9032785 Flow Control 87
APPENDIX C. FLOW CONTROL
Flow control is a mechanism which allows you to protect the switch from
overload conditions and to keep additional traffic off the ne twork when
excessive congestion will result. Figure C-1 illustrates flow control for
both half and full duplex applications.
Figure C-1. Flow Control for Full and Half Duplex Applications
Each port of the switch has a transmit queue which buffers frames to be
sent out on that port. In this example, large amounts of data are being
sent from Workstation A (connected to port X) and other ports on the
switch to Workstation B (connected to port Y).The queue on port Y starts
filling up with data until it reaches a determined threshold. The packet
which causes the threshold to be exceeded triggers the Flow C ontrol
function on the port from which the packet entered the switch, in this case
port X.
Since port X is configured with Flow Control set to Enabled, the switch
responds by initiating a pause frame (full duplex applications) or back-
pressure mechanism (half duplex applications). The pause frame causes
Workstation A to stop sending packets. After a certain amount of time has
elapsed, determined by a value in the pause frame, Workstation A will
resume sending data. Similarly, the back-pressure mechanism forces
Workstation A to stop sending packets by inducing collisions on port X.
The pause frame flow control mechanism supported by the
ELS100-24TXM switch conforms with the IEEE 802.3x specification for
full duplex flow control.