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112 Intel 8255x 10/100 Mbps Ethernet Controller Family Open Source Software Developer Manual
Host Software Interface
A flow control frame is identified by a special type field (bytes transmitted left to right): 88 to 80.
The reception of a FC frame can be done either through the regular individual address filtering
mechanism or by a special multicast address detection mechanism. Since the transmitting MAC
may not know the individual address of the other station, it may not be able to send FC frames to
the individual address of the other node. A special multicast destination address is defined for use
in FC frames. Special address filtering is used to receive frames addressed to that multicast
address. The address is (bytes transmitted left to right): 01-80-C2-00-00-01.
6.6.2.2 Pause Operation
When a station’s receive buffer reaches a high watermark (running out of storage space for
incoming packets), it may send its link partner a pause frame (also known as an XOFF frame).
When the station receives the pause request, it stops transmitting for a number of slot times as
specified by the time parameter in the pause frame. The time parameter is a 2 byte field.
The reaction time for a transmitting station (the congester) is 512 bit times from the end of the
received pause frame. If the congester is transmitting when the time expires, it should complete the
transmitted frame. The congester should not start transmitting a frame after the 512-bit time period.
Transmission is restarted upon expiration of the pause timer or upon reception of a pause request
with a time parameter of zero (also known as an XON frame).
6.6.2.3 Flow Control Functionality
Note: The 82557 does not support flow control functionality.
Table 57. Flow Control Frame Format
S one byte
Pre-amble up to 6 bytes
SFD one byte
Destination
Address
6 bytes
Source Address 6 bytes
Type/Length 2 bytes
MAC Control
Opcode
2 bytes
MAC Control
Parameters
FCS 4 bytes
T one byte