Chapter 7:-GENERAL PACKET RADIO SERVICE:
General Packet Radio Service (GPRS) is a packet oriented Mobile Data Service available to
users of Global System for Mobile Communications (GSM) and IS-136 mobile phones. It
provides data rates from 56 up to 114 kbps.
GPRS can be used for services such as Wireless Application Protocol (WAP) access, Short
Message Service (SMS), Multimedia Messaging Service (MMS), and for Internet
communication services such as email and World Wide Web access. GPRS data transfer is
typically charged per megabyte of throughput, while data communication via traditional circuit
switching is billed per minute of connection time, independent of whether the user actually is
utilizing the capacity or is in an idle state. GPRS is a best-effort packet switched service, as
opposed to circuit switching, where a certain Quality of Service (QoS) is guaranteed during the
connection for non-mobile users.
The multiple access methods used in GSM with GPRS are based on frequency division duplex
(FDD) and TDMA. During a session, a user is assigned to one pair of up-link and down-link
frequency channels. This is combined with time domain statistical multiplexing, i.e. packet mode
communication, which makes it possible for several users to share the same frequency channel.
The packets have constant length, corresponding to a GSM time slot. The down-link uses first-
come first-served packet scheduling, while the up-link uses a scheme very similar to reservation
ALOHA. This means that slotted Aloha (S-ALOHA) is used for reservation inquiries during a
contention phase, and then the actual data is transferred using dynamic TDMA with first-come
QUALITY OF SERVICE
QosThe Quality of Service QoS requirements of typical mobile packet data applications are very
diverse. For example diiferent GPRS applications like realtime multimedia, Web browsing, and
e-mail transfer need a different QoS support. This QoS becomes a very important feature of
GPRS allows defining QoS profiles using the parameters service precedence, reliability, delay,
and throughput. These parameters are described below:
The service precedence is the priority of a service in relation to another service. There exist three
levels of priority: high, normal, and low.
Reliability:The reliability indicates the transmission characteristics required by an application.
Three reliability classes are defined, which guarantee certain maximum values for the probability
of loss, duplication, mis-sequencing, and corruption of packets.
The delay is defined as the end-toend transfer time between two communicating mobile stations
or between a mobile station and the Gi interface to an external packet data network.
This includes all delays within the GPRS network, e.g., the delay for request and assignment of
radio resources and the transit delay in the GPRS backbone network. Transfer delays outside the
GPRS network, e.g., in external transit networks, are not taken into account.
The throughput specifies the maximum/peak bit rate and the mean bit rate.
GPRS is a data network that overlays a second-generation GSM network. This data overlay
network provides packet data transport at rates from 9.6 to 171 kbps. Additionally, multiple users
can share the same air-interface resources simultaneously.
Following is the GPRS Architecture diagram:
GPRS attempts to reuse the existing GSM network elements as much as possible, but to
effectively build a packet-based mobile cellular network, some new network elements,
interfaces, and protocols for handling packet traffic are required.
Therefore, GPRS requires modifications to numerous GSM network elements as summarized
GSM Network Element Modification or Upgrade Required for GPRS.
Mobile Station (MS) New Mobile Station is required to access GPRS
services. These new terminals will be backward
compatible with GSM for voice calls.
BTS A software upgrade is required in the existing base
BSC The base station controller (BSC) requires a
software upgrade and the installation of new
hardware called the packet control unit (PCU). The
PCU directs the data traffic to the GPRS network and
can be a separate hardware element associated with
GPRS Support Nodes
The deployment of GPRS requires the installation of
new core network elements called the serving GPRS
support node (SGSN) and gateway GPRS support
Databases (HLR, VLR, etc.) All the databases involved in the network will
require software upgrades to handle the new call
models and functions introduced by GPRS.
GPRS Mobile Stations:
New Mobile Station are required to use GPRS services because existing GSM phones do not
handle the enhanced air interface or packet data. A variety of MS can exist, including a high-
speed version of current phones to support high-speed data access, a new PDA device with an
embedded GSM phone, and PC cards for laptop computers. These mobile stations are backward
compatible for making voice calls using GSM.
GPRS Base Station Subsystem:
Each BSC requires the installation of one or more Packet Control Units (PCUs) and a software
upgrade. The PCU provides a physical and logical data interface to the base station subsystem
(BSS) for packet data traffic. The BTS can also require a software upgrade but typically does not
require hardware enhancements.
When either voice or data traffic is originated at the subscriber mobile, it is transported over the
air interface to the BTS, and from the BTS to the BSC in the same way as a standard GSM call.
However, at the output of the BSC, the traffic is separated; voice is sent to the mobile switching
center (MSC) per standard GSM, and data is sent to a new device called the SGSN via the PCU
over a Frame Relay interface.
GPRS Support Nodes:
Following two new components, called GPRS support nodes (GSNs), are added:
Gateway GPRS support node (GGSN):
The Gateway GPRS Support Node acts as an interface and a router to external networks. The
GGSN contains routing information for GPRS mobiles, which is used to tunnel packets through
the IP based internal backbone to the correct Serving GPRS Support Node. The GGSN also
collects charging information connected to the use of the external data networks and can act as a
packet filter for incoming traffic.
Serving GPRS support node (SGSN):
The Serving GPRS Support Node is responsible for authentication of GPRS mobiles, registration
of mobiles in the network, mobility management, and collecting information for charging for the
use of the air interface.
Transmission and Signaling Planes
The transmission plane consists of a layered protocol structure providing user data transfer, along
with associated procedures that control the information transfer such as flow control, error
detection, and error correction. Figure 3.2 illustrates the layered protocol structure between the
MS and the GGSN.
The internal backbone is an IP based network used to carry packets between different GSNs.
Tunneling is used between SGSNs and GGSNs, so the internal backbone does not need any
information about domains outside the GPRS network. Signaling from a GSN to a MSC, HLR or
EIR is done using SS7.
Security: Basic security rules
Authentication, key management, ciphering
MO, MT, MO+MT
Interworking with GSM services
Security: based on GSM phase2
SGSN uses same principle as MSC/VLR:
• Get triplet, send RAND to MS, wait for SRES from MS, use Kc
MS can‟t authenticate the network
Key management in MS
Kc generated same way from RAND using Ki as in GSM
Ciphering algorithm is optimized for GPRS traffic („GPRS - A5‟)
Ciphering is done between MS and SGSN
IMSI is only used if a temporary identity is not available
Temporary identity (TLLI) is exchanged over ciphered link
GPRS Attach function is similar to IMSI attach
Authenticate the mobile
Generate the ciphering key
Enable the ciphering
Allocate temporary identity (TLLI)
Copy subscriber profile from HLR to SGSN
After GPRS attach
The location of the mobile is tracked
Communication between MS and SGSN is secured
Charging information is collected
SGSN knows what the subscriber is allowed to do
HLR knows the location of the MS in accuracy of SGSN
Data transfer: Basic rules
• Does not interpret user data, except
• SGSN may perform TCP/IP header compression
• Does not interpret source or destination addresses
• Sends all packets to specified GGSN that handles the PDP context
• Performs optional filtering
Decides where and how to route the packet
IP TECH SCHOOL seminar / 23.11.1998 / slide 27
Data transfer (2/4)
Mobile originated (left when MS in HPLMN, right when in
VPLMN, no filtering/screening)
IP TECH SCHOOL seminar / 23.11.1998 / slide 28
Data transfer (3/4)
Mobile terminated (left when MS in HPLMN, right when in
VPLMN, with/without filtering/screening)
IP TECH SCHOOL seminar / 23.11.1998 / slide 29
3.3. Data transfer (4/4)
Mobile originated and terminated (left MSs in same
PLMN, right MSs in different PLMN)
Instead of Location Area, GPRS uses Routing Areas to group cells. RA is a subset of LA.
• MS is not known by the network (SGSN)
• MS‟s location is known in accuracy of Routing Area
• MS can utilize DRX (to save battery)
• MS must inform its location after every Routing Area change (no need to inform if
MS changes from one cell to another within same Routing Area)
• Before the network can perform MT data transfer MS must be paged within the
• MS may initiate MO data transfer at any time
• MS‟s location is known in accuracy of cell
• MS must inform its location after every cell change
• MS can initiate MO data transfer at any time
• SGSN does not need to page the MS before MT data transfer
• MS listens continuously GPRS PCCCH channel
• DRX in READY state is optional
Mobility management messages:
Cell update (implicit, with any message)
• When MS changes the cell within a Routing Area in READY state
Routing Area update
• When MS changes the cell between two Routing Areas in READY or STANDBY
• Two types of Routing Area Updates (from MS‟s point of view only one type)
– Intra-SGSN Routing Area Update
– Inter-SGSN Routing Area Update
Periodic Routing Area updates are applicable
Internetworking of GSM services
GPRS can interwork with GSM services through Gs-interface
If no Gs interface exists:
• Type of the location update procedure is indicated by the network in the response
message to MS
Effects on different MS classes if Gs does not exist:
• A-class mobiles must use conventional GSM services via normal GSM channels
• B-class mobiles won‟t get simultaneous support from the network. Depending on
• MS can try listen both paging channels simultaneously by themselves
• MS does IMSI detach and use only GPRS service
• No effect on C-class mobiles as simultaneous services are not supported
Combined GPRS and IMSI attach
• To save radio resources
• MS indicates its request for combined attach
• MS sends combined GPRS and IMSI attach to SGSN
• SGSN may authenticate the MS
• SGSN informs MSC/VLR about the new MS
Combined Location and Routing Area update
• To save radio resources
• MS indicates its request for combined update
• This is done when both Location Area and Routing Area changes at the same time
• Combined Location and Routing Area update is not done if MS has CS connection
Paging CS services via GPRS network
• MSC/VLR gets MT call or SMS
• In VLR, presence of SGSN address tells that the MS is in GPRS attached state
• MSC/VLR sends the paging request to SGSN address (not to BSC)
• SGSN checks the location of MS (identified by IMSI)
• SGSN pages the MS via GPRS channels indicating “CS page” status
• MS replies to the page using normal GSM channels.
7.5 DATA SERVICES IN GPRS:-
GPRS services are enabled by a wide range of corporate and consumer
Two modes of the GPRS network:-
The user will be using the GPRS mobile phone to access the
applications running on the phone itself.
The phone here acts as the end user device.
All GPRS phones have WAP browser as an embedded application.
Applications can be developed in C/C++ Java.
This mode is for mobile computing where the user will use the GPRS
interface as an access to the network.
The end user will be PDAs.
The mobile phone will be connected to the device and used as a
modem to access the wireless data network.
A GPRS terminal can be one of three classes: A,B or C.
Class A terminal supports GPRS data and other GSM services
such as SMS and voice simultaneously.
This includes simultaneously attach,activation, monitor,and
A Class A terminal can make or receive calls on two services
A class B terminal can monitor GSM and GPRS channels
simultaneously ,but can support only one of these services at
A Class B terminal can support simultaneous attach,activation
and monitor but not simultaneous traffic.
A Class C terminal supports only nonsimultaneous attach.
The user must select which service to connect to.
A Class C terminal can make or receive calls from only the
manually selected network service.
BEARERS in GPRS
The bearer services of GPRS offer end-to -end packet switched data
GPRS is planned to support two different kinds of data transport services.
GPRS will support the following types of data services:
Short message service was originally designed for GSM
GPRS will continue to support SMS as a bearer.
WAP is a Wireless Application Protocol .
It is a data bearer service over HTTP protocol.
WAP uses WML(Wireless Markup Language) and a WAP
MMS supports multimedia messages .
MMS uses WAP and SMS as its lower layer transport.
Video,audio pictures or clips can be sent through MMS.
7.6APPLICATIONS FOR GPRS:-
It is like information services,Internet access ,email,Web
Browsing,which are very useful while mobile.
Due to high bandwidth mobile Internet browsing will be better suited
Access to corporate Intranet can add new dimension to mobile
Banking over wireless is another generic applications people may
like to use while mobile.
GPRS-SPECIFIC APPLICATIONS :-
It is used as means to communicate and discuss matters of
GPRS will offer ubi quitous chat by integrating Internet chatand
wireless chat using SMS and WAP.
Multimedia objects like photographs ,pictures postcards
,greeting cards and presentations,static web pages can be
sent and received over the mobile network.
Can be used for monitoring parking lots or building sites for
intruders and thieves,This can also be used by law
enforcement agents ,jouranalists and insurance agents for
sendind images of accident site.
Virtual Private Network:-
GPRS is used to provide VPN services.
As the bandwidth is higher so many banks in India are
migrating to GPRS-based networks.
This is expected to reduce the transaction time by about 25%.
This application integrates GPS that tell people where they are.
Anyone with a GPS receiver can receive their satellite position
and thereby find out where they are.
Vehicle Positioning applications can be used to deliver several services including
remote vehicle diagnostics.
Billing and Charging in GPRS
The GPRS specifications stipulate that the minimum charging information
that must be collected are:
Destination and source addresses
Usage of radio interface
Usage of external Packet Data Networks
Usage of the packet data protocol addresses
Usage of general GPRS resources and location of the Mobile Station
Operators are clearly extremely interested in unambiguous and reliable charging
mechanisms. Nevertheless, at the time of writing, charging in GPRS seems to be at
least in some respects fuzzy and incompletely thought through. Specifically, it is
sometimes not clear who pays for a particular data transfer, or indeed whether
anybody will be charged at all.
Here is an example:
•A GPRS subscriber, Alice, is running WAP on her mobile. The mobile has a PDP
context activated and has been assigned an IP address, say, 10.99.12.1. Alice
cannot be pinged from a terminal outside the GPRSintranet, because all 10.x.x.x
addresses are internal to the GPRS intranet (assigned by the GGSN), and not
accessible outside it.
•Another GPRS subscriber, Bob, is using FTP. His mobile, too, has a PDP context
activated and has received an IP address: 10.99.12.2. Like Alice, Bob cannot be
pinged from outside the GPRS intranet.
•However, Alice can ping Bob, and vice versa (assuming that their mobiles have
the Ping protocol built in). Now the question is: Who pays for the ping response?
It is obviously unreasonable that Bob should have to pay; a subscriber should not
be charged for a communication that occurs without his knowledge as a forced
response to an action by another subscriber. Still, in today‟s GPRS networks, it is
in fact not certain exactly what will happen. It is even unclear to what extent the
user will be charged for signaling in general, i.e. including the signaling required to
perform self-initiated actions – for example to establish a PDP context.
The potential charging problems are of course greatly aggravated if GPRS
subscribers are not given network-internal but ordinary IP addresses, accessible to
anyone with an Internet connection. (We mention this because it is known to have
occurred in practice.)