Motorola PowerQUICC II MPC8280 Series Reference Manual page 1125

Freescale Semiconductor, Inc.
AAL1 CES Receiver Overview
The AAL1 PDU header, which consists of the sequence number (SN) and the sequence
number protection (SNP) (CRC-3 and parity bit), is checked and the result is delivered to
the 3-step-SN algorithm. The 3-step-SN algorithm (see Section 32.6.1, "The Three States
of the Algorithm") handles the lost or misinserted cells. This algorithm can detect one lost
or misinserted cell and maintain synchronization. If more than one cell is lost or
misinserted, the 3-step-SN algorithm switches to hunt mode where it stays until a cell with
a valid SN field is received. After the receiver switches to hunt mode, it closes the RxBD,
modifies the receive statistics, generates an optional interrupt to the CPU and performs a
restart sequence.
The restart sequence is implemented only when the ATM channel works in CES mode
(RCT[CESM]). In CAS mode (RCT[CASM]), the ATM receiver channel begins the restart
sequence by dropping all incoming cells and advancing to the beginning of the next super
frame, which is the first BD after the one marked with EOSF (end of super frame). When
this BD is ready and the adaptive counter reaches the ATM_Start threshold, the receiver's
write pointer is not longer in danger of overrunning the read pointer of the MCC
transmitter; that is, it is safe to begin receiving cells again. The ATM receiver then begins
the resynchronization process: for unstructured AAL1 type the ATM receiver waits for the
first valid cell, and for structured AAL1 type the receiver waits for the first valid cell that
contains a valid pointer. The first received octet becomes the first byte of the new BD (new
super frame). (See Section 32.5, "ATM-to-TDM Adaptive Slip Control" and Section 32.4,
"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 32.7, "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
MOTOROLA Chapter 32. ATM AAL1 Circuit Emulation Service 32-5
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