TNETX3151 datasheet(45/65 Pages) TI1

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TNETX3151

ThunderSWITCH

™ 12/3 ETHERNET™ SWITCH

WITH 12 10-MBIT/S PORTS AND 3 10-/100-MBIT/S PORTS

SPWS048A – MARCH 1998 – REVISED APRIL 1999

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

collision-based flow control

Collision-based flow control provides a means of preventing frame reception for ports that are operating in

half-duplex mode. While the number of free buffers is fewer than the specified threshold, ports in this state that

are not currently transmitting, generate collisions when they start to receive a frame. The jam sequence

transmitted (55.55.55.55.55.55.55.5D.DD.DD.DD.DD (hex) starts approximately when the source address

starts to receive. Port 8 begins jam sequence after approximately eight bytes of payload data (i.e., after the

source address) have been received.

These forced collisions are not limited to a maximum of 16 consecutive collisions, and are independent of the

normal backoff algorithm.

IEEE Std 802.3 flow control

IEEE Std 802.3X flow control provides a means of reducing network congestion on ports that are operating in

full duplex mode, via special pause frames. It is symmetrical, so that devices that transmit pause frames must

also respond to received pause frames.

Pause frames and their behavior are fully described in the IEEE Std 802.3X standard, but in essence they

comprise:

D 48-bit multicast destination address 01.80.C2.00.00.01

D 48-bit source address – is read from the Devnode register when transmitted by this device.

D 16-bit length/type field, containing the value 88.08

D 16-bit pause opcode equal to 00.01

D 16-bit pausetime. This specifies a nonzero number of pausequanta. A pausequantum is 512 bit times.

D Padding as required/desired

D 32-bit frame-check sequence (CRC word)

All quantities above are hexadecimal and are transmitted most significant byte first. Within the byte they are

transmitted least significant bit first.

The padding is required to fill out the frame to a minimum of 64 bytes. The standard allows pause frames longer

than 64 bytes to be discarded or interpreted as valid pause frames. This device recognizes any pause frame

that is between 64 bytes and 1531 bytes long. It always transmits 64-byte pause frames.

Each 10-/100-Mbit/s port can request IEEE Std 802.3X pause-frame support via the reqnp (= 0) bit within its

Portxcontrol register. The 100-/1000-Mbit/s port has independent control for transmission and reception of IEEE

Std 802.3X pause frames, and can request IEEE Std 802.3X flow control via the reqntxp (= 0) and reqnrxp (= 0)

bits within its Portxcontrol.

Outgoing pause frames are issued only when:

D pause (10/100) = 1

D The port is operating in the full-duplex mode.

D flow = 1 in Syscontrol

Incoming pause frames are acted on only when:

D pause = 1 or pausetx = 1 (i.e., incoming pause frames are recognized in both full-duplex and half-duplex

modes, regardless of the value of the flow bit)

TNETX3151 数据表(PDF) 45 Page - Texas Instruments

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