Introduction Computers and computer networks have changed the way in which we live, run our lives, communicate with each other and the way we work and produce what makes every commercial organisation function and reach success within its field, and in the same time, continue on the path of that success. The computers as stand-alone machines, or as separated ones, are nothing more than advanced computing machines, but what was required in reality is a way to link all the computers with each other and to allow users to have simultaneous access to databases and information; and this is why networks had to be created. Tanenbaum (2003, p.2) explains this fact by stating that “The merging of computers and communications has had a profound influence on the way computer systems are organized. The concept of the ‘computer centre' as a room with a large computer to which users bring their work for processing is now totally obsolete. The old model of a single computer serving all of the organization's computational needs has been replaced by one in which a large number of separate but interconnected computers do the job. These systems are called computer networks.” The main principle behind Computer networking is the communication between two or more computer systems. Computers within a network might be close to one another (such as the case with Bluetooth for example) or hundreds of kilometres away from each other (through the Internet). The first important step in this field came in 1984, when a completely digitalised, circuit-switched telephony system was introduced; this system was called ISDN; which stands for Integrated Services Digital Network for voice and non-voice data. After that, BellCore started developing the standard for the Synchronous Optical Network (SONET), and by the end of the 1980's, Local Area Networks (LAN) appeared as effective method of transferring data between a number of local computers, which led telephone companies replaces all its analogue multiplexing with digital multiplexing. But it is also essential to point out the element of the Internet; this international linked network, composed of servers and clients all over the world, encouraged the changes in both information technology and mobile computing, and this is why we find most of the indications, whenever we face a new product or application, referring to its characteristics in what concerns wireless connection, Bluetooth link, infrared, and much more. Raidl (2003, p.199) states that “mobile cellular networks are by far the most common of all public wireless communication systems. One of the basic principles is to re-use radio resources after a certain distance.” Walters and Kritzinger (2004) refer to the fact that mobile technology has turned to become one of the fastest, if not the fastest, growing field in the telecommunications industry. To give a clearer idea about the change brought to the world and to every one of us, we can refer to the comments of Furht and Ilyas (2003), as they state that “just a few years ago, the only way to access the Internet and the Web was by using wireline desktop and laptop computers. Today, however, users are traveling between corporate offices and customer sites, and there is a great need to access the Internet through wireless devices. The wireless revolution started with wireless phones and continued with Web phones and wireless handheld devices that can access the Internet”
Types of network Computer networks can vary according to the purpose for which they were created and depending on the area they are supposed to cover geographically. Computer networks can be one of the following: 1) LAN (Local Area Network) is “a small interconnection infrastructure that typically uses a shared transmission medium. Because of such factors as the volume of traffic, the level of security, and cost, the network structure in a local area network can be significantly different from that for a wide area network.” And “LAN is used for communications in a small community in which resources, such as printers, software, and servers, are shared. Each device connected to a LAN has a unique address. Two or more LANs of the same type can also be connected to forward data frames among multiple users of other local area networks” (Mir, 2007, p.102). 2) WAN (Wide Area Network) is “spans a large geographical area, often a country or continent. It contains a collection of machines intended for running user (i.e., application) programs” (Tanenbaum, 2003, p.19). 3) CAN (Campus Area Networks) “are the enterprise networks that serve number of related structure, as in a large company or a college campus.” Lehtinen, Gangemi, Gangemi Sr, and Russel, 2006, p.182). 4) MAN (Metropolitan Area Network) which “covers a city. The best-known example of a MAN is the cable television network available in many cities” (Tanenbaum, p.18). 5) HAN (Home Area Network) is “the connection of a number of devices and terminals in the home on to one or more networks which are themselves connected in such a way that digital information and content can be passed between devices and any access ‘pipe' to the home” (Turnbull & Garrett, 2003, p.46).
Cellular networks In their description of the first cellular radio networks in history, Walters and Kritzinger (2004) state that “in 1946, the first car-based telephone was set up in St. Louis, Missouri, USA. The system used a single radio transmitter on top of a tall building. A single channel was used, therefore requiring a button to be pushed to talk, and released to listen. This half duplex system is still used by modern day CB radio systems utilized by police and taxi operators. In the 1960s, the system was improved to a two-channel system, called the improved mobile telephone system (IMTS)... Cellular radio systems, implemented for the first time in the advanced mobile phone system (AMPS), support more users by allowing reuse of frequencies. AMPS is an analogue system, and is part of first generation cellular radio systems.” Even though it has become one of the most common and popular means of communication between people in the last years, cellular networks still have no specific definition; “Cellular communications has experienced explosive growth in the past two decades. Today millions of people around the world use cellular phones. Cellular phones allow a person to make or receive a call from almost anywhere. Likewise, a person is allowed to continue the phone conversation while on the move. Cellular communications is supported by an infrastructure called a cellular network, which integrates cellular phones into the public switched telephone network” (Zhang and Stojmenovic, 2005, p.654). This difficulty in finding a definition is due to the fact that there are different technologies and networking methods used within the frame of cellular networks. Frantz and Carley (2005, p.5) explain that “cellular networks are a distinct and important network topology. Although there is a growing body of work referring to cellular networks, there is no complete formal definition. However, there are several papers that seek to describe characteristics of cellular networks. Cellular networks are a critical topology to formally characterize, in part, as they are thought to be a common form for covert networks.” Yet, it is possible to find some kind of an explanation of such networks and how they operate: “Cellular networks use a networked array of transceiver base stations, each located in a cell to cover the networking services in a certain area. Each cell is assigned a small frequency band and is served by a base station. Neighbouring cells are assigned different frequencies to avoid interference. However, the transmitted power is low, and frequencies can be reused over cells separated by large distances” (Mir, 2006 p.42). A cellular network, for it to be considered a functional type of communication network, relies “on relatively short-range transmitter/ receiver (transceiver) base stations that serve small sections (or cells) of a larger service area. Mobile telephone users communicate by acquiring a frequency or time slot in the cell in which they are located. A master switching centre called the ‘mobile transport serving office' (MTSO) links calls between users in different cells and acts as a gateway to the PSTN” (Muller, 2003, p.50) Each cellular network is composed of what is can be referred to as Cells; which are defined by Frantz, and Carley, (2005) as “a distinct subgroup of actors within a larger cellular network. The presence of at least one cell is fundamental to a network's distinction of being cellular—without at least one cell, a network is not cellular. Empirically, a cell often consists of relatively few actors and has a distinct topology that is effortless to identify visually. The actors in a cell can be partitioned into two distinct but intertwined subgroups, namely the cell-core and the cell-periphery.” Muller (2003) explains that there are no specific sizes for cells within a cellular network, this is due to the fact that there are many factors that interfere in this element and according to the surrounding environment and obstacles can the cell's size be determined: “Cell boundaries are neither uniform nor constant. The usage density in the area, as well as the landscape, the presence of major sources of interference (e.g., power lines, buildings), and the location of competing carrier cells, contributes to the definition of cell size. Cellular boundaries change continuously, with no limit to the number of frequencies available for transmission of cellular calls in an area. As the density of cellular usage increases, individual cells are split to expand capacity. By dividing a service area into small cells with limited-range transceivers, each cellular system can reuse the same frequencies many times.” According to Muller (2003), a cellular network is composed also of a Master Switching Centre which “operates similar to a telephone central office and provides links to other offices. The switching centre supports trunk lines to the base stations that establish the cells in the service area." Another component is the transmission channels which are, in most cases, two kinds of channels; a control channel and a traffic channel. And, of course, to close the circle within this network, a cellular phone is needed; “cellular telephones incorporate a combination of multi-access digital communications technology and traditional telephone technology and are designed to appear to the user as familiar residential or business telephone equipment.” During their evolution and continuing enhancement, cellular networks went through consecutive levels of development; each of them added more power and functionality to the previous one. Zhang and Stojmenovic (2005, p.654) explain that cellular networks have had three stages that are called generations. The first of those generations is analogue in nature. Then, when more cellular phone subscribers needed to be connected and function simultaneously, digital TDMA (time division multiple access) and CDMA (code division multiple access) technologies appeared and were put to work; and this was the second stage or what is known as the second generation (2G) which was necessary in order to increase the capacity of the cellular network. “With digital technologies, digitized voice can be coded and encrypted. Therefore, the 2G cellular network is also more secure.” With the high importance of applications related to the Internet and their continuous growth, many users required more of the their cellular devices. Then the third generation (3G) arrived. 3G “integrates cellular phones into the Internet world by providing high speed packet-switching data transmission in addition to circuit-switching voice transmission. The 3G cellular networks have been deployed in some parts of Asia, Europe, and the United States since 2002 and will be widely deployed in the coming years.” There are some expectations regarding the future for what concerns the fourth generation wireless networks: “These will evolve towards an integrated system, which will produce a common packet-switched (possibly IP-based) platform for wireless systems, offering support for high-speed data applications and transparent integration with the wired networks” (Nicopolitidis, Obaidat, Papadimitriou and Pomportsis, 2003). Cellular networks make use of certain protocols in order to make communication easier between various entities within the limits of the network. A protocol of communication can be defined as a group of rules which correspond to messages that two or more entities communicate between each other within a network. Protocols used for cellular networks are included within the standard which is covering the service. The first and most popular standard for mobile phones is GSM (Global System for Mobile communications). Other standards are CDMA and TDMA. Another important point concerning cellular networks is what can be called Location Management, which is essential for the network to monitor every registered mobile station's location so that the mobile station can be able to connect to the network upon request. It is important to note the similarities between cellular networks and Wireless LANs, but it is also worthwhile noticing the differences between the two: “Goals for third-generation wireless communication, enunciated in the early 1990s by the International Telecommunications Union Task Group IMT-2000, focused on the first two criteria, bit rate and mobility. Third-generation systems should deliver 2 Mbps to stationary or slowly moving terminals, and at least 144 kbps to terminals moving at vehicular speeds. Meanwhile, WLAN development has confined itself to communications with low-mobility (stationary or slowly moving) terminals, and focused on high-speed data transmission. The relationship of bit rate to mobility in cellular and WLAN systems has been commonly represented in two dimensions” (Furht and Ilyas, 2003, p.33).
Wireless data applications With the continuous growth of mobile devices, different services were created in order to widen the range of the functionality of those devices. For such devices to be able to use the newly offered services, specific types of applications had to be created and deployed or installed on the mobile device, may it be a cell-phone, PDA, or a notebook computer. “Wireless data services use a mix of terrestrial and satellite-based technologies to meet a wide variety of local (in building or campus settings), metropolitan, regional, national, and international communication needs... A number of wireless data applications, in fact, are being designed with fixed users in mind” (Office of Technology Assessment, 1995). To be able to understand how wireless data applications work, it is necessary to have a comprehensive view concerning their delivery methods; as a matter of fact, there are two main delivery methods: “There are two fundamental information delivery methods for wireless data applications: point-to-point access and broadcast. In point-to-point access, a logical channel is established between the client and the server. Queries are submitted to the server and results are returned to the client in much the same way as in a wired network. In broadcast, data are sent simultaneously to all users residing in the broadcast area. It is up to the client to select the data it wants” (Zomaya, 2002) Wireless data applications can be divided into two main groups: Messaging and Remote Access. “Messaging applications can generally tolerate low throughput and long transmission delays. Electronic mail (e-mail) often fits this category, but not always, messages with attached files may strain the capacity of wireless messaging networks,” then there is Remote access which is required to allow access to the resources and services of a network from outside the geographical barriers of the physical establishment of that network (Brodsky, 1997).
Conclusion Throughout this paper, understanding the information presented within it fully, it is accurate to state that a cellular network is definable correctly by presenting the following: “We define a cellular network as a single-component and undirected network of actors and their relationships, strictly consisting entirely of actors who are members of a specific cell, as previously defined; thus a network in which all actors are a member of a cell. For a network to be considered cellular, these conditions must be met: (a) the ties making up the relations in the network may only be undirected, (b) the network consists of a single component, e.g., there are no isolate actors, and (c) the network consists solely of cell subgroups that are connected via spanning ties, e.g., there are no actor in the network who is not a member of a cell subgroup” (Frantz and Carley, 2005, p.10) As for wireless data applications, in 1997 Brodsky stated that if such application are to become widespread and popular exactly as the simple mobile phones were in the end of the 1990s, users should become “readily and reliably send and receive data over paging, cellular and PCs”. And as we can see today, that phase is exactly what we experience today; ten years after the author wrote those words.
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