CMPE 150 Fall 2005 Lecture 7

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

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 e-mail me if you need to meet. • Homework 1 due on Monday. • Homework 2 will be posted between today and Monday.

3. Reading Assignment • Tanenbaum Chapter 2.

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

5. Today • PHY (cont’d).

6. Public Switched Telephone System • Structure of the Telephone System. • The Local Loop: Modems, ADSL and Wireless. • Trunks and Multiplexing. • Switching.

7. Structure of the Telephone System • (a) Fully-interconnected network. • (b) Centralized switch. • (c) Two-level hierarchy.

8. Structure of the Telephone System (2) • A typical circuit route for a medium-distance call.

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

10. PSTN

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.

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.

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.

14. Modulation: Examples Binary signal Amplitude modulation Frequency modulation Phase modulation

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

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.

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.

18. Quadrature Modulation • (a) QPSK. • (b) QAM-16. • (c) QAM-64.

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

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?

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.

22. Digital Subscriber Lines • Bandwidth versus distanced over category 3 UTP for DSL.

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).

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).

25. Typical ADSL Setup • A typical ADSL equipment configuration.

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.

27. Wireless Local Loops • Architecture of an LMDS system. Tower with multiple highly directional antennae; but small range (2-5Km).