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In the Name of God Computer Networks Chapter 2: The Physical Layer. Dr. Shahriar Bijani Shahed University Feb. 2014. Main Reference: A . S. Tanenbaum and D. J. Wetherall , Computer Networks (5th Edition ), Pearson Education, the book slides, 2011. Outline.

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in the name of god computer networks chapter 2 the physical layer

In the Name of GodComputer NetworksChapter 2: The Physical Layer

Dr. ShahriarBijani

Shahed University

Feb. 2014


Main Reference:

A. S. Tanenbaum and D. J. Wetherall, Computer Networks (5th Edition), Pearson Education, the book slides, 2011.

  • The Maximum Data Rate of a Channel (sec 2.1.3)
  • Guided Transmission Media (sec 2.2)
  • Wireless Transmission (sec 2.3)
  • Digital Modulation and Multiplexing (sec 2.5.3-2.5.5)
  • The Mobile Phone System (sec 2.7)
the maximum data rate of a channel
The Maximum Data Rate of a Channel
  • Nyquist’stheorem for a noiseless channel:
    • even a perfect channel has a finite transmission capacity.

Maximum data rate = 2 B log2 V bits/sec

V discrete levels: e.g for binary signals V = 2

  • Shannon’s formula for capacity of a noisy channel
guided transmission media
Guided Transmission Media
  • Magnetic media
  • Twisted pairs
  • Coaxial cable
  • Power lines
  • Fiber optics
1 magnetic media
1. Magnetic Media
  • Write data onto magnetic media
    • Disks (e.g. recordable DVDs)
    • Tapes
  • Data transmission speed
2 twisted pairs
2. Twisted Pairs
  • One of the oldest and most common transmission media.
  • Consists of two twisted insulated copper wires (~1 mm thick).
  • like a DNA molecule!

Category 5 UTP (Unshielded Twisted Pair)cable with four twisted pairs

3 coaxial cable
3. Coaxial Cable

A coaxial cable

  • Better shielding and greater bandwidth than UTP
  • It can span longer distances at higher speed
4 power lines
4. Power Lines
  • Power lines deliver electrical power to houses
  • Despite the difficulties, it is practical to send at least 100 Mbps over typical household electrical wiring

A network that uses household electrical wiring.

5 fiber optics 1
5. Fiber Optics (1)
  • 3 key components of an optical transmission system :
    • the light source,
    • the transmission medium,
    • the detector.

Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.

5 fiber optics 2
5. Fiber Optics (2)

Light trapped by total internal reflection.

transmission of light through fiber
Transmission of Light Through Fiber

Attenuation of light through fiber in the infrared region

fiber cables 1
Fiber Cables (1)

Views of a fiber cable

fiber cables 2
Fiber Cables (2)

A comparison of semiconductor diodesand LEDs as light sources

comparison of fiber copper
Comparison of Fiber & Copper

Advantages of Fiber Optics:

  • Handle much higher bandwidths than copper.
  • Repeaters are needed only about every 50 km (because of the low attenuation), versus about every 5 km for copper.
  • Not being affected by power flows, electromagnetic interference, or power failures.
  • Not affected by acidic chemicals in the air.
  • Thin and lightweight: Many existing cable ducts are completely full, so there is no room to add new capacity.
  • For new routes: much lower installation cost.
  • Do not leak light and are difficult to tap.
comparison of fiber copper 2
Comparison of Fiber & Copper (2)

Disadvantages of Fiber Optics:

  • A less familiar technology requiring skills not all engineers have
  • Can be damaged easily by being bent too much.
  • Since optical transmission is inherently unidirectional, two-way communication requires either two fibers or two frequency bands on one fiber.
  • Cost more than electrical devices.
wireless transmission
Wireless Transmission
  • The Electromagnetic Spectrum
  • Radio Transmission
  • Microwave Transmission
  • Infrared Transmission
  • Light Transmission
the electromagnetic spectrum 1
The Electromagnetic Spectrum (1)

The electromagnetic spectrum and its uses for communication

the electromagnetic spectrum 2
The Electromagnetic Spectrum (2)

Spread spectrum and ultra-wideband (UWB) communication

radio transmission 1
Radio Transmission (1)

In the VLF, LF, and MF bands, radio waves follow the curvature of the earth

radio transmission 2
Radio Transmission (2)

In the HF band, they bounce off the ionosphere.

microwave transmission
Microwave Transmission
  • Above 100 MHz, the waves travel in nearly straight lines, so it can be narrowly focused.
  • parabolic antennagives a much higher signalto-noise ratio.
  • The transmitting and receiving antennas must be accurately aligned with each other.
  • Multiple transmitters lined up in a row to communicate with multiple receivers in a row without interference
  • Repeaters are needed periodically
  • The heart of the long-distance telephone transmission system before fiber optics.
infrared transmission
Infrared Transmission
  • Widely used for short-range communication
  • Relatively directional, cheap, and easy to build but have a
  • Disadvantage: do not pass through solid objects.
light transmission
Light Transmission
  • Optical signaling using lasers is inherently unidirectional.
  • Very high bandwidth at very low cost
  • Relatively secure because it is difficult to tap a narrow laser beam.
  • Disadvantage: heat from the sun during the daytime caused convection currents interfere with laser communication systems.
digital modulation and multiplexing
Digital Modulation and Multiplexing
  • Modulation
    • Baseband Transmission*
    • Passband Transmission*
  • Frequency Division Multiplexing
  • Time Division Multiplexing
  • Code Division Multiplexing
  • Digital modulation: the process of converting between bits and signalsthat represent them.
  • Multiplexing: sharing one channel by Channels are often shared by multiple signals
    • Different methods: time, frequency, and code division multiplexing.
baseband transmission
Baseband Transmission
  • The most straightforward form of digital modulation
  • NRZ (Non-Return-to-Zero): a positive voltage represents a 1 and a negative voltage represents a 0.
passband transmission
Passband Transmission
  • The amplitude, phase, or frequencyof a carrier signal to convey bits.
  • Combining these methods and use more levels to transmit more bits per symbol.
  • Usually, amplitude and phase are modulated in combination.
constellation diagram
Constellation Diagram
  • The phase of a dot: the angle with the positive x-axis.
  • The amplitude of a dot is the distancefrom the origin.
  • QPSK: Quadrature Phase Shift Keying
  • QAM: Quadrature Amplitude Modulation
frequency division multiplexing 2
Frequency Division Multiplexing (2)

Frequency division multiplexing.(a) The original bandwidths. (b) The bandwidths raised in frequency. (c) The multiplexed channel.

frequency division multiplexing 3
Frequency Division Multiplexing (3)

Orthogonal frequency division multiplexing (OFDM).

time division multiplexing
Time Division Multiplexing

Time Division Multiplexing (TDM).

multi access radio techniques
Multi-Access Radio Techniques

Courtesy of Petri Possi, UMTS World

code division multiplexing 1
Code Division Multiplexing (1)

Also known as CDMA (Code Division Multiple Access)



Courtesy of Suresh Goyal & Rich Howard



Courtesy of Suresh Goyal & Rich Howard



Courtesy of Suresh Goyal & Rich Howard



Courtesy of Suresh Goyal & Rich Howard

code division multiplexing 11
Code Division Multiplexing (1)
  • Chip sequences for four stations.
  • Signals the sequences represent
code division multiplexing 2
Code Division Multiplexing (2)
  • Six examples of transmissions.
  • Recovery of station C’s