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Computer Networks I

application

transport

link

physical

network Transmission Mediaapplication

transport

link

physical

network

Fernando.Rincon@uclm.es

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Outline● Some informal definitions● Guide Media● Unguided Media: Wireless● References

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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

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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

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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

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Bandwidth● Measured in bits per second (bps)

● Depends mainly:● on the transmission medium● type of technology● and coding

Cisco

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Bandwidth● For analog communications

● It is expressed using a range:– Ex: 3 kHz to 300 kHz

● And measured in Hertz (Hz)

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Transmission media

Transmission Media

Guided(wired)

Unguided(wired)

Twisted-paircable

Coaxialcable

Fiber-opticcable Free space

Forouzan

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Guided media● Source and destination linked through a conduit

● Metallic conduit– Twisted-pair cable– Coaxial cable

● Glass/plastic conduit– Fiber-optic cable

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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.

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Cable specifications

T: twisted-pairF: Fiber-Optic

10 BASE-T

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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

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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

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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

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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

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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

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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

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Unguided media● Transport electromagnetic waves without a physical conductor● Also referred to as wireless communication● Use a part of the electromagnetic spectrum

Tanen

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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

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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

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Wireless transmission waves

Wirelesstransmission

Radio wave Microwave Infrared

Forouzan

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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

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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

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Infrared● They range from 300 GHz to 400 THz● Used for short range communication● Low interference between different systems● Very high data rate

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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.