== OsmoBTS PHY interface abstraction

The OsmoBTS PHY interface serves as an abstraction layer between given
PHY hardware and the actual logical transceivers (TRXs) of a BTS inside
the OsmoBTS code base.


=== PHY link

A PHY link is a physical connection / link towards a given PHY.  This
might be, for example,

* a set of file descriptors to device nodes in the /dev/ directory
  (sysmobts, litecell15)
* a packet socket for sending raw Ethernet frames to an OCTPHY
* a set of UDP sockets for interacting with OsmoTRX

Each PHY interface has a set of attribute/parameters and a list of 1 to
n PHY instances.

PHY links are numbered 0..n globally inside OsmoBTS.

Each PHY link is configured via the VTY using its individual top-level
vty node.  Given the different bts-model / phy specific properties, the
VTY configuration options (if any) of the PHY instance differ between
BTS models.

The PHY links and instances must be configured above the BTS/TRX nodes
in the configuration file.  If the file is saved via the VTY, the code
automatically ensures this.


=== PHY instance

A PHY instance is an instance of a PHY, accessed via a PHY link.

In the case of osmo-bts-sysmo and osmo-bts-trx, there is only one
instance in every PHY link.  This is due to the fact that the API inside
that PHY link does not permit for distinguishing multiple different
logical TRXs.

Other PHY implementations like the OCTPHY however do support addressing
multiple PHY instances via a single PHY link.

PHY instances are numbered 0..n inside each PHY link.

Each PHY instance is configured via the VTY as a separate node beneath each
PHY link.  Given the different bts-model / phy specific properties, the
VTY configuration options (if any) of the PHY instance differ between
BTS models.


=== Mapping PHY instances to TRXs

Each TRX node in the VTY must use the 'phy N instance M' command in
order to specify which PHY instance is allocated to this specific TRX.