1. G O D D A R D S P A C E F L I G H T C E N T E R 1 The Trade Between CCSDS and HDLC Framing on Global Precipitation Measurement David Everett and Jonathan Wilmot Fourth Space Internet Workshop June 9, 2004 David Everett 301/286-1596 David.F.Everett@nasa.gov Jonathan Wilmot Jonathan.J.Wilmot@nasa.gov Goddard Space Flight Center

2. G O D D A R D S P A C E F L I G H T C E N T E R 2 CCSDS vs. HDLC Framing ; SIW-4; June 9, 2004 Assumptions Link layer protocol options: IP in HDLC compared to IP in CCSDS frames. IP in HLDC stack considered: downlink is one UDP in one IP in one HDLC in RS. CCSDS framing considered is from flight communications card to MOC.

3. G O D D A R D S P A C E F L I G H T C E N T E R 3 CCSDS vs. HDLC Framing ; SIW-4; June 9, 2004 HDLC Advantages HDLC is a commercial standard – Does not require custom hardware/software for implementation on the ground. CCSDS ops concept has telemetry frames put in IP for ground routing back to the MOC, requiring additional ground station software to establish link to MOC and back. IP telemetry in HDLC needs only the IP address set from the spacecraft to determine end- to-end routing via standard equipment. – Allows use of low cost standard commercial equipment for development and ground test systems. HDLC simplifies flight software/enables hardware solution – CCSDS frames are traditionally done with custom flight software due to complexity of frame construction in hardware. – Current implementations require a central manager (C&DH) for channel maintenance. UDP in IP over HDLC does not have this issue. – With HDLC, any box can send data independently within bandwidth constraints, allowing the true peer-to-peer networks we are familiar with in ground systems. – Fill frames not needed HDLC hardware inserts any number of 0x7e bytes. Simple hardware, compared to constructing valid fill frames. – Fill packets not required to fill out frames, less software effort.

4. G O D D A R D S P A C E F L I G H T C E N T E R 4 CCSDS vs. HDLC Framing ; SIW-4; June 9, 2004 HDLC Advantages (continued) IP in HDLC has been used in flight and has been prototyped in development labs. IP in CCSDS has not. HDLC down is independent of uplink stream, CCSDS mixes uplink reliability (COP-1) with downlink framing (CLCW). UDP and IP addressing already provide much of the functionality gained with the CCSDS channel identifiers: i.e. application-to- application addressing; Science in VC3, real-time in VC0, etc. IP in HDLC can use the commercial network stacks provided by all modern operating systems, both flight and ground. CCSDS framing needs custom software on both ends of link. – To provide uplink security, standard IPSEC implementation in flight and ground software can be considered. – IP security over CCSDS will require much more effort than using existing standard IP security protocols.

5. G O D D A R D S P A C E F L I G H T C E N T E R 5 CCSDS vs. HDLC Framing ; SIW-4; June 9, 2004 CCSDS Framing Advantages Escape character (fill byte) needed for HDLC flag bytes, leading to variable frame overhead. – Not clear advantage, since CCSDS framing may also require fill. – Trade between fill frames/packets and byte stuffing overhead based somewhat on data contents. CCSDS layer provides indications of frame loss through use of master and channel frame counters. – Mitigation through use of reliable application protocols, CFDP and COP-1 style commanding over UDP. – Packet sequence counters used for indication of packet loss. CCSDS framing approach removes issue of ground errors – CCSDS ops concept uses CCSDS frames in TCP from ground to MOC. So once the telemetry data hits the ground, it is reliable back to the MOC. – Ops concept for HDLC is UDP/IP direct back to MOC, so real time packets may be lost on the ground. – CFDP used for reliable science avoids this issue, but requires spacecraft to resend frame. – Ground network BER should be very low. CCSDS framing doesn’t require additional sync markers for Reed- Solomon encoding.

6. G O D D A R D S P A C E F L I G H T C E N T E R 6 CCSDS vs. HDLC Framing ; SIW-4; June 9, 2004 Summary GPM chose HDLC framing over CCSDS framing because: Development and I&T can be supported with commercial hardware and software. – Computer-to-computer connections vs. our current approach, which relies on custom interface cards. HDLC removes complexity from the flight system. HDLC replaces space-application-only ground equipment with commercial equipment. Efficiencies associated with CCSDS framing don’t outweigh the long- term cost advantages of HDLC framing for low-earth orbiting spacecraft. – LEO missions typically fly with comfortable link margins. – Commercial sector will continue to push technology capability.