tree 11d9bfa1cd367671ca2f09d7aeb3bf429a12228e
parent b71f46126341ae87804e0c4cdf3e3e7642251f47
author Vadim Yanitskiy <vyanitskiy@sysmocom.de> 1678822431 +0100
committer laforge <laforge@osmocom.org> 1704805864 +0000

isdn: initial implementation of the V.110 TA

ITU-T recommendation V.110 defines Terminal Adaptor (TA) functions
for the connection of Terminal Equipment (TE) having standard V-series
interfaces to the ISDN.  This patch brings "software" implementation
of the TA to libosmoisdn.

The primary user for this soft-TA is the mobile-side implementation
of CSD (Circuit Switched Data) in osmocom-bb.  CSD is heavily based
on V.110, which is not surprising given that GSM is a "wireless ISDN".
Nevertheless, this code will likely also be useful in the context
of retro-networking.

Similarly to the existing V.110 code in libosmoisdn, the present
implementation aims to be functional and correct, rather than
efficient in any way.  It also has several limitations, which are
not critical for the CSD use case, but eventually may be a problem
for other use cases in the context of retro-networking.

Therefore, the V.110 TA API should be considered _unstable_,
and may be subject to change in the future.

  +-------+      +------+   B-channel   +------+      +-------+
  |  TE1  |------|  TA  |~~~~~~~~~~~~~~~|  TA  |------|  TE2  |
  +-------+      +------+               +------+      +-------+

TE (also known as DTE) is basically a computer, having a V-series
(usually RS-232) connection to TA (also known as DCE).  The TA acts
like a regular analog modem, except that it is not performing any
kind of modulation or demodulation itself.

The TE-TA interface is implemented by the user supplied callback
functions, configured during the allocation of a TA instance:

* .rx_cb() - receive call-back of the application,
* .tx_cb() - transmit call-back of the application,
* .status_update_cb() - status line update call-back.

In addition to that, the application (TE) can interact with the
V.24 status lines (circuits) using the following API:

* osmo_v110_ta_{get,set}_status(),
* osmo_v110_ta_{get,set}_circuit().

The Rx and Tx between TE and TA is always driven by the TA itself,
as a result of an interaction with the lower layer implementing
the B-channel interface.  There is currently no buffering and thus
no way for TE to initiate transmission or pull data on its own.

The TA-TA (B-channel) interface is implemented by the following
functions, which are meant to be called by the lower layer
transmitting and receiving V.110 frames over certain medium:

* osmo_v110_ta_frame_in() - indicate a received V.110 frame,
* osmo_v110_ta_frame_out() - pull a V.110 frame for transmission,
* osmo_v110_ta_[de]sync_ind() - indicate a synchronization event.

The lower layer is responsible for finding the synchronization
pattern (if needed), aligning to the frame boundaries, and doing
the V.110 frame coding.

The D-channel signalling is behind the scope of this module.

Initial (Work-in-Progress) implementation by Harald Welte,
completed and co-authored by Vadim Yanitskiy.

Change-Id: I5716bd6fd0201ee7a7a29e72f775972cd374082f
Related: OS#4396