CN8223
2.0 Functional Description
ATM Transmitter/Receiver with UTOPIA Interface
2.6 ATM Cell Processing
Two rate control registers are provided for control of the port sources to allow
programmable rate shaping of cell transmission. The ratio of active to idle cells is
programmable with 0.4 % granularity. The cell generation process maintains
status counts of non-idle cells transmitted for each of the four sources. Table 2-17
lists the four cell generation modes provided by the CN8223.
Table 2-17. Cell Generation Modes
Mode
Function
48-Octet Mode
Provides for full ATM generation. Forty-eight octets are taken from the FIFO interface, the appropriate
header fields are attached, and the payload CRC is overwritten to form the ATM cell.
52-Octet Mode
Allows a 53-octet cell, less the HEC octet, to be transferred from the FIFO interface. The HEC is calculated
and inserted by the CN8223. The payload CRC for AAL3/4 can be inserted, or checked and transferred
without modification. Both the HEC and the payload CRC can be optionally disabled or errored on a
single-event basis. The cell generation process also provides HEC coset generation, ATM payload
scrambling. In each of the above modes, any header field can be overwritten with information from
control registers.
53-Octet Mode
57-Octet Mode
Allows entire 53-octet cells to be transferred from the FIFO interface. This is the mode used when Port 0
is configured for UTOPIA.
Allows the input of entire 57-octet PLCP slots from the FIFO interface. Can be used for external PLCP
insertion or test generation purposes.
2.6.1.1 CELL_GEN_x
Register
Per-port cell generation registers for FIFO Ports 0–3 are in the four
CELL_GEN_x registers [0x04–0x07]. Cell Generation Mode [bits 1,0] selects the
operating mode for the generation circuit. The four modes described in Table 2-17
are provided. In 52-, 53-, or 57-octet modes, the individual header fields obtained
from the FIFO interface can be overwritten. This overwriting is accomplished
with the values found in the TX_HDR registers for a particular port by setting the
appropriate field insertion control bit in the CELL_GEN_x register. In 48-octet
mode the header fields always come from the programmed value in the
corresponding TX_HDR register.
The overhead fields of active cells are taken from the locations listed in
Table 2-18 or set to the indicated values.
100046C
Conexant
2-27
CONEXANT [ CONEXANT SYSTEMS, INC ]
