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

../_images/comms_arch.png

Data Packets

All packets sent to/from the communication device will be encapsulated in several layers.

@startuml package "Radio Protocol" { package "UDP" { rectangle "Message Data" } } @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 UDP packet containing one of the following:

  • GraphQL query or mutation
  • JSON GraphQL responses
  • File/shell service commands or data
  • Any other application data a payload or mission application might need

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, a message handler thread is spawned and assigned one of the available handler UDP ports. This message handler examines the destination port embedded in the message’s UDP packet header to determine the internal message destination and then forwards it on to the appropriate service. The handler then waits for a reply (within a specified timeout duration), wraps the response in a UDP packet, and then sends the packet to the communications device for transmission. Once this transaction has completed, the message handler thread exits, freeing up the UDP port for future use.

@startuml hide footbox actor "Ground Station" as ground participant Radio ground -> Radio : 1. Send command to satellite box "Communications Service" #LightBlue participant "Read Thread" as read Radio <- read : 2. Read data packets from radio read -> read : 3. Deframe data packets read -> read : 4. Reassemble data packet create "Message Handler" as handler read -> handler : 5. Spawn new message handler activate handler end box participant "Kubos Service" as service handler -> service : 6. Send GraphQL query/mutation to service service -> handler : 7. Return result of query/mutation handler -> handler : 8. Wrap result in UDP packet handler -> Radio : 9. Send response packet to radio destroy handler Radio -> ground : 10. Send response packet to ground @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:

  • handler_port_min - Starting port used to define a range of ports that are used in the message handlers that handle messages received from the ground
  • handler_port_max - Ending port used to define a range of ports that are used in the message handlers that handle messages received from the ground
  • 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 - Length of time a message handler should wait for a reply, in milliseconds
  • ground_ip - IP address of the ground gateway
  • ground_port - UDP port of the ground gateway
  • satellite_ip - 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
  • handler_port_min - Should be copied from the corresponding config.toml value
  • handler_port_max - 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
  • ground_ip - Should be copied from the corresponding config.toml value
  • ground_port - Should be copied from the corresponding config.toml value
  • satellite_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: