OsmoBSC: update overview to explain both SCCPlite and SCCP/M3UA
Change-Id: I1f105b3febd4f99f4491e217ff1b1d0b28912980
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Neels Hofmeyr
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@ -7,111 +7,125 @@ aspects of configuring and running the OsmoBSC.
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[[intro_overview]]
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=== About OsmoBSC
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OsmoBSC is one particular version of the OpenBSC software suite.
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Unlike the highly integrated OmsoNITB, OsmoBSC implements a more classic
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GSM Base Station Controller with A-bis interface towards BTSs and A
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interface towards a MSC.
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The difference between classic GSM network architecture and
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the OsmoNITB based GSM network architecture is illustrated in
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<<fig-gsm-classic>> and <<fig-gsm-nitb>>.
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[[fig-gsm-classic]]
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.Classic GSM network architecture (simplified)
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[graphviz]
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----
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digraph G {
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rankdir=LR;
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MS0 [label="MS"];
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MS1 [label="MS"];
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MS2 [label="MS"];
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MS3 [label="MS"];
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BTS0 [label="BTS"];
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BTS1 [label="BTS"];
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BSC;
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MSC [label="MSC/VLR"];
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HLR [label="HLR/AUC"];
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EIR;
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SMSC;
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MS0->BTS0 [label="Um"];
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MS1->BTS0 [label="Um"];
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MS2->BTS1 [label="Um"];
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MS3->BTS1 [label="Um"];
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BTS0->BSC [label="Abis"];
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BTS1->BSC [label="Abis"];
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BSC->MSC [label="A"];
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MSC->HLR [label="C"];
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MSC->EIR [label="F"];
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MSC->SMSC;
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}
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----
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[[fig-gsm-nitb]]
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.GSM system architecture using OsmoNITB
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[graphviz]
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----
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digraph G {
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rankdir=LR;
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MS0 [label="MS"];
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MS1 [label="MS"];
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MS2 [label="MS"];
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MS3 [label="MS"];
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BTS0 [label="BTS"];
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BTS1 [label="BTS"];
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EXTMNCC [label="Linux Call Router / SoftSwitch / PBX\n(optional)"];
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MS0->BTS0 [label="Um"];
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MS1->BTS0 [label="Um"];
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MS2->BTS1 [label="Um"];
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MS3->BTS1 [label="Um"];
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BTS0->BSC [label="Abis"];
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BTS1->BSC [label="Abis"];
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subgraph cluster_nitb {
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label = "OsmoNITB";
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BSC;
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MSC [label="MSC/VLR"];
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SMSC;
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EIR;
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HLR [label="HLR/AUC"];
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BSC->MSC;
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MSC->HLR;
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MSC->EIR;
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MSC->SMSC;
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}
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MSC -> EXTMNCC [label="external MNCC"];
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}
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----
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OsmoBSC is the Osmocom implementation of a Base Station Controller. It
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implements:
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- an 'A-bis' interface towards BTSs and
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- an 'A' interface towards an MSC. It is important to be aware that there are
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two variants of the 'A' interface, see <<a-interface>>.
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=== Software Components
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OsmoBSC contains a variety of different software components, which
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we'll quickly describe in this section.
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we'll briefly describe in this section.
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==== A-bis Implementation
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OsmoBSC implements the ETSI/3GPP specified A-bis interface, including
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TS 08.56 (LAPD), TS 08.58 (RSL) and TS 12.21 (OML). In addition, it
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supports a variety of vendor-specific extensions and dialects in order
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to communicate with BTSs from Siemens, Nokia, Ericsson, ip.access and
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sysmocom.
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OsmoBSC implements the ETSI/3GPP specified A-bis interface, including TS 08.56
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(LAPD), TS 08.58 (RSL) and TS 12.21 (OML). In addition, it supports a variety
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of vendor-specific extensions and dialects in order to communicate with BTSs
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from Siemens, Nokia, Ericsson, ip.access, Octasic and sysmocom, as well as
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various USRP based BTS implementations, using OsmoBTS and OsmoTRX (like the
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Ettus B200 series, the Fairwaves XTRX or the LimeSDR, to name a few).
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For more information, see <<bts>> and <<bts-examples>>.
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[[a-interface]]
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==== A Implementation
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OsmoBSC implements a minimal sub-set of the GSM A interface as specified
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in TS 08.08.
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OsmoBSC implements a sub-set of the GSM A interface as specified in TS 08.08
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(BSSAP) and TS 04.08 (DTAP).
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Unlike classic A interface implementations for E1 interfacs, OsmoBSC
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implements a variant of encapsulating the A interface over IP. To do
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so, the SCCP messages are wrapped in an IPA multiplex and then
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Osmocom offers two variants of the 'A' interface's protocol stacking:
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- 'A/SCCPlite'
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- 'A/SCCP/M3UA'
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Traditionally, OsmoBSC only implemented the A/SCCPlite protocol, but since a
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proper M3UA implementation is available from 'libosmo-sigtran'
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('libosmo-sccp.git'), the stock OsmoBSC now supports only A/SCCP/M3UA. (The
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idea is that SCCPlite support may be added to libosmo-sigtran at some point
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in the future, after which the new `osmo-bsc` would support both variants of
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the A interface.)
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The difference between an A/SCCPlite and A/SCCP/M3UA is illustrated in
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<<fig-sccplite>> and <<fig-sccp-m3ua>>.
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===== A/SCCPlite
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Unlike classic A interface implementations for E1 interfacs,
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`osmo-bsc-sccplite` implements a variant of encapsulating the A interface over
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IP. To do so, the SCCP messages are wrapped in an IPA multiplex and then
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communicated over TCP. The audio channels are mapped to RTP streams.
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This protocol stacking is sometimes called "SCCPlite".
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For more information, see <<alink>>.
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At the time of writing, if you would like to use the old A/SCCPlite protocol,
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look for binary packages named `osmo-bsc-sccplite`, or compile `osmo-bsc` from
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the 'openbsc.git' repository.
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[[fig-sccplite]]
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.`osmo-bsc-sccplite` operation using 'A/SCCPlite'
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[graphviz]
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----
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digraph G {
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rankdir=LR;
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MS0 [label="MS"];
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MS1 [label="MS"];
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MS2 [label="MS"];
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MS3 [label="MS"];
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BTS0 [label="BTS"];
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BTS1 [label="BTS"];
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BSC [label="OsmoBSC-SCCPlite"];
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MSC [label="3rd party MSC"];
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{MS0,MS1}->BTS0 [label="Um"];
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{MS2,MS3}->BTS1 [label="Um"];
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{BTS0,BTS1}->BSC [label="Abis\nTCP\nIP"];
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BSC->MSC [label="A\nSCCP\nTCP\nIP"];
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}
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----
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===== A/SCCP/M3UA
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The default OsmoBSC's A interface uses the M3UA variant of SIGTRAN protocol
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stacking:
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|=====
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|A
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|SCCP
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|M3UA
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|SCTP
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|IP
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|=====
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To use the now-default A/SCCP/M3UA protocol, look for binary packages named
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`osmo-bsc`, or compile `osmo-bsc` from the 'osmo-bsc.git' repository. It is
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recommended to use the M3UA variant, which is required to operate with OsmoMSC.
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To route SCCP/M3UA messages between OsmoBSC and and MSC, an STP instance like
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OsmoSTP is required.
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[[fig-sccp-m3ua]]
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.`osmo-bsc` operation using 'A/SCCP/M3UA'
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[graphviz]
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----
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digraph G {
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rankdir=LR;
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MS0 [label="MS"];
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MS1 [label="MS"];
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MS2 [label="MS"];
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MS3 [label="MS"];
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BTS0 [label="BTS"];
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BTS1 [label="BTS"];
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BSC [label="OsmoBSC"];
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STP [label="OsmoSTP"];
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MSC [label="OsmoMSC\n(or 3rd-party MSC)"];
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{MS0,MS1}->BTS0 [label="Um"];
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{MS2,MS3}->BTS1 [label="Um"];
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{BTS0,BTS1}->BSC [label="Abis\nTCP\nIP"];
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BSC->STP->MSC [label="A\nSCCP\nM3UA\nSCTP\nIP"];
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}
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----
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==== BSC Implementation
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