Flow Control Scenario
BCM5718 Programmer’s Guide
Broadcom
®
January 29, 2016 • 5718-PG108-R
Page 577
Switch Backpressure
The Switch will jam ports configured with half-duplex link to slow frame transmission (see
). In this
case, the Server connection must be half-duplex, and then the switch may apply back pressure 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 back pressure 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 59: 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
). 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 60: 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