OsmoBSC: update overview to explain both SCCPlite and SCCP/M3UA

Change-Id: I1f105b3febd4f99f4491e217ff1b1d0b28912980

1 files changed, 91 insertions, 77 deletions

diff --git a/doc/manuals/chapters/overview.adoc b/doc/manuals/chapters/overview.adoc
index 3bb22ccc0..9b619fb24 100644
--- a/doc/manuals/chapters/overview.adoc
+++ b/doc/manuals/chapters/overview.adoc
@@ -7,18 +7,65 @@ aspects of configuring and running the OsmoBSC.
 [[intro_overview]]
 === About OsmoBSC
 
-OsmoBSC is one particular version of the OpenBSC software suite.
+OsmoBSC is the Osmocom implementation of a Base Station Controller. It
+implements:
 
-Unlike the highly integrated OmsoNITB, OsmoBSC implements a more classic
-GSM Base Station Controller with A-bis interface towards BTSs and A
-interface towards a MSC.
+- an 'A-bis' interface towards BTSs and
+- an 'A' interface towards an MSC. It is important to be aware that there are
+  two variants of the 'A' interface, see <<a-interface>>.
 
-The difference between classic GSM network architecture and
-the OsmoNITB based GSM network architecture is illustrated in
-<<fig-gsm-classic>> and <<fig-gsm-nitb>>.
+=== Software Components
+
+OsmoBSC contains a variety of different software components, which
+we'll briefly describe in this section.
+
+==== A-bis Implementation
+
+OsmoBSC implements the ETSI/3GPP specified A-bis interface, including TS 08.56
+(LAPD), TS 08.58 (RSL) and TS 12.21 (OML). In addition, it supports a variety
+of vendor-specific extensions and dialects in order to communicate with BTSs
+from Siemens, Nokia, Ericsson, ip.access, Octasic and sysmocom, as well as
+various USRP based BTS implementations, using OsmoBTS and OsmoTRX (like the
+Ettus B200 series, the Fairwaves XTRX or the LimeSDR, to name a few).
+
+For more information, see <<bts>> and <<bts-examples>>.
+
+[[a-interface]]
+==== A Implementation
+
+OsmoBSC implements a sub-set of the GSM A interface as specified in TS 08.08
+(BSSAP) and TS 04.08 (DTAP).
+
+Osmocom offers two variants of the 'A' interface's protocol stacking:
 
-[[fig-gsm-classic]]
-.Classic GSM network architecture (simplified)
+- 'A/SCCPlite'
+- 'A/SCCP/M3UA'
+
+Traditionally, OsmoBSC only implemented the A/SCCPlite protocol, but since a
+proper M3UA implementation is available from 'libosmo-sigtran'
+('libosmo-sccp.git'), the stock OsmoBSC now supports only A/SCCP/M3UA. (The
+idea is that SCCPlite support may be added to libosmo-sigtran at some point
+in the future, after which the new `osmo-bsc` would support both variants of
+the A interface.)
+
+The difference between an A/SCCPlite and A/SCCP/M3UA is illustrated in
+<<fig-sccplite>> and <<fig-sccp-m3ua>>.
+
+===== A/SCCPlite
+
+Unlike classic A interface implementations for E1 interfacs,
+`osmo-bsc-sccplite` implements a variant of encapsulating the A interface over
+IP.  To do so, the SCCP messages are wrapped in an IPA multiplex and then
+communicated over TCP.  The audio channels are mapped to RTP streams.
+
+This protocol stacking is sometimes called "SCCPlite".
+
+At the time of writing, if you would like to use the old A/SCCPlite protocol,
+look for binary packages named `osmo-bsc-sccplite`, or compile `osmo-bsc` from
+the 'openbsc.git' repository.
+
+[[fig-sccplite]]
+.`osmo-bsc-sccplite` operation using 'A/SCCPlite'
 [graphviz]
 ----
 digraph G {
@@ -29,26 +76,37 @@ digraph G {
         MS3 [label="MS"];
         BTS0 [label="BTS"];
         BTS1 [label="BTS"];
-        BSC;
-        MSC [label="MSC/VLR"];
-        HLR [label="HLR/AUC"];
-        EIR;
-        SMSC;
-        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;
+        BSC [label="OsmoBSC-SCCPlite"];
+        MSC [label="3rd party MSC"];
+        {MS0,MS1}->BTS0 [label="Um"];
+        {MS2,MS3}->BTS1 [label="Um"];
+        {BTS0,BTS1}->BSC [label="Abis\nTCP\nIP"];
+        BSC->MSC [label="A\nSCCP\nTCP\nIP"];
 }
 ----
 
-[[fig-gsm-nitb]]
-.GSM system architecture using OsmoNITB
+===== A/SCCP/M3UA
+
+The default OsmoBSC's A interface uses the M3UA variant of SIGTRAN protocol
+stacking:
+
+|=====
+|A
+|SCCP
+|M3UA
+|SCTP
+|IP
+|=====
+
+To use the now-default A/SCCP/M3UA protocol, look for binary packages named
+`osmo-bsc`, or compile `osmo-bsc` from the 'osmo-bsc.git' repository. It is
+recommended to use the M3UA variant, which is required to operate with OsmoMSC.
+
+To route SCCP/M3UA messages between OsmoBSC and and MSC, an STP instance like
+OsmoSTP is required.
+
+[[fig-sccp-m3ua]]
+.`osmo-bsc` operation using 'A/SCCP/M3UA'
 [graphviz]
 ----
 digraph G {
@@ -59,60 +117,16 @@ digraph G {
         MS3 [label="MS"];
         BTS0 [label="BTS"];
         BTS1 [label="BTS"];
-        EXTMNCC [label="Linux Call Router / SoftSwitch / PBX\n(optional)"];
-        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"];
-                SMSC;
-                EIR;
-                HLR [label="HLR/AUC"];
-                BSC->MSC;
-                MSC->HLR;
-                MSC->EIR;
-                MSC->SMSC;
-        }
-        MSC -> EXTMNCC [label="external MNCC"];
+        BSC [label="OsmoBSC"];
+        STP [label="OsmoSTP"];
+        MSC [label="OsmoMSC\n(or 3rd-party MSC)"];
+        {MS0,MS1}->BTS0 [label="Um"];
+        {MS2,MS3}->BTS1 [label="Um"];
+        {BTS0,BTS1}->BSC [label="Abis\nTCP\nIP"];
+        BSC->STP->MSC [label="A\nSCCP\nM3UA\nSCTP\nIP"];
 }
 ----
 
-
-=== Software Components
-
-OsmoBSC contains a variety of different software components, which
-we'll quickly describe in this section.
-
-==== A-bis Implementation
-
-OsmoBSC implements the ETSI/3GPP specified A-bis interface, including
-TS 08.56 (LAPD), TS 08.58 (RSL) and TS 12.21 (OML). In addition, it
-supports a variety of vendor-specific extensions and dialects in order
-to communicate with BTSs from Siemens, Nokia, Ericsson, ip.access and
-sysmocom.
-
-For more information, see <<bts>> and <<bts-examples>>.
-
-==== A Implementation
-
-OsmoBSC implements a minimal sub-set of the GSM A interface as specified
-in TS 08.08.
-
-Unlike classic A interface implementations for E1 interfacs, OsmoBSC
-implements a variant of encapsulating the A interface over IP.  To do
-so, the SCCP messages are wrapped in an IPA multiplex and then
-communicated over TCP.  The audio channels are mapped to RTP streams.
-
-This protocol stacking is sometimes called "SCCPlite".
-
-For more information, see <<alink>>.
-
-
 ==== BSC Implementation
 
 The BSC implementation covers the classic functionality of a GSM Base