hose are the easiest ones normally to fix. Then continue with connectors, plug-in units and subsequently backplanes, finally Cell Buffers (B) CLR-1E-8 CLR-1E-6 CLR-1E-4 Offered Load (p) Figure 3.81 Offered load versus cell buffers Transmission Network Optimisation considering the possible transmission media issues and what kind of options there are available to either improve the existing media or change it. Transmission Equipment Failure Fourthly, and supposedly the most rare cause of accessibility problems in transmission, is a very funda- mental one, caused by transmission equipment failures. These are easy to monitor, but are time-consuming and troublesome to fix. Equipment failures are impossible to prevent completely, but effective prevention can be done to reduce the chances of negative quality effects of equipment failure by naturally first of all selecting equipment that promises a long time between failure and also by making the transmission network plan as protected as possible. This can be achieved in many ways. The most cost-efficient and very effective one is to use physical layer loops with cross-connections that monitor the signal and switch over to an other direction if one direction of the loop is failing. If nothing else, it is wise at least to avoid long branches of transmission without any secondary routes. Creating long series of single-point-of-failure routes is asking for trouble. Trying to protect the network in case of equipment failures is again an area where there is practically no limit to how much money and effort can be used to try to get more and more incremental improvements to the network under the banner of optimisation, SO it must be carefully determined from the end user perception viewpoint whether it is worth it. For example, a very safe transmission network would have a physical loop for each base station with two different media, e.g. a fibre and microwave radio link, and two different plug-in units serving these different directions. That would protect even from accidenta