Duplex• Full-duplex transmission: both sides can

transmit simultaneously– Even if only one sends, still full-duplex line– Even if neither is sending, still full-duplex line

A B

Time 1Both can send

Both do

A B

Time 1Both can sendOnly A does

A B

Time 1Both can sendNeither does

Duplex• Half-duplex transmission: only one can

transmit at a time; must take turns– Still half duplex if neither transmits

A B A B

Time 1Only one side

Can sendA does

Time 2Only one side

Can sendNeither does

Duplex• Duplex is a Characteristic of the Transmission

System, Not of Use at a Given Moment

– In full duplex, both sides can transmit at once; in half duplex, only one side can transmit at a time

– Still full duplex system if only one side or neither side actually is transmitting at a moment

– Still half duplex if neither side actually is transmitting at a moment

Radio Propagation

• Broadcast signal– Not confined to a wire

Radio Waves

• When Electron Oscillates, Gives Off Radio Waves– Single electron gives a very weak signal

– Many electrons in an antenna are forced to oscillate in unison to give a practical signal

Radio 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

– But they can be blocked

– There are shadow zonesShadow

Zone

Radio 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

– This is called multipath interference

Radio Propagation: Waves

• Waves

Amplitude(strength)

Wavelength(meters)

Frequency in hertz (Hz)Cycles per Second

One Second7 Cycles

1 Hz = 1 cycle per second

Radio 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

0 Hz

Frequencies

• 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

Radio Propagation: Service Bands• Service Bands

– Divide 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

Radio Propagation: Channels and Bandwidth• Service Bands are Further Divided into

Channels– Like television channels

– Bandwidth of a channel is highest frequency minus lowest frequency

0 Hz

Channel 3

Channel 2

Channel 1

ServiceBand

ChannelBandwidth

Radio Propagation: Channels and Bandwidth• 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

ChannelBandwidth

Radio Propagation: Channels and Bandwidth

• Shannon’s Equation– W is maximum possible (not actual) transmission speed

in a channel

– B is bandwidth of the channel: highest frequency minus lowest frequency

– S/N is the signal-to-noise ratio

W = B Log2 (1 + S/N)

Radio Transmission: Broadband

• Speed and Bandwidth– The wider the channel bandwidth (B), the faster

the maximum possible transmission speed (W)

– W = B Log2 (1+S/N)

MaximumPossible

Speed

Bandwidth

Telephony is Narrowband

• Bandwidth in Telephone Channels is Narrow

– Sounds below about 300 Hz cut off to reduce equipment hum within telephone system

– Sounds above about 3,400 Hz cut off to reduce the bandwidth needed to send a telephone signal

20 kHz300 Hz 3.4 kHz

3.1 kHz

Telephony is Narrowband• Bandwidth in Telephone Channels is

Narrow

– A radio channel would have to be from 0 to 3.4 kHz (3.4 kHz)

– This would mean a maximum possible transmission speed of about 35 kbps

20 kHz300 Hz 3.4 kHz

3.1 kHz

Error in Book

Required Radio Channel

Broadband• Two Uses of the Term “Broadband”

• Technically, the signal is transmitted in a single channel AND the bandwidth of the channel is large

– Therefore, maximum possible transmission speed is high

• Popularly, if the signal is fast, the system is called “broadband” whether it uses channels at all