This chapter provides a brief overview of 2G/2.5G and 3G wireless communication systems, with a focus on security-related aspects. Since the introduction of the first generation cellular networks in the 1980s, wireless communications have experienced tremendous growth. The first commercial GSM network was launched in 1992, marking the beginning of the digital cellular network era. In 2003, Hutchinson launched 3G services in Hong Kong, the UK, and Italy. Today, wireless communication devices are almost everywhere and have become an indispensable part of daily life. GSM is currently the most widely used wireless technology. As of the first quarter of 2004, the number of global GSM customers is estimated to be over 1 billion, accounting for over 70% of the global market share. GSM was proposed in Europe and standardized by the European Telecommunication Standards Institute (ETSI). Currently, the work has largely been transferred to the third generation partnership project (3GPP). GSM operates in frequency bands of 400, 800, 900, 1,800, and 1,900 MHz. A GSM channel has a bandwidth of 200 kHz. The modulation scheme is Gaussian minimum shift keying (GMSK), which is a type of continuous 7-phase modulation scheme. GMSK has a constant amplitude envelope, which is desirable for simple amplifiers. It also has a narrow power spectrum with low adjacent channel interference. The duplexing scheme is frequency division duplexing (FDD), with the uplink and downlink channels located in different frequency bands. The mobile station (MS) does not have to send and receive at the same time, thus reducing system design complexity and cost. In Fig. 2.1, we illustrate the processing of a GSM voice call. At the transmitter, the voice is first digitized and source encoded. Then channel coding (convolutional coding) and interleaving are applied for error correction. To achieve confidentiality over the air interface, encryption is performed. After modulation, the user signal is transmitted over the multipath fading channel. At the receiver, the received signal is first demodulated, and then decrypted. After deinterleaving and channel decoding, source decoding is conducted to restore the speech.This chapter provides a brief overview of 2G/2.5G and 3G wireless communication systems, with a focus on security-related aspects. Since the introduction of the first generation cellular networks in the 1980s, wireless communications have experienced tremendous growth. The first commercial GSM network was launched in 1992, marking the beginning of the digital cellular network era. In 2003, Hutchinson launched 3G services in Hong Kong, the UK, and Italy. Today, wireless communication devices are almost everywhere and have become an indispensable part of daily life. GSM is currently the most widely used wireless technology. As of the first quarter of 2004, the number of global GSM customers is estimated to be over 1 billion, accounting for over 70% of the global market share. GSM was proposed in Europe and standardized by the European Telecommunication Standards Institute (ETSI). Currently, the work has largely been transferred to the third generation partnership project (3GPP). GSM operates in frequency bands of 400, 800, 900, 1,800, and 1,900 MHz. A GSM channel has a bandwidth of 200 kHz. The modulation scheme is Gaussian minimum shift keying (GMSK), which is a type of continuous 7-phase modulation scheme. GMSK has a constant amplitude envelope, which is desirable for simple amplifiers. It also has a narrow power spectrum with low adjacent channel interference. The duplexing scheme is frequency division duplexing (FDD), with the uplink and downlink channels located in different frequency bands. The mobile station (MS) does not have to send and receive at the same time, thus reducing system design complexity and cost. In Fig. 2.1, we illustrate the processing of a GSM voice call. At the transmitter, the voice is first digitized and source encoded. Then channel coding (convolutional coding) and interleaving are applied for error correction. To achieve confidentiality over the air interface, encryption is performed. After modulation, the user signal is transmitted over the multipath fading channel. At the receiver, the received signal is first demodulated, and then decrypted. After deinterleaving and channel decoding, source decoding is conducted to restore the speech.