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