1 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. 2

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 3

Survey on communication architectures 4  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 5  Redundancy –Recommended network topologies –Redundant paths Message Data ACK Rcv buffer

SpW-RT elements: routing 6  Support Path and logical addressing  Detach routing from device identification 1.Different routes depending on QoS 2.Different routes depending on timeslot Address 120Address 121 port1,port 2Port 2

SpW-RT elements: timeliness  Computation of maximum packet delivery time for a simple case with lineal topology. 7 One packet per source: 5*Transmission time Nodes continuously sending: 9*Transmission time

SpW-RT elements: TDM and sending priority 8 Time-SlotSlot1Slot2... Path120, , 200 Channel1-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 PDU Other SpW-RT elements 9 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. 10 Net Manager Router B Node A Router A

SpW-RT: prototyping design space 11 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 13

SpW-RT application layer 14

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. 15

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. 16