Chapter 15 16 Electromagnetic Signals Analog Signal signal

Chapter 15 & 16:

Electromagnetic Signals • Analog Signal – signal intensity varies in a smooth fashion over time. In other words, there are no breaks or discontinuities in the signal • Digital Signal – signal intensity maintains a constant level for some period of time and then changes to another constant level 2

Analog and Digital Waveforms 3

Periodic Signal Characteristics • Peak Amplitude (A) – Maximum signal value (strength), measured in volts • Frequency (f) – Repetition rate – Measured in cycles per second or Hertz (Hz) • Period (T) – Amount of time it takes for one repetition, T=1/f • Phase ( ) – Relative position in time, measured in degrees 4

Frequency Domain Concepts s(t) = (4/ ) (sin (2 ft) + (1/3) sin (2 (3 f)t)) 5

Frequency Domain Concepts • Spectrum of a signal is the range of frequencies that it contains • Absolute bandwidth of a signal is the width of the spectrum • Effective bandwidth contained in a relatively narrow band of frequencies, where most of signal’s energy is found • The greater the bandwidth, the higher the information-carrying capacity of the signal 6

Bandwidth • Width of the spectrum of frequencies that can be transmitted – if spectrum=300 to 3400 Hz, bandwidth=3100 Hz • Greater bandwidth leads to greater costs • Limited bandwidth leads to distortion 7

Analog Signaling 8

Voice/Audio Analog Signals • Easily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltage • Human voice has frequency components ranging from 20 Hz to 20 k. Hz • For practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400 Hz 9

Voice Signals 10

Image/Video: Analog Data to Analog Signals • Image is scanned in lines; each line is displayed with varying levels of intensity • Requires approximately 4 Mhz of analog bandwidth • Since multiple signals can be sent via the same channel, guardbands are necessary, raising bandwidth requirements to 6 Mhz per signal 11

Digital Signals 12

Transmission Media • Physical path between transmitter and receiver (“channel”) • Design factors affecting data rate – bandwidth – physical environment – number of receivers – impairments 13

Impairments and Capacity • Impairments exist in all forms of data transmission • Analog signal impairments result in random modifications that impair signal quality • Digital signal impairments result in bit errors (1 s and 0 s transposed) 14

Transmission Impairments: Guided Media • Attenuation – loss of signal strength over distance • Attenuation Distortion – different losses at different frequencies • Delay Distortion – different speeds for different frequencies • Noise – distortions of signal caused by interference 15

Transmission Impairments: Unguided (Wireless) Media • Free-Space Loss – Signals disperse with distance • Atmospheric Absorption – Water vapor and oxygen contribute to signal loss • Multipath – Obstacles reflect signal creating multiple copies • Refraction - Change in signal speed due to atmospheric conditions • Thermal Noise - White noise, arises from thermal activity of devices 16

Types of Noise • Thermal (aka “white noise”) – Uniformly distributed, cannot be eliminated • Intermodulation – When different frequencies collide (creating “harmonics”) • Crosstalk – Overlap of signals • Impulse noise – Irregular spikes, less predictable Business Data Communications, 5 e 17

Channel Capacity • The rate at which data can be transmitted over a given path, under given conditions • Four concepts – Data rate – Bandwidth – Noise – Error rate 18

Data Communication Components • Data – Analog: Continuous value data (sound, light, temperature) – Digital: Discrete value (text, integers, symbols) • Signal – Analog: Continuously varying electromagnetic wave – Digital: Series of voltage pulses (square wave) • Transmission – Analog: Works the same for analog or digital signals – Digital: Used only with digital signals 19

Analog Data Signal Options • Analog data to analog signal – Inexpensive, easy conversion (e. g. , telephone) – Data may be shifted to a different part of the available spectrum (multiplexing) – Used in traditional analog telephony • Analog data to digital signal – Requires a codec (encoder/decoder) – Allows use of digital telephony, voice mail 20

Digital Data Signal Options • Digital data to analog signal – Requires modem (modulator/demodulator) – Allows use of PSTN to send data – Necessary when analog transmission is used • Digital data to digital signal – Requires CSU/DSU (channel service unit/data service unit) – Less expensive when large amounts of data are involved – More reliable because no conversion is involved 21

Analog and Digital Signaling 22

Transmission Choices • Analog transmission – only transmits analog signals, without regard for data content – attenuation overcome with amplifiers – signal is not evaluated or regenerated • Digital transmission – transmits analog or digital signals – uses repeaters rather than amplifiers – switching equipment evaluates and regenerates signal 23

Analog and Digital Data and Signals Analog Signal Digital Signal Analog Data Two alternatives: (1) signal occupies the same spectrum as the analog data (2) Analog data are encoded to occupy a different spectrum. Analog data are encoded using a codec to produce a digital bit stream. Digital Data Digital data are encoded using a modem to produce analog signal. Two alternatives: (1) signal consists of two voltage levels to represent two binary values (2) digital data are encoded to produce a digital signal with desired properties. 24

Analog and Digital Treatment of Signals Analog Digital Transmission Analog Signal Is propagated through amplifiers; same treatment whether signal is used to represent analog data or digital data. Assumes that the analog signal represents digital data. Signal is propagated through repeaters; at each repeater, digital data are recovered from inbound signal and used to generate a new analog outbound signal. Digital Signal Not used. Digital signal represents a stream of 1 s and 0 s which may represent digital data or may be an encoding of analog data. Signal is propagated though repeaters; at each repeater, stream of 1 s and 0 s is recovered from inbound signal and used to generate a new digital outbound signal.

Advantages of Digital Transmission • Cost – large scale and very large scale integration has caused continuing drop in cost • Data Integrity – effect of noise and other impairments is reduced • Capacity Utilization – high capacity is more easily and cheaply achieved with time division rather than frequency division • Security & Privacy – Encryption possible • Integration – All signals (Voice. Video, image, data) treated the same 26

Analog Encoding of Digital Data • Data encoding and decoding technique to represent data using the properties of analog waves • Modulation: the conversion of digital signals to analog form • Demodulation: the conversion of analog data signals back to digital form 27

Modem • An acronym for modulator-demodulator • Uses a constant-frequency signal known as a carrier signal • Converts a series of binary voltage pulses into an analog signal by modulating the carrier signal • The receiving modem translates the analog signal back into digital data 28

Methods of Modulation • Amplitude modulation (AM) or amplitude shift keying (ASK) • Frequency modulation (FM) or frequency shift keying (FSK) • Phase modulation or phase shift keying (PSK) 29

Voice Grade Modems • Designed for digital transmission over ordinary phone lines • Uses 4 -k. Hz bandwidth • Adheres to ITU-T standards 30

Cable Modems • Permits Internet access over cable television networks. • ISP is at or linked by high-speed line to central cable office • Cables used for television delivery can also be used to deliver data between subscriber and central location • Upstream and downstream channels are shared among multiple subscribers, time-division multiplexing technique • Splitter is used to direct TV signals to a TV and the data channel to a cable modem 31

Cable Modems 32

Asymmetric Digital Subscriber Line (ADSL) • New modem technology for high-speed digital transmission over ordinary telephone wire. • At central office, a combined data/voice signal is transmitted over a subscriber line • At subscriber’s site, twisted pair is split and routed to both a PC and a telephone – At the PC, an ADSL modem demodulates the data signal for the PC. – At the telephone, a microfilter passes the 4 -k. Hz voice signal. • The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques. 33

ADSL Modem Application 34

Digital Encoding of Analog Data • Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form • Best-known technique for voice digitization is pulse-code modulation (PCM) • The sampling theorem: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal. • Good-quality voice transmission can be achieved with a data rate of 8 kbps • Some videoconference products support data rates as low as 64 kbps 35

Pulse-Code Modulation Example 36

Analog Encoding of Analog Information • Voice-generated sound wave can be represented by an electromagnetic signal with the same frequency components, and transmitted on a voice-grade telephone line. • Modulation can produce a new analog signal that conveys the same information but occupies a different frequency band – A higher frequency may be needed for effective transmission – Analog-to-analog modulation permits frequencydivision multiplexing 37

Analog Sine-Wave Signals 38

Asynchronous Transmission • Avoids timing problem by not sending long, uninterrupted streams of bits • Data transmitted one character at a time, where each character is 5 to 8 bits in length. • Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character. • Simple and cheap but requires an overhead of 2 to 3 bits per character 39

Asynchronous Transmission 40

Synchronous Transmission • Block of bits transmitted in a steady stream without start and stop codes. • Clocks of transmitter and receiver must somehow be synchronized – Provide a separate clock line between transmitter and receiver; works well over short distances, – Embed the clocking information in the data signal. • Each block begins with a preamble bit pattern and generally ends with a postamble bit pattern • The data plus preamble, postamble, and control information are called a frame 41

Synchronous Transmission • More efficient than asynchronous transmission • Preamble, postamble and control information are typically < 100 bits • Introduces the need for error checking 42

Error Control Process • All transmission media have potential for introduction of errors • All data link layer protocols must provide method for controlling errors • Error control process has two components – Error detection: redundancy introduced so that the occurrence of an error will be detected – Error correction: receiver and transmitter cooperate to retransmit frames that were in error 43

Error Detection: Parity Bits • Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity) • Good for detecting single-bit errors only • High overhead (one extra bit per 7 -bit character=12. 5%) • Noise impulses are often long enough to destroy more than one bit 44

Error Detection: Cyclic Redundancy Check (CRC) • Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame • 17 -bit divisor leaves 16 -bit remainder, 33 bit divisor leaves 32 -bit remainder • For a CRC of length N, errors undetected are 2 -N • Overhead is low (1 -3%) 45

Error Detection Process 46