 from the medium of downloading. Wireless Network Selection With the support of 4G user terminals, it is possible to choose any available wireless network for each particular communication session. The correct network selection can ensure the QoS required by each Fourth Generation Mobile Networks session. However, it is complicated to select a suitable network for each communication session, since network availability changes from time to time. Terminal Mobility There are two main issues in terminal mobility: location management and hand-off management. With location management, the system tracks and locates a mobile terminal for a possible connection. It involves handling all the information about the roaming terminals, such as the original and current located cells, authentication information and QoS capabilities. On the other hand, hand-off management maintains ongoing communications when the terminal roams. The mobile IPv6 is a standardised IP based mobility protocol for IPv6 wireless systems, where each terminal has an IPv6 home address. Whenever the terminal moves outside the local network, the home address becomes invalid and the terminal obtains a new IPv6 address (called the care-of-address) in the visited network. This hand-off process causes an increase in system load, high handover latency and packet losses. It is very difficult to solve these problems in 4G networks. The reason for this is that besides horizontal hand-off, vertical handover is also needed. Moreover, 4G networks are expected to support real-time multimedia services that are highly time-sensitive. It is very hard to calculate the hand-off between different wireless systems as it is very complicated. Network Infrastructure Existing wireless systems can be classified into two types: non-IP based and IP based. The non-IP based systems are basically optimised for voice delivery (e.g. GSM, CDMA) and the IP based systems are basically optimised for data services (e.g. 802.11, HIPERLAN). In 4G wireless networks, the problem in integrating the