Frame Relay Technology

Frame Relay is operate at physical and data link layers of OSI reference model, which is used as a high-performance Wide Area Network protocol. Originally Frame Relay was designed for use across Integrated Services Digital Network (ISDN) interfaces. Today, most of interfaces use Frame Relay to improve performance.

Frame Relay follows principle of packet-switched technology. Packet-switched networks allow dynamically share the network medium and the available bandwidth to the end to end stations. The two techniques used in packet-switching technology are:

1) Variable-length packets:
For more efficient and flexible data transfers variable-length packets are used. These packets are switched between the various segments in the network until the particular destination is reached.

2) Statistical multiplexing:
In a packet-switched network Statistical multiplexing techniques control network access so that network is prevented from unauthorized user. This technique leads to more flexibility and more efficient use of bandwidth.

Frame Relay competes with X.25 permanent virtual circuit but provides fewer robust capabilities such as windowing and retransmission of lost data that are offered in X.25. This is because Frame Relay typically operates over WAN facilities that offer more reliable connection services and a higher degree of reliability than the facilities available for X.25 WANs. Frame Relay is Layer 2 protocol suite, which enables Frame Relay to offer greater transmission and higher performance and makes Frame Relay suitable for current LAN, WAN, MAN and DQDB applications.

Uses of Frame Relay:
Frame Relay provides a minimal service in which Frame Relay is used as a low cost carrier to replace the networks of leased lines used to connect ATM machines, POS terminals and other devices to mainframes for client-server applications. These applications require protocol conversion to send-up the equipment at both ends. For companies with numerous distributed offices, Frame Relay provides a cost effective secure private IP based network. Frame Relay privacy is guaranteed by the nature of the network backed up by legislation. Many Frame Relay connections are used for end to end Internet connections.

Frame Relay is an advance technology, which is well implemented worldwide. It provides reliability and high performance just like that of point-to-point leased digital lines.

Frame Relay Devices
Devices attached to a Frame Relay are of two types:

1) Data terminal equipment (DTE):
DTEs are located on the premises of a client and considered to be terminating equipment for a specific network. DTE devices are bridges, terminals, personal computers and routers. These devices may be owned by client.

2) Data circuit-terminating equipment (DCE):
DCEs are carrier-owned internetworking device that actually transmit data through the WAN. The purpose of DCE equipment is to provide clocking and switching services in a network.

Physical layer component and link layer component are used between a DTE device and a DCE device. The physical layer component defines the mechanical, electrical, functional and procedural specifications for the connection between the devices. One of the most commonly used physical layer interface specifications is RS-232 standard specifications. The link layer component defines the protocol that establishes the connection between the DTE device such as a router and the DCE device such as a switch.

Congestion-Control Mechanisms:
Frame Relay implements simple congestion control mechanism rather than complicated explicit per-virtual-circuit flow control, which involves certain overhead. It is mainly implemented for reliable network media so that data integrity is not a major problem because flow control can be left to higher-layer protocols.
Frame Relay implements two congestion-control mechanisms:
1) Forward-explicit congestion notification (FECN)
2) Backward-explicit congestion notification (BECN)
Frame Relay frame header contain single important bit that control FECN and BECN. During periods of congestion, Discard Eligibility (DE) bit is used to identify less important traffic that can be dropped. This DE bit is present in header of Frame Relay.
In the Frame Relay frame header FECN bit is part of the Address field. When a DTE device sends Frame Relay frames into the network then FECN mechanism is initiated. DCE devices set the value of the frames FECN bit to 1 when the network is congested. When the frames reach the destination DTE device, the Address field with the FECN bit set indicates that the frame suffered from congestion in the path while reaching to destination. The DTE device depends upon higher-layer protocol for processing this information.

In the Frame Relay frame header BECN bit is part of the Address field. DCE devices set the value of the BECN bit to 1 in frames traveling in the opposite direction of frames with their FECN bit set. This provides information to the receiving DTE device that a particular path is congested in the network. The DTE device depends upon higher-layer protocol for this information processing.

Frame Relay Frame Formats:
The following are basic Frame Relay frame fields.
1) Flags: It indicates the start and end of the frame. This field’s value is always same and is represented in hexadecimal number 7E or in binary number 01111110.
2) Address: This field contains the following information
i) DLCI: The 10-bit DLCI is the very important field of the Frame Relay header. This field represents the virtual connection between the DTE device and the switch. Each virtual connection which is multiplexed onto the physical channel will be represented by a unique DLCI. The DLCI values have local significance only, which means that they are unique only to the physical channel on which they reside.
2) Extended Address (EA): The EA is used to indicate whether the byte in which the EA value is 1 is the last addressing field. If the value is 1, then the current byte is determined to be the last DLCI octet.
3) C/R: It follows the most significant DLCI byte in the Address field. The C/R bit is not currently defined.
4) Discard eligibility (DE): It is set by the DTE device such as a router to indicate that the marked frame is less important than other transmitted frames. Frames that are marked as "discard eligible" should be discarded before other frames in a congested network.
5) Data: It contains encapsulated upper-layer data. Each frame in this field includes a user data or payload field that will vary in length. This field serves to transport the higher-layer protocol packet (PDU) through a Frame Relay network.
6) Frame Check Sequence: It ensures the integrity of transmitted data. This value is computed by the source device and verified by the receiver to ensure integrity of transmission

Advantages of Frame Relay
Frame Relay is alternative to both dedicated leased lines and X.25 networks for providing connection between LANs to bridges and LANs to routers. Frame Relay is based on the following two important factors:
1) While transporting data virtual circuits consume large bandwidth. Many virtual circuits can exist simultaneously across a given transmission line over network. As per requirement each device uses more bandwidth and thus operates at higher speeds which improve the performance of Frame Relay.
2) Frame Relay discard erroneous frames and thus eliminate time-consuming error-handling processing.
These two factors make Frame Relay a one of suitable choice for data transmission.