A studio-transmitter link (or STL) sends a radio station's or television station's audio and video from the broadcast studio to a radio transmitter or television transmitter in another location. This is often necessary because the best locations for an antenna are on top of a mountain, where a much shorter tower is required, but where a studio is, is completely impractical. Even in flat regions, the center of the station's allowed coverage area may not be near the studio location or within a populated area where a transmitter would be frowned upon by the community, so the antenna must be placed several miles or kilometres away. Depending on the locations that must be connected, a station may choose either a point to point (PTP) link on another special radio frequency, or a newer all-digital wired link via a dedicated T1 or E1 (or larger-capacity) line. Radio links can also be digital, or the older analog type, or a hybrid of the two. Even on older all-analog systems, multiple audio and data channels can be sent using subcarriers. Stations that employ an STL usually also have a transmitter-studio link (or TSL) to return telemetry information. Both the STL and TSL are considered broadcast auxiliary services (BAS).