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Introduction to Information Technologies

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1 Introduction to Information Technologies
Fall 2004 Computer Networks Chapter 7 – Transmission Media History of Internet

2 Transmission Media Categories
The transmission medium is the physical path between the transmitter and receiver in a data transmission system The nature of both, the signal and the medium determines the quality of transmission The media can be divided into two categories: Guided media – physical medium exists Unguided media – the air is used as a medium Spring 2006 Computer Networks

3 Twisted-Pair Cable Insulated copper wires in spiral pattern.
Widely used for analog and digital transmission One of the wires transmits the signal, the other is used as ground reference The twist is introduced to reduce the interference Spring 2006 Computer Networks

4 Twisted Pair Cable - Applications
Most common medium for many applications Telephone network Between house and local exchange (subscriber loop) Within buildings To private branch exchange (PBX) For local area computer networks, 10Mbps or 100Mbps, or 1000Mbps Spring 2006 Computer Networks

5 Unshielded vs. Shielded Twisted Pair Cable
Unshielded Twisted Pair (UTP) Ordinary telephone wire Cheapest Easiest to install Suffers from external electrical and mechanical interference Shielded Twisted Pair (STP) Metal braid or sheathing that reduces interference More expensive Harder to handle (thick, heavy) Spring 2006 Computer Networks

6 UTP Categories Category 3 Category 4 Category 5 up to 16MHz
Voice grade found in most offices Used with 10BaseT, IBM Token ring; Arc Net Category 4 up to 20 MHz, the use is same as Cat 3 Category 5 up to 100MHz Commonly pre-installed in new office buildings Used with 10BaseT, Fast Ethernet, Gigabit Ethernet, ATM Spring 2006 Computer Networks

7 UTP Categories-cont. Category 5E (enhanced) Category 6
Up to 100MHz, similar use as Cat 5, Category 6 Up to 250 MHz, similar use as Cat 5 Lower attenuation and longer distances than Cat. 5 Category 6E – enhanced Cat 6 Category 7 (draft) Up to 600 MHz Used for high speed transmissions Spring 2006 Computer Networks

8 UTP Connectors Standard – RJ45 Can be inserted in only one way
Easy to manipulate Spring 2006 Computer Networks

9 Performance of UTP The attenuation depends on how thick the conductors are and the frequency at which is used Spring 2006 Computer Networks

10 Coaxial Cable Central core conductor, enclosed within an insulator sheath which is encased by an outer conductor covered by outer sheath. Spring 2006 Computer Networks

11 Coaxial Cable - Applications
Television distribution Cable TV (RG-59) Lately, only the last part is kept, the rest is replaced by fiber Long distance telephone transmission Can carry 10,000 voice calls simultaneously Being replaced by fiber optic Short distance computer systems links Local area networks, 10Base2 (RG-58), 10Base5 (RG-11) Obsolete (rarely used today) Spring 2006 Computer Networks

12 Coaxial Cable - Connectors
The most common connectors used with coaxial cable are BNC connectors Ordinary BNC connector to connect a single wire T BNC connector – to connect two wires BNC terminator – to terminate the end of the wire Spring 2006 Computer Networks

13 Coaxial Cable - Performance
Analog Amplifiers every few km Closer if higher frequency Up to 500MHz Digital Repeaters every 1km Closer for higher data rates The performance depends on the diameter of the cable and the frequency used Spring 2006 Computer Networks

14 Fiber-Optic Cable Consists of three components:
the light source (laser or light emiting diode) the medium (ultra-thin fiber of glass) the detector (generates electric pulse when light falls on it) Light pulses sent down a fiber spread out in length as they propagate. The attenuation of light through glass depends on the wavelength of the light (0.85, 1.30 and 1.55 micron are used for communication) Wavelength l = c/f , c is the speed of light Spring 2006 Computer Networks

15 Fiber-Optic Cable - Structure
Spring 2006 Computer Networks

16 Optical Fiber - Transmission Characteristics
Act as wave guide for 1014 to 1015 Hz Portions of infrared and visible spectrum Light Emitting Diode (LED) Cheaper Wider operating temperature range Last longer Injection Laser Diode (ILD) More efficient Greater data rate Wavelength Division Multiplexing Spring 2006 Computer Networks

17 Wavelength Division Multiplex (WDM)
Fiber 1 spectrum Fiber 2 spectrum Spectrum on the shared fiber Fiber 1 Prism Prism Fiber 3 Fiber 2 Shared fiber Fiber 4 Spring 2006 Computer Networks

18 Fiber-Optic Cable – Propagation Modes
The density of the core remains constant from the center to the edges The density of the core varies from the center to the edges Uses step-index fiber and highly focused source of light Spring 2006 Computer Networks

19 Fiber-Optic Cable - Connectors
Common connectors ST- used in cable TV SC – used in computer networks MT-RJ – a new connector with a size of RJ-45 Spring 2006 Computer Networks

20 Fiber-Optic Cable - Characteristics
Advantages Greater capacity (data rates of hundreds of Gbps Smaller size & weight (easier to put in the ground than cooper cables) Lower attenuation Electromagnetic isolation (not susceptible to electric interference) Greater repeater spacing (10s of km at least) Disadvantages High cost Difficult and expensive to install and maintain Light is unidirectional – one cable needed for each direction Spring 2006 Computer Networks

21 Fiber –Optic Cable - Applications
Used with wavelength division multiplex (WDM) for long distance transmission of voice channels and data signals Cable TV Local Area Networks, 100Base FX, (Fast Ethernet) and 1000Base X (Gigabit Ethernet) Spring 2006 Computer Networks

22 Electromagnetic Waves in the Air
Besides through guided media, electromagnetic waves can spread through the atmosphere and outer space Hz 104 108 1012 1016 1022 Radio Microwave Infrared UV X-ray Gamma ray Visible light Frequency spectrum of electromagnetic waves Spring 2006 Computer Networks

23 Wireless Transmission
Wireless transmission is used in all types of Wireless communication Mobile devices Satellite communication The frequencies used by the signal and the power of the signal are most important for this type of transmission Frequencies 3KHz to 1 GHz are usually called radio waves Frequencies between 1 and 300 GHz are called microwaves Spring 2006 Computer Networks

24 Antennas Antennas are used for both, transmission and reception of wireless signals To exchange information the antennas need to be tuned to the same frequency Two types of antennas Omnidirectional Directional Spring 2006 Computer Networks

25 Wireless Spectrum Radio waves Micro waves Spring 2006
Computer Networks

26 Problems with Wireless Transmission
Spring 2006 Computer Networks

27 Radio Waves At low frequency, radio waves pass through obstacles well, but the power falls off sharply with distance (AM radio) At high frequency, radio waves tend to travel in streight lines and bounce off obstacles At all frequencies radio waves are subject to inerference from electrical equipment The governments license the users of radio transmitters Spring 2006 Computer Networks

28 Radio Waves (cont.) Radio waves are omnidirectional Frequencies used
Signal spreads in all directions Can be received by many antennae Convenient for broadcasting Frequencies used 30MHz to 1GHz Applications Radio, Television and Paging systems Spring 2006 Computer Networks

29 Microwaves Microwaves are unidirectional Frequencies used Applications
Focused beam Careful alignment required Frequencies used 2GHz to 40GHz Applications Wireless LANs, Satellite communication Spring 2006 Computer Networks

30 Infrared Transmission
A short range communication system – one room Line of sight must be provided Frequencies used 3 x 1011 to 2 x 1014 Hz Application PC-to-PC short range transmission Spring 2006 Computer Networks

31 Spread Spectrum A type of wireless transmission in which signals are distributed over several frequencies simultaneously Developed to provide secure wireless transmission (for military purposes) Used in wireless LAN to reduce propagation effects (multi-path interference and others due to the higher frequencies) Spring 2006 Computer Networks

32 Satellite Microwave Microwave onto which the data is modulated is transmitted to the satelite from the ground Satellite receives on one frequency, amplifies or repeats the signal and transmits it back to earth using on board circuit known as transponder. A typical satelite channel has extremely high bandwidth (500 MHz) Satelites for communication purposes require geo-stationary orbit (Height of 35,784km) Spring 2006 Computer Networks

33 Satellites A single satellite usually contains multiple transponders (typically 6-12) Each transponder consists of a radio receiver and transmitter and uses a different radio frequency (i.e., channel) Multiple communications can proceed simultaneously and independently The degree of collimation of the microwave beem can be: coarse, so that the signal can be picked in a large geografic area focused, so that it can be picked up over a limited area Spring 2006 Computer Networks

34 Geosynchronous Satellites
Place in an orbit that is exactly synchronized with the rotation of the earth Geostationary Earth Orbit (GEO) Distance required for geosynchronous orbit is 36,000 km or 20,000 miles. The entire 360-degree circle above the equator can only hold satellites. This is because satellites need to be separated to avoid interference Spring 2006 Computer Networks

35 Network Connection accross an Ocean via Satelite
Spring 2006 Computer Networks

36 Application of Satelites
Television Long distance telephone Private business networks Internet when there is no other connection Spring 2006 Computer Networks

37 Satelites vs. Fiber Satelite advantages: Satelite disadvantages:
Sites that are not connected can easily use the satelite by installing a ground station Satelite disadvantages: Very large propagation delay (due to big distances) Very low security Quality of transmission can become questionable due to external influences Spring 2006 Computer Networks

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