This is a long distance transmission of messages without physical contact of the sender and the receiver. However, it is a must that the method used for encoding message is familiar by both sender and the receiver. These methods are designed in a way that the can limit medium used in signals (Gray, 2013). In addition, these signals include smoke, flag semaphore and beacons.
This is a device which converts sound into electric signals, which are appropriate for transmission over long distances. From the history, telephone was the first device which enables people to talk directly with one