6.1 Types of communication channels

Electrical communications channels are either wireline or wireless channels. Wireline channels physically connect transmitter to receiverwith a "wire" which could be a twisted pair, coaxial cable or optic fiber. Consequently, wireline channels are more privateand much less prone to interference. Simple wireline channels connect a single transmitter to a single receiver: a point-to-point connection as with the telephone. Listening in on a conversation requires that the wire be tappedand the voltage measured. Some wireline channels operate in broadcast modes: one or more transmitter is connected to several receivers. One simple example of thissituation is cable television. Computer networks can be found that operate in point-to-point or in broadcast modes. Wirelesschannels are much more public, with a transmitter's antenna radiating a signal that can be received by any antennasufficiently close enough. In contrast to wireline channels where the receiver takes in only the transmitter's signal, thereceiver's antenna will react to electromagnetic radiation coming from any source. This feature has two faces: The smileyface says that a receiver can take in transmissions from any source, letting receiver electronics select wanted signals anddisregarding others, thereby allowing portable transmission and reception, while the frowny face says that interference andnoise are much more prevalent than in wireline situations. A noisier channel subject to interference compromises theflexibility of wireless communication.

Maxwell's equations neatly summarize the physics of all electromagnetic phenomena, including circuits, radio, andoptic fiber transmission.