[[chapter_introduction]]
== Overview

[[intro_overview]]
=== About OsmoSGSN

OsmoSGSN is the Osmocom implementation of the GPRS SGSN (Serving Gprs
Support Node) element inside the GPRS network.  The SGSN plays a similar
central function to the GPRS network as the MSC plays in the GSM
network.

The SGSN is connected on the downlink side to Gb interfaces of the BSS,
specifically the PCU inside the BSS.  The SGSN is further connected by
the GTP protocol to the GGSN which terminates the tunnels towards the
external packet data network (e.g. IPv4).

OsmoSGSN supports both a PCU that is co-located with(in) the BTS, as
well as a PCU that is co-located with(in) the BSC.  In combination with
OsmoNITB/OsmoBSC/OsmoBTS, the PCU is co-located within the BTS.

[[fig-gprs-pcubts]]
.GPRS network architecture with PCU in BTS
[graphviz]
----
digraph G {
        rankdir=LR;
        MS0 [label="MS"];
        MS1 [label="MS"];
        MS0->BTS [label="Um"];
        MS1->BTS [label="Um"];
        BTS->BSC [label="Abis"];
        BSC->MSC [label="A"];
        BTS->PCU [label="pcu_sock"];
        PCU->SGSN [label="Gb"];
        SGSN->GGSN [label="GTP"];
}
----

=== Software Components

OsmoSGSN contains a variety of different software components, which
we'll quickly describe in this section.

==== Gb Implementation

OsmoSGSN implements the ETSI/3GPP specified Gb interface, including TS
08.16 (NS), TS 08.18 (BSSGP) and TS 08.64 (LLC) protocols. As transport
layers for NS, it supports NS/IP (NS encapsulated in UDP/IP), as well as
NS/FR/GRE/IP.  The latter is provided in order to use a Router with
Ethernet and Frame Relay interface to convert to actual physical Frame
Relay medium, which is not directly supported by OsmoSGSN.

The actual Gb Implementation is part of the libosmogb library, which is
in turn part of the libosmocore software package.  This allows the same
Gb implementation to be used from osmo-pcu, osmo-gbproxy as well as
OsmoSGSN.


==== GTP Implementation

OsmoSGSN uses the libgtp implementation originating from OsmoGGSN. It
supports both GTPv0 and GTPv1.


==== GMM Implementation

The GPRS Mobility Management implementation is quite simplistic at this
point. It supports the GPRS ATTACH and GPRS ROUTING AREA UPDATE
procedures, as well as GPRS ATTACH and GPRS DETACH.

==== LLC Implementation

The LLC (Logical Link Control) implementation of OsmoSGSN only supports
non-acknowledged mode, as this is the most common use case in real-world
GPRS networks.

It does support both TCP/IP header compression according to RFC1144 and
payload compression according to V.42bis

The LLC implementation does support LLC encryption with ciphers GEA3 and GEA4.
For encryption to work the auth policy needs to be set to remote and the SGSN
connected to an HLR containing the subscriber data including key material.
Other auth policys will not work with encryption.

==== Session Management Implementation

The session management procedures ACTIVATE PDP CONTEXT and DEACTIVATE
PDP CONTEXT are supported.  However, no MODIFY PDP CONTEXT and no
Network-initiated PDP context activation is possible.  This is again
covering the predominant use cases and configurations in GPRS real-world
networks while skipping the more esoteric features.

Multiple PDP contexts can be attached by a single MS.

Multiple GGSNs can be configured and routing to a GGSN can be configured based
on APN. Dynamic lookup of GGSNs though DNS-based APN resolving is also possible.

=== Limitations

At the time of writing, OsmoSGSN still has a number of limitations,
which are a result of the demand-driven Open Source development model.
If you require any of those features, please consider implementing and
contributing them, or contracting the existing OsmoSGSN developers for
performing that work.

Known Limitations include:

* No paging coordination between SGSN and MSC
* No SMS over Ps support