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Version 3/02/12
Computer Networks I
application
transport
link
physical
network Transmission Mediaapplication
transport
link
physical
network
Computer Networks I 2
Outline● Some informal definitions● Guide Media● Unguided Media: Wireless● References
Computer Networks I 3
Transmission medium● Anything that can carry information from a source to a
destination● Background
● 19th century - Morse's telegraph (metallic medium)● 1869 – Bell's telephone (also metallic medium)● 1895 – Hertz radio transmission (wireless)
– Later Marconi applied Hertz discover to the telegraph
Physical layer
Cable or air
Transmission channelPhysical layer
Computer Networks I 4
Transmission channel● Data are transmitted through electromagnetic
waveform propagation● Channels have limited transmission capacity
(bandwidth)● And take certain delay to reach destination● Signals in the channel can be affected by
● Noise● Attenuation● Distortion
Computer Networks I 5
Bandwidth● Amount of information that can flow through a
connection in a limited time● It matters because
● It's limited by physics & technology● It's not free● Requirements grow at a rapid pace● Critical to network performance
Computer Networks I 6
Bandwidth● Measured in bits per second (bps)
● Depends mainly:● on the transmission medium● type of technology● and coding
Cisco
Computer Networks I 7
Bandwidth● For analog communications
● It is expressed using a range:– Ex: 3 kHz to 300 kHz
● And measured in Hertz (Hz)
Computer Networks I 8
Transmission media
Transmission Media
Guided(wired)
Unguided(wired)
Twisted-paircable
Coaxialcable
Fiber-opticcable Free space
Forouzan
Computer Networks I 9
Guided media● Source and destination linked through a conduit
● Metallic conduit– Twisted-pair cable– Coaxial cable
● Glass/plastic conduit– Fiber-optic cable
Computer Networks I 10
Twisted pair● Twisting provides shielding against noise & interferences● The most common type is unshielded twisted-pair (UTP)● The standard categorizes the UTP into 7 classes
Cat1 0.4 MHzCat2 ? MHzCat3 16MHzCat4 20MHzCat5 100MHz 100BASE-TX & 1000BASE-T EthernetCat5e 100MHz 100BASE-TX & 1000BASE-T EthernetCat6 250MHz 1000BASE-T EthernetCat6e 250MHzCat6a 500MHzCat7 600MHzCat7a 1200MHz
Category Bandwidth Applications
Telephone and modem linesOlder terminal systems, e.g. IBM 327010BASE-T and 100BASE-T4 Ethernet16 Mbit/s Token Ring
10GBASE-T (under development) Ethernet10GBASE-T (under development) EthernetNo applications yet.Telephone, CATV, 1000BASE-T in the same cable.
Computer Networks I 12
Twisted pair● The most common UTP connector is the RJ45● Performance:
● Relation between attenuation (db/Km) and frequency● Sharply decreases with frequencies above 100KHz
● Applications● Voice & data through telephone lines● DSL for high-bandwidth● Local area networks
Computer Networks I 13
Coaxial cable● Supports higher frequency signal ranges than twisted-pair● Most common connector BNC● Performance
● Higher bandwidth than twisted-pair● But also higher attenuation
● Applications● Digital telephonic networks (nowadays replaced)● Local area networks (10Base-2)● Cable TV
Computer Networks I 14
Fiber-Optic cable● Made of glass or plastic● Signals are transmitted in the form of light, using
refraction capabilities of the material
Multimode(multiple beams using
different paths)
Single Mode
Computer Networks I 15
Fiber-Optic cable● Performance:
● Up to 1600Gps data transfers. Limited by the electronics, not the medium
● Better attenuation than twisted-pair. 10 times less repeaters for the same cable length
● Applications● Backbone networks (good relationship between
bandwidth & cost)● Cable TV● Local-area networks 100Base-Fx 1000Base-X
Computer Networks I 16
Fiber-Optic cable● Other advantages
● Immunity to electromagnetic interference● Resistance to corrosive materials● Light weight
● Some disadvantages● Installation & maintenance expertise required● Unidirectional light propagation● Cost. Only justifiable for high-bandwidth requirements
Computer Networks I 17
Guided media comparison ● Coaxial cable
● Hardly used nowadays● Fiber-Optic cable
● Highest bandwidth but the most expensive● Immune to electromagnetic distortions
● Twisted-pair cable● Easy to connect● Cheap electronics
Computer Networks I 18
Unguided media● Transport electromagnetic waves without a physical conductor● Also referred to as wireless communication● Use a part of the electromagnetic spectrum
Tanen
Computer Networks I 19
Wireless propagation methods● Ground propagation:
● Lowest portion of the atmosphere● Signals follow the curvature of the earth
● Sky propagation● Higher frequency signals raise up to the
ionosphere● Greater distance with lower output power.
● Line-of-sight (visual) propagation● Highest frequency but shorter range
Forouzan
Computer Networks I 20
Wireless bands
Band Range Propagation Application
VLF (very low frequency) 3-30 Khz ground Long-range radio navigation
LF (low frequency) 30-300 Khz ground Radio beacons & navigational locators
MF (middle frequency) 300 Khz – 3 Mhz sky AM radio
HF (high frequency) 3-30 Mhz sky Citizens band (CB), shift/spacecraft comm.
VHF (very high frequency) 30-300 Mhz Sky & line-of-sight VHF-TV, FM radio
UHF (ultrahigh frequency) 300 Mhz – 3 Ghz Line-of-sight UHF-TV, cellular phones, satellite
SHF (superhigh frequency) 3-30 Ghz Line-of-sight Satellite comm.
EHF (extremely high frequency) 30-300 Ghz Line-of-sight Radar, satellite
Forouzan
Computer Networks I 21
Wireless transmission waves
Wirelesstransmission
Radio wave Microwave Infrared
Forouzan
Computer Networks I 22
Radio wave● They range from 3 kHz to 1 GHz● Transmitted through omnidirectional antennas● Can travel long distances and penetrate walls (good
for AM radio, for instance)● Very sensitive to interference● Almost the entire band is regulated from authorities● Applications:
● TV & radio broadcasting● Cordless phones
Computer Networks I 23
Microwave● They range from 1 GHz to 300 GHz● Unidirectional => Antennas must be aligned● Propagation is line-of-sight (earth curvature is a
problem)● Cannot penetrate walls● Higher data range than radio waves● Part of the spectrum is regulated form authorities● ISM Radio Bands ( 2.4 & 5.7Ghz) used for WiFi
Computer Networks I 24
Infrared● They range from 300 GHz to 400 THz● Used for short range communication● Low interference between different systems● Very high data rate
Computer Networks I 25
ReferencesB.F. Transmisión de datos y redes de comunicaciones, 4th edition 2007.
● Chapter 7A.S. Redes de computadores. Pearson, 4th edition, 2003.
● Chapter 2: Sections 2.2 and 2.3CISCO Networking Academy e-learning.
● Module 8 – CCNA Exploration
All unlabeled figures are taken from the Wikipedia.