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Cell Relay Retreat>List Archive>month:1997-Aug> msg00143



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AAL1 Header

  • From: Hui Wang <hwang@akira.hitel.com>
  • Date: Tue, 19 Aug 1997 14:30:17 EDT

In an AAL1 cell, the first byte of payload is composed of two parts,
4 bits for Sequence Number (SN) and 4 bits for Sequence Number Protection
(SNP). In SNP, 3 bits are devoted to CRC-3 protection, and the last
bit is for parity checking.

My question is, for protection, why two different mechnisms are used
here (CRC + parity), and if it is a great idea, why it is not used
anywhere else? (Like for AAL3/4, it provides CRC-10 in SAR-PDU trailer,
instead of CRC-9 + Parity, why?)

Any suggestion will be appreciated.

Hui Wang