TRANSMISSION TECHNOLOGY AND METHODS

Basically there are two types of transmission facilities: analog and digital. Transmission facilities can also be segmented according to electrical baseband, radio frequency, and optical media. In a simple third set of terms, it’s either wired or wireless. (Wouldn’t Marconi be proud that his term came back in vogue in the 21st Century?) Digital transmission techniques and methods are all within the OSI stack’s lowest layer, most often referred to as layer 1 or the physical layer. In addition to physical transmission, or aspects related to physical movement of energy across a media, layer 1 is sometimes seen as a link in the sense that it behaves like a single link in a chain, or as a link between two other objects. In the case of transmission media in communications networks, the link characterization would apply when used to link together two switches, routers, or other network interface devices.

At the outset, telegraph and telephone technology were based on use of direct current power and metal conductors connecting transmit and receive hardware. As demand for service grew, tubes and later transistors enabled designers to place more than one conversation or transmission on a pair of wires. Thus, the communications industry became part of the electronics evolution along with radio, and later television. The telephone industry was granted rights to use parts of the radio spectrum and viewed by regulators similar to the broadcast industry as being ‘‘in the public interest, convenience, and necessity.’’ At one time, the FCC was organized into bureaus and included separate broadcast and common carrier bureaus. Throughout the 1960s, 1970s, and well into the 1980s, microwave radio spectrum carried the majority of US long distance traffic. Radio transmission capacity became the growth engine of the long distance telephone industry. MCI applied for and received a license to carry long distance traffic between Chicago and St. Louis, eventually building a nationwide analog radio transmission network. AT&T had a combination of digital and analog radio transmission capacity when it entered into to its third consent decree with the justice department in 1982. Most of the digital radio capacity was in the access network in intra-state service. By the end of the century, those radio transmission networks have for the most part been decommissioned and replaced with fiber transmission capacity.

Until computers came into the picture, transmission was all analog. Audio, video, speech, or other information was carried in basic or baseband form on wire, or modulated on to some type of carrier—a separate, single, much higher frequency signal, transmitted, received, demodulated, and applied to a speaker or display device. Long before computers placed demands on the communications network, voice was digitized and the resulting signals multiplexed to gain more usage on common trunk facilities. As computers increased in capability and created demand for connections to terminals and other computers, the digital signal interface to the network was the natural common sense approach; however, that was not to be for many years.

Digital transmission techniques include various forms of carrier modulation in wireless, including amplitude, frequency, and phase. Modems (acronym for modulator–demodulator) are a required function and, in some cases, a separate item of equipment in satellite earth station equipment complements. The function of these devices is to impress upon, or modulate, the carrier signal with the baseband signal, or digital bitstream. Modem functions mirror, or complement, analog to digital and digital to analog converter functions. For example, an analog signal must be converted to digital before it can be transported on a digital network. A digital signal must be converted to analog before it can be transported on an analog network.

The three types of media we should know about are baseband, free space, and fiber. Most baseband transmission is on copper twisted pair or coaxial wire.