BCM5722 Programmer’s Guide
10/15/07
Broadcom Corporation
Page 513 Notes Document 5722-PG101-R
Appendix A: Flow Control
NOTES
Developers can refer to the IEEE 802.3 Annex 31B specification for detailed information on Ethernet flow control
mechanisms.
• Flow control frames use a well-known multicast address, defined in the 802.1D Bridging specification The MAC
destination address is 01-80-C2-00-00-01
• Bridges and Switches will not forward pause frames to downstream ports.
• A pause frame contains a request_operand that contains a pause_time field. Pause_time specifies the number of
quanta, which transmission should be inhibited.
• Pause frames cannot inhibit MAC control Frames
• Pause_time is a two-octet field, which represents a quanta value. The quanta value is based on bit/slot times for the
connection speed. Valid pause_times vary from 0 to 65535.
• The pause frame contains a MAC control opcode. 00-01 is reserved for PAUSE MAC control functions.
• MAC control layers will provide two indicators—paused and not paused.
• The Enet source address equals the unicast address of the MAC sublayer, which transmits the pause_frame.
• The receive engine will set a countdown timer, based on the value of pause_time. When the timer expires, the transmit
engine may resume send operation
• A Mac sublayer may transmit pause frames with pause_time = 0. The zero value will stop a pause count down,
executed by the MAC’s link partner. Effectively, a value of zero restarts a link partner’s transmit engine, assuming the
link partner was inhibited by a previous pause operation.
FLOW CONTROL SCENARIO
This scenario assumes that the Gigabit switch has a 1:1 port mapping, between the Gigabit Server and Client. The switch
does not implement header aligned blocking, nor will it drop packets to alleviate buffer pressure. The following constraints
are placed on this scenario:
• Client
- Full-duplex connection at Gigabit speed.
- Implements flow control.
- Flow control enabled.
• Switch
- Does not drop packets.
- Full-duplex connection to Client.
• Server
- Gigabit connection.
- Either half/full-duplex connection at Gigabit speed (i.e., this scenario will cover two subcases).
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