Air Interface

2Analog Transmission

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

3Digital Transmission

Time is divided into fixed-length clock cycles– Modems: a few thousand clock cycles per second– LANs: millions of clock cycles per second

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

At the end of each time period, the line may change abruptly to another of these few states

4Digital Versus Binary Transmission

Digital transmission: a few states Binary transmission: exactly two states (1 and 0)

– Binary is a special case of digital

Digital Binary

Two StatesFew States

0

1

5Digital Versus Binary Transmission

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: Highest = 11 Second highest = 10 Next highest = 01 Lowest = 00

6Wire Propagation Effects

Propagation Effects– Signal changes as it travels– If change is too great, receiver may not be able to

recognize it

Distance

OriginalSignal

FinalSignal

7Wire Propagation Effects: Attenuation

Attenuation: Signal Gets Weaker as it Propagates– May become too weak for receiver to recognize

Distortion: Signal changes shape as it propagates– Adjacent bits may overlap– May make recognition impossible for receiver

SignalStrength

Distance

Distance

8Wire Propagation Effects: Noise

Noise: Thermal Energy in Wire Adds to Signal– Noise floor is average noise energy– Noise spikes are random energy affecting bits

Noise Floor

SignalStrength

Time

Signal

Noise

Spike

Error

9Wire Propagation Effects

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

Want a high Signal-to-Noise Ratio (SNR)– Signal strength divided by average noise strength– As SNR falls, errors increase

Distance

SignalStrength Signal Noise Floor

SNR

10Wire Propagation Effects: Noise & Speed

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 BitNoiseSpike

Average NoiseDuring Bit

Low Speed(Long

Duration)

One BitNoiseSpike

Average NoiseDuring Bit

High Speed(Short

Duration)

OK Error

11Wire Propagation Effects: Interference

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)

SignalStrength

Signal

Interference

12Wire Propagation Effects: Cross-Talk Interference

Cross-Talk Interference– Multiple wires in a bundle each

radiates its signal– Causes “cross-talk” interference

in nearby wires

Wire Usually is Twisted– Several twists per inch– Interference adds to signal over half twist, subtracts

over other half

Single Twist

Interference- +

Signal

13Practical Issues in Propagation Effects

Distance limits in standards prevent serious propagation effects– Usually 100 meters maximum for ordinary copper wire

Problems usually occur at connectors– Crossed wires– Poor connections– Cross-talk interference

14Radio Propagation

Broadcast signal– Not confined to a wire

15Radio Waves

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

16Radio Propagation Problems

Wires Propagation is Predictable– Signals go through a fixed path: the wire– Propagation problems can be easily anticipated– Problems can be addressed easily

Radio Propagation is Difficult– Signals begin propagating as a simple sphere– Inverse square law attenuation

If double distance, only ¼ signal strength If triple distance only 1/9 signal strength

– Signals can be blocked by dense objects– Creates shadow zones with no reception

ShadowZone

17Radio Propagation Problems

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

18Radio Propagation: Waves

Waves

Amplitude(strength)

Wavelength(meters)

Frequency in hertz (Hz)Cycles per Second

One Second7 Cycles

1 Hz = 1 cycle per second

1

4

3

2

19Radio Propagation: Frequency Spectrum

Frequency Spectrum– Frequencies vary (like strings in a harp)– Frequencies measured in hertz (Hz)– Frequency spectrum: all possible frequencies from 0

Hz to infinity

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

20Radio Propagation: Service Bands

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

ServiceBands

21Radio Propagation: Channels and Bandwidth

Service Bands are Further Divided into Channels– Like television channels– Bandwidth of a channel is highest frequency minus

lowest frequency 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

ServiceBand– FM Radio

ChannelBandwidth

22Radio Propagation: Channels and Bandwidth

Shannon’s Equation -- W = B Log2 (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)

MaximumPossible

Speed

Bandwidth

23Broadband vs. Baseband

Baseband: Inject signal into medium & propagates

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