chapter-2-1 Physical layerof updates.pptx

it show d/f layers of interner transmission

1. The Physical Layer Chapter 2 Theoretical Basis for Data Communications Guided Transmission Media Wireless Transmission Communication Satellites Digital Modulation and Multiplexing Public Switched Telephone Network Mobile Telephone System Cable Television CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Revised: February 2018

2. The Physical Layer Foundation on which other layers build Properties of wires, fiber, wireless limit what the network can do Key problem is to send (digital) bits using only (analog) signals This is called modulation Application Transport Network Link Physical CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

3. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Physical Layer Issues Media: wires, fiber, satellites, radio Signal propagation: bandwidth, attenuation, noise Modulation: how bits are represented as voltage signals Fundamental limits: Nyquist, Shannon

4. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Abstract Model of a Link Bit rate: bits/sec depends on the channel’s bandwidth Delay: how long does it take a bit to get to the end? Error rate: what is the probability of a bit flipping? Sender Channel: bit rate, delay, error rate Receiver

5. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Bandwidth-Delay Product Bits have a physical size on the channel! Storage capacity of a channel is: bit rate x delay Example: 100 Mbps 5000-km fiber, delay = 50 msec In 50 msec we can pump out 5 million bits So the fiber can store 5 million bits in 5000 km 1 km holds 1000 bits so a bit is 1 meter long At 200 Mbps, a bit is 0.5 m long

6. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Signal Propagation over a Wire The signal has a finite propagation speed (2/3 c) The signal is attenuated per km Frequencies above a cutoff are strongly reduced Noise is added to the signal

7. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Digital Modulation and Multiplexing Modulation schemes send bits as signals Multiplexing schemes share a channel among users. Baseband Transmission Passband Transmission Frequency Division Multiplexing Time Division Multiplexing Code Division Multiple Access

8. Baseband Transmission CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Line codes send symbols that represent one or more bits NRZ is the simplest, literal line code (+1V=“1”, -1V=“0”) Other codes tradeoff bandwidth and signal transitions Four different line codes

9. Clock Recovery To decode the symbols, signals need sufficient transitions Otherwise long runs of 0s (or 1s) are confusing, e.g.: 1 CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 0 0 0 0 0 0 0 0 0 0 um, 0? er, 0? Data Code Data Code Data Code Data Code 0000 11110 0100 01010 1000 10010 1100 11010 0001 01001 0101 01011 1001 10011 1101 11011 0010 10100 0110 01110 1010 10110 1110 11100 00 1 1 10101 0111 01111 1011 10111 1111 11101 Strategies: Manchester coding, mixes clock signal in every symbol 4B/5B maps 4 data bits to 5 coded bits with 1s and 0s: Scrambler XORs tx/rx data with pseudorandom bits

10. Passband Transmission (1) Modulating the amplitude, frequency/phase of a carrier signal sends bits in a (non-zero) frequency range NRZ signal of bits Amplitude shift keying Frequency shift keying Phase shift keying CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

11. BPSK QPSK 4 symbols 2 symbols 1 bit/symbol 2 bits/symbol QAM-16 16 symbols 4 bits/symbol Q AM-64 64 symbols 6 bits/symbol QAM varies amplitude and phase CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 BPSK/QPSK varies only phase Passband Transmission (2) Constellation diagrams are a shorthand to capture the amplitude and phase modulations of symbols:

12. Frequency Division Multiplexing FDM (Frequency Division Multiplexing) shares the channel by placing users on different frequencies: Overall FDM channel CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

13. Time Division Multiplexing (TDM) Time division multiplexing shares a channel over time: Users take turns on a fixed schedule; this is not packet switching or STDM (Statistical TDM) Widely used in telephone / cellular systems CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

14. Code Division Multiple Access (1) Chip sequences for four stations. Signals the sequences represent Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

15. Code Division Multiple Access (2) Six examples of transmissions. Recovery of station C’s Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

16. The Public Switched Telephone Network CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Structure of the telephone system Politics of telephones Local loop: modems, ADSL, and FttH Trunks and multiplexing Switching

17. Pulse Code Modulation (2) Multiplexing T1 streams into higher carriers Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

18. Wavelength Division Multiplexing WDM (Wavelength Division Multiplexing), another name for FDM, is used to carry many signals on one fiber: CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

19. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Mobile Telephone System Generations of mobile telephone systems Cellular mobile telephone systems GSM, a 2G system UMTS, a 3G system 4G LTE 4G

20. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Generations of mobile telephone systems 1G, analog voice AMPS (Advanced Mobile Phone System) is example, deployed from 1980s. Modulation based on FM (as in radio). 2G, analog voice and digital data GSM (Global System for Mobile communications) is example, deployed from 1990s. Modulation based on QPSK. 3G, digital voice and data UMTS (Universal Mobile Telecommunications System) is example, deployed from 2000s. Modulation based on CDMA LTE, digital data including voice LTE (Long Term Evolution) is example, deployed from 2010s. Modulation based on OFDM 4G based on CDMA and 802.16m (WiMax)

21. Cellular mobile phone systems All based on notion of spatial regions called cells Each mobile uses a frequency in a cell; moves cause handoff Frequencies are reused across non-adjacent cells To support more mobiles, smaller cells can be used CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

22. 2G GSM – Global System for Mobile Communications (1) Mobile is divided into handset and SIM card (Subscriber Identity Module) with credentials Mobiles tell their HLR (Home Location Register) their current where abouts for incoming calls Cells keep track of visiting mobiles (in the Visitor LR) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

23. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011 Goals for UMTS (3G) Basic services desired High-quality voice transmission. Messaging (replacing email, fax, SMS, chat). Multimedia (music, videos, films, television). Internet access (Web surfing, incl. audio, video).

24. 3G UMTS – Universal Mobile Telecommunications System (1) Architecture is an evolution of GSM; terminology differs Not compatible with 2G GSM Internet CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

25. 3G UMTS – Universal Mobile Telecommunications System (2) Air interface based on CDMA over 5 MHz channels Rates over users <14.4 Mbps (HSPDA) per 5 MHz CDMA permits soft handoff (connected to both cells) Soft hando f f CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

26. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 4G ITU defined spec in 2008, before the technology existed ITU can’t enforce what carriers do or call their services Pure IPv6 packet switching, no circuit switching No voice (except as VoIP) 1 Gbps for stationary user, 100 Mbps for moving user Uses carrier aggregation (multiple bands together) Uses OFDMA (Orthogonal Freq. Div. Mux Access)

27. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 OF DMA 0 1 0 0 1 0 1 1 0 1 1 1 0 0 1 0 1 0 1 1 Channel 1 Channel 2 Channel 3 Channel 4 1 1 1 1 1 1 1 1 1 1 1 1 Each channel is broadcast in parallel on different frequency bands

28. Data Capacity

29. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 End Chapter 2 part 1