Computer Networks Chapter 5 Analog Transmission Spring 2006

Computer Networks Chapter 5 – Analog Transmission Spring 2006 Computer Networks

Why Study Analog Transmission? § Before the ideas for digital transmission came in place, only the analog transmission was used § Usually the analog signals (voice) were transmitted § Computers were not invented, yet § Analog transmission is still used § The infrastructure cannot be changed in a short period of time § Even today digital signals sometimes need to be transmitted using analog transmission Spring 2006 Computer Networks 2

Digital-to-analog and Analog-to-analog Conversion § The process of digital-to-analog and analog-toanalog conversion is known as modulation. Digital-to-analog modulation Digital-to-analog Analog-to-analog modulation Analog-to-analog Spring 2006 Computer Networks 3

Modulation § Modulation is a process in which the high frequency sinusoidal signal (usually called carrier) changes some of its parameters according to the changes of the low frequency signal that carries information § The parameters that can be changed are: § Amplitude § Frequency § Phase Spring 2006 Computer Networks 4

Demodulation § Demodulation is a reverse process to modulation. § The modulated signal is simply multiplied by the carrier signal to get the original signal § Modems are devices that do both MODulatuion. DEModulation Spring 2006 Computer Networks 5

Keying – Modulation of Digital Signals § Modulation of digital signals is called keying § Depending on the parameter that is changed the keying can be: § § Amplitude Shift Keying (ASK) – a change in amplitude Frequency Shift Keying (FSK) – a change in frequency Phase Shift Keying (PSK) – a change in phase Quadrature Amplitude Modulation (QAM) – combination of ASK and PSK Spring 2006 Computer Networks 6

Modulation of a Digital Signal - Example Digital message Sinusoidal carrier Carrier signal parameters to modify Amplitude Spring 2006 Frequency Phase Amplitude & Phase Computer Networks 7

Bit Rate with Analog Transmission § In the PSTN (Public Switched Telephone Network), the available bandwidth for transmitting voice is fixed (300 – 3400 Hz) § The signal-to-noise ratio for the voice channel is known § Using the Shannon theorem the maximum bit rate can be calculated § This bit rate was increased by using modulation techniques in which multi-level pulses are carrying several bits/pulse Spring 2006 Computer Networks 8

Bit Rate vs. Baud Rate § If the parameter (amplitude, frequency or phase) has more than two values than there are more than two signal units in the modulated signal. § The number of signal units send per second is called Baud rate (Baudaut was a French scientist) § When the number of signal units is higher than two, then more then one bit can be sent with a single signal unit. Spring 2006 Computer Networks 9

Characteristics of ASK § The bandwidth of the modulated signal depends on the baud rate and the modulation factor § Susceptible to sudden gain changes § Inefficient § Although AM is used for radio (audio) transmissions, the resulting signal quality is too poor for computer use. Spring 2006 Computer Networks 10

Characteristics of FSK § The frequencies must be reasonably far apart in order the receiver to be able to do the detection. § The bandwidth depends on the baud rate and how far are the two frequencies § Less susceptible to error than ASK § Used for high frequency radio Spring 2006 Computer Networks 11

Characteristics of PSK § The simplest form of phase modulation uses phases of 180 o for 0 and 0 o for 1 or vice versa § PSK is less susceptible to noise and requires the same bandwidth as ASK § This makes it suitable for creating signal units with more different phases § With four phases, 2 bits can be carried with each signal units and bit rate = 2 xbaud rate § With eight phases, 3 bits can be carried with each signal units and bit rate = 3 xbaud rate Spring 2006 Computer Networks 12

Quadrature Amplitude Modulation (QAM) § This is more sophisticated modulation sheme used for high speed voice band telephone line modems. § The idea is to combine amplitude and phase modulation. This allows to increase the number of signal units and increase the bit rate. Spring 2006 Computer Networks 13

Signal Vector Representation Frequency of the carrier s(t) = Ac(t) cos (2 pfct + f(t)) amplitude of the carrier Spring 2006 Q fixed!!! it l p m A e d u S Phase Computer Networks 0 degrees 14

Constellations Two amplitudes, Same phase S 1 Two phases Same amplitude S 2 S 1 Two amplitudes, Two phases S 2 Spring 2006 S 2 S 1 § Constellations show the number of signal units and the type of keying used Computer Networks 15

Constellations - Examples 0111 0100 2 A 0011 0010 0001 A 0110 1001 1011 16 signal units QAM 4 bits / unit Bit rate = 4 xbaud rate Spring 2006 4 signal units PSK 2 bits/ unit Bit rate = 2 xbaud rate Computer Networks 1010 0000 1110 1101 1100 1111 16 signal units 4 bits / unit QAM Bit rate = 4 xbaud rate 16

Bit Rate vs. Baud Rate - Examples 1. Compute the bit rate for a 1000 baud 16 -QAM channel. Since 24 = 16, 4 bits are carried by each of the signals. Therefore the bit rate = 4 x 1000 = 4000 bps 2. Compute the baud rate for a 72000 bps 64 -QAM signal. Since 216 = 64, there are six bits per signal elements. Therefore the baud rate = 72 000/6 = 12 000 baud Spring 2006 Computer Networks 17

Telephone Modems § Used to transmit digital data over analog voice lines § Only bandwidth of 2400 Hz is used § Some terminology used § DTE (Data Terminating Equipment) § Used to denote the computer § DCE (Data Communication Equipment) § Used to denote the modem § The modem modulates at the sender’s side and demodulates at the receiver’s site Spring 2006 Computer Networks 18

Modem Standards § V. 32 and V. 32 bis § uses QAM and achievs 9600 and 14 400 bps. § V. 34 and V. 34 bis § specifies a full-duplex data rate at 28. 8 Kbps and 33. 6 Kbps. These modems use sophisticated error control coding to provide reliable data transmission. In addition to that they use data compression to achieve higer speeds § The most recent, higher speed modems (56 Kbps) would appear to have rate that is in violation with the Shannon limit § These modems exploit the fact that the PSTN for the most part uses digital tehniques § can be used only if one party is using digital signaling (usually the Internet provider) § They are asymmetrical (data from the Internet provider flow at 56 Kbps, while the data from PC flow at 33. 6 Kbps. ). § V 90 and V 92 use these features Spring 2006 Computer Networks 19

Modulation of Analog Signals § Modulation of analog signals is necessary in order to shift the frequency of the signal, usually to the higher frequency § The high frequency sine wave (the carrier) is used § The parameters of the carrier (amplitude, frequency or phase) are varied according to the low frequency signal § Used for radio transmission § Each station in a particular geographic area uses carrier with different frequency § Interference among stations is avoided and listeners can tune to the desired frequency Spring 2006 Computer Networks 20

Amplitude Modulation s(t) S(f) t sm(t) 0 f fl Sm(f) t fc - f l fc fc + f l The frquency has been shifted The bandwidth is twice as large Spring 2006 Computer Networks 21 f

Frequency and Phase Modulation § The bandwidth of frequency modulated signal is 10 times the bandwidth of the low frequency signal § For stereo broadcast a bandwidth of 15 KHz is necessary § Radio stations need 10 x 15=150 KHz § The assigned bandwidth is 200 KHz § The additional bandwidth is called guard bands (its purpose is to protect stations’ bandwidth from overlapping) § Phase modulation is similar to FM Spring 2006 Computer Networks 22