© 2008 The McGraw-Hill Companies 1 Principles of Electronic Communication Systems Third Edition Louis E. Frenzel, Jr.

© 2008 The McGraw-Hill Companies 3 Topics Covered in Chapter 11  11-1: Digital Codes  11-2: Principles of Digital Transmission  11-3: Transmission Efficiency  11-4: Basic Modem Concepts  11-5: Wideband Modulation  11-6: Broadband Modem Techniques  11-7: Error Detection and Correction  11-8: Protocols

© 2008 The McGraw-Hill Companies 4 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 5 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 6 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 7 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 8 11-1: Digital Codes 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 9 11-1: Digital Codes 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 10 11-1: Digital Codes 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 11 11-2: Principles of Digital Transmission Serial Transmission  Data can be transmitted in two ways: 1. Parallel 2. 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 13 11-2: Principles of Digital Transmission 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 14 11-2: Principles of Digital Transmission 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 15 11-2: Principles of Digital Transmission 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 17 11-2: Principles of Digital Transmission 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 18 11-2: Principles of Digital Transmission 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 20 11-2: Principles of Digital Transmission 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 21 11-2: Principles of Digital Transmission 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 22 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 23 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 24 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 25 11-3: Transmission Efficiency 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 27 11-3: Transmission Efficiency Transmission Media and Bandwidth  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 28 11-3: Transmission Efficiency 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 29 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 30 11-3: Transmission Efficiency 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 31 11-4: Basic Modem Concepts  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 32 11-4: Basic Modem Concepts  There are four widely used modem types: 1. Conventional analog dial-up modems. 2. Digital subscriber line (DSL) modems. 3. Cable TV modems. 4. Wireless modems.

© 2008 The McGraw-Hill Companies 34 11-4: Basic Modem Concepts Modulation for Data Communication  The four main types of modulation used in modern modems are: 1. Frequency-shift keying (FSK) 2. Phase-shift keying (PSK) 3. Quadrature amplitude modulation (QAM) 4. Orthogonal frequency division multiplexing (OFDM)

© 2008 The McGraw-Hill Companies 35 11-4: Basic Modem Concepts 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 37 11-4: Basic Modem Concepts 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 39 11-4: Basic Modem Concepts 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 40 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.

© 2008 The McGraw-Hill Companies 41 11-4: Basic Modem Concepts 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 43 11-4: Basic Modem Concepts 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 45 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. ModulationSpectral efficiency, bps/Hz FSK<1 GMSK1.35 BPSK1 QPSK2 8-PSK3 16-QAM4

© 2008 The McGraw-Hill Companies 46 11-4: Basic Modem Concepts Spectral Efficiency and Noise  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 47 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 48 11-5: Wideband Modulation 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 49 11-5: Wideband Modulation 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 50 11-5: Wideband Modulation 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 51 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 54 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 55 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 56 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 57 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 60 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 61 11-5: Wideband Modulation 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 62 11-5: Wideband Modulation 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 63 11-5: Wideband Modulation Orthogonal Frequency-Division Multiplexing (OFDM)  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 64 11-5: Wideband Modulation Orthogonal Frequency-Division Multiplexing (OFDM)  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 67 11-6: Broadband Modem Techniques 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 68 11-6: Broadband Modem Techniques Analog Telephone Modem  Modems are packaged on a single small printed circuit board and are designed to plug into the PC bus.  Most analog modems today are single chip DSPs mounted on the PC motherboard.  The modem takes its power from the PC power supply.  An RJ-11 modular connector attaches the modem to the telephone line.

© 2008 The McGraw-Hill Companies 69 11-6: Broadband Modem Techniques Analog Telephone Modem: Modem Operation  During transmission operations: 1. The data to be transmitted is stored in the computer’s RAM. 2. It is formatted there by the communication software installed with the computer. 3. It is then sent 1 byte at a time to the modem. 4. The modem’s first job is to convert parallel data to serial data. This is done with shift registers. It is usually carried out by a universal asynchronous receiver/transmitter (UART), a digital IC that performs parallel-to-serial conversion for transmission and serial-to-parallel conversion for reception.

© 2008 The McGraw-Hill Companies 70 11-6: Broadband Modem Techniques Analog Telephone Modem: Modem Operation 5. The serial data from the UART is passed through a scrambler circuit to ensure that the data is random. 6. The random serial data is sent to the modulator. 7. The output of the modulator is filtered to band-limit it and then fed to an equalizer circuit.

© 2008 The McGraw-Hill Companies 71 11-6: Broadband Modem Techniques Analog Telephone Modem: Modem Operation  During receive operations: 1. The signal is picked off the telephone line. 2. It is passed through the interface circuits. 3. Then it is fed to the receiver section. 4. It first encounters an adaptive equalizer. The adaptive equalizer adjusts itself automatically to compensate for the amplitude attenuation and distortion of the signal. 5. The signal is then demodulated, resulting in an NRZ serial digital signal.

© 2008 The McGraw-Hill Companies 72 11-6: Broadband Modem Techniques Analog Telephone Modem: Modem Operation 6. This is passed through a descrambler, which produces the opposite effect of the transmit scrambler. 7. The descrambler output is the original serial data signal. This is sent to the UART, where it is translated to a parallel byte that the computer can store and use.  Data compression and decompression circuits are now being used in some modems.  All the newer modem types incorporate circuitry that can detect bit transmission errors and correct them as they occur.

© 2008 The McGraw-Hill Companies 73 11-6: Broadband Modem Techniques Analog Telephone Modem: Modem Classification and Standards  The International Telecommunications Union (ITU) sponsors, negotiates, and maintains modem and other communication standards.  Modem standards are designated by a special V.xx symbol.  Modems are usually capable of operating in several different V.xx modes.

© 2008 The McGraw-Hill Companies 74 11-6: Broadband Modem Techniques Analog Telephone Modem: Modem Classification and Standards  The modem will automatically adjust itself to the highest speed possible but will drop back to a lower speed or different mode if the receiving modem cannot handle the highest speed.  Most modems is use today are the V.90 or V.92 type and are capable of speeds up to 56 kbps.

© 2008 The McGraw-Hill Companies 75 11-6: Broadband Modem Techniques xDSL Modems  The digital subscriber line (DSL) describes a set of standards set by the International Telecommunications Union that greatly extend the speed potential of the common twisted-pair telephone lines.  In the term xDSL, the x designates one of several letters that define a specific DSL standard.  The most widely used form of DSL is called asymmetric digital subscriber line (ADSL), which permits downstream data rates up to 8 Mbps and upstream rates up to 640 kbps using the existing telephone lines.

© 2008 The McGraw-Hill Companies 76 11-6: Broadband Modem Techniques xDSL Modems  The modulation scheme used with ADSL modems is called discrete multitone (DMT), another name for OFDM.  It divides the upper frequency spectrum of the telephone line into 256 channels, each 4 kHz wide.  Each channel, called a bin, is designed to transmit at speeds up to 15 kbps/Bd or 60 kbps.

© 2008 The McGraw-Hill Companies 77 11-6: Broadband Modem Techniques xDSL Modems  Each channel contains a carrier that is simultaneously phase-amplitude-modulated (QAM) by some of the bits to be transmitted.  The serial data stream is divided up so that each carrier transmits some of the bits. All bits are transmitted simultaneously.  All the carriers are frequency-multiplexed into the line bandwidth above the normal voice telephone channel  The system is complex and is implemented with a digital signal processor.

© 2008 The McGraw-Hill Companies 80 11-6: Broadband Modem Techniques Cable Modems  Many cable TV systems are set up to handle high- speed digital data transmission.  The digital data is used to modulate a high-frequency carrier that is frequency-multiplexed onto the cable that also carries the TV signal.  Cable modems provide significantly higher data rates than can be achieved over the standard telephone system.

© 2008 The McGraw-Hill Companies 81 11-6: Broadband Modem Techniques Cable Modems  Television channels extend from 50 MHz (Channel 2) up to 550 MHz. In this 500 MHz of bandwidth, up to 83 channels of 6 MHz can be accommodated.  The spectrum above the TV channels, from 550 to 850 MHz, is available for digital data transmission. Standard 6-MHz channels are used.  Cable modems use 64-QAM for downstream data.  Standard QPSK is used in the upstream channels.

© 2008 The McGraw-Hill Companies 83 11-6: Broadband Modem Techniques Cable Modems  A typical cable modem is a VHF/UHF receiver connected to the cable for downloads and a modulator/transmitter for uploads.  The signal from the cable passes through the diplexer, which is a filter circuit that permits simultaneous transmit and receive operations.  The signal is amplified and mixed with a local oscillator signal from the frequency synthesizer to produce an IF signal.  The frequency synthesizer selects the cable channel. The IF signal is demodulated to recover the data.

© 2008 The McGraw-Hill Companies 84 11-6: Broadband Modem Techniques Cable Modems  Reed Solomon error detection circuitry finds and corrects any bit errors.  The digital data then goes to an Ethernet interface to the PC.  For transmission, the data from the computer is passed through the interface, where it is encoded for error detection.  The data then modulates a carrier that is up-converted by the mixer to the selected upstream channel before being amplified and passed through the diplexer to the cable.

© 2008 The McGraw-Hill Companies 86 11-7: Error Detection and Correction  When high-speed binary data is transmitted over a communication link, whether it is a cable or radio, errors will occur.  These errors are changes in the bit pattern caused by interference, noise, or equipment malfunctions.  Such errors will cause incorrect data to be received.  The number of bit errors that occur for a given number of bits transmitted is referred to as the bit error rate (BER).

© 2008 The McGraw-Hill Companies 87 11-7: Error Detection and Correction  The process of error detection and correction involves adding extra bits to the data characters to be transmitted. This process is generally referred to as channel encoding.  The data to be transmitted is processed in a way that creates the extra bits and adds them to the original data. At the receiver, these extra bits help in identifying any errors that occur in transmission caused by noise or other channel effects.

© 2008 The McGraw-Hill Companies 88 11-7: Error Detection and Correction  A key point about channel encoding is that it takes more time to transmit the data because of the extra bits. These extra bits are called overhead in that they extend the time of transmission.  Channel encoding methods fall into to two separate categories, error detection codes and error correction codes.

© 2008 The McGraw-Hill Companies 89 11-7: Error Detection and Correction Error Detection  Many different methods have been used to ensure reliable error detection:  Redundancy is a method that ensures error-free transmission by sending each character or message multiple times until it is properly received.  Encoding schemes like the RZ-AMI are used whereby successive binary 1 bits in the bit stream are transmitted with alternating polarity.

© 2008 The McGraw-Hill Companies 90 11-7: Error Detection and Correction Error Detection  One of the most widely used systems is known as parity, in which each character transmitted contains one additional bit, known as a parity bit.  The cyclical redundancy check (CRC) is a mathematical technique used in synchronous data transmission that effectively catches 99.9 percent or more of transmission errors.

© 2008 The McGraw-Hill Companies 91 11-7: Error Detection and Correction Error Correction  A number of efficient error-correction schemes have been devised to complement error detection methods.  The process of detecting and correcting errors at the receiver so that retransmission is not necessary is called forward error correction (FEC).  There are two basic types of FEC: block codes and convolutional codes.

© 2008 The McGraw-Hill Companies 92 11-7: Error Detection and Correction Error Correction: Block-Check Character  The block check character (BCC) is also known as a horizontal or longitudinal redundancy check (LRC).  It is the process of logically adding, by exclusive-ORing, all the characters in a specific block of transmitted data.  The final bit value for each horizontal row becomes one bit in a character known as the block-check character (BCC), or the block-check sequence (BCS).

© 2008 The McGraw-Hill Companies 93 11-7: Error Detection and Correction Error Correction: Block-Check Character  The most popular FEC codes are the Hamming and Reed Solomon codes.  These codes add extra parity bits to a transmitted word, process them using unique algorithms, and detect and correct bit errors.  Interleaving is a method used in wireless systems to reduce the effects of burst errors.

© 2008 The McGraw-Hill Companies 94 11-7: Error Detection and Correction Error Correction: Convolutional Codes  Convolutional encoding creates additional bits from the data as do Hamming and Reed Solomon codes, but the encoded output is a function of not only the current data bits but also previously occurring data bits.  Convolutional codes pass the data to be transmitted through a special shift register.  As the serial data is shifted through the shift register flip-flops, some of the flip-flop outputs are XORed together to form two outputs.

© 2008 The McGraw-Hill Companies 95 11-7: Error Detection and Correction Error Correction: Convolutional Codes  These two outputs are the convolutional code, and this is what is transmitted.  The original data itself is not transmitted.  Instead, two separate streams of continuously encoded data are sent.  Since each output code is different, the original data can more likely be recovered at the receiver by an inverse process.

© 2008 The McGraw-Hill Companies 97 11-8: Protocols  Protocols are rules and procedures used to ensure compatibility between the sender and receiver of digital data regardless of the hardware and software being used.  Protocols are used to identify the start and end of a message, identify the sender and receiver, state the number of bytes to be transmitted, state the method of error detection, and for other functions.  Various protocols, and various levels of protocols, are used in data communication.

© 2008 The McGraw-Hill Companies 98 11-8: Protocols Asynchronous Protocols  Three popular protocols for asynchronous ASCII-coded data transmission between personal computers, via modem are:  Xmodem  Kermit  MPN.

© 2008 The McGraw-Hill Companies 99 11-8: Protocols Asynchronous Protocols: Xmodem  In Xmodem, the data transmission procedure begins with the receiving computer transmitting a negative acknowledge (NAK) character to the transmitter.  NAK is a 7-bit ASCII character that is transmitted serially back to the transmitter every 10 seconds until the transmitter recognizes it.  Once the transmitter recognizes the NAK character, it begins sending a 128-byte block of data, known as a frame (packet) of information.

© 2008 The McGraw-Hill Companies 101 11-8: Protocols Asynchronous Protocols: Kermit  The Kermit protocol transmission begins with a start- of-header (SOH) character followed by a length (LEN) character, which tells how long the block of data is.  Next is a packet sequence number (SEQ).  There can be up to 63 blocks, and these are given a sequence number so that both transmitter and receiver can keep track of long messages.  Kermit is reliable because it requires every packet sent be acknowledged by the receiver as being read correctly.

© 2008 The McGraw-Hill Companies 102 11-8: Protocols Asynchronous Protocols: MNP  Microcom Networking Protocols (MNPs) are a series of protocols developed by the manufacturer Microcom to be used with asynchronous modems.  They specify ways to handle error detection and correction and how to specify whether or not data compression is used.  There are 10 classes of protocols.  MNPs are easy to implement because they can be programmed into the control microcomputer used in most modems.

© 2008 The McGraw-Hill Companies 103 11-8: Protocols Synchronous Protocols  Protocols used for synchronous data communication are more complex than asynchronous protocols.  Like asynchronous systems, they use various control characters for signaling purposes at the beginning and ending of the block of data to be transmitted.

© 2008 The McGraw-Hill Companies 104 11-8: Protocols Synchronous Protocols: Bisync  IBM’s Bisync protocol, which is widely used in computer communication, usually begins with the transmission of two or more ASCII sync (SYN) characters.  These characters signal the beginning of the transmission and are also used to initialize the clock timing circuits in the receiving modem.  This ensures proper synchronization of the data transmitted a bit at a time.

© 2008 The McGraw-Hill Companies 106 11-8: Protocols Synchronous Protocols: SDLC  One of the most flexible and widely used synchronous protocols is the synchronous data link control (SDLC) protocol.  SDLC is used in networks that are interconnections of multiple computers.

© 2008 The McGraw-Hill Companies 108 11-8: Protocols Open Systems Interconnection Model  The International Organization for Standardization (ISO) has attempted to standardize data communication procedures.  The ISO has come up with a framework, or hierarchy, that defines how data can be communicated.  This hierarchy, known as the open systems interconnection (OSI) model, is designed to establish general interoperability guidelines for developers of communication systems and protocols.

© 2008 The McGraw-Hill Companies 109 11-8: Protocols Open Systems Interconnection Model  The OSI hierarchy is made up of seven levels, or layers.  Each layer is defined by software (or, in one case, hardware) and is clearly distinct from the other layers.  These layers are not protocols, but they provide a way to define and partition protocols to make data transfers in a standardized way.

© 2008 The McGraw-Hill Companies 110 11-8: Protocols Open Systems Interconnection Model  The layers are:  Layer 1: Physical layer: The physical connections and electrical standards for the communication system are defined here.  Layer 2: Data link: This layer defines the framing information for the block of data.  Layer 3: Network: This layer determines network configuration and the route the transmission can take.

© 2008 The McGraw-Hill Companies 111 11-8: Protocols Open Systems Interconnection Model  Layer 4: Transport: Included in this layer are multiplexing, error recovery, partitioning of data, and addressing and flow control operations.  Layer 5: Session: This layer handles such things as management and synchronization of the data transmission.  Layer 6: Presentation: This layer deals with the form and syntax of the message.  Layer 7: Applications: This layer is the overall general manager of the network or the communication process.

© 2008 The McGraw-Hill Companies 1 Principles of Electronic Communication Systems Third Edition Louis E. Frenzel, Jr.

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© 2008 The McGraw-Hill Companies 1 Principles of Electronic Communication Systems Third Edition Louis E. Frenzel, Jr.

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