Functions of the GMSC
The Gateway MSC (GMSC) is the gateway of the UMTS core network with the PSTN/ISDN
for circuit-switched services.
For mobile terminating services the Home Location Register (HLR) is interrogated for
information how to route and process the call further.
2.1.3 Functions of the GGSN
The GGSN is the gateway of the UMTS PLMN towards the external packet data network
(PDN). It is able to access Internet based networks of several service providers (ISP).
On the other hand the GGSN forwards the packet data units (PDUs) of the external
packet data protocols by using the tunneling protocol to the related SGSN where the
destination user resides. Therefore the GGSN holds a GPRS specific routing table
which is updated by the packet-switched mobility management of the 3G-SGSN. The
information is transferred by the GTP-U. The GGSN can assign dynamic packet data
protocol addresses.
Access management (authentication and accounting)
For authentication of the user in the ISP there is also an so-called non-transparent access
to Internet (RADIUS access) which can be named as ISP authentication. In case
of non-transparent access there is an additional authentication procedure controlled by
the Internet operator (Internet service provider (ISP)). For this the ISP supports a RADIUS
server and the GGSN supports a RADIUS client which is passing the user-ID and
password.
Session management
Session management (SM) provides the functionality required for handling packet data
protocol (PDP) contexts in GGSN node. SM covers the activation, modification and deactivation
of PDP contexts.
GGSN specifics for SM: Only for active PDPs, a context is provided in the GGSN. The
GGSN assigns temporary PDP addresses itself or gets them from the RADIUS server
of the Internet service provider (ISP) if no static PDP address is assigned to the PDP.

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