Communications Service Framework

The hardware used to establish communication between a satellite and the ground varies wildly from mission to mission. As a result, KubOS provides a communications service framework which can be used by developers when creating their specific radio’s hardware service. This framework is used to simplify the process of reading messages from the ground, forwarding them to the appropriate internal destination, and then sending properly formatted messages back to the ground.

Note

The communications service framework is currently only available in Rust. As a result, services wishing to utilize the framework should also be written in Rust.

Architecture

@startuml skinparam linetype polyline skinparam linetype ortho left to right direction rectangle "Telemetry Service" as Telemetry rectangle "Mission Application" as App rectangle "Radio" as Radio package "Communications Service" { rectangle "Read Thread" as Read rectangle "Message Handler" as Message rectangle "Downlink Endpoint" as Downlink } Radio -right-> Read Downlink -up-> Radio Message -left-> Radio Telemetry .left.> Message Message .right.> Telemetry Telemetry .> App App .> Telemetry App .> Downlink @enduml

@startuml hide footbox actor "Ground Control" as ground_control participant "Radio" as radio participant "Communications Services" as comms_service participant "Flight Software" as software ground_control -> radio: 1. Send command to satellite radio -> comms_service: 2. Read data packets from radio comms_service -> software: 3. Send packet payload to appropriate service software -> comms_service: 4. Send response back comms_service -> radio: 5. Send data packet to radio with response radio -> ground_control: 6. Send response to ground control @enduml

Data Packets

All packets sent to/from the communication device, over the wire (or air), will be encapsulated in several layers.

@startuml package "Radio Protocol" { package "Space Packet" { rectangle "Payload" } } @enduml

The first layer will be whatever communication protocol the device requires. For example, AX.25 is frequently used as the header protocol for radio communication.

Inside of this will be a Space Packet containing one of the following payloads:

  • GraphQL query or mutation
  • JSON GraphQL responses
  • File/shell service commands or data
  • Any other application data or payload which can be passed over UDP

Ground Communication

The communications service maintains a constant read thread which listens for messages from the ground via the communications device. Once a message is received, the message’s Space Packet header is examined to determine the payload type. Currently the communications service framework supports two payload types: GrahpQL and UDP. The payload type determines how the message is passed on to the rest of the system.

GraphQL Payloads

When a GraphQL message is received, a message handler thread is spawned. This message handler examines the port embedded in the message’s Space Packet header to determine the internal message destination and then makes an HTTP POST to the appropriate service. The handler then waits for a response (within a specified timeout duration), wraps the response in a Space Packet, and then sends the packet to the communications device for transmission. Once this transaction has completed, the message handler thread exits.

@startuml hide footbox actor Radio box "Communications Service" #LightBlue participant "Read Thread" as read Radio -> read : 1. Read data packets from radio read -> read : 2. Deframe data packets read -> read : 3. Reassemble data packet create "Message Handler" as handler read -> handler : 4. Spawn new message handler activate handler end box participant "Kubos Service" as service handler -> service : 5. Posts GraphQL query/mutation to service service -> handler : 6. Return result of query/mutation handler -> handler : 7. Wrap result in Space Packet handler -> Radio : 8. Send response packet to radio destroy handler @enduml

UDP Payloads

When a UDP message is received, a message handler thread is spawned. This message handler examines the port embedded in the message’s Space Packet header to determine the internal message destination and then sends a UDP packet containing the message to the appropriate service. The message handler thread exits immediately after sending out the UDP packet. UDP payloads sent from the ground to flight are intended to act as a one-way passthrough. Any responses must be sent through the appropriate downlink endpoint.

@startuml hide footbox actor Radio box "Communications Service" #LightBlue participant "Read Thread" as read Radio -> read : 1. Read data packets from radio read -> read : 2. Deframe data packets read -> read : 3. Reassemble data packet create "Message Handler" as handler read -> handler : 4. Spawn new message handler activate handler end box participant "Kubos Service" as service handler -> service : 5. Pass UDP packet to service destroy handler @enduml

Configuration

Most missions tend to have a single communications device which communicates via a single uplink and single downlink method. However, some missions have more complex communications systems; for example, having a slower, but more reliable downlink method for transmitting a small health-and-status beacon and then a faster, but more error-prone method for transmitting more detailed telemetry information.

The communications service framework provides mechanisms to handle these more complex configurations.

The service’s config.toml file should contain the following parameters:

  • max_num_handlers - (Default: 50) The maximum number of concurrent message handlers allowed
  • downlink_ports - (Optional) List of ports used by downlink endpoints that send messages to the ground. Each port in the list will be used by one downlink endpoint
  • timeout - (Default: 1500) Length of time a message handler should wait for a reply, in milliseconds
  • ip - (Required) IP address of the communications service

The service which implements the framework should create a CommsControlBlock, which provides the final configuration to the main communication logic. It contains the following members:

  • read_conn - This is the connection structure which is used for reading from the communications device, for example a UART port or UDP socket
  • write_conn - This is the connection structure which is used for reading from the communications device, for example a UART port or UDP socket
  • read - (Optional) A pointer to the function which should be used to read a message from the communications device
  • write - A list of function pointers for all available ways that messages may be written to the communications device
  • max_num_handlers - Should be copied from the corresponding config.toml value
  • downlink_ports - Should be copied from the corresponding config.toml value or None
  • timeout - Should be copied from the corresponding config.toml value
  • ip - Should be copied from the corresponding config.toml value

Warning

If downlink endpoints are being used, the number of function pointers in the write list must be equal to the number of ports in the downlink_ports list.

Implementation

Because communication methods may vary from mission to mission, it is up to the user to create the final hardware service/s which will be used for communication with the ground.

The service which implements the communications service framework should utilize only a single communication device. If multiple devices are present in the system, a service should be created for each of them.

The service should contain the following components:

  • A function capable of reading messages from the communications device (note: this may be omitted in the case where the communications device will only be used for transmitting data)
  • At least one function capable of writing messages to the communications device
  • Initialized connection structure/s which will be used by the previous read and/or write functions
  • A CommsControlBlock structure containing all of the communications configuration information
  • A CommsTelemetry structure which will be used to gather communication statistics

It is recommended that the service also follow the guidelines for normal hardware services.

Some support for communicating with specific hardware devices has been provided in various hardware APIs.

For more information about how to implement a communications service, please refer to the following resources: