RS8234EBGC Mindspeed Technologies, RS8234EBGC Datasheet - Page 89

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

RS8234
ATM ServiceSAR Plus with xBR Traffic Management
Figure 4-6. AAL3/4 CPCS-PDU Generation
4.2.4 ATM Physical Layer Interface
28234-DSH-001-B
Segmentation (SAR-PDU)
NOTE(S):
1. CPI = Common Part Indicator. In AAL3/4, initially set to all Os.
2. Btag = Beginning Tag. Has the same identifying number as the Etag field. When the receiver reassembles a long PDU,
3. BASIZE = Buffer Allocation Size. Tells the receiver how large the buffer allocation must be to receive and reassemble
4. AL = Aligns the trailer to fit a 4-byte word.
5. Etag = Ending Tag (see Btag above).
6. Length = Contains the exact size of the PDU's payload.
H ST SN MID Information
Convergence Sublayer (CPCS-PDU)
these tags help identify that cells are from the same PDU.
this PDU.
INTERFACE
PHY
4.2.3.3 AAL0
CPI Btag BASIZE
(1-9 Cells)
TX_FIFO
(44 bytes)
Header
LI CRC
The RS8234 also supports a transparent or NULL adaptation layer, AAL0. AAL0
maps CPCS-SDUs directly to CPCS-PDU payloads. The SAR pads the SDU to a
48-byte cell payload boundary, but generates no other overhead. The SAR
generates the ATM header and PDU termination indications in the same manner
as it does with AAL5.
Once the segmentation coprocessor has formed an ATM cell, the RS8234
transfers the cell to the transmit FIFO. The user chooses the length of this FIFO,
with possible sizes from one to nine cells. The FIFO depth is programmable,
since there is a trade-off between absorbing PCI latency with a longer FIFO, and
introducing greater jitter.
Once sent to the transmit FIFO, the cell passes through the FIFO to the PHY layer
interface circuits.
Seqmented
(To Each
Cell)
Figure 4-6
Byte Length Counter
CRC Accumulator
H ST SN MID
Mindspeed Technologies
Internal
Circuits
shows the RS8234’s AAL3/4 PDU generation scheme.
Payload
<64 K
Section 6.2.3.3
LI CRC
SEG VCC Table Entry
™
discusses this trade-off in greater depth.
ATM_ HEADER
PDU_LEN
4.2 Segmentation Functional Description
BETAG
H ST SN MID
MID
SN
Seqmentented
4.0 Segmentation Coprocessor
(To Each
Cell)
PAD
AL
Etag
Trailer
Pad
Length
LI CRC
100074_021
4-11