Computer Networks

Chapter 7 – Transmission Media

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, 10Mbps or 100Mbps, or 1000Mbps

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 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 6

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

Spring 2006 Computer Networks 7

UTP Connectors

  

Standard – RJ45 Can be inserted in only one way Easy to manipulate

Spring 2006 Computer Networks 8



The attenuation depends on how thick the conductors are and the frequency at which is used

Performance of UTP

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, 10Base2 (RG-58), 10Base5 (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 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 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 10 14



to 10 15 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

Spring 2006

Wavelength Division Multiplex (WDM)

Fiber 1 spectrum Fiber 2 spectrum Spectrum on the shared fiber Prism Shared fiber Prism

Computer Networks 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 Computer Networks

Uses step-index fiber and highly focused source of light

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 (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 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, 100Base FX, (Fast Ethernet) and 1000Base X (Gigabit Ethernet)

Spring 2006 Computer Networks 21

Hz

Electromagnetic Waves in the Air



Besides through guided media, electromagnetic waves can spread through the atmosphere and outer space 10 4 10 8 10 12 10 16 10 22

Spring 2006

Radio Microwave Infrared UV Visible light X-ray Frequency spectrum of electromagnetic waves Gamma ray

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 3KHz 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 Micro waves

Spring 2006 Computer Networks 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



30MHz to 1GHz Applications



Radio, Television and Paging systems

Spring 2006 Computer Networks 28

Microwaves

  

Microwaves are unidirectional



Focused beam



Careful alignment required Frequencies used



2GHz to 40GHz 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 10 11 to 2 x 10 14 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 geo stationary orbit (Height of 35,784km)

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