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To convert an analog signal to a digital signal, you must start with a uniform periodic reference called the pulse train. See Figure 2-7. A theorem states that for complete recovery of intelligence, the minimum sampling rate (the "clock" rate or periodicity of the uniform pulse train) is twice the maximum frequency of the analog signal. This minimum rate is called the Nyquist rate.
During the sampling period, the analog signal level is "measured." The measured analog signal level is then represented by a discrete, digital signal whose level has some relationship to the measured analog signal level. This type of digital sampling is called pulse amplitude modulation (PAM). The output of a PAM modulator is a series of pulses whose amplitudes are related to the amplitudes of the analog signal at the point in time the samples were taken.
If the analog signal is a constant frequency and constant level signal, PAM produces a bit stream with no variation in its level. Not very useful for conveying intelligence. However, voice is not a constant frequency source. Voice levels and frequencies are changing as we talk. The PAM digital pulse train that represents analog voice signals is therefore constantly changing its amplitude in accordance with the voice level changes.
Single polarity PAM can easily be generated by offsetting the AC analog voltage with a DC offset voltage so that the minimum AC voltage level is above 0 volts. See Figure 2-8. To recover the original intelligence, the PAM pulse train is applied to a low pass filter. PAM signals are easily distorted, so the technique is rarely used to transmit information directly. But PAM is usually an intermediate step in other forms of pulse modulation.
Note that only the level of the sampling pulse train changes in PAM. In pulse time modulation (PTM), the pulse train amplitude remains constant while the timing of the individual pulses varies with the level of the analog signal at the moment in time the pulse occurs.
In pulse width modulation (PWM), the pulse train amplitude remains constant while the width of each individual pulse varies according to the analog level at the moment in time the pulse occurs. This is also called pulse duration modulation (PDM) and pulse length modulation (PLM).
For any given transmission type and for most users, bandwidth is wasted if a communications channel sits idly waiting for a user to transmit something. How often is a homeowner's telephone offhook, thereby engaging the line for use and compelling the telephone company to assign a switched channel? (Assume for the moment that there are no teenagers in the household.) How often does a client computer on a network demand host services? For much of the world, the answer is, not often. So, data transmission can be characterized as "bursty"; that is, occurring in a random manner. Dedicating communication channels to users would then result in a huge waste of the commodity that is most in demand in communicationsavailable bandwidth.
The operative word is "available." The available bandwidth for communicating between your home and the local access office until now was 4 KHz. Other bandwidth restrictions are present throughout the public and private communication networks. Governing bodies throughout the world define the available bandwidth for the various types of communication services.
Math wizards were able to model the bursty nature of human communications with some degree of accuracy. Using the models for templates, modern communication systems are now designed to provide reliable service while minimizing the use of available bandwidth and minimizing the number of copper wires/fiber links necessary to provide the required quality of service. The ability to perform these feats of magic is embedded in the technique of multiplexing digital and analog signals.
Bandwidth is the limiting factor in communications. Within the confines of communication technology, bandwidth is finite and there is just not enough to go around. As we demand more and more communication services, bandwidth will become even more precious. To accommodate hundreds of millions of users in the public switched communication networks with an individual dedicated channel for their sole use would be impossible due to the need to wire the country and world with sufficient copper and/or fiber to accommodate that many users. The cost alone would be astronomical. Assuming not everyone desires access simultaneously to the switched networks, thereby lessening the demand on the number of available channels at any given moment, even to wire the globe for such service would still be prohibitive. The most economical approach is to place in service as few physical wires or fiber links as possible, and maximize the utilization of the available channels within the parameters of some defined Quality of Service (QoS). Both the public switched telephone network and private networks seek to maximize the use of their available bandwidth. ATM and ADSL help them do just that.
Initially, Plain Old Telephone Service (POTS) was the only customer of the public switched telephone network (PSTN). Before long, teletype machines were developed that allowed the transmission of data faster than a Morse code key operator could tap out messages on a manual keyset. For many years, voice and teletype data transmission were virtually the only two uses of the PSTN. After World War II, developments in electronics paved the way for uses of the PSTN that the original network planners never dreamed of. Faster and faster data rates were the objective as the race for new services, and revenue streams, was under way, especially after deregulation of the telephone industry. Numerous types of data acquisition and transmission technologies came into existence, each demanding its own special treatment by PSTN operating companies. Telephone companies offered the new data transmission services in conjunction with POTS as government agencies insisted that all new services had to be compatible with the extensive in-place POTS network.
Figure 2-9. Transmission channel utilization by PSTN user characteristics
Today, many different services requiring PSTN access are gobbling up the system resources. Typically, the bit rate of any particular service is constant for the period of time the service actually transmits its data. But most services do not transmit continuously, 24 hours per day, seven days per week. From the perspective of PSTN operating companies, these variable and individually unpredictable demands on system resources give the appearance of users with an extremely variable bit rate. While any one particular service or company may have a very constant bit rate, the sum of all services results in a variable one. What we are left with is a batch of user services with variable bit rates that all operate totally without regard for any other service or user.
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