doc/manuals/chapters/code-architecture.adoc

[[code_architecture]]
== Code Architecture

[[fig-code-architecture-general]]
.General overview of main OsmoTRX components
[graphviz]
----
digraph hierarchy {
node[shape=record,style=filled,fillcolor=gray95]
edge[dir=back, arrowtail=empty]

2[label = "{Transceiver|+ constructor()\l+ destructor()\l+ init()\l+ numChans()\l+ receiveFIFO()\l+ setSignalHandler()\l}"]
3[label = "{RadioInterface|...}"]
4[label = "{RadioInterfaceResamp|...}"]
5[label = "{RadioInterfaceMulti|...}"]
6[label = "{RadioDevice|...}"]
7[label = "{UHDDevice|...}"]
8[label = "{LMSDevice|...}"]
9[label = "{USRPDevice|...}"]

2->3[arrowtail=odiamond]
3->4[constraint=false]
3->5[constraint=false]
3->6[arrowtail=odiamond]
6->7
6->8
6->9
}
----

[[fig-code-architecture-threads]]
.Example of thread architecture with OsmoTRX configured to use 2 logical RF channels (Trx=Transceiver, RI=RadioIface)
[graphviz]
----
digraph hierarchy {
node[shape=record,style=filled,fillcolor=gray95]

trans [label="Transceiver"];
radioiface [label="RadioInterface"];
radiodev [label="RadioDevice"];

trans:nw->trans:ne [label="Main"];
trans:w->radioiface:w [label="Trx.TxPriorityQueueServiceLoop_0"];
trans:w->radioiface:w [label="Trx.TxPriorityQueueServiceLoop_1"];
radioiface:e->trans:e [label="Trx.RxServiceLoop_0"];
radioiface:e->trans:e [label="Trx.RxServiceLoop_1"];
radioiface->radiodev[label="RI.AlignRadioServiceLoop"];
radioiface:sw->radiodev:nw [label="Trx.TxLowerLoop"];
radiodev:ne->radioiface:se [label="Trx.RxLowerLoop"];
}
----

[[code_component_transceiver]]
=== Transceiver

The Transceiver is the main component managing the other components running in
the OsmoTRX process. There's a unique instance per process.

This class is quite complex from code point of view, as it starts lots of
different threads and hence the interaction with this class from the outside is
quite limited. Only interaction possible is to:

* `Transceiver()`: Create an instance through its constructor, at this time most
  configuration is handed to it.
* `init()`: Start running all the threads.
* `receiveFIFO()`: Attach a `radioInterface` channel FIFO in order to use it.
* `setSignalHandler()`: Used to set up a callback to receive certain events
  asynchronously from the Transceiver. No assumptions can be made about from
  which thread is the callback being called, which means multi-thread locking
  precautions may be required in certain cases, similar to usual signal handler
  processing. One important event received through this path is for instance
  when the Transceiver detected a fatal error which requires it to stop. Since
  it cannot stop itself (see destructor below), stopping procedure must be
  delegated to the user who created the instance.
* `~Transceiver()`: The destructor, which stops all running threads created at
  `init()` time. Destroying the object is the only way to stop the `Transceiver`
  completely, and must be called from a thread not managed by the
  `Transceiver`, otherwise it will deadlock. Usually it is stopped from the main
  thread, the one that called the constructor during startup.

During `init()` time, `Transceiver` will create a noticeable amount of threads,
which may vary depending on the amount of RF channels requested.

Static amount of Threads (1 per `Transceiver` instance):

* `RxLowerLoop`: This thread is responsible for reading bursts from the
  `RadioInterface`, storing them into its FIFO and sending Clock Indications
  (<<trx_if_clock_ind>>) to _osmo-bts_trx_.
* `TxLowerLoop`: Manages pushing bursts from buffers in the FIFO into the
  `RadioInterface` at expected correct time based on the Transceiver clock.

Dynamic amount of Threads (1 per RF logical channel on the `Transceiver` instance):

* `RxServiceLoop`: Each thread of this type pulls bursts from the
  `RadioInterface` FIFO for one specific logical RF channel and handles it
  according to the slot and burst correlation type, finally sending proper data
  over the TRX Manager UDP socket (<<trx_if>>).
* `TxPriorityQueueServiceLoop`: Blocks reading from one ARFCN specific TRX
  Manager UDP socket (<<trx_if>>), and fills the `RadioInterface` with it
  setting clock related information.

All the Per-ARFCN Control Interface socket (<<trx_if_control>>) commands are
handled by the event loop runnnig on the main thread. This is the only thread
expected to use the private `start()` and `stop()` methods.

[[code_component_radioiface]]
=== RadioInterface

The `RadioInterface` sits between the `Transceiver` and the `RadioDevice`,  and
provides extra features to the pipe like channelizers, resamplers, Tx/Rx
synchronization on some devices, etc.

If the `RadioDevice` it drives requires it (only _USRP1_ so far), the
`RadioIntercace` will start and manage a thread internally called
`AlignRadioServiceLoop` which will align current RX and TX timestamps.

Different features are offered through different `RadioInterface` subclasses
which are selected based on configuration and device detected at runtime. Using
these features may impact on the amount of CPU required to run the entire pipe.

==== RadioInterfaceResamp

This subclass of `RadioInterface` is automatically selected when some known
specific UHD are to be used, since they require resampling to work properly.
Some of this devices are for instance Ettus B100, USRP2 and X3XX models.

==== RadioInterfaceMulti

This subclass of `RadioInterface` is used when <<multiarfcn_mode>> is requested.

[[code_component_radiodev]]
=== RadioDevice

The `RadioDevice` class is responsible for driving the actual Hardware device.
It is actually only an interface, and it is implemented in each backend which in
turn becomes a specific OsmoTRX binary, see <<trx_backends>>.