Motorola PowerQUICC II MPC8280 Series Reference Manual page 1188

AAL2 Receiver
Table 33-5. SSSAR TxBD Field Descriptions (continued)
1
Offset Bits Name
0x02 — Data
Length
0x04 — TXDBPTR
1
Boldfaced entries must be initialized by the user.
33.4 AAL2 Receiver
The following sections describe the AAL2 receiver.
33.4.1 Receiver Overview
The receiver cycle starts after the FCC receives a cell. If the cell header is successfully
mapped to an ATM channel number, the corresponding RCT is fetched and the AAL type
is read. For AAL2 cells, the receiver begins by checking if the last cell received in this
channel (CID) has an uncompleted (split) packet. If so, the receiver first finishes handling
this packet.
The receiver then goes through a validation process. The receiver matches the OSF field in
the STF with the expected OSF based on the actual split packet (if the first packet is not
split, the OSF should be zero). If the two values do not match, an OSF error interrupt is
issued and the receiver drops the last packet. Also, if the STF parity check, the SN check or
the OSF>47 check results in an error, the receiver issues an interrupt and discards the whole
cell. If any of the above errors has occurred and the cell has started with the remainder of
an uncompleted packet, the receiver does the following:
• For a CPS sublayer CID, the packet's RxBD[UP] (uncompleted packet) bit is set.
• For an SSSAR sublayer CID, the buffer is closed with RxBD[RxError = US = 10]
(uncompleted SDU), and the rest of the frame is dropped.
The receiver now begins the process of extracting new CPS packets out of the cell with
another round of error checking. The receiver examines each CPS packet header for the
following errors:
• Incorrect packet HEC. The packet and rest of the cell are discarded.
• Packet length (LI+1) is larger than CPS_Max_SDU_Length. The receiver discards
the packet and then continues to extract the next packet in the cell. However, if the
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Contains the length of the buffer associated with this BD. If this is the last buffer (L=1)
and the UUI bit in the SSSAR TxQD is set, the 5-bit UUI field is located at
(TXDBPTR+Data Length)[3:7] with bit [3] being the msb, that is, in the byte (right
justified) immediately following the last byte of the buffer. For best bandwidth
utilization and optimized partitioning of the SDU to packets of exactly Seglen size
when an SDU is spread over multiple BD's, the application should set Data Length to
be an integer multiple of Seg_Len. (Data Length == n x Seg_Len). For an SDU on a
single BD this restriction does NOT apply.
Tx data buffer pointer. Points to the address of the associated buffer. There are no
byte-alignment requirements for the buffer, and it may reside in either internal or
external memory. This value is not modified by the CP.
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