Global System for Mobile Communication (GSM)

                                                         

                               MOBILE COMMUNICATION SYSTEMS

Global System for Mobile Communication (GSM) – General Packet Radio Service (GPRS) – Universal Mobile Telecommunication System (UMTS).

GSM-Global System For Mobile Communication :

GSM is a present being used in India. It is possibly the most successful digital mobile system to have ever been used till now. An important characteristic of GSM system is that it provides data services in addition to voice services and yet is compatible with 1G systems.

Explain in detail about GSM services?

GSM services (8 Marks):
GSM provides three main categories of services. These are :
1.Bearer Services.
2. Teleservices
3.Supplementary services

Bearer Services

These give the subscribers the capability to send and receive data to or from remote computers or mobile phones. For this reason, bearer services are also known as data services.
These services also enable the transparent transmission of data between GSM and other networks like PSDN, ISDN etc… at rates from 300 bps to 9600bps.
These services are implemented on the lower three layers of the OSI reference model.
GSM supports data transfer rate of up to 9.6Kbps.
They permit either transparent or non-transparent and either synchronous or asynchronous mode of data transmission.
Teleservices:
GSM provides both the voice-oriented teleservices and non-voice teleservices.
 Telephony:
The main goal of GSM was to provide high quality digital voice transmission offering the bandwidth of 3.1kHZ of analog phone system.
Special codecs are used  for voice transmission while other codes are used for transmission of analog data.
Emergency Number:
The same number  is used throughout an area.
This service is free of cost and mandatorily provided by all service providers.
Short Message Services(SMS):
This service offers transmission of text messages of sizes up to 160 characters.
SMS services  use the signaling channels, making possible the duplex systems of sending and receiving the SMS’s messages.
FAX:

Modem Fax data are transmitted as digital data over the analog telephone network to the ITU-T standards T.4 and T.30 

Supplementary services:
GSM provides certain supplementary services such as user identification, call direction and forwarding of on-going calls.
In addition, standard ISDN features such as  ‘close user groups ‘ and  ‘ multiparty’ communication are available.

Explain the architecture of GSM and illustrate with a neat diagram?[8Marks]

System architecture of GSM:

GSM is a typical second-generation system, replacing the first generation analog systems, but not offering the high worldwide data rates that the third generation systems, such as UMTS, are promising.

GSM has initially been deployed in Europe using 890–915 MHz for uplinks and 935–960 MHz for downlinks – this system is now also called GSM 900 to distinguish it from the later versions.

These versions comprise GSM at 1800 MHz (1710–1785 MHz uplink, 1805–1880 MHz downlink), also called DCS (digital cellular system) 1800, and the GSM system mainly used in the US at 1900 MHz (1850–1910 MHz uplink, 1930–1990 MHz downlink), also called PCS (personal communications service) 1900.

 Two more versions of GSM exist. GSM 400 is a proposal to deploy GSM at 450.4–457.6/478.8–486 MHz for uplinks and 460.4–467.6/488.8–496 MHz for downlinks.  The following section describes the architecture, services, and protocols of GSM that are common to all three major solutions, GSM 900, GSM 1800, and GSM 1900.

GSM has mainly been designed for this and voice services and this still constitutes the main use of GSM systems. However, one can foresee that many future applications for mobile communications will be data driven. The relationship of data to voice traffic will shift more and more towards data. A GSM system consists of three subsystems, the Radio Sub System (RSS), the Network and Switching Subsystem (NSS), and the Operation Subsystem (OSS).

As the name implies, the radio subsystem (RSS) comprises all radio specific entities, i.e., the mobile stations (MS) and the base station subsystem (BSS).

An interface (solid lines) and the connection to the OSS via the O interface (dashed lines).

The A interface is typically based on circuit-switched PCM-30 systems (2.048 Mbit/s), carrying up to 30 64 kbit/s connections, whereas the O interface uses the Signalling System No. 7 (SS7) based on X.25 carrying management data to/from the RSS.

Base Station Subsystem (BSS):

 A GSM network comprises many BSSs, each controlled by a base station controller (BSC). The BSS performs all functions necessary to maintain radio connections to an MS, coding/decoding of voice, and rate adaptation to/from the wireless network part. Besides a BSC, the BSS contains several BTSs.

Base Transceiver Station (BTS):

 A BTS comprises all radio equipment, i.e., antennas, signal processing, amplifiers necessary for radio transmission. A BTS can form a radio cell or, use sectorized antennas, several cells are connected to MS via the Um interface (ISDN U interface for mobile use), and to the BSC via the Abis interface.  The Um interface contains all the mechanisms necessary for wireless transmission (TDMA, FDMA etc.) and will be discussed in more detail below.

The Abis interface consists of 16 or 64 kbit/s connections. A GSM cell can measure between some 100 m and 35 km depending on the environment (buildings, open space, mountains etc.) but also expected traffic.

Base Station Controller (BSC):

The BSC basically manages the BTSs. It reserves radio frequencies, handles the handover from one BTS to another within the BSS, and performs paging of the MS. The BSC also multiplexes the radio channels onto the fixed network connections at the A interface.

Mobile Station (MS):  MS (or) cell phone contains two major components :
      *SIM(subscriber- identification-module)
      *Mobile Device

v  The MS comprises all user equipment and software needed for communication with a GSM network.

v   An MS consists of user independent hard- and software and of the subscriber identity module (SIM), which stores all user-specific data that is relevant to GSM.3 While an MS can be identified via the international mobile equipment identity (IMEI), a user can personalize any MS using his or her SIM, Without the SIM, only emergency calls are possible.

v  The SIM card contains many identifiers and tables, such as card-type, serial number, a list of subscribed services, a personal identity number (PIN), a PIN unblocking key (PUK), an authentication key Ki, and the International Mobile subscriber identity (IMSI) (ETSI, 1991c). The PIN is used to unlock the MS. Using the wrong PIN three times will lock the SIM. In such cases, the PUK is needed to unlock the SIM.

Network and switching subsystem: This subsystem forms the heart of the GSM system. It connects the wireless networks to the standard public networks and carries out usage-based charging, accounting and also handles roaming.
NSS consists of a switching center and several databases as described below:

Mobile Services Switching Centre (MSC):
MSC sets up the connection to other MSCs and to other networks such as Public  Data Network(PDN). A gateway MSC (GMSC) has additional connections to other fixed networks, such as PSTN and ISDN. Using additional interworking functions (IWF), an MSC can also connect to public data networks (PDN) such as X.25. An MSC handles all signaling needed for connection setup, connection release and handover of connections to other MSCs. The standard signaling system No. 7 (SS7) is used for this purpose.  An MSC also performs all functions needed for supplementary services such as call forwarding, multi-party calls, reverse charging etc.
Home location registers(HLRs):
An HLR stores in a database important information that is specific to each subscriber.
The information contains subscriber’s IMSI, pre or post paid ,users current location etc. Dynamic information is also needed, e.g., the current location area (LA) of the MS, the mobile subscriber roaming number (MSRN), the current VLR and MSC.

Visitor Location Register(VLR):
It is essentially a temporary database that is updated whenever a new MS enters its area by roaming. The information is obtained from the corresponding HLR database.  The function of the VLR is to reduce the number of queries to the HLR. The VLR associated with each MSC is a dynamic database which stores all important information needed for the MS users currently in the LA that is associated with the MSC (e.g., IMSI, MSISDN, HLR address).

If a new MS comes into an LA the VLR is responsible for, it copies all relevant information for this user from the HLR. This hierarchy of VLR and HLR avoids frequent HLR updates and long-distance signaling of user information. Some VLRs in existence are capable of managing up to one million customers.

Operation and Maintenance Subsystem(OSS):
The OSS contains all the functions necessary for network operation and maintenance .
It consists of the following:
1.Operation and Maintenance Centre (OMC):
It supervises all other network entities.
Its functions are
                        1.Traffic monitoring
                        2.Subscribers
                        3.Security management
                        Account Billing

OMCs use the concept of telecommunication management network (TMN) as standardized by the ITU-T.
2.Authentication center (AuC):
As the radio interface and mobile stations are particularly vulnerable, a separate AUC has been defined to protect user identity and data transmission.

The AUC contains the algorithms for authentication as well as the keys for encryption and generates the values needed for user authentication in the HLR. The AUC may, in fact, be situated in a specially protected part of the HLR.

Equipment identity registers (EIR):

The EIR is a database for all IMEIs, i.e., it stores all device identifications registered for this network. As MSs are mobile, they can be easily stolen. With a valid SIM, anyone could use the stolen MS. The EIR has a blacklist of stolen (or locked) devices. The EIR also contains a list of valid IMEIs (white list), and a list of malfunctioning devices (gray list).

GSM SECURITY[8Marks]:
Security in GSM  is broadly supported at three levels:

1. Operators level
2.Customer‘s level
3.System level

Features of GSM security :
1.Authentication:
The purpose of authentication is to protect the network against unauthorized use.
It helps to protect the GSM subscribers by denying the possibility for intruders to impersonate authorized users. A GSM network operator can verify the identity of the subscriber making it highly improbable to clone someone else’s mobile phone identity
2.Confidentiality:
GSM network protects voice, data, and sensitive signaling information against eavesdropping on the radio path. It is achieved by using encryption techniques prescribed by the GSM designers.
3.Anonymity:
A GSM network protects against someone tracking the location of the users or identifying calls made to the user by eavesdropping on the radio path.
The anonymity of the subscriber in the radio access link in the GSM network is achieved by allocating Temporary Mobile Subscriber Identity (TMSI) instead of permanent identities.
This helps to protect against tracking the user’s location and obtaining information about a user’s calling pattern. 

UMTS (Universal Mobile Telecommunications System):[16Marks]
CDMA2000 and UTMS were developed separately and are 2 separate ITU approved 3G standards. In these networks, coverage is provided by a combination of various cell sizes , ranging from “in building” pico cells to global cells provided by satellites, giving service to the remote regions of the world.

FIVE GROUPS OF 3G RADIO ACCESS TECHNOLOGIES:

● IMT-DS:

Ø  The direct spread technology comprises wideband CDMA (WCDMA) systems.

Ø  Example: NTT DoCoMo for 3G wide area services.
 IMT-DS, ETSI called it UTRA-FDD in the UMTS context, and technology used is called W-CDMA

Ø  Today, standardization of this technology takes place in 3GPP (Third generation partnership project, 3GPP, 2002a).

 ● IMT-TC:

Ø  Initially, this family member, called time code, contained only the

Ø  UTRA-TDD system which uses time-division CDMA (TD-CDMA).
Later on, the Chinese proposal, TD-synchronous CDMA (TD-SCDMA) was added.
Both standards have been combined and 3GPP fosters the development of this technology.
 The initial UMTS installations are based on W-CDMA

.● IMT-MC:

Ø  cdma2000 is a multi-carrier technology standardized by 3GPP2 (Third generation partnership project 2, 3GPP2, 2002), which was formed shortly after 3GPP to represent the second mainstream in 3G technology.

Ø  Version cdma2000 EV-DO has been accepted as the 3G standard.

● IMT-SC:

Ø  The enhancement of the US TDMA systems, UWC-136, is a single carrier technology originally promoted by the Universal Wireless Communications Consortium (UWCC).

Ø  It is now integrated into the 3GPP efforts.
This technology applies EDGE, among others, to enhance the 2G IS-136 standard.

● IMT-FT:

Ø  As frequency time technology, an enhanced version of the cordless telephone standard DECT has also been selected for applications that do not require high mobility.
 ETSI is responsible for the standardization of DECT.

Ø  The main driving forces in the standardization process are 3GPP and 3GPP2.

Ø  ETSI has moved its GSM standardization process to 3GPP and plays a major role there.

UMTS SYSTEM ARCHITECTURE [8Marks]:

Ø  The UTRA network (UTRAN) handles cell level mobility and comprises several radio network subsystems(RNS).
The functions of the RNS include radio channel ciphering and deciphering, handover control, radio resource management etc.
The UTRAN is connected to the user equipment (UE) via the radio interface Uu (which is comparable to the Um interface in GSM).
 Via the Iu interface (which is similar to the A interface in GSM),

Ø  UTRAN communicates with the core network (CN).
The CN contains functions for inter-system handover, gateways to other networks (fixed or wireless), and performs location management if there is no dedicated connection between UE and UTRAN.

Ø  UMTS further subdivides the above-simplified architecture into so-called domains .

Ø  The user equipment domain is assigned to a single user and comprises all the functions that are needed to access UMTS services.

Ø  Within this domain are the USIM domain and the mobile equipment domain.
 The USIM domain contains the SIM for UMTS which performs functions for encryption and authentication of users, and stores all the necessary user-related data for UMTS.
 Typically, this USIM belongs to a service provider and contains a microprocessor

for an enhanced program execution environment (USAT, UMTS SIM application toolkit). The end device itself is in the mobile equipment domain.
 All functions for radio transmission as well as user interfaces are located here.

Ø  The infrastructure domain is shared among all users and offers UMTS services to all accepted users. This domain consists of the access network domain, which contains the radio access networks (RAN), and the core network domain, which contains access network independent functions.
The core network domain can be separated into three domains with specific tasks.
The serving network domain comprises all functions currently used by a user for accessing UMTS services.
All functions related to the home network of a user, e.g., user data look-up, fall into the home network domain.
Finally, the transit network domain may be necessary if, for example, the serving network cannot directly contact the home network.
All three domains within the core network may be in fact the same physical network. These domains only describe functionalities.

UMTS RADIO INTERFACE:

Ø  The biggest difference between UMTS and GSM comes with the new radio interface

(Uu).
The duplex mechanisms are already well known from GSM (FDD) and DECT .

Ø  This technology multiplies a stream of bits with a chipping sequence. This spreads the signal and, if the chipping sequence is unique, can separate different users.

Ø  To separate different users, the codes used for spreading should be (quasi) orthogonal, i.e., their cross-correlation should be (almost) zero.

Ø  UMTS uses a constant chipping rate of 3.84 Mchip/s. Different user data rates can be supported using different spreading factors (i.e., the number of chips per bit).

Ø  26 shows the basic ideas of spreading and separation of different senders in UMTS.

Ø  The first step in a sender is spreading of user data (data) using orthogonal spreading codes.

Ø  Using orthogonal codes separates the different data streams of a sender.

Ø  UMTS uses so-called orthogonal variable spreading factor (OVSF) codes.

Ø  27 shows the basic idea of OVSF. Orthogonal codes are generated by doubling a chipping sequence X with and without flipping the sign of the chips.

Ø  This results in X and –X, respectively. Doubling the chipping sequence also results in spreading a bit twice as much as before. The spreading factor SF=n becomes 2n.

Ø  Starting with a spreading factor of 1,  27 shows the generation of orthogonal codes with different spreading factors.
Two codes are orthogonal as long as one code is never a part of the other code. Looking at the coding tree in  27 and considering the construction of the codes, orthogonality is guaranteed if one code has not been generated based on another.
 For example, if a sender uses the code (1,–1) with spreading factor 2, it is not allowed to use any of the codes located in the subtrees generated out of (1,–1). This means that, e.g., (1,–1,1,–1), (1,–1,–1,1,–1,1,1,–1), or (1,–1,–1,1,–1,1,1,–1,–1,1,1,–1,1,–1,–1,1) cannot be used anymore. However, it is no problem to use codes with different spreading factors if one code has not been generated using the other. Thus, (1,–1) block only the lower subtree in  27, many other codes from the upper part can still be used. An example for a valid combination in OVSF is (1,–1), (1,1,–1,–1), (1,1,1,1,1,1,1,1), (1,1,1,1,–1,–1,–1,–1,

1,1,1,1,–1,–1,–1,–1), (1,1,1,1,–1,–1,–1,–1,–1,–1,–1,–1,1,1,1,1).
This combination occupies the whole code spaces and allows for the transmission of data with different spreading factors (2, 4, 8, and 2*16). This example shows the tight coupling of available spreading factors and orthogonal codes.

Ø  Now remember that UMTS uses a constant chipping rate. Using different spreading factors this directly translates into the support of different data

rates.
 If the chipping rate is constant, doubling the spreading factor means dividing the data rate by two. But this also means that UMTS can only support a single data stream with SF=1 as then no other code may be used. Using the example combination above, a stream with half the maximum data rate, one with a fourth, one with an eighth, and two with a sixteenth are supported at the same time.

Ø  Each sender uses OVSF to spread its data streams as 26 shows.
The spreading codes chosen in the senders can be the same. Using different spreading codes in all senders within a cell would require a lot of management and would increase the complexity.
 After spreading all chip streams are added and scrambled. Scrambling does not spread the chip sequence any further but XORs chips based on a code.

Ø  In the FDD mode, this scrambling code is unique for each sender and separates all senders (UE and base station) in a cell. After scrambling, the signals of different senders are quasi-orthogonal.

            Quasi-orthogonal signals have the nice feature that they stay quasi-orthogonal even if          they are not synchronized.
Using orthogonal codes would require chip-synchronous reception and tight synchronization

(this is done in other CDMA networks).  For TDD the scrambling code is cell specific, i.e., all stations in a cell use the same scrambling code and cells are separated using different codes. The scrambled chips are QPSK modulated and transmitted.

GPRS(General Packet  Radio Service): [16Marks]
GPRS when integrated with GSM ,significantly improves and simplifies internet access.
It transfers data packets from GSM mobile stations to external Public Data Networks (PDNs).
Packets can be directly routed from the GPRS mobile station to packet switched networks  making it easy to connect to the internet. GPRS uses a billing system based on the amount of the transmitted data rather than the duration of the connection. So users can remain continuously connected to the system , and yet get charged only for the amount of the transmitted data.

GPRS SERVICES:

GPRS is a  service, implying variable  and  that depend on the number of other users sharing the service concurrently, as opposed to , where a certain  (QoS) is guaranteed during the connection. In 2G systems, GPRS provides data rates of 56–114 kbit/second. Cellular technology combined with GPRS is sometimes described as, that is, a technology between the second) and third (generations of mobile telephony It provides moderate-speed data transfer, by using unused  (TDMA) channels in, for example, the GSM system. GPRS is integrated into GSM Release 97 and newer releases.

1. Point-to-point (PTP)Service: It is between two users and can either be connectionless or connection oriented.
2. Point-to-Multipoint(PTM)Service

It is a data transfer service from one user to multiple users. There are two types of PTM service:
               Multicast PTM: Data packets are broadcast in a certain area.
               GroupCall PTM: Data packets are addressed to a group of users.

GPRS extends the GSM Packet circuit switched data capabilities and makes the following services possible:

·         SMS messaging and broadcasting

·         "Always on" internet access

·         (MMS)

·         over cellular (PoC)

·         and presence

·         Internet applications for smart devices through  (WAP)

·         (P2P) service: inter-networking with the Internet (IP)

·         (P2M) service: point-to-multipoint multicast and point-to-multipoint group calls

If SMS over GPRS is used, an SMS transmission speed of about 30 SMS messages per minute may be achieved. This is much faster than using the ordinary SMS over GSM, whose SMS transmission speed is about 6 to 10 SMS messages per minute.

GPRS ARCHITECTURE[8Marks]:
GPRS  architecture introduces two new network elements  called Serving GPRS Support Node(SGSN) and Gateway GPRS Support Node(GGSN).

SGSN is essentially a router.
All SGSN are integrated into a standard GSM architecture and defines many new interfaces
The GGSN is the interworking unit between the GPRS network and the external packet data network(PDN).
The GGSN contains routing information for GPRS users performs address connection and tunnels data to a user through encapsulation.
The GGSN is connected to an external network and it transfers packets to the SGSN to an IP-based GPRS backbone network.
SGSN (SERVING GPRS SUPPORT NODE) helps support MS.
SGSN is connected to BSC through frame delay and it is at the same hierarchy level as the MSC.
GPRS register (GR) is a part of a HLR which stores all relevant GPRS data In a part of HLR ,which stores all the relevant data of the GPRS in a mobile IP network, GGSN and SGSN can be compared with home agent and foreign agent respectively.
The data packets are transmitted to the BSS and finally to the MS through the GGSN and SGSN.
The MSC is responsible for data transport in the traditional circuit-switched GSM.

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:

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 Transceiver Station(BTS).
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 the BSC.
GPRS Support Nodes (GSNs)	The deployment of GPRS requires the installation of new core network elements called the serving GPRS support node (SGSN) and gateway GPRS support node (GGSN).
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 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. Signaling 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, fewer radio resources are used While broadcasting a page message.

How is data routing done in GPRS?In what aspect is data routing different from voice routing?State its merits and demerits?[16Marks]

DATA ROUTING

Data routing or routing of data packets to and fro from a mobile user, is one of the pivot requisites in the GPRS network. The requirement can be divided into two areas:

·         Data packet routing

·         Mobility management.

Data Packet Routing

The important roles of GGSN involve synergy with the external data network. The GGSN updates the location directory using routing information supplied by the SGSNs about the location of an MS. It routes the external data network protocol packet encapsulated over the GPRS backbone to the SGSN currently serving the MS. It also decapsulates and forwards external data network packets to the appropriate data network and collects charging data that is forwarded to a charging gateway (CG).

There are three important routing schemes:

·         Mobile-originated message - This path begins at the GPRS mobile device and ends at the host.

·         Network-initiated message when the MS is in its home network - This path begins at the host and ends at the GPRS mobile device.

·         Network-initiated message when the MS roams to another GPRS network - This path begins at the host of visited network and ends at the GPRS mobile device.

The GPRS network encapsulates all data network protocols into its own encapsulation protocol called the GPRS Tunnelling Protocol (GTP). The GTP ensures security in the backbone network and simplifies the routing mechanism and the delivery of data over the GPRS network.

Mobility Management

The operation of the GPRS is partly independent of the GSM network. However, some procedures share the network elements with current GSM functions to increase efficiency and to make optimum use of free GSM resources (such as unallocated time slots).

An MS can be in any of the following three states in the GPRS system. The three-state model is unique to packet radio. GSM uses a two-state model either idle or active.

Active State

Data is transmitted between an MS and the GPRS network only when the MS is in the active state. In the active state, the SGSN knows the cell location of the MS.

Packet transmission to an active MS is initiated by packet paging to notify the MS of an incoming data packet. The data transmission proceeds immediately after packet paging through the channel indicated by the paging message. The purpose of the paging message is to simplify the process of receiving packets. The MS listens to only the paging messages instead of to all the data packets in the downlink channels. This reduces battery usage significantly.

When an MS has a packet to transmit, it must access the uplink channel (i.e., the channel to the packet data network where services reside). The uplink channel is shared by a number of MSs, and its use is allocated by a BSS. The MS requests use of the channel in a random access message. The BSS allocates an unused channel to the MS and sends an access grant message in reply to the random access message.

Standby State

In the standby state, only the routing area of the MS is known. (The routing area can consist of one or more cells within a GSM location area).

When the SGSN sends a packet to an MS that is in the standby state, the MS must be paged. Because the SGSN knows the routing area of the MS, a packet paging message is sent to the routing area. On receiving the packet paging message, the MS relays its cell location to the SGSN to establish the active state.

Idle State

In the idle state, the MS does not have a logical GPRS context activated or any Packet-Switched Public Data Network (PSPDN) addresses allocated. In this state, the MS can receive only those multicast messages that can be received by any GPRS MS. Because the GPRS network infrastructure does not know the location of the MS, it is not possible to send messages to the MS from external data networks.

Routing Updates

When an MS that is in an active or a standby state moves from one routing area to another within the service area of one SGSN, it must perform a routing update. The routing area information in the SGSN is updated, and the success of the procedure is indicated in the response message.

A cell-based routing update procedure is invoked when an active MS enters a new cell. The MS sends a short message containing the identity of the MS and its new location through GPRS channels to its current SGSN. This procedure is used only when the MS is in the active state.

The inter-SGSN routing update is the most complicated routing update. The MS changes from one SGSN area to another, and it must establish a new connection to a new SGSN. This means creating a new logical link context between the MS and the new SGSN and informing the GGSN about the new location of the MS.

ADVANTAGES OF GPRS:
It Supports:
1. machine-to-machine data communication
2.Lower service charges
3.Compatible with Email
4.Broadcast services and
5.Web Browsing 
6.E-Commerce and advertising

LIMITATIONS OF GPRS:
1.Reduced cell capacity:
             Since the radio resources are deployed for both voice and GPRS calls, it affects the capacity of the existing cell.

2.Transit delay:
            When packets are lost, they trigger retransmission and contribute to the overall transit delay.

3.No store:
            SMS services typically provided store and forward services, but GPRS standards have no support for any storage mechanisms.

           

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