s increase in data throughput is at the cost of a decrease in the sensitivity of the radio signal. Therefore the highest data rates are provided within a limited coverage. This is the modulation scheme used in the EGPRS/EDGE networks. Quadrature Phase-shift Keying (QPSK) To demodulate the frequency modulated signal, phase-shift keying or PSK has been used as the preferred scheme. In PSK, the phase of the transmitted waveform is changed instead of its frequency. In PSK, the number of phase changes is two. A step forward is the assumption that the number of phase changes is more than two (i.e. four), which is the case with quadrature phase-shift keying or QPSK. This enables the carrier to carry four bits instead of two, so effectively doubling the capacity of the carrier. For this reason, QPSK is the modulation scheme chosen in WCDMA Networks. 1.3.3 The OSI Reference Model The basic idea behind development of the Open System Interconnection (OSI) reference model by the ITU was to separate the various parts that form a communication system. This was possible by layering and modularisation of the functions that were performed by various layers (parts of the communication system). Although initially developed for communica- tions between computers, this model is being extensively used in the telecommunication field, especially mobile communications. Basic Functions of the OSI Reference Model Each layer shown in Figure 1.3 communicates with the layer above or below it. No two adjacent layers are dependent, so the lower layer does not worry about the content of the information that it is receiving from the layer above. So, communication between adjacent layers is direct, while communication with the other layers is indirect. Each node has this same reference model. When communicating with layers in another node, each layer Application Layer Application Layer Presentation Layer Presentation Layer Session Layer Session Layer Transport Layer Transport Layer Network Layer Network Layer Data Link Layer Data Link Layer Physic