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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

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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