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The GPRS system is used by GSM Mobile phones, as of 2004 the most
common mobile phone system in the world, 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 Core Network Structure The GPRS Core Network (GPRS stands for General Packet Radio
Services) provides mobility management, session management and
transport for Internet packets packet services in GPRS and UMTS
networks. The core network also provides support for other additional
functions such as charging and lawful interception. It was
also proposed, at one stage, to support packet radio services in the
US TDMA system, however, in practise, most of these networks are being
converted to GSM so this option is becoming largely irrelevant.
Like GSM in general, GPRS is an open standards driven system and the
standardisation body is the 3GPP. Most of the patents required to
implement a GPRS network are available with
RAND terms. Most of
the standards required to implement GPRS can be downloaded directly
from the 3GPP web site (http://www.3gpp.org).
GPRS Tunnelling Protocol (GTP)
GPRS Tunnelling Protocol is the defining protocol of the GPRS core
network. It is an IP over IP tunnelling protocol (though, in
principle it can also carry other protocols such as PPP or, in older
versions, X.25). As of 2004 there are two versions defined,
version 0 and version 1. Interestingly version 0 and version 1 are
considerably different in structure. In version 0 the
signalling protocol (the protocol which sets up the tunnels by
activating the PDP context) is combined with the
tunnelling protocol on one port. Version 1 is actually effectively
two protocols, one for control (called GTP-C) and one for user
data tunnelling (called GTP-U).
GTP-U is also used to transport user data from the RNC to the SGSN
in UMTS networks. However, in this case signalling is done using
RANAP instead of GTP-C.
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 Tunnel 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 Local 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.
Common SGSN Functions
- Detunnel GTP packets from the GGSN
- Tunnel IP packets toward the GGSN
- Carry out mobility management as Idle mode mobile moves from Routing Area to Routing Area.
GSM/EDGE Specific SGSN functions
- Carry up to about 60kb/s (150kb/s for Edge) traffic per subscriber
- Connect via frame relay to the PCU using the Gb protocol stack
- Accept uplink data to form IP packets
- Encrypt downlink data, decrypt uplink data
- Carry out mobility management to the level of a cell for connected mode mobiles
UMTS Specific SGSN functions
- Carry up to about 300kb/s traffic per subscriber
- Tunnel/detunnel downlink/uplink packets toward the RNC
- Carry out mobility management to the level of an RNC for connected mode mobiles.
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
An access point is:
- An IP network to which a mobile can be connected
- A set of settings which are used for that connection
- A particular option in a set of settings in a mobile phone
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: bigcompany.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.
- Subscriber's IP address
- Subscriber's IMSI
- Subscriber's
- Tunnel ID (TEID) at the GGSN
- Tunnel ID (TEID) at the SGSN
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 (points of connection from one system to another) and
reference points (logical points of connection which probably share a
common physical connection with other reference points). The names of
these can be seen in the network structure diagram on this page.
External links
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