gure 11.2. It consists of the following major parts: Ground networks: satellites usually act as inter-regional or international 'relay' links. Ground networks connect and switch telephone calls between the satellite system and the regional/national telephony networks. Ground antennas (earth stations): because of the large distance between a satellite and the Earth, a signal travelling between the two loses a large portion of its power. This signal loss is compensated by high-gain antennas on the ground. Satellite Communications Public network Public network Private Customer Customer Private network network Figure 11.2 A satellite communications system (source Intelsat). CPE, customer premises equipment Satellite antennas: satellites transmission and reception coverage can be designed for a particular country or geographic region. Specially designed antennas on-board a satellite can focus their communications for a particular area. Transponder: a signal received from the Earth needs to be amplified and relayed back using a different frequency in order that uplink and downlink signals do not interfere with each other. The devices that carry these functions are called 'transponders'. A satellite typ- ically has several tens of transponders, each of which may be assigned to a region/country. Operating Frequencies Because of the international nature of satellite communications, a number of frequency bands have been internationally assigned for satellite communications. In order to avoid interference, ITU has divided the world into three regions within which frequency spectrum allocation is harmonised. Table 11.1 lists the S, C, X, K and K bands in region 2 used for satellite telephony and data telecommunications [3, 4, 5, 6]. Until recently the K band had mostly been used for military applications. However with the growth of broadband services and the wide available spectrum bandwidth, many satellite operators are using this band for civilian applications. For a satellite operator, the choice of frequency band depends