= OsmoBSC A / SCCPlite / 3GPP AoIP Options

== Introduction

This document serves as a paper to illustrate the different
configurations of OsmoBSC in terms of integration with BTSs and MSCs.

The document should accompany us in the 2017 development cycle which
includes the _death of the NITB_, i.e. the move away from OsmoNITB to
having OsmoBSC in all configurations, whether with a
proprietary/external MSC or with OsmoMSC.

Particular attention is spent on the user plane, including aspects
such as

* user plane transport address handling
* use of MGCP (Media Gateway Control Protocol)
* the (required) evolution of `osmo-bsc_mgcp`
* not loosing classic TDM (E1/T1) BTS support when moving from
  OsmoNITB to split OsmoBSC + OsmoMSC setup


== Overview

=== Classic GSM RAN with E1 based Abis and E1 A

This configuration was actually never supported by OpenBSC, as E1 BTS
support was so far for NITB only, but not for OsmoBSC.

[mscgen]
----
include::{srcdir}/mgw/classic-bsc.msc[]
----

=== OsmoBSC 2010-2017: IPA-style A over SCCPlite

This configuration was introduced as early as 2010 in OpenBSC.  It
allowed the use of IP based BTSs (ip.access nanoBTS as well as all the
OsmoBTS supported BTS models) in combination with third-party MSCs
implementing a pre-standard, proprietary way of transporting the A
interface over IP at a time where the 3GPP specifications only allowed
classic TDM transport.

[mscgen]
----
include::{srcdir}/mgw/osmo-bsc-old-sccplite.msc[]
----


=== OsmoBSC 2017+: 3GPP AoIP + Abis/IP

Release 7 of 3GPP included an official specification on how an
interoperable A-over-IP (AoIP) interface shall look like.

As more modern MSCs at operators tend to favor implementing 3GPP AoIP
rather than the proprietary SCCPlite based A interface, it becomes
neccessary for OsmoBSC to support this.

At the same time, for compatibility reasons, the classic SCCPlite
support shall be kept, if possible with reasonable effort.

[mscgen]
----
include::{srcdir}/mgw/osmo-bsc-new-mgw.msc[]
----


=== OsmoBSC 2017+: 3GPP AoIP + Abis/E1

Since OsmoNITB will soon be deprecated, we will use OsmoBSC in all
Osmocom GSM ntework setups, requiring the support for classic E1/T1
based BTSs from OsmoBSC.

[mscgen]
----
include::{srcdir}/mgw/osmo-bsc-new-mgw-e1.msc[]
----