Freescale Semiconductor MPC8260 - Manual (Page 1019)

ATM AAL1 Circuit Emulation Service
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 31-3
31.2 AAL1 CES Transmitter Overview
The PowerQUICC II supports both structured and unstructured AAL1 cell formats. For unstructured
format, the transmitter reads 47 bytes from the external buffer and inserts them into the AAL1 user data
field. For structured format, the transmitter reads 46 or 47 bytes from the external buffer and inserts them
into the AAL1 user data field.
31.2.1 Data Path
The AAL1 PDU header, which consists of the sequence number (SN) and the sequence number protection
(SNP) (CRC-3 and parity bit), is generated and inserted into the AAL1 Tx cell, as shown in
Figure 31-1 .
Figure 31-1. AAL1 Transmit Cell Format
When the transmitter operates in structured data transfer (SDT) mode, two types of AAL1 cells are
defined: P (pointer) format and non-P format. The two formats are shown below.
Figure 31-2. AAL1 SDT Cells Type
The transmitter generates the structured pointer according to the I.363.1 ITU standard and inserts the
pointer exactly once every cycle (eight successive cells). The transmitter will insert the structured pointer,
at the first opportunity, into a cell with an even sequence count (SC). When the end of the structure is not
present in the current cycle, a P-format cell with a dummy pointer (127) is inserted into the last cell (SC=6)
of the cycle.
The PowerQUICC II supports partially filled cells configured on a per-VC basis. In this mode, only the
valid octets are filled with user information; the rest of the cell is filled with padding octets.
The PowerQUICC II supports synchronous residual time stamp (SRTS) generation using an external PLL.
If this mode is enabled, the PowerQUICC II reads the SRTS code from external logic and inserts it into
four outgoing cells. See
Section 30.15, “SRTS Generation and Clock Recovery Using External Logic.”
31.2.2 Signaling Path
The PowerQUICC II automatically handles the signaling information as part of the interworking function.
The ATM transmitter packs the signaling information at the end of each superframe during the data
segmentation process. Each VC is associated to one signaling block by an internal routing table; see
SN SNP SAR-PDU payload
SAR PDU header
SAR-PDU header ST pointer AAL1 user information
P format AAL1 cell
Non-P format AAL1 cell
SAR-PDU header AAL1 user information