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9033029 Flow Control 19
APPENDIX B. FLOW CONTROL
Flow control is a mechanism that controls the flow of data in both
directions to prevent data loss, buffer deadlock, overload conditions and
minimizing communications overhead during transmission. Figure B-1
illustrates flow control for both half and full duplex applications.
Figure B-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 buffer on port Y starts
filling up with data until it reaches a pre-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 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.
Note that for the full duplex pause frame mechanism to work, the device
connected to the switch port must also support IEEE 802.3x flow control.
The pause frame flow control mechanism supported by the ELS100-
8TXUF2 switch conforms with the IEEE 802.3x specification for full duplex
flow control.
ELS100-8TXUF2.book Page 19 Friday, June 4, 1999 4:01 PM