DM8108 Просмотр технического описания (PDF) - Unspecified

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DM8108

8 port 10/100M Fast Ethernet Switching Controller

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DM8108

8 port 10/100M Fast Ethernet Switching Controller

Media access management

IEEE 802.3 protocols define a media access

mechanism that permits all stations to access the

channel with equality. Any node can attempt to

connect for the channel by waiting a predefined period

of time (Inter Packet Gap) after the last activity before

transmitting on the media. If two nodes

simultaneously contend for the channel, their signals

will interact causing loss of data, defined as collision.

It is the responsibility of the MAC to attempt to avoid

and recover from the end-to-end transmission to the

receiving station.

T Medium allocation (collision avoidance, except

in full-duplex operation)

The MAC will monitor the medium for traffic by

watching for carrier activity. When the carrier is

detected, the media is considered as busy, and the

MAC should defer to the existing message.

The MAC implements the IEEE-802.3 defined two

part deferral algorithm, with Inter-Frame-Spacng-

Part1 (IFS1) time for 64-bit time (6.4 us for10-BASE

and 640 ns for 100-BASE). The Inter-Frame-

Spacing-Part2 (IFS2) interval is, therefore, 32-bit time.

The Inter Packet Gap (IPG) timer will start timing the

96-bit time Inter-Frame-Spacing after the receiving

carrier is de-asserted. During the IFS1, the MAC will

defer any pending transmit frame and respond to the

receive message. The IPG counter will be cleared to

0 continuously until the carrier de-asserts, at which

point the IPG will resume the 96-bit time count again.

Once the IFS1 period has completed and the IFS2

has commenced, the MAC will not defer to the

receiving frame if a transmit frame pending. The MAC

will not attempt to receive the receiving frame, since it

will start transmit and generate a collision at 96-bit

time. The MAC will complete the preamble (64-bit)

and JAM (32-bit) sequence before ceasing

transmission and invoking the random back-off

algorithm.

T Contention resolution (collision handling,

except in full-duplex mode)

If a collision is detected through COL pin 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 preamble/SFD has

been completed, but prior to 512 bits being

transmitted, the MAC will abort the transmission and

append the JAM sequence immediately. The JAM

sequences is a 32-bit all “34” pattern.

The MAC will attempt to transmit a frame a total of 16

times (15-retries) due to normal collisions (those

within the slot time). Detection of collision will cause

the transmission to be re-scheduled to a time

determined by the random back-off algorithm. If 16

attempts experienced collisions, the transmitting

message will be flushed from FIFO.

If a collision is detected after 512-bit times have been

transmitted, the collision is termed “Late” collision.

The MAC will abort the transmission, append the JAM

sequence. No retry attempt will be scheduled on

detection of late collision, and transmit message will

be flushed from the FIFO.

The MAC implements the truncated exponential

back-off algorithm defined by the 802.3 standard.

In full-duplex mode, the MAC transmits

unconditionally.

Preliminary

15

Version: DM8108-DS-P02

November 25, 1999

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