Virtual Um¶

In July 2017, the Osmocom project has inroduced a Virtual Um interface (i.e. virtual radio layer) between OsmoBTS and OsmocomBB. This allows us to run a complete GSM network with 1-N BTSs and 1-M MSs without any actual radio hardware, which is of course excellent for all kinds of testing scenarios.

The Virtual Um layer is based on sending L2 frames (blocks) encapsulated via GSMTAP UDP multicast packets. There are two separate multicast groups, one for uplink and one for downlink. The multicast nature simulates the shared medium and enables any simulated phone to receive the signal from multiple BTSs via the downlink multicast group.

In OsmoBTS, this is implemented via the new osmo-bts-virtual BTS model.

In OsmocomBB, this is realized by adding virtphy virtual L1, which speask the same L1CTL protocol that is used between the real OsmcoomBB Layer1 and the Layer2/3 programs such as [[OsmocomBB:Mobile] and the like.

This page describes how to set up the Virtual Um layer. It assumes that you are famliar with classic operation of OsmocomBB and OsmoBTS and OsmoNITB with a real radio layer. In case of doubt, look at the genric documentation and tutorials for real RF hardware.

The big picture¶

The BTS side¶

The MS side¶