【正文】 he service area and to broadcast some signaling messages. Multiple Access Technique Multiple access is a technique to allow users to share a munication medium so that the overall capacity can be increased. There are three monly used multiple access schemes: FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access). In FDMA, each call is assigned its own band of frequency for the duration of the call. The entire frequency band is divided into many small individual channels for users to access. In TDMA, users share the same band of frequencies. Each call is assigned a different time slot for its transmission. In CDMA, users share the same band of frequencies and time slots. Each call is assigned a unique code, which can spread the spectrum to the entire frequency band. The spectrum spread calls are sent on top of each other simultaneously, and are separated at the receiver by an inverse operation of the unique codes. A bination of the three multiple access schemes can also be applied. Power Control Power control is one of the most important design features in wireless munication including FDMA, TDMA, and CDMA systems. It ensures each user transmits and receives at a proper energy level to convey information successfully while reducing the interference to other users. Power control is needed in FDMA and TDMA systems because of the cochannel interference management. This type of interference is caused by the frequency reuse in the limited available spectrum. Via a proper power level adjustment, the cochannel interference can be reduced. This allows a higher frequency reuse factor and thus increases the system capacity. Owner control is the most essential requirement in CDMA systems. Without power control, all the mobiles transmit to the base station with the same power not taking into account path loss and fading effect. Mobiles close to the base station will cause significant interference to mobiles that are farther away from the base station. This effect is the socalled near/far effect. Therefore, a welldesigned power control algorithm is crucial for proper operation of a CDMA system. In the absence of power control, the system capacity is very low pared to other systems. Another advantage of power control is that it can prolong battery life by using a minimum required transmission power. Power control on a reverse link is more stringent than on a forward link because of the near/far effect. On a forward link, power control is still necessary to reduce the intercell interference. Power control can be operated in a centralized form or a distributed form. A centralized controller obtains the information of all the established connections and channel gains, and controls the transmission power level. The centralized approach can optimize the power usage of the entire or part of the work and thus is very efficient. It requires extensive control signaling in the work, however, and is difficult to apply in practice. 3 GPRS architecture GPRS is considered as a service or feature of GSM. It was designed by ETSI to be implemented over the existing infrastructure of GSM without interfering with the already existing services. The aim is quick GPRS deployment with minor impact on existing GSM PLMN ponents. Fig. 1 illustrates the logical architecture of a GSM work supporting GPRS. Figure 1. Architecture of GPRS work Mobile Station GPRS and GSM systems provide interworking and sharing of resources dynamically between users. For this reason, three types of terminals have been defined: a classA MS can carry a circuitswitched and a packet switched connection simultaneously enabling the subsc