Using PPP for Synchronous
                        Data Compression


                         Stuart Venters
                             SDCC
                          ADTRAN, Inc.

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Synchronous Data Compression Consortium

  o  SDCC - CSU/DSU Vendors

     Open to anyone; primarily a group of DSU/CSU Vendors

     AT&T Paradyne, Racal Datacom, Motorola, Adtran, Cray
       Communications, are active; others attend

  o  Meeting about once a month since February

  o  Email Reflector:  dsu-compress@paradyne.com

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Goal

  To develop a standard for compression of synchronous data

  >> DSU/CSU products

  >> Synchronous data is data at the DTE/DCE interface in 
     bit-synchronous frames with flags and zero-insertion

  >> Provide a framework for extension to support future markets

  >> Demonstrate interoperability

  >> Quickly -- before market confusion

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Decisions

  o  Timeline mandated a phased approach

  o  Default compression:  Stacker-LZS

  o  Runtime bit stream based on V.120 and PPP

  o  Negotiation via PPP with possible short-cut

  o  Limited feature set -- Phase 1

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Phase 1 Features

  o  User Data

     >> HDLC frames / single DTE port

  o  Transport

     >> Point-to-point 56 kb/s DSU application

     >> Leased or circuit switched

  o  Single default compression algorithm

     >> Stacker-LZS

  o  Expandable through architecture and neg.

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Phase 2 and Beyond

  o  User data:  async, other sync protocols

  o  Multi-session; multi-point; ISDN, FR

  o  Higher speed transport: T1/E1 and beyond

  o  Additional compression algorithms

  o  Encryption

  o  Inverse and statistical multiplexing

  o  Others -- unknown but expected

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PPP Plan

  LCP:   Address-and-control-field compression
         Protocol-field-compression

  CCP:   Stacker-LZS: check-mode 3 (sequencing); history-count 1

  SDTP:  Serial Data Transport Protocol 
         Transported data encapsulation and segmentation
             draft-schneider-ppp-sdtp-00.txt
         Negotiation via Serial Data Control Protocol (SDCP)
         Packet format designed for use of compression
         V.120 terminal adaption header at end of packet
         Optional fields for multiport, compound frames

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Sample Frame

  7E             - HDLC Flag
  FD 02          - Comp. PID / Seq. Num.
  001001001      - CW 49  - SDTP PID
  1111111110001  - CW `ABC'
  000101111      - CW 2F - CRC byte 1
  010011111      - CW 9F - CRC byte 2
  010000011      - CW 83 - (V.120 Header)
  11000000       - Stacker-LZS End Marker
  XX XX 7E       - HDLC FCS / Flag

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Next Steps

  o  SDTP Document

     Initial version posted to Internet-Drafts
     Still changing; comments welcome
     IETF status: Standards path or Informational?

  o  Guidelines for SDC-DCE implementors 

     An Informational RFC

  o  TR-30.1

     Present documents as a multi-vendor contribution

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General Interest Items

  o  Macro header compression

     -  Requires a block of PID space

  o  Support for Tandem Devices

     -  DCE identifier for LCP

     -  DCE enters negotiation