9.1 Introduction

In 1979, the World Administrative Radio Conference (WARC‐79) suggested the allocation primarily of the 806–902 MHz frequency band along with the allocation of the 470–512 MHz and 614–806 MHz bands on a secondary basis for land mobiles [1]. The cellular mobile radio became operational for civilian use in the 1980s in various countries across the world. Although the number of users was only in thousands, which is insignificant in terms of today’s user base, still it was a time when the cellular network kept improving to overcome various challenges faced by the cellular mobile technology of the age.

European countries were among the first users of the cellular radio network, with a hundred thousand users in Finland, Denmark, Norway, and Sweden where the operations began in 1981. Bahrain has been reported as the first country to have cellular radio operations in mid‐1978 through the Bahrain Telephone Company, followed by Tokyo in Japan where the operations started at the end of 1978 through the Nippon Telegraph and Telephone Public Corporation. Other cities in Japan followed soon thereafter. There were about 30 000 mobile radio customers in Tokyo during the early 1980s, and 40 000 users in total in the USA by 1984, where the cellular radio was made operational in about 25 US cities. Chicago followed by Washington DC and Baltimore were the first three cities in the USA, where operations started in 1983. Frequency modulation was used in two‐way radio communication.

Although most countries allocated bands in the 800 MHz region of the spectrum, there were countries operating cellular radio in the 400/450 MHz band also. There was a great variation in the radius of the cell, ranging from 15 to 75 km [2]. The Advanced Mobile Phone Service (AMPS) was developed and used in the USA, while the Total Access Communications System (TACS) was adopted in the UK. Some of the other systems [3] in use during that period were Nordic Mobile Telephone (NMT) in Scandinavia (Sweden, Norway, Finland, Denmark, and Iceland), UNITAX in China, Network C in Germany, the Radio Telephone Mobile System (RTMS) in Italy, the Nippon Automatic Mobile Telephone System (NAMTS) in Japan, Radiocom 2000 (R 2000) in France, and a few other systems.

9.2 Basics of Cellular Wireless Networks

The cellular wireless network is an infrastructure‐based wireless network that has emerged as a challenging platform for distributed applications and distributed computing. The cellular network is a terrestrial telecommunication infrastructure. It supports mobile communication with cellular architecture [4] for the coverage area. Coverage areas may or may not overlap. Cellular coverage areas have a telecom infrastructure within them, as well as connecting all other cells so as to enable process calls between various cells. The cellular network can be ‘intrasystem’, wherein all the cells are under a single administrative control with distributed switching. Alternatively, the cellular network can be ‘intersystem’, wherein different cellular networks under the administrative control of different operators are networked together for extension of the range, and these support interoperability among themselves. Although it might not be required or feasible to have coverage of the entire geographic area by having overlapping cells, still a cellular network with coverage in terms of overlapping cells is much better for handling applications, security, and call management in mobile communication.

The performance of a cellular network is measured in terms of voice quality, call failures, and spectrum efficiency. The voice quality demanded is equivalent to that of wired telephony. Mobility may lead to variation in voice quality, and increase in distance from the base station can cause voice impairment due to signal fading. Sometimes a call cannot be set up owing to non‐availability of channels or signaling errors, leading to ineffective attempts. At times ongoing calls may also be aborted because of signal fading, improper handover, or cochannel interference. The geographic distribution of traffic is highly uneven, leading to call failures in regions with dense traffic or regions with sparse cellular coverage. All mobile communication resources, primarily the bandwidth, are very costly, and hence spectrum efficiency measured in terms of number of subscribers per cell is an important measure of cost [5]. However, increase in spectrum efficiency beyond a limit leads to reduction in service quality.

Each cell has a limited capacity that it can handle, and if the number of users in a cell goes beyond the capacity, blocking of certain calls will occur. However, cellular wireless networks are highly scalable, as they are implemented through a number of small cells, which not only avoids single points of failure but also allows the establishment of microcells to support areas with higher user density. Increase in the number of cells based on demand and use also helps in the dynamic distribution of users across cells. The cell infrastructure can also be established on a mobile platform where the cellular tower is on wheels to support a region with higher call density or during disaster management or in areas that cannot support a permanent infrastructure. The high scalability helps the cellular wireless network to cover huge geographic regions. The smaller cells have to use low‐power antennas. Nearness to cellular antennas also saves the transmission power of the mobile devices. Reduced power consumption makes the operations of the cellular wireless network cheaper and affordable. Further, smaller cells help in better reuse of frequency and reduce interference among cells because the adjacent cells have different frequencies.

As the communication in the cellular network is infrastructure based and wireless, it brings along with it associated problems and challenges. The cell towers can generally withstand wind speeds of about 150–200 km/h. Cyclones, hurricanes, and tornadoes can damage the towers. Sandstorms and hail/snow storms can damage the antennas, leading to disruption in service. Compared with wired networks, the transmission of voice and data is not only slow but also costlier in wireless networks. The signal quality or voice quality is also dependent on the location owing to a number of factors such as fading, absorption, attenuation, interference, and handover. Mobile devices inside speeding vehicles or trains gain high mobility, which can be challenging in terms of signal strength, handover, changing topology, and routing. Cellular transmission has to be in the open and in broadcast mode. This not only leads to privacy issues, but the signal can also be hindered by jamming.

Each cell in a cellular wireless network is supported by an infrastructure having an antenna. This antenna with associated infrastructure is known as the base transceiver station (BTS) or the base station. The base station has a transmitter, receiver, and controller with a small range of frequency assigned to it. A number of base stations generally equidistant from each other are installed in the region for uniform coverage of the area. However, in certain cases, special cell sizes are in use, which might be much larger or smaller than the standard cell size, and the antennas are at varying distances in these cells.

A base station controller (BSC) connects a number of base transceiver stations (BTS), and a number of BSCs are connected to a mobile service switching center (MSC). The MSCs are connected to each other on a backhaul public switched telephone network (PSTN). The interconnections between BTS and BSC and between BSC and MSC are also on a wired network or microwave link.

The height of the base station on which the antenna is installed and the power of the antenna are calculated on the basis of location and requirements. Athough the antennas are generally installed on a metallic or concrete tower over the land, they are sometimes installed over rooftops, water tanks, street light poles, or hilltops. Each BSC controls a number of BTSs and is responsible for channel allocation to the BTSs, detection of signal strength of mobile devices, and handovers between the BTSs.

The adjacent cells in the cellular network have different frequencies to avoid interference in overlapped areas. The power level of the transmitter at the base station of the cell has to be optimized to avoid the presence of the signal across neighboring cells, which can lead to interference and cross‐talk. The number of cells also has to be increased to increase the geographical coverage. Each cell has a predefined frequency band and hence a fixed capacity in terms of the number of mobile devices within the cell that can simultaneously communicate. Thus, the cell size has to be reduced to increase the number of cells in a region to support a greater number of devices within the same area. The commonly used techniques to increase the capacity of a cell in the cellular network are as follows: frequency borrowing from adjacent cells, splitting a cell into smaller cells, sectoring, and zone microcells.

As the number of cells increases by cell splitting, it not only increases the capacity of the system, but also reduces the power consumption of the mobile devices and the base station of each cell because the microcell has to operate at a lower radiated power level and signal strength. However, as indicated in Figure 9.1, cell splitting reduces the coverage region of each cell, leading to an increase in the number of handovers for a mobile device that is not stationary.

In sectoring, a cell is covered by directional antennas instead of omnidirectional antennas. Depending on the angle of coverage of the directional antenna, the number of sectors in a cell can be calculated. Generally, a cell is divided into sectors of 120° or 60°, as depicted in Figure 9.2. In sectoring, the size of the cell remains the same, but as each sector has its own channel, which is a subset of the channel of the cell, it does not help in reducing the number of handovers. Sectoring is complex to implement as a number of antennas have to be managed at the base station, and there is deterioration in performance owing to reflections from various structures in an urban environment, leading to interference in the sectors. Sometimes, instead of having 3–6 sectors in the cell to cover the entire 360°, a cell might have only one or two unidirectional antennas for wedge‐shaped coverage for some specific purpose such as linking towards a valley from a hill or coverage over a bridge or inside a train or tunnel. Such cells are known as ‘selective cells’. Figure 9.3 depicts selective cells of 120° to provide coverage only to a highway crossing an uninhabited forest.

Zone microcells help to prevent dead or no‐signal zones within a cell. Dead zones can be inside tunnels, basements, in the shadow area of a hill, in areas with reflectors, or in buildings with thick walls. Antennas are fitted in such microcells inside the cell to provide transmission signals to these dead zones. Microcells thus created by using additional antennas use the same frequency as the cell to which they belong. There can be a number of microcells within a cell, and all the microcells should have their own antenna. The antennas of the microcells in the cell are generally connected to a base station through a wired network. Increase in the number of antennas in a zone microcell makes the control operation in the base station a complex activity. The handover between microcells is also handled by the base station.

Based on the area of coverage of the cell, it may be termed a macrocell, a microcell, a picocell, or a femtocell. Normally, the size of a cell in a cellular wireless network is 5–15 km in an urban environment, and can reach up to 35 km in rural and obstruction‐free regions. Cells with a coverage region greater that the normal cell sizes are referred to as macrocells. A macrocell generally has a coverage of 10 km or more. Macrocells are used in sparsely populated areas and as a means to connect two densely populated areas separated by long distances but with network coverage between them throughout the path.

Microcells have a diameter of about 1–2 km and are used in areas with dense population to increase the capacity of the cell. Picocells, with a coverage of 200–500 m, are used in specific areas such as airports, railway stations, tunnels, basements, bridges, or alleys. The range of a femto cell is about 10 m for personal communication or to provide services in indoor areas or at the edge of the cell.

Frequency reuse. Hundreds of millions of mobile devices have to use the limited frequency range allocated to the cellular wireless network. Each cell is allocated a small frequency range known as a channel. The channels of two adjacent cells cannot be the same, as this could lead to information distortion or packet drop due to mutual interference, which is also known as cochannel interference. Still, frequencies are reused innumerous times with only a constraint that the frequency of the adjacent cells should not be the same. There can be a number of reuse patterns, one of which, commonly referred as the seven‐cell reuse pattern, is shown in Figure 9.4. Frequency reuse allows increase in the capacity of the cellular network and makes it highly scalable to enhance the coverage area of the cellular network.

The transmitter at the base station has limited power, leading to a fixed coverage area, which is also known as the ‘footprint’. The boundary of a cell is not very well defined or sharp, as the signal strength falls slowly over a region, leading to areas with overlapping signals from neighboring cells. As shown in Figure 9.5, the area of a cell is not perfectly circular, but has an irregular pattern, as the signal in different directions may encounter different types of obstruction and terrain, leading to variation in the signal strength in different directions at equal distance from the antenna.

Fading. In a cellular wireless network, the signals are broadcast in all directions and hence the same signal, in addition to direct reception, may reach a particular receiver at different times on account of a number of factors such as reflection, diffraction, scattering, or propagation at different rates owing to a change in medium [6]. These multiple signals increase the signal‐to‐noise ratio, cause attenuation, and can cancel each other as a result of the phase difference leading to the weakening of signal strength. Fading can affect all frequencies equally, or different frequencies can be affected in different proportions.

Handover. Owing to mobility, the location of the mobile device keeps changing and may frequently move out of the range of one cell and enter into the range of another. As the cells are overlapping, as shown in Figure 9.6, the transfer of the mobile device from one cell to another is decided by weakening of the signal strength from one base station and strengthening of the signal from the other base station. This leads the base station to inform the MSC, which then transfers the control and reroutes the call of the mobile device to the base station of the new cell which has the mobile device in its range. There is also a change in the communication channel, as the adjacent cells have different channel frequencies. The process of handover is completed in milliseconds, so that the transmission remains uninterrupted and the process remains transparent to the user of the mobile device. Handover is also referred to as ‘handoff’. Sometimes the mobile device remains in the overlapping cell area with almost equal signal strength from both base stations. In such scenarios, there might be frequent handover of the mobile device between these two base stations in the overlapping cell, and such a phenomenon is known as ‘ping‐ponging’.

Two different threshold parameters are conventionally used to trigger handover – the increased power level used in the mobile device to receive the deteriorated signal or the signal strength (or signal‐to‐interference ratio) [7]. However, the present‐day cellular wireless networks use a number of other parameters for effective decision‐making with respect to cost, QoS, and performance to trigger handover. Some of these additional parameters are available bandwidth, delay, preferred service provider, signal decay, network interface, link capacity, link cost, power consumption, loss rate, packet delay, jitters, and policies [8‐11]. Artificial intelligence and fuzzy logic [12, 13] are also used in decision‐making regarding handovers.

A handover can also occur within the same cell when the mobile device changes the channel inside the cell. Apart from this, the handover can be intra‐BSC, inter‐BSC, or inter‐MSC. In intra‐BSC handover, the mobile device moves from one cell to another cell where the base stations of the two cells are connected to the same BSC. In the case of inter‐BSC handover, the mobile device moves to a new cell from an existing cell such that the base stations of the two cells are connected to two different BSCs. If these two BSCs are connected to the same MSC, this is also known as intra‐MSC handover. A special scenario in inter‐BSC handover is when these two BSCs are in different MSCs, and in such a case it is known as inter‐MSC handover.

The handovers are classified into two categories – hard handover and soft handover. In the case of hard handover, the mobile device releases the channel of the existing cell and thereafter starts using the channel of the new cell. Thus, there is an intermediate break in the signal during changeover from the existing cell to the new cell, hence also referred to as ‘break‐before‐make’. When the mobile device does not release the existing channel but takes on the channel of the new cell, retaining both channels together for some time before releasing the channel of the initial cell, the process is known as soft handover. As there is no instant of time when no channel is available, this phenomenon is also called ‘make‐before‐break’ and is used in the CDMA or 3G networks where the neighboring cells operate on the same frequency, enabling soft handover.

Increase in the number of cells in a region enhances the number of mobile devices that can be supported in the region and reduces the cell size. It also leads to an increase in the number of handovers per unit distance travelled, as the cells are smaller in size. The increase in the number of handovers increases the time delay as well as the requirement of buffers in the base station. The increase in the number of handovers also increases the time spent by the base station and the MSC in processing the handovers. The communication between the base station and the mobile switching centers also increases the time to process the handovers. However, movement of the mobile devices across microcells does not require handover, as all the microcells use the same frequency.

Umbrella cells are used to handle mobile devices that frequently change cells. Consider a thickly populated area served by a good transportation system, e.g. port areas, railway stations, or a market place with good road connectivity. As the number of persons per unit space is high in these areas, these areas have to be served by small cells so as to enhance the capacity of the cellular network. However, there are mobile devices inside the transportation system, i.e. vehicles, steamers, and trains, which move at a high speed in these areas, crossing a number of cells in a very small timeframe. This leads to very frequent handovers for these few specific devices travelling at high speed. To cater for the high mobility requirement of such devices in densely populated areas, umbrella cells are implemented. The base station in the umbrella cell uses high‐power antennas and hence has a much wider coverage area than a few microcells that lie inside the region of the umbrella cell. As the mobility of a device increases, it is handed over from the microcell to the umbrella cell, and thus the handovers for this high‐speed mobile device are reduced, as shown in Figure 9.7.

Multihop cellular network. The basic architecture of a cellular wireless network is based on the single hop, as shown in Figure 9.8a, where the voice or data reaches the mobile device in a single hop from the base station. Two mobile devices in the same cell or in different cells also communicate with each other through the base stations. Unlike an ad hoc network, the mobile devices do not directly communicate with each other. The multihop mobile network [14] optimizes the channel utilization and the number of base stations by incorporating the characteristics of an ad hoc network within the cellular wireless network.

In a multihop cellular network, as shown in Figure 9.8b, two mobile devices within the same cell directly communicate with each other if they are in the transmission range or communicate through other intermediate mobile devices in the cell instead of routing the communication through the base station. If the communicating devices are in different cells as shown in Figure 9.9, then the packet is routed between them through the base stations. However, the packet may not be reachable directly from the source mobile device to its base station, and a few intermediate mobile devices in the cell might be used to forward the packet from the source mobile device to its base station. Similarly, the destination mobile device may not be reachable in a single hop from its base station, and a few intermediate mobile devices in its cell may be used for multihop routing.

A multihop cellular network not only reduces the number of base stations, it also increases the number of simultaneous calls that can be supported within a single cell. Not all calls between the mobile device and base station utilize the channel allocated to the base station. The mobile devices can directly communicate with each other, and there can be a number of simultaneous calls between various mobile devices without utilizing the channel available with the base station.

Communication channel. There are two types of communication channel between the mobile device and the cell infrastructure (the base station) – the traffic channel and the control channel. The traffic channel is used for transmission of voice calls and data transfer between users through the intermediate cell infrastructure. The control channel is used for exchange of information for synchronizing, setting up, maintaining, and disconnecting the call. The control channel handles timing information, phase reference, signal strength, system identification information, and messaging between mobile stations. Interference in the control channel can lead to dropped calls, problems in handovers, or error calls. Error detection and correction information as well as power control information are carried by the traffic channel. The control channel can also be passed through the data channel once the call has been established.

The frequency used for communication from the cellular base station to the master station or from the cellular base station to the mobile device is known as the forward channel or downlink. The frequency used for communication from the master station to the base station or from the mobile device to the base station is known as the reverse channel or uplink. The frequency of the forward channel is different from the frequency of the reverse channel.

9.3 Resource Allocation

The resources in the wireless cellular network can be classified as radio resources and device resources. Although the base station is a consumer as well as a creator of resources, we assume it to possess sufficient power and other resources and hence not to be constrained. The major radio resources are power, code, and bandwidth. The major device resources are battery power and signal strength (receive as well as transmit). The use of resources is optimised for longer use by least battery consumption with reception of QoS signals, as well as to maximize the revenue of the service provider. The neighboring cells also cooperate with each other for resource sharing, as the resources are equally distributed across all cells, but the density of the users may vary widely across the cells.

Traditionally, the resource allocation has been time aware with traffic‐dependent pricing. As the traffic varies with time, with higher demand during peak hours, time‐varying resource allocation is implemented. Certain less‐priority traffic such as updates can be planned for running during less congested traffic hours when the resource prices and QoS requirements are low.

The resource allocation is generally content based. The cellular wireless traffic contents are of two different types – elastic and inelastic traffic [15]. The inelastic traffic pertains to those applications and services that require real‐time flow of voice or data between the source–destination pair. These are generally interactive applications and emergency notifications. A minimum bandwidth, throughput, and QoS have to be ensured for inelastic traffic for the entire duration of the connectivity.

The elastic traffic pertains to those delay‐tolerant applications and services for which best‐effort delivery is attempted for the recipients of voice and data. This type of traffic generally pertains to applications and services performing firmware updates, messaging services, email delivery and Internet browsing.

The category of application where variable bit rate is acceptable and thus the stringent inelastic traffic pattern may be eased leads to an intermediate or semi‐elastic traffic. A video conference held between mobile devices through applications such as Skype, Hangouts or FaceTime is an example of semi‐elastic traffic. Although real‐time traffic is given priority over delay‐tolerant traffic, this does not mean that the elastic traffic gets dropped on account of deprivation of resources.

Resource estimation and allocation become more challenging in a cellular wireless network because it is not a closed system with reference to any cell. A base station has to ensure maximum resource utilization to ensure revenue generation through channel utilization. Bandwidth is the most costly resource in the cellular wireless network. However, the base station has also to keep an optimum level of resources available for handovers from neighboring cells to this base station because a minimum level of success rate for handovers has to be ensured. There may be a large number of active users in the neighboring cell with high resource utilization and inelastic traffic. These users in the neighboring cells may move on to this cell, and the handover of such calls should not be dropped for the lack of availability of sufficient resources as successful handover may be one of the QoS parameters. For the reserved resources, handover calls get preference over any new connection requests because once a mobile device has been granted resources, the rejection is unacceptable [16, 17]. However, the resources cannot be kept reserved for all the ongoing calls in the neighboring cells. For such a case, the requirement will grow manifold and outnumber by a few times the existing resource utilization by the cell as there will be a number of neighboring cells based on the frequency reuse pattern and the radio power. Thus, other parameters such as location of the mobile device, speed, direction, and expected call duration also play a vital role in utilization of reserved resources.

A slow‐moving mobile device creates another typical situation when it is in the overlap area of two base stations [18]. Assume it is connected to one of the base stations and may be one of the high‐resource‐utilization mobile devices. Now, when this mobile device comes into the overlap area, the neighboring cell gets an indication that the mobile device might move from its existing cell to the new cell. Hence, this new cell keeps its resources reserved for this mobile device so that a successful handover can be performed. However, as this mobile device may have very slow mobility speed, it may remain in the overlapped area for a very long time, utilizing the resources of one cell and keeping equivalent resources in the other cell reserved for it.

The requirement becomes more stringent if the QoS is not based on the average of all connections but has to be ensured for each individual connection. The criterion for accepting a mobile device from a neighboring cell is different for voice and data requirements. Voice connections have to ensure a through‐and‐through non‐blocking connectivity, while data connections have to comply with maximum permissible average packet drop and packet delay.

Being in wireless medium, the granted resources are rarely equal to the requested resources. There are losses due to packet drops, attenuation, shadowing, fading, noise, interference, and handovers. Fairness in resource allocation [19] cannot be achieved by equally distributing the resources across all mobile devices, as the nearer devices require less transmit power than the devices at the edge of the cell.

9.4 Routing in GSM Networks

The Global System for Mobiles (GSM) is an international standard for the public land mobile network (PLMN). The standard was published in 1990 and its commercial use started in 1991. The standard belongs to the second generation of cellular networks. It uses digital systems with error detection and correction and encryption for security. The first generation of cellular networks was based on analog systems. The common services provided by the GSM standard are voice telephone, data services, and messaging services. It has also defined supplementary services such as call forwarding, call waiting, conference call, call barring, and caller identification. GSM is based on time division multiple access (TDMA), which operates at a frequency band of around 900 MHz in Asia and Europe and 1900 MHz in America.

In GSM, the user information is contained in the subscriber identity module (SIM). The SIM is small in size and can be swapped across various mobile phones or other GSM devices. The user can change the mobile handset by simply taking out the SIM from the old device and putting it in the new mobile device. The SIM is not only used in handheld mobile devices but also in fixed or portable communication terminals. The power output of the terminal varies with resource availability by virtue of its size and mobility. The power output of a fixed GSM terminal can be 20 W or more, and that of a handheld mobile terminal is generally below 1 W.

The SIM is an integrated circuit that can operate at three different operating voltages: 5, 3, and 1.8 V. There are eight pins in the SIM to provide an interface with the device in which it is inserted. The pin configuration is shown in Table 9.1.

The SIM is available in four different form factors. The first form factor is the full‐size SIM, the others being mini‐SIM, micro‐SIM, and nano‐SIM. The embedded SIM, which is much smaller than a nano‐SIM, is also in use. The internal architecture of a SIM comprises memory, microprocessor, I/O interface, and internal bus. The memory is in the form of RAM, ROM, and EEPROM. The memory of a SIM varies between 32 and 128 kB. This memory stores a number of pieces of information that can be categorized as user‐stored information and GSM or SIM information. The user‐stored information comprises the contact details, SMSs, phone settings, and last dialed numbers. The GSM‐ or SIM‐related information stored in the memory comprise the personal identification number (PIN), authentication key, encryption key, available PLMNs, integrated circuit card identifier (ICCID), location identity, and international mobile subscriber identity (IMSI).

The service provider uniquely identifies each SIM by the IMSI that is mapped to the mobile number and thus uniquely identifies the subscriber. IMSI is generally a 15 digit number that can be even smaller, as its size is governed by the regulations in specific countries. The first three digits represent the mobile country code (MCC), the next two or three digits represent the mobile network code (MNC), and the remaining digits store the mobile subscriber identity number (MSIN). The MCC uniquely identifies a country. In the case of countries with a huge number of mobile service providers, one country may be represented by more than one MCC. MNC uniquely identifies a service provider within a country. Thus, there can be two different service providers in two different countries with the same MNC. The combination of MCC and MNC uniquely identifies the mobile service provider. The 10 digit MSIN is also known as the mobile identification number (MIN) and is the unique 10 digit mobile phone number.

9.4.1 Architecture

The GSM network as depicted in Figure 9.10 comprises three major subsystems – the base station subsystem (BSS), the network and switching subsystem (NSS) and the GPRS core network.

The BSS connects the mobile device to the NSS for access to the telephone network and to the GPRS core network for Internet access. The BSS consists of base transceiver stations (BTSs), the base station controller, and the packet control unit (PCU). Each BTS comprises a radio receiver, a transmitter and an antenna, creating one cell in the cellular network. If a BTS uses a directional antenna, it can create a number of cells. The location of the antenna defines the center of the cell and the transmitting power of the antenna defines the radius of the cell. A BSC controls numerous BTSs or cells. The BSC is responsible for managing the network resources, frequency hopping, allocation of radio channels, and power level to BTSs, managing handovers between the BTSs within the BSS, and connecting to the NSS through the terrestrial channel. The PCU handles the data processing for the BSC and connects it to the GPRS core network.

The NSS interconnects the mobile users among themselves and with the users of the terrestrial fixed‐line telephone. The core of the NSS is the mobile switching center (MSC). The NSS also has a few databases to store user information related to subscription and mobility – home location register (HLR), visitor location register (VLR), equipment identity register (EIR), and authentication center (AUC). The databases may be maintained within the MSC or as separate powerful servers in the case of a large subscriber base.

The MSC performs mobile registration, authentication of users, call routing for roaming, and inter‐BSS call handovers.

HLR contains subscribers’ identity record, the list of services hired by the subscriber, and the location updates to support roaming. Each record is semi‐permanent in nature and is retained until the subscriber is registered with the service provider.

VLR contains information about the BSC to which a mobile device is connected for all the mobile devices in the range of various cells controlled by the MSC. Whenever a new mobile device enters the range of the MSC, the entry of its location area identity (LAI) is made in the VLR and the same is informed to the HLR of the mobile device to enable update of the location information by the HLR and receive data about the mobile device from the HLR. Some other major data corresponding to the mobile device stored in the VLR are the HLR address of the mobile device, its IMSI, the subscribed services, and authentication data. Whenever a mobile device enters within the area of control of an MSC, the VLR informs the HLR and also keeps track of the location of the mobile device with respect to its cell location even when the device in inactive. When the mobile device moves from the area of one MSC to another MSC, the VLR transfers the data to the other VLR in the new MSC. The VLR database is temporary in nature as the database is reset every day.

Mobile equipment is uniquely identified by its international mobile equipment identity (IMEI). Further, each mobile handset has its make and model, which define various functionalities available in the handset. The EIR contains information about the mobile handset being used by the mobile device. The IMEI is also used to prevent the use of stolen, fake, or unauthorized devices in the network for secured use of the cellular system.

The AUC has encryption keys for providing authentication and secured wireless communication.

The GPRS core network provides connectivity and routing of data packets from the GSM network to the Internet. It also keeps a record of the amount of data transmitted per user. The interconnectivity between the GPRS network and the Internet is provided by the gateway GPRS support node (GGSN). The serving GPRS support node (SGSN) provides GPRS data connectivity to mobile devices and is responsible for authentication, mobility management, and tunneling of data packets from mobile devices to GGSN.

9.5 Challenges in Mobile Computing

Mobile phones have become so widespread as devices of common utility that the number of mobile phones worldwide is more than double the number of landline phones or the number of personal computers or Internet connections in use in the world, and more than double the TVs or credit cards in use. This indicates the pace of growth and popularity of the mobile phone, which along with its use for voice calls also has computing capabilities through the availability of a processor and memory in the phone. The mobile phone can be used these days for voice and video communication, website access, email communication, and running different utility applications (apps) and various other utility applications that may use the processing capabilities and memory of the mobile phone with limited or no requirement for mobile connectivity. This breed of computing devices brings a new set of rules and restrictions, common among which are the restricted user interface, cheaper computation and storage, but limited and costly bandwidth. Application development for mobile computing is governed by the communication requirement and driven by innovation and consumer requirements [20]. The new areas of applications introduced by mobile computing include mobile commerce, location‐based services, and dual‐mode operations.

Storage. Data stored in a mobile phone can be categorized into communication‐related data (contacts, SMSs, call logs), configuration data, Internet data (browser cache, email), sound (ringtones, music), calendar (reminder, schedules, tasks), and files (photos, ebooks). The data is stored either in file systems or databases, both of which have been designed specifically for use and storage in the memory of mobile devices which is primarily NAND flash. The flash memory used in mobile phones is non‐volatile and similar to that used in digital cameras, memory cards (SD cards), USB drives/storages, and ultrathin laptops. The flash drives may be based on NAND gates or NOR gates and can be termed as a category of electrical erasable and programmable read only memory (EEPROM), which is non‐volatile. Although flash memory is compact, energy efficient and inexpensive, it has a finite number of write/erase cycles and writes/erases in blocks, as the write operation can only change 1 s to 0 s and hence the complete block has to be changed from 0 s to 1 s during erase. Such erase cycles of converting 0 s to 1 s is longer for NOR flash than for NAND flash, which rules out NOR flash for a lot of applications. The difference between NAND flash and NOR flash is indicated in Table 9.2.

As the software is not updated frequently in a mobile device, the code requires random access, and the storage requirement is much less than the data, these requirements make the costlier NOR flash suitable for storing the code.

The number of write/erase cycles is finite in flash memory and restricted to ten thousand to a million cycles, depending on the type of memory. Thus, suitable writing mechanisms have to be devised for traditional file system and relational databases (RDBMS) to distribute write across the sectors for wear leveling. To overcome these challenges, file systems for use in mobile computing have been devised, such as TrueFFS (True Flash File System), YAFFS (Yet Another Flash File System), JFFS (Journaling Flash File System), and JFFS2, some of which are proprietary while many are open source. With regard to databases for use in mobile devices, either the traditional RDBMS can be retrofitted for mobile devices or new databases optimized for use in flash can be built from scratch to reduce serialization. Excessive modification in the traditional databases to optimize for use in mobile computing may bloat the code and make the database inefficient. Some of the common RDBMSs used in mobile devices are Apache Derby, Sybase iAnywhere, SQLite, and Berkley DB.

Synchronization. Based on update characteristics, the mobile data can be put into two categories – the data that is created and deleted only in the mobile device such as SMS, logs, emails, ringtones, photos, videos, and music, and the data that is not only created and deleted but also updated in the mobile device such as contacts, codes, configuration data, calendar, operating system, and apps data. Some of this second category of data requires synchronization between the data stored in the mobile device and that stored in the servers of the service provider. The data sync can either be required for maintaining the same copy of the data at both ends for codes, configuration, contacts, or calendar, or just version control at the server end and the entire data or software in the mobile device as in the case of the operating system or applications. Synchronization is a complex process in mobile computing owing to unreliable and costly communication channels, and it is supported by change logs to detect the amount of change, date of change, and hash functions to trace similarity.

The sync can either be for the files or for an application. It can either be from the server to the mobile device or bidirectional. The logs have to be maintained in the servers as well as the mobile devices. The logs may contain a variety of information pertaining to creation, deletion, or change in terms of size, timestamps, and hash. The process of sync is termed fast if it takes place between a mobile device and a server, while it is termed slow if the mobile device has to sync with a number of servers. As the mobile devices are power and resource constrained, the sync process confines maximum processing at the server end. In slow syncs, too, the mobile device syncs to a server and the server in turn syncs with other clients and servers.

The hash‐based sync does not require any historical logs and can also be used for detection of substring similarity using rolling hash. Synchronization of code is more complex than data synchronization. Patches should be available for differences in version. However, the patches should be small and reusable for execution over a number of devices. The patches should be highly compressed to save on bandwidth. Further, there is no resource constraint at the server end to compress the patch, but it becomes challenging to decompress the patch at the mobile device with limited resources. The patches should be installable in the limited disk space of the mobile device and capable of fast, flawless, and trace‐free rollback on failure.

Updates. The mechanism for sending updates and notification to the mobile phone should be highly scalable as it has to reach millions of devices. The mechanism generally does not require user intervention, and the service provider or any other server asynchronously sends the update or notification to the mobile device. Some common examples of such notifications and updates are emails, SMSs and IP calls. These notifications should be small to save radio battery but less complex to decode in order to save computational requirements. The latency should be low and scalability should be high. The mechanism should be secure to avoid impersonation, obfuscation, and channel safety and to ensure policy‐compliant firewall passage. The version management should be error free with provision for roll‐back. There are three different mechanisms through which updates are received in mobile devices:

• Round‐the‐clock connectivity. The devices use supporting protocols such as Hypertext Transfer Protocol (HTTP) or Transmission Control Protocol (TCP) to remain connected to the server on a 24 × 7 basis or may connect‐disconnect frequently to receive any data from the server. As the number of devices is very large, this leads to a high number of connections between the server and the devices, which becomes challenging for the protocol to support, and most of the connections remain idle.
• Polling. The device checks for any update from the server after every ‘t’ seconds. By changing the value of ‘t’, the frequency of the polling can be varied to provide high scalability or reduce unwanted polls in the case of rare updates.
• Queuing on inbound port. There are three variants to this methodology of receiving updates on a dedicated port. The device gets a dedicated server socket to which it connects periodically to get an update. The server may also contact the client using SMS or UDP connection. For bulky updates, SMS and UDP may not be suitable, and hence the server uses the ‘poke‐n‐pull’ mechanism to send updates to the device. The server, when it has an update to send to the device, pokes the device using SMS or UDP, following which the device establishes a TCP connection to receive the actual update. These mechanisms ensure highest scalability.

Broadcast. The communication from server to mobile devices is a one‐to‐many communication that is in broadcast mode, while the communication from mobile devices to server is a one‐to‐one communication. Hence, the broadcasts are asymmetric communications with a further complexity of time synchronization, as all the mobiles may not be ON or in the communication range during the broadcast by the server. Thus, broadcast‐on‐demand may be more suitable than live broadcast for version upgrade and code update in mobile devices. The live broadcast may, however, be used for certain information‐related applications in the mobile device, such as streaming information related to news, weather, or traffic. A few other challenges with broadcast, some of which are typical of a mobile computing network, are lack of acknowledgement from the device, each device starts listening to the broadcast at different points in time, and the packets dropped by each device may be different owing to interruption, coverage, jamming, or resource constraint. This requires repeated broadcast of the data in cycles by the server to enable mobile devices to pick up again the relevant data that was dropped or a block of which was deleted in the storage. The retransmission of updates consumes ‘n’ times the bandwidth than a one‐time broadcast, where ‘n’ is the number of times the broadcast has to be looped. Further, the mobile device has to wait for one complete cycle to get the dropped data. Waiting by the mobile device until the last unwanted packet also leads to loss of power owing to active radio listening to the broadcast.

To ensure redundancy in the broadcast data and save waiting time of the devices, multiple tracks may be broadcast by using broadcast disks or data carousel. Erasure codes may also be used where the server pads the broadcast with redundant data, which helps the devices to reconstruct the dropped data from the padded data. The forward error correcting codes encode the message from ‘n’ blocks to ‘n’ + ‘x’ blocks, where x < < n to help to reconstruct the erased data. The value of x decides the bandwidth efficiency of the erasure code. Some of the commonly used erasure codes are Reed Solomon code [21, 22], Tornado codes, and LT codes [23–25].

Device management. Mobile devices are the newest and most popular type of computing equipment that is nearest to the human attention span. The applications in the devices find everyday use, and hence users drive innovation leading to regular updates over the air. The data received by the devices relates to software version upgrade, firmware upgrade, application data, new installations, access codes, and diagnostic data. For effective device management, it is essential to have information about the protocol stack running in the device, details of the device, and details of all the enablers running in the device, such as security overlay, authorization framework, browser, data synchronization, management schedule, domain name system, navigation, identity management, location awareness, services interface, and APIs.

The authorization for device management is held by a number of stakeholders, such as the service provider, equipment manufacturer, application provider, operating system/firmware developer, processor manufacturer, Government, employer/parent, and the user. Each of the stakeholders has to manage its own part of the equipment or service in the device, such as incorporation of new features, bug fixing, monitoring, and service provisioning, and hence wants access control in device management. The device management updates should be atomic and are performed in three phases: (i) identification and authentication, (ii) downloading, and (iii) update. In the case of a failed device update, it should be capable of rolling back to the stable state previous to the failure. The device update is challenging as it should ensure sufficient battery availability before the start of the process, availability of the device in the home network (and not on roaming), memory limitations, and variation in the previous version number in the devices that are undergoing updates.

References

1. 1 WARC‐79 overview, actions, and impacts, Chapter 4, in Radiofrequency Use and Management: Impacts from the World Administrative Radio Conference of 1979, Congress of the US Office of Technology Assessment, 1982.
2. 2 T. E. Bell. Communications: Highlights include the AT&T divestiture, a new long‐distance record in fiber‐optic communications, local‐area networks, and cellular radio. IEEE Spectrum, 22(1):56–59, 1985.
3. 3 D. M. Barnes. The introduction of cellular radio in the United Kingdom. 35th IEEE Vehicular Technology Conference, Volume 35, pp. 147–152, 1985.
4. 4 J. J. McCarthy and G. E. Marco. Cellular networking functionality & application. 37th IEEE Vehicular Technology Conference, Volume 37, pp. 305–311, 1987.
5. 5 J. Whitehead. Cellular system design: an emerging engineering discipline. IEEE Communications Magazine, 24:8–15, 1986.
6. 6 Cellular wireless networks, Chapter 14, in Data and Computer Communications, 8th edition, William Stallings.
7. 7 J. Kim, E. Serpedin, D. R. Shin, and K. Qaraqe. Handoff triggering and network selection algorithms for load‐balancing handoff in CDMA‐WLAN integrated networks. EURASIP Journal on Wireless Communications and Networking, 2008(136939):14, 2008.
8. 8 Q. Zhang, C. Guo, Z. Guo, and W. Zhu. Efficient mobility management for vertical handoff between WWAN and WLAN. IEEE Communications Magazine, 41(11):102–108, 2003.
9. 9 H. J. Wang, R. H. Katz, and J. Giese. Policy‐enabled handoffs across heterogeneous wireless networks. 2nd IEEE Workshop on Mobile Computing Systems and Applications (WMCSA ’99), pp. 51–60, New Orleans, LA, USA, 1999.
10. 10 L. J. Chen, T. Sun, B. Chen, V. Rajendran, and M. Gerla. A smart decision model for vertical handoff. 4th ANWIRE International Workshop on Wireless Internet and Reconfigurability (ANWIRE ’04), Athens, Greece, 2004.
11. 11 S. Balasubramaniam and J. Indulska. Vertical handover supporting pervasive computing in future wireless networks. Computer Communications, 27(8):708–719, 2004.
12. 12 Y. Nkansah‐Gyekye and J. I. Agbinya. Vertical handoff decision algorithm for UMTS‐WLAN. 2nd International Conference on Wireless Broadband and Ultra Wideband Communications (AusWireless ’07), Sydney, Australia, 2007.
13. 13 H. Liao, L. Tie, and Z. Du. A vertical handover decision algorithm based on fuzzy control theory. 1st IEEE International Multi‐Symposiums on Computer and Computational Sciences (IMSCCS ’06), pp. 309–313, Hangzhou, China, 2006.
14. 14 Y. D. Lin and Y. C. Hsu. Multihop cellular: a new architecture for wireless communications. 19th Annual Joint Conference of the IEEE Computer and Communications Societies, Volume 3, pp. 1273–1282, 2000.
15. 15 R. Madan, S. P. Boyd, and S. Lall. Fast algorithms for resource allocation in wireless cellular networks. Transactions on Networking, IEEE/ACM, 18(3):973–984, 2010.
16. 16 D. A. Levine, I. F. Akyildiz, and M. Naghshineh. A resource estimation and call admission algorithm for wireless multimedia networks using the shadow cluster concept. IEEE/ACM Transactions on Networking, 5(1):1–12, 1997.
17. 17 J. Tajima and K. Imamura. A strategy for flexible channel assignment in mobile communication systems. IEEE Transactions on Vehicular Technology, 37:92–103, 1988.
18. 18 S. G. Choi, O. S. Yang, and J. K. Choi. An efficient resource allocation scheme during handoff in mobile wireless networks. 7th International Conference on Advanced Communication Technology, ICACT, Volume 2, pp. 988–992, 2005.
19. 19 L. Wayne and Y. Pan (eds). Resource Allocation in Next Generation Wireless Networks, Volume 5, Nova Publishers, 2006.
20. 20 S. Roy. Lecture Notes in Mobile Computing. IIT, Kharagpur, 2008.
21. 21 I. S. Reed and G. Solomon. Polynomial codes over certain finite fields. SIAM Journal of Applied Mathematics, 8:300–304, 1960.
22. 22 B. Sklar. Digital Communications: Fundamentals and Applications, Prentice Hall, 2nd edition, 2001.
23. 23 J. W. Byers, M. Luby, M. Mitzenmachert, and A. Rege. A digital fountain approach to reliable distribution of bulk data. ACM SIGCOMM Computer Communication Review, 28(4):56–67, 1998.
24. 24 J. W. Byers, M. Luby, and M. Mitzenmachert, A digital fountain approach to asynchronous reliable multicast. IEEE Journal on Selected Areas in Communications, 20(8):1528–1540, 2002.
25. 25 M. Luby and L. T. Codes, Foundations of Computer Science. 43rd Annual IEEE Symposium, pp. 271–280, 2002.

Abbreviations/Terminologies

3G

Third‐Generation (networks)

AMPS

Advanced Mobile Phone Service

apps

(mobile) Applications

AUC

Authentication Center

BSC

Base Station Controller

BSS

Base Station Subsystem

BTS

Base Transceiver Station

CDMA

Code Division Multiple Access

EEPROM

Electrically Erasable Programmable Read‐Only Memory

EIR

Equipment Identity Register

GGSN

Gateway GPRS Support Node

GPRS

General Packet Radio Service

GSM

Global System for Mobiles

HDD

Hard Disk Drive

HLR

Home Location Register

HTTP

Hypertext Transfer Protocol

ICCID

Integrated Circuit Card Identifier

IEEE

Institute of Electrical and Electronics Engineers

IMEI

International Mobile Equipment Identity

IMSI

International Mobile Subscriber Identity

JFFS

Journaling Flash File System

LAI

Location Area Identity

MCC

Mobile Country Code

MIN

Mobile Identification Number

MNC

Mobile Network Code

MSC

Mobile Switching Center

MSIN

Mobile Subscriber Identity Number

NAMTS

Nippon Automatic Mobile Telephone System

NMT

Nordic Mobile Telephone

NSS

Network and Switching Subsystem

PCU

Packet Control Unit

PIN

Personal Identification Number

PLMN

Public Land Mobile Network

PSTN

Public Switched Telephone Network

QoS

Quality of Service

RAM

Random Access Memory

RDBMS

Relational Database Management System

ROM

Read‐only Memory

RTMS

Radio Telephone Mobile System

SD card

Secure Digital Card (memory)

SGSN

Serving GPRS Support Node

SIM

Subscriber Identity Module

SMS

Short Message Service

TACS

Total Access Communications System

TCP

Transmission Control Protocol

TDMA

Time Division Multiple Access

TrueFFS

True Flash File System

VLR

Visitor Location Register

WARC

World Administrative Radio Conference

YAFFS

Yet Another Flash File System

Questions

1. Why is the cellular wireless network termed an ‘infrastructure‐based network’?
2. Mention the advantages and disadvantages of a cellular wireless network.
3. Why does the high user density in a region require cell splitting?
4. With the help of a diagram, explain the concept of various cell sizes.
5. Describe the process of handover in a cellular wireless network. With reference to movement between the same or different BSCs or MSCs., what are the different types of handover?
6. Why is a software handover also termed ‘make‐before‐break’?
7. Explain the process of handling high‐mobility mobile devices using umbrella cells.
8. Draw the broad architecture of a GSM network, properly marking all the components and subsystems.
9. How is a voice call routed in a GSM‐based cellular network?
10. Explain the different mechanisms for receiving updates in a mobile device.
11. Differentiating between NAND flash and NOR flash, explain the applications where each can be used.
12. In the area of a cellular wireless network, ‘Communication is the primary driver, not computation’. Please justify this statement.
13. Write short notes on the following:
    1. cochannel interference,
    2. call handover,
    3. selective cells,
    4. frequency reuse,
    5. network and switching subsystems,
    6. international mobile subscriber identity,
    7. device databases,
    8. device management,
    9. flash memory,
    10. data synchronization.
14. Differentiate between the following:
    1. intrasystem and intersystem cellular networks,
    2. base transceiver station and base station controller,
    3. 60° cell sectoring and 120° cell sectoring,
    4. cell splitting and cell sectoring,
    5. inter‐BSC handover and intra‐BSC handover,
    6. hard handover and soft handover,
    7. single‐hop and multihop cellular networks,
    8. traffic channel and control channel,
    9. forward channel and reverse channel,
    10. elastic traffic and inelastic traffic.
15. For the following, mark the option that is correct:
    1. The performance of a cellular network can be measured in terms of:
       • (a) voice quality, (b) call failure, (c), spectrum efficiency, (d) all of these.
    2. The capacity of a cell in a cellular network cannot be increased by:
       • (a) sectoring, (b) frequency borrowing, (c) scattering, (d) cell splitting.
    3. A parameter that can trigger handover is:
       • (a) speed of the mobile device, (b) signal strength, (c) movement of the mobile device into a cell overlap area, (d) moving out of line‐of‐sight of the BTS.
    4. The same broadcast signal reaches a point at different times owing to:
       • (a) diffraction, (b) scattering, (c) reflection, (d) any of these.
    5. The area or shape of a real cell is:
       • (a) irregular, (b) hexagonal, (c) circular, (d) square.

Exercises

1. A hexagon cell has a radius of 20 km. The cell has to be split into equisized smaller hexagonal microcells. The minimum number of microcells should be created to cover the cell without any overlapping. What will the radius of each microcell be? The microcell has to be further split to one more level to create picocells under the same splitting conditions as for microcells. What will the radius of each picocell be?
2. When did the cellular radio operations start in your city, who was the service provider, what was the radius of each cell, how many voice call charges were there per minute, and what was the approximate number of users? As of today, who are the service providers in your city, what is the average radius of each cell, how many voice call charges are there per minute, and what is the approximate number of users?
3. Present‐day humans carry a number of interconnected devices with them, and these devices communicate with each other through a personal area network. The transmission range of the devices is 5 m. A number of similar devices used by different people operate on the same frequency and hence interference should be avoided. In a convention hall of 50 m × 100 m, how many persons can be accommodated if the transmission frequency of their devices is not known and collision/interference has to be avoided?
4. In a convention hall of 50 m × 100 m with a height of 4 m, transmission antennas can be fitted on the walls and roof. Mounting of antennas on walls is preferred, and hence a roof‐mounted antenna should be used only if a wall‐mounted antenna is not feasible. Each antenna has a range of 5 m. What is the minimum number of antennas required if the entire convention hall has to be under cellular wireless network cover without any non‐covered area? How many of the antennas will be on the roofs and how many of the antennas will be on the walls? The reflection of the transmission signals may be ignored.
5. Please refer to exercise 4 above. Assume that it has been said that there should be no areas with overlapping signals, even if certain areas remain outside the coverage of the cellular network. However, such uncovered areas should be minimized. All the other conditions of mounting the antennas remain the same. How many of the antennas will be on the roofs and how many of the antennas will be on the walls?
6. Attempt a frequency reuse pattern with less than seven cells as well as a frequency reuse pattern with more than seven cells. Any other uniform shape of the cell other than a hexagon may also be assumed if required.
7. Try to identify all the digits of the IMSI with respect to your mobile phone number. Also identify the IMEI of the mobile device and the form factor of the SIM used.
8. A cell switches on in ‘roaming’ and then starts moving and reaches its home network. Indicate the initial value and changes in the values of various registers used in the GSM network. Necessary assumptions may be made.