1. SOIS Standard Services for Communications over 1553 Implementation with ECSS-E-ST-50-13C defined services and protocols
CCSDS-SOIS Fall Meeting 2008, Berlin, Germany
Olivier Notebaert – Astrium Satellites – ASG74
Convenor of the ECSS Working Group for standardization on 1553 MilBus

2. The ECSS Working Group and the drafting process
Summary
Context
The ECSS Working Group and the drafting process
ECSS-E Draft-C Standard Overview
Conclusion
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3. Context Mil-Bus in Space Applications
a proven Standard
Well defined physical and data link layers requirements
But does not provide any “sub-network service layer” (according to CCSDS SOIS model)
But does not provide any user level communication protocol
well adapted to spacecrafts Data-Handling
Very good reliability characteristics
Allows deterministic and efficient real-time command/control
Lot of sensors, commercial products and test equipments
Large return on experience in space applications
used successfully for many space applications
Launcher avionics on Ariane 5 and Vega
Spacecrafts control for many platforms and payloads
In-Orbit infrastructure and manned flight for ISS and ATV
still with good perspective
Evolutions are foreseen to allow higher bandwidth and lower power consumptions
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4. The ECSS Working Group and the drafting process
Summary
Context
The ECSS Working Group and the drafting process
ECSS-E Draft-C Standard Overview
Conclusion
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5. ECSS 1553 Working Group Objectives and Strategy
Physical layer and Data link layers are well defined by the Mil standard but there are several usage variants
Identify best practices for harmonization
Capitalize the acquired experience on 1553 based systems
Adapt requirements from existing projects and take into account existing communication devices
Several services and communication protocols exist with similar properties
Define a standard set of services and protocols suitable to most 1553 based systems taking into account applications needs
Take into account the sub-network services reference defined by CCSDS/SOIS
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6. ECSS 1553 Working Group Members and interfaces
The WG activity started in December 2005 with the initial objective for a Draft standard delivery at the end of 2007
It is ruled by ECSS standards productions procedures and by an agreed Term of Reference
8 members involved in on-board 1553 systems from European space agencies and industry
ECSS Technical Authority
ECSS secretariat
ECSS-E-50
ESA/Industry
Working Group
ECSS-E-50
Engineering panel
CCSDS/SOIS
(ESA: Chris Taylor)
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7. ECSS 1553 Working Group Drafting Process
Mil 1553
CCSDS/SOIS
study
Projects needs and experience
Working Group
Analysis and Drafting
Publication
(end 2008)
Standard engineering - elaboration and maturation
Draft D September 2008
Draft A
April 2007
Draft B
November 2007
Draft C May 2008
Edition
Editorial review
(ECSS Secretariat)
Public review
Technical review (Industry + ESA)
Modeling
Verification
(ESA)
SOIS mapping & prototyping
Standard verification activities
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8. The ECSS Working Group and the drafting process
Summary
Context
The ECSS Working Group and the drafting process
ECSS-E Draft-D Standard Overview
Conclusion
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9. Draft-D Overview Table of Content
Scope
Normative References
Terms, definition and Abbreviated terms
Overview
Physical layer requirements
Data Link layer requirements
Services definition
Protocol Specification
Test and Verification
Annexes
A Tailoring guidelines
B Unreferenced requirements in Mil-Std-1553
C Bibliography
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10. Draft-D Overview Scope
A reference architecture for 1553 systems, a service model and a bus profiling concept are defined
gathering experience of 1553 based projects
aiming at determinism of data bus exchanges thus allowing bandwidth allocation on the bus to ensure:
Guaranteed availability for critical data
Flexibility for asynchronous and sporadic traffic
in Section 4 (Informative)
Harmonisation rules, services and protocols are defined
mapping on SOIS architecture and service model
re-using existing requirements and protocol elements from space projects
in Section 5 to 9 (Normative)
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11. Draft-D Overview Reference Architecture
Send
User Data
Services W, X,Y,Z BC
Application
Receive
Physical Layer Functions: Retry, Error detection…
Elements of Protocol: Synch pattern, parity,...
Bus Controller
Data Link Layer
ECSS Services Functions: Segmentation,…
Elements of Protocol: Block data length, Address tables,…
1553 Functions: Error detection, Delimiting and Synchronisation, …
Elements of Protocol: Message length, Start/end Marker, Status Word,…
Receive
User Data
RT X
Application
Send
Services W, X,Y, Z
Remote Terminal (with ECSS Services)
Data Link Layer
ECSS Services Functions: Segmentation,…
Elements of Protocol: Block data length, Address tables,…
1553 Functions: Error detection, Delimiting and Synchronisation, …
Elements of Protocol: Message length, Start/end Marker, Status Word,…
Physical Layer Functions: Retry, Error detection…
Elements of Protocol: Synch pattern, parity,…
Receive
User Data
RT Y
Application
Services
Y, Z
Send
Remote Terminal (Legacy)
Data Link Layer
1553 Functions: Error detection, Delimiting and Synchronisation, …
Elements of Protocol: Message length, Start/end Marker, Status Word,…
Physical Layer Functions: Retry, Error detection…
Elements of Protocol: Synch pattern, parity,…
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12. Draft-D Overview Service Model and CCSDS/SOIS
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13. Draft-D Overview Physical and Data Link Layer Harmonisation
The ECSS standard adds specific details for the Physical and Data Link Layers of MIL-STD-1553B
On the Physical layer:
More precise definition of the bus cable impedance (using metric units)
Selection of the transformer-coupled stub connection
Application of bus cable and stub discharge resistances
Definition of the connector interface between the terminals and the bus
On the Data Link Layer
Rules on usage of mode codes
RT to RT messages transfer not allowed
1553 Standard automatic retry not allowed
Fixes the response timeout (14 µs to 22 µs without bus repeaters)
Provides subaddress usage harmonisation rules and recommendations
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14. Draft-D Overview Physical and Data Link Layer Harmonisation
Retry function included in the 1553 standard
It is not often used by space projects for several reasons such as:
In systems used cyclic scheduling (90%), an immediate retry modifies the cyclic scheduling of the bus unless the retry transfer is systematically included within the frame which has a high bandwidth cost
The error rate on 1553 is very low – experience shows that retry’s are almost always application errors (and the retried message also fails)
It is often a system choice to handle errors and retries within the FDIR function (at application level) or within higher level services protocols
In most applications, the highest proportion of the traffic is periodic acquisition data and the system application is tolerant to one acquisition failure (even 2 or more). Retrying immediately is not necessary.
As a result, the ECSS standard does not use the built-in retry of the 1553 standard retry
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15. Draft-D Overview Standard Services and protocols
Time Distribution and Synchronisation
distribution of time information
Communication Synchronisation
deterministic time multiplexing of data bus messages
Set Data and Get Data
Classical 1553 transfer oriented with additional built-in functions such as segmentation dedicated to direct RT memory access
Data Block Transfer
Packet Transfer oriented service supported by a handshake protocol providing different levels of QoS
Terminal management
Rules and standard data structures in support to the harmonisation of terminal configuration, test and bus management applications
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16. Standard Services and protocols Time Distribution and Time Synchronisation (1/2)
The On Board Time Master is driving the 1553 Bus controller
Maintains a coherent time data for all users of the service
Remote terminal applications maintain local OBT synchronized through reception of broadcasted messages from OBT Master
The time format is normalized to CCSDS CUC
Time information is distributed at a fixed subaddress (29)
Accuracy is mission dependant (shall be better than 50 µs)
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17. Standard Services and protocols Time Distribution and Time Synchronisation (2/2)
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18. Standard Services and protocols Communication Synchronisation
The standard defines Communication frames which embed all 1553 message sequences within time bounded and synchronized intervals
This supports messages time multiplexing in a deterministic way
It allows to control user services latencies
A bus profile can be implemented for a given mission through:
Communication Frames programming in the Bus Controller at initialization time
Management of Communication Frames and real-time data at run-time
Real-time issues are guaranteed through resource reservation within the bus profile (bandwidth pre-allocation)
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19. Standard Services and protocols Communication synchronisation and bus profiling
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20. Standard Services and protocols Communication service and protocols
Two main use cases  two services with adapted data transfer protocols:
Data transfer for real-time critical applications (Get/Set)
Asymmetrical service provided on Bus controller only and dedicated to command control of sensors by a central computer
Typically sensor acquisition/distribution of unformatted data
Segmentation included up to a configurable and limited maximum limit (few Kbits)
Easily mapped to SOIS Memory access service
Data transfer for on-board communications (Data Block Transfer)
Symmetrical service provided between users on the Bus Controller and users on Remote Terminals
Asymmetrical protocol due to 1553 Master/slave characteristics
Data bloc transmissions, including segmentation, sequence control, flow control, confirmation, up to a maximum of 4 Kbytes
Easily mapped to SOIS Packet service
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21. Standard Services and protocols Set Data and Get Data
Classical 1553 data transfer with built-in functions
Provided on the BC side only:
Set Data provides data transfer function from BC to one RT
Get Data provides data transfer function from one RT to BC
Segmentation
A single access to the service allows transmission of more than 32 words (maximum data size is application dependant)
Quality of Service and error detection
The protocol requires that accesses are time-bounded (maximum is one communication frame)
The destination user application ensures timely retrieval of data before it may be overwritten by new data
Error report provided to the service user based on the 1553 status words of the transfers in support to user FDIR application
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22. Standard Services and protocols Example: Invoked 1553 messages for a Set Data
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23. Standard Services and protocols Data Block Transfer
Capability to transfer data blocks on request of the sender, with completion by a confirmation
Adapted for RT’s that have the capability to handle, segment re-assemble and store large data structures
Data transfer protocol control
Handshake between sender and receiver
Error controls
Protocol Reset
Quality of Service
Best Effort
No check for correctness of the transferred data
Verified Length
Check that the correct number of bytes has been received
Check for data bus transmission error
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24. Data Block Transfer Flat and Deep sub-addressing modes
Flat sub-addressing
Different subaddresses are used to transfer different sections of the transmitted data block.
In this mode the maximum block length that can be transmitted is 1024 bytes
The subaddresses to be used are fixed and defined by the standard (11 to 26)
Deep sub-addressing
A single subaddress is used to transfer different sections of the transmitted data block
Several subaddresses may be used resulting in several channels
In this mode the maximum block length that can be transmitted is 4096 bytes
The subaddress usage is determined by the RT design
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25. Data Block Transfer Data Transfer control
Protocol data structures to be exchanged across the data bus between BC and RT before and after the transfer of user-data
Layout of the Distribution Transfer Descriptor (BC to RT, SA 27R)
Layout of the Distribution Transfer Confirmation (BC to RT, SA 27T)
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26. Data Block Transfer Performance / QoS
Data Distribution timing with Best Effort QoS
Possible delay of one or more complete Communication Frames depending on the selected bus profile and the RT application timing.
Maximum performance is one user data block every second Communication Frames
Data Distribution timing with Verified Length QoS
Maximum performance is one user data block every fourth Communication Frames
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27. Standard Services and protocols Terminal Management
Provides data link layer harmonisation requirements for
RT Health and Monitoring
Capability to receive information concerning health of Remote Terminals across the bus
Alarm Notification
Allows a RT to signal events to the FDIR function of the bus management.
In the MIL-STD-1553B standard, this is covered by the status bits
Terminal Flag Bit concerning RT level problems;
Subsystem Flag Bit concerning RT subsystem, including SW for impact on the transferred data;
Terminal Configuration
Allows the user on BC to be informed on and (re)configure the Remote Terminals
To be extended in support to the CCSDS/SOIS PnP service
The Data Wrap Around Service
Provides support to requirement defined in section 30.7 of MIL-STD-1553B.
It can be used to test BC-to-RT bus connection in support of test services and FDIR applications
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28. The ECSS Working Group and the drafting process
Summary
Context
The ECSS Working Group and the drafting process
ECSS-E Draft-C Standard Overview
Conclusion
19/02/2008

29. SOIS subnetwork 1553 services prototyping subnetwork services mapping to ECSS Standard
SOIS Subnetwork Services are all implemented and mapped on the ECSS-E-ST-50-13C defined services and protocols
Packet transfer
Mapped on Data Block Transfer
Memory Access
mapped on Get Data and Set Data
Synchronisation
mapped on the Time Distribution and Time Synchronisation services
Test
Relies on the Data Wrap-Around function as defined in the Terminal management
Device Discovery
Supported by Get Data and Set Data using the Terminal management reserved sub addresses in order to exchange devices configuration data.
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30. This Standard has completed verification activities and public review
Conclusion
Using the ECSS-E standard draft requirements, a SOIS subnetwork level communication services interface and protocols for 1553 Milbus systems has been defined and implemented
This Standard has completed verification activities and public review
provides feedback on the implementation and performance issues to CCSDS/SOIS and ECSS-E-50 WG
Publication awaited end 2008
Thank you for your attention !
19/02/2008