RS8234EBGC Mindspeed Technologies, RS8234EBGC Datasheet - Page 116

• Manufacturer Part Number: RS8234EBGC
• Description: RS8234EBGC ATM XBR SAR
• Manufacturer: Mindspeed Technologies

5.0 Reassembly Coprocessor
5.3 CPCS-PDU Processing
5.3.2 AAL3/4 Processing
Figure 5-9. AAL3/4 CPCS-PDU Reassembly
5-12
AAL3/4 PDU fields:
CPI = Common Part Indicator. In AAL3/4, initially set to all 0s.
Btag = Beginning Tag. Has the same identifying number as the Etag field. When the receiver reassembles a long PDU,
BASIZE= Buffer Allocation size. Tells the receiver how large the buffer allocation must be to receive and reassemble this PDU.
AL = Aligns the trailer to fit a 4-byte word.
Etag = Ending Tag. (See Btag above.)
Length = Contains the exact size of the PDU's payload.
AAL3/4 ATM cell fields:
ST = Segment Type. 10 = beginning of message, 00 = continuation of message, 01 = end of message, and 11 = single
SN = Sequence Number. The cell number sequence within the PDU. Cycles through 16 values.
MID = Message Identification. Allows many long SDUs to be multiplexed onto a single stream of cells. The reassembly
LI = Length Indication. Dictates the length of pad of the SDU, to a multiple of four octets.
CRC = A CRC check of the 48-byte ATM cell payload.
Received Cells
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.
H ST SN MID
AAL3/4 CPCS-PDU
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.
to b10, and the EOM cell will have the ST field set to b01. All COM cells will
have the ST field set to b00. An ST field value of b11 indicates a Single Segment
Message (SSM), i.e., the cell holds all of a short PDU.
cell stream.
LI CRC
The BOM cell for a PDU will have the Segment Type (ST) field in the cell set
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
28234-DSH-001-B
5.4. The EOM
Trailer
PAD
LI CRC
RS8234
100074_030