In the Name of God Computer Networks Chapter 2: The Physical Layer

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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

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### 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.

Outline
• 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
• 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
• Magnetic media
• Twisted pairs
• Coaxial cable
• Power lines
• Fiber optics
1. Magnetic Media
• Write data onto magnetic media
• Disks (e.g. recordable DVDs)
• Tapes
• Data transmission speed
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

A coaxial cable

• Better shielding and greater bandwidth than UTP
• It can span longer distances at higher speed
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)
• 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)

Light trapped by total internal reflection.

Transmission of Light Through Fiber

Attenuation of light through fiber in the infrared region

Fiber Cables (1)

Views of a fiber cable

Fiber Cables (2)

A comparison of semiconductor diodesand LEDs as light sources

Comparison of Fiber & Copper

• 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)

• 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
• The Electromagnetic Spectrum
• Microwave Transmission
• Infrared Transmission
• Light Transmission
The Electromagnetic Spectrum (1)

The electromagnetic spectrum and its uses for communication

The Electromagnetic Spectrum (2)

Spread spectrum and ultra-wideband (UWB) communication

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

In the HF band, they bounce off the ionosphere.

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
• 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
• 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
• Modulation
• Baseband Transmission*
• Passband Transmission*
• Frequency Division Multiplexing
• Time Division Multiplexing
• Code Division Multiplexing
Definitions
• 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
• 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
• 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
• 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
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)

Orthogonal frequency division multiplexing (OFDM).

Time Division Multiplexing

Time Division Multiplexing (TDM).

Courtesy of Petri Possi, UMTS World

Code Division Multiplexing (1)

Also known as CDMA (Code Division Multiple Access)

CDMA

Courtesy of Suresh Goyal & Rich Howard

CDMA

Courtesy of Suresh Goyal & Rich Howard

CDMA

Courtesy of Suresh Goyal & Rich Howard

CDMA

Courtesy of Suresh Goyal & Rich Howard

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