lecture 1 basics of communication systems
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PPT files of list of lectures.TRANSCRIPT
Communications
Communications The message (data and information) is
communicated via the signal The transmission medium “carries” the signal
Sender ReceiverSignal
Transmissionmedium
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
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.
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
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
Digital Communications
Digital communications A specialized subset of telecommunications that
refers to the electronic collection, processing, and distribution of digital signal.
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
Communication Systems
A Communications Model
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
Digital Communications Model
Communication Technology Applications
voice mail Twitter
e-mailinstant
messaging chat rooms
newsgroups telephony videoconferencing
collaboration groupware global positioning system (GPS)
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.
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)
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.
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
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
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
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.
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.
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
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.
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.
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
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
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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
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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.
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
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
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.
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.
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.
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
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.
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.
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
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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.
Multiplexer
Multiplexer Allows several telecommunications signals to be
transmitted over a single communications medium at the same time
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Communicationslink
Multiplexor Front-endprocessor
Hostcomputer
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.
Front-End Processor
Front-end processor… Special purpose computers that manage
communication to and from a computer system
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Incoming and outgoing jobs
Front-endprocessor
Mainsystem
Job 1
Job 2
Job 3
Job 4
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
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)
ISDN ISDN = Integrated Services Digital Network Technology that uses existing common-carrier lines to
simultaneously transmit voice, video, and image data in digital form
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Carriers and Services
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
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
Communications Software and Protocols
Protocol Rules that ensure communications among computers
of different types and from different manufacturers.
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
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
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
Presentation
Session
Transport
Network
Data link
Physical
Machine#1
Machine#2
Machine#3
Gatewaybox
Machine#4
Bridgebox
Routerbox
Media
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
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
Telecommunications Applications
Videoconferencing Allows participants to conduct long-distance
meetings “face to face” while eliminating
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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
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EDI link
EDI link EDI link
Vendor Customer
Vendor Customer
Third-partyclearing house
(a)
(b)
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
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.
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
Two digital signals: one with two signal levels and the other with four signal levels
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.
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
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
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
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.
The time and frequency domains of periodic and nonperiodic digital signals
Baseband transmission
A digital signal is a composite analog signal with an infinite bandwidth.
Note
Bandwidths of two low-pass channels
Baseband transmission using a dedicated medium
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
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
Rough approximation of a digital signal using the first harmonic for worst case
Simulating a digital signal with first three harmonics
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.
Bandwidth requirements
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
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
Bandwidth of a bandpass channel
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
Modulation of a digital signal for transmission on a bandpass channel
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
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
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
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
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 -
Draw Backs of Digital Communication
Increased Bandwidth Transmission BW = nB
Needs Synchronization Analog doesnot need synchronization
A/D & D/A Conversion
Goals
Increase of Transmission rate Minimizing system Utilization Minimize BER or Probability of bit error Bandwidth Minimization Reduce system complexity Optimization of Eb/No
Transmission Impairments
Noise Thermal (No = KTB) Induce Noise Cross talk Impulse Noise ( Lightning)
Attenuation Delay Distortion
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.
Noise
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.
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:
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.
Two cases of SNR: a high SNR and a low SNR
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.
Distortion
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.
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
Attenuation
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.
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
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
Decibels for Example
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
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
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
RSA-1
Write down a report on the history of telecommunication, listing the important milestones in the development of telecommunication technology.