1 The public switched telephone network (PSTN) is a combination of many central offices throughout the country and the world connected by copper cables,

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The public switched telephone network (PSTN) is a combination of many central offices throughout the country and the world connected by copper cables, fiber optics, microwave and cellular links, satellites and undersea telephone cables.

Each CO has a bank of network switches allowing any two phones to establish a communications link

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The combination of transmission media and switching equipment creates a “switched star network”.

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In the early days of telephony the PSTN was primarily an analog copper cable system that used mechanical switching techniques and was called a telephone exchange

Today it is almost completely digital and includes mobile phones

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Here is a simple diagram of the PSTN, the “last mile” or connection to the home is still CAT 3 copper cable and can be either analog or digital or a combination of analog and digital signals.

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LAST MILE

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Since the time of the telegraph it became apparent that a need to speed up communications needed to be addressed

Multiplexing began with telegraph systems in the 1800s. By 1872, Western Union had duplex operations on their lines.

In 1874, Thomas Edison devised a way to double the line capacity. This allowed two individual messages to travel the same direction on the line at the same time, which was called diplexing.

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It is important to recognize the difference between an analog signal and a digital signal which is depicted below

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In the 1930s, Frequency Division Multiplexing came into existence.

This type of system used a vacuum tube and carrier system with an alternating current.

The use of vacuum tubes improved the speed of transmissions and the integrity of signals.

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There are many forms of multiplexing, we are only going to look at the most commonly used ones in telephony

FDM, TDM, AND CDM

It is important to understand the basic concept of multiplexing, which is the process of sending multiple signals over a single transmission medium such as a pair of copper wires.

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Frequency division multiplexing (FDM)Is the process by which the total bandwidth available to the system is divided into a series of non overlapping frequency sub-bands that are then assigned to each communicating source and user pair

FDM allows for several conversations to travel over the same pair of wires.

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The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium.

The multiplexing divides the capacity of the high-level communication channel into several low-level logical channels, one for each message signal or data stream to be transferred.

A reverse process, known as de-multiplexing, can extract the original channels on the receiver side.

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FDM has also been used by the cable company to provide the broad spectrum of TV channels to users over a single coax cable

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A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a de-multiplexer (DEMUX).

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Time-division multiplexing (TDM) Is a digital (or in rare cases, analog) technology. TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver

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If done sufficiently quickly, the receiving devices will not detect that some of the circuit time was used to serve another logical communication path.

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Code division multiplexing (CDM) or spread spectrum is a class of techniques where several channels simultaneously share the same frequency spectrum, and this spectral bandwidth is much higher than the bit rate or symbol rate.

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In digital communications, symbol rate (also known as baud or modulation rate) is the number of symbol changes (waveform changes or signaling events) made to the transmission medium per second using a digitally modulated signal or a line code.

The Symbol rate is measured in baud (Bd) or symbols/second. In the case of a line code, the symbol rate is the pulse rate in pulses/second. Each symbol can represent or convey one or several bits of data

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Here is a side by side comparison of the three types of multiplexing methods used in telephony

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EXAMPLE 1

EXAMPLE 2

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Each of the methods of multiplexing will be discussed in more detail when we look a cellular networks.

Because of the sheer amount of users on the PSTN network, user access presents additional challenges on the PSTN

These challenges are met through additional coding process that expand on FDM, TDM and CDM technologies

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To deal with the volume of users on the PSTN coding techniques such as FDMA, TDMA and CDMA are utilized

As you can see the abbreviations are very similar to the multiplexing abbreviations with a slight variation.

FDMA: Frequency Division Multiple AccessTDMA: Time Division Multiple AccessCDMA: Code Division Multiple Acces

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Because the PSTN also utilizes fiber optic backbone systems another form of multiplexing specific to fiber is implemented

Wavelength-division multiplexing (WDM): is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e. colours) of laser light.

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WDM enables bidirectional communications over one strand of fiber, as well as multiplication of capacity.

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WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fiber.

By using WDM and optical amplifiers, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network.

Capacity of a given link can be expanded simply by upgrades to the multiplexers and demultiplexers at each end.

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