[[overview]]
== Overview


[[intro_overview]]
=== About OsmoHNBGW

OsmoHNBGW implements the Home NodeB Gateway function in the 3G network architecture.  It serves 
as a gateway between the classic 3G core network (CN) domain with its IuCS and IuPS interface
and the femtocell based RAN.

A typical 3G network consisting of Osmocom components will look as illustrated in the following
diagram:

[[fig-3g]]
.Typical 3G network architecture used with OsmoHNBGW
----
        +------------+           +--------+          +----------+          +---------+
 UE <-->| hNodeB     |<--Iuh---->| HNB-GW |<--IuCS-->| OsmoMSC  |<--GSUP-->| OsmoHLR |
 UE <-->| femto cell |     ...-->|        |    ...-->|          |          |         |
        |            |           |        |          +----------+          +---------|
        +------------+<--GTP-U   |        |
                              \  |        |          +------+           +------+
                              |  |        |<--IuPS-->| SGSN |<--GTP-C-->| GGSN |
                              |  +--------+    ...-->|      |   GTP-U-->|      |
                              |                      +------+  /        +------+
                              \_______________________________/
----

The HNB-GW performs a translation interface between the IuCS/IuPS interfaces on the one hand
side, and the Iuh interface on the or ther hand:

----
                      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      |  /
                      +-----------------+         +---------+
----

On the femtocell (Home NodeB) side, OsmoHNBGW implements the Iuh interface as specified by 3GPP.

=== The Iuh interface

Iuh consists of the following sub-layers:

- HNBAP (Home NodeB Application Part)
- RUA (RANAP User Adaptation, between RANAP and SCTP