Intel IXP1200 3.2 ATM Transmit Microengine

IXP1200 Network Processor Family ATM OC-3/12/Ethernet IP Router Example Design

22 Application Note

Modified on: 3/20/02,

3.2 ATM Transmit Microengine

The ATM Transmit microengine is an AAL5 Unspecified Bit Rate (UBR) Transmitter that uses a

single microengine to move cells at wire-rate in either single OC-12 or up to four OC-3 port

configurations. No attempt is made to mix, schedule, or otherwise ’shape’ the order of the cells on

the wire.

The transmitter consumes PDUs one at a time from beginning to end, resulting in an output stream

in which cells from the same PDU are transmitted "back-to-back" from first through the last cell of

the PDU.

The transmitter is implemented with 3 identical fill threads. Unlike the Ethernet transmitter, the

ATM transmitter does not have a thread dedicated to scheduling the work of the fill threads. Rather,

the fill threads use shared absolute registers to act as a "distributed scheduler". The fourth thread

could also be enabled as a fill thread, but is not needed at the wire rates in this design.

In IXP1240/1250 hardware CRC configurations, the ATM Transmitter generates CRC-32 upon

transferring cells from DRAM to the TFIFO. In the IXP1200 software CRC configurations, CRC-

32 is computed by a dedicated CRC-32 generation microengine.

Figure 13. ATM Transmit High Level Algorithm

while(1)

critsect_enter(@poll_for_new_work_mutex)

if (engine not active sending a PDU)

dequeue a PDU

if (Rosetta not ready to transmit)

goto skip#

critsect_exit(@poll_for_new_work_mutex)

get transmit (cell) assignment from active PDU

sequence_enter(SEQ_TFIFO) - remember TIFO element allocation order

_atm_tfifo_element() to claim the next TIFO element

write payload from DRAM to TFIFO

_build_atm_tx_assignment() set-up TFIFO control word

_my_tfifo_status_write() write control to TFIFO

atm_tx_tfifo_write_cell_header_and_data0() – ATM header into TFIFO

sequence_wait(SEQ_TFIFO) - wait for my element to be next

tfifo_ptr_wait() - don't validate too far ahead of xmit_ptr

tfifo_validate_write()

sequence_exit(SEQ_TFIFO)

continue

skip#: // skip a TIFO element