6 WX Window size 3 Time at the Time at the transmitter receiver Permit returns Flow Control 6.1 INTRODUCTION In most networks,there are circumstances in which the externally offered load is larger than can be handled even with optimal routing.Then,if no measures are taken to restrict the entrance of traffic into the network,queue sizes at bottleneck links will grow and packet delays will increase,possibly violating maximum delay specifications. Furthermore,as queue sizes grow indefinitely,the buffer space at some nodes may be exhausted.When this happens,some of the packets arriving at these nodes will have to be discarded and later retransmitted,thereby wasting communication resources.As a result,a phenomenon similar to a highway traffic jam may occur whereby,as the offered load increases,the actual network throughput decreases while packet delay becomes excessive.It is thus necessary at times to prevent some of the offered traffic from entering the network to avoid this type of congestion.This is one of the main functions of flow control. Flow control is also sometimes necessary between two users for speed matching. that is,for ensuring that a fast transmitter does not overwhelm a slow receiver with more packets than the latter can handle.Some authors reserve the term"flow control"for this 4936 j Time at the transm iller Window size = 3 j Time at the receiver Permit returns Flow Control 6.1 INTRODUCTION In most networks, there are circumstances in which the externally offered load is larger than can be handled even with optimal routing. Then, if no measures are taken to restrict the entrance of traffic into the network, queue sizes at bottleneck links will grow and packet delays will increase, possibly violating maximum delay specifications. Furthermore, as queue sizes grow indefinitely, the buffer space at some nodes may be exhausted. When this happens, some of the packets arriving at these nodes will have to be discarded and later retransmitted, thereby wasting communication resources. As a result, a phenomenon similar to a highway traffic jam may occur whereby, as the offered load increases, the actual network throughput decreases while packet delay becomes excessive. It is thus necessary at times to prevent some of the offered traffic from entering the network to avoid this type of congestion. This is one of the main functions of flow control. Flow control is also sometimes necessary between two users for speed matching, that is, for ensuring that a fast transmitter does not overwhelm a slow receiver with more packets than the latter can handle. Some authors reserve the term "flow control" for this 493