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ComputerNetworking

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CHAPTER 6

DATA COMMUNICATION

FUNDAMENTALS

The first section

Exercises

Online lecture

• 6.1 INTREODUCTION

• In this chapter,we will examine four transmission cases:

• Transmitting analog signals on an analog circuit

• Transmitting analog signals on an digital circuit

• Transmitting digital signals on an digital circuit

• Transmitting digital signals on an analog circuit

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6.2 DEFINITIONS OF ANALOG AND DIGITAL

Signals that are continuous are called analog signals.

Signals that have discrete values are called digital signals.

6.3 ANALOG SIGNALS

6.3.1 Analog Signals

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6.3.1 Analog Signals

Figure 6-1 Analog wave of a typical voice signal.

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6.3.2 Signal Frequency

Frequency (for sound waves) is the number of vibrations per second that cause the particular sound.

This pure, simple signal is commonly diagrammed as a sine wave,as shown in figure 6-2.

Each complete wave is called a cycle,and the frequency of the signal is the number of cycles that occur in one second.The unit of measure for frequency is the Hertz(HZ).

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Figure 6-2 Sine waves of differing frequencies.

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Figure 6-3 The frequency ranges of some common sounds.

When referring to very high frequencies,we commonly use the designations kilohertz(KHZ)for thousands of Hertz,megahertz(MHZ)for millions of Hertz,andgigahertz (GHZ)for billions of Hertz,to more easily describe the frequencies.

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Figure 6-4 The frequency spectrum showing the common names applied to certain frequency ranges.

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Figure 6-5 A more detailed view of the frequency spectrum relevant to telecommunications.

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Figure 6-6 Common abbreviations for very large and very small quantities.

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6.3.3 Bandwidth

The difference between the upper and lower frequency is called the bandwidth.

For example: The FM radio band in the United States extends from 88 MHz to 108 MHz. Its bandwidth is 20MHZ.

The guard channel or guard band provides a buffer area so that telephone conversations or data signals on adjacent circuits don’t interfere with each other.

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Figure 6-7 Bandwidth of a voice channel.

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6.3.4 Signal Amplitude

Another characteristic of analog signals is their loudness, or amplitude.

The amplitude of the signal is also called its level.

Figure 6-8 Analog wave with constant frequency and varying amplitude.

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6.3.4 Signal Amplitude

Analog signal level is measured in decibels (dB), which is a logarithmic ratio of signal input and output power.

Because the dB is a logarithmic measure, doubling the strength of the signal increases its level by 3 dB.

The mathematical formula for the relationship between power and signal strength is

power out

dB = 10 log10 ( )

power in

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Figure 6-9 The relative power of a signal measured in decibels.

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6.3.4 Signal Amplitude

Crosstalk is interference that occurs when the signals from one communications channel interfere with those on another channel.

The loss of signal strength between two points on a communication circuit is called attenuation.

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Figure 6-10 A signal loses strength as the distance it travels increases. This loss of strength is called attenuation.

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6.3.5 Signal Phase

A signal’s phase is the relative position of the sine wave measured in degrees.

Figure 6-11 Example of a phase shift.

The three attributes of an analog signal discussed in the chapter are frequency, amplitude, and phase.

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6.4 ATTRIBUTES OF VOICE SIGNALS

Normal speech is made up of sounds that have frequencies in the range of 100 to 6000 HZ, but most of the speech energy fall in the 300 to 3000 HZ range.

Speech with frequencies above 3000 HZ is attenuated and not transmitted through the public telephone network.

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6.5 FREQUENCY DIVISION MULTIPLEXING(FDM)

This technique of packing several analog signals onto a single wire (or other media) is called frequency division multiplexing(FDM).

When FDM is employed,the signal’s frequency is shifted by equipment at the transmitting end and restored to its original frequency by similar equipment at the receiving end.

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Figure 6-12 Frequency multiplexed voice signals.

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6.6 MODULATION

Changing a communication signal by altering its amplitude, frequency, or phase is called modulation.

If the frequency is changed ,it is called frequency modulation(FM);changing the amplitude is called amplitude modulation(AM) and changing the phase is called phase modulation (PM).

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Figure 6-13 Amplitude and frequency modulation.

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6.7 CIRCUIT SIGNALING RATE

The signaling rate is defined as the number of times per second that the signal on the circuit changes,whether in amplitude,frequency,or phase.

The unit of signaling rate on a communication circuit is called baud.

The signaling rate is measured in baud.

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6.8 CIRCUIT SPEED

Circuit speed is defined as the number of bits that a circuit can carry in one second.

Circuit speed is measured in bits per second (bps).

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6.9 DIGITAL SIGNALS

The signal is made up of discreet,discontinuous voltage pulses.Each pulse represents one of the binary digits,either 1 or 0,which represents the coded data to be transmitted.This type of signal is called a digital signal.

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Figure 6-15 Digital signals. P93Manchester coding and differential Manchester coding,as known as bi-phase coding —— have become very widely used in digital transmission systems,including LANs.

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6.10 SIGNAL AND TRANSMISSION COMBINATIONS

Figure 6-16 The four combinations of analog and digital signals and transmission techniques.

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6.10.1 Analog Transmission of Analog Signals

The standard telephone is a device that converts analog data(sound) to an analog electrical signal that can be transmitted.At the other end of the connection,another telephone performs the reverse conversion,talking the incoming electrical analog and converting it to a sound(also analog) that the listener can hear.

The public switched telephone network (PSTN) is an analog network.

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6.10.2 Digital Transmission of Analog Signals

Figure 6-17 Quantization of an analog voice signal.

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6.10.2 Digital Transmission of Analog Signals

An A/D converter converts analog signals to a digital format.

The process of approximating the actual analog value signal voltage to the predetermined integer steps is called quantization.

The difference between the exact height of an analog signal and the nearest integer value when a digitizing sample is taken is called quantizing noise or digitizing distortion .

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6.10.2 Digital Transmission of Analog Signals

An D/A converter converts a digital format to analog signals.

The name that is commonly applied to A/D and D/A converters in the communications world is a coder/decoder(codec).

Pulse code modulation(PCM) is one relatively old technique for digitizing voice signals,it uses 256 integer values and samples the signal 8000 times per second.

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Figure 6-18 ADPCM codes the difference in signal strength in bits each time a sample is taken.

The type of digital modulation that takes 8,000 samples per second but uses only 4 bits per sample to code the difference between the values of two samples is called adaptive differential pulse code modulation(ADPCM).

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6.10.3 Digital Transmission of Digital Signals

Digital transmission,also called baseband transmission,is simply the transmission of the pulses of digital signal in the form of electrical pulses.

A digital transmitter/receiver sometimes called a data server unit/channel server unit(DSU/CSU) ,and ensures that the signal entering a digital circuit has properly shaped, square pulses, and is precisely timed.

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A network interface card(NIC)provides the interface from a PC to a network.It is a simple kind of a digital transmitter/receiver .

6.10.4 Analog Transmission of Digital Signals

Modems and DSU/CSU are called data circuit-terminating equipment(DCE).DCE provides the interface between DTE and the communications line.

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6.11 MODEMS

A modem (from modulation and demodulation) is a form of a A/D converter and a D/A converter.

Figure 6-19 Location of modems in a communications system.

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Figure 6-20 Block diagram representation of a modem.

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6.11 MODEMS

Equalizer circuitry compensates for variability in the transmission line used.

Fixed equalizers assume that a certain average set of parameters exists,and they shape the transmitted wave accordingly.

Adaptive equalizers are used to adjust the transmission speed and other attributes to the actual parameters of the line being used.

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6.11.1 How Modems Work

Figure 6-21 Frequencies used by a slow-speed frequency shift keying（FSK） modem.

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Figure 6-22 Frequency modulation in a slow-speed FSK modem.

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6.11.2 Phase Modulation(PM)

Phase modulation is the technique of changing a analog signal’s phase in order to modulate it.

Figure 6-23 Angles of a sine wave.

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Figure 6-24 Phase shifts.

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Figure 6-25 Phase shift keying (PSK).

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Figure 6-26 Differential phase shift keying (DPSK).

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QAM uses a combination of phase changes and relative amplitudes to generate quadbits.

Figure 6-27   An example of the phase changes and amplitudes used in one type of modem that uses QAM.

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6.11.4 Trellis Code Modulation(TCM)

Trellis Code Modulation(TCM) is a specialized form of a QAM that codes the data so that many bit combinations are invalid.

The current international standards for modems that are used on dial-up connections.

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6.11.5 V.32 and V.32bis Modem Standards

The ITU-T has published standard V.32,which specifies 9600bps,full-duplex operation using TCM with an echo cancellation technique.

Standard V.32bis,a modem following the standard can transmit data on a dial-up or leased circuit at 14400bps.

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6.11.6 V.33 Modem Standards

The V.33 Standard defines modem transmission at up to 14400 bps on 4-wire leased circuit.

6.11.7 V.34 and V.34bis Modem Standards

V.34 is the ITU-T standard for modems that operate at 28.8Kbps through a standard telephone line.

V.34bis allow a higher data rate of 33.6Kbps,and is known as V.34+.

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V.34 and V.34bis assume that most of the data transmission occurs on digital lines and that analog lines are used only in the first and last parts of the connection-the local loops.

6.11.8 V.90 Modem Standards

The ITU-T adopted the V.90 standard for 56 Kbps modems. V.90 technology assumes that at least one end of the communication line has a pure digital connection to the telephone network.

An typical example is home connection to the Internet.

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Figure 6-28 A V.90 connection is assumed to have this configuration.

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6.11.8 V.90 Modem Standards

V.90 transmission is asymmetric, the all-digital end has the 56 Kbps data rate and the analog end follow the V.34bis standard and occur at maximum rate of 33.6 Kbps.

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• 6.11.9 V.92 Modem Standards

• The top speed for downloads is still 56Kbps,and has a number of enhancements:

• Startup time:the time needed to establish a connection has been reduced by about half.

• The maximum upload speed has been increased from 33.6Kbps to 48Kbps.

• Internet call waiting.

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Figure 6-29 Modem standards.

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6.11.10 Modem Data compression

High-speed modems achieve a high throughput rate by using modulation and Data compressiontechniques.

For example: the ITU-T V.42bis standard

6.11.11 Higher Speeds

P108

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6.11.12 Modems for Fiber-Optic Circuits

The communications circuit is an optical fiber.

6.11.13 Cable Modems

A cable modem links a DTE to a cable television system cable.

Cable modems for each CATV system have a standard, called DOCSIS.

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6.11.14 Null Modems

Figure 6-30 Modem eliminator (simplified).

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Figure 6-31 A RS-232-C cable with male and female 25-pin connectors.

6.12 DTE-DCE INTERFACE STANDARDS

6.12.1 RS-232-C

The most widely used interface standard between PCs and modems is RS-232-C .

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Figure 6-32 RS-232-connector pin assignments.

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6.12.2 RS-232-D

6.12.3 RS-449

6.12.4 RS-336

6.12.5 X.21 and X.21bis

The X.21 interface standard defines the interface to a digital circuit .

6.12.6 Current Loop

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• 6.13 WHY DIGITAL TRANSMISSION IS SUPERIOR

• The reasons that digital transmission is superior to analog transmission are :

• better data integrity

• higher capacity

• easier integration

• better security and privacy

• lower cost

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Figure 6-33 The ways in which increasingly higher data transmission rates have been achieved on dial-up communication lines.

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6.14 OTHER WAYS OF CLASSIFYING TRANSMISSION

6.14.1 Type of Signal Transmitted:Baseband and Broadband

Baseband transmission is for digital transmission.

Broadband transmission is for analog transmission.

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6.14.2 Data Flow

1.Simplex Transmission

A circuit that allows transmission in one direction only is called a simplex circuit.

2. Half-duplex Transmission

A circuit that allows transmission in either direction but not at the same time is called a half-duplex circuit.

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6.14.2 Data Flow

3.Full-duplex transmission

A circuit that allows transmission in both directions simultaneously is called a full-duplex circuit.

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6.14.3 Type of Physical Connection

Parallel Mode

A circuit that allows all of the bits in a character to be transmitted simultaneously is called a parallelcircuit.

Serial Mode

A circuit in which the bits of a character are transmitted one after the other on a single communication path is called a serial circuit.

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Figure 6-35 Parallel and serial transmission.

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6.14.4 Timing

1.Asynchronous Transmission

When each character to be transmitted is surrounded with start and stop bits the transmission is called asynchronous.

The mechanism by which character synchronization occurs in asynchronous transmission is called startand stopbits.

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6.14.4 Timing

The means by which the receiver knows which bits to group together to form a character is called character synchronization.

2.Synchronous Transmission

When each block of data to be transmitted is surrounded with synchronizing and other control characters the transmission is called synchronous.

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A

1.A network interface card （　　）.

A、 provides the interface from a PC to a network

B、 provides the interface from one network to another

×

C、 provides the interface from a modem to a network

×

×

D、 converts signals from analog to digital

Redo

Next

2.The most widely used interface standard between PCs and modems is （　　）.

D

×

A、 X.25

×

B、 USB

×

C、 RS-232-D

D、 RS-232-C

Redo

Next

3.The X.21 interface standard （　　）.

B

A、 is a replacement for RS-232-C

×

B、 defines the interface to a digital circuit

C、 uses a PL-259 connector

×

×

D、 All of the above

Redo

Next

4.The difference between the exact height of an analog signal and the nearest integer value when a digitizing sample is taken is called （　　）.

C

×

A、 digitizing discrepancy

×

B、 digitizing error

C、 quantizing noise

×

D、 digitizing discernment

Redo

Next

5. Hertz is the unit of measure for （　　）.

D

×

A、 baud

×

B、 bit rate

×

C、 data rate

D、 cycles per second

Redo

Next

6.The unit of signaling rate on a communication circuit is called ________.

• baud

two

7.A signal drop of 10 dB represents a drop in power of _______ times.

8.A circuit that allows transmission in one direction only is called a _________ circuit.

• simplex

9.The type of digital modulation that takes 8,000 samples per second but uses only 4 bits per sample to code the difference between the values of two samples is called ____________ .