ansmission network is planned to be able to adapt to loading of the network. In other words if the network load increases to a certain level existing services might be reallocated new narrower bandwidth to be able to accommodate and admit still new service requests. And Transmission Network Planning and Optimisation Received Cells Transmitted Cells ATM Switch Cell Switched To Wrong VPI/VCI Cell Stream Figure 3.82 Retainability/sustainability problems vice versa if the network load decreases existing services might be reallocated new larger bandwidth. This kind of adaptation even if it is of course very useful network feature overall, creates a sort of handover situation between two capacity allocations and like in any handover there is some chance of failure. RNCs that support such adaptation most likely always offer a chance to disable the feature, SO if the quality really main concern then one might consider disabling it. However generally the positive effects of such feature are likely to exceed the negative ones. Transmission Quality Loss This is the main cause of transmission retainability/sustainability quality issues. In a packet switched network the transmission network is increasingly vulnerable to bit errors. In 3G the ATM cells, AAL units and IP packets carry large amounts of data in one cell, unit or packet compared to the 2G era. Any errors in those headers can cause a loss of information and in the worst case misinsertion of information as well. Naturally these layers do provide retransmissions in case information is lost, but that helps only in cases of non-real-time services. Real-time services cannot utilise retransmissions (other than human retransmission: 'What did you say?') and SO are more vulnerable in 3G transmission. Figure 3.82 illustrates one example of retainability/sustainability problems caused by transmission quality loss. Refer to an earlier discussion about how to prevent transmission network quality loss. Transmission Equipment Failure This is supposedly a rare chance to experianc