1. SOIS Workshop The SOIS Communication Service Architecture Chris Plummer

2. Presentation Understanding the problem Understanding the onboard buses The SOIS communication services How the services stack up

3. Understanding the problem The goal of SOIS communication services is to enable ad hoc, asynchronous communications between flight software applications. Ad hoc? Communications are not pre-planned in respect of: Source/destination Amount of data Asynchronous? Communications occur when the users decide, and not in accordance to a pre-determined schedule

4. Understanding the problem The sticking points Spacecraft systems (and software) must be extremely reliable The response to meet this need is to develop fully deterministic systems The traditional way of achieving full determinism is to use synchronous, scheduled systems Spacecraft systems have therefore evolved to support this paradigm Synchronous operation of software based on underlying bus timing All transfers of data pre-planned in respect of source/destination, size, and scheduling

5. Understanding the problem Two problems Overcome behavioural resistance to a new software paradigm Somebody else's problem Need to provide for ad hoc, asynchronous communications over systems currently designed to operate scheduled synchronous communications A SOIS problem

6. Understanding the buses Three different buses are already being used onboard MIL-STD-1553B Most common at the moment Long space heritage Well suited to command and control in centralized systems Not well suited for distributed systems or ad hoc, asynch operation CAN bus Contender for direct replacement of 1553 Similar throughput as 1553 But actually has completely different operational philosophy Well suited to ad hoc asynchronous communications Bit tricky to use for centralized command and control applications

7. Understanding the buses SpaceWire Significantly better throughput (2 orders magnitude over 1553/CAN) Actually a point-to-point link But can be made to look like a bus using routing switches Could have extensive use in future spacecraft Several other buses are also being developed for spacecraft including: IEEE-1394 Ethernet RF wireless (802.11 and others) Optical wireless

8. Understanding the buses Three radically different buses: MIL-STD-1553B 1Mbps signaling, about 400kbps data Max 64-octets per transfer Synchronous operation Single master, multiple slave All transactions fully controlled by master No standard mechanism for slave to initiate a transfer CAN bus 1Mbps signaling, about 400kbps data Max. 8-octets per transfer Contention (any node can transmit at any time) Lossless CDMA based on message priority Built-in multiple retry (non-deterministic) Content labeling (i.e. no node addressing) SpaceWire 100 to 400Mbps signaling, about the same data! Unlimited number of octets per transfer! Point-to-point links formed into meshes Wide range of topologies possible No data link functionality defined yet

9. SOIS communication services 1. Generic data link service The trick is to make all of these buses appear to provide exactly the same functionality, namely: To transfer a variable length unit of data… between any two nodes on the bus… on demand So implementations of this service must deal with all of the problems coming from the choice of bus

10. SOIS communication services 2. Intra-networking service Provides for protocol multiplexing and the proper handling of different grades of service across a single subnetwork So implementations of this service must deal with the explicit transfer of protocol identity and grade-of-service information across a single subnetwork so that the data is handled appropriately at the receiving end of the communication

11. SOIS communication services 3 Inter-networking service: While current spacecraft are connected like this: We want future spacecraft to be connected like this: So the inter-networking service must provide the capability of transferring spacecraft data across multiple, possibly heterogeneous subnetworks Main Comp. Nav. Comp. Payload Comp. Spacecraft bus Main Comp. Nav. Comp. Payload Comp. P/L LAN Nav. LAN Sys LAN GW

12. SOIS service stack