CS 453 Computer Networks Lecture 5 Layer 1 – Physical Layer.

CS 453CS 453Computer NetworksComputer Networks

Lecture 5Lecture 5

Layer 1 – Physical LayerLayer 1 – Physical Layer

Some Responses to HW#1 Q2Some Responses to HW#1 Q2…I can envision two really interesting services. The first service would be similar to Netflix only it would be exclusively an online experience. With this service you would request a movie and it would immediately start streaming to your computer or mobile device in HD quality. The difference between my service and Netflix is that every movie, tv show, and broadcast ever would be available to watch.The second service i can envision is a life recorder. This service is like Tivo for your life. You will wear a small device that records what you see, what you hear, and what your location is. Thedevice is smart enough to recognize speech so all your conversations would be searchable and tags could be applied to every few second of video. This service would make it possible to relive past eventsby yourself or with others.

… three dimensional web conferencing where people could interact with one another as if they were sitting in the same room with them. I can also imagine a crisis or serve weather advisory system that could contact every mobile phone, computer, and television in an area and alert people to nearby danger.

…This new human network would allow anyone to live vicariously through any one else. Your could have all of the sensations of some else’s body any where in the world. It could be the greatest sporting event to see and feel what the starting quarterback of a football game, or live in a 3rd world county to “Walk a mile in some else’s shoes.” This technology could only be possible though a great understanding of the human body, mind and greater bandwidth and wireless communication.

…Doctors specialized in a certain field of surgery may not be able to reach the patients at a different country during emergency. Performing surgeries through video conferencing is an idea which is realized these days. But, it just involves the experienced doctor to give directions to the other doctors performing the operation for the patient.

…online operating systems. In such a model, computers would only need to be complex enough to boot up and open a web browser, and all tasks can be performed with online applications on a virtual desktop. Google is already creating online applications with examples such as the fast and highly functional Gmail and the online editing and file storage suite Google Docs, and there are other corporations and startups persuing similar ideas. In the future perhaps all file storage and applications would be remote, and users can have complete access to all their data and programs from anywhere in the world.

Wireless TransportWireless Transport

Earlier we made a distinction between…Earlier we made a distinction between… Guided and Unguided transmission mediaGuided and Unguided transmission media … … not a good distinction…not a good distinction… Lasers are directional and guided…Lasers are directional and guided… Certain radio bands are directional and guidedCertain radio bands are directional and guided

We will used Wired and Wireless (even though We will used Wired and Wireless (even though fiber is not really wire)fiber is not really wire)

““Stuff you can touch” vs “Stuff you can’t touch” Stuff you can touch” vs “Stuff you can’t touch” seems a little awkward…seems a little awkward…

Wireless Transmission MediaWireless Transmission Media

Wireless transmission media has become Wireless transmission media has become very popular with the advent of WiFi very popular with the advent of WiFi (which we will come back to later)(which we will come back to later) Wifi offers mobility..Wifi offers mobility.. … … and fairly high data ratesand fairly high data rates It has become a common transmission It has become a common transmission

infrastructure in homes, campuses, …infrastructure in homes, campuses, … … … in some citiesin some cities


……But wireless transmission media takes But wireless transmission media takes several forms…several forms… RadioRadio MicrowaveMicrowave Infrared and MillimeterInfrared and Millimeter Laser Laser SatellitesSatellites

Where do you suppose wireless digital Where do you suppose wireless digital communications was invented?communications was invented?


HawaiiHawaii Several populated islandsSeveral populated islands Large amount of ocean separating the islandsLarge amount of ocean separating the islands No good communications system connecting No good communications system connecting

the islandsthe islands Stringing wire was not a very feasible solutionStringing wire was not a very feasible solution


When we talk about wireless When we talk about wireless communication we are talking about using communication we are talking about using the electromagnetic spectrum…the electromagnetic spectrum… Radio, light, xrays,…Radio, light, xrays,…

More specifically, we are talking about More specifically, we are talking about dividing up the electromagnet spectrum…dividing up the electromagnet spectrum…… … and allocating portions of it for and allocating portions of it for communications…communications………but what portions are used for what?but what portions are used for what?


The electromagnetic spectrum can be The electromagnetic spectrum can be discussed in terms of…discussed in terms of…

FrequenciesFrequencies

WavelengthsWavelengths

……or portions (bands) have namesor portions (bands) have names

Wireless Transmission MediaWireless Transmission MediaThe electromagnetic spectrumThe electromagnetic spectrum

From: http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html


The electromagnetic spectrumThe electromagnetic spectrum

From: Tanenbaum (2003) pg. 101


Specific communications applications use Specific communications applications use a portion of electromagnetic spectrum – a portion of electromagnetic spectrum – bandsbandsRecall in a previous discussion-Recall in a previous discussion- The maximum amount of information in a The maximum amount of information in a

signal is related to the bandwidth of its signal is related to the bandwidth of its transmission mediatransmission media

That was expressed by Nyquist’s Theorem –That was expressed by Nyquist’s Theorem –Data rate = 2(Hz)logData rate = 2(Hz)log2222

If we assume 1 bit per Hz….If we assume 1 bit per Hz….


… … the 1 bit per Hz assumption does not the 1 bit per Hz assumption does not always hold, but we will get back to that always hold, but we will get back to that later.later.

So, since a communications applications So, since a communications applications uses a slice of the spectrum…uses a slice of the spectrum…

… … we need to know the width of the band we need to know the width of the band

The wider the band the higher the data The wider the band the higher the data raterate


Most communications applications use a narrow Most communications applications use a narrow frequency bandfrequency bandSome use a frequency wideband…Some use a frequency wideband… One use of wideband communications is…One use of wideband communications is… Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum In FHSS – a wide band is allocated, …In FHSS – a wide band is allocated, … Communications is done using multiple smaller bands Communications is done using multiple smaller bands

within the wideband…within the wideband… Communication senders and receivers hops from Communication senders and receivers hops from

frequency to frequency in a predetermined frequency to frequency in a predetermined sequence…sequence…

Known to both the sender and receiverKnown to both the sender and receiver


OK, but who invented Frequency Hopping OK, but who invented Frequency Hopping Spread Spectrum?Spread Spectrum?


Hedy LamarrHedy Lamarr

Austrian bornAustrian born

Hollywood actressHollywood actress

Movies 1932-1958Movies 1932-1958

Most famous role – Most famous role – Delilah in “Samson Delilah in “Samson and Delilah” 1949and Delilah” 1949

Wireless Transmission MediaWireless Transmission MediaHedy Lamarr (so the story goes) was married to Hedy Lamarr (so the story goes) was married to a weapons manufacturer in Austria…a weapons manufacturer in Austria…who, she learned, was selling weapons to Hitler.who, she learned, was selling weapons to Hitler.She also learned that Germany could jam the She also learned that Germany could jam the radio signals using by the Allies to control radio signals using by the Allies to control torpedoestorpedoesShe escaped to the U.S…She escaped to the U.S…And developed (and has a patent for) Frequency And developed (and has a patent for) Frequency Hopping Spread Spectrum…Hopping Spread Spectrum…As a was to foil the German military’s efforts to As a was to foil the German military’s efforts to jam torpedoes jam torpedoes see - http://en.wikipedia.org/wiki/Hedy_Lamarrsee - http://en.wikipedia.org/wiki/Hedy_Lamarr


Frequency Hopping Spread Spectrum…Frequency Hopping Spread Spectrum………also…also…Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum A bit stream of data is combined with a higher data A bit stream of data is combined with a higher data

rate bit stream -- called chipping coderate bit stream -- called chipping code The chipping code is a redundant patent of the The chipping code is a redundant patent of the

transmitted bitstransmitted bits Combined bit stream is broken up and transmitted in Combined bit stream is broken up and transmitted in

multiple small bands across a widebandmultiple small bands across a wideband See: http://www.webopedia.com/TERM/D/DSSS.htmlSee: http://www.webopedia.com/TERM/D/DSSS.html

We will visit this again in the futureWe will visit this again in the future


RadioRadio Operates in the 10E4 to 10E8 bandOperates in the 10E4 to 10E8 band Long RangeLong Range Easy/Cheap to generateEasy/Cheap to generate Omnidirectional – do not need to align Omnidirectional – do not need to align

transmitters/receiverstransmitters/receivers Omnidirectional problemsOmnidirectional problems

Ohio State Patrol and GM in the 70sOhio State Patrol and GM in the 70sWVU’s PRTWVU’s PRTDental workDental work


RadioRadio At low frequencies – good penetration…At low frequencies – good penetration… … … but power falls over rapidly ) (1/rbut power falls over rapidly ) (1/r22)) Radio spectrum highly regulated and licensed Radio spectrum highly regulated and licensed

by governmentsby governments ……but (more on this)but (more on this)


MicrowaveMicrowave 300 MHz to 300 GHz300 MHz to 300 GHz IEC/IEEE says 1 GHz+ IEC/IEEE says 1 GHz+

Includes UHF, SHF, EHFIncludes UHF, SHF, EHF Normally use up to 10 GHz for communicationsNormally use up to 10 GHz for communications Tend to travel in straight lineTend to travel in straight line DirectionalDirectional Maintains high S/N ration over distanceMaintains high S/N ration over distance Microwave are blocked by EarthMicrowave are blocked by Earth Can’t have senders/receivers/repeaters too far apartCan’t have senders/receivers/repeaters too far apart


MicrowaveMicrowave Above about 4 GHz microwaves are blocked Above about 4 GHz microwaves are blocked

by water (this is why your microwave oven by water (this is why your microwave oven works)works)

……so can be disrupted by rainso can be disrupted by rain Might need reroutedMight need rerouted


MicrowaveMicrowave Widely used in long distance communications Widely used in long distance communications

– particularly telephone– particularly telephone Relatively cheapRelatively cheap

Requires towers, transceivers, maybe repeaters – Requires towers, transceivers, maybe repeaters – every 50 km or soevery 50 km or so

Small footprintSmall footprint

Very few right-of-way issues unlike fiber and Very few right-of-way issues unlike fiber and coppercopper


Government, Bandwidth and the Government, Bandwidth and the Electromagnetic SpectrumElectromagnetic SpectrumGovernments tend to carefully control and Governments tend to carefully control and requlate the spectrum, particularly the radio requlate the spectrum, particularly the radio spectrumspectrumBands of spectrum are may available for specific Bands of spectrum are may available for specific classes of applicationsclasses of applications TV gets a bandTV gets a band Wireless phone carrier get a band,Wireless phone carrier get a band, ..etc...etc.

Users of a band most of these bands must be Users of a band most of these bands must be licensed – in U.S. by FCClicensed – in U.S. by FCC


Governments, etc.Governments, etc.An alternative approach is to allocation band and An alternative approach is to allocation band and not regulate themnot regulate them Unlicensed useUnlicensed use ……but regulate the power used in transmissionbut regulate the power used in transmission For example, in U.S. For example, in U.S.

902 MHz-928 MHz, 902 MHz-928 MHz, 2.4 GHz-2.4835 GHz,2.4 GHz-2.4835 GHz,5.735 GHz-5.860 GHz5.735 GHz-5.860 GHz

Are available for unlicensed use, butAre available for unlicensed use, but Are limited to less than one watt of powerAre limited to less than one watt of power


Governments, etc.Governments, etc.

For example,For example, 900 MHz has a lot of devices – cordless 900 MHz has a lot of devices – cordless

phones, garage door openers, etc.phones, garage door openers, etc. 802.11 and Bluetooth use 2.4 GHz band802.11 and Bluetooth use 2.4 GHz band 802.11a uses 5.7 GHz band802.11a uses 5.7 GHz band And devices use spread spectrum techniquesAnd devices use spread spectrum techniques


Infrared and millimeter wavesInfrared and millimeter waves

Higher frequency than Microwave, lower Higher frequency than Microwave, lower than visible lightthan visible light

Just lower than visible light, so…Just lower than visible light, so…

These waves act like light…These waves act like light…

……do not penetrate solid objectsdo not penetrate solid objects

… … means they can be limited in range (by means they can be limited in range (by walls,etc.walls,etc.

CS 453 Computer Networks Lecture 5 Layer 1 – Physical Layer.

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