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Point-to-Point Communication

Point-to-Point Communication. Terminal-Host Communication. Components Terminal Host (locus of processing) Transmission line (here, phone line and modem) Telephone line acts as a point-to-point link. Host. Terminal. Phone Line. Modem. Modem. Terminal Emulation. People Already Have PCs

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Point-to-Point Communication

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  1. Point-to-Point Communication

  2. Terminal-Host Communication • Components • Terminal • Host (locus of processing) • Transmission line (here, phone line and modem) • Telephone line acts as a point-to-point link Host Terminal Phone Line Modem Modem

  3. Terminal Emulation • People Already Have PCs • Host operating systems only work with terminals • Do not want to buy a terminal as well • PCs can emulate (act like) terminals • Only requires software (a communication program) • Turns an expensive PC into a cheap terminal

  4. VT100 Terminals • VT100 Emulation Only Needs Software • Communications program • Terminal emulation software • Most Hosts Support VT100 Terminals • Lowest Common Denominator • Slow: maximum speed of 19 kbps, usually slower • Uses inefficient asynchronous ASCII transmission, discussed later • No graphics or even multiple fonts: plain text only • No color

  5. Terminal Emulation Software • File Transfer • Transfer whole files with error correction • Upload: from PC to host • Download: from host to PC • Terminal emulation program and host file transfer program must support the same file transfer protocol standard • Kermit • XMODEM, YMODEM • IBM 3270 Terminals

  6. Analog and Digital Transmission Analog Signal Digital Signal (1101) Modem Modulation Analog Signal Digital Signal (1101) Modem Demodulation

  7. Digital and Binary • Digital Transmission • Can have multiple voltage levels, say 4 • Change to one at start of each bit cycle • If 4, changes can represent 2 bits each: • 00, 01, 10, 11 11 Voltage Level 10 01 00 Time

  8. Baud Rate and Bit Rate • Baud Rate • Number of times line changes per second • Let baud rate be 4 (4 changes per second) • Let bits per line change be 2 • Bit rate = 8 bits per second • Bit rate = x2 Baud rate in this example 11 10 01 00 One Second

  9. Wave Characteristics • Amplitude (power) • Frequency (cycles per second, Hertz) • Wavelength (meters) Frequency (Hz) Wavelength (meters) One Second Amplitude (power)

  10. 0o 90o 180o 270o 0 and rising Highest 0 and falling Lowest Wave Characteristics • Phase • Fully cycle is 360 degrees • Phase is degrees different from reference wave • Human ears cannot hear. Equipment can Reference Wave 0o 180 degrees out of phase 180o

  11. Wave Characteristics • Amplitude • Frequency and Wavelength • Not independent • As frequency rises, wavelength falls(Shorter guitar strings produce higher notes) • Their product is constant--the speed of light, sound, etc. • Phase

  12. Frequency Modulation Vary the frequency (wavelength) to represent 1 and 0 Wavelength Low Frequency (0) Wavelength High Frequency (1) 0 1 Frequency Modulation (1011) 1 1

  13. Amplitude Modulation Amplitude (low) Low Amplitude (0) Amplitude (high) High Amplitude (1) Amplitude Modulation (1011)

  14. Phase Modulation In Phase (0) 180 degrees out of phase (1) Frequency Modulation (1011)

  15. Complex Modulation Vary both amplitude and phase Several values (not just two) on each dimension In Phase 90 Degrees Out of Phase, High Amplitude High Amplitude Low Amplitude 180 Degrees Out of Phase

  16. Modem Standards • Modems at Two Ends Must Communicate • Must follow same standards • Most modem standards set by ITU-T • Multiple category of standards: • Modem speed (modulation) • Error correction and compression • Facsimile • Etc. • When buying a modem, must check for standard(s) followed in each category

  17. Modem Speed Standards • Set by the ITU-T • Govern how modulation is done • Standards for speed governs modulation for data transmission • V.92 56.6 kbps plus quick connect, modem on hold, PCM upstream • V.90 56.6 kbps • V.34 28.8 kbps/33.6 kbps • V.32 bis 14.4 kbps

  18. Modem Speed Standards • Most data modems are also fax modems • V.14 14.4 kbps • V.29 9,600 bps

  19. Error Correction and Compression • ITU-T Standards • V.42 Error detection and correction • V.42 bis Data compression (up to 4:1) • V.44 Data compression (20 to 120% more than V.42 bis) • Independent of speed standards (but V.44 only with V.92) • Microcom Standards • Microcom Network Protocol (MNP) • Both error correction and compression • Several levels • Independent of speed standards

  20. Modem Intelligence • ComputerCan Send Commands to Modem • Dial a number, including how long to wait, etc. • Called intelligent modems • Hayes Developed the first Command Set • Most modems follow the same command set • We call them “Hayes compatible” • Commands start with “AT” • Other Standards for Fax Modems • Class 1 and Class 2: extensions to Hayes

  21. Telephone Bandwidth is Limited • Telephone Transmission • Cuts off sounds below 300 Hertz • Cuts off sounds above about 3,400 Hz • Bandwidth is the difference between the highest and lowest frequencies (3400-300): about 3,100 Hz Sound Loudness Bandwidth 3,100 Hz 0 300 3400 20,000 Frequency (Hz)

  22. Telephone Bandwidth is Limited • Speed is Limited • Maximum speed is related to bandwidth (Shannon’s Law) • Maximum speed for phone lines for transmission is a little over 30 kbps • So modems can’t get much faster

  23. Another Look at Compression • With 4:1 Compression, a V.34 Modem Can Receive Data at 115.2 kbps from the PC • However the ~30 kbps limit of the phone system is not exceeded. Still transmit at 33.6 kbps. ~35 kbps Maximum Compression in Modem 33.6 kbps 115.2 kbps

  24. 56 kbps Analog Modems • From home, you transmit • Analog-to-Digital Converter (ADC) • Filters your signal to a bandwidth of ~3.1 kHz • This limits you to 33.6 kbps Telephone Network PC V.34 modem ADC 33.6 kbps

  25. 56 kbps Analog Modems • But ISP Can Connect Digitally • Signal travels through phone system at 56 kbps • At user end, digital-to-analog converter (DAC) • Sends signal to analog modem at wide bandwidth • Modem can receive at 56 kbps Digital Link Telephone Network PC ISP 56 kbps modem DAC 56 kbps

  26. 56 kbps Modems • What they can do • Send at 33.6 kbps (V.92 with PCM upstream can go up to 48 kbps) • Receive at 56 kbps (V92 with V.44 compression can go up to 120 kbps) • Problems • past:competing standards from Rockwell, U.S. Robotics (V.90 ended them) • present: ISPs must support V.92 (all support V.90) • Users and ISPs • Users V.90 analog modem or V.92 • ISPs V.90 digital modem or V.92

  27. 56 Kbps Modems • Telephone company • No changes needed, although ... • Many not have an internal ADC conversion between ISP and customer (some do) • May not have long transmission line from last switch to the customer premises (local loop) • Not all phone lines to customer premises will support 56 kbps modems • Even when they do, speeds may only be 40-50 kbps

  28. A B A B Half-Duplex Transmission • Sender and receiver must take turns sending • Like an old one-lane road • No interruption for error handling or flow control Time 1 Only one side May communicate A does Time 2 Only one side May communicate B does

  29. Full-Duplex Transmission • Both Sides May Transmit Simultaneously • Needed for error correction, flow control • Now almost universal in modem communication A B A B Time 1 Both sides may communicate Both do Time 2 Both sides may communicate A does

  30. Asynchronous Transmission • ASCII Character Set • 7-bit is the standard • 8-bit extended ASCII is popular • Bits transmitted backward • Parity for Error Detection • Only for 7-bit ASCII • Start/Stop Bits for Framing • Each frame is exactly 10 bits long

  31. Asynchronous Transmission • ASCII Character Set • Created for sending printed American text • Each character is a 7-bit code (e.g., 1010101) • This allows 2^7 or 128 possible characters • Printing characters: A, a, !, <, %, etc. • Control codes: XOFF tells other side to pause

  32. Asynchronous Transmission • 8-bit ASCII • Used in PCs: 8 bits per character (10101010) • Used for word processing format codes • Used in graphics that stores data in bytes

  33. PC Serial Port • Bit Transmission of ASCII Characters • Transmits last bit first • If you wish to send 1111000, • The serial port transmits 0001111

  34. Parity • For 7-bit ASCII Only (No Parity = 8-bit ASCII) • Transmit an 8th bit per character • Even parity: sum of data and parity bits is even • To send 1110000 (odd), send 00001111 • To send 1111000 (even), send 00011110 • Odd parity: sum is odd • If error is detected, the character is simply discarded. No way to ask for retransmission

  35. Start and Stop Bits • When the Data Line is at Rest • It is kept in the “1” state • So “11110000” would look like 111111100001111 • “00001111” would also look like 11111100001111 • How can you tell where a character begins? • Solution • Add a start bit (always 0) to change the line state • End with a stop bit (always 1) to guarantee at least a one-bit rest (1) against which to detect the next start bit (0)

  36. The Final Asynchronous Frame • Always 10 bits • Start, 7 data bits, parity, stop, or • Start, 8 data bits, stop Start Parity 0 1 1 1 0 0 0 1 11 7-bit ASCII Character Stop Start 0 1 1 1 0 0 0 1 11 Stop 8-bit ASCII Character

  37. Flow Control • Ask the Other Device to Pause (or Slow Down) • ASCII • In asynch, usually done by sending ASCII control codes • XOFF tells other side to pause • XON tells the other device to resume • Serial Port • Signals on the pins control when PC, modem can transmit

  38. ISDN • Integrated Serviced Digital Network • Planned as eventual replacement for the worldwide analog telephone network • All-digital, but expensive and difficult to install • Integrated services • Automatic number identification • Call waiting • Now available only locally (business and limited) • will ISDN standardize worldwide? - NO!!!

  39. Basic Rate Interface (BRI) • Service Brought to User’s Desk • 3 Channels multiplexed onto one set of wires • Really, bidirectional, so 6 channels! D Channel: control signals B Channel Terminal Adapter 2B+D 1101 Multiplexed Onto One Set of Wires B Channel

  40. ISDN BRI • B Channels • 2 B-channels multiplexed to desktop • 64 kbps each • Simultaneous voice and data connections • Sometimes, a single 128-kbps connection • Moderately faster than modems and comparable increase in cost • D Channel • one 16 kbps channel, usually for control signaling

  41. Connecting a Computer to the ISDN • Requires a Terminal Adapter • Contains a DSU (data service unit) • Converts PC digital format to ISDN digital format DSU 1101 1101 Terminal Adapter

  42. ISDN Costs for Internet Access • Carrier Charges • Installation of the ISDN Line by Carrier • Monthly Service Charge to Carrier • Traffic volume charges (per kilobit) • Purchase or Lease of Terminal Adapter • Internet Service Providers • Charge extra for ISDN access • Even more for 128 kbps service (2 B channels) • ISDN fees do not go to ISP

  43. Beyond ISDN • Faster than analog phone lines but still slow • Better dial-up connections …. V.92-V.44 • Newer alternatives are growing • Cable modems from cable television companies • DSL lines from telephone companies • Much faster • 1-10 Mbps to customer premises • Sometimes slower uplink (33.6 kbps - 1 Mbps) • More expensive than dial-up • equivalent to the cost of two phone lines • installation costs and/or monthly lease of modem

  44. Beyond ISDN • Cable Modems from Cable TV Companies • High-speed digital access offered by cable television companies • Really DSUs but called cable modems • 500 kbps to 10 Mbps downstream (to you) • Downstream speed shared by block of users • ISDN-like speeds upstream (from you) • Typical Cable + ISP charges around $40.00 • Comcast and TCI major players • 10 million users today and growing

  45. Beyond ISDN • Digital Subscriber Link (DSL) • High-speed digital access offered by telephone companies over the ordinary local loop • Requires equipment at telephone company switch • User needs DSL modem (really, a DSU) • 128 kbps to perhaps 8 Mbps downstream • ISDN-like rates upstream are common • Several versions: Asymmetric (ADSL), etc. • Microsoft, Intel, Compaq, and others join Telcos • Keep yourself informed: ADSL Forum

  46. Beyond ISDN • Satellite/Wireless • Satellite link instead of wire link to ISP • Good for mobile users • Good for intranets in a company with multiple dispersed sites • ISDN speeds or higher • High cost

  47. Broadband Market Forecast • Residential Use (2003) • 80% of Households have computers online • 77% dial-up connections • 23% broadband (17 million) • Forecast 2007: 56% ( 41 million) • Broadband • 54% Cable modems • 34% DSL • 8% ISDN • 4% Satellite/wireless • Cnet: Cable vs DSL

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