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== Overview

=== About this manual

This manual should help you getting started with the OsmoBTS software.
It will cover aspects of configuring and running OsmoBTS as well as some
introduction about its internal architecture and external interfaces.

=== About OsmoBTS

OsmoBTS is an implementation of a GSM BTS (Base Transceiver Station).  A
BTS serves  as the interface between the Um radio interface towards
phones  and the wired Abis interface towards the BSC (Base Station
Controller).  It also implements the network side of the Layer 2 of the
Um radio interface: The LAPDm protocol.

OsmoBTS is licensed as Free and Open Source Software (FOSS) under _GNU
AGPLv3_ <<gnu-agplv3>>.  It is developed as one GSM network
infrastructure component part of the overall Osmocom project.

As perhaps the first implementation of a GSM BTS ever in the industry,
OsmoBTS is implemented in a vendor-independent way and supports a large
variety of transceiver hardware and physical layer implementations from
many vendors.

=== Credits

OsmoBTS was originally developed in 2011 by Andreas Eversberg and Harald
Welte.  It has since been maintained by Harald Welte and Holger Freyther
at sysmocom.

=== OsmoBTS in the Osmocom GSM network architecture

OsmoBTS can be used in combination with the various other GSM network
elements developed under the umbrella of the Osmocom project.

Typical configurations either use OsmoBTS with OsmoBSC, or with
OsmoNITB, as can be seen in the following figures.

[[fig-gsm-classic]]
.Classic GSM archtiecture using OsmoBTS with OsmoBTS components
[graphviz]
----
digraph G {
	rankdir=LR;
	MS0 [label="MS"]
	MS1 [label="MS"]
	MS2 [label="MS"]
	MS3 [label="MS"]
	BTS0 [label="OsmoBTS"]
	BTS1 [label="OsmoBTS"]
	BSC [label="OsmoBSC"]
	MSC [label="MSC/VLR"]
	HLR [label="HLR/AUC"]
	MS0->BTS0 [label="Um"]
	MS1->BTS0 [label="Um"]
	MS2->BTS1 [label="Um"]
	MS3->BTS1 [label="Um"]
	BTS0->BSC [label="Abis"]
	BTS1->BSC [label="Abis"]
	BSC->MSC [label="A"]
	MSC->HLR [label="C"]
	MSC->EIR [label="F"]
	MSC->SMSC
}
----


[[fig-gsm-nitb]]
.GSM architecture using OsmoBTS + OsmoNITB
[graphviz]
----
digraph G {
	rankdir=LR;
	MS0 [label="MS"]
	MS1 [label="MS"]
	MS2 [label="MS"]
	MS3 [label="MS"]
	BTS0 [label="OsmoBTS"]
	BTS1 [label="OsmoBTS"]
	MS0->BTS0 [label="Um"]
	MS1->BTS0 [label="Um"]
	MS2->BTS1 [label="Um"]
	MS3->BTS1 [label="Um"]
	BTS0->BSC [label="Abis"]
	BTS1->BSC [label="Abis"]
	subgraph cluster_nitb {
		label = "OsmoNITB";
		BSC
		MSC [label="MSC/VLR"]
		HLR [label="HLR/AUC"]
		BSC->MSC [label="A"]
		MSC->HLR [label="C"]
		MSC->EIR [label="F"]
		MSC->SMSC;
	}
}
----

If intended by the user, it is of course also possible to implement an
OsmoBTS-compatible Abis-over-IP interface in any third party BSC.  The
Abis/IP interface and its protocol are documented in the _OsmoBTS
Abis Protocol Specification_ <<osmobts-abis-spec>>.  However, be advised
that such a configuration is currently not officially supported by
Osmocom.