doc: services: Add modem entry

   resource_management/index.rst

   mem_mgmt/index.rst

   modbus/index.rst

   modem/index.rst

   notify.rst

   pm/index.rst

   portability/index.rst

.. _modem:

Modem modules

#############

This service provides modules necessary to communicate with modems.

Modems are self-contained devices that implement the hardware and

software necessary to perform RF (Radio-Frequency) communication,

including GNSS, Cellular, WiFi etc.

The modem modules are inter-connected dynamically using

data-in/data-out pipes making them independently testable and

highly flexible, ensuring stability and scalability.

Modem pipe

**********

This module is used to abstract data-in/data-out communication over

a variety of mechanisms, like UART and CMUX DLCI channels, in a

thread-safe manner.

A modem backend will internally contain an instance of a modem_pipe

structure, alongside any buffers and additional structures required

to abstract away its underlying mechanism.

The modem backend will return a pointer to its internal modem_pipe

structure when initialized, which will be used to interact with the

backend through the modem pipe API.

.. doxygengroup:: modem_pipe

Modem PPP

*********

This module defines and binds a L2 PPP network interface, described in

:ref:`net_l2_interface`, to a modem backend. The L2 PPP interface sends

and receives network packets. These network packets have to be wrapped

in PPP frames before being transported via a modem backend. This module

performs said wrapping.

.. doxygengroup:: modem_ppp

Modem CMUX

**********

This module is an implementation of CMUX following the 3GPP 27.010

specification. CMUX is a multiplexing protocol, allowing for multiple

bi-directional streams of data, called DLCI channels. The module

attaches to a single modem backend, exposing multiple modem backends,

each representing a DLCI channel.

.. doxygengroup:: modem_cmux