chapter 3

Physical Layer

CHAPTER 3

OutlineRecap2. Application Layer

2.1 Hardware2.2 Application Architecture (Software)2.3 Web2.4 Email

Outline3. Physical Layer

3.1 Circuits3.2 Media3.3 Digital Transmission (Digital Data)3.4 Analog Transmission (Digital Data)3.5 Digital Transmission (Analog Data)

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Network Layers

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Computer 1 Computer 2

Acronyms• FDM – Frequency Division Multiplexing

• TDM – Time Division Multiplexing

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3 Physical Layer - Overview

Includes network hardware and circuits

Types of Circuits Physical circuits connect devices & include actual

wires

Logical circuits refer to the transmission characteristics of the circuit

Physical and logical circuits may be the same or different. For example, in multiplexing, one physical wire may carry several logical circuits.

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Physical Layer

Network Layer

Data Link Layer

3.1.2 Circuit Configurations

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Basic physical layout of the circuit

Configuration types:

Point-to-Point Configuration

Multipoint Configuration

3.1.2.1 Point-to-Point Configuration

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3.1.2.2 Multipoint Configuration

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3.1.2 Data Flow (Transmission)

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How does data flow through the circuit

Configuration types:

Simplex

Half-Duplex

Full-Duplex

3.1.2 Data Flow (Transmission)

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3.1.3 Data Flow (Transmission) – Multiplexing

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Combines many low speed circuits into one high speed transmission

Categories of multiplexing

3.1.3 Frequency Division MultiplexingMakes a number of smaller channels from a larger frequency band by dividing the circuit “horizontally”

circuit

FDMFDM

Four terminals

Host computer

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3.1.3 Time Division Multiplexing

Dividing the circuit “vertically”

• TDM allows terminals to send data by taking turns

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3.1.4 Inverse Multiplexing

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Combines a number of low speed circuits to create a single high speed circuit on the opposite ends

Why would companies choose to do this?

Inverse Multiplexing (IMUX)

Shares the load by sending data over two or more lines

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Digital Subscriber Line (DSL)

Became popular as a way to increase data rates in the local loop.

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3.2 Media

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Physical matter that carries the transmission

Types:

• Guided Media

• Radiated (Unguided) Media

3.2.1.1 Guided: Twisted Pair (TP) WiresCommonly used for telephones and LANsReduced electromagnetic interferenceTP cables have a number of pairs of wires

Price:

Speed:

Distance:

Common Use:

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3.2.1.1 Guided: Twisted Pair (TP) Wires (CAT5e)

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3.2.1.1 Guided: Comparison of Cables

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3.2.1.2 Guided: Coaxial Cable• Less prone to interference than TP due to shield • More expensive than TP, thus quickly disappearing

Price:

Speed:

Distance:

Common Use:

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3.2.1.2 Guided: Problems with Copper

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3.2.1.3 Guided: Fiber Optic Cable• Light created by an LED (light-emitting diode) or

laser is sent down a thin glass or plastic fiber• Has extremely high capacity, ideal for broadband• Works well under harsh environments

Price:

Speed:

Distance:

Common Use:

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3.2.1.3 Guided: Fiber Optic CableFiber optic cable structure (from center):

Core (v. small, 5-50 microns, ~ the size of a single hair) Cladding, which reflects the signal Protective outer jacket

How they are made: http://www.youtube.com/watch?v=llI8Mf_faVoCommunication: http://www.ehow.com/video_4951202_optical-fiber-work_.html

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Types of Optical FiberMultimode (about 50 micron core)

Earliest fiber-optic systems Signal spreads out over short distances (up to ~500m) Inexpensive

Graded index multimode Reduces the spreading problem by changing the

refractive properties of the fiber to refocus the signal Can be used over distances of up to about 1000 meters

Single mode (about 5 micron core) Transmits a single direct beam through the cable Signal can be sent over many miles without spreading Expensive (requires lasers; difficult to manufacture)

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Optical Fiber

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3.2.1.3 Guided: Which is faster – Fiber or Copper?Fiber transmits via light – does that mean it is faster

than copper b/c it travels at the speed of light?

Data Carrying Capacity

What should companies use?

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3.2.2.1 Wireless (Unguided) – WLAN (Radio)

Wireless transmission of electrical waves through air

Each device has a radio transceiver with a specific frequency

Includes

Speed:Distance

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3.2.2.2 Wireless Media - MicrowaveHigh frequency form of radio communications

Performs same functions as cables

Speed:Distance:

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3.2.2.3 Wireless Media - SatelliteSpecial form of microwave communications

Signals travel at speed of light, yet long propagation delay due to great distance between ground station and satellite

Speed:

Distance:

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3.2 Factors Used in Media SelectionType of network

Cost

Transmission distance

Security

Error rates

Transmission speeds

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