TNETX4080GGP datasheet(45/64 Pages) TI1

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TNETX4080

ThunderSWITCH II™ 8-PORT 10-/100-MBIT/S ETHERNET™ SWITCH

SPWS050A – AUGUST 1998 – REVISED NOVEMBER 1998

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

flow control

The switch incorporates two forms of flow control: collision based, and IEEE Std 802.3 pause frames.

In either case, the switch recognizes when it is becoming congested by monitoring the size of the free-buffer

queue. When the number of free buffers drops below the specified threshold, the switch prevents frames from

entering the device by issuing the flow control appropriate to each port’s current mode of operation. This

prevents reception of any more frames on those ports until the frame backlog is reduced and the number of free

buffers has risen above the threshold, at which point, flow control ceases and frames again can be received.

The default free-buffer threshold after a hardware reset is chosen to ensure that all ports simultaneously can

start reception of a maximum-length frame and ensure complete reception.

The purpose of flow control is to reduce the risk of data loss if a long burst of activity caused the switch to backlog

frames to the point where the memory system is full. However, there is no way to prevent frame reception on

those ports operating in full-duplex mode that have not negotiated IEEE Std 802.3 flow control. Such ports can

exhaust the free-buffer queue, with subsequent data loss.

Each 10-/100-Mbit/s port can request collision or IEEE Std 802.3x flow control through internal registers.

Flow control (both forms) is globally enabled/disabled. Each individual port can request half full-duplex or

IEEE Std 802.3 flow to be negotiated by the PHY device.

In full duplex, a port does not start transmitting a new frame if the collision pin is active, although the value of

this pin is ignored at other times.

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) begins 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.

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