CS8900 데이터 시트보기 (PDF) - Cirrus Logic

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CS8900

Highly-Integrated ISA Ethernet Controller

Cirrus Logic 

CS8900

Start Monitoring

Network Activity

Yes Network

Active?

No

Wait

9.6 µs

Tx

Frame

No

Ready and Not

in Backoff?

No

Network Yes

Active?

Yes

Transmit

Frame

Figure 3.8. Simple Deferral

sponds in one of three ways depending on

whether it is a normal collision (within the first

512 bits of transmission) or a late collision (after

the first 512 bits of transmission):

Normal Collisions: If a collision is detected be-

fore the end of the preamble and SFD, the MAC

finishes the preamble and SFD, transmits the jam

sequence (32-bit pattern of all 0’s), and then in-

itiates Backoff. If a collision is detected after the

transmission of the preamble and SFD but before

512 bit times, the MAC immediately terminates

transmission, transmits the jam sequence, and

then initiates Backoff. In either case, if the

Onecoll bit (Register 9, TxCMD, Bit 9) is clear,

the MAC will attempt to transmit a packet a total

of 16 times (the initial attempt plus 15 retrans-

missions) due to normal collisions. On the 16th

collision, it sets the 16coll bit (Register 8,

TxEvent, Bit F) and discards the packet. If the

Onecoll bit is set, the MAC discards the packet

without attempting any re-transmission.

DS150PP2

Late Collisions: 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 (Register 8,

TxEvent, Bit 9). The CS8900 does not initiate

backoff or attempt to re-transmit the frame. For

additional information about Late Collisions, see

Out-of-Window Error in this section.

Backoff: After the MAC has completed transmit-

ting the jam sequence, it must wait, or "Back

off", before attempting to transmit again. The

amount of time it must wait is determined by

one of two Backoff algorithms: the Standard

Backoff algorithm (ISO/IEC 4.2.3.2.5) or the

Modified Backoff algorithm. The host selects

which algorithm through the ModBackoffE bit

(Register 13, LineCTL, Bit B).

Standard Backoff: The Standard Backoff algo-

rithm, also called the "Truncated Binary

Exponential Backoff", is described by the equa-

tion:

0 ≤ r ≤ 2k

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 re-transmission attempts.

Modified Backoff: The Modified Backoff is de-

scribed 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 re-transmission attempts.

The advantage of the Modified Backoff algo-

rithm over the Standard Backoff algorithm is that

it reduces the possibility of multiple collisions

on the first three re-tries. The disadvantage is

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