AT SATCOM WESTAR TDRSS Common ITT WORLDCOM ITT WORLDCOM RCA Americom Western Western Carriers Union Union AETNA RCA GLOBCOM RCA GLOBCOM RCA ALASCOM AMSAT Southern Pacific Service Coverage International Domestic Marine Domestic Domestic Domestic Domestic Fig. 7-2. Operational satellite systems. SATELLITE AND CARRIER SERVICES 111 The satellite-operating organizations are responsible for the de- velopment and continuing operation of their associated satellite sys- tems. They provide common carriers with bulk satellite capacity. The common carriers sell satellite communication services directly to end- user organizations. The carriers are responsible for the development and implementation of the terrestrial stations. They are also respon- sible for the operation of terrestrial facilities connecting earth sta- tions and for the earth station networks. They are generally not responsible for control of the satellite itself. 7.2 SELECTING A TRANSMISSION METHOD Many users depend entirely on the telephone company to provide lines and equipment for all their data communication needs. Of course, the phone companies are the backbone of our communica- tions network, but there are viable alternatives. Bell's telephone lines for data transmission are available in many varieties. The type 3002 voice-grade private line is the most common for high-speed (2.4 kbit/s and greater) data traffic. Even the type 3002 channel is available in several flavors: It may have C-or D-types of conditioning or it may be unconditioned. The unconditioned circuit usually contains repeaters at specific in- tervals, as well as loading coils and echo suppressors. In addition, local telephone companies, such as New York Telephone Company, offer what is called a straight copper circuit, which comprises pairs of copper wires terminated at each end by Bell 829 channel-interface devices. This type of circuit is available only when the entire circuit is within the same telephone-company central office. In essence, the straight copper circuit is composed of two