Frame Relay Network

Frame relay is a high-speed Wide Area Network (WAN) protocol that operates at the physical and data link layers of OSI reference model. Frame relay originally was designed for use across Integrated Service Digital Network (ISDN). Some important features of frame relay are,

·        Operates in the physical and data link layers

·        Operates at a higher speed (1.544 Mbps and 44.376 Mbps).

·        Allows a frame size of 9000 bytes.

·        Less expensive as compare to other WANs

·        Frame relay detect errors at data link layer.

·        No flow control and error control

·        Is a virtual circuit network

·         Every virtual circuit in frame relay represent by using numbers that are usually known as data link connection identifier (DLCI).

Frame relay Network

 

Frame Relay Protocol Data Unit

Each Frame relay frame consist data and some control information. A typical frame relay, frame structure is given below,

Frame Relay FRAME

Frame Relay Address Fields

FCS: Frame Check Sequence 

C/R: Command/response

EA: Extended address

FECN: Forward explicit congestion notification

BECN: Backward explicit congestion notification

DE: Discard eligibility

DLCI: Data link connection identifier

Address (DLCI) field.

 The first 6 bits of the first byte makes up the first part of the  DLCI. The second part of the DLCI uses the first 4 bits of the second byte.

The C/R bit indicates whether the frame is a command frame or a response frame.

The extended address (EA) bit indicates whether the current byte is the final byte of the address.  When it is set to 0, then the next octet contains the rest of the address; otherwise, it indicates the last field of the address. This bit can be used to represent extended addressing.

The BECN and FECN bits are used to avoid congestion. They are used when a congestion situation is about to occur in one direction or in the reverse direction. The source endpoint that receives this information will reduce its throughput. We will discuss congestion later.

The DE bit allows the network nodes to indicate which frames will be eliminated first in case of congestion

Extended Address

To increase the range of DLCIs, the Frame Relay address has been extended from the original 2-byte address to 3- or 4-byte addresses.

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