RELEASED
PM73123 AAL1GATOR-8
DATASHEET
PMC-2000097
ISSUE 2
8 LINK CES/DBCES AAL1 SAR
9.2.1.3 Transmit Adaptation Layer Processor (TALP)
9.2.1.3.1 OAM Cell Generation
When an OAM cell transmission is requested, it is sent at the first available
opportunity. Transmit OAM cells have higher priority than cells scheduled by the
CSD circuit. Because of this, care should be taken to ensure that OAM cells do
not overwhelm the transmitter to such an extent that data cells are starved of
adequate opportunities. The rate of OAM cells must be limited for the AAL1gator-
8 to maintain its maximum CSD data rate.
To send an OAM cell, the microprocessor writes OAM cells into one of two
dedicated cell buffers located in external memory. When the cell is assembled in
the buffer, the microprocessor must set the appropriate bit in the Command
register The TALP sends the cell as soon as possible, then clears the
appropriate attention bit to indicate the requested cell has been sent. If requests
for both OAM cells are active at the time the command register is read by the
AAL1gator-8, OAM cell 0 will always be sent because it is assigned a higher
priority. Therefore, to control the order of OAM cell transmission, the
microprocessor should set only one OAM attention bit at a time and wait until it is
cleared before setting the other attention bit.
OAM cells can optionally have the 48-byte OAM payload CRC-10 protected. This
is accomplished by a CRC circuit that monitors the OAM cell as it is sent to the
TUTOPIA and computes the CRC on the fly. It then substitutes the 10 bit
resultant CRC, preceded by six 0s, for the last two bytes of the cell. The CRC
generation is enabled by setting Bit 0 in Word 2 of the T_OAM_CELL.
9.2.1.3.2 Data Cell Generation
If the TALP receives a request to send a CSD-scheduled data cell and there are
no OAM cell requests pending, it will do so as soon as it is free. It will look up the
predefined ATM header from the T_QUEUE_TBL (refer to section 7.6.8
“T_QUEUE_TBL” on page 122). It will then obtain a sequence number for that
queue from memory, and a structure pointer if necessary. After these bytes are
written to the TUTOPIA interface, the TALP will then go to the data and the
signaling frame buffers, locate the data bytes for the correct channels, and write
them in the correct order to the UTOPIA interface. This cell building process is
described in more detail in the following section.
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