“Communication” is the process of transferring information from a source to a
destination. Communication systems for the most part cover distances between
computers, and may involve the public telephone system, radio, and television.
Wide-area communication systems have become very complex, with all combinations
of voice, data, and video being transferred by wire, optical fiber, radio,
and microwaves. Communication routes may traverse distances over land, under
water, through local radio cells, and via satellite. Data that originates as analog
voice signals may be converted to digital data streams for efficient routing over
long distances, and then converted back to an analog signal, without the awareness
of those communicating.
In this chapter we focus on communication between entities located at distances
ranging within a kilometer for a local area network (LAN), and across much
larger distances for wide area networks (WANs) as typified by the Internet. We
start by discussing a few principles of communication.