Add ascii art protocol overview.
I have my paper notes, but they exist only once.
diff --git a/doc/protocols_around_hnbgw.txt b/doc/protocols_around_hnbgw.txt
new file mode 100644
index 0000000..a72bc54
--- /dev/null
+++ b/doc/protocols_around_hnbgw.txt
@@ -0,0 +1,57 @@
+Who Talks What Around the Home Node B Gateway
+=============================================
+(wip 2016-01-11)
+
+
+ +------------+ +--------+ +----------+
+ UE <-->| hNodeB |<--Iuh---->| HNB-GW |<--IuCS-->| OsmoCSCN |
+ UE <-->| femto cell | ...-->| | +----------+
+ | | | |
+ +------------+<--GTP-U | |
+ \ | | +------+ +------+
+ | | |<--IuPS-->| SGSN |<--GTP-C-->| GGSN |
+ | +--------+ +------+ GTP-U-->| |
+ | / +------+
+ \_______________________________/
+
+
+
+ Iuh IuCS/IuPS
+
+NAS +----+----+ +----+----+
+Non-Access Stratum | CC | MM | | CC | MM |
+- - - - - - - - - - - +----+----+-------+ +----+----+
+ | RANAP | | H | RANAP |
+Access Stratum +---------+ HNBAP | N +---------+ - - SCCP USER SAP
+ | RUA | | B | SUA | \
+ +---------+-------+ - +---------+ |
+ | SCTP | G | SCTP | } SIGTRAN
+ +-----------------+ W +---------+ |
+ | IP | | IP | /
+ +-----------------+ +---------+
+
+
+Various SIGTRAN implementations:
+
+ IuCS/IuPS
+ usual
+ | simplest
+ | |
+ v v
+ +------+------+------+-----+
+ | SCCP | SCCP | | |
+ +------+------+ SCCP | |
+ | MTP3 | MTP3 | | |
+ +------+------+------+ SUA |
+ | MTP2 | | | |
+ +------+ M2UA | M3UA | |
+ | M2PA | | | |
+ +------+------+------+-----+
+ | SCTP |
+ +--------------------------+
+ | IP |
+ +--------------------------+
+
+UE (User Endpoint) == MS (Mobile Subscriber) == mobile device
+CSCN (Circuit Switched Core Network) == OsmoNITB without BSC
+