88 lines
2.8 KiB
Plaintext
88 lines
2.8 KiB
Plaintext
= OsmoBSC A / SCCPlite / 3GPP AoIP Options
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== Introduction
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This document serves as a paper to illustrate the different
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configurations of OsmoBSC in terms of integration with BTSs and MSCs.
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The document was created ahead of the 2017 development cycle which
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included the _death of the NITB_, i.e. the move away from OsmoNITB to
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having OsmoBSC in all configurations, whether with a
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proprietary/external MSC or with OsmoMSC.
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Particular attention is spent on the user plane, including aspects
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such as
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* user plane transport address handling
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* use of MGCP (Media Gateway Control Protocol)
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* the (required) evolution of `osmo-bsc_mgcp`
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* not loosing classic TDM (E1/T1) BTS support when moving from
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OsmoNITB to split OsmoBSC + OsmoMSC setup
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== Overview
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=== Classic GSM RAN with E1 based Abis and E1 A
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This is how GSM was originally specified when it was introduced: E1/T1
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circuits on all interfaces, no IP anywhere.
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This configuration was actually never supported by OpenBSC, as E1
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support was always only on the Abis side (OpenBSC, OsmoNITB and today OsmoBSC).
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We never supported A interface over E1. It could be done if there was a need.
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[mscgen]
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----
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include::{srcdir}/mgw/classic-bsc.msc[]
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----
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=== OsmoBSC 2010-2017: IPA-style A over SCCPlite
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This configuration was introduced as early as 2010 in OpenBSC. It
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allowed the use of IP based BTSs (ip.access nanoBTS as well as all the
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OsmoBTS supported BTS models) in combination with third-party MSCs
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implementing a pre-standard, proprietary way of transporting the A
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interface over IP at a time where the 3GPP specifications only allowed
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classic TDM transport.
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[mscgen]
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----
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include::{srcdir}/mgw/osmo-bsc-old-sccplite.msc[]
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----
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=== OsmoBSC 2017+: 3GPP AoIP + Abis/IP
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Release 7 of 3GPP included an official specification on how an
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interoperable A-over-IP (AoIP) interface shall look like.
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As more modern MSCs at operators tend to favor implementing 3GPP AoIP
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rather than the proprietary SCCPlite based A interface, it became
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necessary for OsmoBSC to support this.
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At the same time, for compatibility reasons, the classic SCCPlite
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support is kept in OsmoBSC as a configuration option.
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[mscgen]
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----
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include::{srcdir}/mgw/osmo-bsc-new-mgw.msc[]
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----
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=== OsmoBSC 2020+: 3GPP AoIP + Abis/E1
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Since OsmoNITB was deprecated in 2017, and OsmoBSC only supported Abis/IP,
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we temporarily lost the ability to use classic E1 based BTSs. In 2020,
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we re-introduced and re-tested the support of Abis/E1.
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Fro the control plane of Abis (RSL, OML) the E1 support via libosmo-abis
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never really ceased to exist. But for the user plane, E1 support had to be
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introduced to osmo-mgw, and osmo-bsc needed to be taught how to configure
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E1 endpoints at the MGW. The related call flow for such setups looks like this:
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[mscgen]
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----
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include::{srcdir}/mgw/osmo-bsc-new-mgw-e1.msc[]
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----
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