1CMPE 150- Introduction to Computer Networks

CMPE 150

Fall 2005Lecture 7

Introduction to Networks and the Internet

2CMPE 150- Introduction to Computer Networks

Announcements• Labs:

– Tue 6-8pm.– W 4-6pm.– First lab is next week.

• TA office hours: T 6-7pm and Th 1-2pm in BME 314.• My office hours will be posted this weekend,• Please e-mail me if you need to meet.• Homework 1 due on Monday.• Homework 2 will be posted between today and

Monday.

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Reading Assignment• Tanenbaum Chapter 2.

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Last Class• PHY (Cont’d).– Guided media.– The PSTN.

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Today• PHY (cont’d).

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Public Switched Telephone System

• Structure of the Telephone System.• The Local Loop: Modems, ADSL and

Wireless.• Trunks and Multiplexing.• Switching.

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Structure of the Telephone System

• (a) Fully-interconnected network.• (b) Centralized switch.• (c) Two-level hierarchy.

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Structure of the Telephone System (2)

• A typical circuit route for a medium-distance call.

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Major Components of the Telephone System

• Local loops Analog twisted pairs going to houses and

businesses• Trunks

Digital fiber optics connecting the switching offices

• Switching offices Where calls are moved from one trunk to

another

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PSTN

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Local Loop• “Last mile”.• End office-subscriber connection.• Analog, twisted pair.• Traditionally, voice but it has been changing:

data transmission.• To transmit data, conversion digital to analog: modem.

• At phone office, data usually converted back to digital for long-distance transmission over trunks.

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Transmission Impairments• Problems that happen with signal as it propagates.• Attenuation: loss of energy as signal propagates.

– Different frequencies suffer different attenuation.– Different Fourier components attenuated by different

amount.• Distortion: different Fourier components shifted in

time.• Noise: unwanted energy from other sources.

– E.g., thermal noise: unavoidable random motion of electrons in wire.

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Modulation• Signal with wide range of frequencies is

undesirable.• Square waves exhibit wide frequency range.• To avoid that, AC signaling is used.– Sine wave “carrier” to carry information.

• Modulation:– Information is encoded in the carrier by

varying either amplitude, frequency, or phase.

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Modulation: Examples

Binary signal

Amplitude modulation

Frequencymodulation

Phasemodulation

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Modem• Modulator-demodulator.• Modulates digital signal at the source and

demodulates received signal at the destination.

• How to transmit faster? – Nyquist says that capacity is achieved at

2*H*log2V.– So there is no point sampling faster than 2*H.– But, can try to send more bits per sample.

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Baud Rate• Baud rate = symbols/sec.• Data rate = bits/sec.• If 2 voltage levels are used, then

– 1 symbol=1bit.– Baud rate = bit rate.

• But, if can encode more than 1 bit in a symbol…– E.g., if voltages 0, 1, 2, and 3, every symbol consists of

2 bits.– Thus, 2400 baud line corresponds to 4800 bps.– The same thing for 4 different frequencies: QPSK.

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Bandwidth, Baud- and Bit Rates• Bandwidth: physical property of medium.– Range of frequencies transmitted with

adequate quality.– Measured in Hz.

• Baud rate is number of samples/sec or symbols/sec.

• Modulation technique determines number of bits/symbol: symbols/sec * bits/symbol.

• Modern modems transmit several bits/symbol frequently combining multiple modulation schemes.

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Quadrature Modulation

• (a) QPSK.• (b) QAM-16.• (c) QAM-64.

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Trellis Coded Modulation

• (a) V.32 for 9600 bps.• (b) V32 bis for 14,400 bps.

(b) 128 points: 6 data and 1 parity32 points: 4data bits + 1 parity

Constellation Diagrams

TCM includes error detection!

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Full Duplex, Half Duplex, Simplex

• Full duplex: traffic in both directions simultaneously.

• Half duplex: traffic in both directions but 1 direction at a time.

• Simplex: traffic allowed only one way.• Examples?

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What’s next?• Modems were getting faster, e.g., 56Kbps.• But, demand for faster access was growing!• CATV and satellite as competitors.• Phone company’s response: DSL.– “Broadband” access.– ADSL: asymmetric digital subscriber line.– When you subscribe to DSL service, you are

connected to the local office without the filter to frequencies below 300Hz and above 3400Hz.

– Physical limitation still exists and depends on thickness, length, etc.

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Digital Subscriber Lines• Bandwidth versus distanced over category 3

UTP for DSL.

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Digital Subscriber Lines (2)• Operation of ADSL using discrete multitone

modulation.

Available 1.1MHz local loop spectrum divided into 256 channels (4.3KHz each).

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ADSL• Typically, 32 channels for upstream and the

rest for downstream traffic.• Usually, 512 Kbps downstream and 64 Kbps

upstream (standard) and 1 Mbps downstream and 256 Kbps upstream (premium).

• Within each channel, modulation scheme is used (sampling at 4000 baud).

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Typical ADSL Setup• A typical ADSL equipment configuration.

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Wireless Local Loop• Last mile is wireless.• Why?• Historically: local telcos had monopoly for

local telephone service.– In the mid 1990’s market open to competition,

e.g., long distance carriers.– Cheaper alternative to stringing cables to

customers is using a wireless local loop.• Mobile telephony?• “Fixed” wireless.

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Wireless Local Loops

• Architecture of an LMDS system.

Tower with multiple highly directionalantennae; but small range (2-5Km).