GPRS

Presented by

Hrudya

Need for GPRS Speed Immediacy New and better applications User friendly billing

History 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 Switched Data). 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 as 2.5G. The GPRS network acts in parallel with the

GSM network, providing packet switched connections to the external networks.

Functions 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 (PDN) 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)

The GPRS network and it’s new elements.

GPRS Architecture

From GSM to GPRS....

Figure shows the architecture of a GPRS network. The GPRS system brings some new network elements to an existing GSM network. These elements are: Packet Control Unit (PCU) Serving GPRS Support Node (SGSN): the MSC of the GPRS

network Gateway GPRS Support Node (GGSN): gateway to external

networks Border Gateway (BG): a gateway to other PLMN (Public Land

Mobile Network) Intra-PLMN backbone: an IP based network inter-connecting all

the GPRS elements Charging Gateway (CG) Legal Interception Gateway (LIG) Domain Name System (DNS) Firewalls: used wherever a connection to an external network is

required.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 network.

The SGSN of the GPRS network is equivalent to the MSC of the GSM network. There must at least one SGSN in a GPRS network.

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.

Handles: 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. Handover/cell Change

Stores: 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 Security:

Authentication, by means of identity or equipment check. Data compression is used to minimise the size of

transmitted data units Charging

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:

Handles: 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

network Activation and Deactivation of PDP-Contexts / Session

Management. Stores:

Subscriber data for active Mobile Stations. Security:

Interfaces to external IP networks and deals with security issues Firewall. Screening.

Charging 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 .

Charging Gateway

GPRS users have to be charged for the use of the network. 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 duration.

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.

Border 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 network.

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

133.44.15.5.

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 Access control Scheduling for all active transmissions including radio

channel management Controlling transmission for checking, buffering and

retransmission

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.

Class A. Equipments that handle voice calls and transfer data at

the same time Class B.

Equipments that can handle voice or data traffic separately, and can put a packet transfer on hold to receive a phone call

Class C. Equipments that handle voice calls and transfer data ,

but has to be disconnected from one mode explicitly inorder to enable the other

GPRS Interfaces

Gb: 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.

Gr: Between an SGSN and the HLR. The Gr gives the SGSN access to subscriber information in

the HLR. The HLR can be located in a different PLMN than the SGSN

(MAP). Gs:

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.

Gf: Between an SGSN and the EIR. The Gf gives the SGSN access to GPRS user equipment

information. 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 .

Gc Between the GGSN and the HLR. The GGSN may request the location of an MS via this optional

interface. The interface can be used if the GGSN needs to forward

packets to an MS that is not active. Gn

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.

Gd: Interface connects the SGSN to an SMS gateway , thus

enabling the SGSN to support SMS services. Gi:

Is a references rather than an interface. It refers to the connection between the GGSN and some

external network IPV4, IPV6, and PPP are supported by GGSN

Um: Modified air interface between mobile device and the GPRS

network.

GPRS Logical channel Logical channels in the GPRS network , are

divided into two categories Traffic channels Signalling/Control Channels

Logical channels are defined to perform multiple functions like broadcast , paging , signalling and payload transport.

They are listed in table

Group Channel

Function Direction

Packet Broadcast Control Channel PBCCH Broadcast Control

BSS MS

Packet Data Traffic Channel PDTCH Data traffic flow

MS BSS

Packet Dedicated Control Channel PACCH Associated Control

MS BSS

MS BSSPTCCH Timing

Cntrl

Packet Common Control Channel PRACH Random Access

MS BSS

PAGCH Access Grant

MS BSS

PPCH Paging MS BSS

PNCH Notification

MS BSS

PDTCH: 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

PBCCH: Packet Broadcast Control Channel. Is a uni-directional point to multi point signalling

channel from base station sub system (BSS) to mobile stations.

PTCCH: Packet Timing Advance Control Channel . used for adaptive frame synchronisation.

PCCCH: Packet Common Control Channel. bi-directional point to multi point channel Transport signalling information for network access

management 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 region.

It is again divided into PTM- Multicast (PTM-M). PTM- Group (PTM-G)

In PTM-M Data packets are broadcast over a certain geographical area.

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 .

GPRS States

Idle

Ready

Standby

GPRS Attach

GPRS Detach

Ready Timer expired

Data transfer or reception

Idle Mode. (MS off or not attached yet.). 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.

Ready Mode. (MS is able to send and receive data). Cell updating is necessary. If no activity within the timer (44s) the MS will fall

back to a stand-by state. 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.