Congestion: Too many packets present (a part of ) the network causes packet delay and loss that degrades performance. This situation is called congestion.

• The network and transport layers share the responsibility for handling congestion.
• Congestion occurs within the network, it is the network layer that directly experiences it and must ultimately determine what to do with excess packets.

Circuit Switching: Two nodes communicate with each other over a dedicated communication path. In this, a circuit is established to transfer the data. These circuits may be permanent or temporary.

In the above figure, when the data packets arrive at Node from senders A, B and C then the node cannot transmit the data to the receiver at a faster rate. There occurs a delay in transmission or maybe data loss to due to heavy congestion. The data waits in the queue line for the transmission. When the queue line is utilized more than 80%, then the queue line is said to be congested.

There are two techniques for congestion control:

1. Open loop: The open loop congestion control mechanism produces protocols to avoid congestion. These protocols are sent to the source and the destination. It has

   1. retransmission policy
   2. window policy
   3. acknowledgment policy
   4. discarding policy
   5. admission policy

2. Closed-loop: The closed-loop congestion control mechanism produces protocols that allow the system to enter the congested state and then detect and remove the congestion. The explicit and implicit feedback methods help in the running of the mechanism. It has

   1. back pressure
   2. choke packet
   3. implicit signalling
   4. explicit signalling