HDLC Link Framing for Future Space Missions

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HDLC Link Framing for Future Space Missions

• Ed Criscuolo - CSC

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Overview

• What is HDLC?
• Simple framing protocol in use for 30 years.
• 1 byte sync pattern (0x7e)
• 4 byte header
• Variable length data
• 2 byte CRC (same CRC as CCSDS)
• Bit stuffing to avoid sync occurring in data (lt1
  overhead)
• Used as the basis for many serial protocols
• SLIP, PPP, Frame-Relay, X.25, LAPB, LAPD, LAPM,
  Packet over SONET, SDLC.

HDR
Var. Len. Body
CRC
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Overview (Cont)

• Why should we use Frame-Relay on space links?
• Commodity network equipment readily available
  from many vendors.
• Widely used in existing networks allowing easy,
  transparent interoperability up to 51Mb/s.
• No forward link required (Same as the current
  system)
• HDLC testing activities at GSFC
• A Laboratory Comparison between HDLC and CCSDS
• TDRSS activities
• A Flight Demonstration on UoSAT-12
• The History of HDLC in Space

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A Laboratory Comparison

• Used existing ITT TDRSS Link simulator
• Simulator included CCSDS framing model
• Added HDLC framing model
• Simulated the following cases for both HDLC and
  CCSDS transfer frame protocols
• Both uncoded and convolutionally coded cases (No
  Reed-Solomon)
• Packet sizes of 64, 256, 1024, 1500, and 4096
  bytes
• Bit error rates from 5x10-4 to 1x10-7

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Simulation Model Functional Diagram
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Throughput for Uncoded Service
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Throughput for Coded Service

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TDRSS Activities

• South Pole TDRSS Relay (SPTR)
• TDRSS Internet Relay Terminal (TILT)
• Educational outreach activities
• Two solar eclipses
• Two North pole expeditions
• Yosemite 99 program
• President Clinton link from New Zealand to
  Antarctica
• U.S. Coast Guard research vessel
• OMNI project activities
• OMNI lab -gt mobile node at GSFC
• JSC -gt mobile node at GSFC
• OMNI lab -gt Experiment platform on a ship in the
  Black Sea
• GSFC -gt Globalstar -gt Globalstar G/S -gt WSGT -gt
  TDRSS -gt GSFC

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Flight Test

• Identified UoSAT-12 as available HDLC based
  platform.
• Ported BSD IP stack to existing operating system
• Ground station modified by addition of a router
• Performed testing with various standard IP
  applications
• Ping - Basic network connectivity
• FTP - Reliable data delivery
• NTP - Spacecraft clock synchronization
• HTTP - Reliable real-time data displays
• UDP - real-time housekeeping data / blind
  commanding

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BER Measurements from UoSAT-12
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FEC Coding and HDLC

• How do you put variable length HDLC frames in
  fixed length Reed-Solomon blocks?
• In the CCSDS protocol the transfer frame and R-S
  code block are one in the same.
• In the commercial network world Reed-Solomon and
  convolutional coding are performed at the
  physical layer.
• R-S encoder operates on a bit stream passed to it
  from the link layer
• It has no knowledge of any framing present in the
  bit stream

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History of HDLC in Space

• Used for over 20 years in small educational and
  experimental spacecraft.
• Over 70 past, present, or planned missions used
  or will use HDLC
• Developed and operated by 23 universities, 8
  amateur groups, and 7 commercial space entities
  in 24 countries
• Also represented are NASA, ESA, the US Air Force,
  the US Navy, and the Chilean Air Force.
• Earth resources satellites by Germany and Turkey,
  and a Disaster Monitoring Constellation (DMC) by
  Algeria, Nigeria, and the UK are currently in
  development and will use HDLC.

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Conclusions

• HDLC performs slightly better than CCSDS in a
  high BER environment.
• CCSDS performs slightly better in a low BER
  environment.
• Based on the comparable performance HDLC is
  preferred for IP links for compatibility and
  interoperability reasons.
• HDLC has been flight proven by 20 years of use in
  space by small educational and experimental
  spacecraft.

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HDLC Spacecraft
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HDLC Spacecraft (cont)