1. 1 CS716 Advanced Computer Networks By A. Wahid Shaikh

2. Lecture No. 5

3. 3 The Big Picture You are here Midterm exam (estimated)

4. 4 What We Know Networks are –Experiencing explosive growth –Providing wide range of services It is attributed to: –General purpose nature of computer networks –Ability to add new functionality with software –High performance computers are now affordable

5. 5 and We Know … Connecting mainframes over long-distance telephone lines has turned into a big business! Lots of competing players –Computing industry –Telephone carriers –Service providers, operators, … Global, ubiquitous, heterogeneous networking ? –Issues of connectivity, service levels, performance, …

6. 6 What We Have Learned Carefully identify what we expect from a network Cost-effective connectivity –Accomplished through nested interconnection of nodes and links –Provides process-to-process communication services –Should offer high performance using the metrics like latency and throughput This results in a packet-switched network

7. 7 What is Our Approach A layered architecture as a guideline for design Protocols are central objects –Provides services to higher-level protocols –Make a message exchange meaningful with peers Implement protocols in software –Define interfaces to invoke services –Socket interface between applications and protocols –“Similar” interface within the network subsystem

8. 8 What Next ? Start with a simplest possible network Two nodes connected directly through some suitable medium

9. 9 Point-to-Point Links Reading: Peterson and Davie, Ch. 2 Outline Hardware building blocks Encoding Framing Error Detection Reliable transmission Sliding Window Algorithm

10. 10 Direct Link Issues in the OSI and Hardware/Software Contexts transport network data link physical session presentation application user-level software kernel software (device drivers) reliability framing, error detection, MAC encoding hardware (network adapter)

11. 11 Hardware Building Blocks Nodes –Hosts: general-purpose computers –Switches: typically special-purpose hardware –Routers (connecting networks): varies Links –Copper wire with electronic signaling –Glass fiber with optical signaling –Wireless with electromagnetic (radio, infrared, microwave) signaling

12. 12 Nodes – A Workstation Architecture CPU (processor) Cache $ Memory I/O bus Network adaptor to network finite memory (implies limited buffer space) Device driver managing network adaptor which is using system’s I/O bus Memory access much slower than CPU speed memory bus

13. 13 Links Physical media –twisted pair cable –coaxial cable –optical fiber –space Media is used to propagate signals Signals are electromagnetic waves of certain frequency, traveling at speed of light

14. 14 Electromagnetic Spectrum Wavelength = speed/frequency = 2 x 10 8 / 300 = 667 meters

15. 15 Signals Over a Link Signal is modulated for transmission –varying frequency/amplitude/phase to receive distinguishable signals Binary data (0s and 1s) is encoded in a signal –make it understandable by the receiving host

16. 16 Bits Over a Link Bit streams may be transmitted both ways at a time on a point-to-point link –full-duplex Sometimes two nodes must alternate link usage –half duplex

17. 17 Which Link to Use ? Cables –same room / building / site insulation braided conductor copper core coax twisted pair glass core (fiber) glass clading plastic jacket

18. 18 Leased Lines Across city / country Dedicated link from the telephone company Appears, but may not be a single link !!! Service: DS1/T1DS3STS-1STS-3STS-12...STS-48 Bandwidth: 1.5M44.7M51.8M155M622M...2.5G (bps)

19. 19 Last-mile Links Most economical Home to network service provider To take benefit of an existing network Service: POTSISDNxDSLCATV Bandwidth: 28.8 - 56 K64 - 128 K16 K - 55.2 M20 - 40 M (bps)

20. 20 ADSL (Asymmetric Digital Subscriber Line) Connects the subscriber to the central office via the local loop Bandwidth depends on length of local loop Central office Subscriber premises 1.554–8.448 Mbps 16–640 Kbps Local loop 2.74 – 5.48 Km

21. 21 VDSL (Very high data rate DSL) Connects the subscriber to the optical network that reaches the neighborhood Runs over short distances Symmetric Central office Neighborhood optical network unit STS-N over fiber Subscriber premises VDSL at 12.96–55.2 Mbps over 1000–4500 feet of copper

22. 22 CATV Uses existing cable TV (CATV) infrastructure –reaches 95% of households in U.S. Single CATV channel has bandwidth of 6 MHz Can be used in asymmetric way Currently achieves on a single channel: –40 Mbps downstream (100 Mbps theoretical capacity) –20 Mbps upstream Multiple access on shared channel (IEEE 802.14)

23. 23 Optical Communication Higher bandwidths Superior attenuation properties Immune from electromagnetic interference No cross-talk between fibers Thin, lightweight and cheap (the fiber, not the optical-electrical interfaces)

24. 24 Wireless Links Satellite links Provide a grid of medium and low orbit satellites –Geosynchronous satellite 600-1000 Mbps –Low Earth Orbit (LEO) array ~400 Mbps Targeted at voice communication  modems Teledesic supports 1440 16 kbps satellite-to- earth channels (~2 Mbps); 155.5 Mbps intersatellite channels

25. 25 Wireless Links Radio and infra-red frequency links 11 Mbps rates, 2.4 GHz band, distances of 50-150 meters –5.2 GHz band, > 55 Mbps: HIPERLAN-1, IEEE 802.11a Bluetooth piconets: Infrared links, 1 Mbps, 10 meters

26. 26 Encoding

27. 27 Point-to-Point Links Reading: Peterson and Davie, Ch. 2 Hardware building blocks Encoding Framing Error Detection Reliable transmission –Sliding Window Algorithm

28. 28 Encoding Signals propagate over a physical medium –modulate electromagnetic waves –e.g., vary voltage Encode binary data onto signals that propagate Signalling component Signal Bits Node Adaptor

29. 29 Encoding Problems with signal transmission –Attenuation: signal power absorbed by medium –Dispersion: a discrete signal spreads in space –Noise: random background “signals” modulator demodulator a string of signals Digital data (a string of symbols)

30. 30 Advantages of Digital Transmission over Analog Reasonably low-error rates over arbitrary distances –Calculate/measure effects of transmission problems –Periodically interpret and regenerate signal Simpler for multiplexing distinct data types (audio, video, e-mail, etc.)

31. 31 Advantages of Digital Transmission over Analog Examples of modulators-demodulators (modems) Electronic Industries Association (EIA) standard RS-232(-C) International Telecommunications Union (ITU) standard V.32 96 kbps modem

32. 32 RS-232(-C) Communication between computer and modem Uses two voltage levels (+15V, -15V), a binary voltage encoding Data rate limited to 19.2 kbps (RS- 232-C); raised in later standards

33. 33 RS-232(-C) Characteristics Serial: one signaling wire, one bit at a time Asynchronous: line can be idle, clock generated from data Character-based: send data in 7- or 8- bit characters

34. 34 RS-232 Timing Diagram +15 -15 voltage Idle start 1 0 0 1 1 0 0 stop idle time

35. 35 RS-232 One bit per clock Voltage never returns to 0V (0V is a dead / disconnected line) -15V is both idle and “1”; initiates the send by pushing to 15V for one clock (start bit)

36. 36 RS-232 Minimum delay between character transmissions idle for one clock at –15V (stop bit) One character leads to 2+ voltage transitions Total of 9 bits for 7 bits of data (78% efficient) Start and stop bits also provide framing

37. 37 Binary Voltage Encoding NRZ (non-return to zero) NRZI (NRZ inverted) Manchester (used by IEEE 802.3, 10 Mbps Ethernet) 4B/5B (8B/10B) in Fast Ethernet

38. 38 Non-Return to Zero (NRZ) Encode binary data onto signals –e.g., 0 as low signal and 1 as high signal –voltage does not return to zero between bits known as Non-Return to Zero (NRZ) Bits NRZ 0010111101000010

39. 39 Problem: Consecutive 1s or 0s Low signal (0) may be interpreted as no signal High signal (1) leads to baseline wander Unable to recover clock –sender’s and receiver’s clock have to be precisely synchronized –receiver resynchronizes on each signal transition –clock drift in long periods without transition sender’s clock receiver’s clock

40. 40 Alternative Encodings Non-Return to Zero Inverted (NRZI) Make a transition from current signal (switch voltage level) to encode/transmit a “one” Stay at current signal (maintain voltage level) to encode/ transmit a “zero” Solves the problem of consecutive ones (shifts to 0s)

41. 41 Alternative Encodings Manchester (in IEEE 802.3 – 10 Mbps Ethernet) Split cycle into two parts –Send high--low for “1”, low--high for “0” –Transmit XOR of NRZ encoded data and the clock Only 50% efficient (1/2 bit per transition)

42. 42 Different Encoding Schemes Bits NRZ Clock Manchester NRZI 0010111101000010

43. 43 4B/5B Encoding Every 4 consecutive bits of data encoded in a 5-bit code (symbol) –4-bit pattern is “translated” to a 5-bit pattern (not addition) 5-bit codes selected to have no more than one leading 0 and no more than two trailing 0s –00xxx (8 symbols) and xx000 (4 symbols) are illegal –5 free symbols (non-data) Thus, never gets more than three consecutive 0s Resulting 5-bit codes are transmitted using NRZI Achieves 80% efficiency

44. 44 Binary Voltage Encoding Problem: wide frequency range required, implying –Significant dispersion –Uneven attenuation Prefer to use narrow frequency band (carrier frequency) Types of modulation –Amplitude (AM) –Frequency (FM) –Phase / phase shift –Combination of these (e.g. QAM)

45. 45 Amplitude Modulation idle idle 1 idleidle0 idle idle time

46. 46 Frequency Modulation idle idle 1 idleidle 0 idle time

47. 47 Phase Modulation idle idle 1 idleidle0 idle idle time

48. 48 Phase Shift in Carrier Frequency 108 degrees difference in phase collapse for 108 degrees shift

49. 49 Review Lecture 5 Simplest possible network – 2 nodes connected directly Building blocks – nodes and links Nodes – workstation architecture Links – several types, optical, wireless Encoding – binary data into signals, RS 232 Binary voltage encoding – NRZ, NRZI, Manchester, 4B/5B Modulation schemes