The basic idea is to avoid data accumulation inside the network.
The frame relay network does not use a link-by-link flow control and therefore has a simplified protocol at each switching node. As a result, the offered load has largely determined the performance of frame relay networks. When there are bursts in some services the offered load is high, causing temporary overload at some frame relay nodes. The result is a collapse in network throughput, and therefore, some effective mechanisms are required to control the congestion.
The congestion control in Frame Relay networks includes the following elements:Admission Control: This is the principle mechanism used in frame relay to ensure that resource requirement, once accepted, can be guaranteed. It is also used to achieve high network performance. The decision whether to accept a new connection request should be based on the relation of the network's residual capacity and the requested traffic descriptor, which defined as a set of parameters communicated to the switching nodes at call set-up time or service subscription time.
The traffic descriptor characterizes the connection's statistical properties, and consists of three elements:Committed Burst Size (BC): The maximum number of information units that can be transmitted during the interval T.
Excess Burst Size (BE): The maximum number of uncommitted information units (in bits) that the network will attempt to carry during the interval T.
Committed Information Rate (CIR): The average rate at which the network guarantees to transfer information units over a measurement interval TC. This TC interval is defined as: TC = Bc/CIR .
Once a connection has been established in the network, the edge node of the frame relay network must monitor the connection's traffic flow. This ensures that the actual usage of network resources does not exceed this specification and the user's information rate is within the restrictions defined. It allows the network to enforce the end user's information rate and discard information when the subscribed access rate is exceeded.
Explicit congestion notification is proposed as the congestion avoidance policy. It tries to keep the network operating at its desired equilibrium point so that a certain QOS for the network can be met. To do so, special congestion control bits have been incorporated into the address field of the frame relay: FECN and BECN (Forward and Backward Explicit Congestion Notification).
The basic idea is to avoid data accumulation inside the network.
A node that decides it's congested on a PVC, sets FECN from 0 to 1 on all frames sent forward in the direction that congestion is seen for that PVC.
If there are frames for that PVC going back toward the source, BECN is set from 0 to 1 by the node that experienced congestion in the opposite direction.
It lets the router know that there is congestion and have that router stop transmission until the condition is reversed.
BECN is a direct notification. FECN is an indirect one.
