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Questions Answered in This Chapter
What is a transmission system?
What is a switching system?
What do transmission and switching systems have to do with ATM and ADSL?
Why does someone working to understand ATM and ADSL require a dose of mundane and flightless transmission and switching systems? Because ATM is a transport and switching technology. Because ADSL is a transport and switching technology. Because ATM and ADSL must exist and work with legacy transmission and switching systems. Because ATM and ADSL must cope with the multitude of transmission and switching seams. Because ATM and ADSL are the wings of a now flight-enabled global communications system that can soar with no known bounds. A believer must have a fundamental idea of the legacy transmission and switching systems empowered by ATM and ADSL to fully appreciate the power of emerging ATM and ADSL technologies.
Transmission systems are necessary to convey intelligent information from source to destination. Knowing all the information in the world does not have much practicality if one cannot transmit some portions to a recipient and therefore make some use of all that information. Whether the transmission system is the voice and ear working together and jointly using sound waves to transmit the information from source to destination (also called end-to-end), or an expensive government network of computers and digital radios spread around the globe, transmission systems are an integral element of our communications networks. Without the ability to transmit our knowledge, the possession of knowledge itself would dramatically lessen in import and there would be an attendant decrease in our standard of living and even our ability to evolve. Without information transmission systems, we might be little better off than chimpanzees.
Given that transmission systems are so important to our ability to communicate, which dictates our ability to evolve, it is no surprise that humans utilize numerous transmission systems. We are concerned with transmission systems that have found particular usage in the world of telecommunications. Since the topic of this book is ATM and ADSL, we will confine the discussion to those systems specifically useful for the transmission of ATM and ADSL information. But first, we need to know something of the past and how we came to where we are today.
In the late 1800s, the dominant communications technology was analog in nature. Analog signals are characterized as being continuous in time with respect to some constantly changing value such as amplitude and/or frequency. Mathematically speaking, an analog signal has at least a first derivative. The advent of the telephone gave rise to analog voice communications using twisted copper pairs for single channel communications transport. That is, one pair of copper wires could carry one-half of a two-way (half duplex transmission) conversation mechanically switched in the central office from the source of the call to the destination.
Development of the vacuum tube in 1907 led to analog systems utilizing FDM (frequency division multiplexing) in 1925. FDM is the process of simultaneously transmitting two or more signals representing intelligent information over a common path by using a different frequency band for each signal. That is, a frequency domain is divided into discrete bandwidths, called channels, with each channel used to transmit a unique signal representing some particular information. Once multiplexed, FDM signals could be transmitted at low cost over long distances. Multiplexing was expensive, so the technology was reserved for long-distance and high-volume service where the multiplexing cost was more than offset by the reduction in transmission cost. FDM technology resulted in the consolidation of the long-distance network into fewer routes with much more capacity while microwave transmission technology, characterized by its analog nature to date, progressed from 4 GHz through 11 GHz frequency regions.
The invention of transistors in 1954 was to have a profound impact on every aspect of our lives including communications systems. Electronic equipment became smaller, faster, less power hungry, easier to maintain, easier to operate, easier to build, and less expensive. Also, the transistor opened the door to a technology-based world full of opportunities that previously was a window that only the interesting characters among us could occasionally peer into.
In 1969, transistor-based digital wireless systems were developed. Wireless systems is another name for transmission systems that exclude wire-based transmission systems, such as copper and coaxial cable, and encompasses RF transmission systems including microwave and satellite.
The concept of PCM (pulse code modulation) was developed in the late '60s. PCM is the process where an analog signal, like voice, is digitally encoded. To generate a PCM signal, an analog signal is sampled at discrete periodic intervals. The level of the analog signal as measured during the sample interval is equated to a binary value. There can be an almost infinite range of binary values to represent the continuous analog signal if exactitude is a requirement, which becomes impractical. Why? Well, our digital world is pretty much based upon either 8-bit, 16-bit, 32-bit, or 64-bit representations of data. And one can represent only so many things with 8, 16, 32, or 64 bits. Practically, discrete binary values of some limited range are chosen to represent the analog signal. Typically, the number of values are closely related to the number of bits available in the digital equipment. An 8-bit machine can represent values from 0 to 255 (0 - 2^8-1). Obviously, every sample must be made to fit one of the available digital values. The process of fitting the sample value to an available digital value is called quantizing and the loss of resolution is called quantizing error, a source of noise generation in digital systems including ATM and ADSL.
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