Chapter 3


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Chapter 3

  1. 1. Chapter 3 E51243.0 MODEMModem also know as modulator-demodulator. A modem is a device or program that enables acomputer to transmit data over, for example, telephone or cable lines. Computer information isstored digitally, whereas information transmitted over telephone lines is transmitted in theform of analog waves. A modem converts between these two forms.Fortunately, there is one standard interface for connecting external modems to computerscalled RS-232. Consequently, any external modem can be attached to any computer that has anRS-232 port, which almost all personal computers have. There are also modems that come asan expansion board that you can insert into a vacant expansion slot. These are sometimescalled onboard or internal modems. Fig 1 Modem as interface between computer and incoming line3.1 ImportanceWhile the modem interfaces are standardized, a number of different protocols for formattingdata to be transmitted over telephone lines exist. Some, like CCITT V.34, are official standards,while others have been developed by private companies. Most modems have built-in supportfor the more common protocols -- at slow data transmission speeds at least, most modems cancommunicate with each other. At high transmission speeds, however, the protocols are lessstandardized. Aside from the transmission protocols that they support, the followingcharacteristics distinguish one modem from another: bps How fast the modem can transmit and receive data. At slow rates, modems are measured in terms of baud rates. The slowest rate is 300 baud (about 25 cps). At higher speeds, modems are measured in terms of bits per second (bps). The fastest modems run at 57,600 bps, although they can achieve even higher data transfer rates by compressing the data. Obviously, the faster the transmission rate, the faster you can send and receive data. Note, however, that you cannot receive data any faster than it is being sent. If, for example, the device sending data to your computer is sending it at 2,400 bps, you must receive it at 2,400 bps. It does not always pay, therefore, to have a 39Sargunan ainal (jke,puo)
  2. 2. Chapter 3 E5124 very fast modem. In addition, some telephone lines are unable to transmit data reliably at very high rates. Voice/data Many modems support a switch to change between voice and data modes. In data mode, the modem acts like a regular modem. In voice mode, the modem acts like a regular telephone. Modems that support a voice/data switch have a built-in loudspeaker and microphone for voice communication. Auto-answer An auto-answer modem enables your computer to receive calls in your absence. This is only necessary if you are offering some type of computer service that people can call in to use. Data compression Some modems perform data compression, which enables them to send data at faster rates. However, the modem at the receiving end must be able to decompress the data using the same compression technique. Flash memory Some modems come with flash memory rather than conventional ROM, which means that the communications protocols can be easily updated if necessary. Fax capability Most modern modems are fax modems, which means that they can send and receive faxes.3.1.1 Types of Modemsi. Internal modem. Most internal modems come installed in the computer you buy. Internal modems are more directly integrated into the computer system and, therefore, do not need any special attention. Internal modems are activated when you run a communications program and are turned off when you exit the program. This convenience is especially useful for novice users. Internal modems usually cost less than external modems, but the price difference is usually small. The major disadvantage with internal modems is their location: inside the computer. When you want to replace an internal modem you have to go inside the computer case to make the switch.ii. PC Card modem.These modems, designed for portable computers, are the size of acredit card and fit into the PC Card slot on notebook and handheldcomputers. These modems are removed when the modem is notneeded. Except for their size, PC Card modems are like acombination of external and internal modems. These devices are 40Sargunan ainal (jke,puo)
  3. 3. Chapter 3 E5124plugged directly into an external slot in the portable computer, so no cable is required otherthan the telephone line connection. The cards are powered by the computer, which is fineunless the computer is battery-operated. Running a PC Card modem while the portablecomputer is operating on battery power drastically decreases the life of your batteries.iii. External modem.This is the simplest type of modem to install because youdont have to open the computer. External modems havetheir own power supply and connect with a cable to acomputers serial port. The telephone line plugs into a socketon the rear panel of the modem.Because external modems have their own power supply, youcan turn off the modem to break an online connection quicklywithout powering down the computer. Another advantageover an internal modem is that an external modems separatepower supply does not drain any power from the computer.You also can monitor your modems connection activity bywatching the status lights.3.1.2 How modems workWhen a modem first makes a connection, you will hear screeching sounds coming from themodem. These are digital signals coming from the computer to which you are connecting beingmodulated into audible sounds. The modem sends a higher-pitched tone to represent the digit Iand a lower-pitched tone to represent the digit 0.At the other end of your modem connection, the computer attached to its modem reverses thisprocess. The receiving modem demodulates the various tones into digital signals and sendsthem to the receiving computer. Actually, the process is a bit more complicated than sendingand receiving signals in one direction and then another. Modems simultaneously send andreceive signals in small chunks. The modems can tell incoming from outgoing data signals by thetype of standard tones they use.Another part of the translation process involves transmission integrity. The modems exchangean added mathematical code along the way. This special code, called a checksum, lets bothcomputers know if the data segments are coming through properly. If the mathematical sumsdo not match, the modems communicate with each other by resending the missing segments ofdata. Modems also have special circuitry that allows them to compress digital signals beforemodulating them and then decompressing them after demoduating the signals. Thecompression/decompression process compacts the data so that it can travel along telephonelines more efficiently. 41Sargunan ainal (jke,puo)
  4. 4. Chapter 3 E5124 Fig 2 Modem communicationModems convert analog data transmitted over phone lines into digital data computers canread; they also convert digital data into analog data so it can be transmitted. This processinvolves modulating and demodulating the computer’s digital signals into analog signals thattravel over the telephone lines. In other words, the modem translates computer data into thelanguage used by telephones and then reverses the process to translate the responding databack into computer language.3.1.3 Difference between digital and analog signalsA computer performs its tasks by turning on and off a series of electronic switches representedby the numerical digits of 0 and 1. A 0 is the code for off, and a 1 is the code for on.Combinations of these digital codes represent text, computer commands, and graphics insidethe computer. By comparison, the telephone works by sending sounds in a continuous analogsignal sent along an electronic current that varies in frequency and strength3.2 Analogue modulationThe modulated signal consists pure sine wave "carrier" signal which is modified to conveyinformation. A pure carrier sine wave, unchanging in frequency and voltage, provides no flow ofinformation at all (except that a carrier is present). To make it convey information we modify(or modulate) this carrier. There are 3 basic types of modulation: frequency, amplitude, andphase. 42Sargunan ainal (jke,puo)
  5. 5. Chapter 3 E5124i. Frequency ModulationThe simplest modulation method is frequency modulation. Frequency is measured in cycles persecond (of a sine wave). Its the count of the number of times the sine wave shape repeats itselfin a second. This is the same as the number of times it reaches it peak value during a second.The word "Hertz" (abbreviated Hz) is used to mean "cycles per second".A simple example of frequency modulation is where one frequency means a binary 0 andanother means a 1. For example, for some obsolete 300 baud modems 1070 Hz meant a binary0 while 1270 Hz meant a binary 1. This was called "frequency shift keying". Instead of just twopossible frequencies, more could be used to allow more information to be transmitted. If wehad 4 different frequencies (call them A, B, C, and D) then each frequency could stand for a pairof bits. For example, to send 00 one would use frequency A. To send 01, use frequency B; for 10use C; for 11 use D. In like manner, by using 8 different frequencies we could send 3 bits witheach shift in frequency. Each time we double the number of possible frequencies we increasethe number of bits it can represent by 1. Fig 3 Types Analogue Modulationii. Amplitude ModulationOnce one understands frequency modulation example above (Fig 3) including the possibilitiesof representing a few bits by a single shift in frequency, its easier to understand bothamplitude modulation and phase modulation. For amplitude modulation, one just changes theheight (voltage) of the sine wave analogous to changing the frequency of the sine wave. For asimple case there could only be 2 allowed amplitude levels, one representing a 0-bit and 43Sargunan ainal (jke,puo)
  6. 6. Chapter 3 E5124another representing a 1-bit. As explained for the case of frequency modulation, having morepossible amplitudes will result in more information being transmitted per change in amplitude.iii. Phase ModulationTo change the phase of a sine wave at a certain instant of time, we stop sending this old sinewave and immediately begin sending a new sine wave of the same frequency and amplitude. Ifwe started sending the new sine wave at the same voltage level (and slope) as existed when westopped sending the old sine wave, there would be no change in phase (and no detectablechange at all). But suppose that we started up the new sine wave at a different point on thesine wave curve. Then there would likely be a sudden voltage jump at the point in time wherethe old sine wave stopped and the new sine wave began. This is a phase shift and its measuredin degrees (deg.) A 0 deg. (or a 360 deg.) phase shift means no change at all while a 180 deg.phase shift just reverses the voltage (and slope) of the sine wave. Put another way, a 180 deg.phase shift just skips over a half-period (180 deg.) at the point of transition. Of course we couldjust skip over say 90 deg. or 135 deg. etc. As in the example for frequency modulation, themore possible phase shifts, the more bits a single shift in phase can represent.3.3 Digital ModulationDigital modulation schemes transform digital signals like the one shown below into waveformsthat are compatible with the nature of the communications channel. There are two majorcategories of digital modulation. One category uses a constant amplitude carrier and the othercarries the information in phase or frequency variations (FSK, PSK). The other category conveysthe information in carrier amplitude variations and is known as amplitude shift keying (ASK).3.3.1 Why use DigitalThe move to digital modulation provides more information capacity, compatibility with digitaldata services, higher data security, better quality communications, and quicker systemavailability. Developers of communications systems face these constraints: available bandwidth permissible power inherent noise level of the system The RF spectrum must be shared, yet every day there are more users for that spectrum as demand for communications services increases. Digital modulation schemes have greater capacity to convey large amounts of information than analogue modulation schemes. 44Sargunan ainal (jke,puo)
  7. 7. Chapter 3 E51243.3.2 Frequency Shift Keying – FSKFSK DefinitionThe two binary states, logic 0 (low) and 1 (high), are each represented by an analoguewaveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is representedby a wave at a different frequency. Below shows the basic representation, Fig 4. Fig 4 FSK modulationWith binary FSK, the centre or carrier frequency is shifted by the binary input data. Thus theinput and output rates of change are equal and therefore the bit rate and baud rate equal. Thefrequency of the carrier is changed as a function of the modulating signal (data), which is beingtransmitted. Amplitude remains unchanged. Two fixed-amplitude carriers are used, one for abinary zero, the other for a binary one.Uses of FSK.Today FSK Modems are used for short haul data communication over private lines or anydedicated wire pair. These are many used for communication between industrial applicationslike railroad signaling controls and mobile robotic equipment. The short haul modem offers thefollowing specs; Speeds of up to 9600 bps Full-duplex or half duplex operation. Distance up to 9.5 miles3.3.3 Phase Shift Keying - PSKPhase shift keying (PSK) is a method of transmitting and receiving digital signals in which thephase of a transmitted signal is varied to convey information.The simplest from of PSK has onlytwo phases, 0 and 1. It is therefore a type of ASK with (t) taking the values -1 or 1, and itsbandwidth is the same as that of ASK. The digital signal is broken up time wise into individual 45Sargunan ainal (jke,puo)
  8. 8. Chapter 3 E5124bits (binary digits).The state of each bit is determined according to the state of the precedingbit. If the phase of the wave does not change, then the signal state stays the same (low or high).If the phase of the wave changes by 180 degrees, that is, if the phase reverses, then the signalstate changes (from low to high or from high to low)If the phase of the wave changes by 180 degrees, that is, if the phase reverses, then the signalstate changes (from low to high or from high to low). Because there are two possible wavephases, this form of PSK is sometimes called bi-phase modulation. If two or more of the samelogic level are received in secession the frequency will remain the same until the logic levelchanges. Fig 5 PSK ModulationUses of PSK. Binary Phase Shift Keying (BPSK) a. BPSK is mainly used in deep space telemetry and also cable modems Quadrature Phase Shift Keying (QPSK) and it Variants a. Satellites CDMA, (Code-Division Multiple Access) refers to any of several protocols used in so- called second-generation (2G) and third-generation (3G) wireless communications3.3.3 Quadrature Amplitude Modulation- QAMQAM is the encoding of information into a carrier wave by variation of the amplitude of boththe carrier wave and a quadrature carrier that is 90° out of phase with the main carrier inaccordance with two input signals. Alternately, this can be regarded (using complex numbernotation) as simple amplitude modulation of a complex-valued carrier wave by a singlecomplex-valued signal. What this actually means is that the amplitude and the phase of thecarrier wave are simultaneously changed according to the information you want to transmit.The table (Fig 6) below shows the different amplitude and the phase changes for different bitsequences for 8-bit QAM and animates it. 46Sargunan ainal (jke,puo)
  9. 9. Chapter 3 E5124QAM is simply a combination of amplitude modulation and phase shift keying. It use a signalthat is transmitting at 3600 bps, or 3 bits per baud. This means that we can represent 8 binarycombinations. It will use 2 measures of amplitude, 1 and 2. Also use 4 possible phase shifts, likewe did before. Combining the two, we have 8 possible waves that we can send.First step is to generate a table to show us which waves correspond to which binarycombination. This can basically be done at random, although modem manufacturers haveagreed on standards.Bit value Amplitude Phase shift 000 1 None 001 2 None 010 1 1/4 011 2 1/4 100 1 1/2 101 2 1/2 110 1 3/4 111 2 3/4(a) QAM amplitude with phase shift (b) Phase shift in waveform Fig 6 QAM Lets encode a big bit stream: 001010100011101000011110 First, break it up into 3-bit triads: 001-010-100-011-101-000-011-110Uses of QAM: i. 16 QAM o Microwave digital radio. o Modems. o DVB-C, (Digital Video Broadcasting - Cable) is the digital TV broadcasting method thats done via digital cable networks(mostly used in Europe) 47Sargunan ainal (jke,puo)
  10. 10. Chapter 3 E5124 o -DVB-T, (Digital Video Broadcasting - Terrestrial) and means basically the; digital TV broadcasting method thats done via terrestrial networks -- normally existing analogue TV antennas are used to receive the transmissions. (mostly used in Europe) ii. 32 QAM o Terrestrial microwave. o DVB-T. iii. 64 QAM o DVB-C. o modems. o broadband set top boxes. o MMDS, (Multi-channel Multipoint Distribution Service) is a broadcasting and communications service that operates in the ultra-high-frequency (UHF) portion of the radio spectrum between 2.1 and 2.7 GHz. MMDS is also known as wireless cable. It was conceived as a substitute for conventional cable television (TV). However, it also has applications in telephone/fax and data communications. iv. 256 QAM o Modems o DVB-C (Europe) o Digital Video (US)3.4 Asynchronous ModemsAsynchronous transmission allows data to be transmitted without the sender having to send aclock signal to the receiver. Instead, the sender and receiver must agree on timing parameters inadvance and special bits are added to each word which are used to synchronize the sending andreceiving units.When a word is given to the UART for Asynchronous transmissions, a bit called the "Start Bit"is added to the beginning of each word that is to be transmitted. The Start Bit is used to alert thereceiver that a word of data is about to be sent, and to force the clock in the receiver intosynchronization with the clock in the transmitter. These two clocks must be accurate enough tonot have the frequency drift by more than 10% during the transmission of the remaining bits inthe word. (This requirement was set in the days of mechanical teleprinters and is easily met bymodern electronic equipment.)After the Start Bit, the individual bits of the word of data are sent, with the Least Significant Bit(LSB) being sent first. Each bit in the transmission is transmitted for exactly the same amount oftime as all of the other bits, and the receiver “looks” at the wire at approximately halfwaythrough the period assigned to each bit to determine if the bit is a 1 or a 0. For example, if ittakes two seconds to send each bit, the receiver will examine the signal to determine if it is a 1 ora 0 after one second has passed, then it will wait two seconds and then examine the value of the 48Sargunan ainal (jke,puo)
  11. 11. Chapter 3 E5124next bit, and so on. The sender does not know when the receiver has “looked” at the value of thebit. The sender only knows when the clock says to begin transmitting the next bit of the word.When the entire data word has been sent, the transmitter may add a Parity Bit that the transmittergenerates. The Parity Bit may be used by the receiver to perform simple error checking. Then atleast one Stop Bit is sent by the transmitter, refer Fig 7. Fig 7 Asynchronous data transmissionWhen the receiver has received all of the bits in the data word, it may check for the Parity Bits(both sender and receiver must agree on whether a Parity Bit is to be used), and then the receiverlooks for a Stop Bit. If the Stop Bit does not appear when it is supposed to, the UART considersthe entire word to be garbled and will report a Framing Error to the host processor when the dataword is read. The usual cause of a Framing Error is that the sender and receiver clocks were notrunning at the same speed, or that the signal was interrupted. Regardless of whether the data wasreceived correctly or not, the UART automatically discards the Start, Parity and Stop bits. If thesender and receiver are configured identically, these bits are not passed to the host. If anotherword is ready for transmission, the Start Bit for the new word can be sent as soon as the Stop Bitfor the previous word has been sent. Because asynchronous data is “self synchronizing”, if thereis no data to transmit, the transmission line can be idle. 49Sargunan ainal (jke,puo)
  12. 12. Chapter 3 E5124 Fig 8 Asynchronous block diagramFunction of each block as in Fig 8: UART (Universal Asynchronous Receiver/Transmitter) o Parallel data from computer converted to serial data for transmitting o Serial data and MOD/DEMOD circuit converted to original data while receiving. MOD/DEMOD circuit o Modulate transmitting data to telephony signal for transmission o Convert telephony signal to digital signal at reciver Transmitter/Receiver Filter o Eliminate unwanted signal like distortion, noise and interruption signal while receiving or transmitting. Buffer o Set transmitting signal level o Impedance matching for telephone system o It works under same fundamental for transmitting and receiving Carrier Detector o Detect data carrier signal and inform computer the status. 50Sargunan ainal (jke,puo)
  13. 13. Chapter 3 E51243.5 Synchronous MODEMTwo basic techniques are employed to ensure correct synchronization. In synchronous systems,separate channels are used to transmit data and timing information. The timing channel transmitsclock pulses to the receiver. Upon receipt of a clock pulse, the receiver reads the data channeland latches the bit value found on the channel at that moment. The data channel is not read againuntil the next clock pulse arrives. Because the transmitter originates both the data and the timingpulses, the receiver will read the data channel only when told to do so by the transmitter (via theclock pulse), and synchronization is guaranteed.Techniques exist to merge the timing signal with the data so that only a single channel isrequired. This is especially useful when synchronous transmissions are to be sent through amodem. Fig 9 Synchronous MODEMFunction (Fig 9): Clocking/Timer Circuit o Produce synchronous clocking for both side transmitting & receiving Descrambler o Convert random signal to original shape. Phase Demod o Phase demodulate carried out to remove carrier signal and restore original signal. ADC (Analog-Digital-Converter) o Convert analog signal to digital signal so that computer can understand Equalizer o To amplify signal at high frequency. 51Sargunan ainal (jke,puo)