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4.0 Segmentation Coprocessor
4.2 Segmentation Functional Description
RS8234
Datasheet for RS8234 xBR ServiceSAR
N8234DS1B
CPI = Common Part Indicator. In AAL3/4, initially set to all zeros.
Btag = Beginning Tag. Has the same identifying number as the Etag field. When the receiver
reassembles a long PDU, these tags help identify that cells are from the same 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 four-byte word.
Etag = Ending Tag. (see Btag above).
Length = contains the exact size of the PDU’s payload.
4.2.3.3 AAL0
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 gener-
ates the ATM header and PDU termination indications in the same manner as it
does with AAL5.
4.2.4 ATM Physical Layer Interface
Once the segmentation coprocessor has formed an ATM cell, the RS8234 trans-
fers 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. Section 6.2.3.3 in the Traffic Management chapter discusses this
trade-off in greater depth. Once sent to the transmit FIFO, the cell passes through
the FIFO to the PHY layer interface circuits.
4.2.5 Status Reporting
The RS8234 informs the Host of segmentation completion using segmentation
status queues. The Host assigns each VCC to one of 32 status queues, enabling a
multiple peer architecture as described in Section 3.2. The RS8234 reports status
entry on either PDU or buffer boundaries, selectable on a per-VCC basis by set-
ting the STM_MODE bit. PDU boundary status is referred to as Message Mode,
while buffer status reporting is called Streaming Mode. Error conditions also gen-
erate status queue entries, though this is a rare occurrence within a RS8234 sub-
system’s segmentation block. The segmentation status queues operate according
to the write-only Host interface, defined in Section 3.3.2.
The RS8234 returns a user supplied field (USER_PNTR) from the first SBD
associated with the status entry. The SAR does NOT use this field for any internal
purpose; it simply circulates the information back to the Host. The value of
USER_PNTR must uniquely describe the segmented buffer associated with the
SBD. USER_PNTR can contain the address of the buffer or of a Host data struc-
ture describing the buffer. To simplify Host management, the RS8234 also returns
the VCC_INDEX of the VCC on which the buffer was transmitted.
4.2.6 Virtual FIFOs
In addition to gathering PDU data from buffers, the RS8234 provides an optional
method to segment from a fixed PCI address, or Virtual FIFO. The RS8234 sup-
ports AAL0, CBR Virtual FIFO segmentation.
The Host configures the channel for Virtual FIFO operation by setting the
CURR_PNTR and RUN fields to zero in the Seg VCC Table entry. The Host