 network elements (ARs - old and new - and QoSBs). When an MT starts to lose the signal to the current AR (called 'old AR', or AR1), it will start a handover procedure to a neighbouring AR (now called 'new AR', or AR2), from which it receives a beacon signal with a network prefix advertisement. The MT builds its CoA and initiates the handover procedure, sending a handover request to its new AR, still through the old AR. The old AR will forward this request to both the new AR (known by the network prefix) and to the QoS broker. The old QoS broker (QoSB1) sends a handover request to the new QoS broker, indicating the user's NVUP and the list of services currently being used. This acts as a context transfer from the old QoS broker to the new QoS broker. With this information, the new QoS broker (QoSB2) will verify the availability of resources and send a message to the new AR indicating whether the MT may or may not perform the handover. If the handover is possible, the new AR will be configured with a copy of the policies that the old AR has for that user. If the handover is not possible, multiple operator policies can be supported, according to network usage and user preferences. Meanwhile, the new AR will forward the answer to the old AR, which will then send it to the MT. Upon a positive answer, the MT may perform Challenges and Limitations of 4G Networks the layer 2 handover because all the layer 3 resources are then reserved. During this phase, both ARs are bicasting, to minimise packet loss. 5.7 Challenges and Limitations of 4G Networks No technology is complete in itself and this is also the case with 4G. Up to now the discussion has been about the 4G technologies and its advantages over the previous generations, but a careful look shows that it also faces some serious limitations and challenges. There are three major limitations/challenges in 4G networks: Mobile station Wireless network Quality of service 5.7.1 Mobile Station For a large variety of services and wireless networks in 4G systems the multimode 