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An analog signal is sampled at the same repetitious rate known as the sample rate which must be at least twice the maximum frequency of the analog signal for full recovery of the signal. The minimum sampling rate is called the Nyquist rate. A signal sampled at less than the Nyquist rate will result in less than full reproduction of the original intelligence. Since telephone systems limit the audio (analog) signal to a maximum of 4 KHz, the sampling rate must be 2 times 4 KHz, or 8 KHz. Granny Bell does not really care about the higher order frequencies that add the warmth to our voices.

By 1972, a digital channel bank combining digital wireless transmission systems and PCM was developed that offered toll quality service. Toll quality refers to the minimum measure of service quality sufficient to charge for the service. No one wants to pay for less than toll quality service.

A technique called time division multiplexing (TDM) was developed to maximize the number of voice channels that could be transmitted over a single transmission line. An analysis of communications revealed that during a connection much of the time was empty space where no information was sent or received, especially if the call was between two lovelorn individuals. Determining optimum rates of multiplexing based upon statistical methods, diverse signals were combined on the same channel at the local exchange in such a manner that the daw signal still contained all the intelligence of the original signals.

Soon after the appearance of digital wireless and PCM, a high bit rate digital system employing TDM was developed. TDM is the process of dividing time into slots and assigning specific channels to particular slots. After the analog signal was pulse code modulated at 8 KHz, the resultant bit stream could be assigned a slot (channel) in a much faster transmission system. The number of digital channels available depended upon the speed of the transmission system.

TDM was an expensive technology. Only upon the deployment of optical transmission systems was TDM economically employed as a coding method. Optical transmission is accomplished by combining the intelligent information with an unintelligent carrier (modulating) signal. The modulated electrical signal that now includes the unintelligent carrier and the intelligent information is converted to a light wave signal by a laser diode or light emitting diode (LED). The modulated signal is carried over optical fiber from source to destination. The intelligent information is then demodulated (carrier is removed). The intelligent information is reconstructed electrically from the light wave by the action of pin diodes or avalanche photo diodes.

PCM and TDM encoding methods are used in optical communications systems which have increased in data rates from 45 Mbps to 1.2 Gbps, 1.7 Gbps, and 2.4 Gbps. These communications systems are identified in the United States and Canada as Synchronous Optical Network, or SONET. Elsewhere, particularly in Europe, fiber optic systems became known as Synchronous Digital Hierarchy (SDH). Now, we commonly see the two acronyms combined as SONET/SDH. SDH is the ITU-TSS international standard for transmitting information over optical fiber. SONET is the American National Standards Institute standard for optical fiber transmission. A SONET system deviates slightly from an SDH system.

Before SONET/SDH, the most common digital encoding scheme in use, in the mid-80s, was Plesciosynchronous Digital Hierarchy (PDH). PDH is still used in the form of DS-1/2/3/4 signals. PDH is called “plescio” (meaning “almost”) because the transmission method is neither wholly synchronous nor asynchronous. Synchronous in this context refers to the use of a specific time relationship between two or more signals that must be maintained for the successful transmission of the data and its eventual recovery by the end user. Asynchronous in this context refers to the lack of a time relationship. PDH was replaced by SDH. SDH multiplexed the PDH signals into a synchronous time division multiplexer (STM) of n = STM-n signal where n represents some integer value.

There are two common transmission methods utilized by the majority of the communications networks today. One method is called synchronous transmission and the other is asynchronous transmission. Some confusion may result from the usage of these two terms as they mean two very different things in RF communications and in telecommunications. The following discussion endeavors to explain what the terms mean in telecommunications.

Synchronous

Synchronous in RF communications, and in digital systems, refers to the relationship of the data to some time reference that is used to either generate and/or regulate the transmission of the information between the sender and the receiver. A synchronous signal is “clocked” by some repetitious reference electrical signal. The “clock” is like a drummer keeping the beat for a group of professional dancers. See Figure 3-1. Like the dancers moving simultaneously to and fro to the beat of the drum, clocked signals do not do anything until the proper clock signal provides the go-ahead. This is because many electrical circuits are “gated.” Think of it as a complex maze with a locked gate at the end of each section with all manner of people trying to get to the finish, except that no one can enter a new section of the maze without a gatekeeper opening the gate. But it is a fixed race, as the gatekeepers have specific instructions concerning who gets through first.


Figure 3-1.  A synchronous bit stream

In telecommunications, especially in ADSL terminology, synchronous refers to the simultaneous transmission and reception of data over the same physical medium (wires, coaxial cable, fiber, etc.). See Figure 3-2.


Figure 3-2.  Another synchronous bit stream

A user can receive data from a source while transmitting data to the source at the same time. However, in RF communications, the simultaneous transmission and reception of information over the same physical medium is called duplex operation. Usually, one can tell by the context which synchronous is referred to. But not always.

Asychronous

Asynchronous in RF communications, and in digital systems, refers to the lack of a timing relationship between the sender and the receiver of the data. Serial communication through the voice frequency analog modem is a good example of asynchronous transmission. See Figure 3-3.


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