General Packet Radio Service

GENERAL PACKET RADIO SERVICE

General Packet Radio System is also known as GPRS is a third-generation step toward internet access. GPRS is also known as GSM-IP that is a Global-System Mobile Communications Internet Protocol as it keeps the users of this system online, allows to make voice calls, and access internet on-the-go. Even Time-Division Multiple Access (TDMA) users benefit from this system as it provides packet radio access.

GPRS also permits the network operators to execute an Internet Protocol (IP) based core architecture for integrated voice and data applications that will continue to be used and expanded for 3G services.

GPRS supersedes the wired connections, as this system has simplified access to the packet data networks like the internet. The packet radio principle is employed by GPRS to transport user data packets in a structure way between GSM mobile stations and external packet data networks. These packets can be directly routed to the packet switched networks from the GPRS mobile stations.

In the current versions of GPRS, networks based on the Internet Protocol (IP) like the global internet or private/corporate intranets and X.25 networks are supported.

Who owns GPRS ?

The GPRS specifications are written by the European Telecommunications Standard Institute (ETSI), the European counterpart of the American National Standard Institute (ANSI).

Key Features

Following three key features describe wireless packet data:

·   The always online feature - Removes the dial-up process, making applications only one click away.

·   An upgrade to existing systems - Operators do not have to replace their equipment; rather, GPRS is added on top of the existing infrastructure.

·   An integral part of future 3G systems - GPRS is the packet data core network for 3G systems EDGE and WCDMA.

Goals of GPRS

GPRS is the first step toward an end-to-end wireless infrastructure and has the following goals:

·        Open architecture

·        Consistent IP services

·        Same infrastructure for different air interfaces

·        Integrated telephony and Internet infrastructure

·        Leverage industry investment in IP

·        Service innovation independent of infrastructure

Benefits of GPRS

Higher Data Rate

GPRS benefits the users in many ways, one of which is higher data rates in turn of shorter access times. In the typical GSM mobile, setup alone is a lengthy process and equally, rates for data permission are restrained to 9.6 kbit/s. The session establishment time offered while GPRS is in practice is lower than one second and ISDN-line data rates are up to many 10 kbit/s.

Easy Billing

GPRS packet transmission offers a more user-friendly billing than that offered by circuit switched services. In circuit switched services, billing is based on the duration of the connection. This is unsuitable for applications with bursty traffic. The user must pay for the entire airtime, even for idle periods when no packets are sent (e.g., when the user reads a Web page).

In contrast to this, with packet switched services, billing can be based on the amount of transmitted data. The advantage for the user is that he or she can be "online" over a long period of time but will be billed based on the transmitted data volume.

GPRS Architecture

GPRS architecture works on the same procedure like GSM network, but, has additional entities that allow packet data transmission. This data network overlaps a second-generation GSM network providing packet data transport at the rates from 9.6 to 171 kbps. Along with the packet data transport the GSM network accommodates multiple users to share the same air interface resources concurrently.

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 below:

GPRS Mobile Stations

New Mobile Stations (MS) 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 Gateway GPRS Support Nodes (GSNs) and, Serving GPRS Support Node (SGSN) are added:

Gateway GPRS Support Node (GGSN)

The Gateway GPRS Support Node acts as an interface and a router to external networks. It 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 on charging for the use of the air interface.

Internal Backbone

The internal backbone is an IP based network used to carry packets between different GSNs. Tunnelling is used between SGSNs and GGSNs, so the internal backbone does not need any information about domains outside the GPRS network. Signalling from a GSN to a MSC, HLR or EIR is done using SS7.

Routing Area

GPRS introduces the concept of a Routing Area. This concept is similar to Location Area in GSM, except that it generally contains fewer cells. Because routing areas are smaller than location areas, less radio resources are used while broadcasting a page message.

GPRS Protocol Stack

The flow of GPRS protocol stack and end-to-end message from MS to the GGSN is displayed in the below diagram. GTP is the protocol used between the SGSN and GGSN using the Gn interface. This is a Layer 3 tunnelling protocol.

The process that takes place in the application looks like a normal IP sub-network for the users both inside and outside the network. The vital thing that needs attention is, the application communicates via standard IP, that is carried through the GPRS network and out through the gateway GPRS. The packets that are mobile between the GGSN and the SGSN use the GPRS tunnelling protocol, this way the IP addresses located on the external side of  the GPRS network do not have deal with the internal backbone. UDP and IP are run by GTP.

SubNetwork Dependent Convergence Protocol (SNDCP) and Logical Link Control (LLC) combination used in between the SGSN and the MS. The SNDCP flattens data to reduce the load on the radio channel. A safe logical link by encrypting packets is provided by LLC and the same LLC link is used as long as a mobile is under a single SGSN.

In case, the mobile moves to a new routing area that lies under a different SGSN; then, the old LLC link is removed and a new link is established with the new Serving GSN X.25. Services are provided by running X.25 on top of TCP/IP in the internal backbone.

GPRS Applications

GPRS has opened a wide range of unique services to the mobile wireless subscriber. Some of the characteristics that have opened a market full of enhanced value services to the users. Below are some of the characteristics:

·        Mobility - The ability to maintain constant voice and data communications while on the move.

·        Immediacy - Allows subscribers to obtain connectivity when needed, regardless of location and without a lengthy login session.

·        Localization - Allows subscribers to obtain information relevant to their current location.

·        Using the above three characteristics varied possible applications are being developed to offer to the mobile subscribers. These applications, in general, can be divided into two high-level categories:

o  Corporation

o  Consumer

These two levels further include:

·        Communications - E-mail, fax, unified messaging and intranet/internet access, etc.

·        Value-added services - Information services and games, etc.

·        E-commerce - Retail, ticket purchasing, banking and financial trading, etc.

·        Location-based applications - Navigation, traffic conditions, airline/rail schedules and location finder, etc.

·        Vertical applications - Freight delivery, fleet management and sales-force automation.

·        Advertising - Advertising may be location sensitive. For example, a user entering a

mall can receive advertisements specific to the stores in that mall.

Along with the above applications, non-voice services like SMS, MMS and voice calls are also possible with GPRS. Closed User Group (CUG) is a common term used after GPRS is in the market, in addition, it is planned to implement supplementary services, such as Call Forwarding Unconditional (CFU), and Call Forwarding on Mobile subscriber Not Reachable (CFNRc), and closed user group (CUG).

GPRS Quality Of Service

Quality of Service (QoS) requirements of conventional mobile packet data applications are in assorted forms. The QoS is a vital feature of GPRS services as there are different QoS support requirements for assorted GPRS applications like realtime multimedia, web browsing, and e-mail transfer.

GPRS allows defining QoS profiles using the following parameters :

·        Service Precedence

·        Reliability

·        Delay and

·        Throughput

These parameters are described below:

Service Precedence

The preference given to a service when compared to another service is known as Service Precedence. This level of priority is classified into three levels called:

·        high

·        normal

·        low

When there is network congestion, the packets of low priority are discarded as compared to high or normal priority packets.

Reliability

This parameter signifies the transmission characteristics required by an application. The reliability classes are defined which guarantee certain maximum values for the probability of loss, duplication, mis-sequencing, and corruption of packets.

Delay

The delay is defined as the end-to-end 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.

Throughput

The throughput specifies the maximum/peak bit rate and the mean bit rate. Using these QoS classes, QoS profiles can be negotiated between the mobile user and the network for each session, depending on the QoS demand and the available resources. The billing of the service is then based on the transmitted data volume, the type of service, and the chosen QoS profile.

GPRS Mobile Station Class

Mobile Station Classes talk about the globally-known equipment handset which is also known as Mobile Station (MS) and its three different classes. This equipment, more popular as handset, is used to make phone calls and access data services. The MS comprises of Terminal Equipment (TE) and Mobile Terminal (MT).

TE is the equipment that accommodates the applications and the user interaction, while the MT is the part that connects to the network.

In the following example, Palm Pilot is TE and Mobile phone is MT.

In order to take advantage of the new GPRS services, we need new GPRS enabled handsets. There are three different classes of GPRS terminal equipments:

Class A

Class A terminals can manage both packet data and voice simultaneously. Which means, one needs two transceivers, as the handset has to send or receive data and voice at the same time. This is the main reason why class A terminals are high-priced to manufacture than class B and C terminals.

Class B

Class B terminals do not play the same role like Class A. These terminals can manage either packet data or voice at a time. One can use a single transceiver for both, resulting in the low cost of terminals.

For example, If a user is using the GPRS session (like WAP browsing, file transfer, etc.) then this session is halted if he or she receives a call. This terminal does not allow both the sessions active in one go. This backlog needs rectification thereby giving the user a facility of both receiving a call and maintaining the data session.

Class C

Class C terminals can manage either only packet data or only voice. Examples of class C terminals are GPRS PCM/CIA cards, embedded modules in vending machines, and so on. Due to the high cost of class A handsets, most handset manufacturers have announced that their first handsets will be class B. Currently, work is going on in 3GPP to standardize a light weight class A in order to make handsets with simultaneous voice and data available at a reasonable cost.

GPRS Access Mode

The GPRS access modes specify whether or not the GGSN requests user authentication at the access point to a Public Data Network (PDN). The available options are:

·                                Transparent - No security authorization/authentication is requested by the GGSN.

·                                Non-transparent - In this case, GGSN acts as a proxy for authenticating.

The GPRS transparent and non-transparent modes relate only to PDP type IPv4.

Transparent Mode

Transparent access pertains to a GPRS PLMN that is not involved in subscriber access authorization and authentication. Access to PDN-related security procedures are transparent to GSNs. In transparent access mode, the MS is given an address belonging to the operator or any other addressing space of domain. The address is given either at subscription as a static address or at PDP context activation, as a dynamic address.

The dynamic address is allocated from a Dynamic Host Configuration Protocol (DHCP) server in the GPRS network. Any user authentication is done within the GPRS network. No RADIUS authentication is performed; only IMSI-based authentication (from the subscriber identity module in the handset) is done.

Non Transparent Mode

Non-transparent access to an intranet/ISP means that the PLMN plays a role in the intranet/ISP authentication of the MS. Non-transparent access uses the Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP) message issued by the mobile terminal and piggybacked in the GTP PDP context activation message. This message is used to build a RADIUS request toward the RADIUS server associated with the access point name (APN).

GPRS Access Point Name

The GPRS standards define a network identity called an Access Point Name (APN). An APN identifies a PDN that is accessible from a GGSN node in a GPRS network. In GPRS, only the APN is used to select the target network. To configure an APN, the operator configures three elements on the GSN node:

·        Access point - Defines an APN and its associated access characteristics, including security (RADIUS), dynamic address allocation (DHCP), and DNS services.

·        Access point list - Defines a logical interface that is associated with the virtual template.

·        Access group - Defines whether access is permitted between the PDN and the MS.

GPRS Billing

As packet data is introduced into mobile systems, the question of how to bill for the services arises. Always online and paying by the minute does not sound all that appealing. Here, we describe the possibilities but it totally depends on different service providers, how they want to charge their customers.

The SGSN and GGSN register all possible aspects of a GPRS user's behaviour and generate billing information accordingly. This information is gathered in so-called Charging Data Records (CDR) and is delivered to a billing gateway.

The GPRS service charging can be based on the following parameters:

·      Volume - The amount of bytes transferred, i.e., downloaded and uploaded.

·      Duration - The duration of a PDP context session.

·      Time - Date, time of day, and day of the week (enabling lower tariffs at off peak hours).

·      Final destination - A subscriber could be charged for access to the specific network, such as through a proxy server.

·      Location - The current location of the subscriber.

·      Quality of Service - Pay more for higher network priority.

·      SMS - The SGSN will produce specific CDRs for SMS.

·      Served IMSI/subscriber - Different subscriber classes (different tariffs for frequent users, businesses, or private users).

·      Reverse charging - The receiving subscriber is not charged for the received data; instead, the sending party is charged.

·      Free of charge - Specified data to be free of charge.

·      Flat rate - A fixed monthly fee.

·      Bearer service - Charging based on different bearer services (for an operator who has several networks, such as GSM900 and GSM1800, and who wants to promote usage of one of the networks). Or, perhaps the bearer service would be good for areas where it would be cheaper for the operator to offer services from a wireless LAN rather than from the GSM network.