Chapter 3
- 1. Chapter 3 E5124 3.0 MODEM Modem also know as modulator-demodulator. A modem is a device or program that enables a computer to transmit data over, for example, telephone or cable lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms. Fortunately, there is one standard interface for connecting external modems to computers called RS-232. Consequently, any external modem can be attached to any computer that has an RS-232 port, which almost all personal computers have. There are also modems that come as an expansion board that you can insert into a vacant expansion slot. These are sometimes called onboard or internal modems. Fig 1 Modem as interface between computer and incoming line 3.1 Importance While the modem interfaces are standardized, a number of different protocols for formatting data 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 support for the more common protocols -- at slow data transmission speeds at least, most modems can communicate with each other. At high transmission speeds, however, the protocols are less standardized. Aside from the transmission protocols that they support, the following characteristics 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 39 Sargunan ainal (jke,puo)
- 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 Modems i. 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 a credit card and fit into the PC Card slot on notebook and handheld computers. These modems are removed when the modem is not needed. Except for their size, PC Card modems are like a combination of external and internal modems. These devices are 40 Sargunan ainal (jke,puo)
- 3. Chapter 3 E5124 plugged directly into an external slot in the portable computer, so no cable is required other than the telephone line connection. The cards are powered by the computer, which is fine unless the computer is battery-operated. Running a PC Card modem while the portable computer 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 you don't have to open the computer. External modems have their own power supply and connect with a cable to a computer's serial port. The telephone line plugs into a socket on the rear panel of the modem. Because external modems have their own power supply, you can turn off the modem to break an online connection quickly without powering down the computer. Another advantage over an internal modem is that an external modem's separate power supply does not drain any power from the computer. You also can monitor your modem's connection activity by watching the status lights. 3.1.2 How modems work When a modem first makes a connection, you will hear screeching sounds coming from the modem. These are digital signals coming from the computer to which you are connecting being modulated into audible sounds. The modem sends a higher-pitched tone to represent the digit I and a lower-pitched tone to represent the digit 0. At the other end of your modem connection, the computer attached to its modem reverses this process. The receiving modem demodulates the various tones into digital signals and sends them to the receiving computer. Actually, the process is a bit more complicated than sending and receiving signals in one direction and then another. Modems simultaneously send and receive signals in small chunks. The modems can tell incoming from outgoing data signals by the type of standard tones they use. Another part of the translation process involves transmission integrity. The modems exchange an added mathematical code along the way. This special code, called a checksum, lets both computers know if the data segments are coming through properly. If the mathematical sums do not match, the modems communicate with each other by resending the missing segments of data. Modems also have special circuitry that allows them to compress digital signals before modulating them and then decompressing them after demoduating the signals. The compression/decompression process compacts the data so that it can travel along telephone lines more efficiently. 41 Sargunan ainal (jke,puo)
- 4. Chapter 3 E5124 Fig 2 Modem communication Modems convert analog data transmitted over phone lines into digital data computers can read; they also convert digital data into analog data so it can be transmitted. This process involves modulating and demodulating the computer’s digital signals into analog signals that travel over the telephone lines. In other words, the modem translates computer data into the language used by telephones and then reverses the process to translate the responding data back into computer language. 3.1.3 Difference between digital and analog signals A computer performs its tasks by turning on and off a series of electronic switches represented by 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 inside the computer. By comparison, the telephone works by sending sounds in a continuous analog signal sent along an electronic current that varies in frequency and strength 3.2 Analogue modulation The modulated signal consists pure sine wave "carrier" signal which is modified to convey information. A pure carrier sine wave, unchanging in frequency and voltage, provides no flow of information 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, and phase. 42 Sargunan ainal (jke,puo)
- 5. Chapter 3 E5124 i. Frequency Modulation The simplest modulation method is frequency modulation. Frequency is measured in cycles per second (of a sine wave). It's the count of the number of times the sine wave shape repeats itself in 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 and another means a 1. For example, for some obsolete 300 baud modems 1070 Hz meant a binary 0 while 1270 Hz meant a binary 1. This was called "frequency shift keying". Instead of just two possible frequencies, more could be used to allow more information to be transmitted. If we had 4 different frequencies (call them A, B, C, and D) then each frequency could stand for a pair of bits. For example, to send 00 one would use frequency A. To send 01, use frequency B; for 10 use C; for 11 use D. In like manner, by using 8 different frequencies we could send 3 bits with each shift in frequency. Each time we double the number of possible frequencies we increase the number of bits it can represent by 1. Fig 3 Types Analogue Modulation ii. Amplitude Modulation Once one understands frequency modulation example above (Fig 3) including the possibilities of representing a few bits by a single shift in frequency, it's easier to understand both amplitude modulation and phase modulation. For amplitude modulation, one just changes the height (voltage) of the sine wave analogous to changing the frequency of the sine wave. For a simple case there could only be 2 allowed amplitude levels, one representing a 0-bit and 43 Sargunan ainal (jke,puo)
- 6. Chapter 3 E5124 another representing a 1-bit. As explained for the case of frequency modulation, having more possible amplitudes will result in more information being transmitted per change in amplitude. iii. Phase Modulation To change the phase of a sine wave at a certain instant of time, we stop sending this old sine wave and immediately begin sending a new sine wave of the same frequency and amplitude. If we started sending the new sine wave at the same voltage level (and slope) as existed when we stopped sending the old sine wave, there would be no change in phase (and no detectable change at all). But suppose that we started up the new sine wave at a different point on the sine wave curve. Then there would likely be a sudden voltage jump at the point in time where the old sine wave stopped and the new sine wave began. This is a phase shift and it's measured in 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 could just skip over say 90 deg. or 135 deg. etc. As in the example for frequency modulation, the more possible phase shifts, the more bits a single shift in phase can represent. 3.3 Digital Modulation Digital modulation schemes transform digital signals like the one shown below into waveforms that are compatible with the nature of the communications channel. There are two major categories of digital modulation. One category uses a constant amplitude carrier and the other carries the information in phase or frequency variations (FSK, PSK). The other category conveys the information in carrier amplitude variations and is known as amplitude shift keying (ASK). 3.3.1 Why use Digital The move to digital modulation provides more information capacity, compatibility with digital data services, higher data security, better quality communications, and quicker system availability. 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. 44 Sargunan ainal (jke,puo)
- 7. Chapter 3 E5124 3.3.2 Frequency Shift Keying – FSK FSK Definition The two binary states, logic 0 (low) and 1 (high), are each represented by an analogue waveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is represented by a wave at a different frequency. Below shows the basic representation, Fig 4. Fig 4 FSK modulation With binary FSK, the centre or carrier frequency is shifted by the binary input data. Thus the input and output rates of change are equal and therefore the bit rate and baud rate equal. The frequency of the carrier is changed as a function of the modulating signal (data), which is being transmitted. Amplitude remains unchanged. Two fixed-amplitude carriers are used, one for a binary zero, the other for a binary one. Uses of FSK. Today FSK Modems are used for short haul data communication over private lines or any dedicated wire pair. These are many used for communication between industrial applications like railroad signaling controls and mobile robotic equipment. The short haul modem offers the following specs; Speeds of up to 9600 bps Full-duplex or half duplex operation. Distance up to 9.5 miles 3.3.3 Phase Shift Keying - PSK Phase shift keying (PSK) is a method of transmitting and receiving digital signals in which the phase of a transmitted signal is varied to convey information.The simplest from of PSK has only two phases, 0 and 1. It is therefore a type of ASK with (t) taking the values -1 or 1, and its bandwidth is the same as that of ASK. The digital signal is broken up time wise into individual 45 Sargunan ainal (jke,puo)
- 8. Chapter 3 E5124 bits (binary digits).The state of each bit is determined according to the state of the preceding bit. 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 signal state 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 signal state changes (from low to high or from high to low). Because there are two possible wave phases, this form of PSK is sometimes called bi-phase modulation. If two or more of the same logic level are received in secession the frequency will remain the same until the logic level changes. Fig 5 PSK Modulation Uses 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 communications 3.3.3 Quadrature Amplitude Modulation- QAM QAM is the encoding of information into a carrier wave by variation of the amplitude of both the carrier wave and a 'quadrature' carrier that is 90° out of phase with the main carrier in accordance with two input signals. Alternately, this can be regarded (using complex number notation) as simple amplitude modulation of a complex-valued carrier wave by a single complex-valued signal. What this actually means is that the amplitude and the phase of the carrier 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 bit sequences for 8-bit QAM and animates it. 46 Sargunan ainal (jke,puo)
- 9. Chapter 3 E5124 QAM is simply a combination of amplitude modulation and phase shift keying. It use a signal that is transmitting at 3600 bps, or 3 bits per baud. This means that we can represent 8 binary combinations. It will use 2 measures of amplitude, 1 and 2. Also use 4 possible phase shifts, like we 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 binary combination. This can basically be done at random, although modem manufacturers have agreed 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 Let's encode a big bit stream: 001010100011101000011110 First, break it up into 3-bit triads: 001-010-100-011-101-000-011-110 Uses of QAM: i. 16 QAM o Microwave digital radio. o Modems. o DVB-C, (Digital Video Broadcasting - Cable) is the digital TV broadcasting method that's done via digital cable networks(mostly used in Europe) 47 Sargunan ainal (jke,puo)
- 10. Chapter 3 E5124 o -DVB-T, (Digital Video Broadcasting - Terrestrial) and means basically the; digital TV broadcasting method that's 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 Modems Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver. Instead, the sender and receiver must agree on timing parameters in advance and special bits are added to each word which are used to synchronize the sending and receiving 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 the receiver that a word of data is about to be sent, and to force the clock in the receiver into synchronization with the clock in the transmitter. These two clocks must be accurate enough to not have the frequency drift by more than 10% during the transmission of the remaining bits in the word. (This requirement was set in the days of mechanical teleprinters and is easily met by modern 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 of time as all of the other bits, and the receiver “looks” at the wire at approximately halfway through the period assigned to each bit to determine if the bit is a 1 or a 0. For example, if it takes two seconds to send each bit, the receiver will examine the signal to determine if it is a 1 or a 0 after one second has passed, then it will wait two seconds and then examine the value of the 48 Sargunan ainal (jke,puo)
- 11. Chapter 3 E5124 next bit, and so on. The sender does not know when the receiver has “looked” at the value of the bit. 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 transmitter generates. The Parity Bit may be used by the receiver to perform simple error checking. Then at least one Stop Bit is sent by the transmitter, refer Fig 7. Fig 7 Asynchronous data transmission When 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 receiver looks for a Stop Bit. If the Stop Bit does not appear when it is supposed to, the UART considers the entire word to be garbled and will report a Framing Error to the host processor when the data word is read. The usual cause of a Framing Error is that the sender and receiver clocks were not running at the same speed, or that the signal was interrupted. Regardless of whether the data was received correctly or not, the UART automatically discards the Start, Parity and Stop bits. If the sender and receiver are configured identically, these bits are not passed to the host. If another word is ready for transmission, the Start Bit for the new word can be sent as soon as the Stop Bit for the previous word has been sent. Because asynchronous data is “self synchronizing”, if there is no data to transmit, the transmission line can be idle. 49 Sargunan ainal (jke,puo)
- 12. Chapter 3 E5124 Fig 8 Asynchronous block diagram Function 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. 50 Sargunan ainal (jke,puo)
- 13. Chapter 3 E5124 3.5 Synchronous MODEM Two basic techniques are employed to ensure correct synchronization. In synchronous systems, separate channels are used to transmit data and timing information. The timing channel transmits clock pulses to the receiver. Upon receipt of a clock pulse, the receiver reads the data channel and latches the bit value found on the channel at that moment. The data channel is not read again until the next clock pulse arrives. Because the transmitter originates both the data and the timing pulses, the receiver will read the data channel only when told to do so by the transmitter (via the clock pulse), and synchronization is guaranteed. Techniques exist to merge the timing signal with the data so that only a single channel is required. This is especially useful when synchronous transmissions are to be sent through a modem. Fig 9 Synchronous MODEM Function (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. 51 Sargunan ainal (jke,puo)