Fundamentals of Data Communications Source Destination Telephone Lines Modem.

Fundamentals of Data Communications

Source DestinationTelephone Lines

Modem


•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

Voltage

Time

Crest

Trough

1/2 Cycle

1 Cycle1 Oscillation

Wavelength

Amplitude

Analog Signal

Higher Frequency

Shorter Wavelength

Lower Frequency

Longer Wavelength


•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.


•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 SpectrumClick here


•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

Voltage

Time

Signal Element

1

0

1

0

1

Data Element


•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

Voltage

Time

1

0

1

0

Voltage

Time

1

0

1

0

1

Voltage

Time

11

00

11

00

11

Signal elements Per Second

Data elements Per Second





Bit Rate

Baud Rate

[Could be compared with Train Bogey and Passengers.


•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.


•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).


Source DestinationTelephone Lines

Modem

Source Destination

Digital Network

No need of conversion from Analog to Digital and vice-versa.

Need to convert Digital to Analog and vice-versa.

Modem


•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

External Modem

Internal 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 ConnectionsTo 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

External Modem

Internal Modem

Modem Line Connections

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 connectionat the same time.

Dial-Up Connection

DSL Broadband Connection

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.


•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 bitsMaximum number of

characters that can berepresented

Standard ASCII

Extended ASCII

EBCDIC

Unicode

7

8

8

16

27 = 128

28 = 256

28 = 256

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 Decimal Value Character

0000000 0 NUL

0110000 48 0

0110001 49 1

1000001 65 A

1000010 66 B

1000011 67 C

1100001 97 a

1100010 98 b

1111111 127 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.


Telephone Lines

TELEPHONE SYSTEM

Source (Caller) Destination (Callee)

Common Language

Transmitter ReceiverMedium

Rotary Dial Touch Tone Dial

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.


•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


•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.


•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.


•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 SystemTHE TELEPHONE NETWORK

Local Loop

Local Loop

End Office

Central Office (CO) Exchange Office (EO)

Routing / Switching takes place here

Source Destination

Dials a number

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.

The Telephone SystemTELEPHONE CALLS

Customer

End Office

End Office

End Office

Tandem Office

Tandem Office

Toll Office

Toll Office

Long-DistanceSwitching CenterLocal Loop Trunks

Toll Trunks

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


•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


•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 SystemTELEPHONE LINES / CHANNEL

0 Hz

300 Hz

3,300 Hz

4,000 Hz

Transmitting Data Bandwidth: 3,000 Hz

Guardband

Guardband

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.

3,300 – 300 =

(300 - 0) = 300 Hz

(4,000 – 3,300) = 700 Hz


•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


•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.


•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.


•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.


•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.


•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.


•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.


•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.


•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


•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.


•Government and Regulatory Agencies:–FCC:

•Federal Communications Commission

–PUC:•Public Utility Commissions

–NTIA:•National Telecommunications and Information Administration

–TRAI:•Telecom Regulatory Authority of India


•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


•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:–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.

Fundamentals of Data Communications Source Destination Telephone Lines Modem.

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