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Mixed SpaceWire - SpaceFibre Networks

Mixed SpaceWire - SpaceFibre Networks . Martin Suess (1 , Steve Parkes (2 (1 European Space Agency, (2 University of Dundee E-mail: martin.suess at esa.int, sparkes at computing.dundee.ac.uk. Overview. Introduction SpaceWire – SpaceFibre comparison SpaceFibre Virtual Channels

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Mixed SpaceWire - SpaceFibre Networks

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  1. Mixed SpaceWire - SpaceFibre Networks Martin Suess(1, Steve Parkes(2 (1European Space Agency, (2University of Dundee E-mail: martin.suess at esa.int, sparkes at computing.dundee.ac.uk 2nd International SpaceWire Conference in Nara, Japan

  2. Overview • Introduction • SpaceWire – SpaceFibre comparison • SpaceFibre Virtual Channels • Priorities and Group Adaptive Routing • Mixed Network Examples • SpaceFibre Outlook • Conclusion 2nd International SpaceWire Conference in Nara, Japan

  3. Introduction • SpaceWire supports bi-directional traffic of up to 200Mbit/sec over a distance up to 10m. • SpaceFibre shall be capable to improve to both figures by at least a factor of 10: • Data rates ≥ 2.5Gbit/sec • Distance ≥ 100m • Provide additional features like galvanic isolation • This requires a number of modifications at different levels of the protocol stack. • The aim is to maintain compatibility between SpaceWire and SpaceFibre on Packet and Network level. • In the following the solutions implemented in the SpaceFibre breadboard are presented 2nd International SpaceWire Conference in Nara, Japan

  4. Physical & Signal Level - Optical • SpaceWire uses cables with 4 twisted pairs with nine-pin micro miniature D-type connectors. • SpaceFibre uses two optical fibres as medium: • The Draka MaxCap 300 radhard graded-index multimode fibre has been selected after testing • A cable protecting the fibre was designed based on expanded polytetrafluoethylene • Diamond AVIM connectors where already qualified for space • Electro optical transceivers produce 850-nm laser light with a power of 3dBm peak 2nd International SpaceWire Conference in Nara, Japan

  5. Diamond AVIM connectors and electro optical transceiver breadboard 2nd International SpaceWire Conference in Nara, Japan

  6. Physical & Signal Level - Electrical • In addition an electrical version for short distances forseen. • Prototype used 4 coaxial cables with SMA connectors. • The electrical interface of the transceivers use Current Mode Logic (CML). • CML is directly used as signal level in the electrical version. • Tolerance to common mode voltage differences can be improved by blocking capacitors. • More investigations are needed before physical & signal level definition of electrical SpaceFibre. AC coupling of a CML transmitter and receiver 2nd International SpaceWire Conference in Nara, Japan

  7. Character Level - 8B10B Encoding • SpaceWire defines data and 4 control characters • FCT, EOP, EEP, ESC • The combination of ESC with FCT and Data Characters defines the Null control code and the Time-Codes • SpaceFibre characters are based on 8B10B encoding • DC balanced data signal plus 12 special characters for control functions • Three of these special characters are comma characters • Comma characters contain a unique sequence of ones and zeroes that are used for character alignment 2nd International SpaceWire Conference in Nara, Japan

  8. Character Level - Ordered Sets • Ordered Sets are a sequence of 4 characters starting with a comma character (K28.5) • The second character defines the type of ordered set • The last two characters can carry additional information • Ordered Sets greatly extend the possibility to embed control information in the data steam • Several types of ordered sets are defined for SpaceFibre: • Link-level, power management, reset, flow control, faming and user ordered sets • User ordered sets are used to propagate time codes and interrupts though the network 2nd International SpaceWire Conference in Nara, Japan

  9. Exchange Level - Flow Control & Framing • SpaceWire uses flow control to prevent receive buffer over flow. • Each FCT indicates 8 Bytes of free buffer space. • SpaceFibre maintains the concept of flow control. • Granularity of flow control is increased due to higher bandwidth. • Each FCT ordered set controls the flow of one frame of maximum 255 data words. • A frame starts with a Start of Frame ordered set and ends with an End of Frame ordered set. • The End of Frame ordered set contains the 16 bit CRC of the frame for error detection. • SpaceWire packets are segmented into frames and reassembled at link level. 2nd International SpaceWire Conference in Nara, Japan

  10. Virtual Channels in SpaceFibre • Flow control and start of frame ordered sets contain the virtual channel number. • With separate frame buffers the virtual channel data flow is logically separated while using the same medium. • Congestion in one virtual channel does not influence the traffic in the other virtual channels. • A SpaceWire packet in one virtual channel can pass a packet in another virtual channel. • Priorities can be used to control the access of a virtual channel to the physical medium so that the higher priority channel has always direct access. 2nd International SpaceWire Conference in Nara, Japan

  11. Virtual Link 1 Virtual Link 2 Virtual Link 3 Virtual Link 4 Virtual Channels in SpaceFibre 2nd International SpaceWire Conference in Nara, Japan

  12. Number of Virtual Channels in a SpaceFibre Link • Maximum number of virtual channels is 256. • In practice less will be used to limit the number of buffers needed. • The individual virtual channels must be accessible without blockage or bottle neck. • In a SpaceWire router the total number of ports for path addressing is limited to 31. • The SpaceWire standard allows to use two consecutive address bytes for path addressing in large routers. • The first path address byte indicates the SpaceFibre link. • The second path address byte indicates the virtual channel number. • Beyond this some of the virtual channels could be accessed by logical addressing only. 2nd International SpaceWire Conference in Nara, Japan

  13. SpaceWire - SpaceFibre Router Router example with: • 3 SpaceFibre links with 6 virtual channels each • 7 SpaceWire links • 2 External ports • Non blocking crossbar switch provides direct access to every virtual channel 2nd International SpaceWire Conference in Nara, Japan

  14. Virtual Channel Priorities & Group Adaptive Routing • Each virtual channel can provide the full bandwidth of the SpaceFibre link. • If two or more virtual channels request a bandwidth higher than the full bandwidth of the link some arbitration is required. • Priorities can be assigned to virtual channels: • The virtual channel with higher priority is allowed to send the next frame. • Round robin arbitration is performed between virtual channels of the same priority. • User ordered sets for time-code and interrupt distribution have priority and are sent in the middle of the frame currently transmitted. • SpaceWire-RT protocol should be used to provide Quality of Service beyond priorities. 2nd International SpaceWire Conference in Nara, Japan

  15. Group Adaptive Routing • If packets are routed through the same virtual channel the access is arbitrated by the router. • Routers can provide group adaptive routing among virtual channels with the same priority: • Two packets to the same logical address can use parallel virtual channels. • The receiving side can then decide which should be processed first. • Available overall bandwidth is not increased. • Routers can provide group adaptive routing among several SpaceFibre links. • This can be used to increase the available bandwidth. 2nd International SpaceWire Conference in Nara, Japan

  16. Network Example – Single SpaceFibre Link 2nd International SpaceWire Conference in Nara, Japan

  17. SpaceFibre as Backbone 2nd International SpaceWire Conference in Nara, Japan

  18. Mixed Networks 2nd International SpaceWire Conference in Nara, Japan

  19. SpaceFibre Breadboarding 2nd International SpaceWire Conference in Nara, Japan

  20. SpaceFibre Breadboarding 2nd International SpaceWire Conference in Nara, Japan

  21. SpaceFibre Outlook • A first SpaceFibre prototype covering all levels has been implemented and tested. • A first outline specification has been published and discussed in the frame of the SpaceFibre working group. • The experience gained will be consolidated and used for the development of a SpaceFibre demonstrator. • The SpaceFibre Demonstrator activity will target: • Development of a SpaceFibre IP core, • Test and validation of IP core using existing prototype, • SpaceFibre Demonstrator implementation based on Actel FPGA and Wizard Link SerDes, • Preparation of a SpaceFibre specification as input for the standardisation process. 2nd International SpaceWire Conference in Nara, Japan

  22. Conclusion • The different levels of SpaceFibre have been compared with SpaceWire. • SpaceWire and SpaceFibre is intended to be fully compatible on Packet and Network level. • This allows the easy implementation of mixed SpaceWire – SpaceFibre networks. • Some examples of those networks have been presented. • This compatibility is seen as essential feature of SpaceFibre. • Experience has been gained with a prototype implementation. • As next step a demonstrator will be developed based on space qualified components. • Standardisation in ECSS is envisioned in corporation with the other space agencies. 2nd International SpaceWire Conference in Nara, Japan

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