Frame relay
Encyclopedia
Frame Relay is a standardized wide area network
technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching
methodology. Originally designed for transport across Integrated Services Digital Network
(ISDN) infrastructure, it may be used today in the context of many other network interfaces.
Network providers commonly implement Frame Relay for voice (VoFR
) and data as an encapsulation
technique, used between local area network
s (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.
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.
With the advent of Ethernet
over fiber optics, 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 non-dial-up 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.
The designers of Frame Relay aimed to 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 (such as re-transmission 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
figures out the route each frame travels to its destination and can charge based on usage.
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 signal
s, 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 network
s (LANs) with major backbone
s as well as on public wide-area network
s (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 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 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.
(PDU) consists of the following fields:
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.
provides quality of service and error-free delivery, whereas, Frame Relay was designed to relay data as quickly as possible over low error networks. Frame Relay eliminates a number of the higher-level procedures and fields used in X.25. Frame Relay was designed for use on links with error-rates far lower than available when X.25 was designed.
X.25 prepares and sends packets, while Frame Relay prepares and sends frames. X.25 packets contain several fields used for error checking and flow control
, most of which are not used by Frame Relay. 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. The elimination of functions and fields over X.25 allows Frame Relay to move data more quickly, but leaves more room for errors and larger delays should data need to be retransmitted.
X.25 packet switched networks typically allocated a fixed bandwidth through the network for each X.25 access, regardless of the current load. This resource allocation approach, while apt for applications that require guaranteed quality of service, is inefficient for applications that are highly dynamic in their load characteristics or which would benefit from a more dynamic resource allocation. Frame Relay networks can dynamically allocate bandwidth at both the physical and logical channel level.
protocol, Frame Relay is most commonly implemented at Layer 2 (data link layer
) of the Open Systems Interconnection (OSI) seven layer 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 are analogous to the circuit-switching concepts of the public switched telephone network
(PSTN), the global phone network.
) in 1984. Lack of interoperability and standardization, 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 (DLCI
s) 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 subinterface
s).
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 line
s; 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
).
As of June 2007 AT&T
was the largest Frame Relay service provider in the USA, with local networks in 22 states, plus national and international networks.
Wide area network
A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching
Packet switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
methodology. Originally designed for transport across Integrated Services Digital Network
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
(ISDN) infrastructure, it may be used today in the context of many other network interfaces.
Network providers commonly implement Frame Relay for voice (VoFR
VoFR
Voice over Frame Relay is a protocol to transfer voice over Frame Relay networks. VoFR uses two sub-protocols, FTF.11 and FRF.12. FRF.11 defines the frame format of VoFR, and FRF.12 is used for packet fragmentation and reassembly....
) and data as an encapsulation
Encapsulation (networking)
In computer networking, encapsulation is a method of designing modular communication protocols in which logically separate functions in the network are abstracted from their underlying structures by inclusion or information hiding within higher level objects....
technique, used between local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
s (LANs) over a wide area network
Wide area network
A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
(WAN). Each end-user gets a private line
Private line
In wired telephony, a private line or tie line is a service that involves dedicated circuits, private switching arrangements, and/or predefined transmission paths, whether virtual or physical, which provide communications between specific locations. Most private lines connect only two locations...
(or leased line
Leased line
A leased line is a service contract between a provider and a customer, whereby the provider agrees to deliver a symmetric telecommunications line connecting two or more locations in exchange for a monthly rent . It is sometimes known as a 'Private Circuit' or 'Data Line' in the UK or as CDN in Italy...
) to a Frame Relay node
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
. The Frame Relay network handles the transmission over a frequently-changing path transparent to all end-users.
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.
With the advent of Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
over fiber optics, MPLS
Multiprotocol Label Switching
Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...
, VPN
Virtual private network
A virtual private network is a network that uses primarily public telecommunication infrastructure, such as the Internet, to provide remote offices or traveling users access to a central organizational network....
and dedicated broadband
Broadband
The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...
services such as cable modem
Cable modem
A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...
and DSL
Digital Subscriber Line
Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
, 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 non-dial-up 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.
Technical description
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
s (LANs) and between end-points in a wide area network
Wide area network
A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
(WAN). Frame Relay puts data in variable-size units called "frames" and leaves any necessary error-correction (such as re-transmission 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
Leased line
A leased line is a service contract between a provider and a customer, whereby the provider agrees to deliver a symmetric telecommunications line connecting two or more locations in exchange for a monthly rent . It is sometimes known as a 'Private Circuit' or 'Data Line' in the UK or as CDN in Italy...
, while the service-provider
Service provider
A service provider is an entity that provides services to other entities. Usually, this refers to a business that provides subscription or web service to other businesses or individuals. Examples of these services include Internet access, Mobile phone operators, and web application hosting...
figures out the route each frame travels to its destination and can charge based on usage.
An enterprise can select a level of service quality
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
- prioritizing some frames and making others less important. Frame Relay can run on fractional T-1
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
or full T-carrier
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
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
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(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
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
technology, designed for transmitting data on analog voice lines. Unlike X.25, whose designers expected analog signal
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
s, Frame Relay offers a fast packet technology
Fast packet switching
In telecommunications, fast packet switching is a packet switching technique that increases the throughput by eliminating overhead. Overhead reduction is accomplished by allocating flow control and error correction functions to either the user applications or the network nodes that interface with...
, 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 network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
s (LANs) with major backbone
Backbone network
A backbone network or network backbone is a part of computer network infrastructure that interconnects various pieces of network, providing a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same building, in different...
s as well as on public wide-area network
Wide area network
A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
s (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 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 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
Link Access Procedures, D channel
Link Access Procedures on the D channel , specified in ITU-T Q.920 and ITU-T Q.921, is the second layer protocol on the ISDN protocol stack in the D channel.It is heavily based on HDLC.-External links:*http://www.protocols.com/pbook/pdf/isdn.pdf...
. Traffic analysis can distinguish Frame Relay format from LAP-D by its lack of a control field.
Protocol data unit
Each Frame Relay Protocol data unitProtocol data unit
In telecommunications, the term protocol data unit has the following meanings:#Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data....
(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 and destuffingBit stuffingIn data transmission and telecommunication, bit stuffing is the insertion of noninformation bits into data...
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.
- DLCI-Data Link Connection Identifier Bits. The DLCIDLCIA data link connection identifier is a Frame Relay 10 bit wide link-local virtual circuit identifier used to assign frames to a specific PVC or SVC. Frame Relay networks use DLCIs to statistically multiplex frames...
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 multiplexMultiplexingThe multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
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
- DLCI-Data Link Connection Identifier Bits. The DLCI
- 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 erred frames. The error detection mechanism used in Frame Relay uses the cyclic redundancy checkCyclic redundancy checkA cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
(CRC) as its basis.
Congestion control
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 RateCommitted Information RateCommitted information rate or CIR in a Frame relay network is the average bandwidth for a virtual circuit guaranteed by an ISP to work under normal conditions. At any given time, the bandwidth should not fall below this committed figure...
(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
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
(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.
Frame Relay versus X.25
X.25X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
provides quality of service and error-free delivery, whereas, Frame Relay was designed to relay data as quickly as possible over low error networks. Frame Relay eliminates a number of the higher-level procedures and fields used in X.25. Frame Relay was designed for use on links with error-rates far lower than available when X.25 was designed.
X.25 prepares and sends packets, while Frame Relay prepares and sends frames. X.25 packets contain several fields used for error checking and flow control
Flow control
In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
, most of which are not used by Frame Relay. The frames in Frame Relay contain an expanded link layer
Link Layer
In computer networking, the link layer is the lowest layer in the Internet Protocol Suite , the networking architecture of the Internet . It is the group of methods or protocols that only operate on a host's link...
address field that enables Frame Relay nodes to direct frames to their destinations with minimal processing. The elimination of functions and fields over X.25 allows Frame Relay to move data more quickly, but leaves more room for errors and larger delays should data need to be retransmitted.
X.25 packet switched networks typically allocated a fixed bandwidth through the network for each X.25 access, regardless of the current load. This resource allocation approach, while apt for applications that require guaranteed quality of service, is inefficient for applications that are highly dynamic in their load characteristics or which would benefit from a more dynamic resource allocation. Frame Relay networks can dynamically allocate bandwidth at both the physical and logical channel level.
Virtual circuits
As a WANWide area network
A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
protocol, Frame Relay is most commonly implemented at Layer 2 (data link layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
) of the Open Systems Interconnection (OSI) seven layer model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. 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 are analogous to the circuit-switching concepts of the public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
(PSTN), the global phone network.
Frame Relay origins
Frame Relay began as a stripped-down version of the X.25 protocol, releasing itself from the error-correcting burden most commonly associated with X.25. When Frame Relay detects an error, it simply drops the offending packet. Frame Relay uses the concept of shared-access and relies on a technique referred to as "best-effort", whereby error-correction practically does not exist and practically no guarantee of reliable data delivery occurs. Frame Relay provides 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.Local Management Interface (LMI)
Initial proposals for Frame Relay were presented to the Consultative Committee on International Telephone and Telegraph (CCITTITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
) in 1984. Lack of interoperability and standardization, prevented any significant Frame Relay deployment until 1990 when Cisco
Cisco Systems
Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
, Digital Equipment Corporation
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
(DEC), Northern Telecom, and StrataCom
StrataCom
StrataCom, Inc. was founded in Cupertino, California, USA, in January 1986 by 26 former employees of the failing Packet Technologies, Inc. StrataCom produced the first commercial cell switch, also known as a fast-packet switch. Its product was the working proof of the technology which became...
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
Local Management Interface
Local Management Interface is a signaling standard used between routers and frame relay switches. Communication takes place between a router and the first frame relay switch it's connected to...
(LMI).
Datalink connection identifiers (DLCI
DLCI
A data link connection identifier is a Frame Relay 10 bit wide link-local virtual circuit identifier used to assign frames to a specific PVC or SVC. Frame Relay networks use DLCIs to statistically multiplex frames...
s) 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 subinterface
Subinterface
In telecommunications and computer networking, a subinterface is a division of one physical interface into multiple logical interfaces. Routers commonly employ subinterfaces for a variety of purposes, most common of these are for routing traffic between VLANs, and in Non Broadcast Multiple Access...
s).
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
Data terminal equipment
Data Terminal Equipment is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment...
and DCE
Data circuit-terminating equipment
A data circuit-terminating equipment is a device that sits between the data terminal equipment and a data transmission circuit. It is also called data communications equipment and data carrier equipment...
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.
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. All traffic exceeding the CIR is marked discard eligible.Market reputation
Frame Relay aimed to make more efficient use of existing physical resources, permitting the over-provisioning of data services by telecommunications companies to their customers, as 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.Telcos often sell Frame Relay to businesses looking for a cheaper alternative to dedicated line
Dedicated line
In computer networks and telecommunications, a dedicated line is a communications cable or other facility dedicated to a specific application, in contrast with a shared resource such as the telephone network or the Internet....
s; 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
StrataCom
StrataCom, Inc. was founded in Cupertino, California, USA, in January 1986 by 26 former employees of the failing Packet Technologies, Inc. StrataCom produced the first commercial cell switch, also known as a fast-packet switch. Its product was the working proof of the technology which became...
(later acquired by Cisco Systems
Cisco Systems
Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
) and Cascade Communications
Cascade Communications
Cascade Communications was a Westford, Massachusetts based manufacturer of communications equipment.-Product:Cascade made a compact Frame Relay and Asynchronous Transfer Mode communication switches that were sold to telecommunication service providers worldwide...
(later acquired by Ascend Communications
Ascend Communications
Ascend Communications was an Alameda, California-based manufacturer of communications equipment that was later purchased by Lucent Technologies in 1999....
and then by Lucent Technologies
Lucent Technologies
Alcatel-Lucent USA, Inc., originally Lucent Technologies, Inc. is a French-owned technology company composed of what was formerly AT&T Technologies, which included Western Electric and Bell Labs...
).
As of June 2007 AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
was the largest Frame Relay service provider in the USA, with local networks in 22 states, plus national and international networks.
See also
- Multiprotocol label switchingMultiprotocol Label SwitchingMultiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...
- FRF.12FRF.12The FRF.12 specification describes the method of fragmenting Frame Relay frames into smaller frames.-Background:When multiplexing packet data from different virtual circuits or flows, Quality of Service concerns often arise...
- describes the method of fragmenting Frame Relay frames into smaller frames. - List of device bit rates
External links
- RFC 1490 - Multiprotocol Interconnect over Frame Relay
- RFC 1973 - PPP in Frame Relay
- RFC 2427 - Multiprotocol Interconnect over Frame Relay
- The IP/MPLS Forum (MPLS, Frame Relay, and ATM)
- Cisco Frame Relay Tutorial
- Frame Relay animation