Computer Networks

1. Computer Networks Chapter 7 – Transmission Media

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 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 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 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) Computer Networks

6. UTP Categories • Category 3 • 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 Computer Networks

7. UTP Categories-cont. • Category 5E (enhanced) • 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 Computer Networks

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

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

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

15. Fiber-Optic Cable - Structure 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 Computer Networks

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

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

25. Wireless Spectrum Radio waves Micro waves Computer Networks

26. Problems with Wireless Transmission 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 Computer Networks

28. Radio Waves (cont.) • Radio waves are omnidirectional • 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 Computer Networks

29. Microwaves • Microwaves are unidirectional • Focused beam • Careful alignment required • Frequencies used • 2GHz to 40GHz • Applications • Wireless LANs, Satellite communication 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 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) 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) 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 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 45-90 satellites. • This is because satellites need to be separated to avoid interference Computer Networks

35. Network Connection accross an Ocean via Satelite Computer Networks

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

37. Satelites vs. Fiber • Satelite advantages: • 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 Computer Networks