Data Communications

1
Data Communications

• Telecommunications
• LANs, WANs and Intranets

2
Telecommunications

• Hardware
• Software
• Impact

3
Telecommunications

• Two computers linked together through the
  telephone system
• in a computer network computers are linked
  together with high-speed cables

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)

5
Computers

• use digital parallel transmission
• a bunch of bits (8) move around a once

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

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

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

9
Data Transfer Rates Compared

• 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.

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

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

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

13
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

14
Channels

• communication channel - the physical link between
  two computer
• two important characteristics are
• transmission medium
• transmission mode

15
Transmission Media

• wire media
• wireless media

16
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

17
Wireless Transmission

• Cellular telephone system -
• microwave transmission - longer distances,
  straight-line transmission - cannot pass through
  buildings or mountains
• satellite transmission - uses satellites in space

18
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

19
Transmission Mode

• enables the receiving computer to know where one
  byte ends and next byte begins.
• Synchronous/asynchronous transmission

20
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

21
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

22
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

23
Uses of Telecommunications

• Telecommuting
• E-mail
• BBSs - bulletin board systems
• on-line information services
• teleconferencing

24
Computer Network

• Computer Networks need direct, high-speed
  connections
• These connections have more much more bandwidth
  than a standard telephone line

25
LANs

• Local Area Network
• direction connections
• doesnt use wires owned by telephone company
• can connect from two to hundreds of computers

26
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.

27
Network Topology

• The physical layout
• star
• ring
• bus

28
Network Models

• Client/server (CS network)
• peer-to-peer

29
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

30
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

31
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