Freescale Semiconductor MPC8260 - Manual (Page 1083)

ATM AAL2
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 32-21
The receiver issues an interrupt for each of the above errors.When a SSSAR buffer is closed with
RxBD[RxError = US or OS], indicating Uncompleted SDU or Oversized, then RxBD[L] is set, and if
RxQD[RFM]=1 then the receiver also issues an RXF interrupt.
Then, if no errors have occurred in the packet header, the packet CID is used to match the PHY | VP | VC
| CID with an RxQD; see
Section 32.4.2, “Mapping of PHY | VP | VC | CID.” The match process yields
an RxQD pointer. The RxQD indicates the type of SAR operation to be performed on the PHY | VP | VC
| CID. Three SAR operation modes are supported:
• For the CPS sublayer, each packet from the CID is stored, as is, in a one packet buffer.
• For switching, the packet is stored directly into the transmit buffer, and the CID is translated.
• For the SSSAR sublayer, all packets received from the CID are reassembled into an SSSAR SDU
similar to the BD and buffer structures used for AAL5 frames.
• For the SSSAR sublayer, last packet UUI indication is stored in the last RxBD of the SDU.
For the SSSAR sublayer, two additional parameters are verified for each new packet received:
• RAS_Timer expiration. The RAS_Timer_Duration defined in the AAL2 parameter RAM limits the
time (starting when the first packet is received) allowed to receive a complete SSSAR SDU. If this
time limit is exceeded, the receiver closes the current buffer with RxBD[RxError = TE = 01]
(Ras_Timer expired) and starts a new SSSAR frame with the next packet. When a buffer is closed
with RxBD[RxError = TE = 01], RxBD[L] is not set and the receiver does not issue an RXF
interrupt.
• SSSAR_Max_SDU_Length. With each new packet the receiver checks whether the current
accumulated length of the SSSAR SDU exceeds the SSSAR_Max_SDU_Length. If so, the
receiver closes the current buffer with RxBD[RxError = OS = 11] (oversized), discards the rest of
the SSSAR SDU, RxBD[L] is set, and if RxQD[RFM]=1 issues an RXF interrupt.
NOTE
CIDs that have the same number but that are from different AAL2
connections cannot use the same RxQD, unless they never have split
packets.
32.4.2 Mapping of PHY | VP | VC | CID
The AAL2 mapping mechanism translates a PHY | VP | VC | CID combination into an RxQD. An RxQD
can be unique per PHY | VP | VC | CID.
The mapping mechanism, shown in Figure 32-13 , can be broken down as follows:
• Each ATM channel number (RCT) has its own CID mapping table. The mapping table can be
placed in internal or external memory (according to RCT[MAP]) and is pointed to by RCT[CID
Mapping Table Base]. The CID of the received packet is used as an index into the mapping table.
• Each entry in the mapping table contains a 2-byte RxQD offset. This offset, multiplied by 4, is the
offset to an RxQD in either the internal or external RxQD table.
• The two RxQD tables serve all the ATM channel numbers of an FCC. (RxQD_Base_Int and
RxQD_Base_Ext are defined in the FCC parameter RAM.)