Computer Networking for Engineers: Understanding the Telephone Network, Modems, and DSL - , Study notes of Engineering

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CSE394F

Computer Networking for EngineersComputer Networking for Engineers

Lecture

Jeffrey Miller, Ph.D.

Outline

• Chapter 2.5-2.8•^

All images with white backgrounds are from Andrew

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Tanenbaum’s Computer Networks, 4

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Edition, Prentice

Hall, 2003.

Structure of Telephone Network

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Local Loops–

Analog twisted pairs going into buildings

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Trunks–

Digital fiber optics connecting switching offices

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Switching Offices–

Calls are moved from one trunk to another

The Local Loop

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The local loop is sometimes called “the last mile”even though it can be more than a mile long– This is the connection between a computer and the end

This is the connection between a computer and the endoffice (or central office)

Modems

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The square waves (DC) used in digital signals have a wide frequency spectrum, subjecting them toattenuation and delay distortion

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With telephone lines, AC signaling is used–

A continuous tone between 1000-2000Hz is used with different types of modulation to represent the

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binary values

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The number of samples per second is measured in baud – an n baud line transmits n symbols/second^ Binary SignalBinary

Signal

Amplitude Modulation Frequency Modulation

Phase Modulation

Digital Subscriber Lines

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DSL is the phone company’s way of offering digital service over thelocal loop

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The capacity of the local loop depends on length, thickness, andThe capacity of the local loop depends on length, thickness, andgeneral quality

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The following graph shows the bandwidth versus distance over a Cat-3UTP for DSLUTP for DSL

Multiplexing

• As a cost savings mechanism,

communication companies would like to

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transmit multiple conversations over thesame line without collisions

• Multiplexing allows multiple conversations

over a single physical trunkover a single physical trunk

Frequency Division Multiplexing•^

Frequency Division Multiplexing (FDM) raises thefrequency of each channel by a different amount, whichallows them to be combined since no two channels occupyallows them to be combined since no two channels occupythe same portion of the spectrum

Time Division Multiplexing

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Time Division Multiplexing (TDM) allocates a smallpercentage of each channel to different conversationsTh

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TDM

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The following figure shows how a T1 carrier uses TDM

Switching

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Circuit switching is used primarily with voice conversations–

When a voice conversation is initiated, the switching equipment within thetelephone system needs to find a physical path from your telephone to thereceiver’s telephone

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Message switching is used primarily with data–

When the sender has a block of data to send, it is stored in the firstswitching office and forwarded one hop at a time (called a store-and-forward network)

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Message switching has no limit to the size of the data though, so lines canb^

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be tied up for long periods of time

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Packet switching is used primarily with data–

Packet switched networks have an upper limit on the block size, allowingpackets to be buffered in router main memory rather than on disk at thepackets to be buffered in router main memory rather than on disk at therouter

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This allows the handling of interactive traffic

Message Switching

Mobile Telephone System

• Mobile phones have gone through three

generationsg – Analog voice– Digital voice

Digital voice

– Digital voice and data

Analog Phones (cont.)

Homework

• Chapter 2 homework is posted!