Transcript Document 7545645

SpW-RT prototyping
QoS for SpaceWire networks
Albert Ferrer-Florit, Steve Parkes
Space Technology Centre
University of Dundee
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Protocol design objectives
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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.
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
 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
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SpW-RT elements: reliability
 Cause of packet errors
– Link error or faulty router
– Network congestion
 Retry mechanism
– Sliding sending window
Message
Data
Rcv buffer
ACK
 Redundancy
– Recommended network topologies
– Redundant paths
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SpW-RT elements: routing
 Support Path and logical addressing
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 Detach routing from device identification
1. Different routes depending on QoS
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Address 120
Address 121
port1,port 2
Port 2
2. Different routes depending on timeslot
SpW-RT elements: timeliness
 Computation of maximum packet delivery
time for a simple case with lineal topology.
One packet per source: 5*Transmission time
Nodes continuously sending: 9*Transmission
time
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SpW-RT elements: TDM and sending priority
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
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Time-Slot
Slot1
Slot2
Path
120, 200
121, 200
Channel
1-Command
2-Data
1-Command
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Other SpW-RT elements
• Segmentation limits the maximum packet size
• SDU segments are encapsulated in RT PDUs
Message (SDU)
RT PDU
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
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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.
Router A
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Router B
Node A
Net
Manager
SpW-RT: prototyping design space
Basic requirements Met
Prototyping analysis
Prototyping
implementation
Current
Initial specification
specification
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SpW-RT Prototypes
 Two major prototypes have been developed :
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Complete implementation of the first draft of the SpW-RT
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Complete operative RT protocol that evaluate alternative
approaches to help on the SpW-RT design trade off.
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To check that the SpW-RT concepts are valid
To provide preliminary performance and complexity figures
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.
SpW-RT prototype scheme
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SpW-RT application layer
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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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