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The GPRS system is used by GSM Mobile phones, the mostcommon mobile phone system in the world (as of 2004), for transmitting IP packets. The GPRS Core Network is the centralised part of the GPRS system and also provides support for UMTS based 3G networks. The GPRS core network is an integrated part of the GSM core network. GPRS Core Network in General
GPRS Tunnelling Protocol (GTP) main article GPRS Tunnelling Protocol GPRS Tunnelling Protocol is the defining IP protocol of the GPRS core network. Primarily it is the protocol which allows end users of a GSM or UMTS network to move from place to place whilst continuing to connect to the internet as if from one location at the GGSN. It does this by carrying the subscriber's data from the subscriber's current SGSN to the GGSN which is handling the subscriber's session. Three forms of GTP are used by the GPRS core network. GGSNs and SGSNs (collectively known as GSNs) listen for GTP-C messages on UDP port 2123 and for GTP-U messages on port 2152. This communication happens within a single network or may, in the case of international roaming, happen internationally, probably across a GPRS Roaming Exchange (GRX). The "Charging Gateway Function" (CGF) listens to GTP' messages sent from the GSNs on UDP port 3386. The core network sends charging information to the CGF, typically including PDP context activation times and the quantity of data which the end user has transferred. However, this communication which occurs within one network is less standardised and may, depending on the vendor and configuration options, use proprietary encoding or even an entirely proprietary system. GPRS Support Nodes (GSN) A GSN is a network node which supports the use of GPRS in the GSM core network. All GSNs should have a Gn interface and support the GPRS tunnelling protocol. There are two key variants of the GSN; the GGSN and the SGSN defined below. GGSN - Gateway GPRS Support Node The GGSN is the node which carries out the role in GPRS equivalent to the Home Agent in Mobile IP. It is a router which detunnels user data from GPRS Tunnelling Protocol and sends out normal user data IP packets. SGSN - Serving GPRS Support Node The SGSN is the node which in some sense carries out the same function as the Foreign Agent in Mobile IP. However, an SGSN is actually considerably more complex since it also does the full set of interworking with the connected radio network. This means that the functions carried out by the SGSN vary quite considerably between GSM and UMTS. The GGSN provides the connectivity to the IP network and the SGSN. It is responsible for IP address assignment and is the default router for the connected UE. Common SGSN Functions GSM/EDGE Specific SGSN functions UMTS Specific SGSN functions These differences in functionality have led some manufacturers to create specialist SGSNs for each of UMTS and GSM which do not support the other networks, whilst other manufacturers have succeeded in creating both together, but with a performance cost due to the compromises required. Access Point Main article Access Point Name An access point is: When a GPRS mobile phone sets up a PDP context, the access point is selected. At this point an Access Point Name (APN) is determined Example: flextronics.mnc012.mcc345.gprs. Example: internet Example: mywap. This access point is then used in a DNS query to a private DNS network. This process (called APN resolution) finally gives the IP address of the GGSN which should serve the access point. At this point a PDP context can be activated.. PDP Context The PDP context is a data structure present on both the SGSN and the GGSN which contains the subscriber's session information when the subscriber has an active session. When a mobile wants to use GPRS, it must first attach and then activate a PDP context. This allocates a PDP context data structure in the SGSN that the subscriber is currently visiting and the GGSN serving the subscribers access point. The data recorded includes. The tunnel ID (TEID) is a number allocated by the GSN which identifies the tunnelled data related to a particular PDP context. Reference Points and Interfaces Within the GPRS core network standards there are a number of interfaces and reference points (logical points of connection which probably share a common physical connection with other reference points). Some of these names can be seen in the network structure diagram on this page. Interfaces in the GPRS network | |||||||||
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