ains are connected, it would become a loop (as shown in Figure 3.62). This means that additional equipment required to create a single link would be required. However, this would give an increased protection to the links. When loop topology is used, equipment redundancy is not used. Thus, the cost required to put up one additional link results in increased protection and increased availability. Figure 3.62 Loop topology Transmission Network Planning and Optimisation Interference Interference Adjacent Channel Co-channel Figure 3.63 Co-channel and adjacent channel interference 3.8 Frequency Planning and Interference Once the frequency bands are defined by international bodies such as ITU, IEEE, etc., it is up to the national regulatory bodies to determine the distribution of the frequency bands (which is usually done by paying a fee) to different bodies intending to use them for public or private applications. This has an impact on the type of equipment that would be required in the network, leading to an impact on link budget calculations, topology planning, etc. In some networks the number of bands available for microwave planning engineers might be less and hence frequency planning plays an important role. A bad 'frequency planned' network would see a drop in its quality and subsequently a drop in revenue for the operator. The main idea behind frequency planning in the microwave network is to make sure that the wanted 'carrier' signal reaches the receiving end without getting hindered by any other 'interfering' signal. The closer the two signals are in terms of frequency and signal strength, the higher will be the level of interference. Interference may take place due to many phenomena taking place in the network, some of which are described below: (a) Both the interfering and carrier signals have the same frequency channel but different polarisations. (b) The interfering signal has an adjacent frequency channel to the carrier frequency channel and the same polarisation. (c) The interfering signal has an adjacen