Course overview
Skills: noneConcepts: wired hardware characteristics, wireless hardware characteristics (frequency and power), modulation schemes
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License. The physical layer1The physical layer standards specify hardware characteristics and ways to differentiate between ones and zeros.
This presentation covers wired and wireless connections.
We will focus on frequency and power, key characteristics of a wireless link.
That will be followed by a discussion of ways of ssssssssssssssssssssssssssssWhere does this topic fit?Internet conceptsApplicationsTechnology (communication)ImplicationsInternet skillsApplication developmentContent creationUser skills2The presentation deals with communication technology.The physical layerLayerFunctionApplicationDo useful work like Web browsing, email, and file transfer TransportTransport data between application programs running on two hostsInternetRoute packets between networks (inter network)Data linkSend data within the local area networkPhysicalSpecify hardware characteristics and ways to differentiate between ones and zeros3At the physical layer, we consider hardware characteristics and also the ways to differentiate between one and zero bits.
Lets begin with hardware characteristics.
Well consider wired media first, then wireless.
Copper and fiber
4There are physical layer standards governing the lengths, size and transmission properties of wires and cables.
On the left we see copper wires, which are typically twisted around each other for improved performance, and a coaxial cable with a copper wire in the middle.
Varying electric current signals ones and zeros.
We see optical fibers on the right.
The single strand passes through the eye of a needle.
When used in undersea cables, they are protected by layers of reinforcing materials.
The standards determine maximum lengths for cables and the speed at which they can transmit data.
The colors of wires and the sizes and shapes of connectors are standardized to make sure things plug together.
FrequencyPower and frequency are key characteristics of wireless links.
Lets take frequency first.
Consider your car radio.
Hundreds of radio transmitters are broadcasting all over the city, but you only hear one station as you drive.
The transmission frequency your radio is tuned to determines the station you will hear.
If you push a different button, the radio switches to a different frequency and you hear a different program.
The same is true of a WiFi radio or any other data radio.
The WiFi radio in a laptop has to be transmitting and receiving on the same frequency as the radio in the WiFi access point.5ApplicationFrequency rangeLicenseAM radio535-1,700 KHzYesFM radio88 to 108 MHzYesTV54 to 216 MHzYesWiFi 802.11 b, g, n2.401 to 2.473 GHzNoWiFi 802.11 a5.15 to 5.825 GHzNoVisible light400 to 700 THzNo
Frequency range examplesAs you see here WiFi radios communicate at higher frequencies than AM radio, FM radio or television.
The FCC requires licenses to transmit at many frequencies in a given city or geographic area.
A license is not required to operate a WiFi transmitter because they use very low power, so are unlikely to interfere with others.
The frequency range for visible light is much higher than WiFi.
Note that the unit of frequency is the hertz.
One hertz equals one cycle per second.
The usual prefixes kilo, mega, giga, tera, etc. designate one thousand, million, billion, trillion, etc.6Electromagnetic radiation applications
WiFi, radio and television occupy just a small portion of the electromagnetic spectrum.
Each of these applications use different electromagnetic frequency ranges or bands.
WiFi uses higher frequencies than AM and FM radio or TV, so you can use both at home without interference.
X-rayAM~radioFM~radioTelevisionAmateur~radioSubmarine~communicationMarine~radioCB~radioWiFiWimaxZigBeeGPSRFIDMicrowave~radioShortwave~radioRadio~telescopeVisible~lightHeart~monitorMobile~phoneBluetoothSatelliteMicrowave~ovenWireless~telephoneInfraredUltravioletGamma~rays7
What is power?
Lets turn our attention from frequency to power.
Power is the rate at which energy is consumed, and it is measured in watts.
Here are three light sources a searchlight, a bulb and a flashlight.
The searchlight is brightest, and consumes energy at a faster rate than the others.
The flashlight consumes energy slowly, but emits less light.
8
Radio powerTransmission power determines the maximum length of a radio link.
Lets go back to your car radio.
What happens as you drive out of the city?
At some point, the radio begins to sound bad you hear static or perhaps a bit of sound from another station.
The further away you get from the transmitter the worse it gets until you can no longer hear the station.
The more powerful the transmitter, the further away you can hear it.
Lets look at two radio stations as examples. 9
Estimated coverage for 600 watt KPCC
10KPCC is a Los Angeles radio station with a 600 watt transmitter.
If you are inside the red line, you hear it well.
If you are outside the blue line, reception will be poor.
Reception is good within about 25 miles of the transmitter and after about 60 miles, you cannot get KPCC with a typical car radio.
The further you get away from a radio transmitter, the weaker the signal becomes.
The geek term for signal becoming weaker is to attenuate.
To say that a signal is attenuated just means that it is weakened.
Wolfman Jack, 250,000 watt XERFClick to hear the Wolfman11Compare 600 watt KPCC with 250,000 watt XERF.
In the 1960s, Wolfman Jack broadcast using a 250,000 watt transmitter.
The transmitter was so powerful he could be heard throughout the southwest United States.
But, because XERF was so powerful, no local station could broadcast at their frequency of 1570KHz.
The XERF transmitter was in Mexico, near the United States border.
If XERF had been located in the United States, they would not have been licensed to broadcast with so much power.Coverage 802.11g WiFi access point, < 1 watt
200 feet125 feet
20-22 Mbps5-10 Mbps10-15 Mbps15-20 Mbps12In contrast to a commercial radio station, a WiFi access point transmits at 1 watt or less, depending upon the characteristics of its antenna.
For that reason, a typical WiFi link can only be one or two hundred feet long.
As shown here, the signal is strongest near the access point, which allows faster data transfer rates.
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Inside my houseThis radio signal detection program found 12 WiFi access points near my house.
As you see, the strongest signal is from my own access point, named lpress.
The signal is much stronger than those of my neighbors.
The access point named Gaffney has the second strongest signal on WiFi channel 11.
At first glance, it looks like my signal is only two or three times as strong as that of Gaffney, but the power scale is not linear.
The signal from the lpress access point is actually about 15 times stronger.13
In the back yardHere is the radio signal scan from my back yard.
The lpress signal is substantially weaker than it was inside the house.
That is because I am about 80 feet from the access point and the signal was further attenuated as it passed through three walls.
14Antenna focus
15A final point with respect to power is that antennas, like flashlights with reflectors, can be designed to focus their power relatively narrowly or can radiate in all directions like an open light bulb.
A narrowly focused WiFi radio is allowed to transmit at a higher level because the likelihood of interference is reduced.
Like a focused flashlight, the signal will also travel farther.
orModulation mehtod16The second physical layer concern is the method for differentiating between one and zero bits or, as a geek would say, the method of modulation.
Modulation means change.
For example, actors modulate their voices varying pitch and stress to convey emphasis.
Modulation transmit ones and zerosAmplitude modulation 0: low amplitude1: high amplitude Frequency modulation 0: low frequency1: high frequency Phase shift modulation 0: no phase shift during a clock period1: 180 degree phase shift
17This figure shows three basic ways to represent 0s and 1s by modulating a signal.
In the first, the amplitude is increased to signal a 1 and decreased to signal a zero.
With frequency modulation, the frequency is modulated.
A phase shift is an abrupt change in the regular oscillation of the transmitted signal.
Regardless of the modulation method, the trasnmitter must be able to generate the change and the receiver must be able to detect the change.
SignalAMFMAM and FM modulation101http://som.csudh.edu/fac/lpress/presentations/Amfm.gif18Here we see the signal changes for alternating zeros and ones using amplitude and frequency modulation.
If we were signaling using sound instead of electromagnetic waves, we would hear the volume change when amplitude changed and we would hear pitch change with changes in frequency.
SummaryWe saw that the two major functions of the physical layer are defining the hardware and the modulation method.
We discussed both wired and wireless hardware specifications, noting that wireless links are characterized by their frequency and transmission power level.
Self study questionsWe mentioned two wired technologies, what were they?WiFi is commonly used for wireless network links. Can you think of other wireless technologies that are used for network links?In our modulation example, the AM transmitter and receiver could differentiate two different frequencies one high the other low. In other words, we could transmit one bit during each clock cycle. If a more sensitive radios could differentiate between four different frequencies, how many bits could we transmit during a clock cycle?Why isnt the signal strength at a given distance the same in all directions?What is the unit of measure for radio power?What is the unit of measure for radio signal frequency?The FCC allows WiFi radios with narrowly focused antennae to transmit at higher power levels than WiFi radios which transmit In all directions. Why is that?
ResourcesLocate radio stations and unused FM frequency bands:http://www.radio-locator.com/
