A paper on the cellular technology
                         General Packet Radio Service
                                 ...
Contents

GENERAL PACKET RADIO SERVICE.......................................................................1
   RAJVIKRA...
1. Introduction

        The omnipresent cellular phone technologies seem to have captured the fancy of
the consumer in a ...
performance “guarantees” but also has to mutually co-exist with the older GSM
infrastructure without disrupting the effici...
NMSI                           National Mobile Station Identification number
O&M                            Operations & M...
3. Reasoning behind development of GPRS networks

        The GPRS network can be described as a supplement to the GSM net...
And also since the GPRS provides an upgrade to the GSM networks, it is
backward compatible with the old GSM handsets i.e. ...
• Dispatching—Communication of job details such as location and scheduling;
     permitting interrogation of information t...
Gi                Between the SGSN and the host PDN network (typically the Internet).
                  Uses TCP/IP for tr...
Apart from the apparent hardware that needs to be added, the software on almost all the
present nodes has to be upgraded s...
4.3 BTS (Base Transceiver System)
        This unit will have to undergo a software upgrade. The other sub units within a
...
unreachable for packet-switched traffic and vice versa. A special case of the class
               C mobile is a packet-on...
 Laptops running TCP/IP-enabled operating system such as Linux, MacOS and
      Windows 95/NT/CE.

     Cell phones empl...
throughput and low-delay to succeed even under moderately heavy network load
conditions.

         The support of differen...
received, and they are released after the transmission. For bursty traffic this results in a
much more efficient usage of ...
Explanation for such a delay: By originating the GPRS session, users confirm
their agreement to pay for the delivery of co...
7. Some observations and recommendations

       The step to be taken for the various GSM vendors to a GPRS network does n...
7.3 Network Management tools

       The present mobile communication networks all have comprehensive network
management t...
8. References

References used for this paper:

[1] IS-95 CDMA and cdma2000 Cellular/PCS System s Implementation by V. K. ...
[8] Principle and Applications of GSM, by V.Garg & Wilkes, Prentice Hall, 1998

[9] GPRS and EDGE - Comprehensive informat...
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A paper on the cellular technology General Packet Radio Service

  1. 1. A paper on the cellular technology General Packet Radio Service by Rajvikram Singh General Packet Radio Service Author : Rajvikram Singh 1
  2. 2. Contents GENERAL PACKET RADIO SERVICE.......................................................................1 RAJVIKRAM SINGH...............................................................................................................1 Contents.......................................................................................................................2 1. INTRODUCTION..........................................................................................................3 2. SOME ABBREVIATIONS AND ACRONYMS USED IN THIS PAPER...............4 3. REASONING BEHIND DEVELOPMENT OF GPRS NETWORKS.....................6 4. GPRS ARCHITECTURE: CHANGES OVER 2ND GENERATION GSM...........8 4.1 SGSN (SERVING GPRS SERVICE NODE) .....................................................................10 4.2 GGSN (GATEWAY GPRS SERVICE NODE)....................................................................10 4.3 BTS (BASE TRANSCEIVER SYSTEM)...............................................................................11 4.4 BSC (BASE STATION CONTROLLER)..............................................................................11 4.5 DATABASES.................................................................................................................11 4.6 MOBILE STATIONS (MS)..............................................................................................11 5. OTHER KEY CONCEPTS OF A GPRS SYSTEM.................................................13 5.1 MOBILITY MANAGEMENT..............................................................................................13 5.2 QUALITY OF SERVICE (QOS).........................................................................................13 5.3 HIGH DATA SPEEDS IN GPRS........................................................................................14 6. LIMITATIONS OF A PRACTICAL GPRS SYSTEM............................................15 6.1 UPPER BOUND ON THE CELL CAPACITY.............................................................................15 6.2 PRACTICAL THROUGHPUT MUCH LOWER THAN EXPECTED.....................................................15 6.3 LIMITED GPRS TERMINAL ABILITIES...............................................................................15 6.4 TRANSIT DELAYS DUE TO INHERENT BROADCASTING..........................................................16 6.5 NO STORE AND FORWARD............................................................................................16 7. SOME OBSERVATIONS AND RECOMMENDATIONS.....................................17 7.1 OPERATING SYSTEMS UPGRADE.......................................................................................17 7.2 PARALLEL PROCESSING SYSTEMS.....................................................................................17 7.3 NETWORK MANAGEMENT TOOLS.....................................................................................18 8. REFERENCES.............................................................................................................19 General Packet Radio Service Author : Rajvikram Singh 2
  3. 3. 1. Introduction The omnipresent cellular phone technologies seem to have captured the fancy of the consumer in a big way. The need to constantly be in touch with friends, colleagues or relatives has been the driving force behind the success of such wireless technologies. The telephone is widely regarded as one of the greatest inventions of the last century and even Alexander Graham Bell himself would never have had imagined the impact of his creation on the modern society. But in spite of the popularity of Bell’s "electrical speech machine," one thing was still missing – and that was that a person still had to go to a phone to make a call. The dream of having a communication device on a person, which allows him to communicate “anytime, anywhere” has led to a very sophisticated evolution of the telephone. This ‘evolution’, like all other modern technologies, has been a result of significant advancement in other related fields. Research aimed at building more efficient wireless circuits, better antennas, more robust coding and error-correcting techniques, more compact and efficient batteries and power cells, designing of higher bandwidth land networks, faster and energy efficient microprocessors etc have all contributed towards the development of the cellular technology as we see in its present form. The most popular mobile technologies like the GSM, CDMA or Iridium are considered as extremely complex systems, which have layers and layers of logic embedded in them but at the same time provide a transparent interface to the user. These technologies are evolving by the day and the market is soon expected to see the 3rd generation of cellular technologies like EDGE and UMTS that will provide the user with services and option never before imagined. In this paper I shall try to present a brief but an insightful look in to the General Packet Radio Service (GPRS). The GPRS is considered to be an important stepping-stone in moving towards 3rd generation systems. It is also called as the 2.5th generation of cellular phones in this respect and is a huge leap in terms of the services and bandwidths that were offered to the 2nd generation users of GSM networks. GPRS is not a system, which has been designed from scratch – Instead it can be considered as an add-on system to the present GSM networks. The reasons behind such an approach will be discussed in detail later in this paper. But the reader can think of it as a very clever way to boost the capacity of the existing GSM infrastructure taking into consideration the time, money and effort that has already gone in setting up the present 2nd generation GSM networks. GPRS provides many classes of services and for the first time in cellular networks implements the concept of QoS (Quality of Service). We’ll also discuss the various components of a GPRS network as defined by ETSI and other institutions. We also try to highlight the difficulties involved with setting up such a complex system, which not only has to work under stringent network General Packet Radio Service Author : Rajvikram Singh 3
  4. 4. performance “guarantees” but also has to mutually co-exist with the older GSM infrastructure without disrupting the efficiency of the ‘host’ network. 2. Some abbreviations and acronyms used in this paper Though I’ll clarify and explain any abbreviation or notation used in the subsequent sections, the reader is encouraged to go through this section, as these abbreviations will be used generously throughout the paper. The following list gives the ones used most often in this paper. AuC Authentication Centre AUT(H) AUThentication BSC Base Station Controller BSS Base Station System BTS Base Transceiver Station CFB Call Forwarding on mobile subscriber Busy supplementary service CRC Cyclic Redundancy Check CSPDN Circuit Switched Public Data Network DCE Data Circuit terminating Equipment DL Data Link (layer) DTE Data Terminal Equipment ETSI European Telecommunications Standards Institute (www.etsi.org ) FEC Forward Error Correction GGSN Gateway GPRS Serving node GMM Global Mobility Management GSM Global System for Mobile Communication GPA GSM PLMN Area GPRS General Packet Radio Service GTP GPRS Tunneling Protocol HDLC High level Data Link Control HLR Home Location Register HLR Home Location Register IMEI International Mobile station Equipment Identity IMSI International Mobile Subscriber Identity ISDN Integrated Services Digital Network ISO International Organization for Standardization LAN Local Area Network LAPB Link Access Protocol Balanced LLC Low Layer Compatibility (Also stands for Logical Link Control layer – in a protocol stack on the SGSN) MS Mobile Station. MSC Mobile-services Switching Centre, Mobile Switching Centre NMC Network Management Centre General Packet Radio Service Author : Rajvikram Singh 4
  5. 5. NMSI National Mobile Station Identification number O&M Operations & Maintenance OSI Open System Interconnection PLMN Public Lands Mobile Network PDP Protocol Data Packet PSTN Public Switched Telephone Network RR Radio Resource RTOS Real Time Operating Systems SDU Service Data Unit SGSN Serving GPRS Support Node TMSI Temporary Mobile Subscriber Identity VLR Visitor Location Register Note: ETSI, which is the body controlling and maintaining the GPRS standards, has prepared a reference document (GSM 01.04 version 8.0.0) for specifying the various abbreviations and acronyms used in the standards’ documents. The reader is advised to have a quick perusal of the document before reading any of the GPRS standards. General Packet Radio Service Author : Rajvikram Singh 5
  6. 6. 3. Reasoning behind development of GPRS networks The GPRS network can be described as a supplement to the GSM networks for enabling the implementation of a packet switched network over the circuit switched architecture of GSM. One of the main concerns was that already a lot of investment had gone into setting up the GSM networks in many countries all over the world. So the designers of GPRS have tried to use the existing GSM infrastructure as much as possible and have added extra nodes in the network to allow packet switching over GSM. The GSM networks were originally designed as digital systems intended for carrying voice for users at a rate of 9.6 kbps plus a small amount of data. The digital-data carrying capability of GSM is very small as user data is piggybacked along with the voice in the network. This data piggybacking is termed as the SMS or the Short Messaging Service and is not very useful by itself for transferring large amounts of data to and from the mobile terminal. And though service providers have been using SMS for implementing simple services like alphanumeric paging for stock market updates, weather forecasts, to even implementing WAP (Wireless Application Protocol), GSM in a true sense can never be considered suitable for ‘packet switched’ services because of its extremely small bandwidth and lack of basic services for digital data transfer. With the explosion of information technology and with data communication exceeding the amount of voice carried over networks, it was inevitable that the market would demand high speed and reliable data transfers to and from the mobile handsets. Yes it is true that one can connect to the Internet using a laptop, modem and a cell phone on a basic GSM network. But the data rates and reliability of the connection leaves much to be desired. The GPRS networks are intended to provide the users with data rates from 9.6 kbps to 171 kbps. And since it also implements Quality of Service (QoS) within the mobile network, the network can now provide network performance guarantees to the users. Thus finally it will be possible to implement bandwidth hungry real-time multimedia applications such as video-conferencing or video streaming on mobile terminals. Another attractive feature for the consumer is that since GPRS implements a packet switched network, which has been designed for handling bursty traffic, users can exploit this feature and can efficiently share the network bandwidth at the same time. So the users only have to pay for the ‘amount’ of data transacted and not for the ‘time’ for which they are connected to the network. A GPRS subscriber is always connected to the network but is billed only when data is sent or received over the connection. The GPRS network can be considered akin to a mobile LAN in which the user is always online and unlike circuit switched network, does not have to establish a connection, whenever he needs to access the network. General Packet Radio Service Author : Rajvikram Singh 6
  7. 7. And also since the GPRS provides an upgrade to the GSM networks, it is backward compatible with the old GSM handsets i.e. the users can still use their old GSM cell-phones in a GPRS network albeit only for voice communication. So if they want to use the GPRS services, they’ll have to buy a completely new terminal. GPRS specifies three classes of handsets, which allow users to use the packet switched services. One of the final goals has been to ultimately give at least one IP-address to every mobile handset in the network, so that the handset becomes a part of the Internet. Thus all the applications and protocols that have been developed for the World Wide Web will also be available on a mobile terminal. And the users have access to virtually infinite amounts of information from their handset. The applications on such a system is left totally to one’s imagination. Apart from accessing the Internet, the corporate consumer is also interested in applications involving private intranets. GPRS has been designed taking into consideration the possibilities of providing a range of such applications. The prospective applications can be broadly classified as the following: - Communications: These include services pertaining to typical applications on the Internet private intranets of companies. This kind might prove to provide the maximum number of applications ranging from private/public email services with a unified messaging system (like MIME) to ensure the consistency of data across platforms. Other applications can be multimedia applications like video streaming, video/tele-conferencing, etc, which require real time guarantees from the network. Value Added Services: The term “value added services” is used to denote the services offered by the network service providers, to increase the value of their services to the consumer. These kinds of services typically include options like stock market or weather updates at regular intervals; secure transactions for carrying out business deal or any other financial trading. These applications require a strong security support from the underlying network for transmission of sensitive data. Location-Based Services: Location-based services provide the ability to link push or pull information services related to a user’s location. Examples include hotel and restaurant finders, roadside assistance, and city-specific news and information. This technology also has interesting corporate applications such as workforce management and vehicle tracking. Vertical Applications: In the mobile environment, vertical applications apply to systems utilizing mobile architectures to support the carrying out of specific tasks within the value chain of a company, as opposed to applications that are then being offered for sale to a consumer. Examples of vertical applications include: • Sales support—Provision of stock and product information for sales staff, as well as integration of their use of appointment details and the remote placing of orders General Packet Radio Service Author : Rajvikram Singh 7
  8. 8. • Dispatching—Communication of job details such as location and scheduling; permitting interrogation of information to support the job • Fleet management—Control of a fleet of delivery or service staff, monitoring their locations and scheduling work • Parcel delivery—Tracking the locations of packages for feedback to customers and performance monitoring Advertising: Advertising services will be offered as a push type information service. The push services are services, which can be best explained as unsolicited data that can be sent to mobile handsets without requiring the users consent or intervention. 4. GPRS architecture: Changes over 2nd Generation GSM In this sub-section, we will have a look at the extra components that need to be added to a 2nd generation GSM network to upgrade it to carry packet switched data. We are assuming that the reader is aware of the general concepts and architecture of a GSM network. Fig. 1 shows a GPRS network built on top of a GSM network. The gray colored components denote the original GSM architecture and the blue ones denote the extra components that had to be added to upgrade the system to a GPRS network. The various interfaces as specified by ETSI are as shown in the figure, the important ones are briefly described below: Gd Interface between the SMS GMSC (Short Messaging Service Gateway MSC) and the SGSN Gn Between the SGSN and the GGSN. Uses the GTP (GPRS tunneling protocol) to shuttle data between the two nodes. IP routing is used for transferring information, as the link between the two nodes may not be direct. This is because the SGSN and the GGSN can have a many-to-many relationship between them i.e. there could be more than one GGSN supporting multiple SGSNs. Gb Defines the interface between the SGSN and the BSC. Uses frame-relay at the data-link level to transfer packets. Provides logical link control and defines virtual circuits and virtual pipes to distinguish the data intended for different BSCs and BTSs in its domain. Gp Interface between the SGSN and an external GGSN (of some other GPRS network). General Packet Radio Service Author : Rajvikram Singh 8
  9. 9. Gi Between the SGSN and the host PDN network (typically the Internet). Uses TCP/IP for transferring data to and from external hosts. The SGSN here acts as a proxy. It is supposed to take care of any address translation before letting the data through. Other GPRS PLMN SMS Gateway MSC and SMS GGSN Inter Networking MSC SMS GMSC Gp SMS INMSC Gd SGSN Gn PDN GGSN Gb Gi B Gc S C Gf Gr Gs B HLR T S D B EIR VLR T S User Data and signaling data Signaling data Fig 1: GPRS system Architecture General Packet Radio Service Author : Rajvikram Singh 9
  10. 10. Apart from the apparent hardware that needs to be added, the software on almost all the present nodes has to be upgraded so that they can handle user and signaling information for packet-switched data. The ETSI standards define many such software interfaces as protocol stacks. An uppercase letter and a lower-case subscript denote each interface. The notations besides the links, between the various components, represent the interfaces. Enabling GPRS on a GSM network requires the addition of two main modules: 4.1 SGSN (Serving GPRS Service Node) The SGSN provides packet routing to and from the SGSN service area for all users in that service area. The SGSN can be considered to be on the same level of hierarchy as a MSC in a GSM network. In fact it can be considered as a packet-switching MSC, which is responsible for delivering the data to the various Mobile Stations in its domain. Some other responsibilities are:  SGSNs send queries to home location registers (HLRs) to obtain profile data of GPRS subscribers.  SGSNs detect new GPRS MSs in a given service area, process registration of new mobile subscribers, and keep a record of their location inside a given area. Therefore, the SGSN performs mobility management functions such as mobile subscriber attach/detach and location management.  The SGSN is connected to the base-station subsystem via a Frame Relay connection to the PCU in the BSC. 4.2 GGSN (Gateway GPRS Service Node) As the word Gateway in its name suggests, the GGSN acts as a gateway between the GPRS network and Public Data Networks such as IP and X.25. GGSNs also connect to other GPRS networks to facilitate GPRS roaming. Thus it broadly takes care of the following:  GGSNs maintain routing information that is necessary to tunnel the protocol data units (PDUs) to the SGSNs that service particular MSs  Network and subscriber screening and address mapping. One (or more) GGSNs may be deployed to support multiple SGSNs. Note : All GSNs are connected via an IP-based GPRS backbone network. Within this backbone, the GSNs encapsulate the PDN packets and transmit (tunnel) them using the GPRS Tunneling Protocol GTP. There are two kinds of GPRS backbones:  Intra-PLMN backbone networks connect GSNs of the same PLMN and are therefore private IP-based networks of the GPRS network provider.  Inter-PLMN backbone networks connect GSNs of different PLMNs. A roaming agreement between two GPRS network providers is necessary to install such a backbone. Apart from these two major additions to the network, following are the changes that need to be made are in the other components: General Packet Radio Service Author : Rajvikram Singh 10
  11. 11. 4.3 BTS (Base Transceiver System) This unit will have to undergo a software upgrade. The other sub units within a BTS, like the Radio Link control and the Medium Access Layer, which basically take care of the actual radio interface, will remain as they are. 4.4 BSC (Base Station Controller) The present BSCs will have to undergo a major software upgrade in order for it to handle packet switched data. This includes the addition of Packet Control Units (PCU); often hosted in the Base Station Subsystems. Also the BSCs will have to run protocols stacks for handling the new Gb interface with the SGSN. This might not be purely a software upgrade as adding extra features in the BSC essentially means more computation and logic per unit of data. Hence a hardware upgrade might not be avoidable. 4.5 Databases The databases in the network such as the HLR, VLR, EIR etc in the network will also have to be upgraded as now they have to handle more information. The billing information per user is also more complicated in GPRS as it not only depends upon the amount of data sent but also on the class of data or service desired for sending the information. Some service providers might also like to charge according to time-of-day during which data was transacted for e.g. the charges might be higher in peak business hours. The databases also have to provide the various interfaces with the SGSN and GGSN within the network and have to handle the new call models and functions defined by GPRS. Apart from the unavoidable software upgrade, a hardware upgrade might also be required. 4.6 Mobile Stations (MS) The new network has been designed to be backward compatible with the GSM standards. So the consumers can still use their GSM handsets for voice communication within a GPRS network. Apart from this, three classes of GPRS terminals have been defined:  Class A terminals are able to handle circuit-switched and packet-switched services simultaneously. Both types of services are handled independently.  Class B terminals handle one service at the time, circuit switched or packet switched, and have a capability for automatic switching between the two modes. The class B terminal may suspend a packet transfer when it gets an incoming circuit-switched call, and resume it afterwards.  Class C terminals must be manually set to either circuit-switched mode or packet- switched mode. When a class C mobile is in the circuit-switched mode, it is General Packet Radio Service Author : Rajvikram Singh 11
  12. 12. unreachable for packet-switched traffic and vice versa. A special case of the class C mobile is a packet-only terminal. GPRS terminals can be typical mobile phones, smart-phones, PC cards or specific modules that can be built into a machine. A variety of different mobile terminals will be able to take advantage of GPRS. But there is a common set of requirements that is expected from a GPRS terminal and these requirements have been summarized and shown in Fig. 2. These features are desired as the terminal is expected to run sophisticated applications over WAP/HTTP and at least one TCP/IP stack. High-speed network connection with a theoretical limit of 171kbps Large color display with a high resolution. GPRS TERMINAL A moderately powerful An inbuilt storage media microprocessor running (semiconductor or magnetic) advanced operating systems for storing large amounts of like Palm OS or mobile information. Linux An easy to use keypad for Efficient battery power keying in alphanumeric utilization and should also be characters. Preferably with a capable of using alternate pointing device power sources Compact design. Should be convenient to carry around. Fig. 2 GPRS terminal Thus the mobile terminal should behave as a PC and should be mobile as well. At the same time they should ideally be able to connect to a LAN as easily as they can to the mobile networks. It is because of this reason that notebooks with GPRS adapters have become quite popular as terminals. Though it is difficult to imagine all the combinations of hardware and software that would qualify as GPRS MSs but some options are: General Packet Radio Service Author : Rajvikram Singh 12
  13. 13.  Laptops running TCP/IP-enabled operating system such as Linux, MacOS and Windows 95/NT/CE.  Cell phones employing micro browsers using the Wireless Application Protocol.  Other embedded systems that communicate with a central system such as traffic, weather or stock market update systems. Or dedicated e-mail/fax terminals used for providing personal information.  Palmtops and PDAs with GPRS capable modules. This is the direction towards which market trend has been shifting. 5. Other key concepts of a GPRS System 5.1 Mobility Management The main task of mobility management is to keep track of the user’s current location, so that incoming packets can be routed to his or her MS. For this purpose, the MS frequently sends location update messages to its current SGSN. If the MS sends updates less frequently, its location (e.g., its current cell) is not known exactly and paging is necessary for each downlink packet, resulting in a significant delivery delay. On the other hand, if location updates happen very often, the MS’s location is well known to the network, and the data packets can be delivered without any additional paging delay. However, quite a lot of uplink radio capacity and battery power is consumed for mobility management in this case. Thus, a good location management strategy must be a compromise between these two extreme methods. In the GPRS network the SGSN is primarily responsible for this task. ETSI has defined a GMM (Global Mobility Management) layer, which is supposed to contain the logic for managing mobility of the subscribers. 5.2 Quality of Service (QoS) QoS can be defined as the network performance guarantees given by the service provider. Such guarantees are absolutely necessary for running real-time applications, where constant throughput and delay should be maintained throughout a session. The Quality of Service QoS requirements of typical mobile packet data applications are very diverse (e.g., consider real-time multimedia, Web browsing, and e- mail transfer). This kind of distinction is necessary in guaranteeing network performance for real-time applications. Owing to limited network resources, one cannot allow all packets to be given a fair chance and then expect the applications requiring high- General Packet Radio Service Author : Rajvikram Singh 13
  14. 14. throughput and low-delay to succeed even under moderately heavy network load conditions. The support of different QoS classes, which can be specified for each individual session, is therefore an important feature. GPRS allows defining of 12 different QoS profiles using the following parameters:  Service precedence is the priority of a service in relation to another service. There exist three levels of priority: high, normal, and low.  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 (an undetected error) of packets.  Delay parameters define maximum values for the mean delay and the 95- percentile delay. The latter is the maximum delay guaranteed in 95 percent of all transfers. 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 PDNs, are not taken into account.  Throughput specifies the 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 current available resources. The billing of the service is then based on the transmitted data volume, the type of service, and the chosen QoS profile. 5.3 High data speeds in GPRS The GSM system, on which the General Packet Radio Service is built, is essentially a TDMA system that allocates a time slot per user on the physical layer for establishing a communication link for voice transmission. Each frequency on the GSM system is divided into 8 equal time slots. The channel allocation in GPRS is different from the original GSM. GPRS allows a single mobile station to transmit on multiple time slots of the same TDMA frame (multi-slot operation). This results in a very flexible channel allocation: one to eight time slots per TDMA frame can be allocated for one mobile station. Moreover, uplink and downlink are allocated separately, which efficiently supports asymmetric data traffic (e.g., Web browsing). In conventional GSM, a channel is permanently allocated for a particular user during the entire call period (whether data is transmitted or not). In contrast to this, in GPRS the channel s are only allocated when data packets are sent or General Packet Radio Service Author : Rajvikram Singh 14
  15. 15. received, and they are released after the transmission. For bursty traffic this results in a much more efficient usage of the scarce radio resources. With this principle, multiple users can share one physical channel. 6. Limitations of a practical GPRS system All said and done, the performance of a complex system like GPRS can never be truly evaluated, till it has either been implemented or some simulation has been done to gauge the efficacy of the system in practical scenarios. Many company’s had joined the fray to implement GPRS systems in many countries and the following drawbacks of the system have been most glaring. We shall now briefly take a look at them: 6.1 Upper bound on the cell capacity There are only limited pools of radio resources, which can be used and divided amongst the various systems and increase in usage of radio resources by one negatively impacts the other. And since voice and GPRS calls both use the same network resources. The extent of the impact depends upon the number of timeslots, if any, that are reserved for exclusive use of GPRS. However, GPRS does dynamically manage channel allocation and allow a reduction in peak time signaling channel loading by sending short messages over GPRS channels instead. But the bottom-line is that GPRS does impact a network's existing cell capacity 6.2 Practical throughput much lower than expected Achieving the theoretical maximum GPRS data transmission speed of 172.2 kbps would require a single user taking over all eight timeslots. And the way the theoretical speed is calculated, the user will have to use each and every bit in the 8 slots i.e in other words there is no error protection or synchronization bits, which is a impractical assumption. Also, it is unlikely that a network operator will allow all timeslots to be used by a single GPRS user. Additionally, the initial GPRS terminals are expected to be severely limited ones, supporting only one, two or three timeslots. The bandwidth available to a GPRS user will therefore be severely limited. 6.3 Limited GPRS terminal abilities At the time of writing of this paper, it is not clear whether in the near future, the hardware vendors have any intention of providing the ability for mobile terminated GPRS calls (i.e. receipt of GPRS calls on the mobile phone). This feature is critical for implementing many non-voice applications. General Packet Radio Service Author : Rajvikram Singh 15
  16. 16. Explanation for such a delay: By originating the GPRS session, users confirm their agreement to pay for the delivery of content from that service. This origination may well be performed using a Wireless Application Protocol (WAP) session using the WAP micro-browser that will be built into GPRS terminals. However, mobile terminated IP traffic might allow unsolicited information to reach the terminal. Internet sources originating such unsolicited content may not be chargeable. A possible worse case scenario would be that mobile users would have to pay for receiving unsolicited junk content. This is a potential reason for a mobile vendor NOT to support GPRS Mobile Terminate in their GPRS terminals. 6.4 Transit Delays due to inherent broadcasting GPRS packets are sent in all different directions to reach the same destination due to the uncertainty of user location. This opens up the potential for one or some of those packets to be lost or corrupted during the data transmission over the radio link. The GPRS standards recognize this inherent limitation of wireless packet technologies and incorporate data integrity and retransmission strategies. However, the result is that potential transit delays can occur. 6.5 No Store and Forward The nodes in the network do not provide any facility to buffer (or cache) the network data. The incoming data is either forwarded to the next node in the network or is dropped in the event of network congestion. Though each node has sufficient storage capacity to momentarily hold the data when it is processing it, it does not hold it after the processing is done. The mobile terminal and the external hosts/servers running the application are supposed to take care of this. General Packet Radio Service Author : Rajvikram Singh 16
  17. 17. 7. Some observations and recommendations The step to be taken for the various GSM vendors to a GPRS network does not appear to be one a simple and inexpensive one. The GSM network consists of nodes, which though are supposed to handle a lot of digital information, are not designed to provide the extra amount of processing and computation, as required by GPRS nodes. So in such a scenario, a simple software upgrade may not be sufficient. Some changes that might be required are: 7.1 Operating systems upgrade The new system provides QoS guarantees to the consumers and thus should be capable of ensuring that the data packets received by it have been ‘disposed’ off in a timely fashion. This is not a very simple goal to achieve, especially when the nodes are heavily loaded and the operating systems on them are trying to handle hundred of thousands of packet every second. It is very much possible that in such a system some high-priority packets may get delayed and thus fail to meet the QoS guarantees. Enter Real Time Operating Systems (or RTOS). These are special versions of the operating systems that we come across everyday on our desktops or servers that have been designed to handle mission critical operations. They can provide guarantees on the computation time taken irrespective of the load on the system. Thus the GPRS nodes will have to use something like an RTOS to ensure that packets are processed and forwarded to the next node within the set limits. There is no other way that a network can think of providing service guarantees. 7.2 Parallel processing systems Most of the present GSM systems are already distributed processing machines with large amounts of processing power. The GPRS upgrade may require a vendor to add more processors to the present nodes (e.g. in a BSC) in order to take care of the extra amount of processing. Nodes like the SGSN or GGSN, which are best implemented as completely different sub-systems need not considered here. They may have to be bought and installed from scratch. Also extra hardware/software might be needed to implement load balancing on such machines with many nodes. Some desirable properties of such a system are that they should be scalable and should have a large MTBF (Mean Time Between Failures) General Packet Radio Service Author : Rajvikram Singh 17
  18. 18. 7.3 Network Management tools The present mobile communication networks all have comprehensive network management tools for remotely configuring/rebooting a node in the network. These tools need to be further enhanced to cater to non-voice data now. Some possibilities are re- routing facility for data packets, bringing up extra processors at the SGSN/GGSN in case of overloading and vice-versa, providing statistics of missed QoS guarantees on packets etc. Also, since now the mobile consumer will be using a lot of non-voice applications over the internet, the companies providing these services would like to analyze the behavior of their customer-base. The GPRS vendors may want to provide data-mining and analysis services to such corporate clients. One example could be a company wanting to know about how many of its customers are accessing their on-line services through a mobile network or through a PDN, so that they can plan their future services accordingly. General Packet Radio Service Author : Rajvikram Singh 18
  19. 19. 8. References References used for this paper: [1] IS-95 CDMA and cdma2000 Cellular/PCS System s Implementation by V. K. Garg, Prentice Hall, 2000, ISBN0-13-087112-5; [2] Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Service description; Stage 1 (GSM 02.60 version 6.3.1) http://webapp.etsi.org/exchangefolder/en_301113v060201p.pdf [3] Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Service description; Stage 2 (GSM 03.60 version 7.4.1 Release 1998) http://webapp.etsi.org/exchangefolder/en_301344v070401p.pdf [4] Digital cellular telecommunications system (Phase 2+); Abbreviations and acronyms (GSM 01.04 version 8.0.0 Release 1999) http://webapp.etsi.org/exchangefolder/tr_101748v080000p.pdf [5] GSM Phase 2+, General Packet Radio Service (GPRS): Architecture, Protocols and Air Interface by Christian Bettstetter, Hans-Jorg Vogel and Jorg Eberspacher, Techbische Universitat Munchen (TUM) http://www.comsoc.org/pubs/surveys/3q99issue/pdf/Bettstetter.pdf [5] Mobile Applications Initiative Developers' Guide by Ericsson http://www.mobileapplicationsinitiative.com/document/filer/developersguide.pdf [6] General Packet Radio Service (GPRS) White Paper, Trillium Digital Systems, Inc. http://www.trillium.com/whats-new/wp_gprs.html [7] GPRS White Paper by CISCO http://www.ieng.com/warp/public/cc/so/neso/gprs/gprs_wp.pdf Other books/papers on GSM and GPRS that the reader might be interested in: General Packet Radio Service Author : Rajvikram Singh 19
  20. 20. [8] Principle and Applications of GSM, by V.Garg & Wilkes, Prentice Hall, 1998 [9] GPRS and EDGE - Comprehensive information about GPRS and Edge http://www.3g-generation.com/gprs_and_edge.htm [10] An Introduction to the General Packet Radio Service by Simon Buckingham, hosted on the GSM World web-site. http://www.gsmworld.com/technology/yes2gprs.html General Packet Radio Service Author : Rajvikram Singh 20

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