nfiguration will usually resemble the INTELSAT global net- 1980a; CCIR, 1980b; CCIR, 1980c). Foreseeable levels of inter- work, although on a smaller scale, using FDMA and with ference from other satellite networks are allowed for, and a margin analogue telephone channels in frequency division multiplexed is left for interference from terrestrial radio systems operating in the basebands frequency modulating the radio carriers, or digital same frequency bands. The more important of these criteria are as channels in time division multiplexes phase shift modulating follows: the carriers. Networks serving isolated communities are likely to use voice switched carriers for single telephone channels, 1. For analogue telephone channels, the total noise, including analogue or digital. interference reckoned as noise, psophometrically weighted, at Big corporations operating in well separated localities have a point of zero relative level in the telephone network, shall not found it worth while to lease satellite transponders for private exceed 10000pW for more than 20% of the month. Nor shall telecommunications networks linking their establishments. In this noise exceed 50000pW for more than 0.3% of any month. principle these networks resemble the national trunk networks Within (1), the margins allowed for interference from terrestrial described in (1) above, but they may be more flexible in their radio systems are 1000pW for 20% of any month and 50000pW configuration, being designed to cater for needs such as tele- for 0.03% of any month (CCIR, 1980d). conferencing and high speed data transfer which are not well For 64kbit/s digital channels which are to form part of an supplied at present by the public telecommunications networks. Integrated Services Digital Network (ISDN), the BER should 3. The largest single use of satellite transponders in the fixed not exceed one in 107 for more than 10% of any month. For satellite service, at present, is for the distribution of television and 0.03% of any month, the BER should not ex