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CIS 349 Introduction to Data Communications and Distributed Networks Day 12

CIS 349 Introduction to Data Communications and Distributed Networks Day 12. Rhys Eric Rosholt Gillet 301C 718-960-8663 http://comet.lehman.cuny.edu/rosholt/ rosholt@lehman.cuny.edu. Instructor: Office: Office Phone: Web Site: Email Address:. Chapters about one each week.

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CIS 349 Introduction to Data Communications and Distributed Networks Day 12

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  1. CIS 349Introduction to Data Communications and Distributed NetworksDay 12 Rhys Eric Rosholt Gillet 301C 718-960-8663 http://comet.lehman.cuny.edu/rosholt/ rosholt@lehman.cuny.edu Instructor: Office: Office Phone: Web Site: Email Address:

  2. Chaptersabout one each week 1. An Overview of Data Communications 2. Terminal Devices 3. Messages and Transmission Channels 4. Asynchronous Modems and Interfaces 5. Synchronous Modems, Digital Transmission, and Service Units 6. Multiplexing Techniques 7. Fiber-Optic and Satellite Communications 8. Protocols and Error Control 9. PC Communications Software 10. WAN Architectures and Packet Networks 11. Local Area Networks 12. The Internet 13. ISDN 14. Asynchronous Transfer Mode 15. Wireless Transmission 16. Network Security 17. Network Design and Management

  3. Chapter 4Asynchronous Modems and Interfaces Why Data Can't Be Transmitted Directly Solving the Problem with Modems Analog Modulation V.21 Standard Modem Features Interface and Signaling Standards The RS-232 and V.24 Interface Other Interfaces Asynchronous Modem Operations Smart Modems Viewing and Changing Modem Settings Extended Command Set Variances Fax Modems

  4. LZW Algorithm (V.42bis) Regular compression algorithm: INPUT is converted to OUTPUT OUTPUT is converted back to INPUT No other data needs to be transmitted! Book example: (page 142) INPUT: a b a b c Five Symbols OUTPUT: 97 98 256 99 Four Symbols CODING  DATA STREAM  DECODING

  5. Modem Features Data Compression and the V.42bis Standard The LZW Algorithm Compression Parameters Bit Errors on Compressed Data Error Detection and Correction The MNP Protocol V.42 Recommendation Data Compression Interface Versus Operating Rate

  6. Modem Features V.42 Recommendation CRC MNP as a secondary standard Data Compression Interface Versus Operating Rate Interface Rate must be faster than Operating Rate 4x Operating Rate, if possible

  7. Interface and Signaling Standards Many standards and recommended practices Used for purposes never originally intended Serve to shortcut the engineering design process

  8. The RS-232 and V.24 Interface(Text Pages 150 to 164) The Mechanical Interface Electrical and Functional Signals Subset of Signals Examples of RS-232 Connections Asynchronous Modem Control Nonstandard Use of the RS-232 Standard Limitations of RS-232

  9. RS-232 Subset of Signals(used in both DB-25 and DB-9) TD Transmitted Data T C RD Received Data T  C RTS Request To Send T C CTS Clear To Send T  C DTR Date Terminal Ready T C DSR Data Set Ready T  C DCD Data Carrier Detect T  C RI Ring Indicator T  C SG Signal (Frame) Ground T  C

  10. Examples of RS-232 Connections Computer to Modem Interface DTE to DTE or DCE to DCE require 2-3 swap! Computer to Video Display Terminal Interface Computer to Serial Printer Interface

  11. Asynchronous Modem Control Flow Control Have to stop DTE before it floods DCE with data Two Methods: CTS / RTS XON / XOFF Control Circuits Why the other handshaking signals are used!

  12. Nonstandard Useof the RS-232 Standard Printers and DTE Standard BOTH computers and printers are DTE ! Nonstandard Flow Control DTR / DSR RD

  13. Limitations of RS-232 Distance Limitations 50 feet Speed Limitations 20 Kbps 56 Kbps modems only allow 12 foot cables Otherwise need an “enhanced” serial port Ground Limitations Only one ground for frame and signal

  14. Other Interfaces An Old Standby: The Current Loop The RS Series ITU X.21 USB and FireWire Interfaces

  15. An Old Standby: The Current Loop Signaling by presence/absence of current Active side generates current (20 or 60 ma) Passive side receives (switches) current Transmitter or receiver can be “passive” Advantages Convenient only one power supply Longer runs up to 1500 feet Faster up to 9600 bps Needs converter if not active-to-passive

  16. The RS Series RS-422A RS-423A RS-449 RS-530 RS-366

  17. The RS Series Similar to RS-232 RS-422A Balanced transmission Lower voltages, so PC-friendly Not compatible with RS-232 circuits RS-423A Operates in RS-232 and RS-422A circuits RS-449 Intended successor to RS-232 Mechanical connectors: 37-pin or 9-pin

  18. The RS Series Continued RS-530 Uses DB-25 connector Balanced transmission No RI (Ring Indicator) so not for PSTN RS-366 Automatic calling units Intended uses: private-line circuits automatic dialing of remote terminals Superceded by modem technology

  19. ITU X.21 Up to 64,000 bps (BT’s Kilostream) Six signals T Transmit T C R Receive T  C C Control T C I Indication T  C S Signal Element Timing T C B Byte Timing T  C Serial digital coding of control signals Part of X.25 packet-switching protocol X.21bis, very close to V.24 and RS-232

  20. USB and FireWire Interfaces Universal Serial Bus / FireWire (IEEE 1394) Both allow multiple PC peripheral chaining USB: 127devices / 1.5Mbps or 12Mbps USB 2.0 added high speed 480Mpbs FireWire: 67 devices / 100,200,400 Mbps IEEE 1394a High-Performance Serial Bus IEEE 1394b added 1.6 Gbps Both are Plug-and-Play interfaces FireWire started at Apple Computer in 1986

  21. Asynchronous Modem Operations Asynchronous Autoanswer Modem Answer: DTR, RI, DCD, and RTS / CTS Asynchronous Private Line Modem No autoanswer Full-Duplex Asynchronous Uses TD, RD, DCD and DSR Half-Duplex Asynchronous Needs more signals for turnaround 200 ms turnaround delay Software protocol controls turnarounds

  22. Smart Modems Hayes Smartmodem 1200 Command Types Hayes Command Set Hayes Communication in Practice Extended Command Sets Modem Registers

  23. Command Types Basic Commands start with capital letter Extended Commands start with “&” prefix to capital letter Proprietary Commands usually start with “%” or “\” Register Commands S registers Use of S0 through S12 are standardized

  24. Hayes Command Set Table 4.6 Pages 178-179 Commands start with attention code: AT 40 character command buffer Examples: A Answer call D Dial a telephone number S Set modem register values Z Reset the modem

  25. Hayes Communication in Practice Basic command set is insufficient Modern features require extended commands or use of S registers Most modem operations are fully automated by operating systems or communications programs

  26. Extended Command Sets Vary widely between manufacturers Examples: &B Control automatic retraining &K Flow control &M Select error control mode &T Perform modem test &W Store modem configuration &Y Set configuration at power-on &Z Store phone number

  27. Modem Registers Table 4.8 S Register Control Parameters (page 182) First 13 S registers are standardized Modern modems have 100+ registers Read register n ATSn? Set register n to x ATSn=x

  28. Viewing and Changing Modem Settings Need a communications program HyperTerminal (MicroSoft OS versions) Configure program for connection View / modify settings (AT commands) Store modified configuration in modem

  29. Extended Command Set Variances Allows features not supported by Hayes Hayes was late adding features Many different extended command sets Extended Commands Different functions might use same command Modem Self Testing Loopback Tests

  30. Modem Self Testing Send output of transmitter to receiver Pattern Generator to Transmitter Transmitter to Receiver Receiver to Pattern Comparator Pattern compared with Generator

  31. Loopback Tests Local digital loopback Local analog loopback Remote analog loopback Remote digital loopback

  32. Fax Modems Allows transfer of image data over analog telephone circuits The ITU-T T.4 Standard The ITU-T T.30 Standard Fax Modem Commands

  33. The ITU-T T.4 Standard Defines data formatting 1728 dpl (dots per line) 1143 lpp (lines per page) Optional fine resolution 2287 lpp

  34. The ITU-T T.30 Standard Defines transmission parameters Also defines call setup procedures

  35. Fax Modem Commands Fax Modem Service Class 1 six commands using T.30 implemented in software timing problems Fax Modem Service Class 2 unapproved >50 commands fax modem does T.30 work Fax Modem Service Class 2.0 based on Class 2 officially approved

  36. Reminder! Mid-Term ExaminationMonday, March 24, 2003Twelve Days from Today!Covers Chapters 1, 2, 3, and 4

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