The British Army's Wireless Set, Number 10, was the world's first microwave relay telephone system. It transmitted eight full-duplex (two-way) telephone channels between two stations limited only by the line-of-sight, often on the order of 25 to 50 miles (40 to 80 km). The stations were mounted in highly mobile trailers and were set up simply by aiming the two parabolic antennas on the roof at the next station.