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{{ :doc:crazyflie:dev:arch:crazyflie-comm-stack.png?direct&400|}}
====== Communication stack ======
The Crazyflie communication stack is divided into three layers:
  * **Module**: End receiving service of data
  * **CRTP**: Protocol for communication
  * **Link**: Physical communication link

Each layer communicates on a logical level with the same layer on the other side.

===== Module =====
Each module is assigned a port, which works much like the ports in UDP. Sending data on a specific port will make sure it's routed to the registered module for this port. The data that is sent to/from a specific port also contains a channel id of 3 bits. This channel id can be used to more easily route the data once inside the module.

===== CRTP =====
The protocol used for sending data to/from the Crazyflie is named CRTP (Crazyflie RealTime Protocol). In it's current state the protocol isn't real-time, but there's bits over in the header to implement this functionality.

Each CRTP packet is 32 bytes, of which 1 byte is taken by the header. This gives a total payload of 31 bytes per packet. The header holds the port (8 bits), channel (2 bits) and reserved (2 bits).

===== Link =====
The link is responsible for transferring the packet to/from the Crazyflie. Currently there's two types of physical links implemented: A radio link using the Crazyradio and a wired link using USB directly to the Crazyflie (only implemented for 2.0).

Since no CRC/checksum is added to the CRTP packet it's the link layers responsibility to check the integrity of the packets.

====== Architecture ======
===== Modules =====
In the firmware most modules (that are connected to ports) are implemented as tasks. Each task is blocking on a message delivery queue where incoming CRTP packets are delivered. At start up each of these tasks (and other modules) register a callback with the communication layer for their pre-defined ports.

For every module that is used to send/receive CRTP packets there's a Python API object that is used to interact with it. For instance for logging there's a module named //log.c// in the Crazyflie firmware and a class named //Log// in the Python API. This part of the API is then used to directly interact with the logging functionality in the Crazyflie.
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