It is important to mention some of the basic concepts of mobile telephony (2G) in order to build and understand the ideas discussed hereafter.
2.1.1 Basic Network Architecture
BSC: A number of base stations are connected to and controlled by a Base Station Controller (BSC).The BSC manages the handoff of calls from one base station to another as subscribers move from cell to cell.
MSC: Mobile Switching Centre (MSC) is connected to the BSC. It is a switch that manages the set up and teardown of calls to and from mobile subscribers. In addition to numerous similarities with standard PSTN switch, it contains a number of functions that are specific to mobile communications. For example, it interacts with a number of BSCs, deals with mobile subscribers, and acts as an interface to one or more HLRs.
HLR: Home Location Register (HLR) contains subscription information related to a number of subscribers. It plays a critical role in mobility management, the tracking of a subscribers as he or she moves around the network. "As a subscriber moves from one MSC to another, each MSC in turn notifies the HLR. When a call is received from the PSTN, the MSC that receives the call queries the HLR for the latest information regarding the subscriber location so that the call could be correctly routed to the subscriber." 
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Figure 1 - Basic Network Architecture
The network depicted in Figure 1 can be considered as representing the bare minimum needed to provide a mobile telephony service. In addition, there are VLR and IN systems that deal with visiting subscribers and network intelligence (authentication, security, billing, etc.) respectively.
2.1.2 Cell and Sectors
The concept of cellular mobile telephony was the breakthrough in telecommunication technology. "Before the advent of cellular technology, capacity was enhanced through a division of frequencies and the resulting addition of available channels. However, this reduced the total bandwidth available to each user, affecting the quality of service. Cellular technology allowed for the division of geographical areas, rather than frequencies, leading to a more efficient use of the radio spectrum. This geographical re-use of radio channels is known as "frequency reuse." 
2.1.3 Air Interface Access Techniques
"Radio spectrum is a precious and finite resource. Unlike other transmission media such as copper or fiber facilities, it is not possible to simply add radio spectrum when needed. Only a certain amount of spectrum is available and it is critical that it be used efficiently, and be reused as much as possible. Such requirements are at the heart of the radio access techniques used in mobile communications." 
FDMA: Frequency Division Multiple Access (FMDA) chunks up available spectrum into predefined bandwidths and then assigns one to each user. The result is a scenario where the network has one user per band of frequency or per channel as shown in figure 3. The channel, therefore, is closed to other conversations until the initial call is finished or until it is handed off to a different channel. "In most FDMA systems, separate channels are used in each direction one from network to subscriber (downlink) and the other from subscriber to network (uplink). For example, in analog AMPS 30-kHz channels implies two 30-kHz channels, one in each direction. Such an approach is known as Frequency Division Duplex (FDD) and normally a fixed separation exists between the frequency used in the uplink and that used in the downlink. This fixed separation is known as the duplex distance." 
TDMA: "Time Division Multiple Access (TDMA) improves spectrum capacity by splitting each frequency into time slots. TDMA allows each user to access the entire radio frequency channel for the short period of a call. Other users share this same frequency channel at different time slots. The base station continually switches from user to user on the channel. TDMA was the dominant technology for the second generation mobile cellular networks." 
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CDMA: Originating from the spread spectrum technology which involves spreading the signal over a wide bandwidth, Code Division Multiple Access (CDMA) increases spectrum capacity by allocating the entire radio band to all users in a given instant of time. User communication (voice and data) is based on a unique code assigned to individual user. "CDMA allows for a 'soft hand-off,' which means that terminals can communicate with several base stations at the same time." 
Figure 2 - Access Technologies
Roaming converts a wireless communication network into a mobile network. "Mobility implies that subscribers be able to move freely around the network and from one network to another. This requires that the network tracks the location of a subscriber to certain accuracy so that calls destined for the subscriber may be delivered and allow him to do so while engaged in a call." 
It is defined as the ability of a subscriber to maintain a call while moving within the network. Handoff implies that a subscriber travels from one cell to another while engaged in a call, and that call is maintained during the transition (ideally without the subscriber noticing any change). Depending on the two cells in question, the handoff can be between two sectors on the same base station, between two channels within the same cell, between two BSCs, between two MSCs belonging to the same operator, or even between two networks.
2.2 First Generation of Mobile Communication "1G"
Based purely on analog communication, 1G marked the advent of wireless communication. The three major 1G technologies that gained popularity are:
AMPS: It began with the implementation of a trial system in Chicago in 1978 using a technology known as Advanced Mobile Phone Service (AMPS), operating in the 800-MHz band based on FDMA.
NMT: The Europeans 1G systems were launched in 1981 in Sweden, Norway, Denmark, and Finland using a technology known as Nordic Mobile Telephony (NMT), operating in the 450-MHz band. Later, another version was developed to operate in the 900-MHz band known as NMT900.
TACS: UK introduced a modified version of AMPS in 1985 called the Total Access Communications System (TACS) that operated 900-MHz band.
The success of 1G was beyond what anyone had expected. This very success eventually exposed shortcomings like limited capacity and vulnerability to fraud. "The systems were able to handle large number of subscribers, but when the subscribers started to number in millions, cracks started to appear, particularly since subscribers tend to be densely clustered in metropolitan areas. Consequently, significant effort was dedicated to the development of second-generation systems."