✓ ✏ ✒ ✑ 4 - Transmission Media Faculty of Information Technology BUS3150 - Computer Facilities Network Management 4 - Transmission Media Faculty of Information Technology Monash University ✓ ✏ ✒ ✑ 4 - Transmission Media 2 Faculty of Information Technology This Lecture • Different transmission media. – Guided transmission media: · Twisted Pair · Coaxial Cable · Optical Fibre – Wireless transmission: · Terrestrial Microwave · Satellite Microwave · Broadcast Radio · Infrared ✓ ✏ ✒ ✑ 4 - Transmission Media 3 Faculty of Information Technology Overview • Guided - wire /optical fibre • Unguided - wireless • Characteristics and quality of a data transmission determined by medium and signal. – For guided, the medium is more important. – For unguided, the bandwidth produced by the antenna is more impor- tant. ✓ ✏ ✒ ✑ 4 - Transmission Media 4 Faculty of Information Technology Overview • Key concerns are data rate and distance. • These depend on: – Bandwidth: · Higher bandwidth gives higher data rate. – Transmission impairments: · Attenuation limits distance. · Interference and noise causes bit errors. – Number of receivers in guided media. · More receivers (multi-point) introduce more attenuation. • Also need to consider cost (materials, labour and operating), security, op- erating environment, safety, reliability, maintenance and expansibility.
10
Embed
This Lecture BUS3150 - Computer Facilities Network Management · 2011-06-25 · BUS3150 - Computer Facilities Network Management 4 - Transmission Media Faculty of Information Technology
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
• It is possible to store data on magnetic (tapes or disks) or optical (CD orDVD) media, package the media and send using a courier.
• For extremely large sets of data, this is the cheapest method of transmis-sion. What is the down side?Andrew S. Tanenbaum, “Never underestimate the bandwidth of a station
wagon full of tapes hurtling down the highway”.
• For guided transmission, we typically consider:
– Twisted pair
– Coaxial cable
– Optical fibre
� �
� �
4 - Transmission Media
7 Faculty of Information Technology
Copper Twisted Pair
Outer insulatoror PVC
Solid copperconductors
Twist length
• Twisted pair cable consists of two insulated copper wires twisted together.
• Twisting allows each wire to have approximately the same noise level andreduces crosstalk.
• Typically installed in buildings during construction.
• Twisted pair comes in two varieties: STP and UTP.
� �
� �
4 - Transmission Media
8 Faculty of Information Technology
Effect of Noise on Parallel and Twisted-Pair Lines
Total effect is16 − 12 = 4
ReceiverSender
Noise effect = 12 units
Noise source
Noise effect = 16 units
16
12
Noise source
14
14Receiver
4
3
4 4 4
3 3 3
Sender
Total effect is14 − 14 = 0
� �
� �
4 - Transmission Media
9 Faculty of Information Technology
Twisted Pair - Applications, Pros and Cons
• Applications:
– Most common medium for both analog and digital.
– Telephone network - Between house and local exchange (subscriberloop).
– Within buildings - To private branch exchange (PBX).
– For local area networks (LAN) - 10Mbps to 1Gbps.
• Pros - Cheap and easy to work with.
• Cons - Low bandwidth/data rate and short range.
� �
� �
4 - Transmission Media
10 Faculty of Information Technology
Twisted Pair - Transmission Characteristics
• Analog:
– Needs amplifiers every 5km to 6km.
• Digital:
– Can use either analog or digital signals.
– Needs a repeater every 2 to 3km (depends on data rate).
• Limited distance.
• Limited bandwidth (1MHz).
• Limited data rate (100MHz).
• Susceptible to interference and noise.
� �
� �
4 - Transmission Media
11 Faculty of Information Technology
Unshielded and Shielded TP
• Unshielded Twisted Pair (UTP):
– Ordinary telephone wire.
– Cheapest.
– Easiest to install.
– Suffers from external EM interfer-ence.
• Shielded Twisted Pair (STP):
– Insulated twisted pairs encased in ametal braid or sheathing that reducesinterference.
– More expensive.
– Harder to handle and work with(thick, heavy).
Twisted pairs5 pairs
Plastic cover
Plastic cover Insulation CopperMetal shield
� �
� �
4 - Transmission Media
12 Faculty of Information Technology
UTP Categories
• Cat 3:
– Up to 16 MHz and 16 Mbps.
– Voice grade found in abundance in most offices.
– Twist length of 7.5 cm to 10 cm.
• Cat 4:
– Up to 20 MHz.
• Cat 5:
– Up to 100 MHz and 100 Mbps.
– Data-grade commonly pre-installed in new office buildings.
– Tighter twist (more expensive): twist length 0.6 cm to 0.85 cm.
• Cat 5E (enhanced), Cat 6, Cat 7.
� �
� �
4 - Transmission Media
13 Faculty of Information Technology
Near End Crosstalk
• Coupling of signal from one pair to another.
• Coupling takes place when transmit signal entering the link couples backto receiving pair.
• i.e. near transmitted signal is picked up by near receiving pair.
� �
� �
4 - Transmission Media
14 Faculty of Information Technology
Twisted Pair - Category Characteristics
Attenuation (dB per 100 m) Near−end Crosstalk (dB)
– A metallic outer conductor (shield), serves as aground.
– An insulator covering the centre conductor.
– A plastic jacket.
� �
� �
4 - Transmission Media
16 Faculty of Information Technology
Coaxial Cable - Applications
• Most versatile transmission medium.
• Television distribution:
– Ariel to TV.
– Cable TV (hundreds of TV channels up to a few tens of km).
• Long distance telephone transmission:
– Can carry 10,000 voice calls simultaneously (FDM).
– Mostly replaced by fibre optic, microwave and satellite.
• Short distance computer systems links.
• Local area networks.
� �
� �
4 - Transmission Media
17 Faculty of Information Technology
Coaxial Cable - Transmission Characteristics
• Superior frequency characteristics to twisted pair.
• Performance limited by attenuation and noise.
• Analog signals:
– Amplifiers every few km, closer if higher frequency.
– Up to 500 MHz
• Digital signals:
– Repeater every 1km, closer for higher data rates.
� �
� �
4 - Transmission Media
18 Faculty of Information Technology
Refraction and Reflection (Snell’s law)
More dense medium(water or glass)
BeamLess dense medium (air)
I
R
(a) From less dense to more dense medium
More dense medium(water or glass)
Less dense medium (air)
I
R
Beam
(b) From more dense to less dense medium
angleCritical
Angle ofincidence reflection
Angle of
� �
� �
4 - Transmission Media
19 Faculty of Information Technology
Refraction and Reflection Examples
You cannot see the pavement
� �
� �
4 - Transmission Media
20 Faculty of Information Technology
Refractive Index
• The refractive index of a medium is a measure of the speed at which lighttravels in it.
• It is proportional to the density of the medium.
• Index of refraction, n =
Speed of light in vacuumSpeed of light in the medium.
• For example:
– Vacuum: n = 1.0 ←−Fast
– Air: n = 1.0002926
– Water: n = 1.333
– Glass: n = 1.5
– Diamond: n = 2.419 ←−Slow
� �
� �
4 - Transmission Media
21 Faculty of Information Technology
Optical Fiber
Light at less thancritical angle isabsorbed in jacket
Angle ofreflection
Angle ofincidence
Cladding
Plastic coating
Core
• Glass or plastic core surrounded by a cladding with different optical prop-erties and coated in acrylate (plastic) for protection.
• Refractive index (density) of the core is higher than the refractive index ofthe cladding. Acts as a wave guide (total internal reflection) to trap lightin the core.
� �
� �
4 - Transmission Media
22 Faculty of Information Technology
Optical Fiber - Advantages
• Mechanical characteristics of fiber:
– Small diameter, light weight.
– Flexible.
– Resistance to corrosive materials.
• Electrical characteristics of fiber:
– Low attenuation.
– Wide bandwidth.
– Electromagnetic immunity.
– Ground loop elimination.
– No electromagnetic radiation.
• Greater repeater spacing.
• Large capacity.
• Space efficiency.
• Easy installation.
• Higher security.
� �
� �
4 - Transmission Media
23 Faculty of Information Technology
Optical Fiber - Applications
• Long-haul trunks:
– Average about 1500 km with high capacity (20 - 60,000 voice chan-nels).
– Undersea optical fiber being used.
• Metropolitan trunks:
– Average about 12 km with 100,000 voice channels per trunk group.
– Join telephone exchanges.
• Rural exchange trunks:
– Ranging from 40 to 160 km with fewer than 5000 voice channels.
• Cable TV networks.
• Subscriber loops: telephone, data, video, etc.
• LANs: 100 Mbps to 1 Gbps.
� �
� �
4 - Transmission Media
24 Faculty of Information Technology
Optical Fiber - Transmission Characteristics
• Act as wave guide (total internal reflection) 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: multiple light beams at different fre-quencies (colours).
� �
� �
4 - Transmission Media
25 Faculty of Information Technology
Optical Fiber - Transmission Modes
Source Destinationindex (n)
Refractive
(a) Step-index Multimode
Source Destinationindex (n)
Refractive
(b) Graded-index Multimode
Source Destinationindex (n)
Refractive
(c) Single Mode
� �
� �
4 - Transmission Media
26 Faculty of Information Technology
Optical Fiber - Dimensions
125
50
Multi-mode Fiber
125
7
Single-mode FiberDimensions in Micrometers (µm)
• Due to the dimensions:
– Multi-mode enables the use of cheap LED light sources.
– Single-mode requires more expensive laser sources.
� �
� �
4 - Transmission Media
27 Faculty of Information Technology
Optical Fiber - Cable� �
� �
4 - Transmission Media
28 Faculty of Information Technology
Optical Fiber - Attenuation
800 900
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
1000 1100 1200 1300Wavelength (nanometer)
850nmBand
1300nmBand
1550nmBand
Atte
nuat
ion
(dB
/km
)
1400 1500 1600 1700 1800
• Depends on the light wavelength and the physical properties of the glass.
• Three low attenuation windows centred on 850, 1300 and 1550nm.