== OsmoBTS Interfaces

OsmoBTS offers a set of interfaces to interact with external entities:

* A-bis/IP interface to talk to the BSC
* bts_model specific PHY interface
* VTY interface
* Osmocom control interface
* GSMTAP interface
* PCU interface


=== OsmoBTS Abis/IP Interface

OsmoBTS implements the GSM A-bis interface as described in the relevant
3GPP specifications:

* A-bis RSL according to 3GPP TS 08.58
* A-bis OML according to 3GPP TS 12.21

As the 3GPP specifies A-bis only over E1 interfaces and not over IP,
significant enhancements and modifications to the 3GPP specifications are
employed.  Nevertheless, the implementation tries to stay as close as
possible to the 3GPP specifications.

Please see the _OsmoBTS Abis Protocol Specification_
<<osmobts-abis-spec>> for more information on this subject.


=== bts_model specific PHY interface

This interface is specific to the bts_model that OsmoBTS was compiled
for.  It can take any form as required by the respective hardware.

Please see the PHY documentation of your respective BTS hardware for more
details.


=== OsmoBTS VTY Interface

See <<vty>> for further information.


=== OsmoBTS Control Interface

The general structure of the Osmocom control interface is described in
<<common-control-if>>.

The number of control interface commands/attributes is currently quite
limited and largely depends on the bts_model used.

==== trx.N.thermal-attenuation

The idea of this parameter is to attenuate the system output power as part of
thermal management.  In some cases the PA might be passing a critical level,
so an external control process can use this attribute to reduce the system
output power.

Please note that all values in the context of transmit power calculation
are integers in milli-dB (1/10000 bel), so the below example is setting
the attenuation at 3 dB:

----
bsc_control.py -d localhost -p 4238 -s trx.0.thermal-attenuation 3000
Got message: SET_REPLY 1 trx.0.thermal-attenuation 3000
----

----
bsc_control.py -d localhost -p 4238 -g trx.0.thermal-attenuation
Got message: GET_REPLY 1 trx.0.thermal-attenuation 3000
----



=== OsmoBTS GSMTAP Interface

GSMTAP is a standard created by Osmocom to UDP-encapsulate GSM protocol
messages normally communicated over non-IP interfaces for the primary
purpose of protocol analysis in the wireshark dissector.

The initial purpose was to encapsulate GSM Um frames including some
meta-data like ARFCN and GSM frame number into something that can be
parsed and dispatched within the wireshark dissector.

This interface has since been extended to many other
GSM/GPRS/UMTS interfaces and protocols, and even to TETRA and GMR.

In OsmoBTS, it is possible to export both uplink and downlink Um
messages via GSMTAP.  There is a set of VTY configuration options to
specify for which logical channels of the Um interface GSMTAP messages
shall be emitted, and to which destination IP address they shall be
sent.

Using GSMTAP it is possible to place a virtual tap at the air interface
between BTS and MS, without going through the trouble of setting up an
actual radio receiver at the same frequencies.  Also, GSMTAP export is
performed before the Um air-interface encryption (A5) is performed, so
all frames are always in plain text.

Please refer to <<gsmtap>> for more information on how to configure and
use this interface.


=== OsmoBTS PCU Socket Interface

In order to assist the provisioning of GPRS services over the same radio
interface as circuit-switched GSM, OsmoBTS exposes a Unix domain socket
based interface towards OsmoPCU.

OsmoPCU is the Osmocom implementation of the GPRS Packet Control Unit
(PCU), which is co-located with the BTS in the Osmocom implementation.
Contrary to that, many classic E1-based implementations of the GSM RAN
co-locate the PCU with the BSC.  However, the GSM specifications keep
the location up to the implementor.

The GPRS network architecture is shown in <<fig-gprs-pcubts>>.

[[fig-gprs-pcubts]]
.GPRS network architecture
[graphviz]
----
digraph G {
        rankdir=LR;
        MS0 [label="MS"];
        MS1 [label="MS"];
        BTS;
        BSC;
        MSC;
        PCU;
        SGSN;
        GGSN;
        MS0->BTS [label="Um"];
        MS1->BTS [label="Um"];
        BTS->BSC [label="Abis"];
        BSC->MSC [label="A"];
        BTS->PCU [label="pcu_sock"];
        PCU->SGSN [label="Gb"];
        SGSN->GGSN [label="GTP"];
}
----

The PCU socket interface serves the following purposes:

* to pass PCU relevant configuration from BTS to PCU
* to forward paging requests from BTS to PCU
* to forward RACH Requests from BTS to PCU

Depending on your bts_model, the PCU may also be passing actual
PH-DATA.request / PH-DATA.indication / PH-RTS.indication primitives for
the PDCH.  This is considered sub-optimal, and some BTS models offer a
direct interface by which the PCU can exchange those primitives directly
with the PHY.

The default PCU socket interface name is `/tmp/pcu_sock`, but this can
be overridden by the `pcu-socket` VTY command in the BTS configuration
VTY node.
