What is Frame Relay Technology?

Frame relay is a packet-switched technology which uses routers, bridges or Frame Relay Access Devices (FRADs). These devices are used to convert data into Frame-Relay packets at 56kbps, FT1, and T1 speeds.
Originally, frame relay was designed for use across the Integrated Services Digital Network (ISDN) interface. However, today, most interfaces use this technology to improve performance. It follows the principle of packet-switched technology, which allows sharing the network medium and the available bandwidth to end to end stations. The two techniques used in this 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 the network is prevented from unauthorized users. This technique leads to more flexibility and 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 it 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. It is Layer 2 protocol suite, which enables it to offer greater transmission and higher performance. This makes it suitable for current LAN, WAN, MAN, and DQDB applications.

Uses

Frame relay provides a minimal service in which it 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, it provides a cost-effective secure private IP-based network. Privacy is guaranteed by the nature of the network, backed up by legislation. It provides reliability and high performance just like that of point-to-point leased digital lines.

Frame Relay Devices

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

2.] Data Circuit-terminating Equipment (DCE)
DCEs are carrier-owned internetworking devices 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 the 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 overheads. 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.

The two congestion-control mechanisms implemented are:

1.] Forward-explicit Congestion Notification (FECN)
2.] Backward-explicit Congestion Notification (BECN)

Frame Relay frame headers contain important bits that control FECN and BECN. During periods of congestion, a Discard Eligibility (DE) bit is used to identify less important traffic that can be dropped. This DE bit is present in header. When a DTE device sends frames into the network, the 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 while reaching the destination. The DTE device depends upon higher-layer protocol for processing this information.

In the frame header, the 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 fields.

1.] Flags: It indicates the start and end of the frame. This field's value is always the 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 a very important field of the 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.

ii.] 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.

iii.] C/R: It follows the most significant DLCI byte in the Address field. The C/R bit is not currently defined.

3.] 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.

4.] 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.

5.] 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

Frame Relay is alternative to both dedicated leased lines and X.25 networks for providing connection between LANs to bridges and LANs to routers. It 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. As per requirement, each device uses more bandwidth and thus operates at higher speeds which improve performance.

2.] It discards erroneous frames and thus eliminates time-consuming error-handling processing.

These two factors make this technology a suitable choice for data transmission.