de Des- ignator) or E.164 ATM format). The IDI tells the network addressing domain where values of the DSP are allocated; the network addressing authority is responsible for allocating values of the DSP from that domain. The HO-DSP describes the hierarchy of the addressing authority and also conveys topological Transmission Network Planning and Optimisation significance. The ESI is 6 octets long and identifies an end system. Although SEL (1 octet long) is not used for ATM routing, it may be used by end systems. 3.4 Microwave Link Planning Microwave links generally operate between frequencies of 2 and 58 GHz. Initially analogue links were used, but now far superior digital microwave links are used. Digital microwave links have many advantages over the analogue microwave links, some of which are: high tolerance against interference; high tolerance against deep fading; high signal carrying capacity ranging from 2 to 155 Mbps; high frequency range (2-58 GHz); easy, rapid (and hence economical) installations. As the frequency increases, the length of the link decreases. Due to the high frequency range (2-58 GHz), the microwave links can be classified into three main categories: (a) Long haul (b) Medium haul (c) Short haul Long Haul The frequency of operation of these links is usually 2-10 GHz. In the best of climatic conditions and frequency of operation, the distance covered by the links could range from 80 km to 45 km. These links are affected by multipath fading (explained later). Frequency band 2 GHz Maximum path length 80 km Multipath fading Antenna diameters up to 370 cm for an antenna gain of 36 dB Both vertical and horizontal polarisations used Frequency band 7 GHz Maximum path length about 50 km Multipath fading Antenna diameters up to 370 cm for an antenna gain of 46.8 dB Both vertical and horizontal polarisations used Frequency band 10 GHz Maximum path length about 45 km Multipath fading Antenna diameters 60-120 cm for a gain range of 34-40 dB Both vertical and horizontal polarisations used Microwave Link P