Mixed SpaceWire - SpaceFibre Networks

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

2
Overview

• Introduction
• SpaceWire SpaceFibre comparison
• SpaceFibre Virtual Channels
• Priorities and Group Adaptive Routing
• Mixed Network Examples
• SpaceFibre Outlook
• Conclusion

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

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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

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• Diamond AVIM connectors and electro optical
  transceiver breadboard

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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
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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

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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

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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.

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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.

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Virtual Channels in SpaceFibre
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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.

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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

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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.

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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.

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Network Example Single SpaceFibre Link
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SpaceFibre as Backbone
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Mixed Networks
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SpaceFibre Breadboarding
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SpaceFibre Breadboarding
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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.

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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.