1 CMPE 150- Introduction to Computer Networks CMPE 150 Fall 2005 Lecture 7 Introduction to Networks and the Internet
Dec 20, 2015
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).