Fundamentals of Data
Communications
Source
Modem
Destination
Telephone Lines
Fundamentals of Data
Communications
•Fundamentals of Data Communications:
–When data is sent over a network,
•It is transmitted by means of a,
–Signal / Wave
–Characteristics and properties of a signal
•Would be different in different forms of
communication.
•Example:
–The way a person’s voice is sent over a telephone
line is different from,
–The way a computer transmits data over a
communications line.
Analog Signal
Crest
Wavelength
1 Cycle
1 Oscillation
Voltage
1/2 Cycle
Amplitude
Time
Trough
Analog Signal
Shorter Wavelength
Higher Frequency
Longer Wavelength
Lower Frequency
Fundamentals of Data
Communications
•Analog Signal:
–Crest:
•Any part of the wave that is pointing up like
mountains.
–Trough:
•Any part of the wave that is sloping down like a
valley.
–Amplitude:
•Measure of how big the wave is.
–The height from the equilibrium point to the highest
point of a crest.
–The depth from the equilibrium point to the lowest
point of a trough.
Fundamentals of Data
Communications
•Analog Signal:
–Frequency:
•Measurement of how many cycles can happen in a
certain amount of time.
–Cycles per second.
•Measured in unit called:
–Hertz (Hz).
•Determines how much energy a wave carries.
–Wavelength:
•Distance from a particular height on the wave to the next
spot on the wave where,
–It is at the same height and going in the same direction.
•Meters per cycle.
•Usually measured in Meters.
•Determines how much distance a wave can cover/travel.
Measurement System
Electromagnetic Spectrum
Click here
Fundamentals of Data
Communications
•Digital Signal:
–A signal in which measurement is
represented in the form of:
•Digits
–Digits that are used in computers are 2 (bi):
•1 or 0
•So they are called:
–Binary Digit / Bit
Digital Signal
Signal Element
Data Element
Voltage
1
1
1
0
0
Time
Fundamentals of Data
Communications
•Digital Signal:
–Bit:
•Short form for ‘Binary Digit’.
•Smallest unit of information used in computers.
•Represented either as 1 or a 0 in the computer.
–1:On State / Presence of electric current.
–0: Off State / Absence of electric current.
•To create characters (such as A, 2, *) used by
the computer,
–Combination of such Bits (0 and 1) are used.
•Combination of ‘8’ bits is called,
–A Byte.
Digital Signal
1
1
Signal elements Per Second
Voltage
0
0
Time
1
Baud Rate
Data elements Per Second
Bit Rate
1
1
Signal elements Per Second
Voltage
0
0
Data elements Per Second
Time
11
11
11
Signal elements Per Second
Voltage
00
00
Time
Data elements Per Second
[Could be compared with Train Bogey and Passengers.
Fundamentals of Data
Communications
•Digital Signal:
–Bit Rate:
•Refers to the,
–Speed at which binary digits are transmitted over a network.
•Usually expressed in:
–Bits per second (bps).
•Example:
–If the communications network can transmit data at a bit rate of 103
bits per second,
»1,000 bits can be transferred from the sending computer to
the receiving computer each second.
•Today’s communications networks transmit data in,
–Millions of bits per second (Mbps) or
–Billions of bits per second (Gbps) range.
–Baud Rate:
•Refers to the,
–Number of events, or signals transmitted in one second.
Fundamentals of Data
Communications
•Common (Analog, Digital) Term:
–From source to destination,
•The number or cycles/oscillations (Analog) OR
•The number of bits (Digital) that can pass in 1 second depends
on,
–One basic component of communication called:
»Medium.
–Bandwidth:
•Capacity of the ‘medium’ to carry information OR
•The speed at which data can be transmitted over a specific
channel.
•Wider the bandwidth,
–Faster is the communication.
•Measurement of Bandwidth might be different in Analog and
Digital communication.
–Analog Communication:
»Measured in Hertz (Hz)
–Digital Communication:
»Measured in Bits per second (bps).
Fundamentals of Data
Communications
Digital Network
Source
Destination
No need of conversion from Analog to Digital and vice-versa.
Source
Modem
Modem
Telephone Lines
Need to convert Digital to Analog and vice-versa.
Destination
Fundamentals of Data
Communications
•Modems:
–If a Digital network is used to send Digital data (Only between
2 computers),
•Conversion to an Analog signal is not needed.
–It Digital data is sent over the telephone network (Analog),
•Some type of conversion must take place at both ends.
–Device that is used to,
•Convert a,
–Digital signal into an Analog signal (at the Sender) and conversely,
–From an Analog signal to a Digital signal (at the Receiver) is called:
»Modem (MOdulator / DEModulator)
–Depending on where a modem is placed relative to a CPU
box, it can be either:
•External
•Internal
Modems
Internal Modem
External Modem
Modems
•External Modem:
–Sits next to the computer outside the CPU box and,
•Is connected to it by a cable on the serial port.
–Modem is powered by:
•External power source (supply) with its own power cord
(cable/wire).
–Have lights on the front that indicate,
•When the modem is on,
•When data is being transmitted etc.
–Advantage:
•It is portable.
–If you want to change to a different type of modem,
»You simply can unplug and replace the modem without
disassembling the entire computer.
–Disadvantage:
•It occupies physical space on the desk.
•Takes up a serial port on the computer and requires a power
plug.
Modems
•Internal Modem:
–Sits inside the CPU box.
–It is a PCB,
•Printed Circuit Board and is inserted directly into the
motherboard.
–Commonly called a,
•Card.
–Cables are not needed to connect the card to the computer
because,
•The modem is installed directly into a slot on the motherboard
inside the computer.
–Modem is powered by,
•The power supply connected to the computer.
–Advantage:
•Does not take up desk space.
–Disadvantage:
•Computer must be opened to insert/remove the modem.
•Will use up an expansion slot inside the computer.
Modem Line Connections
To use Internet connection
To use Telephone
Line cord is either connected with
The Modem or with the Telephone depending on the use.
Disadvantage:
Every time the application needs to be changed,
the cable connection needs to be changed.
Modems
Internal Modem
External Modem
Modem Line Connections
Dial-Up Connection
DSL Broadband
Connection
Advantage:
There was no need to change the cord
if you wanted to use Telephone or PC
Disadvantage:
Both Telephone and PC cannot use the connection
at the same time.
Modem Line Connections
Modems
•Line Connections:
–Line cord is connected,
•Either with the modem or
•With the telephone depending on,
–What needs to be used.
•Disadvantage was:
–Every time the application needs to be changed, the cable
connection needs to be changed.
–To overcome the disadvantage,
•Modem started coming with 1 extra jack to connect
Telephone also.
•Advantage was:
–There was no need to change the cord if you wanted to use
telephone or PC.
•Disadvantage was:
–Both Telephone and PC could not use the connection at
the same time.
Modems
•Data Transmission Speed:
–Typical modem speeds range from,
•300 bps to
•56,000 bps (56Kbps).
–Technique that allows a modem to transmit
more data in a given period is:
•Data Compression
–Maximum transmission rate is called:
•Throughput
Modems
•Usage of Modems:
–Modems always are used in:
•Pairs.
–2 modems are necessary.
•Modem at the sending end converts,
–A computer’s digital signal into an analog signal for
transmission over a telephone line.
•Modem at the receiving end then converts,
–The analog telephone signal back into a digital signal
to be entered into the computer at the receiving end.
Modems
•Types of Modems:
–Cable Modem:
•Used a type of cable that is used to bring cable television
into the home.
•Transmission rate is much faster than that for a 56K
modem,
–Reaching speeds of 40Mbps.
–Modem Eliminator / Null Modem:
•Can be used for direct short-distance communication
between 2 devices.
•Provides high speeds communication for distances of
lesser than 50 feet.
•Transmission speeds range from,
–1,200 bps to 38,400 bps.
Types of Modems
Null Modem
Short-haul Modem
Modems
•Types of Modems:
–Short-haul Modem:
•Permits communications between 2 devices
that are up to 20 miles apart.
•Advantage of this type of modem is that,
–You could connect computers that are distance apart
without using telephone lines.
•Commonly used to connect computers within
buildings.
–Fax Modem:
•Provided facsimile (fax) service also.
–Included a software needed to send a fax.
•It could also have a facility for Voice Mail.
Fundamentals of Data
Communications
•Data Codes:
–Computers store and represent all kind of data in the form of,
•Bits (Binary Digits),
•0 and 1
–To store, understand and represent various characters (a-z,
A-Z, 0-9, * etc.),
•A combination/sequence of several bits is required.
–This combination/sequence of several bits is called:
•Data Code
–Common Data Codes used in modern computers are:
•Standard ASCII
•Extended ASCII
•EBCDIC
•Unicode
–In order for 2 computers to communicate,
•Same data codes must be used by both of them.
Data Codes
Code
Number of bits
Maximum number of
characters that can be
represented
Standard ASCII
7
27 = 128
Extended ASCII
8
28 = 256
EBCDIC
8
28 = 256
Unicode
16
216 = 65,536
Data Codes
•ASCII:
–Stands for,
•American Standard Code for Information Interchange.
–Widely used data transmission codes because,
•Most personal computers store data in ASCII format.
–A string of 7 bits makes up an ASCII code,
•Providing a possibility of creating,
–128 (27) unique combinations / characters.
–128 unique combinations vary from:
•0000000
•0000001
•0000010
•0000011
•0000100
•...
•...
•...
•1111111
ASCII Table
Binary Data Code
0000000
0110000
0110001
1000001
1000010
1000011
1100001
1100010
1111111
Decimal Value
0
48
49
65
66
67
97
98
127
Character
NUL
0
1
A
B
C
a
b
DEL
ASCII Table
Data Codes
•Extended ASCII Code:
–Many PCs make use of an,
•8-bit ASCII code which is called,
–Extended ASCII code.
–It can create/represent,
•256 (28) unique characters.
–It added 128 special symbols to the
existing ASCII table.
Extended ASCII Table
Data Codes
•EBCDIC:
–Stands for,
•Extended Binary Coded Decimal Interchange
Code.
–8 bit code used by IBM.
–It can represent a total of,
•256 (28) different characters.
–Many combination of bits have no values
assigned to them.
•This is called:
–Gap in the Table.
EBCDIC
Data Codes
•Unicode:
–Modern data code that was developed in
1993.
–It is a 16-bit code that supports,
•65,536 (216) characters.
–First 128 codes in Unicode are,
•Identical to the ASCII table.
–Apart from that, it also includes alphabets
for languages such as:
•Chinese, Japanese, Korean, Hebrew, Greek,
Russian, Sanskrit etc.
Fundamentals of Data
Communications
Rotary Dial
TELEPHONE SYSTEM
Touch Tone Dial
Common Language
Source (Caller)
Transmitter
Telephone Lines
Medium
Destination (Callee)
Receiver
Data Communication Networks
•The Telephone System:
–Main purpose of the telephone is,
•To take the human voice and,
•Convert it into a signal,
•That can be transmitted over a telephone line or
other communications channel.
–At the receiving end,
•Another telephone converts the signal back into
a voice.
–2 (Two) telephones are needed for this
network to operate properly.
The Telephone System
•Basic Components:
–Sender:
•Person who makes the call or ‘Caller’.
–Receiver:
•Person receiving the call or ‘Callee’.
–Communications Medium:
•Telephone wires / lines.
–Protocol:
•Language of conversation.
The Telephone System
•Working:
–Telephone contains devices called ‘Converters’
that change,
•Outgoing Human speech into an Analog electrical signal
and,
•The incoming Analog signal into Human speech.
–Each telephone has 2 converters:
•One is in the portion of the telephone headset used for
speaking.
–The Transmitter
•Other is in the portion of the telephone handset used for
hearing.
–The Receiver
The Telephone System
•Transmitter:
–Converts human voice into an electrical signal.
–Transmitter contains a membrane called:
•Diaphragm
–Sound waves of a person’s voice cause vibrations,
•Which in turn cause ‘Diaphragm’ to vibrate.
–Diaphragm is connected to a chamber filled with
carbon.
–Each vibration of the diaphragm causes,
•The carbon granules to compress, which causes more
electrical current to flow through the circuit.
The Telephone System
•Receiver:
–Takes the electrical signal and converts it back into
a human voice.
–Receiver contains,
•A Diaphragm,
•A magnet on the outer ring of the diaphragm and,
•An opposing magnet.
–Electrical signals from the telephone line cause the
2 magnets to interact, which in turn,
•Causes the diaphragm to vibrate.
–Movement of the diaphragm causes a vibration in
the air,
•Which generates the human voice over the receiver.
The Telephone System
•Dials:
–Telephone also contains a mechanism that,
•Allows the user to access the telephone network.
–2 types of such dialing mechanisms exist:
•Rotary Dial:
–Each number on a rotary-dial telephone generates a
unique code, or series of electrical pulses/signals as it
rotates back to position.
•Touch Tone Dial:
–Generates tone signals rather than electrical pulses.
–Technique is also called:
»DTMF: Dual Tone Multifrequency Signaling.
The Telephone System
THE TELEPHONE NETWORK
Routing / Switching takes place here
Local Loop
Local Loop
Source
Dials a number
Destination
End Office
Central Office (CO)
Exchange Office (EO)
The Telephone System
•The Telephone Network:
–Switching:
•Routing of the telephone call onto the correct set of wires
–In order to connect it to the proper receiver.
–Local End Office / Central Office (CO) / Exchange
Office (EO):
•Place where call switching takes place.
–Local Loop:
•Wires / Cables that connect a home or business to the
local end office.
–Session:
•Time during which the call remains connected.
•When the conversation finishes and telephone handsets
are hung up,
–The circuit is disconnected and the session ends.
Customer
The Telephone System
Toll Office
TELEPHONE CALLS
End Office
Tandem Office
Trunks
Local Loop
End Office
Long-Distance
Switching Center
Toll Trunks
Tandem Office
End Office
Toll Office
The Telephone System
•Telephone Calls:
–Local Call / IntraOffice Call:
•A call placed within local end offices.
•Cable that connects a customer to an End Office is called:
–Local Loop
–InterOffice Call:
•If a customer makes a call to a telephone,
–Not connected to the local end office but,
–In the same area.
•Routed through different types of offices called:
–Tandem Office
»Does not have any line connected directly to customer.
»Able to handle a large volume of calls between exchanges.
•Cable that connects End Office to a Tandem office is called:
–Trunk
The Telephone System
•Telephone Calls:
–Long-Distance Call:
•Placing a call to a different area code.
•Routed through special offices called:
–Toll Office
•Cable that connects End Office to a Toll Office
is called:
–Toll Trunk
The Telephone System
•Telephone Calls:
–Complexity of the telephone system has
advantage and disadvantage.
–Advantage:
•Virtually impossible to disrupt (stop) all the
communications simultaneously due to:
–Redundancy
–Disadvantage:
•Complexity of the system makes it difficult to resolve
problems quickly.
•Example:
–Serious problems can be caused in areas by telephone
cables being cut,
»During construction, by lightning strikes on telephone
lines, or by fires in switching offices.
The Telephone System
TELEPHONE LINES / CHANNEL
4,000 Hz
Guardband (4,000 – 3,300) = 700 Hz
3,300 Hz
Transmitting Data Bandwidth: 3,300 – 300 = 3,000 Hz
300 Hz
Guardband (300 - 0) = 300 Hz
0 Hz
Sometimes, multiple signals could be sent over the same physical line/channel.
There is a possibility of overlapping of signals / interference of one signal with other.
Cover / Protection / Guard is needed on both sides of the actual signal transmission.
The Telephone System
•Telephone Lines / Channel:
–Telephone wire transmits in a frequency range,
•From 0 Hz to 4,000 Hz.
–In addition to being used for telephone calls,
•These telephone lines are also used for data
transmission.
–To protect the information from interfering with
signals of higher or lower frequencies,
•Special bands of frequencies are provided at the outer
edges of the bandwidth.
•These bands are called:
–Guardbands
The Telephone System
•Telephone Lines / Channel:
–Guardband:
•Range of frequencies that,
–Prevents the current containing the data from one
transmission,
–Interfering with another transmission current on the
same line.
•Needed to protect,
–The data from interference with other signals when,
–Many signals are combined.
The Telephone System
•Telephone Lines / Channel:
–For each signal carried on the line, the 4,000 Hz
signal is divided into 3 bands.
•A 300 (0 to 300) Hz guardband is placed at the bottom of
the signal.
•A 700 Hz (3,300 to 4,000) guardband is placed at the top
of the signal.
•A 3,000 Hz (300 to 3,300) band for actual transmission.
–Thus, the telephone line can transmit signals in,
•A bandwidth of 3,000 Hz or
•3,000 cycles per second.
The Telephone System
•Telephone Lines / Channel:
–Voice-Grade Circuits:
•Lines used for voice telephone calls.
•Voice-Grade circuits have,
–A bandwidth of 3,000 Hz and
–Can transmit in a range,
»From 300 to 3,300 Hz.
•The human ear, by comparison can hear sounds in a
range,
–From 20 to 20,000 Hz for,
–A bandwidth of 19,080 Hz (20,000 – 20).
–Conclusion:
•There might be some sounds which,
–Could be heard by the human ear directly but,
–Cannot be heard over the telephone line.
The Telephone System
•Telephone Signals:
–In addition to the signal used to transmit
the actual telephone call,
•The telephone network uses signals to,
•Indicate the various states of the network and
the status of calls on the network.
Telephone Signals
Signal
Meaning
Dial Tone
Indicates that dialing can begin.
Touch-Tone keypad tones
Indicate that DTMF signals are being sent to the
switching office.
Ringing
Verifies that the call has gone through and that
the called telephone is ringing.
Normal busy signal
(60 tones per minute)
Indicates that the called telephone is off the
hook.
Fast busy signal
(120 tones per minute)
Indicates that the long-distance trunks are busy.
Loud, pulsing noise
Denotes occurrences of problem while
attempting to complete the call.
Caller should hang up and try the call again.
The Telephone System
•Telephone Numbering System:
–Area Codes:
•Special number that needs to be dialed before dialing the actual
phone number to,
–Connect to a different area in the same country.
•Telephone network was designed with area codes to,
–Facilitate long-distance telephone calls.
–Country Codes:
•Special number that needs to be dialed before dialing anything
to,
–Connect to a different country.
•To make an international long-distance call,
–The caller must dial a country code with the rest of the number.
•Example:
–Country code for US is 1.
–Country code for Australia is 61,
–Country code for India is 91.
The Telephone System
•Telephone Numbering System:
–800 Service:
•Start with 800 area code.
•Telephone calls made to these numbers are,
–Toll-free to the caller.
–The called party pays for the call.
•Use:
–It is a tremendous marketing tool for businesses in today’s market.
•Also now use an 888, 877, 866 area code.
–900 Service:
•Telephone number uses a 900 area code.
•Unique feature is that,
–Around 7,000 calls can be handled simultaneously on a single
telephone number.
•Uses could be:
–Take opinions / feedback from users.
–Provide recorded or live messages.
The Telephone System
•History of Telephone System:
–When the Telephone was invented,
•One company dominated the market and so,
•Everyone had to use the services provided by that
company.
–That company which held the monopoly was:
•AT&T (American Telephone & Telegraph)
–As a result,
•All customers used the same type of telephones,
•The same type of telephone numbers and
•The same connections to the telephone network.
The Telephone System
•History of Telephone System:
–AT&T (American Telephone & Telegraph):
•Advantage of the monopoly/dominance:
–Standardization
•Disadvantage of monopoly:
–Unfair practices and lack of control / competition.
–So, after a few years, this domination
finally ended.
•Removal of monopoly resulted in,
–Increase of free competition providing consumers,
»More choices of telephone companies.
The Telephone System
•History of Telephone System:
–Common Carriers / Carriers / Telecom
Carriers:
•Profit-oriented companies,
–Regulated/Controlled by the Government,
–That provide telephone services.
•Some common carriers operating in US and
Canada are:
–AT&T
–Verizon
–Sprint etc.
The Telephone System
•History of Telephone System:
–LATA (Local Access Transport Area):
•Every carrier / telephone company has a service area for
telephone calls.
•These service areas are broken up into what are called:
–LATAs
»Local Access Transport Areas
•Call done in a same service area (within a LATA) is
called:
–intraLATA call
•Call done between customers in different service areas is
called:
–interLATA call
LATA Map
The Telephone System
•Regulatory Agencies & Standards
Organizations:
–Because of increasing carriers / telephone
companies, main problems could be:
•Unfair practices and pricing by the companies.
–Government & Regulatory Agencies are needed to
ensure,
»Public is protected from such unfair practices.
•Lack of common standards for devices or
operation.
–Standards Organizations are needed to ensure,
»There is a common standard of communication
which is followed by all the companies.
The Telephone System
•Government and Regulatory Agencies:
–FCC:
•Federal Communications Commission
–PUC:
•Public Utility Commissions
–NTIA:
•National Telecommunications and Information
Administration
–TRAI:
•Telecom Regulatory Authority of India
The Telephone System
•Standards Organization:
–ANSI:
•American National Standards Institute
–ISO:
•International Organization for Standardization
–COS:
•Corporation for Open Systems
–ITU:
•International Telecommunication Union
–IEEE:
•Institute of Electrical and Electronics Engineers
The Telephone System
•Standards Organization:
–EIA:
•Electronics Industries Alliance
–TIA:
•Telecommunications Industry Association
–PCIA:
•Personal Communications Industry Association
–CTIA:
•Cellular Telecommunications Industry Association
–COSMAT:
•Communications Satellite Corporation
–INTELSAT:
•International Telecommunications Satellite Organization
The Telephone System
•The Telephone System:
–Question:
•If Telephone is an electrical device,
–Why it keeps on working even when electrical power
is cut?
–Answer:
•Electricity is supplied by,
–The telephone company,
–Not the electric company,
»Which is why a telephone operates even when
electrical power is cut.
•When the telephone is installed,
–It includes a wire that carries electricity from the
telephone company to the telephone in your house.