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2010 1
The Physical Layer
Chapter 2
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2010 2
Bandwidth-Limited Signals
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2010 3
Maximal Data Rate
Shannon –Hartley law (1948):• a channel with a bandwidth of H Hz and random noise
• maximum bps (bits per second) is: H log2 (1+S/N)
• S/N: signal power to noise power (dB: 10 log10 (S/N) )
• current coding techniques approaches the limit
To achieve higher speed:• better cables and electronics
• higher bandwith (less attenuation of higher frequencies)
• lower internal noise
• decrease influence of external EM radiation
• light via fiber optics
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2010 4
Coax, Twisted Pair, fiber
Category 5 UTP
Category 3 UTP
Signal is difference in voltage
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2010 5
The Electromagnetic Spectrum
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2010 6
The Telephone Local Loop: Modems
The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the
modems and codecs.
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2010 7
Modems
Binary signal
Amplitude modulation
Frequency modulation
Phase modulation
Modern methods combine these modulation modesand use more amplitudes, frequencies and phasesto approach the Shannon limit
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2010 8
(Asymmetric)Digital Subscriber Lines
A typical (A)DSL equipment configuration.
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2010 9
ADSL frequency bands
Operation of ADSL using discrete multitone modulation.
gap, larger for ISDN
In each channel a “modem: of maximal 56 kbps,reduced automatically when S/N is too highADSL2+ goes upto 2,2 GHz
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2010 10
Internet over Cable
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2010 11
TV Cable Spectrum Allocation
Frequency allocation in a typical cable TV system used for Internet access
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2010 12
Wireless Local LoopsArchitecture of an LMDS (IEEE 802.16) system.
Superseded by ADSL and cable TV
WiMAX (IEEE 802.16) is more promising now
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2010 13
Frequency Division Multiplexing
(a) The original bandwidths.
(b) The bandwidths raised in frequency.
(b) The multiplexed channel.
With fibers:different wavelength of light
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2010 14
Time Division Multiplexing
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2010 15
CDMA – Code Division Multiple Access
Each sender has an unique code of m bits, called chips“1”: chip sequence is send“0”: complement of it is send
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2010 16
CDMA – Chip decoding
(a) Binary chip sequences(b) Bipolar chip sequences (c) Six transmissions(d) Recovery of C’s signal
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2010 17
The Mobile Telephone System
• First-Generation Mobile Phones: Analog Voice
• Second-Generation Mobile Phones: Digital Voice (GSM)
• Third-Generation Mobile Phones:Digital Voice and Data (UMTS)
• Fourth-Generation:based on LTE ?
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2010 18
Global System for Mobile Communications
GSM uses 2 * 124 frequency channels, each of which uses an eight-slot TDM system
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2010 19
GSM data framing
other framing:
• Control (base to mobile) to manage the system
• Paging (base to mobile) to alert users to calls for them
• Access (bidirectional) for call setup and channel assignment
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2010 20
Neighbouring cells
Different frequencies for neighbouring cells (fixed
sender / receiver)
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2010 21
Energy, environment
Prediction over 4 year:•1/3 of IT budget goes to energy bills•2/3 of that for cooling
How to dispose of 512 million old PC’s