Air Interface

2 Analog Transmission n In analog transmission, the state of line can vary continuously and smoothly among an infinite number of states – States can be signal strengths, voltages, or other measurable conditions – Human voice is analog; telephone mouthpiece generates analogous electrical signal Time Strength

3 Digital Transmission n Time is divided into fixed-length clock cycles – Modems: a few thousand clock cycles per second – LANs: millions of clock cycles per second n The line is kept in one of only a few possible states (conditions) during each time period – this is why the signal must be kept constant n At the end of each time period, the line may change abruptly to another of these few states

4 Digital Versus Binary Transmission n Digital transmission: a few states n Binary transmission: exactly two states (1 and 0) – Binary is a special case of digital DigitalBinary Two StatesFew States 0 1

5 Digital Versus Binary Transmission n Sender and Receiver associate one or more bits with each state – Simplest case: High state = 1, Low state = 0 – If four states, might have the following: n Highest = 11 n Second highest = 10 n Next highest = 01 n Lowest = 00

6 Wire Propagation Effects n Propagation Effects – Signal changes as it travels – If change is too great, receiver may not be able to recognize it Distance Original Signal Final Signal

7 Wire Propagation Effects: Attenuation n Attenuation: Signal Gets Weaker as it Propagates – May become too weak for receiver to recognize n Distortion: Signal changes shape as it propagates – Adjacent bits may overlap – May make recognition impossible for receiver Signal Strength Distance

8 Wire Propagation Effects: Noise n Noise: Thermal Energy in Wire Adds to Signal – Noise floor is average noise energy – Noise spikes are random energy affecting bits Noise Floor Signal Strength Time Signal Noise Spike Error

9 Wire Propagation Effects n Noise and Attenuation – As signal attenuates, gets closer to noise floor – Smaller spikes can harm the signal – So noise errors increase with distance, even if the average noise level is constant n Want a high Signal-to-Noise Ratio (SNR) – Signal strength divided by average noise strength – As SNR falls, errors increase Distance Signal Strength Signal Noise Floor SNR

10 Wire Propagation Effects: Noise & Speed n Noise and Speed – As speed increases, each bit is briefer – Noise fluctuations do not average out as much – So noise errors increase as speed increases One Bit Noise Spike Average Noise During Bit Low Speed (Long Duration) One Bit Noise Spike Average Noise During Bit High Speed (Short Duration) OK Error

11 Wire Propagation Effects: Interference n Interference – External signal converted to electrical energy – Adds to signal, like noise – Often intermittent (comes and goes), so hard to diagnose – Often called electromagnetic interference (EMI) Signal Strength Signal Interference

12 Wire Propagation Effects: Cross-Talk Interference n Cross-Talk Interference – Multiple wires in a bundle each radiates its signal – Causes “cross-talk” interference in nearby wires n Wire Usually is Twisted – Several twists per inch – Interference adds to signal over half twist, subtracts over other half Single Twist Interference -+ Signal

13 Practical Issues in Propagation Effects n Distance limits in standards prevent serious propagation effects – Usually 100 meters maximum for ordinary copper wire n Problems usually occur at connectors – Crossed wires – Poor connections – Cross-talk interference

14 Radio Propagation n Broadcast signal – Not confined to a wire

15 Radio Waves n When Electron Oscillates, Gives Off Radio Waves (electromagnetic waves) – Single electron gives a very weak signal – Many electrons in an antenna are forced to oscillate in unison to give a practical signal

16 Radio Propagation Problems n Wires Propagation is Predictable – Signals go through a fixed path: the wire – Propagation problems can be easily anticipated – Problems can be addressed easily n Radio Propagation is Difficult – Signals begin propagating as a simple sphere – Inverse square law attenuation n If double distance, only ¼ signal strength n If triple distance only 1/9 signal strength – Signals can be blocked by dense objects – Creates shadow zones with no reception Shadow Zone

17 Radio Propagation Problems n Radio Propagation is Difficult – Signals are reflected – May arrive at a destination via multiple paths – Signals arriving by different paths can interfere with one another: called multipath interference – Can be constructive or destructive interference – Very different reception characteristics with in a few meters or centimeters

18 Radio Propagation: Waves n Waves Amplitude (strength) Wavelength (meters) Frequency in hertz (Hz) Cycles per Second One Second 7 Cycles 1 Hz = 1 cycle per second

19 Radio Propagation: Frequency Spectrum n Frequency Spectrum – Frequencies vary (like strings in a harp) – Frequencies measured in hertz (Hz) – Frequency spectrum: all possible frequencies from 0 Hz to infinity n Metric system – kHz (1,000 Hz) kilohertz; note lower-case k – MHz (1,000 kHz) megahertz – GHz (1,000 MHz) gigahertz – THz (1,000 GHz) terahertz 0 Hz

20 Radio Propagation: Service Bands n Service Bands – Divide frequency spectrum into bands for services – A band is a contiguous range of frequencies – FM radio, cellular telephone service bands etc. 0 Hz Cellular Telephone FM Radio AM Radio Service Bands

21 Radio Propagation: Channels and Bandwidth n Service Bands are Further Divided into Channels – Like television channels – Bandwidth of a channel is highest frequency minus lowest frequency n Example – Highest frequency of a radio channel is 43 kHz – Lowest frequency of the radio channel is 38 kHz – Bandwidth of radio channel is 5 kHz (43-38 kHz) 0 Hz Channel 3 Channel 2 Channel 1 Service Band– FM Radio Channel Bandwidth

22 Radio Propagation: Channels and Bandwidth n Shannon’s Equation -- W = B Log 2 (1+S/N) – W is maximum possible (not actual) transmission speed in channel – B is bandwidth of channel: highest frequency - lowest frequency – S/N is the signal-to-noise ratio – The wider the channel bandwidth (B), the faster the maximum possible transmission speed (W) Maximum Possible Speed Bandwidth

23 Broadband vs. Baseband n Baseband: Inject signal into medium & propagates n Broadband: Different signals sent different channels – Begin with baseband signal – Modulate to fit in radio frequency signal (RF) – Channel bandwidth is wide = broadband transmission – Channel bandwidth is narrow = narrowband transmission