Shortwave - radio frequencies (from 1.605 to 30 MHz) that fall above the commercial broadcast band and are used for communication over long distances.
Solidaridad - geosynchronous satellites in Mexico's system of international telecommunications in the Western Hemisphere.
Statsionar - Russia's geostationary system for satellite telecommunications.
Submarine cable - a cable designed for service under water.
TAT - Trans-Atlantic Telephone; any of a number of high-capacity submarine coaxial telephone cables linking Europe with North America.
Telefax - facsimile service between subscriber stations via the public switched telephone network or the international Datel network.
Telegraph - a telecommunications system designed for unmodulated electric impulse transmission.
Telex - a communication service involving teletypewriters connected by wire through automatic exchanges.
Tropospheric scatter - a form of microwave radio transmission in which the troposphere is used to scatter and reflect a fraction of the incident radio waves back to earth; powerful, highly directional antennas are used to transmit and receive the microwave signals; reliable over-the-horizon communications are realized for distances up to 600 miles in a single hop; additional hops can extend the range of this system for very long distances.
Trunk network - a network of switching centers, connected by multichannel trunk lines.