se two systems becomes apparent when end-to-end QoS services are concerned. Security and Privacy The key concern in security designs for 4G networks is flexibility. Since the existing security systems are basically designed for voice services it becomes very difficult to implement them for heterogeneous environments. Moreover, the key sizes and encryption and decryption algorithms of existing schemes are also fixed. They become inflexible when applied to different technologies and devices. Fault Tolerance and Survivability There is presently an inadequate study on the survivability of wireless access networks, though they are more vulnerable than wired networks. A cellular wireless network is typically designed as a tree like topology. A major weakness of this topology is that when one level fails, all the other levels below it are affected. The situation becomes worse when multiple-tree topology networks work together in 4G sys- items. Their fault-tolerant designs should consider power consumption, user mobility, QoS management, security, system capacity and link error rates of different wireless networks. There are two different ways to achieve fault-tolerant architectures to support QoS in failures. The first is to use a hierarchical cellular network system; the second is to use collaborated or overlapping heterogeneous wireless systems. Multiple Operators and Billing Systems Presently, an operator usually charges customers with a simple billing and accounting scheme, where a flat rate based on subscribed services, call durations, and transferred data volume is usually enough. However, with the increase in service varieties in 4G systems, more comprehensive billing and accounting systems are needed. The customers may no longer belong to only one operator, but instead subscribe to many services from a number of service providers at the same time. Along with this, equalisation on different charging schemes is also needed as a result of different billing schemes (e.g. charging based on data, time or information).