (WAN). Each end-user gets a private line (or leased line) to a frame relay node. The frame relay network handles the transmission over a frequently-changing path that is transparent to all end-users.
As of 2005, Frame relay is slowly being displaced by ATM and native IP-based protocols. With the advent of the VPN and other dedicated broadband services such as cable modem and DSL, the end may be in sight for frame relay protocol and encapsulation.
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X.25 origins
Frame relay began as a stripped-down version of the X.25 protocol, releasing itself of the error-correcting burden most commonly associated with X.25. When an error is detected, the packet is simply dropped. Frame relay uses the concept of shared-access and relies on a technique refered to as best-effort, whereby error-correction is practically non-existent and reliable data delivery is practically unguaranteed. It is an industry-standard encapsulation utilizing the strengths of high-speed, packet-switched technology able to service multiple virtual circuits and protocols between connected devices, such as two routers.
Virtual circuits
As a WAN protocol, frame relay is most commonly implemented at Layer 2 (data link layer) of the OSI model. Two types of circuits exist: permanent virtual circuits (PVCs) which are used to form logical end-to-end links mapped over a physical network, and switched virtual circuits (SVCs). The latter analogous to the circuit-switching concepts of the public-switched telephone network (or PSTN), the global phone network we are are most familiar with today. While SVCs exist and are part of the frame relay specification, they are rarely applied to real-world scenarios. SVCs are most often considered harder to configure and maintain and are generally avoided without appropriate justification.
Local Management Interface (LMI)
Initial proposals for Frame Relay were presented to the Consultative Committee on International Telephone and Telegraph (CCITT) in 1984. Lack of interoperability and standardisation, prevented any significant, Frame Relay deployment until 1990 when Cisco, Digital Equipment Corporation (DEC), Northern Telecom, and StrataCom formed a consortium to focus on its development. They produced a protocol that provided additional capabilities for complex inter-networking environments. These Frame Relay extensions are referred to as the Local Management Interface (LMI).
Datalink Connection Identifiers (or DLCIs) are numbers that refer to paths through the frame relay network. They are only locally significant, which means that when device-A sends data to device-B it will most-likely use a different DLCI than device-B would use to reply. Multiple virtual circuits can be active on the same physical end-points (performed by using subinterfaces).
Committed Information Rate (CIR)
Frame relay connections are often given a Committed Information Rate (CIR) and an allowance of burstable bandwidth known as the Extended Information Rate (EIR). The provider guarantees that the connection will always support the CIR rate, and sometimes the EIR rate should there be adequate bandwidth. Frames that are sent in excess of the CIR are marked as "discard eligible" (DE) which means they can be dropped should congestion occur within the frame relay network. Frames sent in excess of the EIR are dropped immediately.
Market reputation
Frame relay was designed to make more efficient use of existing physical resources, which allow for the overprovisioning of data services by telecommunications companies (telcos) to their customers, as most clients were unlikely to be utilizing a data service 100 percent of the time. In more recent years, frame relay has acquired a bad reputation in some markets because of excessive bandwidth overbooking by these telcos.
Frame relay is/was often sold by telcos to businesses looking for a cheaper alternative to dedicated lines; its use in different geographic areas depended greatly on governmental and telecommunication companies policies. Some of the early companies to make frame relay products were StrataCom (later acquired by Cisco Systems) and Cascade Communications (later acquired by Ascend Communications and then Lucent Technologies).
See Also
- Multiprotocol Label Switching
- FRF.12 - describes the method of fragmenting Frame Relay frames into smaller frames.
External links
- The Frame Relay Forum (http://www.mplsforum.org/frame/)de:Frame Relay
fr:Relais de trames ja:フレームリレー pl:Frame Relay fi:Frame Relay zh:帧中继