ATM AAL1 Circuit Emulation Service
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 31-5
received octet becomes the first byte of the new BD (new super frame). (See Section 31.5, “ATM-to-TDM
Adaptive Slip Control,” and Section 31.4, “Interworking Functions.”)
Note that when the ATM channel is not in CES mode, no restart sequence is performed; the ATM receiver
immediately starts hunting for the first valid cell. The first received octet becomes the first byte of the next
BD.
During reassembly, the ATM receiver channel’s 3-step-SN algorithm status is delivered to the pointer
verification mechanism. (See Section 31.7, “Pointer Verification Mechanism.”) If the receiver operates in
unstructured data format, the 3-step-SN algorithm status is delivered directly to the bit count integrity
module. When partially filled cells arrive, the bit count integrity module copies only the valid octets from
the received cell (or from the dummy cell if that cell is lost) to the current receive buffer.
In unstructured AAL1 format, when the receive process begins, the receiver hunts for the first cell with a
valid sequence number (SN field). When one arrives, the receiver leaves the hunt state and begins
receiving incoming cells.
In structured AAL1 format, when the receive process begins, the receiver hunts for the first cell with a
valid structured pointer (not a dummy pointer), that points to the start of a new structure. When one arrives,
the receiver leaves the hunt state, opens a new buffer and begins receiving. The structured pointer points
to the first octet of the structured block, which then becomes the first byte of the new buffer.
During the reassembly process, if the receiver switches to hunt mode (due to the 3-step-SN algorithm or
due to receiving two successive mismatched pointers), the ATM receiver closes the current RxBD,
discards incoming cells, modifies the receive statistics accordingly and initiates a restart sequence. The
receiver then waits for a cell with a valid structured pointer to regain synchronization and start receiving
incoming cells again.
The PowerQUICC II supports SRTS clock recovery using an external PLL. The PowerQUICC II tracks
the SRTS from the four incoming cells and writes the SRTS code to external logic. See Section30.15,
“SRTS Generation and Clock Recovery Using External Logic.” The data flow for an AAL1 CES receiver
is summarized in Figure31-4.