Programmer’s Guide BCM5722
10/15/07
Broadcom Corporation
Document 5722-PG101-R Flow Control Scenario Page 516
SWITCH BACKPRESSURE
The Switch will jam ports configured with half-duplex link to slow frame transmission (see Figure 71). In this case, the Server
connection must be half-duplex, and then the switch may apply backpressure to the port. The Switch will transmit a jamming
pattern, which will prevent the Server from transmitting further packets. The Server’s MAC will detect a collision situation,
and will back off for a specified interval. The Switch will continue to apply backpressure to the Server Ingress, until the Client
egress is available. The Client port will be available when the pause interval expires, and no further pause packets are sent
by the Gigabit Client.
Figure 71: File Transfer Scenario: Switch Backpressure
SWITCH FLOW CONTROL
The Switch can only use IEEE 802.3x flow control when the link is configured for full-duplex operation (see Figure 72). When
buffers are near exhaustion, the switch will send a pause frame to the Server. The Server’s MAC will be inhibited for a
pause_time interval. During the pause interval, the Switch has the opportunity to empty the buffered packets. Once the
buffered packets fall below the high water mark, the Switch may send another pause frame, with pause_time = 0, to
terminate the Server’s pause interval. The Switch may also allow the Server’s pause interval to expire. Either way, the Switch
no longer will inhibit the Server from sending packets.
Figure 72: File Transfer Scenario: Switch Flow Control
Gigabit Server
Switch
Gigabit Client
Jamming
Empty Buffered
Frames
Gigabit Server
Switch
Gigabit Client
Pause packets
Empty Buffered
Frames
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