lecture 1 basics of communication systems

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Communications Communications The message (data and information) is communicated via the signal The transmission medium “carries” the signal Sender Receiver Signal Transmissi on medium

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Page 1: Lecture 1 Basics of Communication Systems

Communications

Communications The message (data and information) is

communicated via the signal The transmission medium “carries” the signal

Sender ReceiverSignal

Transmissionmedium

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Telecommunications

Tele (Far) + Communications Early telecommunications

smoke signals and drums visual telegraphy (or semaphore in 1792)

Telegraph and telephone Telegraph (1839) Telephone (1876)

Radio and television Telephony

Voice and Data

Page 3: Lecture 1 Basics of Communication Systems

Communications

The transmission of data from one computer to another, or from one

device to another. A communications device, therefore, is any

machine that assists data transmission. For example, modems,

cables, and ports are all communications devices. Communications

software refers to programs that make it possible to transmit data.

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Telecommunications

Telecommunications The electronic transmission of signals for communications,

including such means as: Telephone Radio Television

Telecommunication medium Anything that carries an electronic signal and interfaces

between a sending device and a receiving device

Page 5: Lecture 1 Basics of Communication Systems

Communications and Telecommunications In human speech, the sender transmits a signal through

the transmission medium of the air In telecommunications, the sender transmits a signal

through the transmission medium of a cable

Schematic

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Digital Communications

Digital communications A specialized subset of telecommunications that

refers to the electronic collection, processing, and distribution of digital signal.

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Communication Systems Process describing transfer of information, data, instructions

between one or more systems through some media Examples

people, computers, cell phones, etc. Computer communication systems

Signals passing through the communication channel can be Digital, or analog Analog signals: continuous electrical waves Digital signals: individual electrical pulses (bits)

Receivers and transmitters: desktop computers, mainframe computers, etc.

TX

RXRX

RX

Communication channel

Communicationmedia

Amp/Adaptor

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Communication Systems

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A Communications Model

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Communications Tasks

Transmission system utilization Addressing

Interfacing Routing

Signal generation Recovery

Synchronization Message formatting

Exchange management Security

Error detection and correction Network management

Flow control

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Digital Communications Model

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Communication Technology Applications

voice mail Twitter

e-mailinstant

messaging chat rooms

newsgroups telephony videoconferencing

collaboration groupware global positioning system (GPS)

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Communication Technologies - Applications

Different technologies allowing us to communicate Examples: Voice mail, fax, email, instant message, chat rooms, news groups,

telephony, GPS, and more Voice mail: Similar to answering machine but digitized Fax: Sending hardcopy of text or photographs between computers using

fax modem Email: electronic mail – sending text, files, images between different

computer networks - must have email software More than 1.3 billion people send 244 billion messages monthly!

Chat rooms: Allows communications in real time when connected to the Internet

Telephony: Talking to other people over the Internet (also called VoIP) Sends digitized audio signals over the Internet Requires Internet telephone software

Groupware: Software application allowing a group of people to communicate with each other (exchange data) Address book, appointment book, schedules, etc.

GPS: consists of receivers connected to satellite systems Determining the geographical location of the receiver Used for cars, advertising, hiking, tracking, etc.

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Communication Devices Any type of hardware capable of transmitting data, instructions, and

information between devices Functioning as receiver, transmitter, adaptor, converter Basic characteristics: How fast, how far, how much data!

Examples: Dial-up modems, ISDN, DSL modems, network interface cards Dial-up modem: uses standard phone lines

Converts digital information into analog Consists of a modulator and a demodulator Can be external, internal, wireless

ISDN and DSL Modem: Allows digital communication between networks and computers Requires a digital modem Digital is better than analog – why?

Cable modem: a modem that transmits and receives data over the cable television (CATV) network Also called broadband modem (carrying multiple signals) The incoming signal is split Requires a cable modem

Network interface cards: Adaptor cards residing in the computer to transmit and receiver data over the network (NIC) Operate with different network technologies (e.g., Ethernet)

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Communication Software Examples of applications (Layer 7) take advantage of the

transport (Layer 4) services of TCP and UDP Hypertext Transfer Protocol (HTTP): A client/server

application that uses TCP for transport to retrieve HTML pages. Domain Name Service (DNS): A name-to-address translation

application that uses both TCP and UDP transport. Telnet: A virtual terminal application that uses TCP for

transport. File Transport Protocol (FTP): A file transfer application that

uses TCP for transport. Trivial File Transfer Protocol (TFTP): A file transfer application

that uses UDP for transport. Network Time Protocol (NTP): An application that

synchronizes time with a time source and uses UDP for transport.

Border Gateway Protocol (BGP): An exterior gateway routing protocol that uses TCP for transport. BGP is used to exchange routing information for the Internet and is the protocol used between service providers.

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Communication Channels A channel is a path between two communication

devices Channel capacity: How much data can be

passed through the channel (bit/sec) Also called channel bandwidth The smaller the pipe the slower data transfer!

Consists of one or more transmission media Materials carrying the signal Two types:

Physical: wire cable Wireless: Air

destination network server

T1 lines

T1 lines

T1 lines

T3 lines

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Physical Transmission Media A tangible media

Examples: Twisted-pair cable, coaxial cable, Fiber-optics, etc. Twisted-pair cable:

One or more twisted wires bundled together (why?) Made of copper

Coax-Cable: Consists of single copper wire surrounded by three layers of

insulating and metal materials Typically used for cable TV

Fiber-optics: Strands of glass or plastic used to transmit light Very high capacity, low noise, small size, less suitable to natural

disturbances

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Types of Telecommunications Media

Twisted pair wire cable Insulated pairs of wires historically used in

telephone service and to connect computer devices

Coaxial cable Consists of an inner conductor wire

surrounded by insulation, called the dielectric The dielectric is surrounded by a conductive

shield, which is surrounded by a non-conductive jacket. Coaxial cable has better data transmission rate than twisted pair

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Twisted-pair

A type of cable that consists of two independently insulates wires

twisted around one another. One wire carries the signal while the

other wire is grounded and absorbs signal interference. Twisted pair

cable is used by older telephone networks and is the least expensive

type of local-area network (LAN) cable. Other types of cables used

for LANs include coaxial cables and fiber optic cables.

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Coaxial Cable

A type of wire that consists of a centre wire surrounded byinsulation and then a grounded shield of braided wire. The shieldminimizes electrical and radio frequency interference.

Coaxial cabling is the primary type of cabling used by the cabletelevision industry and is also widely used for computer networks.Although more expensive than standard telephone wire, it is muchless susceptible to interference and can carry much more data.Because the cable television industry has already connected millionsof homes with coaxial cable, many analysts believe that they arethe best positioned to capitalize on the much-heralded informationhighway.

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Types of Telecommunications Media

Fiber-optic Cable Many extremely thin

strands of glass or plastic bound together in a sheathing which transmits signals with light beams

Can be used for voice, data, and video

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Fiber Optics

A technology that uses glass (or plastic) threads (fibers) to transmitdata. A fiber optic cable consists of a bundle of glass threads, eachof which is capable of transmitting messages modulated onto lightwaves.

Fiber optics has several advantages over traditional metalcommunications lines:

Fiber optic cables have a much greater bandwidth than metal cables. This means that they can carry more data

Fiber optic cables are less susceptible than metal cables to interference

Fiber optic cables are much thinner and lighter than metal wires Data can be transmitted digitally (the natural form for computer data)

rather than analogically.

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Fiber Optic ‘Discussion’

The main disadvantage of fiber optics is that the cables areexpensive to install. In addition, they are more fragile than wire andare difficult to split.

Fiber optics is a particularly popular technology for local-areanetworks. In addition, telephone companies are steadily replacingtraditional telephone lines with fiber optic cables. In the future,almost all communications will employ fiber optics.

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Types of Telecommunications Media

Microwave Communications Line-of-sight devices which must be placed in

relatively high locations Microwave usage

Information is converted to a microwave signal, sent through the air to a receiver, and recovered

Pretty picture

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Types of Telecommunications Media

Satellite transmission Communications satellites are relay stations that

receive signals from one earth station and rebroadcast them to another

They use microwave signals

Pretty picture

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Types of Telecommunications Media (5)

Cellular transmission Signals from cells are transmitted to a receiver and

integrated into the regular network

Pretty picture

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Cellular

Refers to communications systems, especially the Advance MobilePhone Service (AMPS), that divide a geographic region into sections,called cells. The purpose of this division is to make the most use outof a limited number of transmission frequencies. Each connection, orconversation, requires its own dedicated frequency, and the totalnumber of available frequencies is about 1,000. To support morethan 1,000 simultaneous conversations, cellular systems allocate aset number of frequencies for each cell. Two cells can use the samefrequency for different conversations so long as the cells are notadjacent to each other.

For digital communications, several competing cellular systems exist,including GSM and CDMA.

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Types of Telecommunications Media (6)

Infrared transmission Involves sending signals through the air via light

waves Requires line-of-sight and short distances (a few

hundred yards) Used to connect various computing devices such as

handheld computers

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Terminology

Analog Signal A continuous, curving signal

Digital Signal A signal represented by bits

Modems Devices that translate data from digital to analog

and analog to digital

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Analog

Almost everything in the world can be described or represented inone of two forms: analog or digital. The principal feature of analogrepresentations is that they are continuous. In contrast, digitalrepresentations consist of values measured at discrete intervals.

Digital watches are called digital because they go from one valueTo the next without displaying all intermediate values. Consequently, they can display only a finite number of times of day. In contrast, watches with hands are analog, because the hands move continuously around the clock face.

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Analog

As the minute hand goes around, it not only touches the numbers 1 through

12, but also the infinite number of points in between.

Early attempts at building computers used analog techniques, but

accuracy and reliability were not good enough. Today, almost all

computers are digital.

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Digital

Describes any system based on discontinuous data or events.Computers are digital machines because at their most basic levelthey can distinguish between just two values, 0 and 1, or off andon. There is no simple way to represent all the values in between,such as 0.25. All data that a computer processes must be encodeddigitally, as a series of zeroes and ones.

The opposite of digital is analog. A typical analog device is a clock inwhich the hands move continuously around the face. Such a clock iscapable of indicating every possible time of day.

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Digital

In contrast, a digital clock is capable of representing only a finite number of times (every tenth of a second, for example).

In general, humans experience the world analogically. Vision, for example, is an analog experience because we perceive infinitely smooth gradations of shapes and colors. Most analog events, however, can be simulated digitally. Photographs in newspapers, for instance, consist of an array of dots that areeither black or white. From afar, the viewer does not see the dots (the digital form), but only lines and shading, which appear to be continuous. Although

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Digital

digital representations are approximations of analog events, they areuseful because they are relatively easy to store and manipulateelectronically. The trick is in converting from analog to digital, andback again.

This is the principle behind compact discs (CDs). The music itselfexists in an analog form, as waves in the air, but these sounds arethen translated into a digital form that is encoded onto the disk.When you play a compact disc, the CD player reads the digital data,translates it back into its original analog form, and sends it to the amplifier and eventually the speakers.

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Digital

Internally, computers are digital because they consist of discrete

units called bits that are either on or off. But by combining many

bits in complex ways, computers simulate analog events. In one

sense, this is what computer science is all about.

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How a Modem Works

Modem Modulates a digital signal into an analog signal for

transmission via analog medium, then demodulates the signal into digital for receiving

Pretty picture

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ModemAcronym for modulator-demodulator. A modem is a device orprogram that enables a computer to transmit data over telephonelines. Computer information is stored digitally, whereas informationtransmitted over telephone lines is transmitted in the form of analogwaves. A modem converts between these two forms.

Fortunately, there is one standard interface for connecting externalmodems to computers called RS-232. Consequently, any externalmodem 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.

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Multiplexer

Multiplexer Allows several telecommunications signals to be

transmitted over a single communications medium at the same time

Pretty picture

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Communicationslink

Multiplexor Front-endprocessor

Hostcomputer

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Multiplexor

A communications device that multiplexes (combines) several signals

for transmission over a single medium. A demultiplexor completes

the process by separating multiplexed signals from a transmission

line. Frequently a multiplexor and demultiplexor are combined into a

single device capable of processing both outgoing and incoming

signals.

A multiplexor is sometimes called a mux.

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Front-End Processor

Front-end processor… Special purpose computers that manage

communication to and from a computer system

Pretty picture

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Incoming and outgoing jobs

Front-endprocessor

Mainsystem

Job 1

Job 2

Job 3

Job 4

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Carriers and Services Carriers

Organizations that take the responsibility of ensuring telecommunications can effectively take place between enterprises

Common carriers Long-distance telephone companies

Value-added carriers Companies that have developed private telecommunications systems and

offer their services for a fee Switched lines

Lines that use switching equipment to allow one transmission device to be connected to other transmission devices (e.g., standard telephone line)

Dedicated line A line that provides constant connection between two points. No switching or

dialing is needed

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Carriers and Services Private branch exchange (PBX)

Communication system that can manage both voice and data transfer within a location (e.g. a building) and to outside lines

Wide area telecommunication service (WATS) Billing method for heavy users of voice services

Phone and dialing services Includes automatic number identification (a.k.a. caller ID) Integration of telephones and personal computers Access code screening Call priorities One number portability (use anywhere) Intelligent dialing (auto re-dial for a busy number)

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ISDN ISDN = Integrated Services Digital Network Technology that uses existing common-carrier lines to

simultaneously transmit voice, video, and image data in digital form

Pretty picture

Carriers and Services

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T1 carriers An expensive service developed by AT&T to increase

the number of voice calls that could be handled through existing cables

Digital subscriber lines (DSL) Uses existing phone wires going into today’s homes

and businesses to provide transmission speeds exceeding 500 Kbps at a cost of $100 - $300 per month

Carriers and Services

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Communications Software and Protocols

Communications software Provides error checking, message formatting,

communications logs, data security and privacy, and translation capabilities for networks

Network operating system (NOS) Systems software that controls the computer systems and

devices on a network and allows them to communicate with each other

Network management software Enables a manager on a networked desktop to monitor the

use of individual computers and shared hardware, scan for viruses, and ensure compliance with software licenses

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Communications Software and Protocols

Protocol Rules that ensure communications among computers

of different types and from different manufacturers.

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Communications Software and Protocols

Open Systems Interconnection (OSI) Model Serves as a standard model for network architectures and

is endorsed by the International Standards Committee Communication functions are represented in seven layers

to promote the development of modular networks. Designed to permit communication among different computers from different operating systems

Seven layers

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Communications Software and Protocols TCP/IP

TCP/IP = Transmission Control Protocol/Internet Protocol Standard originally developed by the U.S. government to link defense research agencies; it is the primary communication protocol of the Internet

Systems Network Architecture (SNA) IBM communication protocol

Ethernet Protocol standard developed forLANs using a bus topology

X.400 and X.500 An international standard for message handling and

network directories

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Bridges, Routers,Gateways and Switches

Bridge Connects two or more networks, with the same protocol, at

the media control portions of the data link layer Router

Operates at the network level of the OSI model and features more sophisticated addressing software than bridges. Can determine preferred paths

Gateway Operates at or above the OSI transport layer and links

LANs or networks that employ different architectures and use dissimilar protocols

Switch Routes or switches data to its destination Schematic

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Application

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Machine#1

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Gatewaybox

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Media

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Telecommunications Applications

Linking personal computers to mainframe computers Download and upload information

Voice mail Enables users to leave, receive, and store verbal

messages for and from other users Electronic mail (e-mail)

Enables a sender to connect a computer to a network, type messages, and send it to another person on the network

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Telecommunications Applications Electronic software distribution

Involves installing software on a file server for users to share by signing onto the network and requesting that the software be downloaded onto their computers over a network

Electronic document distribution Transporting documents -- such as sales reports, policy

manuals, and advertising brochures -- over communications lines and networks

Telecommuting Enables employees to work away from the office using

personal computers and networks to communicate via electronic mail with other workers and to pick up and deliver results

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Telecommunications Applications

Videoconferencing Allows participants to conduct long-distance

meetings “face to face” while eliminating

Pretty picture

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Telecommunications Applications

Electronic Data Interchange (EDI) Uses network systems and follows standards and

procedures that allow output from one system to be processed directly as input to other systems, without human intervention

Pretty picture

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EDI link

EDI link EDI link

Vendor Customer

Vendor Customer

Third-partyclearing house

(a)

(b)

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Telecommunications Applications (5)

Public network services Services that give personal computer users access to vast

databases and other services, usually for an initial fee plus usage fees

Specialized and regional information services Specialized electronic bulletin boards and e-mail services

targeting particular interests.

Distance learning Use of telecommunications to extend the classroom

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Types of Digital Signals

Binary : only two bits either 1 or 0 M-ary : M signals such that M=2^n

- n is the Digits in Groups.

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DIGITAL SIGNALSDIGITAL SIGNALS

In addition to being represented by an analog signal, In addition to being represented by an analog signal, information can also be represented by a information can also be represented by a digital signaldigital signal. . For example, a 1 can be encoded as a positive voltage For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit than two levels. In this case, we can send more than 1 bit for each level.for each level.

Bit Rate Bit Length Digital Signal as a Composite Analog Signal Application Layer

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Two digital signals: one with two signal levels and the other with four signal levels

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A digital signal has eight levels. How many bits are needed per level? We calculate the number of bits from the formula

Example

Each signal level is represented by 3 bits.

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A digital signal has nine levels. How many bits are needed per level? We calculate the number of bits by using the formula. Each signal level is represented by 3.17 bits. However, this answer is not realistic. The number of bits sent per level needs to be an integer as well as a power of 2. For this example, 4 bits can represent one level.

Example

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Assume we need to download text documents at the rate of 100 pages per sec. What is the required bit rate of the channel?SolutionA page is an average of 24 lines with 80 characters in each line. If we assume that one character requires 8 bits (ascii), the bit rate is

Example

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A digitized voice channel, as we will see in Chapter 4, is made by digitizing a 4-kHz bandwidth analog voice signal. We need to sample the signal at twice the highest frequency (two samples per hertz). We assume that each sample requires 8 bits. What is the required bit rate?

SolutionThe bit rate can be calculated as

Example

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What is the bit rate for high-definition TV (HDTV)?

SolutionHDTV uses digital signals to broadcast high quality video signals. The HDTV screen is normally a ratio of 16 : 9. There are 1920 by 1080 pixels per screen, and the screen is renewed 30 times per second. Twenty-four bits represents one color pixel.

The TV stations reduce this rate to 20 to 40 Mbps through compression.

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The time and frequency domains of periodic and nonperiodic digital signals

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Baseband transmission

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A digital signal is a composite analog signal with an infinite bandwidth.

Note

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Bandwidths of two low-pass channels

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Baseband transmission using a dedicated medium

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Baseband transmission of a digital signal that preserves the shape of the

digital signal is possible only if we have a low-pass channel with an infinite or

very wide bandwidth.

Note

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An example of a dedicated channel where the entire bandwidth of the medium is used as one single channel is a LAN. Almost every wired LAN today uses a dedicated channel for two stations communicating with each other. In a bus topology LAN with multipoint connections, only two stations can communicate with each other at each moment in time (timesharing); the other stations need to refrain from sending data. In a star topology LAN, the entire channel between each station and the hub is used for communication between these two entities.

Example

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Rough approximation of a digital signal using the first harmonic for worst case

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Simulating a digital signal with first three harmonics

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In baseband transmission, the required bandwidth is proportional to

the bit rate;

if we need to send bits faster, we need more bandwidth.

Note

In baseband transmission, the required bandwidth is proportional to the bit rate;if we need to send bits faster, we need

more bandwidth.

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Bandwidth requirements

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What is the required bandwidth of a low-pass channel if we need to send 1 Mbps by using baseband transmission?

SolutionThe answer depends on the accuracy desired.a. The minimum bandwidth, is B = bit rate /2, or 500 kHz. b. A better solution is to use the first and the third harmonics with B = 3 × 500 kHz = 1.5 MHz.

c. Still a better solution is to use the first, third, and fifth harmonics with B = 5 × 500 kHz = 2.5 MHz.

Example

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We have a low-pass channel with bandwidth 100 kHz. What is the maximum bit rate of thischannel?

SolutionThe maximum bit rate can be achieved if we use the first harmonic. The bit rate is 2 times the available bandwidth, or 200 kbps.

Example

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Bandwidth of a bandpass channel

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If the available channel is a bandpass channel, we cannot send the digital

signal directly to the channel; we need to convert the digital signal to an analog signal before transmission.

Note

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Modulation of a digital signal for transmission on a bandpass channel

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An example of broadband transmission using modulation is the sending of computer data through a telephone subscriber line, the line connecting a resident to the central telephone office. These lines are designed to carry voice with a limited bandwidth. The channel is considered a bandpass channel. We convert the digital signal from the computer to an analog signal, and send the analog signal. We can install two converters to change the digital signal to analog and vice versa at the receiving end. The converter, in this case, is called a modem.

Example

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A second example is the digital cellular telephone. For better reception, digital cellular phones convert the analog voice signal to a digital signal (see Chapter 16). Although the bandwidth allocated to a company providing digital cellular phone service is very wide, we still cannot send the digital signal without conversion. The reason is that we only have a bandpass channel available between caller and callee. We need to convert the digitized voice to a composite analog signal before sending.

Example

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Why Digital Communication

More Rugged system (noise, distortion resistant)- More Relaiable

Viability to Regenerative Repeaters Hardware Flexibility Easy to MUX

In Analog systems , it is done on the basis of BW.

SNR Bandwidth relationship PCM

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Why Digital Communication

Storage Easy to Search Remote Access Variation of cost with technology

Communication cost has been reduced, as technology has advanced.

Digital devices have become more cheaper

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Background of Digital communication

Started in 1948 IT theory of Claud Shannon also known as

the Mathemetical Theory of Communication. It addresses

“ Rate of Reliable information transfer over noisy

Channels”.- I = log 1/P ---- P is Probability -

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Draw Backs of Digital Communication

Increased Bandwidth Transmission BW = nB

Needs Synchronization Analog doesnot need synchronization

A/D & D/A Conversion

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Goals

Increase of Transmission rate Minimizing system Utilization Minimize BER or Probability of bit error Bandwidth Minimization Reduce system complexity Optimization of Eb/No

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Transmission Impairments

Noise Thermal (No = KTB) Induce Noise Cross talk Impulse Noise ( Lightning)

Attenuation Delay Distortion

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Noise There are different types of noise

Thermal - random noise of electrons in the wire creates an extra signal

Induced - from motors and appliances, devices act are transmitter antenna and medium as receiving antenna.

Crosstalk - same as above but between two wires.

Impulse - Spikes that result from power lines, lighning, etc.

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Noise

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Signal to Noise Ratio (SNR)

To measure the quality of a system the SNR is often used. It indicates the strength of the signal wrt the noise power in the system.

It is the ratio between two powers. It is usually given in dB and referred to as

SNRdB.

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The power of a signal is 10 mW and the power of the noise is 1 μW; what are the values of SNR and SNRdB ?

SolutionThe values of SNR and SNRdB can be calculated as follows:

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The values of SNR and SNRdB for a noiseless channel are

Example

We can never achieve this ratio in real life; it is an ideal.

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Two cases of SNR: a high SNR and a low SNR

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Distortion Means that the signal changes its form or shape Distortion occurs in composite signals Each frequency component has its own

propagation speed traveling through a medium. The different components therefore arrive with

different delays at the receiver. That means that the signals have different

phases at the receiver than they did at the source.

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Distortion

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Attenuation

Means loss of energy -> weaker signal When a signal travels through a medium it

loses energy overcoming the resistance of the medium

Amplifiers are used to compensate for this loss of energy by amplifying the signal.

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Measurement of Attenuation

To show the loss or gain of energy the unit “decibel” is used.

dB = 10log10P2/P1

P1 - input signal

P2 - output signal

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Attenuation

Page 111: Lecture 1 Basics of Communication Systems

Suppose a signal travels through a transmission medium and its power is reduced to one-half. This means that P2 is (1/2)P1. In this case, the attenuation (loss of power) can be calculated as

Example

A loss of 3 dB (–3 dB) is equivalent to losing one-half the power.

Page 112: Lecture 1 Basics of Communication Systems

A signal travels through an amplifier, and its power is increased 10 times. This means that P2 = 10P1 . In this case, the amplification (gain of power) can be calculated as

Example

Page 113: Lecture 1 Basics of Communication Systems

One reason that engineers use the decibel to measure the changes in the strength of a signal is that decibel numbers can be added (or subtracted) when we are measuring several points (cascading) instead of just two. In Figure a signal travels from point 1 to point 4. In this case, the decibel value can be calculated as

Example

Page 114: Lecture 1 Basics of Communication Systems

Decibels for Example

Page 115: Lecture 1 Basics of Communication Systems

Sometimes the decibel is used to measure signal power in milliwatts. In this case, it is referred to as dBm and is calculated as dBm = 10 log10 Pm , where Pm is the power in milliwatts. Calculate the power of a signal with dBm = −30.

SolutionWe can calculate the power in the signal as

Example

Page 116: Lecture 1 Basics of Communication Systems

The loss in a cable is usually defined in decibels per kilometer (dB/km). If the signal at the beginning of a cable with −0.3 dB/km has a power of 2 mW, what is the power of the signal at 5 km?SolutionThe loss in the cable in decibels is 5 × (−0.3) = −1.5 dB. We can calculate the power as

Example

Page 117: Lecture 1 Basics of Communication Systems

The loss in a cable is usually defined in decibels per kilometer (dB/km). If the signal at the beginning of a cable with −0.3 dB/km has a power of 2 mW, what is the power of the signal at 5 km?SolutionThe loss in the cable in decibels is 5 × (−0.3) = −1.5 dB. We can calculate the power as

Example

Page 118: Lecture 1 Basics of Communication Systems

RSA-1

Write down a report on the history of telecommunication, listing the important milestones in the development of telecommunication technology.