CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 7 Introduction to Networks and the Internet

CMPE 150- Introduction to Computer Networks 2 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 me if you need to meet. Homework 1 due on Monday. Homework 2 will be posted between today and Monday.

CMPE 150- Introduction to Computer Networks 3 Reading Assignment Tanenbaum Chapter 2.

CMPE 150- Introduction to Computer Networks 4 Last Class PHY (Cont’d). –Guided media. –The PSTN.

CMPE 150- Introduction to Computer Networks 5 Today PHY (cont’d).

CMPE 150- Introduction to Computer Networks 6 Public Switched Telephone System Structure of the Telephone System. The Local Loop: Modems, ADSL and Wireless. Trunks and Multiplexing. Switching.

CMPE 150- Introduction to Computer Networks 7 Structure of the Telephone System (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

CMPE 150- Introduction to Computer Networks 8 Structure of the Telephone System (2) A typical circuit route for a medium-distance call.

CMPE 150- Introduction to Computer Networks 9 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

CMPE 150- Introduction to Computer Networks 10 PSTN

CMPE 150- Introduction to Computer Networks 11 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.

CMPE 150- Introduction to Computer Networks 12 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.

CMPE 150- Introduction to Computer Networks 13 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.

CMPE 150- Introduction to Computer Networks 14 Modulation: Examples Binary signal Amplitude modulation Frequency modulation Phase modulation

CMPE 150- Introduction to Computer Networks 15 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*log 2 V. –So there is no point sampling faster than 2*H. –But, can try to send more bits per sample.

CMPE 150- Introduction to Computer Networks 16 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.

CMPE 150- Introduction to Computer Networks 17 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.

CMPE 150- Introduction to Computer Networks 18 Quadrature Modulation (a) QPSK. (b) QAM-16. (c) QAM-64.

CMPE 150- Introduction to Computer Networks 19 Trellis Coded Modulation (a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps. (b) 128 points: 6 data and 1 parity 32 points: 4data bits + 1 parity Constellation Diagrams TCM includes error detection!

CMPE 150- Introduction to Computer Networks 20 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?

CMPE 150- Introduction to Computer Networks 21 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.

CMPE 150- Introduction to Computer Networks 22 Digital Subscriber Lines Bandwidth versus distanced over category 3 UTP for DSL.

CMPE 150- Introduction to Computer Networks 23 Digital Subscriber Lines (2) Operation of ADSL using discrete multitone modulation. Available 1.1MHz local loop spectrum divided into 256 channels (4.3KHz each).

CMPE 150- Introduction to Computer Networks 24 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).

CMPE 150- Introduction to Computer Networks 25 Typical ADSL Setup A typical ADSL equipment configuration.

CMPE 150- Introduction to Computer Networks 26 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.

CMPE 150- Introduction to Computer Networks 27 Wireless Local Loops Architecture of an LMDS system. Tower with multiple highly directional antennae; but small range (2-5Km).