TNETX3151 datasheet(24/65 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

receive versus transmit priority

The queue manager prioritizes receive and transmit traffic as follows:

D Highest priority is given to frames that currently are being transmitted. This ensures that transmitting frames

do not underrun.

D Next priority is given to frames that are received if the free-buffer stack is not empty. This ensures that

received frames are not dropped unless it is impossible to receive them.

D Lowest priority is given to frames that are queued for transmission but have not yet started to transmit.

These frames are promoted to the highest priority only when there is spare capacity on the memory bus.

D The NM port receives the lowest priority to prevent frame loss during busy periods.

The memory bus has enough bandwidth to support the two highest priorities. The untransmitted frame queues

grow when frames received on different ports require transmission on the same port(s) and when frames are

repeatedly received on ports that are at a higher speed than the ports on which they are transmitted. This is likely

to be exacerbated by the reception of multicast frames, which typically require transmission on several ports.

When the backlog grows to such an extent that the free buffer stack is nearly empty, flow control is initiated (if

it has been enabled) to limit further frame reception.

uplink pretagging

TNETX3151 can be incorporated into a switch where routing decisions can be made at a higher level. To

facilitate this, two forms of tags are provided on ports 12–14;

D Source-port pretag on transmission

D Port-routing-code pretag on reception

source-port pretag on transmission

Ports 12–14 provide the frame’s source-port-number pretag one cycle before MxxTXEN goes high (this tag is

ignored by an externally connected PHY). The 5-bit tag appears as an encoding on terminals MxxTXER and

MxxTXD3 to MxxTXD0 (most significant bit to least significant bit). This is shown in Figure 3 and Table 5.

MxxTCLK

MxxTXER

Transmit Error

Tag MSB

MxxTXD3–MxxTXD0

Preamble and Frame

Tag LS 4 Bits

MxxTXEN

Figure 3. Source-Port Pretag

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