doc: Document SBc-AP support and config

Change-Id: Ic14ec2766fce251a6fe52a77d7bc0bbcd8d3afbf

doc/manuals/chapters/configuration.adoc

@@ -1,7 +1,7 @@
 [[configuration]]
 == Configuration
 
-=== CBSP / Peer Configuration
+=== CBSP Peer Configuration
 
 CBSP is the BSC-CBC interface within the 3GPP architecture.  It serves
 to communicate CSB and ETWS messages from the CBC to the BSC, who then
@@ -43,8 +43,8 @@ cbc
   remote-port 46133
   remote-ip 1.2.3.4
  peer my-bsc-2
-  remote-port 46133
-  remote-ip 1.2.3.4
+  remote-port 46134
+  remote-ip 1.2.3.5
 ----
 
 For more details on the available configuration commands, please check the OsmoCBC VTY Reference.
@@ -64,6 +64,71 @@ cbc
   local-port 48049
 ----
 
+
+=== SBc-AP Peer Configuration
+
+SBc-AP is the MME-CBC interface within the 3GPP architecture.  It serves
+to communicate CSB and ETWS messages from the CBC to the MME, who then
+subsequently distributes it among the (matching) cells (eNodeBs) within the MME.
+
+[[config-sbcap]]
+==== Configuring the SBc-AP connections
+
+An MME typically operates as an SCTP server, and the CBC connects as SCTP
+client.  This would require the CBC to have out-of-band knowledge of all the
+MMEs in the network (and their IP addresses).
+
+In order to comply with the specifications, OsmoCBC supports this mode
+of operation as SBc-AP SCTP client.  However, to make network operation and
+configuration more simple, it also can operate in SCTP server mode,
+accepting incoming connections from the MMEs.  This way the MMEs need to
+know the CBC IP address, but not vice-versa.
+
+The default port number for the SBc-AP server is 29168, according to the SBc-AP
+specification. It uses SCTP payload protocol identifier 24.
+
+In order to make use of SCTP muti-homing capabilitites, simply configure several
+IP addresses when configuring SBc-AP.
+
+.Example: Configure SCTP server mode and allow arbitrary MMEs to connect
+----
+cbc
+ unknown-peers accept
+----
+
+
+.Example: Configure SCTP client mode and define each BSC
+----
+cbc
+ peer my-mme-1
+  protocol sbcap
+  remote-port 334455
+  remote-ip 1.2.3.4
+  remote-ip 1.2.3.5
+ peer my-mme-2
+  protocol sbcap
+  remote-port 334456
+  remote-ip 1.2.3.10
+----
+
+For more details on the available configuration commands, please check the OsmoCBC VTY Reference.
+
+==== Configuring the IP/Port for SBc-AP to bind to
+
+It can be configured to which IP and SCTP port the SBc-AP protocol binds to.
+
+The default is to bind to the 3GPP standard port number 29168 for SBc-AP at the
+loopback IP address 127.0.0.1 and ::1.
+
+.Example: Configure SBc-AP to bind to (127.0.0.1,::1):48049
+----
+cbc
+ sbcap
+  local-ip 127.0.0.1
+  local-ip ::1
+  local-port 48049
+----
+
 [[config-ecbe]]
 === ECBE (REST Interface) Configuration
 

doc/manuals/chapters/overview.adoc

@@ -10,7 +10,7 @@ It acts as a gateway between external applications / users, such as government
 authorities for civil protection, and the various components within the 3GPP
 network to actually deliver those broadcast and/or emergency messages.
 
-.Role of the CBC inside the 3GPP network architecture
+.Role of the CBC inside the 3GPP network architecture (3GPP TS 23.041 §3)
 [graphviz]
 ----
 include::../images/cbc-in-network.dot[]
@@ -23,6 +23,7 @@ OsmoCBC is the Osmocom implementation of a Cellular Broadcast Centre (CBC). It
 implements:
 
 - the BSC-CBC interface using the CBSP protocol
+- The MME-CBC interface using the SBc-AP protocol
 - a HTTP/JSON/RESTful interface for external applications
 - a telnet-based command line interface for configuration and introspection called VTY
 
@@ -30,12 +31,10 @@ OsmoCBC provides mainly the following functionality:
 
 - establishing communication with the various Radio Access Network elements such as BSCs within the network
 - receiving requests to start or remove CBS and ETWS messages from external applications
-- distributing the CBS and/or ETWS messages
-has connection with the various RANs of the 3GPP cellular network
+- distributing the CBS and/or ETWS messages to the various RANs of the 3GPP cellular network
 
-Future versions of OsmoCBC are expected to contain the RNC-CBC interface
-with the SABP protocol, as well as the MME-CBC interface using the SBcAP
-protocol.  Should you be interested in contributing to this effort,
+Future versions of OsmoCBC are expected to contain the RNC-CBC interface with
+the SABP protocol.  Should you be interested in contributing to this effort,
 please contact the author of this document.
 
 === CBSP implementation
@@ -57,6 +56,20 @@ simply only accepts incoming CBSP connections.
 
 For more information, see <<config-cbsp>> on how CBSP is configured.
 
+=== SBc-AP implementation
+
+SBc-AP (SBc Application Part) is the  interface between the Mobility Management
+Entity (MME) and the Cell Broadcast Centre (CBC) in 4G networks. It is also the
+interface between the Public Warning System – Inter Working Function (PWS-IWF)
+and the CBC in 5G networks. It is specified in 3GPP TS 29.168.
+
+Similarly to what's done with CBSP, OsmoCBC also supports operating SBc-AP as an
+SCTP client or an SCTP server. In the first case, OsmoCBC must be passed a list
+of MMEs to connect to, while in the second one it will only accept incoming SCTP
+connections.
+
+For more information, see <<config-sbcap>> on how SBc-AP is configured.
+
 === ECBE REST interface
 
 3GPP does not specify the external interface by which competent

doc/manuals/images/cbc-in-network.dot

@@ -5,13 +5,17 @@ digraph G {
   CBC -> BSC [label="CBSP"];
   CBC -> RNC [label="SABP"];
   CBC -> MME [label="SBc-AP"];
+  CBC -> PWSIWF [label="SBc-AP"];
+
+  PWSIWF -> AMF [label="N50"];
 
   BSC -> BTS [label="A-bis RSL"];
   RNC -> NodeB [label="Iub"];
   MME -> eNodeB [label="S1-AP"];
+  AMF -> gNodeB [label="N2"];
 
   BTS -> UE [label="CBCH"];
   NodeB -> UE [label="BMC/CTCH"];
   eNodeB -> UE [label="SIB10/11/12"];
-
+  gNodeB -> UE [label="Uu"];
 }