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Reliable Transport Over Spacewire for James Webb Space Telescope (JWST) Focal Plane Electronics (FPE) Network

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Reliable Transport Over Spacewire for James Webb Space Telescope (JWST) Focal Plane Electronics (FPE) Network Reliable Transport over SpaceWire for James Webb Space Telescope (JWST) Focal Plane Electronics (FPE) Network Glenn Rakow Richard Schnurr Christopher Dailey Kamdin Shakoorzadeh NASA GSFC NASA GSFC QSS Group, Inc. QSS Group, Inc. Code 561 Code 560 GSFC Code 561 Code Code 561 Greenbelt, MD Greenbelt, MD Greenbelt, MD Greenbelt, MD 301-286-5993 301-286-1 852 30 1-286-3302 30 1-286-9854 [email protected] [email protected] [email protected] [email protected] Abstract-NASA's James Webb Space Telescope (JWST) well as the loss of science data itself may cause inefficiency faces difficult technical and budgetary challenges to in the use of the telescope that are unacceptable given the overcome before it is scheduled launch in 2010. The high cost of the observatory. This combination of Integrated Science Instrument Module (ISIM), shares these requirements necessitates a redundanvfault tolerant high- challenges. The major challenge addressed in this paper is speed, low mass, low power network with a low Bit error the data network used to collect, process, compresses and Rate(1E-9- 1E-12). store Infrared data. A total of 114 Mbps of raw information must be collected from 19 sources and delivered to the two The ISIM systems team performed many studies of the redundant data processing units across a twenty meter various network architectures that meeting these deployed thermally restricted interface. Further data must requirements. The architecture selected uses the Spacewire be transferred to the solid-state recorder and the spacecraft. protocol, with the addition of a new transport and network layer added to implement end-to-end reliable transport. The JWST detectors are kept at cryogenic temperatures to The network and reliable transport mechanism must be obtain the sensitivity necessary to measure faint energy implemented in hardware because of the high average sources. The Focal Plane Electronics (FPE) that sample the information rate and the restriction on the ability of the detector, generate packets from the samples, and transmit detectors to buffer data due to power and size restrictions. these packets to the processing electronics must dissipate little power in order to help keep the detectors at these cold This network and transport mechanism was designed to be temperatures. compatible with existing Spacewire links and routers so that existing equipment and designs may be leveraged upon. Separating the low powered front-end electronics from the The transport layer specification is being coordinated with higher-powered processing electronics, and using a simple European Space Agency (ESA), Spacewire Working Group high-speed protocol to transmit the detector data minimize and the Consultative Committee for Space Data System the power dissipation near the detectors. Low Voltage (CCSDS) PlK Standard Onboard Interface (SOIF) panel, Differential Signaling (LVDS) drivers were considered an with the intent of developing a standard for reliable obvious choice for physical layer because of their high transport for Spacewire. Changes to the protocol presented speed and low power. are likely since negotiations are ongoing with these groups. The mechanical restriction on the number cables across the A block of RTL VHDL that implements a multi-port thermal interface force the Image packets to be concentrated Spacewire router with an external user interface will be upon two high-speed links. These links connect the many developed and integrated with an existing Spacewire Link image packet sources, Focal Plane Electronics (FPE), design. The external user interface will be the local located near the cryogenic detectors to the processing interface that sources and sinks packets onto and off of the electronics on the spacecraft structure. network (Figure 3). The external user interface implements the network and transport layer and handles From 12 to 10,000 seconds of raw data are processed to acknowledgements and re-tries of packets for reliable make up an image, various algorithms integrate the pixel transport over the network. Because the design is written in data Loss of commands to configure the detectors as RTL, it may be ported to any technology but will initially be targeted to the new Actel Accelerator series (AX)part. ' 0-7803-765 1 -W03/$17.00 8 2003 IEEE Each link will run at 160 Mbps and the power will be about * IEEEAC paper # 1264, Updated December 9,2002 0.165 Watt per link worst case in the Actel AX. 1 TABLEOF CONTENTS 6.2 Packet Format................................................ 6 1. INTRODUCTION............................................... 2 6.3 Data Packet Format Type 0............................... 6 2. JWST MISSIONOVERVIEW .......................... 2 6.4 Data Packet Format Type 1 ............................... 7 3. ISIM OVERVIEW............................................ 3 6.5 Packet Type Bit Definitions ............................... 7 4. ISIM FPE NETWORK ARCHITECTURE........ 3 6.5.1 Acknowledgment Bit ................................. 7 4.1 Breakdown of Network Components ................... 3 6.5.2 Segmentation Bit (Last Packet Marker) ........ 7 4.3 Architecture Considerations .............................. 3 6.5.3 Sequence Number Field ............................. 7 4.4 Protocol Trades............................................... 4 6.5.4 Channel Field .......................................... 7 4.5 Topology Trades ............................................. 4 6.6 Channel Configuration .................................... 7 4.5.1 Point-to-Point .......................................... 4 6.6.I Channel 0 Payload .................................... 7 4.5.2 Central Routers with Point-to-Point ............. 4 6.7 Channel Establishment .................................... 8 4.5.2 Router Only ............................................. 4 6.8 Channel Shunt down ....................................... 8 5. OVERVIEW OF SPACEWIRE PROTOCOL ...... 4 6.9 Channel Reset ................................................ 8 5.1 Strengths ....................................................... 4 6.10 Sending Packets on Channel ........................... 9 5.2 Characters ..................................................... 4 6.IO. 1Normal Packets ....................................... 9 5.3 Overhead ....................................................... 5 6.10.2 Acknowledgement Packets ........................ 9 5.4 Initialization ................................................... 5 6.10.3 Timeout and Retransmission ..................... 9 5.5 Link Error Recovery ........................................ 5 6.10 Overhead...................................................... 9 5.6 Space Wire Router ........................................... 5 7.USER LAYER.................................................... 9 5.7 Wormhole Routing .......................................... 5 7SDifferent Types of Users.................................... 9 5.8 Routing Scheme .............................................. 5 7.2 ICDH Reliable Transport ................................. 9 5.9 Hardware Addressing ...................................... 5 7.3 ZCDH Unreliable Transport .............................. 9 5.10 Logical Addressing ........................................ 6 7.4 SCEM Reliable Transport................................. 9 5.11 Configuration o space .................................... 6 8. IMPLEMENATION............................................ 9 5.12 Packet Recovery from Error ............................ 6 9. CONCLUSION................................................ 13 5.13 External Port ................................................ 6 10. ACRONYMLIST .......................................... 13 6. SPACEWIRETRANSPORT LAYER.................. 6 11. BIOGRAPHY................................................. 14 6.1 Transparency ................................................. 6 coordinated with the Europe ESA, the Spacewire working 1. INTRODUCTION Group, and the CCSDS PlK SOIF panel with the intent of developing a standard transport layer for Spacewire. There are minor differences that need to be worked out, mostly This paper describes the implementation and operation of consolidating upon a single type (remove type 0, and keep the high-speed instrument network for the JWST. This type 1 and one other TBD type, see section 6, Spacewire network named the Focal Plan Electronics (FPE) network Transport Layer). Mostly though the differences have to do uses the Spacewire specification and a transport layer that is with byte order and names (the basic concept is the same). not part of the SpaceWire Specification. The transport layer is implemented in hardware because of the fast recovery An overview of the JWST mission will be provided time necessary for the application, and its use is necessary to followed by an overview of the instrument for JWST. With make Spacewire robust enough to be used for high reliable this background the architecture of the high-speed network applications. for the ISIM will be described with detailed sections on each topic. This paper will not describe the detailed The FPE network provides a high-speed network fabric of analysis performed to arrive at the solution for the FPE scalable point-to-point links (Spacewire) for the ISIM to network operation. communicate. Specifically, it provides the communications between all the Instrument Sensors and the Integrated Science Instrument Module (ISIM) Command & Data 2. JWST MISSIONOVERVIEW Handling (ICDH). The
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