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Transcript of 1 Albert Ferrer-Florit, Steve Parkes Space Technology Centre University of Dundee QoS for SpaceWire...
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Albert Ferrer-Florit, Steve Parkes
Space Technology Centre
University of Dundee
QoS for SpaceWire networks
SpW-RT prototyping
Protocol design objectives
High performance, high reliability, timely Simple to implement, simple to understand Software and Hardware implementations Support Intelligent and dumb nodes Compatibility with significant SpW devices. Flexibility to accommodate multiple user
cases under a unique protocol definition.
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SpW-RT user cases
Asynchronous (usually also reliable)– Dedicated links: Timely– Shared links: Not timely
Scheduled and timely (usually also reliable)– Periodic messages (i.e. status messages)– Sporadic messages (i.e. command & control messages)
Multiple user cases can be accommodated within the same SpW network
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Survey on communication architectures
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Aeronautics– MIL-STD-1553– Fibre Channel– SAFEBus
Automotive industry– CAN bus– FlexRay– TTP
Cluster computing– Myrinet
System On Chip– AMBA bus, Core-Connect– Network On Chip
SpW-RT elements: reliability
Cause of packet errors– Link error or faulty router– Network congestion
Retry mechanism– Sliding sending window
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Redundancy– Recommended network topologies– Redundant paths
Message Data
ACK
Rcv buffer Rcv buffer
SpW-RT elements: routing
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Support Path and logical addressing
Detach routing from device identification
1. Different routes depending on QoS
2. Different routes depending on timeslot
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2
2
1
1
2
Address 120 Address 121
port1,port 2 Port 2
SpW-RT elements: timeliness
Computation of maximum packet delivery time for a simple case with lineal topology.
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One packet per source: 5*Transmission time
Nodes continuously sending: 9*Transmission time
SpW-RT elements: TDM and sending priority
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Time-Slot Slot1 Slot2 ...
Path 120, 200 121, 200
Channel 1-Command 2-Data
1-Command
TDM is very efficient if network traffic is periodic and known
Sending priority can be used to avoid wasting resources: payload data traffic is sent when command and control is not required.
Node A
RT PDURT PDU
Other SpW-RT elements
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Message (SDU)
• Segmentation limits the maximum packet size• SDU segments are encapsulated in RT PDUs
SDU segment
• Flow control assures that
1. SDU segment will be buffered or consumed
2. Low priority messages are completely receive by non parallel processing protocols
SpW-RT elements: Network manager
Device that supervise network status (nodes, links and routers) periodically.– Using best effort service– Using reserved timeslots in scheduled networks
Could be the only device that can write routing tables and open/close RT channels
Could implement PnP and FDIR.
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Net Manager
Router BNode ARouter A
SpW-RT: prototyping design space
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Basic requirements Met
Prototyping analysis
Prototyping implementation
Initial specification
Current specification
SpW-RT Prototypes
Two major prototypes have been developed:
1. Complete implementation of the first draft of the SpW-RT
• To check that the SpW-RT concepts are valid• To provide preliminary performance and complexity figures
2. Complete operative RT protocol that evaluate alternative approaches to help on the SpW-RT design trade off.
• Use of bidirectional channels, piggybacking acknowledgment, data and flow control.
• Connectionless protocol using control flags to reset sequence numbers.
• Support for zero configuration channels opening.• For synchronous systems, flow control and acknowledgement of
multiple channels is provided in the same PDU.12
Lessons learned
From the prototype work and further theoretical analysis, the trade off results in:– Control PDU format should not be equal for synchronous
and scheduled – Bidirectional channels are not very efficient for scheduled
systems– Connection oriented protocol offers more robustness– In scheduled networks flow control information should be
sent before the sender performs the arbitration and the acknowledgement should be received as soon as possible.
– Scheduled systems should send multiple SDU segments per timeslot. Seven segments of 256 bytes is optimum.
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Conclusions and future work
SpW-RT prototyping efforts have provided an important role in the definition of the SpW-RT protocol.– The specification is a result of a thoroughly analysis of
different options, some of them being prototyped.
Future work will focus on:– Prototyping of latest RT specification for EGSE and space
qualified components.– Development of network design tools and reference
architectures with user cases.
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