Editing Frame Relay (section)

== Technical description ==
The designers of Frame Relay aimed to provide a telecommunication service for cost-efficient data transmission for intermittent traffic between [[local area network]]s (LANs) and between end-points in a wide area network (WAN). Frame Relay puts data in variable-size units called "frames" and leaves any necessary [[error correction|error-correction]] (such as retransmission of data) up to the end-points. This speeds up overall data transmission. For most services, the network provides a [[permanent virtual circuit]] (PVC), which means that the customer sees a continuous, dedicated connection without having to pay for a full-time [[leased line]], while the [[service provider|service-provider]] figures out the route each frame travels to its destination and can charge based on usage.

An enterprise can select a [[quality of service|level of service quality]], prioritizing some frames and making others less important. Frame Relay can run on fractional [[Digital Signal 1|T-1]] or [[E-carrier#E1|E1]], or full [[T-carrier]] or [[E-carrier]] system carriers. Frame Relay complements and provides a mid-range service between basic rate [[ISDN]], which offers bandwidth at 128&nbsp;kbit/s, and [[Asynchronous Transfer Mode]] (ATM), which operates in somewhat similar fashion to Frame Relay but at speeds from 155.520&nbsp;Mbit/s to 622.080&nbsp;Mbit/s.<ref>{{cite web|title=Definition of "Frame Relay" on SearchEnterpriseWAN|url=http://searchenterprisewan.techtarget.com/definition/frame-relay|access-date=9 April 2012|archive-date=9 April 2012|archive-url=https://web.archive.org/web/20120409171824/http://searchenterprisewan.techtarget.com/definition/frame-relay|url-status=dead}}</ref>

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 signal]]s with a relatively high chance of transmission errors, Frame Relay is a [[fast packet switching]] technology operating over links with a low chance of transmission errors (usually practically lossless like [[Plesiochronous Digital Hierarchy|PDH]]), 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 network]]s offer an incidence of error extraordinarily small relative to that of analog networks.)

Frame Relay often serves to connect [[local area network]]s (LANs) with major [[backbone network|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 originated as an extension of integrated services digital network (ISDN). Its designers aimed to enable a [[packet-switched network]] to transport over 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 data between the switches is variable and may differ among carriers (i.e., to function, a practical Frame Relay implementation need not rely solely on its own transportation mechanism).

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 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.<ref>{{Cite patent|title=Method and system for Ethernet and frame relay network interworking|gdate=2004-12-09|country=US|number=7333508|pubdate=2008-02-19|assign=[[Nortel Networks Ltd.]]|inventor1-last=Rabie |inventor1-first=Sameh|inventor2-last=Magd |inventor2-first=Osama Aboul|inventor3-last=Abdullah |inventor3-first=Bashar|inventor4-last=Barka|inventor4-first= Baghdad}}</ref>

=== Protocol data unit ===
Each Frame Relay [[protocol data unit]] (PDU) consists of the following fields:

# '''Flag Field'''. The flag is used to perform high-level data link synchronization which indicates the beginning and end of the frame with the unique pattern 01111110. To ensure that the 01111110 pattern does not appear somewhere inside the frame, [[bit stuffing|bit stuffing and destuffing]] procedures are used.
# '''Address Field'''. Each address field may occupy either octet 2 to 3, octet 2 to 4, or octet 2 to 5, depending on the range of the address in use. A two-octet address field comprises the EA=ADDRESS FIELD EXTENSION BITS and the C/R=COMMAND/RESPONSE BIT.[[File:En-tête Relais de Trame.png|thumb|Address field (2 octets)]]
## '''DLCI'''-Data Link Connection Identifier Bits. The [[DLCI]] serves to identify the virtual connection so that the receiving end knows which information connection a frame belongs to. Note that this DLCI has only local significance. A single physical channel can [[multiplexing|multiplex]] several different virtual connections.
## '''FECN, BECN, DE''' bits. These bits report congestion:
##* '''FECN'''=Forward Explicit Congestion Notification bit
##* '''[[BECN]]'''=Backward Explicit Congestion Notification bit
##* '''DE'''=Discard Eligibility bit
# '''Information Field'''. A system parameter defines the maximum number of data bytes that a host can pack into a frame. Hosts may negotiate the actual maximum frame length at call set-up time. The standard specifies the maximum information field size (supportable by any network) as at least 262 octets. Since end-to-end protocols typically operate on the basis of larger information units, Frame Relay recommends that the network support the maximum value of at least 1600 octets in order to avoid the need for segmentation and reassembling by end-users.
# '''Frame Check Sequence (FCS) Field'''. Since one cannot completely ignore the bit error-rate of the medium, each switching node needs to implement error detection to avoid wasting bandwidth due to the transmission of ''err''ed frames. The error detection mechanism used in Frame Relay uses the [[cyclic redundancy check]] (CRC) as its basis.

=== Congestion control ===
{{redirect|FECN|the compound|FeCN}}
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 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:

# Admission Control. This provides the principal mechanism used in Frame Relay to ensure the guarantee of resource requirement once accepted. It also serves generally to achieve high network performance. The network decides whether to accept a new connection request, based on the relation of the requested traffic descriptor and the network's residual capacity. The traffic descriptor consists of a set of parameters communicated to the switching nodes at call set-up time or at service-subscription time, and which characterizes the connection's statistical properties. The traffic descriptor consists of three elements:
# [[Committed information rate]] (CIR). The average rate (in bit/s) at which the network guarantees to transfer information units over a measurement interval T. This T interval is defined as: T = Bc/CIR.
# [[Committed burst size]] (BC). The maximum number of information units transmittable during the interval T.
# Excess burst size (BE). The maximum number of uncommitted information units (in bits) that the network will attempt to carry during the interval.

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.