In many cases, multiconductor cables are enclosed in a heavy sheath made up of several layers of aluminum or plastic. Some thick cables also contain steel wire to provide strength. Communications cables are laid underground and along the ocean floor, or they are mounted on poles. Burying cable protects it from harsh weather and keeps the land aboveground uncluttered. As a result, few new communications cables are strung aboveground today.
Underwater cables serve as a communications link between continents.
These underwater cables need tough outer coverings so that they can withstand strong ocean currents. Most cables are designed to carry more than one message at a time. For example, a process called carrier transmission enables two pairs of wires to transmit as many as 96 telephone conversations simultaneously. Electronic filters at each end of the cable sort out the various conversations. Engineers have developed two kinds of cables that can carry especially large quantities of messages at once.