Transcript The Physical Layer

Chapter 2
The Physical Layer
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Bandwidth-Limited Signals
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Maximal Data Rate
Shannon –Hartley law (1948):
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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:
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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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Coax, Twisted Pair, fiber
Category 3 UTP
Category 5 UTP
Signal is difference in voltage
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The Electromagnetic Spectrum
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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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Modems
Binary signal
Amplitude
modulation
Frequency
modulation
Phase modulation
Modern methods combine these modulation modes
and use more amplitudes, frequencies and phases
to approach the Shannon limit
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(Asymmetric)Digital Subscriber Lines
A typical (A)DSL equipment configuration.
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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 high
ADSL2+ goes upto 2,2 GHz
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Internet over Cable
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TV Cable Spectrum Allocation
Frequency allocation in a typical cable TV
system used for Internet access
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Wireless Local Loops
Architecture 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Superseded by ADSL and cable TV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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Frequency Division Multiplexing
With fibers:
different wavelength of light
(a) The original bandwidths.
(b) The bandwidths raised in frequency.
(b) The multiplexed channel.
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Time Division Multiplexing
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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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CDMA – Chip decoding
(a) Binary chip sequences
(b) Bipolar chip sequences
(c) Six transmissions
(d) Recovery of C’s signal
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The Mobile Telephone System
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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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Global System for Mobile Communications
GSM uses 2 * 124 frequency channels, each
of which uses an eight-slot TDM system
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GSM data framing
other framing:
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Control (base to mobile) to manage the system
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Paging (base to mobile) to alert users to calls for them
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Access (bidirectional) for call setup and channel assignment
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Neighbouring cells
Different frequencies for
neighbouring cells (fixed
sender / receiver)
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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
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