Handout 3 Telecommunication 1

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CS-105 Fundamentals of Computer Engineering FE (EE) Session-2010

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Chapter Four

Telecommunication

Data Communications:

Essentials of data communications Modem: Digital Analog Medium: Communication Channels

o Guided Media: Twisted Pair, Coaxial, Fiber Optics cableso Electromagnetic Spectrumo Unguided Media: Broadcast radio, Microwave radio, Communication

satellites, Infrared, Bluetooth, WiFi, GPS

Factors affecting data Transmissiono Line configurationo Serial & Parallel Transmissiono Direction of Transmissiono Modes of Transmissiono Circuit switching & Packet switchingo Multiplexingo Protocols


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Essentials of Data Communication system:

To be called a complete data communication system, it must have the following components:

MessageSenderReceiverMediumUnderstandability: Format of the dataError detectionSecurity

Modem:

Modem performs Modulation and De-modulation.

Modulation: Transfer from digital to analog signals.De-Modulation: Transfer from Analog to Digital signals.Modem is short for modulate/demodulate; a sending modem modulates digital signals into analog signals for transmission

over phone lines. A receiving modem demodulates the analog signals back to digital signals.

Modem provides means for computers to communicate with one another using the standard copper wire telephonenetwork.

Data Communication System

Message 00110010101..


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Communication Channels

Communication channel is the path, the physical medium over which information travels in a telecommunication systemfrom source to destination.

Also called linkers, cables and wires.Two basic types.Wired Media (Guided)Wireless Media (Unguided)

Guided Media

Transmits data over physical mediumWires/cables are usedThree basic types

Twisted-Pair cablesShielded twisted-pair cables.Unshielded twisted-pair cables.

Co-axial cablesThinnetThicknet

Fiber Optic cablesTwisted Pair Cables:

Consists of two strands of copper wire twisted around each otherTwisted pair configuration somewhat reduces signal interference and crosstalk from neighboring wires.The greater the number of twists, the more crosstalk is reducedCommunication speed from 1 to 128 MbpsUsed mostly in telephone lines and computer networkingRelatively slower than other media typesTwo major types:

Shielded twisted-pair cables (STPs)

Often shielded to prevent external EMI (electromagnetic interferences).Shielding also serves as a ground.Shielding can be applied to individual pairs, or to the collection of pairs.When shielding is applied to the collection of pairs, this is referred to as screening.


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Unshielded twisted-pair cables (UTPs):

UTP cables are not shielded.This lack of shielding results in a high degree of flexibility (ability to bend) but increased EMI.UTP cables are found in many Ethernet networks and telephone systems.

Coaxial Cable

Commonly called Coax, consists of insulated copper wire wrapped in external cover.Widely used for cable TV.Much better than twisted-pair at resisting interference.Can carry voice and data at a faster rate (up to 200 Mbps).


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Two types:

Thinnet:Thicknet

Following chart shows comparison between these two types of coaxial cables:

Properties Thinnet Thicknet

Diameter 0.25 inches 0.5 inches

Distance to which

signals can be carried

185 meters 500 meters

Impedance, resistance to the alternating current 50 75

Characteristics More flexible, easy to install

And work with

Rigid, harder to install, more expensive

Fiber Optic

An optical fiber or fiber is a thin, transparent fiber, usually made of glass or plastic, for transmitting light.An optical fiber transmits light along its axis, by the process of total internal reflection.Glass optical fibers are almost always made from silica.Optical fibers are commonly used in telecommunication systems, as well as in illumination, sensors, and imaging optics.Some of its advantages and benefits are as follows

No sparks, important in flammable or explosive gas environments.Strands are thin like human hair, can transmit up to 2 billion pulses per second (2 Gbps)

Light pulses are not affected by electromagnetic interferenceLower error rateMore durableCant easily be wire tapped, so transmission is secure.

Electromagnetic spectrum:

Spectrum: - Division of frequencies into different energy bands.

Electromagnetic spectrum: - Full range of frequencies from radio waves to gamma rays.

Radio wave: - Electromagnetic radiation which has the lowest frequency and thus the longest wavelength.

Frequency: - Cycles per second.

Wavelength: - Distance between two adjacent troughs or crests is called wavelength.


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Electromagnetic Spectrum: Definition

The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength.

The sun, earth, and other bodies radiate electromagnetic energy of varying wavelengths. Electromagnetic energy passes through

space at the speed of light in the form of sinusoidal waves. Below is given the diagram for electromagnetic spectrum.

Unguided Media (Wireless Communication channels)

Transmission over wireless mediaData can flow in any of the directions, that is why they are named unguided media


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Broadcast Radio

Broadcast radio signals are quickly propagated in all possible directions and they are commonly not affected by obstaclesor any other medium between transceivers (Transmitter and Receiver).

Broadcast radio is a wireless transmission medium that sends data over long distances.Used in radio signal transmissions.The lower frequencies of radio spectrum are used and reserved not only for AM/FM radio but also for broadcast cellular

phones and land mobile services etc.

Communication speed is up to 2 Mbps.Microwave Radio

Communication speed up to 45 Mbps.Transmits voice and data through the atmosphere as super-high frequency radio called micro waves.Vibrate at 1 GHz/sec (frequency).Also used to transmit message between ground based stations and satellite communication systems.Microwave communication is also in line of sight like infrared.

Microwaves are easily absorbed in water. That is why they are also used in microwave ovens for heating up the food.

Microwaves are also used in radar communications.Communication Satellites

Specially used to avoid some of the limitations of microwave earth stations.May be called sky stationsSatellites are microwave relay stations in orbit around the earth.

Up-linking: sending data from ground to satellite.

Down-linking: Sending data from satellite to ground station.

May occupy one of the three orbits

GEO (Geo Stationary Earth Orbit)Geo stationary because the GEO Satellite rotates with the speed of the earth, and to the observers it seems static22,300 miles (35,863 km) up and above the equatorThe orbiting satellite has solar powered transceivers to receive signals, amplify them and retransmit them to another

earth station

Fewer (normally three) satellites are used for global coverageQuarter second delay makes two way conversation difficult


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MEO (Medium Earth Orbit)5,000-10,000 miles up (8,000 km to 18,000 km)More satellites needed for global coverage as compared to GEO satellites.Longer time delay and weaker signal than a LEO satellite.

LEO (Low Earth Orbit)400-1000 miles up (500 to 1,500 km).No signal delaySmaller and cheaper to launch.

Infrared Transmission

1 to 4 MbpsLine of sight communicationLimited range of transmission

Blue Tooth

Short-range wireless digital standard aimed at linking cell phones, PDAs, computers and peripherals up to distance of 30feet.

Principally designed to replace cable connecting PCs to printersSeveral devices use blue tooth technology like printers, mice and keyboards

Wi-Fi (Wireless Fidelity)

Short-range wireless digital standard aimed at helping machine inside offices to communicate at high speedsShare internet connections up to 300 feetSupported by Windows XPPrincipally used for internet connections

GPS (Global Positioning System)

Developed by the U.S military over the past three decades to aid ship and plane navigation.GPS consists of 24 to 27 solar-powered earth-orbiting satellites continuously transmitting timed radio signals that can be

used to identify earth locations.


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Location of a point on the earth is calculated by calculating its latitude, longitude and altitude with reference to equator.The satellites complete their earth orbit twice in 24 hours.At least four of these 27 satellites are always visible from any location on the earth.The GPS receivers accept the signals from at least these four satellites to calculate their latitude, altitude and longitude.GPS comprises three sub-systems:Satellite sub-system----Space segment

The satellite sub-system is composed of 24 to 27 satellites that orbit the earth in every 12 hours at a height of 500km. They

continuously transmit timed radio signals at the speed of light which are used by GPS receivers in locating their position on

the earth.

Control sub-system----Control segment

The control sub-system is made up of several stations on earth around the world that monitor the satellites and send them

information about their orbital position. The primary control station is situated in the United States, in the state of Colorado.

User sub-system--------User segment

User segment consists of radio receptor with processing unit that receives information sent by at least four satellites in real

time and calculates the position. To determine position accurately, we need at least 12 and half minutes for reception of the

different types of signals. The receivers clock isnt nearly as accurate as the atomic clock in the satellites, but mathematical

adjustments are made to account for inaccuracies.

Factors affecting data transmission

There are several factors which affects how data is transmitted. They include the following.

1. Line configuration2. Serial And Parallel Transmission3. Direction Of Transmission4. Transmission Mode5. Circuit Switching And Packet Switching6. Multiplexing7. Protocols


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Line configurationThere are two ways of connecting communication lines which are as follows.

Point to point

A point to point line directly connects the sending and receiving devices such as terminal with a central computer.More secure transmission because of direct connection between source and destination.

Multipoint

A multipoint line is a straight line that interconnects several communication devices to one computer.Less secure as compared to point-to-point line configuration.

Serial and Parallel Transmission

Serial Transmission

Send bits one at a timeSlow transmission of dataSuitable for transmission over long distanceSlow devices are connected

Parallel Transmission

Transmitting fast data over short distanceEfficient transmission over 15 feet


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Direction of Transmission: -

Simplex Mode

Data travels always in one directionExample include data transmission from TV station to home TV or from remoteControl to television.

Half Duplex

Data travels in only one direction at a time. Examples may include one lane road traffic system, walkie talkie etc. The word over is used to indicate the end of transmission.

Full Duplex

Back and forth transmission of data at the same time. Examples may include two lane road with one lane for each direction Most telephone networks, cellular phones etc. Mostly modems present today are full duplex.

Transmission Modes: -

Two modes: Asynchronous and synchronous Transmission.

Asynchronous Transmission

Also called start stop transmission.Used with most microcomputersData is sent one byte (character) at a time.Because of one character transmission at a time, it is relatively slow.


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Advantage: Data can be transmitted whenever it is needed.

Synchronous Transmission

Synchronous transmission sends data in blocks (multiple bytes at a time) or frames.Start and stop bit patterns called synchronous bytes are transmitted at the beginning and end of the blocks.Appropriate for huge amount of data transmission.

Circuit Switching and Packet Switching: -

Circuit Switching

Used by telephone company for its voice network.For each connection, physical switches are set in the telephone network to create a physical circuitSwitches are set up at the beginning of the connection and maintained throughout the connectionNot a very efficient strategy because a connection holds a physical line even during silence periods (when there is

nothing to transmit)


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Packet Switching

Packet is a fixed length block of data divided into two parts: Payload: The actual data part. Header: That contains information like source and destination address, packet size, sequence number, error checking

information etc.

Electronic messages are divided into packets of typically 1000 octets (Bytes)

Several packets thus, may be transmitted through several routes and may be sandwiched in between packets from othermessages.

Packets are reassembled at destination.Appropriate for huge and long distant data.

Multiplexing

Multiplexing is the transmission of multiple signals over a single communication channel.Three devices used to achieve multiplexing

MultiplexersConcentratorsFront and Processors

Multiplexers

A device that merges several low speed signals into one high speed transmission channel.Depending on the type of multiplexer, 32 or more devices can communicate using single communications line


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Sending and receiving multiplexers must be of the same type.Receiving multiplexers called multiplexers then rearrange the signals and transmit to proper destinations.

Concentrators

Concentrate means to direct towards a common centre.A device that combines several communication signals into one.May temporarily store data while as multiplexes does not.

Front end Processors

A computer used to process data before it is sent to a mainframe computer for analysis or further processing.Devices that interface number of peripheral devices to the host computer

Protocols

These are the rules of data transmission.A protocol or communication protocol is a set of conventions governing the exchange of data between hardware and

software components in a communication network.

Protocols will also specify the type of electrical connections used, the timing of message exchange, error detection anderror correction techniques and so on.

TCP/IP, POP, SMTP, FTP, HTTP etc are the examples of protocols

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Handout 3 Telecommunication 1