TUe Computer Science, System Architecture and Networking

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2IC15 Computer Networks
Physical layer
Igor Radovanovic
Thanks to B. A. Forouzan A. Tanenbaum
Igor Radovanovic, i.radovanovic_at_tue.nl
09/10/2009
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09/10/2009
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TU/e Computer Science, System Architecture and
Networking
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Physical layer
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Analog versus digital signals
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Fourier Coefficients
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Fourier transformation
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Fourier transformation (cntd)
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Bandwidth
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Signal impairments
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Signal distortion
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Digital signal
QUESTION What is the bandwidth of this periodic
signal?
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Throughput
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Propagation time
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Wavelength
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Transmission media
Magnetic tapes?
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Twisted pair
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Twisted pair (cntd)- bandwidth issue -
(a) Category 3 UTP (b) Category 5 UTP
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Using twisted pair cables in a network
Cat 3
Cat 3 or Cat 5
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Using twisted pair cables in a network (cntd)
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Twisted pair networks -example -

• ADSL
• Ethernet networks
• - 10BASE-T
• - 100BASE-TX
• - 1000BASE-T
• - 1000BASE-TX (Cat5e (enhanced))

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Twisted pair - pros and cons -

• Pros
• easy to understand
• mass production - low cost
• most widely used medium
• Cons
• prone to electromagnetic interference
• in power plants, airport buildings, military
  facilities, cars
• Note
• In-building networks at our university are almost
  all twisted pair

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Transmission media
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Coax cable
Longer distances, higher bit-rates
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Coax cable network - example -
10Base2 Ethernet
Note Almost obsolete
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Coax cable network - example 2-
cable modem
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Transmission media
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Optical fiber fundamentals
Bending the light ray
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Optical fiber fundamentals
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Optical fiber fundamentals (cntd)
50 microns
5-10 microns
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Optical fiber fundamentals (cntd)
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Optical fiber network - example 1-
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Optical fiber network-example 2-
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Optical fiber - pros and cons -

• Pros
• Low attenuation
• Large bandwidth
• Cons
• Relatively new technology
• Expensive

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Comparing optical fiber to UTP

• Pros
• Immune to electro-magnetic interference
• no crosstalk
• Reduced need for error detection and correction
• Enables longer link distances
• Attenuation unaffected by transmission rate
• Easier network upgrade
• Can combine different services telephony, TV,
  internet
• Cons
• Optical components have higher cost
• Expensive deploying protocols

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Comparing coax to UTP
coax cable performance
UTP performance
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(No Transcript)
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Wireless

• Modern wireless digital communication began in
  the Hawaiian Islands
• What is the best frequency to use for
  communication?

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ISM bands in US
IEEE 802.11b (11Mb), (22Mb) IEEE 802.11g (54 Mb)
Bluetooth
IEEE 802.11a (100 Mbps)

• Industrial Scientific and Medical band (ISM)
• Government regulated
• Transmission power is limited so as to limit
  interference

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Bandwidth delivery capability by technology
http//www.pafiber.net/ftth/comm-benefits
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Multiplexing
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Frequency Division Multiplexing
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Wavelength Division Multiplexing
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Time Division Multiplexing
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Inverse Time Division Multiplexing
Example High-speed Ethernet networks
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TDM network-example-

• T1
• high-speed digital network (1.544 Mbps) developed
  by ATT in 1957 and implemented in the early
  1960's
• supports long-haul voice transmission
• digitally representing analog telephone system

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TDM/FDM network-example-
GSM phones
uplink
downlink
890MHz
915MHz
935MHz
960MHz
0
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0
124
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Code Division Multiplexing
Direct Sequence Spread Spectrum
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Code Division Multiple Access

• Patent holder
• Hedy Lamarr

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Comparing different techniques
frequency
frequency
TDM
CDM
FDM

• TDM 802.11 CSMA/CA
• TDM/FDM GSM
• CDM UMTS

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Internet _at_ home

• Dial-up
• ADLS
• Cable modem
• Fiber-to-the-Home?

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Modems

• (a) A binary signal
• (b) Amplitude modulation

• (c) Frequency modulation
• (d) Phase modulation

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Bit encoding (1)

• Asynchronous data transmission

• Used for
• character oriented devices
• large indeterminate intervals between characters
• receiver resynchronizes with sender on start and
  stop bits
• polarity of stop bit different from polarity of
  start bit

1 0 0 1 0 0 1 0
Start bit
Stop bit(s)
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Bit encoding (2)

• (a) Binary encoding, (b) Manchester encoding,
  (c) Differential Manchester encoding.

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Modems (2)

• (a) QPSK.
• (b) QAM-16.
• (c) QAM-64.

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ADSL

• TDM/FDM
• Asymmetric higher bit-rate for downloading
• Use local loops (1.1 Mbps)
• Adapted data rate based on the conditions on the
  line

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ADSL (cntd)
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ADSL protocol stack
Link layer
handling timing sensitive transmission of video
and audio services  
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Cable modem
Hybrid Coax-Fiber system
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Cable modem (cntd)

• Theoretical downstream data rate 30 Mbps,
  upstream - 12 Mbps

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Cable modem (cntd)
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Fiber-to-the-Home

• TDM in combination with WDM

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Fiber-to-the-Home (cntd)

• Passive fiber networks
• Easy to upgrade
• No electrical powering
• Immune to lightening, EMI
• What will happen to FTTH?
• Cost?
• Lifetime of telephone lines?

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Conclusions

• Physical layer is the basis of all networks
• limitations attenuation and bandwidth
• Transmission media guided unguided
• Multiplexing techniques introduced to increase
  bandwidth
• TDM FDM (WDM) CDM
• Broadband access ADSL Cable modem FTTH?