Chapter 11
112
© 2008 The McGraw-Hill Companie 1 Principles of Electronic Principles of Electronic Communication Systems Communication Systems Third Edition Louis E. Frenzel, Jr.
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Introduction
Transcript of Chapter 11
Principles of Electronic Communication Systemsin Communication Systems
11-1: Digital Codes
11-3: Transmission Efficiency
11-8: Protocols
11-1: Digital Codes
The proliferation of applications that send digital data over communication channels has resulted in the need for efficient methods of transmission, conversion, and reception of digital data.
© 2008 The McGraw-Hill Companies
11-1: Digital Codes
Data processed and stored by computers can be numerical or text.
The signals used to represent computerized data are digital.
Even before the advent of computers, digital codes were used to represent data.
© 2008 The McGraw-Hill Companies
11-1: Digital Codes
Early Digital Codes
The Morse code was originally designed for wired telegraph, but was later adapted for radio communication.
The Morse code consists of a series of “dots” and “dashes” that represent letters of the alphabet, numbers, and punctuation marks.
The Baudot code was used in the early teletype machine, a device for sending and receiving coded signals over a communication link.
© 2008 The McGraw-Hill Companies
11-1: Digital Codes
Modern Binary Codes
For modern data communication, information is transmitted using a system in which the numbers and letters to be represented are coded, usually by way of a keyboard, and the binary word representing each character is stored in a computer memory.
© 2008 The McGraw-Hill Companies
Modern Binary Codes: American Standard Code for Information Interchange
The most widely used data communication code is the 7-bit binary code known as the American Standard Code for Information Interchange (ASCII).
ASCII code can represent 128 numbers, letters, punctuation marks, and other symbols.
ASCII code combinations are available to represent both uppercase and lowercase letters of the alphabet.
Several ASCII codes have two- and three-letter designations which initiate operations or provide responses for inquiries.
© 2008 The McGraw-Hill Companies
Modern Binary Codes: Hexadecimal Values
Binary codes are often expressed using their hexadecimal, rather than decimal values.
To convert a binary code to its hexadecimal equivalent, first divide the code into 4-bit groups.
Start at the least significant bit on the right and work to the left. (Assume a leading zero on each of the codes.)
© 2008 The McGraw-Hill Companies
Modern Binary Codes: Extended Binary Coded Decimal Interchange Code
The Extended Binary Coded Decimal Interchange Code (EBCDIC) was developed by IBM.
The EBDIC is an 8-bit code allowing a maximum of 256 characters to be represented.
The EBCDIC is used primarily in IBM and IBM-compatible computing systems and is not widely used as ASCII.
© 2008 The McGraw-Hill Companies
Serial Transmission
Parallel
Serial
Data transfers in long-distance communication systems are made serially.
In a serial transmission, each bit of a word is transmitted one after another.
Parallel data transmission is not practical for long-distance communication.
© 2008 The McGraw-Hill Companies
Figure 11-4: Serial transmission of the ASCII letter M.
© 2008 The McGraw-Hill Companies
Serial Transmission: Expressing the Serial Data Rate
The speed of data transfer is usually indicated as number of bits per second (bps or b/s).
Another term used to express the data speed in digital communication systems is baud rate.
Baud rate is the number of signaling elements or symbols that occur in a given unit of time.
A signaling element is simply some change in the binary signal transmitted.
© 2008 The McGraw-Hill Companies
Asynchronous Transmission
In asynchronous transmission each data word is accompanied by start and stop bits that indicate the beginning and ending of the word.
When no information is being transmitted, the communication line is usually high, or binary 1.
In data communication terminology, this high level is referred to as a mark.
To signal the beginning of a word, a start bit, a binary 0 or space is transmitted.
© 2008 The McGraw-Hill Companies
Asynchronous Transmission
Most low-speed digital transmission (the 1200- to 56,000-bps range) is asynchronous.
Asynchronous transmissions are extremely reliable.
The primary disadvantage of asynchronous communication is that the extra start and stop bits effectively slow down data transmission.
© 2008 The McGraw-Hill Companies
Figure 11-6: Asynchronous transmission with start and stop bits.
© 2008 The McGraw-Hill Companies
Synchronous Transmission
The technique of transmitting each data word one after another without start and stop bits, usually in multiword blocks, is referred to as synchronous data transmission.
To maintain synchronization between transmitter and receiver, a group of synchronization bits is placed at the beginning and at the end of the block.
Each block of data can represent hundreds or even thousands of 1-byte characters.
© 2008 The McGraw-Hill Companies
Synchronous Transmission
The special synchronization codes at the beginning and end of a block represent a very small percentage of the total number of bits being transmitted, especially in relation to the number of start and stop bits used in asynchronous transmission.
Synchronous transmission is therefore much faster than asynchronous transmission because of the lower overhead.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
Encoding Methods
Whether digital signals are being transmitted by baseband methods or broadband methods, before the data is put on the medium, it is usually encoded in some way to make it compatible with the medium.
© 2008 The McGraw-Hill Companies
Encoding Methods
In the nonreturn to zero (NRZ) method of encoding the signal remains at the binary level assigned to it for the entire bit time.
In return to zero (RZ) encoding the voltage level assigned to a binary 1 level returns to zero during the bit period.
Manchester encoding, also referred to as biphase encoding, is widely used in LANs. In this system a binary 1 us transmitted first as a positive pulse, for one half of the bit interval, and then as a negative pulse for the remaining part of the bit interval.
© 2008 The McGraw-Hill Companies
11-3: Transmission Efficiency
Transmission efficiency is the accuracy and speed with which information, whether it is voice or video, analog or digital, is sent and received over communication media.
It is the basic subject matter of the field of information theory.
© 2008 The McGraw-Hill Companies
11-3: Transmission Efficiency
Hartley’s Law
The amount of information that can be sent in a given transmission is dependent on the bandwidth of the communication channel and the duration of transmission.
Mathematically, Hartley’s law is
C = 2B
Where C is the channel capacity (bps) and B is the channel bandwidth.
© 2008 The McGraw-Hill Companies
11-3: Transmission Efficiency
Hartley’s Law
The greater the number of bits transmitted in a given time, the greater the amount of information that is conveyed.
The higher the bit rate, the wider the bandwidth needed to pass the signal with minimum distortion.
© 2008 The McGraw-Hill Companies
Transmission Media and Bandwidth
The two most common types of media used in data communication are wire cable and radio.
The two types of wire cable used are coaxial and twisted pair.
Coaxial cable has a center conductor surrounded by an insulator over which is a braided shield. The entire cable is covered with a plastic insulation.
A twisted-pair cable is two insulated wires twisted together.
© 2008 The McGraw-Hill Companies
11-3: Transmission Efficiency
Figure 11-10: Types of cable used for digital data transmission. (a) Coaxial cable.
(b) Twisted-pair cable, unshielded (UTP).
© 2008 The McGraw-Hill Companies
Twisted-pair is available as unshielded (UTP) or shielded.
Coaxial cable and shielded twisted-pair cables are usually preferred, as they provide some protection from noise and cross talk.
Cross talk is the undesired transfer of signals from one unshielded cable to another adjacent one caused by inductive or capacitive coupling.
© 2008 The McGraw-Hill Companies
Transmission Media and Bandwidth
The bandwidth of any cable is determined by its physical characteristics.
All wire cables act as low-pass filters because they are made up of wire that has inductance, capacitance, and resistance.
© 2008 The McGraw-Hill Companies
11-3: Transmission Efficiency
Multiple Coding Levels
Channel capacity can be modified by using multiple-level encoding schemes that permit more bits per symbol to be transmitted.
It is possible to transmit data using more than just two binary voltage levels or symbols.
Multiple voltage levels can be used to increase channel capacity.
Other methods, such as using different phase shifts for each symbol, are used.
© 2008 The McGraw-Hill Companies
Impact of Noise in the Channel
An important aspect of information theory is the impact of noise on a signal.
Increasing bandwidth increases the rate of transmission but also allows more noise to pass.
Typical communication systems limit the channel capacity to one-third to one-half the maximum to ensure more reliable transmission in the presence of noise.
© 2008 The McGraw-Hill Companies
Digital data are transmitted over the telephone and cable television networks by using broadband communication techniques involving modulation, which are implemented by a modem, a device containing both a modulator and a demodulator.
Modems convert binary signals to analog signals capable of being transmitted over telephone and cable TV lines and by radio, and then demodulate such analog signals, reconstructing the equivalent binary output.
© 2008 The McGraw-Hill Companies
Conventional analog dial-up modems.
Cable TV modems.
11-4: Basic Modem Concepts
Figure 11-12: How modems permit digital data transmission on the telephone network.
© 2008 The McGraw-Hill Companies
11-4: Basic Modem Concepts
Modulation for Data Communication
The four main types of modulation used in modern modems are:
Frequency-shift keying (FSK)
Phase-shift keying (PSK)
© 2008 The McGraw-Hill Companies
Modulation for Data Communication: Frequency-Shift Keying (FSK)
Frequency-shift keying (FSK) is the oldest and simplest form of modulation used in modems.
In FSK, two sine-wave frequencies are used to represent binary 0s and 1s.
A binary 0, usually called a space, has a frequency of 1070 Hz.
A binary 1, referred to as a mark, is 1270 Hz.
These two frequencies are alternately transmitted to create the serial binary data.
© 2008 The McGraw-Hill Companies
Figure 11-13: Frequency-shift keying. (a) Binary signal. (b) FSK signal.
© 2008 The McGraw-Hill Companies
Modulation for Data Communication: Phase-Shift Keying
In phase-shift keying (PSK), the binary signal to be transmitted changes the phase shift of a sine-wave character depending upon whether a binary 0 or binary 1 is to be transmitted.
A phase shift of 180°, the maximum phase difference that can occur, is known as a phase reversal, or phase inversion.
During the time that a binary 0 occurs, the carrier is transmitted with one phase; when a binary 1 occurs, the carrier is transmitted with a 180° phase shift.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
Modulation for Data Communication: QPSK
One way to increase the binary data rate while not increasing the bandwidth required for the signal transmission is to encode more than 1 bit per phase change.
In the system known as quadrature, quarternary, or quadra phase PSK (QPSK or 4-PSK), more bits per baud are encoded, the bit rate of data transfer can be higher than the baud rate, yet the signal will not take up additional bandwidth.
In QPSK, each pair of successive digital bits in the transmitted word is assigned a particular phase.
Each pair of serial bits, called a dibit, is represented by a specific phase.
© 2008 The McGraw-Hill Companies
11-4: Basic Modem Concepts
Figure 11-24: Quadrature PSK modulation. (a) Phase angle of carrier for different pairs of bits. (b) Phasor representation of carrier sine wave. (c) Constellation diagram
of QPSK.
Modulation for Data Communication: QPSK
The QPSK modulator consists of a 2-bit shift register implemented with flip-flops, commonly known as a bit splitter.
The serial binary data train is shifted through the register.
The bits from the flip-flops are applied to balanced modulators.
The carrier oscillator is applied to one balanced modulator and through a 90° phase shifter to another balanced modulator.
The outputs of the balanced modulators are linearly mixed to produce the QPSK signal.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
Modulation for Data Communication: QAM
One of the most popular modulation techniques used in modems for increasing the number of bits per baud is quadrature amplitude modulation (QAM).
QAM uses both amplitude and phase modulation of a carrier.
In 8-QAM, there are four possible phase shifts and two different carrier amplitudes.
Eight different states can be transmitted.
With eight states, 3 bits can be encoded for each baud or symbol transmitted.
Each 3-bit binary word transmitted uses a different phase-amplitude combination.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
11-4: Basic Modem Concepts
Spectral Efficiency and Noise
Spectral efficiency is a measure of how fast data can be transmitted in a given bandwidth (bps/Hz).
Different modulation methods give different efficiencies.
Modulation
The signal-to-noise (S/N) ratio clearly influences the spectral efficiency.
The greater the noise, the greater the number of bit errors.
The number of errors that occur in a given time is called the bit error rate (BER).
The BER is the ratio of the number of errors that occur to the number of bits that occur in a one second interval.
© 2008 The McGraw-Hill Companies
11-5: Wideband Modulation
While most modulation methods are designed to be spectrally efficient, there is another class of modulation methods that does just the opposite.
These methods are designed to use more bandwidth. The transmitted signal occupies a bandwidth many times greater than the information bandwidth.
The two most widely used wideband modulation methods are spread spectrum and orthogonal frequency-division multiplexing.
© 2008 The McGraw-Hill Companies
Spread Spectrum
Spread spectrum (SS) is a modulation and multiplexing technique that distributes a signal and its sidebands over a very wide bandwidth.
After World War II, spread spectrum was developed by the military because it is a secure communication technique essentially immune to jamming.
Currently, unlicensed operation is permitted in the 902- to 928-MHz, 2.4- to 2.483-GHz, and 5.725- to 5.85-GHz ranges, with 1 W of power.
© 2008 The McGraw-Hill Companies
Spread Spectrum
Spread spectrum on these frequencies is being widely incorporated into a variety of commercial communication systems, particularly wireless data communication.
Numerous LANs and portable personal computer modems use SS techniques, as does a class of cordless telephones.
The most widespread use of SS is in cellular telephones. It is referred to as code-division multiple access (CDMA).
© 2008 The McGraw-Hill Companies
Spread Spectrum
There are two basic types of spread spectrum: frequency-hopping (FH) and direct-sequence (DS).
In frequency-hopping SS, the frequency of the carrier of the transmitter is changed according to a predetermined sequence, called pseudorandom, at a rate higher than that of the serial binary data modulating the carrier.
In direct-sequence SS, the serial binary data is mixed with a higher-frequency pseudorandom binary code at a faster rate, and the result is used to phase-modulate a carrier.
© 2008 The McGraw-Hill Companies
11-5: Wideband Modulation
Frequency-Hopping Spread Spectrum
In a frequency-hopping SS transmitter, the serial binary data to be transmitted is applied to a conventional two-tone FSK modulator.
The modulator output is applied to a mixer.
Also driving the mixer is a frequency synthesizer.
The output signal from the bandpass filter after the mixer is the difference between one of the two FSK sine waves and the frequency of the frequency synthesizer.
The synthesizer is driven by a pseudorandom code generator, which is either a special digital circuit or the output of a microprocessor.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
11-5: Wideband Modulation
Frequency-Hopping Spread Spectrum
In a frequency-hopping SS system, the rate of synthesizer frequency change is higher than the data rate.
This means that although the data bit and the FSK tone it produces remain constant for one data interval, the frequency synthesizer switches frequencies many times during this period.
© 2008 The McGraw-Hill Companies
11-5: Wideband Modulation
Frequency-Hopping Spread Spectrum
The time that the synthesizer remains on a single frequency is called the dwell time.
The frequency synthesizer puts out a random sine wave frequency to the mixer, and the mixer creates a new carrier frequency for each dwell interval.
The resulting signal, whose frequency rapidly jumps around, effectively scatters pieces of the signal all over the band.
© 2008 The McGraw-Hill Companies
11-5: Wideband Modulation
Direct-Sequence Spread Spectrum
In a direct-sequence SS (DSSS) transmitter, the serial binary data is applied to an X-OR gate along with a serial pseudorandom code that occurs faster than the binary data.
One bit time for the pseudorandom code is called a chip, and the rate of the code is called the chipping rate. The chipping rate is faster than the data rate.
The signal developed at the output of the X-OR gate is then applied to a PSK modulator, typically a BPSK device.
© 2008 The McGraw-Hill Companies
11-5: Wideband Modulation
Direct-Sequence Spread Spectrum
The carrier phase is switched between 0 and 180° by the 1s and 0s of the X-OR output.
The PSK modulator is generally some form of balanced modulator.
The signal phase modulating the carrier, being much higher in frequency than the data signal, causes the modulator to produce multiple, widely spaced sidebands whose strength is such that the complete signal takes up a great deal of the spectrum. Thus the resulting signal is spread.
Because of its randomness, the signal looks like wideband noise to a conventional narrowband receiver.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
11-5: Wideband Modulation
Direct-Sequence Spread Spectrum
Direct-sequence SS is also called code-division multiple access (CDMA), or SS multiple access.
The term multiple access applies to any technique that is used for multiplexing many signals on a single communication channel.
CDMA is used in satellite systems so that many signals can use the same transponder.
It is also widely used in cellular telephone systems. It permits more users to occupy a given band than other methods.
© 2008 The McGraw-Hill Companies
Benefits of Spread Spectrum
Spread spectrum is being used increasingly in data communication as its benefits are discovered and as new components and equipment become available to implement it.
Security: SS prevents unauthorized listening.
Resistance to jamming and interference: Jamming signals are typically restricted to a single frequency, and jamming one frequency does not interfere with an SS signal.
© 2008 The McGraw-Hill Companies
Benefits of Spread Spectrum
Band sharing: Many users can share a single band with little or no interference.
Resistance to fading and multipath propagation: SS virtually eliminates wide variations of signal strength due to reflections and other phenomena during propagation.
Precise timing: Use of the pseudorandom code in SS provides a way to precisely determine the start and end of a transmission, making it a superior method for radar and other applications that rely on accurate knowledge of transmission time to determine distance.
© 2008 The McGraw-Hill Companies
A wideband modulation method called OFDM is growing in popularity.
OFDM is also known as multicarrier modulation (MCM).
Although OFDM is known as a modulation method, the term frequency-division multiplexing is appropriate because the method transmits data by simultaneously modulating segments of the high-speed serial bit stream onto multiple carriers spaced throughout the channel bandwidth.
© 2008 The McGraw-Hill Companies
The carriers are frequency-multiplexed in the channel.
The data rate on each channel is very low, making the symbol time much longer than predicted transmission delays.
This technique spreads the signals over a wide bandwidth, making them less sensitive to the noise, fading, reflections, and multipath transmission effects common in microwave communication.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
Analog Telephone Modem
The most commonly used modem is one that connects personal computers to the telephone line.
A typical dial-up modem consists of both transmitter and receiver sections.
Most modern modems are implemented using digital signal processing (DSP) techniques.
© 2008 The McGraw-Hill Companies
Analog Telephone Modem
Modems…
11-1: Digital Codes
11-3: Transmission Efficiency
11-8: Protocols
11-1: Digital Codes
The proliferation of applications that send digital data over communication channels has resulted in the need for efficient methods of transmission, conversion, and reception of digital data.
© 2008 The McGraw-Hill Companies
11-1: Digital Codes
Data processed and stored by computers can be numerical or text.
The signals used to represent computerized data are digital.
Even before the advent of computers, digital codes were used to represent data.
© 2008 The McGraw-Hill Companies
11-1: Digital Codes
Early Digital Codes
The Morse code was originally designed for wired telegraph, but was later adapted for radio communication.
The Morse code consists of a series of “dots” and “dashes” that represent letters of the alphabet, numbers, and punctuation marks.
The Baudot code was used in the early teletype machine, a device for sending and receiving coded signals over a communication link.
© 2008 The McGraw-Hill Companies
11-1: Digital Codes
Modern Binary Codes
For modern data communication, information is transmitted using a system in which the numbers and letters to be represented are coded, usually by way of a keyboard, and the binary word representing each character is stored in a computer memory.
© 2008 The McGraw-Hill Companies
Modern Binary Codes: American Standard Code for Information Interchange
The most widely used data communication code is the 7-bit binary code known as the American Standard Code for Information Interchange (ASCII).
ASCII code can represent 128 numbers, letters, punctuation marks, and other symbols.
ASCII code combinations are available to represent both uppercase and lowercase letters of the alphabet.
Several ASCII codes have two- and three-letter designations which initiate operations or provide responses for inquiries.
© 2008 The McGraw-Hill Companies
Modern Binary Codes: Hexadecimal Values
Binary codes are often expressed using their hexadecimal, rather than decimal values.
To convert a binary code to its hexadecimal equivalent, first divide the code into 4-bit groups.
Start at the least significant bit on the right and work to the left. (Assume a leading zero on each of the codes.)
© 2008 The McGraw-Hill Companies
Modern Binary Codes: Extended Binary Coded Decimal Interchange Code
The Extended Binary Coded Decimal Interchange Code (EBCDIC) was developed by IBM.
The EBDIC is an 8-bit code allowing a maximum of 256 characters to be represented.
The EBCDIC is used primarily in IBM and IBM-compatible computing systems and is not widely used as ASCII.
© 2008 The McGraw-Hill Companies
Serial Transmission
Parallel
Serial
Data transfers in long-distance communication systems are made serially.
In a serial transmission, each bit of a word is transmitted one after another.
Parallel data transmission is not practical for long-distance communication.
© 2008 The McGraw-Hill Companies
Figure 11-4: Serial transmission of the ASCII letter M.
© 2008 The McGraw-Hill Companies
Serial Transmission: Expressing the Serial Data Rate
The speed of data transfer is usually indicated as number of bits per second (bps or b/s).
Another term used to express the data speed in digital communication systems is baud rate.
Baud rate is the number of signaling elements or symbols that occur in a given unit of time.
A signaling element is simply some change in the binary signal transmitted.
© 2008 The McGraw-Hill Companies
Asynchronous Transmission
In asynchronous transmission each data word is accompanied by start and stop bits that indicate the beginning and ending of the word.
When no information is being transmitted, the communication line is usually high, or binary 1.
In data communication terminology, this high level is referred to as a mark.
To signal the beginning of a word, a start bit, a binary 0 or space is transmitted.
© 2008 The McGraw-Hill Companies
Asynchronous Transmission
Most low-speed digital transmission (the 1200- to 56,000-bps range) is asynchronous.
Asynchronous transmissions are extremely reliable.
The primary disadvantage of asynchronous communication is that the extra start and stop bits effectively slow down data transmission.
© 2008 The McGraw-Hill Companies
Figure 11-6: Asynchronous transmission with start and stop bits.
© 2008 The McGraw-Hill Companies
Synchronous Transmission
The technique of transmitting each data word one after another without start and stop bits, usually in multiword blocks, is referred to as synchronous data transmission.
To maintain synchronization between transmitter and receiver, a group of synchronization bits is placed at the beginning and at the end of the block.
Each block of data can represent hundreds or even thousands of 1-byte characters.
© 2008 The McGraw-Hill Companies
Synchronous Transmission
The special synchronization codes at the beginning and end of a block represent a very small percentage of the total number of bits being transmitted, especially in relation to the number of start and stop bits used in asynchronous transmission.
Synchronous transmission is therefore much faster than asynchronous transmission because of the lower overhead.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
Encoding Methods
Whether digital signals are being transmitted by baseband methods or broadband methods, before the data is put on the medium, it is usually encoded in some way to make it compatible with the medium.
© 2008 The McGraw-Hill Companies
Encoding Methods
In the nonreturn to zero (NRZ) method of encoding the signal remains at the binary level assigned to it for the entire bit time.
In return to zero (RZ) encoding the voltage level assigned to a binary 1 level returns to zero during the bit period.
Manchester encoding, also referred to as biphase encoding, is widely used in LANs. In this system a binary 1 us transmitted first as a positive pulse, for one half of the bit interval, and then as a negative pulse for the remaining part of the bit interval.
© 2008 The McGraw-Hill Companies
11-3: Transmission Efficiency
Transmission efficiency is the accuracy and speed with which information, whether it is voice or video, analog or digital, is sent and received over communication media.
It is the basic subject matter of the field of information theory.
© 2008 The McGraw-Hill Companies
11-3: Transmission Efficiency
Hartley’s Law
The amount of information that can be sent in a given transmission is dependent on the bandwidth of the communication channel and the duration of transmission.
Mathematically, Hartley’s law is
C = 2B
Where C is the channel capacity (bps) and B is the channel bandwidth.
© 2008 The McGraw-Hill Companies
11-3: Transmission Efficiency
Hartley’s Law
The greater the number of bits transmitted in a given time, the greater the amount of information that is conveyed.
The higher the bit rate, the wider the bandwidth needed to pass the signal with minimum distortion.
© 2008 The McGraw-Hill Companies
Transmission Media and Bandwidth
The two most common types of media used in data communication are wire cable and radio.
The two types of wire cable used are coaxial and twisted pair.
Coaxial cable has a center conductor surrounded by an insulator over which is a braided shield. The entire cable is covered with a plastic insulation.
A twisted-pair cable is two insulated wires twisted together.
© 2008 The McGraw-Hill Companies
11-3: Transmission Efficiency
Figure 11-10: Types of cable used for digital data transmission. (a) Coaxial cable.
(b) Twisted-pair cable, unshielded (UTP).
© 2008 The McGraw-Hill Companies
Twisted-pair is available as unshielded (UTP) or shielded.
Coaxial cable and shielded twisted-pair cables are usually preferred, as they provide some protection from noise and cross talk.
Cross talk is the undesired transfer of signals from one unshielded cable to another adjacent one caused by inductive or capacitive coupling.
© 2008 The McGraw-Hill Companies
Transmission Media and Bandwidth
The bandwidth of any cable is determined by its physical characteristics.
All wire cables act as low-pass filters because they are made up of wire that has inductance, capacitance, and resistance.
© 2008 The McGraw-Hill Companies
11-3: Transmission Efficiency
Multiple Coding Levels
Channel capacity can be modified by using multiple-level encoding schemes that permit more bits per symbol to be transmitted.
It is possible to transmit data using more than just two binary voltage levels or symbols.
Multiple voltage levels can be used to increase channel capacity.
Other methods, such as using different phase shifts for each symbol, are used.
© 2008 The McGraw-Hill Companies
Impact of Noise in the Channel
An important aspect of information theory is the impact of noise on a signal.
Increasing bandwidth increases the rate of transmission but also allows more noise to pass.
Typical communication systems limit the channel capacity to one-third to one-half the maximum to ensure more reliable transmission in the presence of noise.
© 2008 The McGraw-Hill Companies
Digital data are transmitted over the telephone and cable television networks by using broadband communication techniques involving modulation, which are implemented by a modem, a device containing both a modulator and a demodulator.
Modems convert binary signals to analog signals capable of being transmitted over telephone and cable TV lines and by radio, and then demodulate such analog signals, reconstructing the equivalent binary output.
© 2008 The McGraw-Hill Companies
Conventional analog dial-up modems.
Cable TV modems.
11-4: Basic Modem Concepts
Figure 11-12: How modems permit digital data transmission on the telephone network.
© 2008 The McGraw-Hill Companies
11-4: Basic Modem Concepts
Modulation for Data Communication
The four main types of modulation used in modern modems are:
Frequency-shift keying (FSK)
Phase-shift keying (PSK)
© 2008 The McGraw-Hill Companies
Modulation for Data Communication: Frequency-Shift Keying (FSK)
Frequency-shift keying (FSK) is the oldest and simplest form of modulation used in modems.
In FSK, two sine-wave frequencies are used to represent binary 0s and 1s.
A binary 0, usually called a space, has a frequency of 1070 Hz.
A binary 1, referred to as a mark, is 1270 Hz.
These two frequencies are alternately transmitted to create the serial binary data.
© 2008 The McGraw-Hill Companies
Figure 11-13: Frequency-shift keying. (a) Binary signal. (b) FSK signal.
© 2008 The McGraw-Hill Companies
Modulation for Data Communication: Phase-Shift Keying
In phase-shift keying (PSK), the binary signal to be transmitted changes the phase shift of a sine-wave character depending upon whether a binary 0 or binary 1 is to be transmitted.
A phase shift of 180°, the maximum phase difference that can occur, is known as a phase reversal, or phase inversion.
During the time that a binary 0 occurs, the carrier is transmitted with one phase; when a binary 1 occurs, the carrier is transmitted with a 180° phase shift.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
Modulation for Data Communication: QPSK
One way to increase the binary data rate while not increasing the bandwidth required for the signal transmission is to encode more than 1 bit per phase change.
In the system known as quadrature, quarternary, or quadra phase PSK (QPSK or 4-PSK), more bits per baud are encoded, the bit rate of data transfer can be higher than the baud rate, yet the signal will not take up additional bandwidth.
In QPSK, each pair of successive digital bits in the transmitted word is assigned a particular phase.
Each pair of serial bits, called a dibit, is represented by a specific phase.
© 2008 The McGraw-Hill Companies
11-4: Basic Modem Concepts
Figure 11-24: Quadrature PSK modulation. (a) Phase angle of carrier for different pairs of bits. (b) Phasor representation of carrier sine wave. (c) Constellation diagram
of QPSK.
Modulation for Data Communication: QPSK
The QPSK modulator consists of a 2-bit shift register implemented with flip-flops, commonly known as a bit splitter.
The serial binary data train is shifted through the register.
The bits from the flip-flops are applied to balanced modulators.
The carrier oscillator is applied to one balanced modulator and through a 90° phase shifter to another balanced modulator.
The outputs of the balanced modulators are linearly mixed to produce the QPSK signal.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
Modulation for Data Communication: QAM
One of the most popular modulation techniques used in modems for increasing the number of bits per baud is quadrature amplitude modulation (QAM).
QAM uses both amplitude and phase modulation of a carrier.
In 8-QAM, there are four possible phase shifts and two different carrier amplitudes.
Eight different states can be transmitted.
With eight states, 3 bits can be encoded for each baud or symbol transmitted.
Each 3-bit binary word transmitted uses a different phase-amplitude combination.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
11-4: Basic Modem Concepts
Spectral Efficiency and Noise
Spectral efficiency is a measure of how fast data can be transmitted in a given bandwidth (bps/Hz).
Different modulation methods give different efficiencies.
Modulation
The signal-to-noise (S/N) ratio clearly influences the spectral efficiency.
The greater the noise, the greater the number of bit errors.
The number of errors that occur in a given time is called the bit error rate (BER).
The BER is the ratio of the number of errors that occur to the number of bits that occur in a one second interval.
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11-5: Wideband Modulation
While most modulation methods are designed to be spectrally efficient, there is another class of modulation methods that does just the opposite.
These methods are designed to use more bandwidth. The transmitted signal occupies a bandwidth many times greater than the information bandwidth.
The two most widely used wideband modulation methods are spread spectrum and orthogonal frequency-division multiplexing.
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Spread Spectrum
Spread spectrum (SS) is a modulation and multiplexing technique that distributes a signal and its sidebands over a very wide bandwidth.
After World War II, spread spectrum was developed by the military because it is a secure communication technique essentially immune to jamming.
Currently, unlicensed operation is permitted in the 902- to 928-MHz, 2.4- to 2.483-GHz, and 5.725- to 5.85-GHz ranges, with 1 W of power.
© 2008 The McGraw-Hill Companies
Spread Spectrum
Spread spectrum on these frequencies is being widely incorporated into a variety of commercial communication systems, particularly wireless data communication.
Numerous LANs and portable personal computer modems use SS techniques, as does a class of cordless telephones.
The most widespread use of SS is in cellular telephones. It is referred to as code-division multiple access (CDMA).
© 2008 The McGraw-Hill Companies
Spread Spectrum
There are two basic types of spread spectrum: frequency-hopping (FH) and direct-sequence (DS).
In frequency-hopping SS, the frequency of the carrier of the transmitter is changed according to a predetermined sequence, called pseudorandom, at a rate higher than that of the serial binary data modulating the carrier.
In direct-sequence SS, the serial binary data is mixed with a higher-frequency pseudorandom binary code at a faster rate, and the result is used to phase-modulate a carrier.
© 2008 The McGraw-Hill Companies
11-5: Wideband Modulation
Frequency-Hopping Spread Spectrum
In a frequency-hopping SS transmitter, the serial binary data to be transmitted is applied to a conventional two-tone FSK modulator.
The modulator output is applied to a mixer.
Also driving the mixer is a frequency synthesizer.
The output signal from the bandpass filter after the mixer is the difference between one of the two FSK sine waves and the frequency of the frequency synthesizer.
The synthesizer is driven by a pseudorandom code generator, which is either a special digital circuit or the output of a microprocessor.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
11-5: Wideband Modulation
Frequency-Hopping Spread Spectrum
In a frequency-hopping SS system, the rate of synthesizer frequency change is higher than the data rate.
This means that although the data bit and the FSK tone it produces remain constant for one data interval, the frequency synthesizer switches frequencies many times during this period.
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11-5: Wideband Modulation
Frequency-Hopping Spread Spectrum
The time that the synthesizer remains on a single frequency is called the dwell time.
The frequency synthesizer puts out a random sine wave frequency to the mixer, and the mixer creates a new carrier frequency for each dwell interval.
The resulting signal, whose frequency rapidly jumps around, effectively scatters pieces of the signal all over the band.
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11-5: Wideband Modulation
Direct-Sequence Spread Spectrum
In a direct-sequence SS (DSSS) transmitter, the serial binary data is applied to an X-OR gate along with a serial pseudorandom code that occurs faster than the binary data.
One bit time for the pseudorandom code is called a chip, and the rate of the code is called the chipping rate. The chipping rate is faster than the data rate.
The signal developed at the output of the X-OR gate is then applied to a PSK modulator, typically a BPSK device.
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11-5: Wideband Modulation
Direct-Sequence Spread Spectrum
The carrier phase is switched between 0 and 180° by the 1s and 0s of the X-OR output.
The PSK modulator is generally some form of balanced modulator.
The signal phase modulating the carrier, being much higher in frequency than the data signal, causes the modulator to produce multiple, widely spaced sidebands whose strength is such that the complete signal takes up a great deal of the spectrum. Thus the resulting signal is spread.
Because of its randomness, the signal looks like wideband noise to a conventional narrowband receiver.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
11-5: Wideband Modulation
Direct-Sequence Spread Spectrum
Direct-sequence SS is also called code-division multiple access (CDMA), or SS multiple access.
The term multiple access applies to any technique that is used for multiplexing many signals on a single communication channel.
CDMA is used in satellite systems so that many signals can use the same transponder.
It is also widely used in cellular telephone systems. It permits more users to occupy a given band than other methods.
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Benefits of Spread Spectrum
Spread spectrum is being used increasingly in data communication as its benefits are discovered and as new components and equipment become available to implement it.
Security: SS prevents unauthorized listening.
Resistance to jamming and interference: Jamming signals are typically restricted to a single frequency, and jamming one frequency does not interfere with an SS signal.
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Benefits of Spread Spectrum
Band sharing: Many users can share a single band with little or no interference.
Resistance to fading and multipath propagation: SS virtually eliminates wide variations of signal strength due to reflections and other phenomena during propagation.
Precise timing: Use of the pseudorandom code in SS provides a way to precisely determine the start and end of a transmission, making it a superior method for radar and other applications that rely on accurate knowledge of transmission time to determine distance.
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A wideband modulation method called OFDM is growing in popularity.
OFDM is also known as multicarrier modulation (MCM).
Although OFDM is known as a modulation method, the term frequency-division multiplexing is appropriate because the method transmits data by simultaneously modulating segments of the high-speed serial bit stream onto multiple carriers spaced throughout the channel bandwidth.
© 2008 The McGraw-Hill Companies
The carriers are frequency-multiplexed in the channel.
The data rate on each channel is very low, making the symbol time much longer than predicted transmission delays.
This technique spreads the signals over a wide bandwidth, making them less sensitive to the noise, fading, reflections, and multipath transmission effects common in microwave communication.
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
© 2008 The McGraw-Hill Companies
Analog Telephone Modem
The most commonly used modem is one that connects personal computers to the telephone line.
A typical dial-up modem consists of both transmitter and receiver sections.
Most modern modems are implemented using digital signal processing (DSP) techniques.
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Analog Telephone Modem
Modems…
