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DLCIs

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PVC DLCIs

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SVC

• Each time two DTEs want to communicate a new VC should be established

• This is done by the services of another protocol that has network layer and network layer addresses (such as ISDN or IP)

• The signaling mechanism of this other protocol makes a connection request using the network layer address.

• After connection phase the VC is established DTEs can exchange data.

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SVC Setup and Release

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SVC DLCIs

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DLCI Inside the Network

• The DLCI are also assigned between two DCEs (switches) inside the network

• A switch assigns a DLCI to each VC in an interface which is unique for a particular interface.

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DLCIs Inside a Network

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Frame Relay Switch

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Frame Relay Layers

• Has only physical and datalink layer.

• Frame Relay has 1.5 layers as compared to X.25 which has 3 layer.

• Frame Relay eliminates all the network layer functions and portion of the conventional data link functions.

• It therefore save processing time of complete 1.5 layers.

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Frame Relay Layers

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Comparing Layers in Frame Relay and X.25

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Frame Relay Frame

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Congestion Control

• Occurs if data rate is greater than that allowed by network resources. (limited buffer size)

• Frame Relay does not use flow control at data link layer and does not have network layer.

• Frame Relay allows users to transmit bursty traffic and therefore has potential to congest the network.

• Two approaches are adopted for congestion control one is congestion avoidance and the other is traffic control.

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Congestion Avoidance• Backward Explicit Congestion Notification

(BECN)– Warns sender of congestion so that it can

decrease the data rate for transmission.– In full duplex mode response frames from the

receiver or predefined connection (DLCI=1023) frame, from switch are used for this specific purpose.

– Sender’s response to the warning is by reducing the rate.

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• Forward Explicit Congestion Notification (FECN)– Warns receiver of congestion in network.

– Frame Relay assumes that sender and receiver are in communication and are using some kind of flow control at higher level.

• Discarding– If user does not respond to congestion notice the Frame

Relay discards frames.

– Which Frames are discarded are subjected to section called traffic control.