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Multi-Service Access Device For Channelized Interfaces
Telecom Standard Product Data Sheet
Production
Proprietary and Confidential to PMC-Sierra, Inc., and for its Customers’ Internal Use
Document ID: PMC-1990823, Issue 4
88
Cell Filter and HCS Verification
Cells are filtered (or dropped) based on HCS errors and/or a cell header pattern. Cell filtering is
optional and is enabled through the RTDP registers. Cells are passed to the receive FIFO while
the cell delineation state machine is in the SYNC state as described above. When both filtering
and HCS checking are enabled, cells are dropped if HCS errors are detected, or if the header
contents match the pattern contained in the RTDP Idle Cell Header and Mask register. Idle cell
filtering is accomplished by writing the appropriate cell header pattern into the RTDP Idle Cell
Header and Mask Pattern and register. Idle/Unassigned cells are assumed to contain the all zeros
pattern in the VCI and VPI fields. The RTDP Idle Cell Header and Mask register allow filtering
control over the contents of the GFC, PTI, and CLP fields of the header.
The HCS is a CRC-8 calculation over the first 4 octets of the ATM cell header. The RTDP block
verifies the received HCS using the polynomial, x
8
+ x
2
+ x + 1. The coset polynomial, x
6
+ x
4
+ x
2
+ 1, is added (modulo 2) to the received HCS octet before comparison with the calculated
result.
Performance Monitor
The Performance Monitor consists of two 16-bit saturating HCS error event counters, a 32-bit
saturating receive cell counter, and a 32-bit saturating Idle cell counter. The first error counter
accumulates HCS errors. A 32-bit receive cell counter counts all cells written into the receive
FIFO. Filtered Idle cells are counted in another 32-bit counter.
Each counter may be read through the microprocessor interface. Circuitry is provided to latch
these counters so that their values can be read while simultaneously resetting the internal counters
to 0 or 1, if appropriate, so that a new period of accumulation can begin without loss of any
events. It is intended that the counter be polled at least once per second so as not to miss any
counted events.
11.14.2 RTDP Packet Processor
The RTDP performs PPP and HDLC packet extraction, provides FCS error detection, performs
packet payload descrambling, and provides performance monitoring functions.
Descrambler
When enabled, the self-synchronous descrambler operates on the PPP Frame data, descrambling
the data with the polynomial x
43
+ 1. Descrambling is performed on the raw HDLC/PPP data
stream, before any PPP frame delineation or byte destuffing is performed. Data scrambling can
provide for a more robust system preventing the injection of hostile patterns into the data stream.