FUNDAMENTAL CONCEPTS

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FUNDAMENTAL CONCEPTS

• Analog versus Digital
• Bandwidth (or channel capacity)
• Switched vs. leased lines
• Transmission Media
• Transmission mode (parallel/serial,
  synchronous/asynchronous
• Transmission method (simplex, half-duplex,
  full-duplex)

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Analog Channels
Digital versus Analog

• Digital channels are better.
• Increasingly the phone network is becoming
  digital.
• More than 90 lines in the US telephone network
  are digital now (e.g., Sprint is 100 digital).

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Bandwidth of channel
Digital Channel
B.W. (or channel capacity) is defined in
Bits/sec. that can
flow through it. (Also called DATA RATE.)
Analog Channel
B.W. defined in terms of cycles/sec. (or Hertz)
e.g. B.W. of
telephone line to carry speech is approx. 3000 Hz.
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Note
1)
Highest frequency component in speech is about
4000 Hz.
2)
Bandwidth required to transmit an analog signal
across
an analog channel is determined by the highest
frequency component in the signal.
3)
Rapid changes in signal level correspond to
high frequency.
gradual changes in signal level correspond
to low frequency.

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Electromagnetic Spectrum
Source Tannenbaum, Computer Networks, 3rd Ed.,
1996
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Bandwidth
Human ear 20 Hz to 20,000 Hz. FM radio 50 Hz to
15 KHz. Human voice - 100 Hz to 4000
Hz. Telephone 300 Hz. To 3 KHz. Color television
6 MHz.
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Properties of Analog Signals

• Amplitude (volume) is the height of a wave
• Frequency (pitch) is measured in cycles per
  second
• Phase corresponds to the waves relative position
• Bandwidth is the difference between the lowest
  and highest
• frequency (measured in cycles per second or
  Hz.)
• In general, voice contains many different
  frequencies.

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Kai Larsen When does switching make sense? When
does leasing make sense?
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Transmission Media - Copper wire
STP - shielded twisted pair UTP - unshielded
twisted pair
- Pairs of wire from home to local telephone
company office
- Limited bandwidth
- Deteriorates at high frequency
- Ubiquitous on the local loop. - Wires are
twisted together in helical pattern to minimize
noise and interference.
Kai Larsen Try twisting the power-cord on your
clock-radio, check receptions at different levels
of twisting!
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Transmission media - Twisted Pair Copper

• Bandwidth related to distance and thickness but
  Mbps are possible over small distance.
• Two Types commonly used in networks are Category
  3 and Category 5 (UTP).
• Most buildings wired before 1988 are Cat 3 (2 or
  4 pairs).
• After 1988 generally use Cat 5 (more twists per
  inch, less cross-talk).
• Cat 5E

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Transmission media - Coaxial Cable
Two types 1) Baseband. 2) Broadband.
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Transmission media - Coaxial

• Baseband
• Single digital channel.
• Shielding provides high bandwidth and good noise
  immunity.
• 2 Gbps on a 1 Kilometer cable.
• Used for original Ethernet.
• Broadband
• Divides bandwidth into multiple channels
• Can transmit 300 MHz for long distances (100 km)
• Uses TV (6 MHz per channel) CD audio data.

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Satellitessuccess and failure
Kai Larsen Two Norwegians who started skiing
towards the North Pole, 255 km into their 1,741
km long journey relying only on their Iridium
satellite telephone receives the final call
Iridium was going downMotorola, however,
decided to provided mission critical serviceJune
3rd 2000 they reached the pole. Ever since, the
pole has been without telephone coverage
The Iridium story Rune Gjeldnes and Torry
Larsen attempting to be the first explorers to
reach the North Pole completely un- supported.
Start weight of sledges 175 kilos, each 1741
km long journeyNine expeditions had tried
beforeall failed
Source http//www.geom.umn.edu/worfolk/SaVi/inte
ractive.html
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Transmission media - Wireless Satellite

-
Transmit signal to satellite 22,230 miles in
space (GEOs)
-
0.5 - 0.6 sec. delay in transmission.
-
Various bands between 1 and 40 GHz are used for
satellite transmission.
-
600 MHz band at 12 GHz gt 200,000 telephone
channels or 100 TV channels. Other types
of satellites LEO, MEO, HALE.
Kai LarsenDid you know that GEO -gt
Geo-stationary orbitMEO -gt Medium Earth
OrbitLEO -gt Low Earth OrbitHALE -gt High
Altitude Long Endurance
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More on Satellites

• Various bands between 1 and 40 GHz are used for
  satellite transmission (C-, Ku-, Ka- frequency
  bands most common).
• L-band (1 to 2 GHz) mobile satellite services,
  cell phones C-band (4 to 8 Ghz) fixed satellite
  services, microwave Ku-band and K-band broadcast
  and fixed services (12.5 to 26.5 GHz)

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Transmission media - Microwave

• No physical medium - i.e., wireless.
• Based on radio waves above 100 MHz.
• Signal is beamed from transmission tower.
• Microwaves travel in straight lines
• Usually place 100m towers 50 miles apart.
• Used for long distance telephone
• Subject to weather interference (e.g., rain)

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Transmission media - Fiber Optics
Light
Phone

Coder
Source






Decoder
Regenerator
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Transmission media - Fiber Optics
Plastic Sheathing
Cladding
Glass Core
Multi-mode Fiber (50 micron core/125 micron
cladding)
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Transmission media - Fiber Optics
Single mode Fiber (8/125 microns)
Plastic Sheathing
Cladding
Glass Core
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Transmission media - Fiber Optics

• Refraction property prevents loss of light and
  allows the signal to travel long distances.
• Multi-mode fiber - 50 micron core. Multiple paths
  for a given light ray.
• Single-mode fiber - 8 micron core. Single path.
• Multi-mode is cheaper and more common and gives
  about 1 Gbps at 5 to 10 miles.

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Transmission media - Fiber Optics

• Single-mode is more expensive and gives up to 2
  Gbps to about 20 miles.
• Attenuation depends on wavelength of light and
  there are three wavelengths that are more popular
  (.85, 1.3 and 1.55 micron - most devices use
  first or second).
• Light disperses (signal smears out) especially in
  multi-mode since individual rays take paths of
  different lengths.

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Transmission Modes
I. Mode of Transmission
Serial - bit at a time
or
Parallel - Byte at a time
Serial is the most common mode of transmission.
II.

Types of serial transmissions.

1. Synchronous

Synchronization character synchronizes the two
ends - then, entire message is sent
8-bit flag
8-bit flag
Control fields
Control fields
Data field
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Asynchronous

Send separate start and stop bits with each
character
S
S
S
S
T
8-BIT
T
T
8-BIT
T
A
CHARACTER
A
BLANK
O
CHARACTER
O
R
R
P
P
T
T
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Kai Larsen Think about how the class situation
works between professor and students
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CHANNEL AND SIGNAL TYPE COMBINATIONS
CHANNEL
TYPE
Digital
Analog
Send with
Convert
slight
signal to
Digital
modification
analog form -
Analog
SIGNAL
Modulation
(modem)
TYPE
Digitize
signal
Analog
(Pulse Code
Send as is
Modulation)
Modulating a signal means to adapt it so that it
can
be transmitted over the type of transmission
media
in use.
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Cable Selection Criteria

• Bandwidth How fast must the network be?
• Budget How much money can you spend on cabling?
• Capacity How much traffic must the network
  carry? How will the traffic flow?
• Environmental considerations How noisy is the
  deployment environment? How important is data
  security?
• Placement Where will the cables run? How tight
  are the spaces?
• Scope How many devices must be connected to the
  network?
• Span What kind of distance does the network need
  to span?

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Assignment

• Please find a partner or two (or three)
• Your task
• Select the home of one group member
• Design a home network around the seven cable
  selection criteria
• Write up your solution on a transparency
• Select one person to present solution
• Time?