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April, 2002
L84302 Quad 4-Port Ethernet Controller - Technical Manual
Copyright 1997-2002 by LSI Logic Corporation. All rights reserved.
3.16 MAC Control Frames
3.16.1 General
MAC Control frames are special packets which only contain signaling
information. MAC Control frames are specified in IEEE 802.3 Clause 31
for Full Duplex operation only and are used primarily for Full Duplex flow
control.
MAC Control frames defined in IEEE 802.3 Clause 31 are differentiated
from other packets by having the unique value of 8808
H
in the
length/type field. MAC Control frames have the same packet format as
normal Ethernet packets except the Data field is composed of an opcode
field and a parameter field. The opcode field contains an opcode
command, the parameter field contains a value associated with the
opcode command. The only opcode command defined to date by IEEE
802.3x is the Pause opcode; the parameter field for the Pause opcode
defines the pause time. MAC Control frames with the Pause opcode,
referred to as Pause frames, are only allowed to have a destination
address equal to a specific reserved multicast address or the address of
the receive station itself. The value of the reserved multicast address is
01-80-C2-00-00-01
H
.
The L84302 normally treats MAC Control frames according to the IEEE
802.3 Clause 31 algorithm. When the receive MAC detects a MAC
Control frame with a Pause opcode and with the destination address
equal to the reserved multicast address or the address stored in the MAC
Station Address 0-5 registers, then the transmitter is paused for a time
equal to the number of pause times specified in the parameter field. Each
unit of pause time equals 512 bits (5.12 uS for 100 Mbps, 51.2 uS for 10
Mbps). If a Pause frame is received while another packet is being
transmitted, then the transmission is completed for the current packet
being transmitted, and then the transmitter is paused. If other packets are
in the transmit FIFO, their transmission is delayed until the pause timer
has expired. MAC Control frames are not normally passed into the
receive FIFO; they are terminated in the receive MAC.
The L84302 also has incorporated some additional features to facilitate
MAC Control frame operation. These features are described in the
following sections.