Cirrus Logic EP93xx - Manual (Page 307)

DS785UM1 9-5
Copyright 2007 Cirrus Logic
1/10/100 Mbps Ethernet LAN Controller
EP93xx User’s Guide
9
9
9
9.1.3 Packet Transmission Process
This section explains the complete packet transmission process as seen on the Ethernet line.
This process includes: carrier deference, back-off, packet transmission, transmission of EOF,
and SQE test. Refer to
Figure 9-3 .
Figure 9-3. Packet Transmission Process
The Ethernet/ISO/IEC 8802-3 topology is a single shared medium with several stations. Only
one station can transmit at a time. The access method is called Carrier Sense Multiple
Access with Collision Detection (CSMA/CD). This method is a “listen before talk” mechanism
that has an added feature to end transmissions when two, or more, stations start
transmissions at nearly the same time.
The CSMA portion of this method provides collision avoidance. Each station monitors its
receiver for carrier activity. When activity is detected, the medium is busy, and the MAC
defers (waits) until the medium no longer has a carrier.
9.1.3.1 Carrier Deference
Refer to Figure 9-4 . Once sufficient bytes have been written to the transmit FIFO, the MAC
layer immediately moves to the Carrier Deference State Diagram. The Carrier Deference
state is independent of entry into the state diagram. The MAC layer may enter the state
diagram in any of its five states. The MAC layer exits the Carrier Deference only from the IFG
The Packet Transmission Process
Start of Transmit
Frame in fifo
Carrier Deference is
detailed in the next diagram.
Transmission
Transmission ends with
either completion of the
frame, or a collision.
Transmission
Complete
Transmit EOF
Report Applicable
Transmit
Status
Late
Collision
?
NO
YES
There was
a collision
Max
number of
Collisions
?
NO
YES
Wait for the duration
of the BackOff Timer
Carrier
Deference
Transmit JAM
The maximum number of
collisions is either 16 or 1
depending on the Onecoll bit
in the transmit descriptor.
The backoff time is a computed
random number based on either
the standard algorithm or the
modified back-off algorithm.
See the ModBackoffE bit
in TxCTL.
Report Applicable
Transmit
Status
Report Applicable
Transmit
Status