allows the interconnection between the network and the Internet. As there are the same protocols, the GPRS network can be viewed as a subnetwork of the Internet, with GPRS capable mobile phones being viewed as mobile hosts. Cellular Networks However, there are some limitations in the GPRS network, such as low speed (practical speed is much lower than theoretical speeds). 1.3.5 EDGE (Enhanced Data Rate for GSM Evolution) The limitation of the GPRS network was eliminated to a certain extent by the introduction of the EDGE technology. EDGE works on TDMA and GSM systems. It is considered to be a subset of the GPRS as it can be installed on any system that has GPRS deployed on it. It is not an alternative to UMTS but a complimentary technology for it. In EDGE, 3G services can be given at a lower but similar data rate as UMTS, with the data rates going up to 500 kbps (theoretically). This is done by introducing a new modulation scheme 8-PSK (phase-shift keying) and will coexist with the GMSK that is used in GPRS. However, the major advantage is that existing GSM networks can be upgraded for the same, thus preventing huge costs needed to roll-out the 3G networks and at the same time giving services like 3G. General features of EDGE include enhanced throughput per timeslot (8.8-59.2 kbps/timeslot), modulation changes from GMSK to 8-PSK, decreased sensitivity of the 8-PSK signal and higher capacity and coverage. Though, not many changes in the hardware are required by EDGE, except for some hardware upgrades in the BTS and some software upgraded in the network. However, the second generation system lacked capacity, global roaming and quality, not to mention the amount of data that could be sent. This all led to the industry working on a system that had more global reach (e.g. the user did not need to change phones when going to Japan or the US from SE Asia or Europe). This was the beginning of the evolution of third generation systems. 1.4 Third Generation Cellular Networks The third generation cellular networks were devel