 times the transmission rate, rounded up to the next highest odd integer multiple of 22.5 kHz increment (for information rates less than or equal to 10 Mbps) or 125-kHz increment (for information rates greater than 10 Mbps). Rate 3/4 FEC is mandatory for all IDR carriers. OH = overhead. Source: IESS-308, Rev. 7, Ref. 5. Courtesy of INTELSAT. are given in Table 9.6. These parameters are for the intermediate data rate (IDR) digital carrier system. All IDR carriers are required to use at least R = 3/4, where R is the code rate. 16 Reference 3, Chapter 4, provides a detailed description of various FEC channel coding schemes. The occupied satellite bandwidth unit for IDR carriers is approximately equal to 0.6 times the transmission rate. The transmission rate is defined as the coded symbol rate. To provide guardbands between adjacent carriers on the same transponder, the nominal satellite bandwidth unit is 0.7 times the transmission rate. 9.3.8 Very-Small-Aperture Terminal (VSAT) Networks 9.3.8.1 Rationale. VSATs are defined by their antenna aperture (diameter of the parabolic dish), which can vary from 0.5 m (1.6 ft) to 2.5 m (8.125 ft). Such apertures are considerably smaller than conventional earth stations. A VSAT network consists of one comparatively large hub earth terminal and remote VSAT terminals. Some networks in the United States have more than 5000 outlying VSAT terminals (a large drugstore chain). Many such networks exist. 16 R = (information bit rate)/(coded symbol rate). When R = 3/4 and the information rate is 1.544 Mbps, the coded symbol rate is 4/3 that value, or 2,058,666 symbols a second. FEC coding simply adds redundant bits in a systematic manner such that errors may be corrected by the distant-end decoder. CONCEPTS IN TRANSMISSION TRANSPORT There are three underlying reasons for the use of VSAT networks: 1. An economic alternative to establish a data network, particularly if traffic flow is to/from a central facility, usually a corporate headquarters to/from outlying remotes. To bypass telephone