CS8900A
Crystal LAN™ ISA Ethernet Controller
3.9.5.6 Late Collisions
gorithms: the Standard Backoff algorithm
(ISO/IEC 4.2.3.2.5) or the Modified Backoff algo-
rithm. The host selects which algorithm through
the ModBackoffE bit (Register 13, LineCTL,
Bit B).
If a collision is detected after the first 512 bits have
been transmitted, the MAC immediately terminates
transmission, transmits the jam sequence, discards
the packet, and sets the Out-of-window bit (Regis-
ter 8, TxEvent, Bit 9). The CS8900A does not ini-
tiate backoff or attempt to retransmit the frame. For
additional information about Late Collisions, see
Out-of-Window Error in this section.
3.9.5.8 Standard Backoff
The Standard Backoff algorithm, also called the
"Truncated Binary Exponential Backoff", is de-
scribed by the equation:
3.9.5.7 Backoff
0 ≤ r ≤ 2k
After the MAC has completed transmitting the jam
sequence, it must wait, or "Back off", before at-
tempting to transmit again. The amount of time it
must wait is determined by one of two Backoff al-
where r (a random integer) is the number of slot
times the MAC must wait (1 slot time = 512 bit
times), and k is the smaller of n or 10, where n is the
number of retransmission attempts.
3.9.5.9 Modified Backoff
Start Monitoring
Network Activity
The Modified Backoff is described by the equation:
0 ≤ r ≤ 2k
where r (a random integer) is the number of slot
times the MAC must wait, and k is 3 for n < 3 and
k is the smaller of n or 10 for n ≥ 3, where n is the
number of retransmission attempts.
Yes
Network
Active?
No
The advantage of the Modified Backoff algorithm
over the Standard Backoff algorithm is that it re-
duces the possibility of multiple collisions on the
first three retries. The disadvantage is that it ex-
tends the maximum time needed to gain access to
the network for the first three retries.
Wait
9.6 µs
The host may choose to disable the Backoff algo-
rithm altogether by setting the DisableBackoff bit
(Register 19, TestCTL, Bit B). When disabled, the
CS8900A only waits the 9.6 µs IPG time before
starting transmission.
No
T
x
Fra e
m
Ready and Not
ac off?
No
Yes
Network
Active?
i
n
B
k
3.9.5.10 SQE Test
Yes
If the CS8900A is transmitting on the AUI, the ex-
ternal transceiver should generate an SQE Test sig-
nal on the CI+/CI- pair following each
transmission. The SQE Test is a 10 MHz signal
lasting 5 to 15 bit times and starting within 0.6 to
Transmit
Frame
Figure 11. Simple Deferral
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