How does frame relay provide congestion control and quality of service? Explain with its frame structure?

Subject: Advanced Network Technologies

Topic: WAN Technologies

Difficulty: Medium

1 Answer

Congestion Control:

Congestion in Frame Relay decreases throughput and increases delay. A high throughput and low delay is the main goal of Frame Relay protocol. Frame Relay does not have flow control and it allows user to transmit burst data. This means that a Frame Relay network has potential to be really congested with traffic, requiring congestion control. Frame Relay uses congestion avoidance by means of two bit fields present in the Frame Relay frame to explicitly warn source and destination of presence of congestion:


Backward Explicit Congestion Notification (BECN) warns the sender of congestion present in the network. This is achieved by resending the frame in reverse direction with the help of switches in the network. This warning can be responded by the sender by reducing the transmission data rate, thus reducing congestion effects in the network.


Forward Explicit Congestion Notification (FECN) is used to warn the receiver of congestion in the network. It might appear that receiver cannot do anything to relieve the congestion, however the Frame Relay protocol assumes that sender and receiver are communicating with each other and when it receives FECN bit as 1 receiver delays the acknowledgement. This forces sender to slow down and reducing effects of congestion in the network.

Frame Relay :-

Frame Format is shown below:-

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his 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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