Despite the rapid change in cellular technologies, Mobile Network Operators (MNOs) keep a high percentage of their deployed infrastructure using Global System for Mobile communications (GSM) technologies. With about 3.5 billion subscribers, GSM remains as the de facto standard for cellular communications. However, the security criteria envisioned 30 years ago, when the standard was designed, are no longer sufficient to ensure the security and privacy of the users. Furthermore, even with the newest fourth generation (4G) cellular technologies starting to be deployed, these networks could never achieve strong security guarantees because the MNOs keep backwards- compatibility given the huge amount of GSM subscribers. In this paper, we present and describe the tools and necessary steps to perform an active attack against a GSM-compatible network, by exploiting the GSM protocol lack of mutual authentication between the subscribers and the network. The attack consists of a so-called man-in-the- middle attack implementation. By using Software Defined Radio (SDR), open-source libraries and open- source hardware, we setup a fake GSM base station to impersonate the network and therefore eavesdrop any communications that are being routed through it and extract information from their victims. Finally, we point out some implications of the protocol vulnerabilities and how these can not be mitigated in the short term since 4G deployments will take long time to entirely replace the current GSM infrastructure.

The Internet boom has led to the image of a global village, a totally communicated planet where national borders have no meaning and where the Internet and the information highway have become the path to solve any problem. Technological and scientific research has been conducted in order to exchange information through the telephone lines, which provide a solid platform for the new services related to the Internet such as E-mail, electronic commerce and Internet telephony. Non-technical issues should be taken into account in order to notice that not all the people an Earth are aware of the Internet and the new technological society. Many persons are not educated for those new technologies that are out of their reach. Indeed, even if they should be able to access a specific communications network, the new services would not be affordable for them economically and/or intellectually. This article presents indicators that magnify the problem: the gap between the rich and the poor, the educated and non-educated, the Internet-literate and illiterate. Reflections on several important issues are presented, and ultimately, the global village is questioned as a reality or just a new myth.

It is shown that, for a multiloop sigma-delta modulator driven by a DC input, the statistical behavior (described by long-term time averages) of the error process from an inner (multibit) quantizer is consistent with that of a signal-independent. white. uniform noise. The same result also holds for sinusoidal inputs, provided that the modulator has three or more loops. This behavior is exploited to obtain trade-offs between asymptotic system performances and oversampling ratios, for several sigma-delta modulator/decoding filter pairs. For an L-loop modulator with a simple sinc/sup L+1/ decoding filter, it is shown that doubling the oversampling ratio can result in an improvement of 3.01(2L+1) dB in the signal-to-quantization-noise ratio.>

A discrete-time model having a two-bit (2-b) quantizer is analyzed exactly, and an analytical expression for the quantizer noise sequence is found. Rigorous answers are then provided to two fundamental questions for a double-loop sigma-delta modulation system with DC input: (1) What is the long-term statistical behavior of the internal quantizer noise? (2) How does the asymptotic mean square sigma-delta quantization error vary as a function of the oversampling ratio?.>

New and simpler achievability proofs that are based on the backward decoding technique are presented for the well-known coding theorems for the multiple-access channel (MAC) with perfect feedback and the degraded relay channel. A class of MACs with different generalized feedback signals is also considered, and achievable-rate regions that are larger than those previously presented in the literature are established.>

An exact spectral analysis of oversampled discrete-time L-loop (L>or=2) sigma-delta modulator ( Sigma Delta M) with DC (constant) input is presented. It is shown by a novel application of ergodic theory that the embedded quantizer error sequence is asymptotically uniformly distributed, white and uncorrelated with the input, a fact usually assumed a priori in the linearized models. It was found that for the following linear demodulators: (1) optimum FIR (finite-impulse response) filter, (2) sinc/sup k/ filters (k>or=L+1), and (3) the ideal low-pass filter, the overall system ( Sigma Delta M plus demodulator) can provide a normalized average quantization noise power inversely proportional to the (2L+1)-st power of the oversampling ratio, confirming the performance superiority of multiloop Sigma Delta M over single-loop Sigma Delta M.>

The companding model for quantizer design and analysis has been widely applied in the scalar quantization case. However, if the signal to be quantized is a vector, then the optimum companding system can be designed for only a limited number of distributions. On the other hand, multidimensional piecewise linear companders can be designed for any signal density, generating quantizers that are uniform on each region of the compander. These systems, while not optimal, can have asymptotic performance arbitrarily close to the optimum. Their analysis and implementation can be simpler than those of optimal systems. Piecewise linear companders for asymptotic multidimensional quantization are analyzed, and a method for their design is suggested.>

The companding model for quantizer design and analysis has been frequently used in various instances. However, if the signal to be quantized is a vector, then the optimum companding system can be designed for only a limited number of distributions. On the other hand, piecewise linear companders can be designed for any signal density, generating quantizers that are uniform on each region of the compander. These systems, while not optimal, can have performances which are close to the optimum, and their analysis and implementation can be simpler than those of optimal systems. In this paper we analyze piecewise linear companders for asymptotic multidimensional quantization, and we suggest methods for their design.

The discrete short-time Fourier transform (DS TFT) has been widely used to study and analyze several speech signal characteristics. However, this Scheme has not been used as successfully in speech compression based on scalar quantization. On the other hand, most speech perception concepts have very interesting frequency-domain interpretations, which suggest the design of compression schemes based on frequency domain analysis. In this paper we apply vector quantization techniques for designing and simulating discrete short-time Fourier transform based speech compression systems. The main conclusion we can draw is that a low to medium rate compressor, in which the reproduction signal has at least communications quality, and in which the complexity is moderate, is at least in principle possible.

In many linear predictive coding (LPC) speech compression systems the encoding of the LPC parameters is performed using a product code book scheme. One possible approach consists in organizing the set of LPC reproduction models as the Cartesian product of a vector code book describing the shape of each reproduction LPC model and a scalar codebook describing the gain or energy. We first present a formal development of rate-distortion theoretic results for distortion functions based on quotients of input and reproduction symbols. We also obtain theoretical bounds for the rate-distortion function of the gain term of LPC systems (which, depending on the quantization scheme, can use about10-25percent of the transmission rate), as well as asymptotic (small distortions) approximations to the performance of the optimal scalar quantizer for these gain terms, when the overall fidelity criterion is the Itakura-Saito distortion measure, which is a member of the class of quotient distortion measures. The approximations and bounds are compared finally with experimental results.

The twin tools of asymptotic quantization theory and rate-distortion theory are used to find an expression for the approximate total mean-squared distortion of the series connection of a nonuniform quantizer and a noisy channel. The nonuniform quantizer is modeled as a compressor followed by a uniform quantizer. The approximate total mean-squared error distortion is minimized over a class of parameterized compressor characteristics for input processes whose univariate probability density functions are members of the generalized Gaussian family.

In this paper we present the derivation of the bivariate distribution function for median filtered sequences of independent, arbitrary, second-order random variables. This result is then used to qualitatively analyze second moment properties of median filtered data. The results hint towards a low-pass characteristic of the median filters and a low dependency of the output spectrum on input alphabet size and distribution.