1. 第6章Communications and Networks
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2. 第6章 Communications and networks
本章学习目的
Define the components required for successful communications
Describe commonly used communications devices
Identify uses of communications
Explain difference between a local area network and a wide area network
Explain various physical and wireless transmission media
Identify uses of intranets and extranets
Describe the purpose of communications applications
Understand the various communications protocols
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3. 6.1 Communications What are computer communications?
Process in which one computer transfers data, instructions, and information to another computer(s)
Communications requires:
a device that initiates the transfer(a sending device)
a signals capable of being carried by a communications channel
a communications channel over which the signals are sent
a communications device that receives the signals and converts them into a form understood by the receiving device
a device that accepts the sent material(a receiving device)
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4. 6.1 Communications What is a sending and receiving device?
Initiates or accepts transmission of data, instructions, and information
notebook computers
mid-range servers
mainframe computers
desktop computers
Internet appliances and Web-enabled devices
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5. 6.2 How is Communications used
What are some uses of communications technology?
voice mail
fax
instant messaging
chat rooms
newsgroups
telephony
videoconferencing
collaboration
groupware
global positioning system (GPS)
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6. 6.3 Communications Channel
What is a channel?
Communications path between two devices
bandwidth
Width of communications channel
transmission media
Materials or techniques capable of carrying one or more signals
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7. 6.3 Communications Channel
Sending a request over the Internet
1: Sending device requests information
2: When request leaves the ISP, it travels over a variety of lines until it reaches Internet backbone
3: Request travels over T3 lines along Internet backbone
4: Request travels over T1 lines until it reaches destination network server
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8. 6.4 Physical Transmission Media
What is physical transmission media?
Wire, cable, and other tangible (touchable) materials used to send communications signals
twisted-pair cable
coaxial cable
fiber-optic cable
is typically used within buildings or underground
Optical cable
Coaxial cable
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9. 6.4 Physical Transmission Media
What is twisted-pair cable?
Transmission media used by telephone system and network cabling
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10. 6.4 Physical Transmission Media
What is coaxial cable?
Single copper wire surrounded by three layers：
an insulating material
a woven or braided metal
a plastic outer coating
Often used for cable television wiring
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11. 6.4 Physical Transmission Media
What is fiber-optic cable?
Contains core of dozens or hundreds of thin strands of glass or plastic
Uses light to transmit signals
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12. 6. 5 Wireless Transmission Media
What is wireless transmission media?
Send communications signals through air or space
Used when inconvenient, impractical, or impossible to install cables
Channel Transfer Rates
Broadcast radio Up to 2 Mbps
Microwave radio Mbps
Communications satellite Mbps
Cellular radio bps to 14.4kbps
Infrared to 4 Mbps
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13. 6. 5 Wireless Transmission Media
What is broadcast radio?
a wireless transmission medium that distributes radio signals through the air over long distances such as between cities, regions, and countries
Some networks use a transceiver, which both sends and receives signals from wireless devices
Bluetooth and Cellular Radio are examples of Broadcast radio
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14. 6.5 Wireless Transmission Media
What is an example of a wireless local area network?
Radio 2011 PC card
Server
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15. 6.5 Wireless Transmission Media
What is Bluetooth ?
Radio frequency specification that portable devices will use for short range communications
10 m –100 m range
Originally designed for hands free headsets with cellular phones.
Potentially going to replace all current remotes and allow all manner of devices to communicate with each other. Devices with bluetooth transceivers can automatically detect each others presence and start communicating with each other.
Blue tooth is a new protocol for devices to communicate with each other over short distances ~ 10m, comes with built in encryption – devices started to come on the market in the last quarter of Expect > a billion within 3 years and a new way of interacting with each other. More on this in a later lecture.
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16. 6.5 Wireless Transmission Media
What is cellular radio?
Form of broadcast radio used widely for mobile communications
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17. 6.5 Wireless Transmission Media
What is a microwave station?
Earth-based reflective dish that contains antenna, transceivers, and other equipment necessary for microwave communications
Uses line-of-sight transmission
Must transmit in straight line with no obstructions between microwave antennas
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18. 6.5 Wireless Transmission Media
What is a communications satellite?
Space station that receives microwave signals from an earth-based station, amplifies the signals, and then broadcasts the signals back over a wide area to any number of earth-based stations
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19. 6.5 Wireless Transmission Media
What is infrared (IR)?
Wireless transmission media that sends signals using infrared light waves
Requires line-of-sight transmission
Many computers and devices have an IrDA port that enables transfer of data using infrared light rays
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20. 6.6 Transmission Characteristics
What are digital signals and analog signals?
analog signal
Continuous electrical wave
digital signal
Distinct pulses that represent bits grouped together into bytes
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21. 6.6 Transmission Characteristics
What are broadband and baseband ?
baseband
Transmits only one signal at a time
Media that use baseband transmission transmit signals at a lower speed than those that use broadband transmission
broadband
Transmit multiple signals simultaneously
Satellite is an example of a media that uses broadband transmission
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22. Asynchronous transmission
6.6 Transmission Characteristics
Transmission Modes: Asynchronous and Synchronous
Synchronous Transmission
Requires that both the sending and receiving devices be synchronized before any bytes are transmitted
Timing signals synchronize the communications devices at the sending and receiving ends
Asynchronous transmission
Usually involves communications in which data can be transmitted intermittently instead of in a steady stream
Relatively slow
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23. 6.6 Transmission Characteristics
Transmission Direction: Simplex, Half-Duplex, and Full-Duplex
Data can flow in either direction-from sender to receiver and back-but only in one direction at a time
Simplex
Full-Duplex
Half-Duplex
Data flows only in one direction-from the sending device to the receiving device
Data can flow in both directions at the same time
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24. 6.6 Transmission Characteristics
What is transfer rate?
Speed at which a line carries data and information
Faster the transfer rate, the faster you can send and receive data and information
Usually expressed as a measure of bits per second
bits per second (bps)
kilobits per second (Kbps)
megabits per second (Mbps)
gigabits per second (Gbps)
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25. 6.7 The Telephone Network
What is the public switched telephone network (PSTN)?
Worldwide telephone system that handles voice-oriented telephone calls
                                                                                                                                           
satellite
most of the PSTN use digital technology
analog signal
final link from local telephone company to home or office
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26. 6.7 The Telephone Network What is a dial-up line and a dedicated line?
Dial-up line is temporary connection using telephone line for communications
Dedicated line is line always connected between two communications devices
Advantages
of dial-up line
1. Costs no more than making regular call
2. Computers at any two locations can establish a connection using modems and telephone network
Advantages of dedicated line
1. Quality and consistency of connection are better
2. Computer locations are fixed
3. Can be digital or analog
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27. 6.7 The Telephone Network
What are popular types of digital dedicated lines?
Type of line Transfer Rate
Dial-Up Up to 56kbps
ISDN(BRI) Up to 128kbps
ADSL kbps-9Mbps
Cable TV(CATV) kbps -25Mbps
T Mbps
T Mbps
ATM Mbps -622Mbps
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28. 6.8 Communications Software
What is communications software?
Programs that help establish connection to another computer or network
supports file transfer allows you to send one or more files from one computer to another
resides in the main memory of the sending and receiving computers while the connection is established
The Internet access feature allows you to use the computer to connect to the Internet to send , participate in chat rooms, visit World Wide Web sites
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29. 6.9 Networks What is a local area network (LAN) ?
Network in limited geographical area such as home, school computer laboratory, or office building
nodes in network
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30. printer may be used by all computers on network
6.9 Networks
network operating system and application software installed on each computer
What is a peer-to-peer LAN?
Small network that shares hardware, data, or information located on any other computer in network
Each computer stores files on its own storage devices
printer may be used by all computers on network
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31. 6.9 Networks What is a client/server LAN?
Network in which one or more computers act as a server and other computers on the network can request services from server
client
server
laser printer
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32. 6.9 Networks What is a wide area network (WAN)?
Network that covers large geographic area
Internet is world’s largest WAN
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33. 6.9 Networks What is an Internet use of peer-to-peer (P2P)?
Enables users with same networking software to connect to each other’s hard disks and exchange files directly
Internet
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34. 6.9 Networks What is a network topology? bus ring star
Configuration, or physical arrangement, of devices in a communications network
Networks usually use combinations of three topologies
bus
ring
star
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35. 6.9 Networks What is a bus network?
Consists of a single central cable, to which all computers and other devices connect
Bus is physical cable or backbone
Inexpensive and easy to install
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36. 6.9 Networks What is a ring network?
Cable forms closed ring, or loop, with all computers and devices arranged along ring
Data travels from device to device around entire ring, in one direction
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37. 6.9 Networks What is a star network?
All devices connect to a central computer, called the hub
All data transferred from one computer to another passes through hub
Personal computer
Central computer
Personal computer
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38. 6.9 Networks What is a network communications Protocol? TCP/IP
a set of rules and procedures for exchanging information among computers
Using the same protocols, different types and makes of computers can communicate with each other
TCP/IP
token ring
Ethernet
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39. 6.9 Networks What is Ethernet?
LAN technology that allows personal computers to contend for access to network
Based on bus technology
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40. 6.9 Networks What is token ring?
LAN technology that controls access to network by requiring network devices to share or pass a special signal, called a token
Device with token can transmit data over network
Only one token exists per network
Based on ring topology, although it can use star topology
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41. 6.9 Networks What is TCP/IP? packets
Data divided into small pieces called packets
What is TCP/IP?
Short for transmission control protocol/Internet protocol
Transmits data by breaking it up into packets
Commonly used for Internet transmissions
routers
Devices that direct packets along fastest available path
packet switching
Technique of breaking a message into packets, sending packets, and then reassembling data
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42. accessible via a Web browser
6.9 Networks
has a Web server
What is an intranet?
Internal network that uses Internet technologies
Lets company make information accessible to employees and facilitate working in groups
Typically includes a connection to Internet
accessible via a Web browser
users can post and update information on intranet by creating and posting a Web page
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43. 6.9 Networks What is an extranet?
an intranet that extends to the authorized users outside the company
facilitates communications among a company's customers or suppliers
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44. Reading Materials: Communications Devices
What is a communications device?
Any type of hardware capable of transmitting data, instructions, and information between sending device and receiving device
cable modems
dial-up modems
ISDN and DSL modems
network interface cards
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45. Reading Materials: Communications Devices
What is a dial-up modem?
Communications device that converts computer's digital signals to analog signals and analog signals to digital signals
ISDN and DSL use digital modem with a digital telephone line
mo + dem
modulate
To convert a digital signal into an analog signal
demodulate
To convert an analog signal into a digital signal
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46. External and internal MODEMS
Reading Materials: Communications Devices
External and internal MODEMS
External MODEMS
internal MODEMS
COMMUNICATION DEVICES Modems
External and internal
Analog to digital – demodulate
Digital to analog – modulate
ISDN and DSL modems are digital modems
sends and receives digital data over digital channels
shouldn’t really be called a modem
permanent – creates a security risk
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47. Reading Materials: Communications Devices
What is a cable modem?
Modem that sends and receives data over cable television network
Much faster than modem or ISDN
Usually attaches to network card
cable modem
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48. Reading Materials: Communications Devices
What is a multiplexer ?
sometimes referred to as a MUX
a device that combines two or more input signals from various devices into a single stream of data and then transmits it over a single transmission medium
both the sending and the receiving devices need a multiplexer for data transmission to occur
The multiplexer at the receiving end takes the transmitted signal, uses the character identifiers to separate the combined data stream into its original parts, and sends the data to the appropriate device
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49. Reading Materials: Communications Devices
What is a network interface card (NIC)?
Card inserted into expansion slot of personal computer or other device, enabling it to connect to a network
NIC for desktop computer
NIC for notebook computer
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50. Reading Materials: Communications Devices
What are connecting devices?
To interconnect many types of networks that exist, various types of communications devices are used, including hubs, repeaters, bridges, gateways, and routers.
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51. Reading Materials: Communications Devices
What is a hub?
Device that provides a central point for cables in a network
                                                                  
hub
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52. Reading Materials: Communications Devices
Other connecting devices:
Repeaters: a device that accepts a signal from a transmission medium, amplifies it, and retransmits it over the medium
bridges: a device that connects two LANs using the same protocol, such as Ethernet
gateway: a combination of hardware and software that connects networks that use different protocols
router: a device that connects multiple networks - including those with differing protocol
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53. Summary of Communications and Networks
Sending and receiving devices
Communications channel
Physical transmission media
Wireless transmission media
Telephone network
Communications software
Networks
Communications protocols
Communications devices
Chapter 6 Complete
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