9032947 Flow Control 81APPENDIX C. FLOW CONTROLFlow control is a mechanism which allows you to protect the switch fromoverload conditions and to keep additional traffic off the network whenexcessive congestion will result. Figure C-1 illustrates flow control for bothhalf and full duplex applications.Figure C-1. Flow Control for Full and Half Duplex ApplicationsEach port of the switch has a transmit queue which buffers frames to besent out on that port. In this example, large amounts of data are beingsent from Workstation A (connected to port X) and other ports on theswitch to Workstation B (connected to port Y).The queue on port Y startsfilling up with data until it reaches a determined threshold. The packetwhich causes the threshold to be exceeded triggers the Flow Controlfunction on the port from which the packet entered the switch, in this caseport X.Since port X is configured with Flow Control set to Enabled, the switchresponds by initiating a pause frame (full duplex applications) or back-pressure mechanism (half duplex applications). The pause frame causesWorkstation A to stop sending packets. After a certain amount of time haselapsed, determined by a value in the pause frame, Workstation A willresume sending data. Similarly, the back-pressure mechanism forcesWorkstation A to stop sending packets by inducing collisions on port X.The pause frame flow control mechanism supported by the ELS100-24TXG switch conforms with the IEEE 802.3x specification for full duplexflow control.
