ComCcsds (CCSDS Framing) Subtopology — Software Design Document (SDD)

The ComCcsds subtopologies implement F´’s CCSDS communications stack for framing/deframing on the flight side. There are two variants in the same module:

1. A variant that supplies a Svc::ComStub implementation of Svc.ComInterface and expects to be wired to a Drv::ByteStreamDriverModel (TCP/UDP/UART, etc.), and
2. A variant that expects an external implementation of Svc.ComInterface provided by the deployment.

Both variants provide the standard router + ComQueue + CCSDS framers/deframers path and are tuned through ComCcsdsConfig instance properties.

[!IMPORTANT] ComCcsds provides framing/deframing for CCSDS SpacePackets inside TM/TC data transfer frames.

1. Requirements

2. Design & Core Functions

2.1 Instance Summary

Two variants: A. “With ComStub”: Subtopology includes Svc::ComStub and exposes ByteStream ports to your driver. B. “With External ComInterface”: Subtopology does not include Svc::ComStub; you provide one in the deployment.

2.1.1 Layered Topologies

The module also exposes the two layers of the stack as importable topologies, allowing projects to compose alternative stacks (e.g., inserting an SDLS security layer between them) while reusing the same instances:

Each layer topology exposes its open boundary as topology ports (e.g. SpacePacketFraming.dataOut, TmTcFraming.framedDataIn, FramingSubtopology.comStatusIn), so composing topologies and deployments wire to the layer's ports rather than to individual component instances.

2.2 Required Inputs for Operation

• Rate Groups: Connect a rate group to comQueue.run. This is not required for the subtopology to function, but defines the rate at which ComQueue will send telemetry.

• Transport Endpoint:

  • Variant A: Wire ByteStream send/recv between your Drv::ByteStreamDriverModel and the subtopology’s ComStub.
  • Variant B: Provide your own Svc::ComInterface and wire it to the CCSDS framer/deframer ports in the subtopology.

• Flight-side hookups: Wire the router outputs (commands/files) into your CDH stack (e.g., command dispatcher, file uplink), and feed packet sources (telemetry/events/file downlink) into comQueue.

2.3 Limitations

These subtopologies focus on the CCSDS framing and deframing setup and does not provide wider CDH.

3. Usage

Below are two usage patterns, one for each variant. Replace identifiers/ports with the exact names in ComCcsds.fpp.

3.1 Variant A — ComCcsds with Svc::ComStub (expects a ByteStream driver)

topology Flight {
  instance ComCcsds.Subtopology

instance comDriver: <ByteStreamDriverInterface>

# (A1) Schedule ComQueue telemetry downlink (optional)
  connections RateGroups {
    rg.RateGroupMemberOut[0] -> ComCcsds.Subtopology.comQueueRun
  }

  # (A2) Wire ByteStream driver <-> ComStub supplied by the subtopology
  connections Link {
    comDriver.$recv                                -> ComCcsds.Subtopology.drvReceiveIn
    ComCcsds.Subtopology.drvReceiveReturnOut       -> comDriver.recvReturnIn
    ComCcsds.Subtopology.drvSendOut                -> comDriver.$send
    comDriver.ready                                -> ComCcsds.Subtopology.drvConnected
  }
}

[!TIP] ComCcsds.commsBufferManager and ComCcsds.commsBufferSendIn can be used if the ByteStreamDriver requires buffer management.

3.2 Variant B — ComCcsds without Svc::ComStub

topology Flight {
  import ComCcsds.FramingSubtopology

  # (B1) Provide your own ComInterface
  instance radio: <YourComInterface>

  # (B2) Schedule ComQueue
  connections RateGroups {
    rg.RateGroupMemberOut[0] -> ComCcsds.comQueue.run
  }

  # (B3) Wire your ComInterface between the driver and the ComCcsds framer/deframer
  connections Link {
    # Downlink: framing layer -> your ComInterface
    ComCcsds.FramingSubtopology.dataOut       -> radio.dataIn
    radio.dataReturnOut                       -> ComCcsds.FramingSubtopology.dataReturnIn
    radio.comStatusOut                        -> ComCcsds.FramingSubtopology.comStatusIn

    # Uplink: your ComInterface -> framing layer
    radio.dataOut                             -> ComCcsds.FramingSubtopology.dataIn
    ComCcsds.FramingSubtopology.dataReturnOut -> radio.dataReturnIn
  }
}

4. Configuration

Configure only the instance properties owned by the ComCcsds subtopologies. All knobs live under: Svc/Subtopologies/ComCcsds/ComCcsdsConfig/ComCcsdsConfig.fpp

4.1 Component properties (ComCcsdsConfig.fpp)

• Base ID — Base identifier for the subtopologies; instance IDs are offset from this base.
• Queue sizes — Depths for ComQueue and any other active/queued elements defined by the subtopology.
• Stack sizes — Task stack allocations for active components (if any beyond ComQueue).
• Priorities — RTOS priorities for active/queued components as applicable.
• CPU affinities — Core pinning for active component tasks; defaults to TASK_DEFAULT (no pinning).

4.2 Buffer Manager Bin Configuration

module BuffMgr provides constants for the bins configured for commsBufferManager.

5. Traceability Matrix