To create a packet data service in the cellular GSM network we can follow
two different methods:
The separate system method
The integrated system method
In the first method the infrastructure required for supporting the
service is in addition to that of the GSM network, whilst in the second
method we have the addition of those functions necessary for
supporting the GPRS to the entities composing the GSM network. In fact
even in the integrated system method we have to introduce new entities,
but also guarantee, from an economical point of view, a smaller impact on
the costs required for implementing the service.
The entities that have to be added in view of the integration of the
GPRS service in the GSM network are:
GSN (Gateway Support Node), that are the nodes
supporting the GPRS service.
GPRS register
Modello di Network GPRS
The GSN nodes can be seen as the entities where the great
majority of functions needed to support the GPRS are found.
In the GPRS PLMN (Public Land Mobile Network), there are
generally more GSN nodes and the infrastructure connecting them, referred
to as backbone network, allows the routing of the packets transmitted by
or addressed to users of the network.
With regard to the positioning of the generic mobile station GPRS,
the HSN (Home Support Node) and the VSN (Visited
Support Node) are used. The HSN is the node of the backbone
network where packets addressed to the mobile arrive according to the
network address; moreover, when the mobile is situated in the area
supervised by another node of the backbone network, the HSN sends
to this other node the packets addressed to the mobile.
The VSN is the node of the backbone network in the area where the
mobile is currently found.
The backbone network can be a public packet data network, thus
reducing overhead costs, or a dedicated purpose built packet data network
optimised for supporting the service. The first solution has,
compared to the second, greater transit delays when packets are exchanged
between users of GPRS PLMN and users of another network, while the
second has much greater start up costs.
Internetworking entities are also connected to the backbone network which
guarantee the interconnection between the GPRS PLMN and other data
networks, such as, for example, the Internet and the PSPDN (Public
Switched Packet Data Network), the private packet data networks and
others.
The main functions served by these entities are: protocol conversion and
network address mapping for the entities engaged in data communication.
Another new entity necessary for supporting the service is the GPRS
register. This should not necessarily be thought of as a new physical
entity, as we can extend the functions of the VLR/HLR in the GSM
network.
The functions served by a GPRS register are basically to memorise
information regarding the subscriber to the GPRS service; in particular
each GPRS register contains:
Information required for the routing of the packets addressed to a
mobile GPRS; for example the network address of the mobile for a given
network protocol and the type of network protocol the address refers
to.
· Information regarding the subscription profile of the subscriber;
for example, information characterising the quality of service
required by the user (QOS=Quality Of Service).
The access code to this information relative to the generic subscriber is
the IMSI (International Mobile Subscriber Identity). The
introduction to the GSM network of new entities produces the definition of
new interfaces; among these the Gr supports signalling alone, while
the others support both signalling and data.
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