which the SQE Test message is expected, the
Am79C978 controller will not respond to receive carrier
sense.
first collision. In this case, only the RTRY bit will be set,
and the transmit message will be flushed from the
FIFO.
See ANSI/IEEE Std 802.3-1993 Edition, 7.2.4.6 (1):
If a collision is detected after 512 bit times have been
transmitted, the collision is termed a late collision. The
MAC engine will abort the transmission, append the
jam sequence, and set the LCOL bit. No retry attempt
will be scheduled on detection of a late collision, and
the transmit message will be flushed from the FIFO.
“At the conclusion of the output function, the DTE
opens a time window during which it expects to see
the signal_quality_error signal asserted on the
Control In circuit. The time window begins when
the
CARRIER_STATUS
becomes
CARRIER_OFF. If execution of the output function
does not cause CARRIER_ON to occur, no SQE
test occurs in the DTE. The duration of the window
shall be at least 4.0 µs but no more than 8.0 µs.
During the time window the Carrier Sense Function
is inhibited.”
The ISO 8802-3 (IEEE/ANSI 802.3) Standard requires
use of a “truncated binary exponential backoff” algo-
rithm, which provides a controlled pseudo random
mechanism to enforce the collision backoff interval,
before retransmission is attempted.
See ANSI/IEEE Std 802.3-1990 Edition, 4.2.3.2.5:
TheAm79C978 controller implements a carrier sense
“blinding” period of 4.0 µs length starting from the deas-
sertion of carrier sense after transmission. This effec-
tively means that when transmit two-part deferral is
enabled (DXMT2PD is cleared), the IFS1 time is from
4 ms to 6 ms after a transmission. However, since IPG
shrinkage below 4 ms will rarely be encountered on a
correctly configured network, and since the fragment
size will be larger than the 4 ms blinding window, the
IPG counter will be reset by a worst case IPG shrink-
age/fragment scenario and the Am79C978 controller
will defer its transmission. If carrier is detected within
the 4.0 to 6.0 ms IFS1 period, the Am79C978 controller
will not restart the “blinding” period, but only restart
IFS1.
“At the end of enforcing a collision (jamming), the
CSMA/CD sublayer delays before attempting to re-
transmit the frame. The delay is an integer multiple
of slot time. The number of slot times to delay be-
fore the nth retransmission attempt is chosen as a
uniformly distributed random integer r in the range:
0 ≤ r < 2k Where k = Min (N,10).”
TheAm79C978 controller provides an alternative algo-
rithm, which suspends the counting of the slot time/IPG
during the time that receive carrier sense is detected.
This aids in networks where large numbers of nodes
are present, and numerous nodes can be in collision. It
effectively accelerates the increase in the backoff time
in busy networks and allows nodes not involved in the
collision to access the channel, while the colliding
nodes await a reduction in channel activity. Once chan-
nel activity is reduced, the nodes resolving the collision
time-out their slot time counters as normal.
Collision Handling
Collision detection is performed and reported to the
MAC engine via the COL input pin.
If a collision is detected before the complete preamble/
SFD sequence has been transmitted, the MAC engine
will complete the preamble/SFD before appending the
jam sequence. If a collision is detected after the pream-
ble/SFD has been completed, but prior to 512 bits
being transmitted, the MAC engine will abort the trans-
mission and append the jam sequence immediately.
The jam sequence is a 32-bit all zeros pattern.
This modified backoff algorithm is enabled when EMBA
(CSR3, bit 3) is set to 1.
Transmit Operation
The transmit operation and features of the Am79C978
controller are controlled by programmable options.
TheAm79C978 controller offers a large transmit FIFO
to provide frame buffering for increased system la-
tency, automatic retransmission with no FIFO reload,
and automatic transmit padding.
The MAC engine will attempt to transmit a frame a total
of 16 times (initial attempt plus 15 retries) due to normal
collisions (those within the slot time). Detection of colli-
sion will cause the transmission to be rescheduled to a
time determined by the random backoff algorithm. If a
single retry was required, the 1 bit will be set in the
transmit frame status. If more than one retry was re-
quired, the MORE bit will be set. If all 16 attempts ex-
perienced collisions, the RTRY bit will be set (1 and
MORE will be clear), and the transmit message will be
flushed from the FIFO. If retries have been disabled by
setting the DRTY bit in CSR15, the MAC engine will
abandon transmission of the frame on detection of the
Transmit Function Programming
Automatic transmit features such as retry on collision,
FCS generation/transmission, and pad field insertion
can all be programmed to provide flexibility in the (re-)
transmission of messages.
Disable retry on collision (DRTY) is controlled by the
DRTY bit of the Mode register (CSR15) in the initializa-
tion block.
68
Am79C978
AMD [ AMD ]
