Network providers commonly implement frame relay 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 transparent to all end-users.
With the advent of MPLS, VPN and dedicated broadband services such as cable modem and DSL, the end may loom for the frame relay protocol and encapsulation. However many rural areas remain lacking DSL and cable modem services. In such cases the least expensive type of "always-on" connection remains a 64-kbit/s frame-relay line. Thus a retail chain, for instance, may use frame relay for connecting rural stores into their corporate WAN.
An enterprise can select a level of service quality - prioritizing some frames and making others less important. Frame relay can run on fractional T-1 or full T-carrier system carriers. Frame relay complements and provides a mid-range service between basic rate ISDN, which offers bandwidth at 128 kbit/s, and Asynchronous Transfer Mode (ATM), which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbit/s to 622.080 Mbit/s.
Frame relay has its technical base in the older X.25 packet-switching technology, designed for transmitting data on analog voice lines. Unlike X.25, whose designers expected analog signals, frame relay offers a fast packet technology, which means that the protocol does not attempt to correct errors. When a frame relay network detects an error in a frame, it simply drops that frame. The end points have the responsibility for detecting and retransmitting dropped frames. (However, digital networks offer an incidence of error extraordinarily small relative to that of analog networks.)
Frame relay often serves to connect local area networks (LANs) with major backbones as well as on public wide-area networks (WANs) and also in private network environments with leased lines over T-1 lines. It requires a dedicated connection during the transmission period. Frame relay does not provide an ideal path for voice or video transmission, both of which require a steady flow of transmissions. However, under certain circumstances, voice and video transmission do use frame relay.
Frame relay relays packets at the data link layer (layer 2) of the Open Systems Interconnection (OSI) model rather than at the network layer (layer 3). A frame can incorporate packets from different protocols such as Ethernet and X.25. It varies in size up to a thousand bytes or more.
Frame Relay originated as an extension of Integrated Services Digital Network (ISDN). Its designers aimed to enable a packet-switched network to transport the circuit-switched technology. The technology has become a stand-alone and cost-effective means of creating a WAN.
Frame Relay switches create virtual circuits to connect remote LANs to a WAN. The Frame Relay network exists between a LAN border device, usually a router, and the carrier switch. The technology used by the carrier to transport the data between the switches is variable and changes between carrier (i.e. Frame Relay does not rely directly on the transportation mechanism to function).
The sophistication of the technology requires a thorough understanding of the terms used to describe how Frame Relay works. Without a firm understanding of Frame Relay, it is difficult to troubleshoot its performance.
Frame Relay has become one of the most extensively-used WAN protocols. Its cheapness (compared to leased lines) provided one reason for its popularity. The extreme simplicity of configuring user equipment in a Frame Relay network offers another reason for Frame Relay's popularity.
Frame-relay frame structure essentially mirrors almost exactly that defined for LAP-D. Traffic analysis can distinguish frame relay format from LAP-D by its lack of a control field.
Each frame relay PDU consists of the following fields:
The frame relay network uses a simplified protocol at each switching node. It achieves simplicity by omitting link-by-link flow-control. As a result, the offered load has largely determined the performance of frame relay networks. When high offered load is high, due to the bursts in some services, temporary overload at some frame relay nodes causes a collapse in network throughput. Therefore, frame-relay networks require some effective mechanisms to control the congestion.
Congestion control in frame-relay networks includes the following elements:
Once the network has established a connection, the edge node of the frame relay network must monitor the connection's traffic flow to ensure that the actual usage of network resources does not exceed this specification. Frame relay defines some restrictions on the user's information rate. It allows the network to enforce the end user's information rate and discard information when the subscribed access rate is exceeded.
Explicit congestion notification is proposed as the congestion avoidance policy. It tries to keep the network operating at its desired equilibrium point so that a certain Quality of Service (QOS) for the network can be met. To do so, special congestion control bits have been incorporated into the address field of the frame relay: FECN and BECN. The basic idea is to avoid data accumulation inside the network. FECN means Forward Explicit Congestion Notification. The FECN bit can be set to 1 to indicate that congestion was experienced in the direction of the frame transmission, so it informs the destination that congestion has occurred. BECN means Backwards Explicit Congestion Notification. The BECN bit can be set to 1 to indicate that congestion was experienced in the network in the direction opposite of the frame transmission, so it informs the sender that congestion has occurred.
X.25 specifies processing at layers 1, 2 and 3 of the OSI model, while frame relay operates at layers 1 and 2 only. This means that frame relay has significantly less processing to do at each node, which improves throughput by an order of magnitude.
X.25 prepares and sends packets, while frame relay prepares and sends frames. X.25 packets contain several fields used for error and flow control, none of which frame relay needs. The frames in frame relay contain an expanded link layer address field that enables frame relay nodes to direct frames to their destinations with minimal processing .
X.25 has a fixed bandwidth available. It uses or wastes portions of its bandwidth as the load dictates. Frame relay can dynamically allocate bandwidth during call setup negotiation at both the physical and logical channel level.
Sprint International (as of 2005 a part of Sprint Nextel) contracted with StrataCom for the first implementations, and deployed StrataCom hardware in its public data network to offer the first frame relay public service.
Datalink connection identifiers (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).
The LMI global addressing extension gives Frame Relay data-link connection identifier (DLCI) values global rather than local significance. DLCI values become DTE addresses that are unique in the Frame Relay WAN. The global addressing extension adds functionality and manageability to Frame Relay internetworks. Individual network interfaces and the end nodes attached to them, for example, can be identified by using standard address-resolution and discovery techniques. In addition, the entire Frame Relay network appears to be a typical LAN to routers on its periphery.
LMI virtual circuit status messages provide communication and synchronization between Frame Relay DTE and DCE devices. These messages are used to periodically report on the status of PVCs, which prevents data from being sent into black holes (that is, over PVCs that no longer exist).
The LMI multicasting extension allows multicast groups to be assigned. Multicasting saves bandwidth by allowing routing updates and address-resolution messages to be sent only to specific groups of routers. The extension also transmits reports on the status of multicast groups in update messages.
Telcos often sell frame relay 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 included StrataCom (later acquired by Cisco Systems) and Cascade Communications (later acquired by Ascend Communications and then by Lucent Technologies).
AT&T is currently (as of June 2007) the largest frame relay service provider in the USA, with local networks in 22 states, plus national and international networks.