Explain Frame format of Frame Relay?

Subject: Advanced Network Technologies

Topic: WAN Technologies

Difficulty: Medium

1 Answer

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Fig. Shows the frame relay frame format. This frame is very similar to the HDLC frame except for the missing control field here.

• The control field is not needed because flow and error control are not needed.

• The Flag, FCS and information fields are same as those of HDLC.

• The address field defines the DLCI along with some other bits required for congestion control and traffic control.

• Their description is as follows:

1. DLCI field:

The first part of DLCI is of 6 bits and the second part is of 4 bits. They together form a 10 bit data link connection identifier.

2. Command / Response (C / R):

The C/R bit allows the upper layers to identify a frame as either a command or response. It is not used by the frame relay protocol.

3. Extended Address (EA):

• This bit indicates whether the current byte is the final byte of the address.

• If EA = 1 it indicates that the current byte is the final one but if EA = 0, then it tells that another address byte is going to follow.

4. Forward Explicit Congestion Notification (FECN):

• This bit can be set by any switch to indicate that traffic is congested in the direction of travel of the frame.

• The destination is informed about the congestion via this bit.

5. Backward Explicit Congestion Notification (BECN):

• This bit indicates the congestion in the direction opposite to the direction of frame travel.

• It informs the sender about the congestion.

6. Discard Eligibility (DE):

• The DE bit indicates the priority level of the frame. In the overload situations a frame may have to be discarded.

• If DE = 1 then that frame can be discarded in the event of congestion.

• DE bit can be set by the sender or by any switch in the network.

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