l cable cut caused by a careless excavator or an unexpected squall of rain. However, even with such a very expensive arrangement there could be the possibility of failure. Retainability/Sustainability The transmission network should not be vulnerable when it comes to retainability/sustainability. The radion interface due to mobility of the end user is much more likely to have these issues due to coverage differences. However, the transmission network will also face challenges to keep the ongoing service running if the user is constantly moving, i.e. moving from one base station area to another, or if the radio interface planning is done SO that there is some underlay-overlay plan with, for example, 900 and 1800 networks or 2G and 3G networks. In such a case the new transmission connection will face the same accessibility issues as discussed earlier; they just seem like retainability/sustainability issues from the end user perspective. If the end user is stationary and there is no need for handovers due to other abov-ementioned reasons, then the situation is simplier for the transmission network. Transmission Signalling Transmission signalling can still cause some retainability/sustainability quality problems, because some services rely on signalling links themselves to provide the service. This means that from an end user per- spective a well-started service can have problems during the end user experience. From the transmission network viewpoint this results from a series of instances of signalling network use, but again the end user does not know or care about that. Refer to an earlier discussion about how to optimise the transmission signalling network. Transmission Capacity Transmission capacity should be protected in 3G by admission control SO that once the connection(s) for the end user service is/are admitted and established then the capacity should be guaranteed according to the service level agreement as long as the service is ongoing and untill it is dissolved. There is, however, possibility that the tr