cn8236 Mindspeed Technologies, cn8236 Datasheet - Page 122

• Manufacturer Part Number: cn8236
• Description: Atm Servicesar Plus With Xbr Traffic Management
• Manufacturer: Mindspeed Technologies

5.0 Reassembly Coprocessor
5.3 CPCS-PDU Processing
5.3.2 AAL3/4 Processing
Figure 5-10. AAL3/4 CPCS—PDU Reassembly
5-12
AAL3/4 PDU fields:
CPI
Btag
BASIZE = Buffer Allocation size. Tells the receiver how large the buffer allocation must be to receive and reassemble this PDU.
AL
Etag
Length
AAL3/4 ATM cell fields:
ST
SN
MID
LI
CRC
Received Cells
H ST SN MID
AAL3/4 CPCS-PDU
= Common Part Indicator. In AAL3/4, initially set to all 0s.
= Beginning Tag. Has the same identifying number as the Etag field. When the receiver reassembles a long PDU,
= Aligns the trailer to fit a 4-byte word.
= Ending Tag. (See Btag above.)
= Contains the exact size of the PDU's payload.
= Segment Type. 10 = beginning of message, 00 = continuation of message, 01 = end of message, and 11 = single
= Sequence Number. The cell number sequence within the PDU. Cycles through 16 values.
= Message Identification. Allows many long SDUs to be multiplexed onto a single stream of cells. The reassembly
= Length Indication. Dictates the length of pad of the SDU, to a multiple of four octets.
= A CRC check of the 48-byte ATM cell payload.
these tags help identify that cells are from the same PDU.
segment message.
processor sorts cells by MID and reassembles PDUs from cells with the same MID.
CPI Btag BASIZE
Header
(44 bytes)
Information
The BOM cell contains the header information for the AAL3/4 PDU, and the
EOM cell contains the trailer information for the PDU. All of the other cell
payloads for that PDU contain user data. The reassembly coprocessor writes all
header, trailer, and user data to memory as described in
cell contains both user data and CPCS-PDU overhead, and delineates the end of
an AAL3/4 PDU.
and the EOM cell sets the ST field set to b01. All COM cells set the ST field set
to b00. An ST field value of b11 indicates a Single Segment Message (SSM); that
is, the cell holds all of a short PDU.
cell stream.
LI CRC
The BOM cell for a PDU sets the Segment Type (ST) field in the cell to b10,
Figure 5-10
H ST SN MID
Mindspeed Technologies
Payload
illustrates an AAL3/4 CPCS-PDU reassembled from the received
<64 K
ATM ServiceSAR Plus with xBR Traffic Management
LI CRC
™
H ST SN MID
Section
PAD
AL Etag Length
28236-DSH-001-B
5.4. The EOM
Trailer
PAD
LI CRC
CN8236
8236_104