This patch adds a virtual physical layer designed to simulate the
Um air interface between BTS and MS. It does so by encapsulating MAC
blocks (Layer 2 PDUs) via GSMTAP and sending them through multicast UDP
streams, both in uplink and in downlink.
The purpose of this is enable testing without any radio hardware or
related licenses.
OsmocomBB has recently received as similar patch-set, adding a virty_phy
executable that can be run on a PC instead of the classic 'layer1'
firmware on a real phone.
Using GSMTAP means that one can use unmodified wireshark to decode the
messages exchanged on the virtual Um layer.
This code was originally started by Harald in January 2016, continued
by Sebastian Stumpf in late 2016 and early 2017, and finally completed
by Harald in July 2017.
Change-Id: I1bf7670975b1e367c1c62983020865a043542622
