1 / 49

CS716 Advanced Computer Networks By Dr. Amir Qayyum

CS716 Advanced Computer Networks By Dr. Amir Qayyum. 1. Lecture No. 5. The Big Picture. Midterm exam (estimated). You are here. What We Know. Networks are Experiencing explosive growth Providing wide range of services It is attributed to:

sari
Download Presentation

CS716 Advanced Computer Networks By Dr. Amir Qayyum

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CS716 Advanced Computer Networks By Dr. Amir Qayyum 1

  2. Lecture No. 5

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

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

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

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

  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. Electromagnetic Spectrum Wavelength = speed/frequency = 2 x 108 / 300 = 667 meters

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

  18. Leased Lines • Across city / country • Dedicated link from the telephone company • Appears, but may not be a single link !!! Service:DS1/T1 DS3 STS-1 STS-3 STS-12 ... STS-48 Bandwidth:1.5M 44.7M 51.8M 155M 622M ... 2.5G (bps)

  19. Last-mile Links • Most economical • Home to network service provider • To take benefit of an existing network Service:POTS ISDN xDSL CATV Bandwidth:28.8 - 56 K 64 - 128 K 16 K - 55.2 M 20 - 40 M (bps)

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

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

  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. 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. 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. 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. Encoding

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

  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 Node Adaptor Adaptor Node Bits

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

  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. 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. 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. 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. RS-232 Timing Diagram +15 voltage -15 Idle start 1001100 stop idle time

  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. 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. 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. Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 NRZ 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)

  39. sender’s clock receiver’s clock 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

  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. 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. Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 NRZ Clock Manchester NRZI Different Encoding Schemes

  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. 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. time Amplitude Modulation idle idle 1 idle idle 0 idle idle

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

  47. time Phase Modulation idle idle 1 idle idle 0 idle idle

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

  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

More Related