Electronic communications began as digital technology with Samuel Morse’s invention of the telegraph in 1845. The dots and dashes of his famous code were the binary ones and zeroes of the current through the long telegraph wires. Radio communications also started out as digital, with Morse code producing the off and on transmission of continuous-wave spark-gap pulses.
Then analog communications emerged with the telephone and amplitude-modulation (AM) radio, which dominated for decades. Today, analog is slowly fading away, found only in the legacy telephone system; AM and FM radio broadcasting; amateur, CB/family, and shortwave radios; and some lingering two-way mobile radios. Nearly everything else, including TV, has gone digital. Cell phones and Internet communications are digital. Wireless networks are digital.
Though the principles are generally well known, veteran members of the industry may have missed out on digital communications schooling. Becoming familiar with the basics broadens one’s perspective on the steady stream of new communications technologies, products, trends, and issues.
The Fundamentals
All communications systems consist of a transmitter (TX), a receiver (RX), and a transmission medium (Fig. 1). The TX and RX simply make the information signals to be transmitted compatible with the medium, which may involve modulation. Some systems use a form of coding to improve reliability. In this article, consider the information to be non-return-to-zero (NRZ) binary data. The medium could be copper cable like unshielded twisted pair (UTP) or coax, fiber-optic cable, or free space for wireless. In all cases, the signal is greatly attenuated by the medium and noise is superimposed. Noise rather than attenuation usually determines if the communications medium is reliable.
Communications falls into one of two categories—baseband or broadband. Baseband is the transmission of data directly over the medium itself, such as sending serial digital data over an RS-485 or I2C link. The original 10-Mbit/s Ethernet was baseband. Broadband implies the use of modulation (and in some cases, multiplexing) techniques. Cable TV and DSL are probably the best examples, but cellular data is also broadband.
Communications may also be synchronous or asynchronous. Synchronous data is clocked as in SONET fiber-optical communications, while asynchronous methods use start and stop bits as in RS-232 and a few others.
Furthermore, communications links are simplex, half duplex, or full duplex. Simplex links involve one-way communications, or, simply, broadcasting. Duplex is two-way communications. Half duplex uses alternating TX and RX on the same channel. Full duplex means simultaneous (or at least concurrent) TX and RX, as in any telephone.
Topology is also fundamental. Point-to-point, point-to-multipoint, and multipoint-to-point are common. Networking features buses, rings, and mesh. They all don’t necessarily work for all media.
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