# Presentation: 388532

## A Standard Link-Layer Protocol for Space Mission Communications

***A Standard Link-Layer Protocol for Space Mission Communications***

**Ed Criscuolo - CSC**

## 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 (<1% overhead)
Used as the basis for many serial protocols
SLIP, PPP, Frame-Relay, X.25, LAPB, LAPD, LAPM, Packet over SONET, SDLC.

- 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 (<1% overhead)
- Used as the basis for many serial protocols
- SLIP, PPP, Frame-Relay, X.25, LAPB, LAPD, LAPM, Packet over SONET, SDLC.

**Overview**

**HDR**

**Var. Len. Body**

**CRC**

- ITC 2002

- Oct. 2002

## 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

- ITC 2002

- Oct. 2002

## 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

- ITC 2002

- Oct. 2002

## Simulation Model Functional Diagram

- ITC 2002

- Oct. 2002

## Throughput for Uncoded Service

- ITC 2002

- Oct. 2002

## Throughput for Coded Service

- ITC 2002

- Oct. 2002

## 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 -> mobile node at GSFC
- JSC -> mobile node at GSFC
- OMNI lab -> Experiment platform on a ship in the Black Sea
- GSFC -> Globalstar -> Globalstar G/S -> WSGT -> TDRSS -> GSFC

- ITC 2002

- Oct. 2002

## 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

- ITC 2002

- Oct. 2002

## BER Measurements from UoSAT-12

- ITC 2002

- Oct. 2002

## 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

- ITC 2002

- Oct. 2002

## 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.

- ITC 2002

- Oct. 2002

## 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**.**

- ITC 2002

- Oct. 2002

## HDLC Spacecraft 

- ITC 2002

- Oct. 2002

## HDLC Spacecraft (cont)

- ITC 2002

- Oct. 2002