Xilinx LOGICORE UG144 - Manual (Page 55)

1-Gigabit Ethernet MAC v8.5 User Guide www.xilinx.com 55
UG144 April 24, 2009
Overview of Flow Control R
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Pause Control Frames
Control frames are a unique type of Ethernet frame, defined in clause 31 of the IEEE 802.3-
2005 standard. Control frames are differentiated from other frame types by a defined value
placed in the length/type field (MAC Control Type code).
Figure 6-2 illustrates the control
frame format.
A pause control frame is a unique type of control frame, identified by a defined value
placed in the MAC Control opcode field.
Note: MAC Control opcodes other than for pause (flow control) frames have recently been defined
for Ethernet Passive Optical Networks.
The MAC control parameter field of the pause control frame contains a 16-bit field
containing a binary value directly related to the pause duration. This defines the number of
pause_quantum
(512 bit times of the particular implementation). For 1-Gigabit Ethernet,
a single pause_quantum corresponds to 512 ns.
Figure 6-2: MAC Control Frame Format
DESTINATION
ADDRESS
SOURCE
ADDRESS
LENGTH/TYPE
MAC CONTROL
OPCODE
MAC CONTROL
PARAMETERS
RESERVED
(transmitted as zeroes)
6 OCTETS
6 OCTETS
2 OCTETS
2 OCTETS
FRAME
TRANSMITTED
TOP-TO-BOTTOM
44 OCTETS
FRAME CHECK
SEQUENCE
4 OCTETS