Introduction
From the customer's point of view, it is mobile systems that are perhaps the most exciting telecommunications development since the invention of the telephone. The developments in optical fiber technology might sound very impressive and the statistics involved are mind-boggling, but the average subscriber does not usually appreciate the full extent of the benefits at a personal level. The developments in this field are taking place mainly behind the scenes, and are not really tangible service improvements to the subscriber. The pocket telephone, on the other hand, is a revelation that is center-stage and whose benefits can be instantly appreciated by anyone who purchases one of these devices. The cellular telephone industry has experienced explosive growth over recent years. It is an area of telecommunications that has benefited not only the developed world, but also many developing countries. From the service provider's point of view, cellular systems are very fast to install compared to extending new cables to customer premises. When a leading telecommunications company forecast in 1996 that by the year 2000 there would be 350 to 400 million mobile radio units worldwide, there was widespread disbelief. So far, all forecasts of this nature have turned out to be conservative.
Cellular mobile telephone systems are not easy to classify. They could be considered as part of the local loop because they extend out to the subscriber handset, or because of the long distances that can be bridged between a fixed
The analog systems that have been around for a number of years are giving way to digital technology. Narrowband TDMA is currently seeing widespread deployment, and CDMA started deployment in 1996. Significant equipment incompatibilities are encountered when going from one system to another or trying to incorporate two or three of the same network. Many of these problems stem from the difference in outputs. High-mobility cellular radios transmit at relatively high power in the region of 1 to 10 W, whereas the latest low-mobility portable units transmit at relatively low power levels of 1 to 10 mW. While this is fine for the customer, it makes coexistence of the two systems a network planner's nightmare High tier is a term often associated with high-mobility systems, whereas Iow tier is associated with low-mobility systems.
In summary, cellular telephony is the culmination of several technologies which have progressed in parallel over the past two decades. In fact. the progress has been so rapid that the standards bodies have had difficulty organizing meetings fast enough to determine standards that are consistent with the new technology. Go to Top
Historically, technology has lagged behind the design of mobile telephone systems, and it was only by 1983 that the first good-quality systems were put into operation. Since those early, low-capacity pioneering systems subscriber demand has mushroomed, despite the higher cost of calling from a mobile telephone. The different types of mobile radio systems, in terms of frequency spectrum usage. They differ primarily in modulatic technique and carrier spacing.
Analog FM. The first-generation cellular systems in operation were analog F radio systems that allocated a single carrier for each call. Each carrier frequency modulated by the caller. The carriers were typically spaced at 25-kl
Digital FDMA.
FDMA systems resemble analog FM, with the exception that the carrier is modulated by a digitally encoded speech signal. The bandwidth of each carrier is similar to the analog FM systems (typically 25 kHz).
Digital narrowband TDMA.
TDMA systems operate with several customers sharing one carrier. Each user is allocated a specific time slot for transmission and reception of short bursts or packets of information. The bandwidth of each carrier is typically 200 kHz, and the total bandwidth available is in the region of 10 to 30 MHz, so many FDMA carriers each contain several customers on a TDMA shared basis. This access combination allows a reasonably large channel capacity in the region of 500 to 1000 channels, before frequency reuse.
In the United States, for example, the 824- to 849-MHz frequency band is allocated for one-way transmission from the base station to the user, and the 869- to 894-MHz band is allocated for transmission from the user to the base station. To enable two competitive systems to operate simultaneously, only half of each of these bands is available to each operator. Each system therefore has 12.5 MHz available for transmission and 12.5 MHz for reception. Each of these 12.5-MHz bands is subdivided into several carriers in an FDMA manner. Each carrier is operated in a TDMA mode having time slots for voice or data channels. Digital wideband (spread spectrum). One form of digital wideband operation is CDMA.
In these systems there is a single carrier that is modulated by the speech signals of many users. Instead of allocating each user a different time slot, each is allocated a different modulation code. Mobile users in adjacent cells all use the same frequency band. Each user contributes some interfering energy to the receivers of fellow users, the magnitude of which depends on the processing gain. In addition to interference from users within a given cell, there is also interference from users in adjacent cells. The distance between adjacent cell users attenuates the interference considerably more than users within the same cell. Frequency reuse is therefore unnecessary. Consequently, each cell can use the full available bandwidth (12.5 MHz) for CDMA operation.
Analog Cellular Radio Go to Top
Analog cellular systems were used exclusively in the early days of mobile communications. Although they are being superseded by digital technology, a large number of systems are still in service and will probably remain in use for several years to come.
Analog FM.
Analog FM cellular radio systems are relatively old technology (1980 to 1985) in this fast-paced industry. These are the first-generation cellular radio systems. However, it is informative to discuss some of their features briefly, because they provide insight into how future systems are evolving. Analog cellular radio was initially designed for vehicle-mounted operation. By 1990, already more than 50 percent of mobile radios (stations) in most networks were hand-held portables, and the demand was growing.
As far back as 1979, Bell Labs designed and installed a trial cellular mobile system called the Advanced Mobile Phone Service (AMPS). This was really the birth of cellular radio in the United States, and is still the basis of the analog systems in operation today. The AMPS system uses the hexagonal cell structure, with a base station in each cell.
The cells are clustered into groups of seven cells (i.e., a seven-cell repeat pattern). AMPS covers large areas with large-sized cells, and high-traffic-density areas are covered by subdividing cells.
Sectorization is also used to enhance capacity. The overall control of the system is by a mobile telephone switching office (MTSO) in each metropolitan area. This digital switch connects into the regular telephone network and provides fault detection and diagnostics in addition to call processing. The mobile unit was originally installed in a car, truck, or bus. The frequencies for AMPS are 870 to 890 MHz from base to mobile, and 825 to 845 MHz from mobile to base. Each radio channel has a pair of one-way channels separated by 45 MHz. The spacing between adjacent channels is 30 kHz. The AMPS system uses FM with 12-kHz maximum deviation. FM has a convenient capture mechanism. If a receiver detects two different signals on the same frequency, it will lock onto the stronger signal and ignore the weaker, interfering signal.
Mobile units are microprocessor controlled. The MTSO periodically monitors the carrier signal quality coming from the active mobile. If, during a call, a mobile moves to the edge of a cell boundary and crosses the boundary, the signal quality from the adjacent cell gradually becomes better than the existing service provider, so handoffs initiated. The handoff command is a "blank and burst" message sent over the voice channel to the designated cell. A brief data burst is transmitted from the base providing service to instruct the mobile microprocessor to retune the radio to a new channel (carrier). The voice connection is momentarily blanked during the period of data transmission and base station switching. This interruption is so brief it is hardly noticeable, and most customers are unaware of its occurrence. Go to Top
All of the call setup is done by a separate channel. There are dedicated signaling channels that transmit information only in the form of binary data. These channels are monitored by all mobiles that do not have a call in progress. When a mobile is first switched to the on mode or is at the end of a call, it is in the idle state. It scans the frequencies used for call setup and monitors the one providing it with the strongest signal. Each cell has its own setup channel. The mobile periodically makes a scan to see if its change of position has made the
The AMPS system has nationwide roaming capability. This is possible by cooperation between the service providers in different parts of the country. The AMPS system has been very successful, but its main disadvantage is that its total system capacity is inferior to the more advanced digital cellular radio systems.
The AMPS cellular radio network structure.
Digital Cellular Radio Go to Top
Digital cellular radio systems can be divided into two categories, narrowband and wideband. Narrowband systems are often considered to be the second generation of cellular radio. Although the digital narrowband TDMA systems in North America and Europe have developed along similar lines, there remain many features that are different. Because of its global success, in this text the main focus of attention for digital narrowband TDMA cellular radio will be the European-designed system called GSM, to which the U.S. system called IS-54 will be compared.
The acronym GSM originally stood for the French name Groupe Speciale Mobile, the planning organization that did much of the groundwork for the TDMA cellular system.
GSM now stands for global system for mobile communications, A description of its features serves to highlight some of the intricacies of present-day cellular radio systems.
GSM operates in the primary spectrum range of 890–915 MHz (uplink) and 935–960 MHz (downlink), with subsequent adaptations to operate in 1800 MHz (Digital Cellular System or GSM 1800) and 1900 MHz (Personal Communications Services or GSM 1900). GSM 450 and GSM 800 (part of the IS-136(TDMA cellular standards) 850 Band) are planned to utilize the 450 MHz and 800 MHz spectra in the future.
GSM is the basis of a powerful family of platforms for the future - providing a direct link into next generation solutions including GPRS (General Packet Radio Services) EDGE (Enhanced Data for GSM Evolution) and 3G(3GSM).In the tables below will show frequency and the speed for the wireless transmission.
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GSM terminals may incorporate one or more of the GSM frequency bands listed below to facilitate roaming on a global basis.
In the above bands mobile stations transmit in the lower frequency sub-band and base stations transmit in the higher frequency sub-band. |
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Theoretical vs. Actual Wireless Transmission Speeds
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| Generation | Technology | Theoretical Top Speed | Avg. Delivered Speed |
| 1G | AMPS | 19.2 Kbps | Less than 9 Kbps |
| 1G | CDPD | 19.2 Kbps | 9.2 Kbps |
| 2G | TDMA, CDMA, iDEN, GSM | 19.2 Kbps | 9.6-19.2 Kbps |
| 2.5G | GPRS | 115 Kbps | 20-40 Kbps |
| 3G | 1xRTT | 153 Kbps | 60-80 Kbps |
| 3G | EDGE Phase II | 384 Kbps | 80-100 Kbps expected |
| 3G | 1xEV-DO | 2.4 Mbps | 200-300 Kbps |
| 3G | W-CDMA | 384 Kbps | 200-300 Kbps |
| 3G | 1xEV-DV | 4.8 Mbps | 200-300 Kbps |
The Wireless Generation is a function of speed and maturity of technology and is usually representative of a family of similar technologies, Theoretical Throughput is the best-case attainable speed over the network, and is typically 50 to 100% faster than real-world performance. 3G An industry term used to describe the next generation of public wireless voice + data networks. To qualify as 3G, a network must meet certain requirements for speed, availability, reliability and other criteria set forth by the International Telecommunications Union. There are many 3G network technologies being developed, generally they are packet-based "always on" networks.
GSM's unrivalled success can be attributed to many factors, including the
unparalleled co-operation and support between all those supplying, running and
exploiting the platform. It is based upon a true end-to-end solution, from
infrastructure and services to handsets and billing systems.
GSM is a standard that embraces all areas of technology, resulting in global,
seamless wireless services for all its customers. It's all part of the Wireless
Evolution.
The idea of cell-based mobile radio systems appeared at Bell Laboratories (in USA) in the early 1970s. However, mobile cellular systems were not introduced for commercial use until the 1980s. During the early 1980s, analog cellular telephone systems experienced a very rapid growth in Europe, particularly in Scandinavia and the United Kingdom. Today cellular systems still represent one of the fastest growing telecommunications systems.
But in the beginnings of cellular systems, each country developed its own system, which was an undesirable situation for the following reasons:
In 1989 the responsability for the GSM specifications passed from the CEPT to the European Telecommunications Standards Institute (ETSI). The aim of the GSM specifications is to describe the functionality and the interface for each component of the system, and to provide guidance on the design of the system. These specifications will then standardize the system in order to guarantee the proper interworking between the different elements of the GSM system. In 1990, the phase I of the GSM specifications were published but the commercial use of GSM did not start until mid-1991
The cellular structure
In a cellular system, the covering area of an operator is divided into cells. A cell corresponds to the covering area of one transmitter or a small collection of transmitters. The size of a cell is determined by the transmitter's power.
The concept of cellular systems is the use of low power transmitters in order to enable the efficient reuse of the frequencies. In fact, if the transmitters used are very powerful, the frequencies can not be reused for hundred of kilometers as they are limited to the covering area of the transmitter.
The frequency band allocated to a cellular mobile radio system is distributed over a group of cells and this distribution is repeated in all the covering area of an operator. The whole number of radio channels available can then be used in each group of cells that form the covering area of an operator. Frequencies used in a cell will be reused several cells away. The distance between the cells using the same frequency must be sufficient to avoid interference. The frequency reuse will increase considerably the capacity in number of users.
In order to work properly, a cellular system must verify the following two main conditions:
Cluster
The cells are grouped into clusters. The number of cells in a cluster must be determined so that the cluster can be repeated continuously within the covering area of an operator. The typical clusters contain 4, 7, 12 or 21 cells. The number of cells in each cluster is very important. The smaller the number of cells per cluster is, the bigger the number of channels per cell will be. The capacity of each cell will be therefore increased. However a balance must be found in order to avoid the interference that could occur between neighboring clusters. This interference is produced by the small size of the clusters (the size of the cluster is defined by the number of cells per cluster). The total number of channels per cell depends on the number of available channels and the type of cluster used.
Types of cells
The density of population in a country is so varied that different types of cells are used:
Microcells
These cells are used for densely populated areas. By splitting the existing areas into smaller cells, the number of channels available is increased as well as the capacity of the cells. The power level of the transmitters used in these cells is then decreased, reducing the possibility of interference between neighboring cells.
Selective cells
It is not always useful to define a cell with a full coverage
of 360 degrees. In some cases, cells with a particular shape and coverage are
needed. These cells are called selective cells. A typical example of selective
cells are the cells that may be located at the entrances of tunnels where a
coverage of 360 degrees is not needed. In this case, a selective cell with a
coverage of 120 degrees is used.
Umbrella cells
A freeway crossing very small cells produces an important number of handovers among the different small neighboring cells. In order to solve this problem, the concept of umbrella cells is introduced. An umbrella cell covers several microcells. The power level inside an umbrella cell is increased comparing to the power levels used in the microcells that form the umbrella cell. When the speed of the mobile is too high, the mobile is handed off to the umbrella cell. The mobile will then stay longer in the same cell (in this case the umbrella cell). This will reduce the number of handovers and the work of the network.
A too important number of handover demands and the propagation characteristics of a mobile can help to detect its high speed.
The transition from analog to digital technology Go to Top
In the 1980s most mobile cellular systems were based on analog systems. The GSM system can be considered as the first digital cellular system. The different reasons that explain this transition from analog to digital technology are presented in this section.
The capacity of the system
As it is explained in section 1, cellular systems have experienced a very important growth. Analog systems were not able to cope with this increasing demand. In order to overcome this problem, new frequency bands and new technologies were proposed. But the possibility of using new frequency bands was rejected by a big number of countries because of the restricted spectrum (even if later on, other frequency bands have been allocated for the development of mobile cellular radio). The new analog technologies proposed were able to overcome the problem to a certain degree but the costs were too important.
The digital radio was, therefore, the best option (but not the perfect one) to handle the capacity needs in a cost-efficiency way.
Compatibility with other systems such as ISDN
The decision of adopting a digital technology for GSM was made in the course of developing the standard. During the development of GSM, the telecommunications industry converted to digital methods. The ISDN network is an example of this evolution. In order to make GSM compatible with the services offered by ISDN, it was decide that the digital technology was the best option.
Additionally, a digital system allows,
easily than an analog one, the implementation of future improvements and the
change of its own characteristics.
Aspects of quality
The quality of the service can be considerably improved using a digital technology rather than an analog one. In fact, analog systems pass the physical disturbances in radio transmission (such as fades, multipath reception, spurious signals or interferences) to the receiver. These disturbances decrease the quality of the communication because they produce effects such as fadeouts, crosstalks, hisses, etc. On the other hand, digital systems avoid these effects transforming the signal into bits. This transformation combined with other techniques, such as digital coding, improve the quality of the transmission. The improvement of digital systems comparing to analog systems is more noticeable under difficult reception conditions than under good reception conditions.
The GSM network
Architecture of the GSM network
The GSM technical specifications define the different entities that form the GSM network by defining their functions and interface requirements.
The GSM network can be divided into four main parts:
Mobile Station
A Mobile Station consists of two main elements:
There are different types of terminals distinguished principally by their power and application:
The SIM is a smart card that identifies the terminal. By inserting the SIM card into the terminal, the user can have access to all the subscribed services. Without the SIM card, the terminal is not operational.
The SIM card is protected by a four-digit Personal Identification Number (PIN). In order to identify the subscriber to the system, the SIM card contains some parameters of the user such as its International Mobile Subscriber Identity (IMSI).
Another advantage of the SIM card is the mobility of the users. In fact, the only element that personalizes a terminal is the SIM card. Therefore, the user can have access to its subscribed services in any terminal using its SIM card.
The Base
Station Subsystem
The BSS connects the Mobile Station and the NSS. It is in charge of the transmission and reception. The BSS can be divided into two parts:
The BTS corresponds to the transceivers and antennas used in each cell of the network. A BTS is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between one and sixteen transceivers depending on the density of users in the cell.
The Base Station Controller
The BSC controls a group of BTS and manages their radio resources. A BSC is principally in charge of handovers, frequency hopping, exchange functions and control of the radio frequency power levels of the BTSs.
The Network and Switching Subsystem
Its main role is to manage the communications between the mobile users and other users, such as mobile users, ISDN users, fixed telephony users, etc. It also includes data bases needed in order to store information about the subscribers and to manage their mobility. The different components of the NSS are described below.
The Mobile services Switching Center (MSC)
It is the central component of the NSS. The MSC performs the switching functions of the network. It also provides connection to other networks.
The Gateway Mobile services Switching Center (GMSC)
A gateway is a node interconnecting two networks. The GMSC is the interface between the mobile cellular network and the PSTN. It is in charge of routing calls from the fixed network towards a GSM user. The GMSC is often implemented in the same machines as the MSC.
Home Location Register (HLR)
The HLR is considered as a very important
database that stores information of the subscribers belonging to the covering
area of a MSC. It also stores the current location of these subscribers and the
services to which they have access. The location of the subscriber corresponds
to the SS7 address of the Visitor Location Register (VLR) associated to the
terminal.
Visitor Location Register (VLR)
The VLR contains information from a subscriber's HLR necessary in order to provide the subscribed services to visiting users. When a subscriber enters the covering area of a new MSC, the VLR associated to this MSC will request information about the new subscriber to its corresponding HLR. The VLR will then have enough information in order to assure the subscribed services without needing to ask the HLR each time a communication is established.
The VLR is always implemented together with a MSC; so the area under control of the MSC is also the area under control of the VLR.
The Authentication Center (AuC) Go to Top
The AuC register is used for security purposes. It provides the parameters needed for authentication and encryption functions. These parameters help to verify the user's identity.
The Equipment Identity Register (EIR)
The EIR is also used for security purposes. It is a register containing information about the mobile equipments. More particularly, it contains a list of all valid terminals. A terminal is identified by its International Mobile Equipment Identity (IMEI). The EIR allows then to forbid calls from stolen or unauthorized terminals (e.g, a terminal which does not respect the specifications concerning the output RF power).
The GSM Interworking Unit (GIWU)
The GIWU corresponds to an interface to various networks for data communications. During these communications, the transmission of speech and data can be alternated.
The Operation
and Support Subsystem (OSS)
The OSS is connected to the different components of the NSS and to the BSC, in order to control and monitor the GSM system. It is also in charge of controlling the traffic load of the BSS.
However, the increasing number of base stations, due to the development of cellular radio networks, has provoked that some of the maintenance tasks are transfered to the BTS. This transfer decreases considerably the costs of the maintenance of the system.
The geographical areas of the GSM network
The figure 2 presents the different areas that form a GSM network.
As it has already been explained a cell,
identified by its Cell Global Identity number (CGI), corresponds to the radio
coverage of a base transceiver station. A Location Area (LA), identified by its
Location Area Identity (LAI) number, is a group of cells served by a single MSC/VLR.
A group of location areas under the control of the same MSC/VLR defines the MSC/VLR
area. A Public Land Mobile Network (PLMN) is the area served by one network
operator.
The GSM functions
In this paragraph, the description of the GSM network is focused on the different functions to fulfill by the network and not on its physical components. In GSM, five main functions can be defined:
Transmission
The transmission function includes two sub-functions:
Radio
Resources management (RR)
The role of the RR function is to establish, maintain and release communication links between mobile stations and the MSC. The elements that are mainly concerned with the RR function are the mobile station and the base station. However, as the RR function is also in charge of maintaining a connection even if the user moves from one cell to another, the MSC, in charge of handovers, is also concerned with the RR functions.
The RR is also responsible for the management of the frequency spectrum and the reaction of the network to changing radio environment conditions. Some of the main RR procedures that assure its responsabilities are:
Handover
The user movements can produce the need to change the channel or cell, specially when the quality of the communication is decreasing. This procedure of changing the resources is called handover. Four different types of handovers can be distinguished:
The mobile station is the active participant in this procedure. In order to perform the handover, the mobile station controls continuously its own signal strength and the signal strength of the neighboring cells. The list of cells that must be monitored by the mobile station is given by the base station. The power measurements allow to decide which is the best cell in order to maintain the quality of the communication link. Two basic algorithms are used for the handover:
The MM function is in charge of all the aspects related with the mobility of the user, specially the location management and the authentication and security.
Location management
When a mobile station is powered on, it performs a location update procedure by indicating its IMSI to the network. The first location update procedure is called the IMSI attach procedure.
The mobile station also performs location updating, in order to indicate its current location, when it moves to a new Location Area or a different PLMN. This location updating message is sent to the new MSC/VLR, which gives the location information to the subscriber's HLR. If the mobile station is authorized in the new MSC/VLR, the subscriber's HLR cancells the registration of the mobile station with the old MSC/VLR.
A location updating is also performed periodically. If after the updating time period, the mobile station has not registered, it is then deregistered.
When a mobile station is powered off, it performs an IMSI detach procedure in order to tell the network that it is no longer connected.
Authentication and security
The authentication procedure involves the SIM card and the Authentication Center. A secret key, stored in the SIM card and the AuC, and a ciphering algorithm called A3 are used in order to verify the authenticity of the user. The mobile station and the AuC compute a SRES using the secret key, the algorithm A3 and a random number generated by the AuC. If the two computed SRES are the same, the subscriber is authenticated. The different services to which the subscriber has access are also checked.
Another security procedure is to check the equipment identity. If the IMEI number of the mobile is authorized in the EIR, the mobile station is allowed to connect the network.
In order to assure user confidentiality, the user is registered with a Temporary Mobile Subscriber Identity (TMSI) after its first location update procedure.
Enciphering is another option to guarantee a very strong security but this procedure is going to be described later on.
Communication
Management (CM)
The CM function is responsible for:
Call Control (CC) Go to Top
The CC is responsible for call establishing, maintaining and releasing as well as for selecting the type of service. One of the most important functions of the CC is the call routing. In order to reach a mobile subscriber, a user dials the Mobile Subscriber ISDN (MSISDN) number which includes:
Supplementary Services management
The mobile station and the HLR are the only components of the GSM network involved with this function. The different Supplementary Services (SS) to which the users have access are presented in section 6.3.
Short Message Services management
In order to support these services, a GSM network is in contact with a Short Message Service Center through the two following interfaces:
Operation, Administration and Maintenance (OAM)
The OAM function allows the operator to monitor and control the system as well as to modify the configuration of the elements of the system. Not only the OSS is part of the OAM, also the BSS and NSS participate in its functions as it is shown in the following examples:
The GSM radio interface
The radio interface is the interface between the mobile stations and the fixed infrastructure. It is one of the most important interfaces of the GSM system.
One of the main objectives of GSM is roaming. Therefore, in order to obtain a complete compatibility between mobile stations and networks of different manufacturers and operators, the radio interface must be completely defined.
The spectrum efficiency depends on the radio interface and the transmission, more particularly in aspects such as the capacity of the system and the techniques used in order to decrease the interference and to improve the frequency reuse scheme. The specification of the radio interface has then an important influence on the spectrum efficiency.
Frequency allocation
Two frequency bands, of 25 Mhz each one, have been allocated for the GSM system:
Multiple access scheme
The multiple access scheme defines how different simultaneous communications, between different mobile stations situated in different cells, share the GSM radio spectrum. A mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA), combined with frequency hopping, has been adopted as the multiple access scheme for GSM.
FDMA and TDMA
Using FDMA, a frequency is assigned to a user. So the larger the number of users in a FDMA system, the larger the number of available frequencies must be. The limited available radio spectrum and the fact that a user will not free its assigned frequency until he does not need it anymore, explain why the number of users in a FDMA system can be "quickly" limited.
On the other hand, TDMA allows several users to share the same channel. Each of the users, sharing the common channel, are assigned their own burst within a group of bursts called a frame. Usually TDMA is used with a FDMA structure.
In GSM, a 25 Mhz frequency band is divided, using a FDMA scheme, into 124 carrier frequencies spaced one from each other by a 200 khz frequency band. Normally a 25 Mhz frequency band can provide 125 carrier frequencies but the first carrier frequency is used as a guard band between GSM and other services working on lower frequencies. Each carrier frequency is then divided in time using a TDMA scheme. This scheme splits the radio channel, with a width of 200 khz, into 8 bursts. A burst is the unit of time in a TDMA system, and it lasts approximately 0.577 ms. A TDMA frame is formed with 8 bursts and lasts, consequently, 4.615 ms. Each of the eight bursts, that form a TDMA frame, are then assigned to a single user.
Channel
structure
A channel corresponds to the recurrence of one burst every frame. It is defined by its frequency and the position of its corresponding burst within a TDMA frame. In GSM there are two types of channels:
Full-rate traffic channels (TCH/F) are defined using a group of 26 TDMA frames called a 26-Multiframe. The 26-Multiframe lasts consequently 120 ms. In this 26-Multiframe structure, the traffic channels for the downlink and uplink are separated by 3 bursts. As a consequence, the mobiles will not need to transmit and receive at the same time which simplifies considerably the electronics of the system.
The frames that form the 26-Multiframe structure have different functions:
Control channels
Go to Top
According to their functions, four different classes of control channels are defined:
Broadcast channels (BCH)
The BCH channels are used, by the base station, to provide the mobile station with the sufficient information it needs to synchronize with the network. Three different types of BCHs can be distinguished:
Common Control Channels (CCCH)
The CCCH channels help to establish the calls from the mobile station or the network. Three different types of CCCH can be defined:
Dedicated Control Channels (DCCH)
The DCCH channels are used for message exchange between several mobiles or a mobile and the network. Two different types of DCCH can be defined:
Associated Control Channels
The Fast Associated Control Channels (FACCH) replace all or part of a traffic channel when urgent signaling information must be transmitted. The FACCH channels carry the same information as the SDCCH channels.
Burst structure
As it has been stated before, the burst is the unit in time of a TDMA system. Four different types of bursts can be distinguished in GSM:
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The tail bits (T) are a group of three bits
set to zero and placed at the beginning and the end of a burst. They are used to
cover the periods of ramping up and down of the mobile's power.
The coded data bits corresponds to two groups, of 57 bits each, containing signaling or user data.
The stealing flags (S) indicate, to the receiver, whether the information carried by a burst corresponds to traffic or signaling data.
The training sequence has a length of 26 bits. It is used to synchronize the receiver with the incoming information, avoiding then the negative effects produced by a multipath propagation.
The guard period (GP), with a length of 8.25 bits, is used to avoid a possible overlap of two mobiles during the ramping time.
Frequency hopping Go to Top
The propagation conditions and therefore the multipath fading depend on the radio frequency. In order to avoid important differences in the quality of the channels, the slow frequency hopping is introduced. The slow frequency hopping changes the frequency with every TDMA frame. A fast frequency hopping changes the frequency many times per frame but it is not used in GSM. The frequency hopping also reduces the effects of co-channel interference.
There are different types of frequency hopping algorithms. The algorithm selected is sent through the Broadcast Control Channels.
Even if frequency hopping can be very useful for the system, a base station does not have to support it necessarily On the other hand, a mobile station has to accept frequency hopping when a base station decides to use it.
From source information to radio waves
The figure 4 presents
the different operations that have to be performed in order to pass from the
speech source to radio waves and vice versa.
If the source of information is data and
not speech, the speech coding will not be performed.
Speech coding
The transmission of speech is, at the moment, the most important service of a mobile cellular system. The GSM speech codec, which will transform the analog signal (voice) into a digital representation, has to meet the following criterias:
Channel coding
Channel coding adds redundancy bits to the original information in order to detect and correct, if possible, errors occurred during the transmission.
Channel coding for the GSM data TCH channels
Go to Top
The channel coding is performed using two codes: a block code and a convolutional code.
The block code corresponds to the block code defined in the GSM Recommendations 05.03. The block code receives an input block of 240 bits and adds four zero tail bits at the end of the input block. The output of the block code is consequently a block of 244 bits.
A convolutional code adds redundancy bits in order to protect the information. A convolutional encoder contains memory. This property differentiates a convolutional code from a block code. A convolutional code can be defined by three variables : n, k and K. The value n corresponds to the number of bits at the output of the encoder, k to the number of bits at the input of the block and K to the memory of the encoder. The ratio, R, of the code is defined as follows : R = k/n. Let's consider a convolutional code with the following values: k is equal to 1, n to 2 and K to 5. This convolutional code uses then a rate of R = 1/2 and a delay of K = 5, which means that it will add a redundant bit for each input bit. The convolutional code uses 5 consecutive bits in order to compute the redundancy bit. As the convolutional code is a 1/2 rate convolutional code, a block of 488 bits is generated. These 488 bits are punctured in order to produce a block of 456 bits. Thirty two bits, obtained as follows, are not transmitted :
C (11 + 15 j) for j = 0, 1, ..., 31
The block of 456 bits produced by the convolutional code is then passed to the interleaver.
Channel coding for the GSM speech channels
Before applying the channel coding, the 260 bits of a GSM speech frame are divided in three different classes according to their function and importance. The most important class is the class Ia containing 50 bits. Next in importance is the class Ib, which contains 132 bits. The least important is the class II, which contains the remaining 78 bits. The different classes are coded differently. First of all, the class Ia bits are block-coded. Three parity bits, used for error detection, are added to the 50 class Ia bits. The resultant 53 bits are added to the class Ib bits. Four zero bits are added to this block of 185 bits (50+3+132). A convolutional code, with r = 1/2 and K = 5, is then applied, obtaining an output block of 378 bits. The class II bits are added, without any protection, to the output block of the convolutional coder. An output block of 456 bits is finally obtained.
Channel coding for the GSM control channels
In GSM the signalling information is just contained in 184 bits. Forty parity bits, obtained using a fire code, and four zero bits are added to the 184 bits before applying the convolutional code (r = 1/2 and K = 5). The output of the convolutional code is then a block of 456 bits, which does not need to be punctured.
Interleaving
An interleaving rearranges a group of bits in a particular way. It is used in combination with FEC codes in order to improve the performance of the error correction mechanisms. The interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors. Being the errors less concentrated, it is then easier to correct them.
Interleaving for the GSM control channels
A burst in GSM transmits two blocks of 57 data bits each. Therefore the 456 bits corresponding to the output of the channel coder fit into four bursts (4*114 = 456). The 456 bits are divided into eight blocks of 57 bits. The first block of 57 bits contains the bit numbers (0, 8, 16, .....448), the second one the bit numbers (1, 9, 17, .....449), etc. The last block of 57 bits will then contain the bit numbers (7, 15, .....455). The first four blocks of 57 bits are placed in the even-numbered bits of four bursts. The other four blocks of 57 bits are placed in the odd-numbered bits of the same four bursts. Therefore the interleaving depth of the GSM interleaving for control channels is four and a new data block starts every four bursts. The interleaver for control channels is called a block rectangular interleaver.
Interleaving for the GSM speech channels
The block of 456 bits, obtained after the channel coding, is then divided in eight blocks of 57 bits in the same way as it is explained in the previous paragraph. But these eight blocks of 57 bits are distributed differently. The first four blocks of 57 bits are placed in the even-numbered bits of four consecutive bursts. The other four blocks of 57 bits are placed in the odd-numbered bits of the next four bursts. The interleaving depth of the GSM interleaving for speech channels is then eight. A new data block also starts every four bursts. The interleaver for speech channels is called a block diagonal interleaver.
Interleaving for the GSM data TCH channels
A particular interleaving scheme, with an interleaving depth equal to 22, is applied to the block of 456 bits obtained after the channel coding. The block is divided into 16 blocks of 24 bits each, 2 blocks of 18 bits each, 2 blocks of 12 bits each and 2 blocks of 6 bits each. It is spread over 22 bursts in the following way :
Burst assembling
The burst assembling procedure is in
charge of grouping the bits into bursts. Section 5.2.3 presents the different
bursts structures and describes in detail the structure of the normal burst.
Ciphering
Ciphering is used to protect signaling and
user data. First of all, a ciphering key is computed using the algorithm A8
stored on the SIM card, the subscriber key and a random number delivered by the
network (this random number is the same as the one used for the authentication
procedure). Secondly, a 114 bit sequence is produced using the ciphering key, an
algorithm called A5 and the burst numbers. This bit sequence is then XORed with
the two 57 bit blocks of data included in a normal burst.
In order to decipher correctly, the receiver has to use the same algorithm A5 for the deciphering procedure.
Modulation Go to Top
The modulation chosen for the GSM system is the Gaussian Minimum Shift Keying (GMSK).
The aim of this section is not to describe precisely the GMSK modulation as it is too long and it implies the presentation of too many mathematical concepts. Therefore, only brief aspects of the GMSK modulation are presented in this section.
The GMSK modulation has been chosen as a compromise between spectrum efficiency, complexity and low spurious radiations (that reduce the possibilities of adjacent channel interference). The GMSK modulation has a rate of 270 5/6 kbauds and a BT product equal to 0.3. Figure 5 presents the principle of a GMSK modulator.
Discontinuous transmission (DTX)
This is another aspect of GSM that could have been included as one of the requirements of the GSM speech codec. The function of the DTX is to suspend the radio transmission during the silence periods. This can become quite interesting if we take into consideration the fact that a person speaks less than 40 or 50 percent during a conversation. The DTX helps then to reduce interference between different cells and to increase the capacity of the system. It also extends the life of a mobile's battery. The DTX function is performed thanks to two main features:
Timing advance
The timing of the bursts transmissions is very important. Mobiles are at different distances from the base stations. Their delay depends, consequently, on their distance. The aim of the timing advance is that the signals coming from the different mobile stations arrive to the base station at the right time. The base station measures the timing delay of the mobile stations. If the bursts corresponding to a mobile station arrive too late and overlap with other bursts, the base station tells, this mobile, to advance the transmission of its bursts.
Power control
At the same time the base stations perform the timing measurements, they also perform measurements on the power level of the different mobile stations. These power levels are adjusted so that the power is nearly the same for each burst.
A base station also controls its power level. The mobile station measures the strength and the quality of the signal between itself and the base station. If the mobile station does not receive correctly the signal, the base station changes its power level.
Discontinuous reception
It is a method used to conserve the mobile station's power. The paging channel is divided into subchannels corresponding to single mobile stations. Each mobile station will then only 'listen' to its subchannel and will stay in the sleep mode during the other subchannels of the paging channel.
Multipath and equalisation
At the GSM frequency bands, radio waves reflect from buildings, cars, hills, etc. So not only the 'right' signal (the output signal of the emitter) is received by an antenna, but also many reflected signals, which corrupt the information, with different phases.
An equalizer is in charge of extracting the 'right' signal from the received signal. It estimates the channel impulse response of the GSM system and then constructs an inverse filter. The receiver knows which training sequence it must wait for. The equalizer will then , comparing the received training sequence with the training sequence it was expecting, compute the coefficients of the channel impulse response. In order to extract the 'right' signal, the received signal is passed through the inverse filter.
General
Packet Radio Service (GPRS)
Go to Top
The General Packet Radio System (GPRS) is a new service that provides actual packet radio access for mobile Global System for Mobile Communications (GSM) and time-division multiple access (TDMA) users. The main benefits of GPRS are that it reserves radio resources only when there is data to send and it reduces reliance on traditional circuit-switched network elements. The increased functionality of GPRS will decrease the incremental cost to provide data services, an occurrence that will, in turn, increase the penetration of data services among consumer and business users. In addition, GPRS will allow improved quality of data services as measured in terms of reliability, response time, and features supported. The unique applications that will be developed with GPRS will appeal to a broad base of mobile subscribers and allow operators to differentiate their services. These new services will increase capacity requirements on the radio and base-station subsystem resources. One method GPRS uses to alleviate the capacity impacts is sharing the same radio resource among all mobile stations in a cell, providing effective use of the scarce resources. In addition, new core network elements will be deployed to support the high burstiness of data services more efficiently.
GPRS (General Packet Radio Service) is a step between GSM and 3G cellular networks. GPRS offers faster data transmission via a GSM network within a range 9.6Kbits to 115Kbits. This new technology makes it possible for users to make telephone calls and transmit data at the same time. (For example, if you have a mobile phone using GPRS, you will be able to simultaneously make calls and receive e-mail massages.) The main benefits of GPRS are that it reserves radio resources only when there is data to send and it reduces reliance on traditional circuit-switched network elements. The GPRS infrastructure and mobile phones support a data transmission speed of up to 13.4Kbits per channel.
In addition to providing new services for today's mobile user, GPRS is important as a migration step toward third-generation (3G) networks. GPRS will allow network operators to implement an IP-based core architecture for data applications, which will continue to be used and expanded upon for 3G services for integrated voice and data applications. In addition, GPRS will prove a testing and development area for new services and applications, which will also be used in the development of 3G services.
A complete understanding of the application availability and GPRS timeline requires understanding of terminal types and availability. The term "terminal equipment" is generally used to refer to the variety of mobile phones and mobile stations that can be used in a GPRS environment; the equipment is defined by terminal classes and types. Cisco Gateway GPRS Serving Node (GGSN) and data network components interoperate with GPRS terminals that follow the GPRS standards.
A GPRS terminal can be one of three classes: A, B, or C. A Class A terminal supports GPRS and other GSM services (such as SMS and voice) simultaneously. This support includes simultaneous attach, activation, monitor, and traffic. As such, a Class A terminal can make or receive calls on two services simultaneously. In the presence of circuit-switched services, GPRS virtual circuits will be held or placed on busy rather than being cleared.
A Class B terminal can monitor GSM and GPRS channels simultaneously, but can support only one of these services at a time. Therefore, a Class B terminal can support simultaneous attach, activation, and monitor, but not simultaneous traffic. As with Class A, the GPRS virtual circuits will not be closed down when circuit-switched traffic is present. Instead, they will be switched to busy or held mode. Thus, users can make or receive calls on either a packet or a switched call type sequentially, but not simultaneously.
A Class C terminal supports only non-simultaneous attach. The user must select which service to connect to. Therefore, a Class C terminal can make or receive calls from only the manually (or default) selected service. The service that is not selected is not reachable. Finally, the GPRS specifications state that support of SMS is optional for Class C terminals.
In addition to the three variables, each handset will have a unique form factor. Some of the form factors will be similar to current mobile wireless devices, while others will evolve to use the enhanced data capabilities of GPRS.
The earliest available type will be closely related to the current mobile phone. These will be available in the standard form factor with a numeric keypad and a relatively small display.
PC Cards are credit card-sized hardware devices that connect via a serial cable to the bottom of a mobile phone. Data cards for GPRS phones will enable laptops and other devices with PC Card slots to be connected to mobile GPRS-capable phones. Card phones provide functionality similar to that offered by PC Cards, without needing a separate phone. These devices may need an earpiece and microphone to support voice services.
Smart phones are mobile phones with built-in voice, non-voice, and Web-browsing services. Smart phones integrate mobile computing and mobile communications into a single terminal. They come in various form factors, which may include a keyboard or an icon drive screen. The Nokia 9000 series is a popular example of this form factor.
The increase in machine-to-machine communications has led to the adoption of application-specific devices. These "black-box" devices lack a display, keypad, and voice accessories of a standard phone. Communication is accomplished through a serial cable. Applications such as meter reading utilize such black-box devices.
Personal digital assistants (PDAs) such as the Palm Pilot series or Handspring Visor are data-centric devices that are adding mobile wireless access. These devices can either connect with a GPRS-capable mobile phone via a serial cable or have GPRS capability built in.
A final category of GPRS terminals is handheld communications. Again, these are primarily data-centric devices that are adding mobile wireless access. Access can be gained via a PC Card or via a serial cable to a GPRS-capable phone.
From a high level, GPRS can be thought of as an overlay network onto a second-generation GSM network. This data overlay network provides packet data transport at rates from 9.6 to 171 kbps. Additionally, multiple users can share the same air-interface resources.
GPRS attempts to reuse the existing GSM network elements as much as possible, but in order to effectively build a packet-based mobile cellular network, some new network elements, interfaces, and protocols that handle packet traffic are required. Therefore, GPRS requires modifications to numerous network elements.
New terminals (TEs) are required because existing GSM phones do not handle the enhanced air interface, nor do they have the ability to packetize traffic directly. A variety of terminals will exist, as described in a previous section, including a high-speed version of current phones to support high-speed data access, a new kind of PDA device with an embedded GSM phone, and PC Cards for laptop computers. All these TEs will be backward compatible with GSM for making voice calls using GSM.
Each BSC will require the installation of one or more PCUs and a software upgrade. The PCU provides a physical and logical data interface out of the base station system (BSS) for packet data traffic. The BTS may also require a software upgrade, but typically will not require hardware enhancements.
When either voice or data traffic is originated at the subscriber terminal, 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.
In the core network, the existing MSCs are based upon circuit-switched central-office technology, and they cannot handle packet traffic. Thus two new components, called GPRS Support Nodes, are added:
The SGSN can be viewed as a "packet-switched MSC;" it delivers packets to mobile stations (MSs) within its service area. 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.
GGSNs are used as interfaces to external IP networks such as the public Internet, other mobile service providers' GPRS services, or enterprise intranets. GGSNs maintain routing information that is necessary to tunnel the protocol data units (PDUs) to the SGSNs that service particular MSs. Other functions include network and subscriber screening and address mapping. One (or more) GGSNs may be provided to support multiple SGSNs. More detailed technical descriptions of the SGSN and GGSN are provided in a later section.
Mobility management within GPRS builds on the mechanisms used in GSM networks; as a MS moves from one area to another, mobility management functions are used to track its location within each mobile network. The SGSNs communicate with each other and update the user location. The MS profiles are preserved in the visitor location registers (VLRs) that are accessible by the SGSNs via the local GSM MSC. A logical link is established and maintained between the MS and the SGSN in each mobile network. At the end of transmission or when a MS moves out of the area of a specific SGSN, the logical link is released and the resources associated with it can be reallocated
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).
States of GPRS in a Mobile Station
An MS has three states in the GPRS system: idle, standby, and active . The three-state model represents the nature of packet radio relative to the GSM two-state model (idle or active).
Data is transmitted between a 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. However, in the standby state, the location of the MS is known only as to which routing area it is in. (The routing area can consist of one or more cells within a GSM location area.)
When the SGSN sends a packet to a MS that is in the standby state, the MS must be paged. Because the SGSN knows the routing area in which the MS is located, a packet paging message is sent to that routing area. After receiving the packet paging message, the MS gives its cell location to the SGSN to establish the active state.
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 packet paging message is to simplify the process of receiving packets. The MS has to listen to only the packet paging messages, instead of all the data packets in the downlink channels, reducing battery use significantly.
When an MS has a packet to be transmitted, access to the uplink channel is needed. 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 packet random access message. The transmission of the packet random access message follows Slotted Aloha procedures. The BSS allocates an unused channel to the MS and sends a packet access grant message in reply to the packet random access message. The description of the channel (one or multiple time slots) is included in the packet access grant message. The data is transmitted on the reserved channels.
The main reasons for the standby state are to reduce the load in the GPRS network caused by cell-based routing update messages and to conserve the MS battery. When a MS is in the standby state, there is no need to inform the SGSN of every cell change—only of every routing area change. The operator can define the size of the routing area and, in this way, adjust the number of routing update messages.
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.
A cell-based routing update procedure is invoked when an active MS enters a new cell. In this case, the MS sends a short message containing information about its move (the message contains 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.
When an MS in an active or a standby state moves from one routing area to another in the service area of one SGSN, it must again 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.
The inter-SGSN routing update is the most complicated of the three routing updates. In this case, 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, as well as informing the GGSN about the new location of the MS.
KEY USER FEATURES OF GPRS
The General Packet Radio Service (GPRS) is a new non-voice value added service that allows information to be sent and received across a mobile telephone network. It supplements today’s Circuit Switched Data and Short Message Service. GPRS is NOT related to GPS (the Global Positioning System), a similar acronym that is often used in mobile contexts. GPRS has several unique features which can be summarized as:
SPEED
Theoretical maximum speeds of up to 171.2 kilobits per second (kbps) are achievable with GPRS using all eight timeslots at the same time. This is about three times as fast as the data transmission speeds possible over today’s fixed telecommunications networks and ten times as fast as current Circuit Switched Data services on GSM networks. By allowing information to be transmitted more quickly, immediately and efficiently across the mobile network, GPRS may well be a relatively less costly mobile data service compared to SMS and Circuit Switched Data.
IMMEDIACY
GPRS facilitates instant connections whereby information can be sent or received immediately as the need arises, subject to radio coverage. No dial-up modem connection is necessary. This is why GPRS users are sometimes referred to be as being "always connected". Immediacy is one of the advantages of GPRS (and SMS) when compared to Circuit Switched Data. High immediacy is a very important feature for time critical applications such as remote credit card authorization where it would be unacceptable to keep the customer waiting for even thirty extra seconds.
NEW APPLICATIONS, BETTER APPLICATIONS
GPRS facilitates several new applications that have not previously been available over GSM networks due to the limitations in speed of Circuit Switched Data (9.6 kbps) and message length of the Short Message Service (160 characters). GPRS will fully enable the Internet applications you are used to on your desktop from web browsing to chat over the mobile network. Other new applications for GPRS, profiled later, include file transfer and home automation- the ability to remotely access and control in-house appliances and machines.
SERVICE ACCESS
To use GPRS, users specifically need:
·
a mobile phone or terminal that supports GPRS·
a subscription to a mobile telephone network that supports GPRS·
use of GPRS must be enabled for that user. Automatic access to the GPRS may be allowed by some mobile network operators, others will require a specific opt-in·
knowledge of how to send and/ or receive GPRS information using their specific model of mobile phone, including software and hardware configuration (thiscreates a customer service requirement)
·
a destination to send or receive information through GPRS. Whereas with SMS this was often another mobile phone, in the case of GPRS, it is likely to be anInternet address, since GPRS is designed to make the Internet fully available to mobile users for the first time. From day one, GPRS users can access any web
page or other Internet applications- providing an immediate critical mass of uses. Having looked at the key user features of GPRS, lets look at the key features from a network operator perspective.
model of mobile phone, including software and hardware configuration .
KEY NETWORK FEATURES OF GPRS
Go to TopPACKET SWITCHING
GPRS involves overlaying a packet based air interface on the existing circuit switches
GSM network. This gives the user an option to use a packet-based data service. To supplement a circuit switched network architecture with packet switching is quite a major upgrade. However, as we shall see later, the GPRS standard is delivered in a very elegant manner- with network operators needing only to add a couple of new infrastructure nodes and making a software upgrade to some existing network elements. With GPRS, the information is split into separate but related "packets" before being transmitted and reassembled at the receiving end. Packet switching is similar to a jigsaw puzzle- the image that the puzzle represents is divided into pieces at the manufacturing factory and put into a plastic bag. During transportation of the now boxed jigsaw from the factory to the end user, the pieces get jumbled up. When the recipient empties the bag with all the pieces, they are reassembled to form the original image. All the pieces are all related and fit together, but the way they are transported and assembled varies. The Internet itself is another example of a packet data network, the most famous of many such network types.SPECTRUM EFFICIENCY
Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data. Rather than dedicating a radio channel to a mobile data user
for a fixed period of time, the available radio resource can be concurrently shared between several users. This efficient use of scarce radio resources means that large numbers of GPRS users can potentially share the same bandwidth and be served from a single cell. The actual number of users supported depends on the application being used and how much data is being transferred. Because of the spectrum efficiency of GPRS, there is less need to build in idle capacity that is only used in peak hours. GPRS therefore lets network operators maximize the use of their network resources in a dynamic and flexible way, along with user access to resources and revenues. GPRS should improve the peak time capacity of a GSM network since it simultaneously:·
allocates scarce radio resources more efficiently by supporting virtual connectivity·
migrates traffic that was previously sent using Circuit Switched Data to GPRS instead, and·
reduces SMS Center and signalling channel loading by migrating some traffic that previously was sent using SMS to GPRS instead using the GPRS/ SMS interconnect that is supported by the GPRS standards.INTERNET AWARE
For the first time, GPRS fully enables Mobile Internet functionality by allowing
interworking between the existing Internet and the new GPRS network. Any service that is used over the fixed Internet today- File Transfer Protocol (FTP), web browsing, chat, email, telnet- will be as available over the mobile network because of GPRS. In fact, many network operators are considering the opportunity to use GPRS to help become wireless Internet Service Providers in their own right. The World Wide Web is becoming the primary communications interface- people access the Internet for entertainment and information collection, the intranet for accessing company information and connecting with colleagues and the extranet for accessing customers and suppliers. These are all derivatives of the World Wide Web aimed at connecting different communities of interest. There is a trend away from storing information locally in specific software packages on PCs to remotely on the Internet. When you want to check your schedule or contacts, instead of using something like "Act!", you go onto the Internet site such as a portal. Hence, web browsing is a very important application for GPRS. Because it uses the same protocols, the GPRS network can be viewed as a sub-network of the Internet with GPRS capable mobile phones being viewed as mobile hosts. This means that each GPRS terminal can potentially have its own IP address and will be addressable as such.SUPPORTS TDMA AND GSM
It should be noted right that the General Packet Radio Service is not only a service
designed to be deployed on mobile networks that are based on the GSM digital mobile phone standard. The IS-136 Time Division Multiple Access (TDMA) standard, popular in North and South America, will also support GPRS. This follows an agreement to follow the same evolution path towards third generation mobile phone networks concluded in early 1999 by the industry associations that support these two network types.
LIMITATIONS OF GPRS
It should already be clear that GPRS is an important new enabling mobile data service
which offers a major improvement in spectrum efficiency, capability and functionality compared with today’s non-voice mobile services. However, it is important to note that there are some limitations with GPRS, which can be summarized as:LIMITED CELL CAPACITY FOR ALL USERS
GPRS does impact a network’s existing cell capacity. There are only limited radio
resources that can be deployed for different uses- use for one purpose precludes simultaneous use for another. For example, 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 signalling channel loading by sending short messages over GPRS channels instead. RESULT: NEED FOR SMS as a complementary bearer that uses a different type of radio resource.SPEEDS MUCH LOWER IN REALITY
Achieving the theoretical maximum GPRS data transmission speed of 172.2 kbps would
require a single user taking over all eight timeslots without any error protection. Clearly, it is unlikely that a network operator wil
