Need for GPRS
and better applications
User friendly billing
In 1994, a Special Mobile Group started to think
about a High Speed Data upgrade for GSM.
The first step was HSCSD (High Speed Circuit
HSCSD is a circuit-switched extension to GSM.
The next step was GPRS.
GPRS is a packet-switched extension to GSM.
What is HSCSD?
HSCSD (High Speed Circuit Switched Data).
The simplest high speed data upgrade for GSM.
Provides GSM users with a bandwidth up to 57.6 Kbps.
Does not require a hardware upgrade within BSS or core
network (NSS), but different MS are needed.
What is GPRS?
GPRS is a packet oriented data service for IP and
X.25 over GSM networks.
GPRS is a step towards 3G and is often referred to
The GPRS network acts in parallel with the GSM
network, providing packet switched connections to
the external networks.
Traditional mobile communication network +
Traditional packet switched computer network.
Capability to separate circuit switched and packet
switched traffic from mobile station (MS)
Interfaces to Internet, intranets, Public Data Networks
Routing of packets to appropriate destination
Allocation of static or dynamic address for packets
originating from MS
Protection of the GPRS network from security threats
Translation between logical names and IP addresses
using Domain Name System (DNS)
Figure shows the architecture of a GPRS network. The GPRS system
brings some new network elements to an existing GSM network. These
Packet Control Unit (PCU)
Serving GPRS Support Node (SGSN): the MSC of the GPRS
Gateway GPRS Support Node (GGSN): gateway to external
Border Gateway (BG): a gateway to other PLMN (Public Land
Intra-PLMN backbone: an IP based network inter-connecting all
Charging Gateway (CG)
Legal Interception Gateway (LIG)
Domain Name System (DNS)
Firewalls: used wherever a connection to an external network is
Not all of the network elements are compulsory for every GPRS network.
SGSN (Service GPRS Support Node )
The SGSN is the most important element of the GPRS
The SGSN of the GPRS network is equivalent to the MSC of
the GSM network. There must at least one SGSN in a GPRS
There is a coverage area associated with a SGSN. As the
network expands and the number of subscribers increases,
there may be more than one SGSN in a network.
PDP contexts for Mobile Stations.
Determines Quality of Service assigned to user.
Routes packets to Mobile Stations.
“Pages” Mobile Stations when data is to be sent.
Subscriber data for all Mobile Stations in the location area.
Store not-acknowledged packets in case of a cell change during
an ongoing packet data transfer
Authentication, by means of identity or equipment check.
Data compression is used to minimise the size of transmitted data
The SGSN collects CDR's (Call Data Records) for the use of the
own network resources.
GGSN (Gateway GPRS Support Node)
The GGSN is the gateway to external networks.
Every connection to a fixed external data network
has to go through a GGSN.
The GGSN acts as the anchor point in a GPRS data
connection even when the subscriber moves to
another SGSN during roaming.
There are usually two or more GGSNs in a network
for redundancy purposes, and they back up each
other up in case of failure.
The functions of a GGSN are given below:
Interconnects a PLMN to the external world (Internet).
Routes IP packets to the appropriate SGSN.
Routing packets originating from a mobile to the correct external
Activation and Deactivation of PDP-Contexts / Session
Subscriber data for active Mobile Stations.
Interfaces to external IP networks and deals with security issues
The GGSN will, in addition to the SGSN, collect CDRs and forward
them to the charging gateway (CG).
GGSN will collect call data records based on the usage of external
network resources .
GPRS users have to be charged for the use of the
In a GSM network, charging is based on the destination,
duration, and time of call.
However, GPRS offers connectionless service to users, so it
not possible to charge subscribers on the connection
Charging has to be based on the volume, destination, QoS,
and other parameters of a connectionless data transfer.
These GPRS charging data are generated by all the SGSNs
and GGSNs in the network. This data is referred to as
Charging Data Records or CDRs.
One data session may generate a number of CDRs, so
these need to collected and processed.
The Charging Gateway (CG) collects all of these
records, sorts them, processes it, and passes it on to
the Billing System.
The GPRS subscriber is billed for the data transaction.
All CDRs contain unique subscriber and connection
identifiers to distinguish it.
A protocol called GTP„ (pronounced GTP prime) is
used for the transfer of data records between GSNs
and the Charging Gateway.
The Border Gateway (BG) is a router that can provide a
direct GPRS tunnel between different operators' GPRS
networks. This is referred to as an inter- PLMN data
It is more secure to transfer data between two operators„
PLMN networks through a direct connection rather than
via the public Internet.
The Border Gateway will commence operation once the
GPRS roaming agreements between various operators
have been signed.
It will essentially allow a roaming subscriber to connect to
company intranet through the Home GGSN via the
visiting PLMN network.
Domain Name Servers
These devices convert IP names into IP addresses
for example, server.nokia.com to 184.108.40.206.
Lawful Intercept Gateway
This is for monitoring traffic for law-enforcement
agencies that require court order.
When data packets traverse the GPRS network , it
may be intercepted and forwarded to the agencies.
Packet Control Unit
GPRS cell phone will trasmit data in Packet switched
mode and voice in circuit switched mode .
So there needs to be a network to differenciate the
different kinds of calls and send them to respective core
networks – voice call to MSC and data calls to SGSN.
PCU does this . PCU is placed at the Base station
subsystem (BSS) .
The functions of PCU are
Packet segmentation and reassembly
Scheduling for all active transmissions including radio channel
Controlling transmission for checking, buffering and
Classes Of GPRS Equipments
As GPRS users need to handle both voice and data
services, the following classifications are made in the
perspective of GPRS equipment.
Equipments that handle voice calls and transfer data at
the same time
Equipments that can handle voice or data traffic
separately, and can put a packet transfer on hold to
receive a phone call
Equipments that handle voice calls and transfer data , but
has to be disconnected from one mode explicitly inorder
to enable the other
Between a SGSN and a BSS.
The Gb interface carries the GPRS traffic and signalling
between the GSM radio network (BSS) and the GPRS network.
Frame Relay based network services is used for this interface.
Between an SGSN and the HLR.
The Gr gives the SGSN access to subscriber information in the
The HLR can be located in a different PLMN than the SGSN
Between a SGSN and a MSC.
The SGSN can send location data to the MSC or receive
paging requests from the MSC via this optional interface.
Between an SGSN and the EIR.
The Gf gives the SGSN access to GPRS user equipment
The EIR maintains three different lists of mobile equipment:
black list for stolen mobiles, grey list for mobiles under
observation and white list for other mobiles .
Between the GGSN and the HLR.
The GGSN may request the location of an MS via this optional
The interface can be used if the GGSN needs to forward
packets to an MS that is not active.
Between two GSNs (GPRS support node) within the same
PLMN. (GP interface is used if they are in different PLMN)
It consist of a protocol stack that includes IP and GPRS
Tunnelling Protocol (GTP)
GTP has two parts , the GTP-U which is used to carry user
data and GTP-C to carry control data.
Interface connects the SGSN to an SMS gateway , thus
enabling the SGSN to support SMS services.
Is a references rather than an interface.
It refers to the connection between the GGSN and some
IPV4, IPV6, and PPP are supported by GGSN
Modified air interface between mobile device and the GPRS
GPRS Logical channel
Logical channels in the GPRS network , are divided
into two categories
Logical channels are defined to perform multiple
functions like broadcast , paging , signalling and
They are listed in table
Packet Broadcast Control Channel
Packet Data Traffic Channel
Packet Dedicated Control Channel
Packet Common Control Channel
Packet Data Traffic Channel
Is employed for the transfer of user data, assigned to one
mobile station (MS) during data transfer.
Each MS can use several PDTCHs simultaneously
Packet Broadcast Control Channel.
Is a uni-directional point to multi point signalling channel
from base station sub system (BSS) to mobile stations.
Packet Timing Advance Control Channel .
used for adaptive frame synchronisation.
Packet Common Control Channel.
bi-directional point to multi point channel
Transport signalling information for network access
It consist of for sub channels as shown in table
GPRS Service Types
A PLMN provider is responsible for the data transfer
between the GPRS Service Access Points
Two kinds of services are presents
PTP (Point to Point):
Used for single packet transfer between two subscribers.
Operated in connection oriented (CONS) and connection
less mode (CLNS)
CONS for X.25
CLNS for IP
PTM (Point to Multipoint):
Supports transmission of data packets between a service
user and a specified group inside a certain geographical
It is again divided into
PTM- Multicast (PTM-M).
PTM- Group (PTM-G)
In PTM-M Data packets are broadcast over a certain
A group identifier indicates whether the packets are intended
for all users or a group of users
PTM- Group (PTM-G) is meant for only certain group of users
The messages are addressed explicitly to a specified group
and are sent to all geographical region where the group
members are located .
Data transfer or
Idle Mode. (MS off or not attached yet.).
Ready Mode. (MS is able to send and receive data).
If the MS is on, and is a Class B or Class C MS, the MS will
listen to the network, but not make any updating of where
the MS is. It is not possible to page an MS.
Cell updating is necessary.
If no activity within the timer (44s) the MS will fall back to a
NOTE: an MS can be forced back to standby mode due to
lack of recourses.
Standby Mode. (MS is listening to the Network).
It is possible to page the MS.