 tion and multiple access to achieve a certain degree of information efficiency and protection. Initially developed for military applications, it gradually developed into a system that gave the promise of better bandwidth and service quality in an environment of spectral conges- tion and interference. In this technology, every user is assigned a separate code/s depending upon the transaction. One user may have several codes in certain conditions. Thus, separa- tion is not based on frequency or time, but on the basis of codes. These codes are nothing but very long sequences of bits having a higher bit rate than the original information. The major advantage of using CDMA is that there is no plan for frequency re-use, the number of channels is greater, there is optimum utilisation of bandwidth, and the confidentiality of information is well protected. 1.3.2 Modulations Gaussian Minimum Phase-shift Keying (GMSK) GMSK is the modulation method for signals in GSM. It is a special kind of modulation method derived from minimum phase-shift keying (MSK). Thus it a falls under the frequency modulation scheme. The main disadvantage of MSK is that it has a relatively wide spectrum of operation, SO GMSK was chosen to be the modulation method as it utilises the limited frequency resources better. GMSK modulation works with two frequencies and is able to shift easily between the two. The major advantage of GMSK is that it does not contain any amplitude modulation portion, and the required bandwidth of the transmission frequency is 200 kHz, which is an acceptable bandwidth by GSM standards. This is the modulation scheme used in GSM and GPRS networks. Octagonal Phase-shift Keying (8-PSK) The reason behind the enhancement of data handling in the 2.5-generation networks such as GPRS/EGPRS is the introduction of octagonal phase-shift keying or 8-PSK. In this scheme, the modulated signal is able to carry 3 bits per modulated symbol over the radio path as OVERVIEW OF MOBILE NETWORKS compared to 1 bit in the GMSK modulated path. But thi