PM73121ꢀAAL1gator II
Data Sheet
PMC-Sierra, Inc.
PMC-980620
,VVXHꢀꢁ
AAL1 SAR Processor
3.3.2 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 134). 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 sig-
naling 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 fol-
lowing section.
3.3.2.1 Header Construction
The entire header is fixed per queue. Headers are maintained in the memory, one per queue. These
headers include a Header Error Check (HEC) character for the fifth byte. The queue should be
deactivated during header replacement to prevent cells from being constructed with incorrect
header values. A queue can be paused by setting the SUPPRESS_TRANSMISSION bit in IDLE_
CONFIG register. Emissions are still scheduled, just the transmissions are suppressed. For any
cells that are suppressed, the T_SUPPRESSED_CELL_CNT is incremented.
3.3.2.2 Payload Construction
Payload construction is the most complex task the TALP circuit performs. The signaling require-
ments define much of the process, which is as follows:
1. The first byte of the payload is provided by a lookup into the T_QUEUE_TBL. This first byte
consists of the CSI bit, a 3-bit sequence number, and a 4-bit sequence number protection field.
The CSI bit is set depending on SRTS and pointer requirements. The sequence number is
incremented every time a new cell is sent for the same VPI/VCI.
2. If the line is in one of the two structured modes, a structure pointer is needed in one of the
even-numbered cells. The TALP inserts structure pointers according to the following rules:
•
•
•
Only one pointer is inserted in each 8-cell sequence.
A pointer is inserted in the first possible even-numbered cell of every 8-cell sequence.
A pointer value of 0 is inserted when the structure starts in the byte directly after the
pointer itself.
•
•
A pointer value of 93 is inserted when the end of the structure coincides with the end of
the 93-octet block of AAL-user information.
A dummy pointer value of 127 is inserted in cell number six if no start-of-structure or
end-of-structure occurs within the 8-cell sequence.
3. This algorithm supplies a constant number of structure pointers and, therefore, data bytes,
regardless of the structure size. The pointer is inserted in the seventh byte location of the cell.
To force the TALP to build a structure consisting of a single DS0 with no signaling nibble and
ꢂꢈ
PMC [ PMC-SIERRA, INC ]
