GPRS Call Flow

            GPRS (General Packet Radio Service) is a packet based communication service for mobile devices that allows data to be sent and received across a mobile telephone network. It's a step towards 3G and is often referred to as 2.5G.
            It’s an upgrade to the existing network that sits along side the GSM network. Many of the devices such as the BTS and BSC are still used. Often devices need to be upgraded be it software, hardware or both. When deploying GPRS many of the software changes can be made remotely.
           There are however 2 New Functional Elements which play a major role in how GPRS works - SGSN & GGSN. In simple terms there are in practice two different networks working in parallel, GSM and GPRS.
           In any GSM network there will be several BSC’s. When implementing GPRS a software and hardware upgrade of this unit is required. The hardware upgrade consists of adding a PCU (Packet Control Unit). This extra piece of hardware differentiates data destined for the standard GSM network or Circuit Switched Data and data destined for the GPRS network or Packet Switched Data.
PCU can be a separate entity.

SGSN (Serving GPRS Support Node) - It takes care of some important tasks, including Routing, Handover and IP address assignment. Its a logical connection to the GPRS device. One job of the SGSN is to make sure the connection is not interrupted as you make your journey passing from cell to cell. It works out which BSC to “route” your connection through. If the user moves into a segment of the network that is managed by a different SGSN it will perform a handoff to the new SGSN, this is done extremely quickly and generally the user will not notice this has happened. Any packets that are lost during this process are retransmitted. The SGSN converts mobile data into IP and is connected to the GGSN via a tunneling protocol.

GGSN (Gateway GPRS support node) - It is the “last port of call” in the GPRS network before a connection between an ISP (Internet Service Provider) or corporate network’s router occurs. The GGSN is basically a gateway, router and firewall. It also confirms user details with RADIUS servers for security, which are usually situated in the IP network and outside of the GPRS network.
            The connection between the two GPRS Support Nodes is made with a protocol called GPRS Tunneling Protocol (GTP). GTP sits on top of TCP/IP and is also responsible for the collection of mediation and billing information. GPRS is billed on per megabyte basis.

GPRS Call Scenario :-

• A subscriber accesses the Internet with GPRS mobile phone to set the APN (Access Point Names) & gateway IP address defined on subscription. In fact, APN is a logical name indicating the external data network in GGSN. A subscriber can select different GGSNs via different APNs. Currently, however, only one APN can be activated at a time. The purpose of selecting different APNs is to access the external network via different GGSNs, because without GGSN, a subscriber cannot access the PDN (Public Data Network). An APN consists of a fully qualified DNS (Domain Name Server) name e.g. airtellive.com.cn., which should be parsed by DNS to get the real IP address of GGSN.
• The call reaches the SGSN of the GPRS network. The SGSN triggers the service in the corresponding SCP (Service Control Point) according to subscriber's authentication information on the HLR interconnected to the corresponding home SCP for processing.
• The DNS parses the APN and get the IP address of the GGSN.
• The call is routed to the GGSN according to the IP address.
• The GGSN assigns the IP address to the subscriber.
• After SCP verifies the subscriber, the subscriber begins to transmit data and log in to the external web sites via the gateway whose IP address is set in the mobile phone.
• The subscriber may select the service from the portal web site to connect the SP/CP web site that provides the service, or enter the IP address of the SP/CP in the mobile phone to access the SP/CP web site.

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