Computer Networks Chapter 7 Transmission Media Spring

Computer Networks Chapter 7 – Transmission Media Spring 2006 Computer Networks

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 2

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 3

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, 10 Mbps or 100 Mbps, or 1000 Mbps Spring 2006 Computer Networks 4

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 5

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

UTP Categories-cont. § Category 5 E (enhanced) § Up to 100 MHz, 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 6 E – enhanced Cat 6 § Category 7 (draft) § Up to 600 MHz § Used for high speed transmissions Spring 2006 Computer Networks 7

UTP Connectors § Standard – RJ 45 § Can be inserted in only one way § Easy to manipulate Spring 2006 Computer Networks 8

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

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 10

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, 10 Base 2 (RG-58), 10 Base 5 (RG -11) § Obsolete (rarely used today) Spring 2006 Computer Networks 11

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 12

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

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 14

Fiber-Optic Cable - Structure Spring 2006 Computer Networks 15

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 16

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

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 Spring 2006 Uses step-index fiber and highly focused source of light Computer Networks 18

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 19

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 (10 s 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 20

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, 100 Base FX, (Fast Ethernet) and 1000 Base X (Gigabit Ethernet) Spring 2006 Computer Networks 21

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

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 3 KHz to 1 GHz are usually called radio waves § Frequencies between 1 and 300 GHz are called microwaves Spring 2006 Computer Networks 23

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 24

Wireless Spectrum Radio waves Spring 2006 Computer Networks Micro waves 25

Problems with Wireless Transmission Spring 2006 Computer Networks 26

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 27

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

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

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 30

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 31

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 geostationary orbit (Height of 35, 784 km) Spring 2006 Computer Networks 32

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 33

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 Spring 2006 Computer Networks 34

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

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

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 Spring 2006 Computer Networks 37