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28229-DSH-001-B
Mindspeed Technologies
5
-7
CX28224/5/9 Data Sheet
Transmission Convergence Block
5.2.4 Cell Scrambler
The ATM standard requires cell scrambling to ensure that only valid headers are found
in the cell delineation process. Scrambling randomizes any repeated patterns or other
data strings that could be mistaken for valid headers. The CX2822x supports two
types of scrambling as defined by ITU-T I.432:
1.
Self Synchronizing Scrambler (SSS)
2.
Distributed Sample Scrambler (DSS). Typically, SSS is used and is, therefore, the
CX2822x’s default method. However, xDSL in asynchronous format generally
use DSS.
5.2.4.1
SSS Scrambling
SSS scrambling uses the polynomial
×43 + 1 to scramble the payload, leaving the five
header bytes untouched. It can be enabled in EnTxCellScr, bit 5, of the CGEN register
(0x08).
Descrambling uses the same polynomial to recover the 48-byte cell payload. It can be
enabled in EnRxCellScr, bit 4, of the CVAL register (0x0C). SSS scrambling runs at
up to 45 Mbps.
5.2.4.2
DSS Scrambling
DSS scrambling uses the
×31 + ×28 + 1 polynomial to scramble the entire cell, except
the HEC byte. HEC is calculated after the first four bytes of the header have been
scrambled. DSS scrambling is enabled in EnTxDSSScr, bit 1, of the CGEN register
(0x08).
Descrambling uses the first six bits of the HEC for alignment. Once alignment is
found, all eight bits of the HEC are sampled. Descrambling uses the same polynomial
to recover the 48-byte cell payload. It is enabled in EnRxDSSScr, bit 0, of the CGEN
register (0x08). If DSS descrambling fails, the CX2822x defaults to unscrambled
mode.
NOTE:
If both SSS and DSS are enabled, SSS overrides DSS.