Chapter 15 & 16:

2

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

3

Analog and Digital Waveforms

4

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

5

s(t) = (4/) (sin (2ft) + (1/3) sin (2(3f)t))

Frequency Domain Concepts

6

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

7

Bandwidth

• Width of the spectrum of frequencies that can be transmitted– if spectrum=300 to 3400Hz,

bandwidth=3100Hz

• Greater bandwidth leads to greater costs

• Limited bandwidth leads to distortion

8

Analog Signaling

9

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 20Hz to 20kHz

• For practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400Hz

Voice Signals

10

11

Image/Video: Analog Data to Analog Signals

• Image is scanned in lines; each line is displayed with varying levels of intensity

• Requires approximately 4Mhz of analog bandwidth

• Since multiple signals can be sent via the same channel, guardbands are necessary, raising bandwidth requirements to 6Mhz per signal

Digital Signals

12

13

Transmission Media

• Physical path between transmitter and receiver (“channel”)

• Design factors affecting data rate– bandwidth– physical environment– number of receivers– impairments

14

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 (1s and 0s transposed)

15

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

16

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

Business Data Communications, 5e 17

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

18

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

19

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

20

Analog DataSignal 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

21

Digital DataSignal 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

Analog and Digital Signaling

22

23

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

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 Transmission

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 1s and 0s which may represent digital data or may be an encoding of analog data. Signal is propagated though repeaters; at each repeater, stream of 1s and 0s is recovered from inbound signal and used to generate a new digital outbound signal.

26

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

27

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

28

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

29

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)

Voice Grade Modems

• Designed for digital transmission over ordinary phone lines

• Uses 4-kHz bandwidth

• Adheres to ITU-T standards

30

31

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

Cable Modems

32

33

Asymmetric DigitalSubscriber 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-kHz voice signal.

• The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques.

ADSL Modem Application

34

35

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

Pulse-Code Modulation Example

36

37

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 frequency-

division multiplexing

Analog Sine-Wave Signals

38

39

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

Asynchronous Transmission

40

41

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

Synchronous Transmission

• More efficient than asynchronous transmission

• Preamble, postamble and control information are typically < 100 bits

• Introduces the need for error checking

42

43

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

44

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

45

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%)

Error Detection Process

46