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Data Communications

Data Communications. Telecommunications LANs, WANs and Intranets. Telecommunications. Hardware Software Impact. Telecommunications. Two computers linked together through the telephone system in a computer network computers are linked together with high-speed cables. Telephone System.

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Data Communications

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  1. Data Communications • Telecommunications • LANs, WANs and Intranets Chapter 1, Unit c

  2. Telecommunications • Hardware • Software • Impact Chapter 1, Unit c

  3. Telecommunications • Two computers linked together through the telephone system • in a computer network computers are linked together with high-speed cables Chapter 1, Unit c

  4. Telephone System • very extensive • most every household has a telephone or the very least a line a few hundred feet away • cheap • used for voice transfer • voice is a continuous series of analog signals (electrical representations of sound) Chapter 1, Unit c

  5. Computers • use digital parallel transmission • a bunch of bits (8) move around a once Chapter 1, Unit c

  6. Translation • for computer data to transmitted over an analog telephone line - must be converted form digital data to analog signals • the device that allows computer to connect to telephones is a modem Chapter 1, Unit c

  7. Modem • modulation/demodulation • modulation-transfer the data from computer buses in parallel fashion to serial form so that one binary digit at a time can be released to the phone line • demodulation-accepts the serial data from the phone line and transforms them back into parallel form Chapter 1, Unit c

  8. Rate of Transmission • or transmission speed of a modem • measured in bits per second (bps) • early modems - 300 bps • newer models - 14,400 or 28,800 bps • a text of type page is about 2800 chars - at 14,400 can send a full page of text in 2 seconds Chapter 1, Unit c

  9. Data transfer rate (bps) 1,200 2,400 9,600 14,400 28,800 Time to send a 20 page wp document 10 min. 5 min. 1.25 min. 50 sec. 25 sec. Data Transfer Rates Compared Chapter 1, Unit c

  10. Types of Modems • internal/external • fax modem - the sending fax machine makes a digital image (pixels) of the document and converts it to analog - the receiving fax converts the analog to digital • when received the document is just a bunch of black and white dots to the computer - need OCR software Chapter 1, Unit c

  11. Protocols • how to behave - standards • modulation protocols - specifies all the details for communication • set by international standards org. • one example is the rate at which they exchange data • V.34 - regulates the 24,400 • V.32bis - regulates the 14,400 Chapter 1, Unit c

  12. Protocols • modems can communicate only if both follow the same protocol • fall back - two modems communicate and establish a protocol they both share - so they can fall back to slower rate Chapter 1, Unit c

  13. Voice Mail • messaging system with many of the features of an electronic mail system • responses are recorded and placed in voice mail • user can dial the number plus some id or password and listen, respond, forward copies to others and delete messages • cannot edit message or attach to other computer documents Chapter 1, Unit c

  14. Video Teleconferencing • allows people to communicate face to face over long distances by combining video and computer technology • need video camera, microphone and television monitor • specially equipped computers allow callers to see each other on their computer networks Chapter 1, Unit c

  15. Channels • communication channel - the physical link between two computer • two important characteristics are • transmission medium • transmission mode Chapter 1, Unit c

  16. Transmission Media • wire media • wireless media Chapter 1, Unit c

  17. Wire Media • Twisted pair - simple, inexpensive, but slow and susceptible to errors • Coaxial Cable - faster and stronger , more expensive • Fiber-Optic - glass fibers smaller than a human hair are used - light pulses rather than electrical signals are sent Chapter 1, Unit c

  18. Wireless Transmission • Cellular telephone system - • microwave transmission - longer distances, straight-line transmission - cannot pass through buildings or mountains • satellite transmission - uses satellites in space Chapter 1, Unit c

  19. Bandwidth • relates to the data transfer capacity of the medium • measured in kilobits per second (Kbps), megabits per second (Mbps) or gigabits per second (gbps) • from slow to fast - twisted pair, coax then fiber optic Chapter 1, Unit c

  20. Transmission Mode • enables the receiving computer to know where one byte ends and next byte begins. • Synchronous/asynchronous transmission Chapter 1, Unit c

  21. Synchronous • the computer are synchronized - there is a set unit of time to relay a unit of data • more complex to implement • used mainly in networks Chapter 1, Unit c

  22. Asynchronous • each byte is marked with a start and stop bit • more data to be sent - thus 20% slower than synchronous (2/10) • simpler to implement • modems Chapter 1, Unit c

  23. Digital Telephony • most of the long distance telephone service is already digital • local service is still analog • ISDN - Integrated Service Digital Network - interim solution - can transmit digital over twisted pair - no conversion necessary - ( digital modem) • you can have this service but costs extra Chapter 1, Unit c

  24. Telecommunications Software • Telecommunications software is software that transforms a computer into a terminal capable of connecting to a multi-user computer system by means of the telephone Chapter 1, Unit c

  25. Features of Communications Program • directs the modem to dial the telephone number • choose the communications settings • emulate many computer terminals • enable user to store and print received data Chapter 1, Unit c

  26. Uses of Telecommunications • Telecommuting (5.5 million people) • E-mail • BBSs - bulletin board systems • on-line information services • teleconferencing Chapter 1, Unit c

  27. LANs, WANs and Intranets • Local Area Networks • Wide Area Networks • Networks within Company Chapter 1, Unit c

  28. Computer Network • Computer Networks need direct, high-speed connections • These connections have more much more bandwidth than a standard telephone line Chapter 1, Unit c

  29. LANs • Local Area Network • direction connections • doesn’t use wires owned by telephone company • can connect from two to hundreds of computers Chapter 1, Unit c

  30. LAN Hardware • Cable - twisted pair, coaxial and fiber optic cables • NIC card (Network Interface Card) • Hub or Cable Interface Unit - sends and receives signals on the network cable • can connect LANs together by using a bridge, router or gateway. Chapter 1, Unit c

  31. Network Topology • The physical layout • star • ring • bus Chapter 1, Unit c

  32. Network Models • Client/server (CS network) • peer-to-peer Chapter 1, Unit c

  33. WANs • Wide Area Networks - distance is more than a mile and as much as half the globe • require special media (normally provided by telephone companies) • require special hardware Chapter 1, Unit c

  34. WAN Hardware • Special-purpose telephone wires, fiber-optic cables, microwaves, or satellites • dedicated, private, or leased line • multiplexors - combines computers signals and passes them on and one on the other end takes them apart and sends them directly • routers- sends data the right way • front-end processors Chapter 1, Unit c

  35. Intranets • Network within an organization • “private” web • keeps the learning curve low when moving either back to the “real” web or if you move from the “real” web to an intranet Chapter 1, Unit c

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