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Second-generation (2G) cellular networks marked the beginning of digital mobile cellular communications. A combination of factors gave rise to the development of these technologies. First, the success of the original analog cellular networks, retroactively dubbed first generation (1G), proved that the newly developed cellular industry was indeed a profitable sector of wireless radio communications. Second, the spectral inefficiency of 1G networks combined with a growing subscriber base meant that the existing cellular networks were physically constrained by the available spectrum-meaning that, by the late 1980s, 1G networks were quickly approaching the saturation point at which technology limitations would prevent network providers from supplying the user capacity to meet growing market demand. The 2G cellular standards generally approached the need for increased spectral efficiency by using time division multiple access (TDMA) methods to multiplex multiple users onto a single carrier frequency. In addition to increased spectral efficiency, the switch to digital transmission technology brought along other benefits. Advances in signal processing and compression coupled with powerful error control coding techniques allowed for improved speech quality in harsh channel conditions. Improved digital speech codecs eventually led to landline-quality voice, which was a key improvement that precipitated the trend of using the mobile phone as a replacement forrather than a supplement to-the landline phone in the home. Digital transmission also allowed for improved security through the use of encryption and digital security algorithms. This was a tremendous advantage in digital cellular networks, as eavesdropping and phone cloning (i.e., reproducing another subscriber's credentials, usually captured over the air, for the purposes of illegal service access without charge) were major drawbacks of 1G networks that proved very costly to cellular network operators in the 1990s. Advances in semiconductor technology also made it possible for pocket-sized mobile handsets to replace their brick-sized predecessors. All of these improvements, combined with more affordable prices due to the growing

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2.20 Auto-covariances for hold time sequences for class 2 stations in a network of five class 1 and five class 2 saturated stations with D = 2, 12, 20 & 32. ns-2. .. .. .. .. .. 2.21 Empirical and theoretical probability density for the length of a hold period for class 2 stations in a network of five class 1 and five class 2 saturated station with D = 2. ns-2 2.22 Empirical and theoretical probability density for the length of a hold period for class 2 stations in a network of five class 1 and five class 2 saturated station with D = 12. ns-2 2.23 Empirical and theoretical probability density for the length of a hold period for class 2 stations in a network of five class 1 and five class 2 saturated station with D = 20. ns-2 2.24 Empirical and theoretical probability density for the length of a hold period for class 2 stations in a network of five class 1 and five class 2 saturated station with D = 32. ns-2 2.25 Largest discrepancy between empirical and predicted distributions, sup k |F n (k)−F (k)|, as a function of sample size n. D = 2, 4, 8 & 12.

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