Chapter 6

Multiplexing

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Objectives

Discuss the differences between frequency division multiplexing (FDM) and time division multiplexing (TDM).

Discuss the various steps required to prepare a signal for TDM.

Understand amplitude modulation (AM). Understand pulse code modulation (PCM).

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Objectives (continued)

Describe how a DS0 signal is formed. Describe how a DS1 signal is formed. Know the medium required for various levels

of multiplexed signals. Explain why all TDM systems have 8000

frames per second.

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Objectives (continued)

Explain the purpose of quantizing. Explain wave division multiplexing (WDM). Describe the various levels of SONET and

know how many DS0 signals each contains.

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Multiplexing

In telecommunications, multiplexing means to combine many signals (voice or data) so they can be sent over one transmission medium. Space Division Multiplexing Frequency Division Multiplexing (FDM) Time Division Multiplexing (TDM)

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Bandwidth

Bandwidth (Bw) refers to the width of a signal, which is determined by taking the difference between the highest frequency of the signal and its lowest frequency.

A voice signal is usually though of as a signal between 0 and 4000 Hz (Bw = 4000 Hz).

In the United States, AT&T designed its FDM systems to handle the band of signals between 200 and 3400 Hz (Bw = 3200 Hz).

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Twelve-Channel Frequency Division Multiplexing

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6.1 The Channel Unit of a FDM System

Each signal fed to a FDM system interfaces to the multiplexer through a device called a channel unit.

The channel unit makes changes to the input signal so it can be multiplexed with other signals for transmission. Voice signals arrive between 0 and 4000 Hz. A sharp cutoff bandpass filter passes only 200 – 3400 Hz. A modulator takes the 200 – 3400 Hz signal and shifts it to

another frequency.

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6.2 Transmit and Receive Paths: The Four-Wire System

The first multiplexers used four wires for the transmission medium. One pair of wires was used for the signal going in one

direction. A separate pair of wires was used for the signal going

in the opposite direction. Two fiber optic strands are required to connect

multiplexers. Two microwave frequencies are required to

connect multiplexers.

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6.3 Why Use Multiplexing?

Multiplexing was first used to reduce the number of transmission media needed between cities and towns.

This resulted in significantly reduced costs for trunk circuits.

Fiber optic cable allows the multiplexer to combine as many as 6 million signals in one direction on one fiber strand.

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6.4 Amplitude Modulation

In amplitude modulation (AM) the amplitude of the carrier frequency coming out of the mixer will vary according to the changing amplitude of the input voice signal.

Amplitude modulation is used by AM broadcast stations. Carrier frequencies are between 540 kHz and

1,650 kHz. Carrier frequencies are licensed by the FCC.

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Amplitude Modulation

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6.5 Frequency Division Multiplexing

Using FDM, many telephone calls can be multiplexed over two pairs of wires.

If we wish to multiplex 12 calls over two pairs of wires between cities A and B: We hook up a multiplexer containing 12 transmitters to the first

wire pair in city A; at the other end we attach 12 receivers in city B.

We hook up a multiplexer containing 12 transmitters to the second wire pair in city B; at the other end we attach 12 receivers in city A.

The two simplex circuits make up a full-duplex circuit between A and B.

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6.6 Hybrid Network

The circuit from a telephone arrives at the input to the channel unit of a multiplexer as a two-wire circuit.

The channel unit contains a hybrid network that interfaces the two-wire input to a four-wire transmit/receive path. The transmit path connects to the transmitter for

this channel. The receive path connects to the receiver for this

channel.

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Channel Unit

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6.7 Amplitude Modulation Technology

Amplitude modulation is a technique in which the amplitude of the high-frequency signal changes when the high-frequency signal is mixed with a low-frequency signal in a device called a mixer or modulator. The high-frequency signal is termed the carrier

frequency. The low-frequency signal is called the modulating

signal.

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Amplitude Modulation Example

If a 64,000 Hz signal is modulated by a 4,000 Hz signal, the outputs of the modulator will be: 64,000 Hz 4,000 Hz 68,000 Hz (64,000 + 4,000) 60,000 Hz (64,000 – 4,000)

The two additional signals generated by the mixing process are called sidebands. The higher signal is called the upper sideband. The lower signal is called the lower sideband.

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6.8 Bandwidth and Single-Sideband Suppressed Carrier

The bandwidth of an AM signal is twice the bandwidth of the modulating signal. When a low-frequency signal is present as an

input to the mixer, the amplitude of both sideband signals changes as the input signal changes.

Since both sidebands have been modulated, they both contain the frequency changes of the modulating signal.

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Single-Sideband (SSB) systems

We only need to demodulate one of the sidebands to recapture the intelligence of the modulating signal.

Since only one sideband is needed to demodulate the modulating signal, some systems will transmit only one sideband with the carrier frequency.

Systems that do not transmit the main carrier frequency, but transmit the sideband only, are called Single-Sideband Suppressed Carrier (SSB-SC) systems.

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6.9 CCITT Standards for Frequency Division Multiplexing

Telecommunications standards are set by several organizations.

Predominant organizations: American National Standards Institute (ANSI)

Establishes standards for North America Consultative Committee on International

Telegraphy and Telecommunications (CCITT) Organization within the International

Telecommunications Union (ITU) Creates worldwide standards

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6.10 Applications for Frequency Division Multiplexing

We now use FDM on fiber optic facilities to place multiple TDM systems on a fiber.

Time Division Multiplexing (TDM) places each signal to be multiplexed on the medium for a brief instant of time at recurring intervals of time.

Wave Division Multiplexing (WDM) uses many different lightwaves to connect many TDM systems on one fiber.

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6.11 Wave Division Multiplexing

The first commercial use of WDM was to use different frequencies of light for transmitters on each end of one fiber.

The velocity of propagation is equal to the product of the wavelength and the frequency vp = λ * f In a vacuum, the speed of light, C, is 3x108 m/s

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6.12 Dense Wave Division Multiplexing (DWDM)

The development of the erbium-doped fiber amplifier (EDFA) made DWDM possible.

C band EDFAs operate from 1530 nm to 1560 nm.

The bandwidth of a C Band system is 4 trillion Hz.

DWDM is available with transmitters that use 4, 8, 16, 32, and 180 lasers.

Frequencies used chosen from the ITU Grid.

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ITU Grid

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6.13 TDM Using Pulse Amplitude Modulation (PAM) Signals

Early TDM signals multiplexed samples of the human voice from different channels over one common facility.

Nyquist made some studies of voice sampling techniques and developed the Nyquist Theorem.

The original signal can be reproduced if the sampling rate is at least twice the highest frequency present in the original signal.

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Pulse Amplitude Modulation Samples

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Creation of a Time Division Multiplexing Signal

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6.14 Time Division Multiplexing Using PCM Signals

TDM systems using PAM are susceptible to noise interference.

The industry standard for converting an analog signal into a digital signal is called Pulse Code Modulation (PCM).

PCM/TDM has many advantages over FDM and PAM/TDM Digital signals are less susceptible to noise. Digital circuitry lends itself readily to integrated circuit

design, which makes digital circuits cheaper than analog circuits.

Digital-to-digital interface is easily achieved.

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6.15 Pulse Code Modulation

The industry standard method for converting one analog voice signal into a digital signal is PCM.

There are five basic steps to PCM: Sampling Quantization Companding Encoding Framing

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6.15.1 Sampling

Sampling refers to how often measurements are taken of the input analog signal.

The Nyquist Theorem states that an analog signal should be sampled at a rate at least twice its highest frequency.

In telecommunications, the network was designed to handle signals between 0 and 4000 Hz.

Every 1/8000 of a second (125 μs) a voltage measurement is taken of the input signal.

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6.15.2 Quantization

In PCM, each voltage measurement is converted to an 8-bit code, and the 8-bit coded is sent instead of the actual voltage. The input signal can be any level. The 8-bit coded limits the conversion process to

the recognition of 256 (28 = 256) levels. Quantization is the process to convert the

input level to one of the 256 discrete codes available.

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6.15.3 Companding

Quantizing a signal will result in some distortion because we do not code the exact voltage of the input signal.

This distortion is called quantizing noise, it is greater for low-amplitude signals.

Companding is used to reduce quantizing noise. The signal is compressed at the transmitter to divide

low-amplitude signals into more levels. When the signal is decoded at the receiving system, it

is expanded by reversing the compression process.

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6.15.4 Encoding

After compression and quantization of the input signal, it will be one of the 256 discrete signal levels that can be assigned an 8-bit code.

The process of assigning an 8-bit code to represent the signal level is known as encoding.

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6.15.5 Framing

The encoded 8-bit signal is time division multiplexed with 23 other 8-bit signals to generate 192 bits for 24 signals.

A single framing bit is added to these 192 bits to make a 193-bit frame.

Framing Bits Follow an established pattern of 1s and 0s for 12 frames

(1,0,0,0,1,1,0,1,1,1,0,0) and then repeat in each of the succeeding 12 frames.

This pattern is used by the receiving terminal to stay synchronized with the received frames.

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6.16 North American DS1 System

The basic building block for digital transmission standards begins with the DS0 signal level. One voice equivalent 8 bits/sample x 8,000 samples/second

The DS1 signal has 24 voice equivalents 193 bits per frame

24 x 8 bits per channel 1 framing bit

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One Frame of a DS1 Signal

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6.17 T1 Carrier Systems

The multiplexing system of choice to connect two central offices in a North American DS1 system is called T1 carrier.

It is necessary to remove load coils from pairs being used for T1 to improve high-frequency response.

Repeaters (pulse regenerators) are placed where the load coils were and at each central office.

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Superframe

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Extended Superframe

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Alternate Mark Inversion

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Bipolar Violation

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6.18 European TDM 30 + 2 System

The European TDM system multiplexes 32 DS0 channels together. Channel 0 is used for synchronizing (framing) and

signaling. Channels 1-15 and 17-31 are used for voice. Channel 16 is reserved for future use as a

signaling channel. The total signal rate is 2.048 Mbps (64 kbps *

32 channels)

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6.19 Statistical Time Division Multiplexing (STDM)

The T1 and E1 systems are referred to as synchronous time division multiplexing systems.

The statistical time division multiplexer improves the efficiency of a TDM system. Channel units do not have reserved time slots. Time slots are dynamically assigned.

Also called stat muxs, intelligent multiplexers, and asynchronous multiplexers.

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6.20 Higher Levels of TDM

The 24-channel DS1 signal developed in the early 1960s is the basic multiplex building block in North America.

Two DS1 signals can be combined into a 48-channel DS1C signal.

Four DS1 signals can be combined into a 96-channel DS2 signal.

Seven DS2 signals can be combined into a 672-channel DS3 signal.

Six DS3 signals can be combined into a 4032-channel DS4 signal.

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Implementation of DS Signals

DS is the signal, T is the physical implementation. T1 can use wire to send a DS1 signal. T1C can use wire to send a DS1C signal. T2 can use wire to send a DS2 signal. T3 must use coaxial cable, fiber optic cable, or

digital microwave radio to send a DS3 signal. T4 must use coaxial cable, fiber optic cable, or

digital microwave radio to send a DS4 signal.

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6.21 SONET Standards

Fiber optics use synchronous optical network (SONET) standards.

The initial SONET standard is OC-1, this level is known as synchronous transport level 1 (STS-1). It has a synchronous frame structure at a speed

of 51.840 Mbps. OC-1 is an envelope containing a DS3 signal (28

DS1 signals or 672 channels).

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Table 6-2 TDM Comparisons

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Wave Division Multiplexing (Courtesy of Sprint)

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6.22 Summary

Multiplexing of signals onto one medium can be accomplished using FDM and TDM.

When fiber optics was first introduced, one light signal was used on the fiber.

Today instead of connecting the TDM system directly to the fiber, the TDM system is connected to a channel unit of a DWDM and the DWDM is connected to the fiber.