9032579-03 Flow Control 81
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 network 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 Workstati on B (connected to por t 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 Control
function on the port from which the packet entered the switch, in thi s 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 app licat ions) . The p ause fram e cause s
Workstation A to stop sending packe ts. After a c ertain a mount of ti me 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.