|
The SGSN Routing
Area (RA)
The SGSN Routing Area (RA) is a subset of the MSC (CS) Location
Area (LA). An MSC Location Area is a group of BSS cells, the
system uses the Las to search for subscribers in the active state.
An LA is the part of the network in which an MS may move around
with out reporting its location to the network.
One MSC/VLR Service Area (SA) is made up of a number of LAs,
the SA is the part of the network that is covered by one MSC.
However there can be more than one MSC corresponding to one SGSN,
one MSC can also be connected to several SGSNs.
Gateway Mobile Services Switching Center (GMSC)
The GMSC switches the circuit switched calls between GSM circuit
switched network and the PSTN which is the fixed telephony network,
hence it serves the function of routing incoming calls to the
MSC where the mobile subscriber is currently registered, it is
normally integrated in the same node as the MSC/VLR. The GMSC
does not need any upgrading for GPRS. |
|
The Home Location Register (HLR)
As stated in the section
about the GSM Network the HLR is the database that holds all
the subscription information for every person who has bought
a from the GSM operator. The HLR stores information for the CS
and PS communication, information stored the HLR includes, for
example supplementary services, authentication parameters, Access
Point Name (APN) such as subscribers ISP, and whether a static
IP address is allocated to the MS. In addition, the HLR also
includes information about the location of the MS. The main difference
between this and the GSM system is that the information from
the HLR is exchanged between the HLR and the SGSN.
The information that is exchanged between the HLR and the SGSN
has been set up by the operator for the user, this information
transfer is done when the operator changes the subscriber information,
or when a new SGSN needs to have data for a subscriber after
the MS has connected or in roaming, the old SGSN is also informed
if the MS is roaming. The information that is going from the
HLR to the SGSN is basically the routing information that is
transferred upon an MS action, e.g. attach or roaming. For a
roaming MS, the HLR may be in a different PLMN that the SGSN
that is serving the MS. |
|
Visitor
Location Register (VLR)
The VLR database contains all the information about all MSs that
are currently located in the MSC LA or the SGSN routing area
respectively. The SGSN actually contains the VLR functionality
for packet-switched communications, similarly, the circuit-switched
VLR is an integrated component of the MSC. Another function of
the VLR is that it contains the temporary subscriber information
needed by the MSC or SGSN to provide services for visiting subscribers.
For MSs that support GPRS (PS) and GSM (CS), both the SGSN and
the MSC will obtain location information from the HLR when the
MS is combined-attached, i.e. both GPRS- and IMSI/CS-attached.
The GPRS VLR consists of software in a serving GRPS Support Node,
the VLR contains information about the SGSN that is used.
The MSC/VLR is connected to the SGSN directly using the Gs interface,
and indirectly via the BBS using the A and the Gb interfaces. |