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Functional Operation
Chapter 3
AMD-8111 HyperTransport I/O Hub Data Sheet
24674
Rev. 3.00
April 2003
AMD Preliminary Information
The MAC engine can be programmed to try to transmit the same frame again after a FIFO underflow.
The transmitter backoff logic can also be programmed to treat late collisions just like normal
collisions.
The status of each receive message is available in the appropriate Receive Message Descriptor
(RMD). All received frames are passed to the host regardless of any error.
During the reception, the FCS is generated on every nibble (including the dribbling bits) coming from
the MII, although the internally saved FCS value is only updated on each byte boundary. The MAC
engine ignores an extra nibble at the end of a message, which corresponds to dribbling bits on the
network medium. A framing or alignment error is reported to the user if an FCS error is detected and
there is an extra nibble in the message. If there is an extra nibble but no FCS error, no framing error is
reported.
3.10.4.3
Media Access Management
A basic requirement for all stations on the network is to provide fair access to the network. The IEEE
802.3/Ethernet protocols define a media access mechanism which permits all stations to access the
channel with equality. Any node can attempt to contend for the channel by waiting for a
predetermined time (Inter Packet Gap) after the last activity, before transmitting on the media. The
channel is a multidrop communications medium (with various topological configurations permitted),
which allows a single station to transmit and all other stations to receive. If two nodes simultaneously
contend for the channel, their signals interact causing loss of data, defined as a collision. It is the
responsibility of the MAC to attempt to avoid and to recover from collisions.
3.10.4.3.1 Medium Allocation
The IEEE standard 802.3, 1998 Edition, requires that the CSMA/CD MAC monitor the medium for
traffic by watching for carrier activity. When carrier is detected, the medium is considered busy, and
the MAC should defer to the existing message.
The standard allows an optional two-part deferral after a receive message. For details, see the IEEE
standard 802.3, 1998 Edition, 4.2.3.2.1.
The MAC engine implements the optional receive two part deferral algorithm, with an
InterFrameSpacing-Part1 (IFS1) time of 60 bit times and an InterFrameSpacingPart 2 time of 36 bit
times. The Inter Packet Gap (IPG) timer starts timing the 96-bit InterFrameSpacing after the receive
carrier is deasserted.
During the first part deferral (IFS1), the MAC engine defers any pending transmit frame and responds
to the receive message. If carrier sense or collision is detected during the first part of the gap, the IPG
counter is cleared to 0 continuously until carrier sense and collision are both deasserted, at which
point the IPG counter resumes the 96-bit time count once again. Once the IPG counter reaches the
IFS1 count (60-bit times), the MAC engine does not defer to a receive frame if a transmit frame is
pending. Instead, when the IPG count reaches 96-bit times, the transmitter starts transmitting, which
causes a collision. The MAC engine completes the preamble (64-bit) and jam (32-bit) sequence
before ceasing transmission and invoking the random backoff algorithm.