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Connecting with Computer
Science, 2e
Chapter 4
Networks
Objectives
• In this chapter you will:
– Learn how computers are connected
– Become familiar with different types of transmission
media
– Learn the differences between guided and unguided
media
– Learn how protocols enable networking
– Learn about the ISO OSI reference model
– Understand the differences between network types
– Learn about local area networks (LANs)
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Objectives (cont’d.)
• In this chapter you will (cont’d.):
–
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Learn about wide area networks (WANs)
Learn about wireless local area networks (WLANs)
Learn about network communication devices
Learn how WANs use switched networks to
communicate
– Learn how devices can share a communication
medium
– Learn about DSL, cable modems, and satellite
communications
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Why You Need to Know About...
Networks
• Networks
– Connect computers together
– Allows sharing of resources
– Used in government research centers, universities,
large corporations, and home computing
– Integral part of computing for homes and enterprises
– Include the Internet
• Integrated into personal computers
• Computing professional
– Incorporates network technology into everything
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Connecting Computers
• Linking computers through system bus is impractical
– PCI bus has 98 wires
• Requires a very thick cable
– Connection problem solved with technology
– Connecting requires a medium (e.g., wire)
• Carries electric signal and a communications protocol
to manage processes
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Transmission Medium
• Transmission medium
– Material conducting electrical and/or electromagnetic
signals
– Most popular: copper
• Rating transmission media
– Bandwidth: medium speed measured in bits/second
– Signal-to-noise ratio: 10 log10 (signal/noise)
– Bit error rate: ratio of incorrect bits to total number of
bits in unit time
– Attenuation: signal weakening over distance
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Transmission Medium (cont’d.)
• Two general transmission media types
– Guided
• Physical media
– Copper wire: form of twisted pair or coaxial cable
– Fiber-optic: uses glass and light to transmit data
– Unguided
• Air and space carry radio frequency (RF) or infrared
(IR) light signals
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Transmission Medium (cont’d.)
Figure 4-1, Coaxial, twisted pair, and
fiber-optic cable are guided media
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Copper Wire: Coaxial and Twisted Pair
• Two wires used to transmit data: signal and ground
• Coaxial cable (coax)
– Copper surrounded by metal shield to reduce noise
– Support bandwidths up to 600 MHz
• 10BaseT
• Twisted pair
– Replacing copper
– Twists dampens effects of inductance
– All copper wiring is subject to impedance
• Attenuation: reduction in signal
– Two types: shielded and unshielded (UTP)
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Copper Wire: Coaxial and Twisted Pair
(cont’d.)
Table 4-1, EIA/TIA twisted pair cable categories
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Copper Wire: Coaxial and Twisted Pair
(cont’d.)
• Twisted pair (cont’d.)
– Unshielded twisted pair (UTP)
• More popular than shielded twisted pair (STP)
– Category 5 (Cat 5)
• Most common twisted pair cable
• Maximum frequency: 100 MHz
• Good for business and home use
• Computer industry has turned to optical media
– Provides faster data transmission
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Fiber-Optic Cable
• Glass fibers guide light pulses along a cable
– Thin strand of nearly pure glass
• Surrounded by reflective material and a tough outer
coating
• Transmission speeds much higher than with copper
– Fiber-optic cables are much less susceptible to
attenuation and inductance
– Principle of inductance does not apply
– Bandwidths hundreds of times faster than copper
• Economies of scale bringing price down
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Unguided Media: Wireless
Technologies
• Benefits of wireless technology
– Eliminates cables and cabling costs
– Provides device mobility
• Basis of wireless technology: radio transmission
– Examples: cell phones, microwave ovens, etc.
– Electronic signal is amplified
• Radiated from an antenna as electromagnetic waves
• Receiving antenna converts back to electronic signal
• Electromagnetic waves
– Transmitted at many different frequencies
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Unguided Media: Wireless
Technologies (cont’d.)
Table 4-2, Wireless technologies
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Unguided Media: Wireless
Technologies (cont’d.)
Figure 4-2, Wireless technologies
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Unguided Media: Wireless
Technologies (cont’d.)
• Difference between low-pitched sound and a highpitched sound
– Frequency of the sound waves, or vibrations
• Uses same technology as car radio and cell phone
• Industry standards (based on 2.4 GHz range)
– IEEE 802.11 series: most common
– Bluetooth specification
• Short-range RF links between mobile computers,
mobile phones, digital cameras, other portable devices
• Maximum distance between devices: three inches to
328 feet
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Light Transmission
• Infrared light used over short distances
• Uses transmission media of air
– Requires clear line of sight between devices
• Used in PDAs, cell phones, notebook computers,
wireless keyboards, and mice
• Pulses of infrared light represent 1s and 0s of binary
transmission
– Speeds up to 4 Mbps
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Protocols
• Set of rules facilitating communication
– Example: classroom questioning
• Often represented with a timing diagram
• Transmission protocol
– Provides for an orderly flow of information
– HTTP, TCP/IP, FTP
• Transmission Control Protocol (TCP)
– Allows two computers to establish a communication
connection, transfer data, and terminate the
connection
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Protocols (cont’d.)
• Must have provisions to check for errors and
retransmit
• International Organization for Standardization (ISO)
– Coordinates worldwide standards development
• Comité Consultatif International Téléphonique et
Télégraphique (International Telegraph and
Telephone Consultative Committee, CCITT)
– Formulated the ISO Open Systems Interconnect
reference model (ISO OSI reference model)
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Protocols (cont’d.)
Table 4-4, Timing diagram for a communication protocol
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ISO OSI Reference Model
• Conceptual model with seven discrete layers
– Physical
• Defines electrical, mechanical, procedural, and
functional specifications for activating and maintaining
the physical link between end systems
– Data Link
• Provides reliable data transit, physical addressing, data
error notification, ordered delivery of frames, and flow
control
– Network
• Provides connectivity and path selection, and assigns
addresses to messages
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ISO OSI Reference Model (cont’d.)
• Conceptual model with seven discrete layers
(cont’d.)
– Transport
• Guarantees delivery of datagrams
• Provides fault detection, error recovery, and flow
control
• Manages virtual circuits
– Session
• Establishes, maintains, and terminates communication
session between applications
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ISO OSI Reference Model (cont’d.)
• Conceptual model with seven discrete layers
(cont’d.)
– Presentation
• Formats data for presentation to an application
• Performs character format translation: ASCII to
Unicode
• Syntax selection
– Application
• Provides network access to applications
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ISO OSI Reference Model (cont’d.)
• Layers are defined by two components
– Header
• Layer and message information
• Sending side of the communication creates the header
• Corresponding layer on the receiving side uses the
header
– Protocol Data Unit (PDU)
• Communicates information about the message to the
next layer on the same side
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ISO OSI Reference Model (cont’d.)
Figure 4-3, How the OSI model processes data
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Network Types
• May be classified according to size and proximity
• Local area network (LAN)
– Small number of computers connected in close
proximity
– Usually confined to building or complex
– Uses copper wire
• Wide area network (WAN)
– Many computers spread over large geographical area
– Typically spans cities, states, or continents
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Network Types (cont’d.)
• MAN (metropolitan area network)
– Spans a city or metropolitan area
• Distinction between WAN and MAN
– No standardized definition
– LAN is confined to a single building
– Internet is classified as largest WAN
• WLAN (wireless local area network)
– LAN using wireless transmission medium
– IEEE 802.11 protocol family often used
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Network Types (cont’d.)
Figure 4-4, Example of a WAN configuration
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LAN Topologies
• Network configurations
• Node: computer attached to a network
– Addressable device
• Three basic LAN topologies
– Ring: connects computers in a loop with cable
– Star: computers connected to hub (central point)
– Bus: configured like a system bus on a
computer (most popular)
• Internet and home networking
– Increasing star topology popularity
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LAN Topologies (cont’d.)
Figure 4-5, LAN topologies
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LAN Communication Technologies
• Ethernet
– Widely used technology
– Industry standard
– Based on a bus topology
– Can be wired in a star pattern (star/bus) topology
– Original Ethernet transferred data at 10 Mbps
– Fast Ethernet transfers data at 100 Mbps
– Gigabit Ethernet transfers data from 1 to 10 Gbps
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LAN Communication Technologies
(cont’d.)
• Token ring
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Second most popular LAN technology
Uses a ring topology
Controls access to the network by passing token
Capable of data transfer of 4 or 16 Mbps
• FDDI and ATM
– Generally faster and more expensive
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LAN Communication Technologies
(cont’d.)
Table 4-5, Bandwidths of LAN technologies
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Network Communication Devices
• LANs, WANs, and WLANs can be connected to form
larger, more complex WANs
• Devices used to created connectivity
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Network interface cards
Repeaters, hubs, and switches
Bridges
Gateways
Routers and firewalls
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Network Communication Devices
(cont’d.)
• NIC
– Required for physical device connected to network
– Usually in motherboard expansion slot or card slot in
notebook
– Includes external port
– Has unique 48-bit address (physical or MAC address)
• Repeater
– Alleviates attenuation problem
– Amplifies signal along cable between nodes
– Does not alter data content
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Network Communication Devices
(cont’d.)
• Hub
– Special repeater with multiple inputs and outputs
– Allows multiple nodes to share same repeater
• Switch
– Repeater with many input and output ports
– Inputs and outputs are not connected
– Examines header and makes point-to-point
connection to output addressed by packet
– Assumes Data Link duties (OSI Layer 2)
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Network Communication Devices
(cont’d.)
• Bridge
– Similar to a switch
– Divides network into segments to reduce traffic
• Gateway
– Similar to a bridge
– Can interpret and translate different network protocols
– Can connect networks of different types
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Network Communication Devices
(cont’d.)
• Router
– Like bridges and gateways
– Function at higher OSI Layer 3
– Directs network traffic based on logical address
• Firewall
– Protects network
• Filters potentially harmful incoming and outgoing traffic
– May be router based
– Examines/restricts inbound and outbound traffic
– Implemented in hardware or software
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Copy editor:
“3Kbps”-should
this be
“30Kbps”?
Switched Networks
• Telephone network adapted to carry digital data
• Modems convert binary signals into audio signals
• Telephone companies split a copper conductor
bandwidth into multiple ranges or bands
– Frequency modulation (FM)
– Amplitude modulation (AM)
– Phase modulation (PM) boost speed to 3Kbps
• Combined compression techniques and rearranged
transitions are used to reach 56Kbps limit
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Switched Networks (cont’d.)
Figure 4-6, Frequency modulation, amplitude
modulation, and phase modulation
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Switched Networks (cont’d.)
• High-speed WANs
– Demand for higher access speeds
• Extend system bus
– Copper capable of speeds up to 1.5 Mbps
• Leasing all wire bandwidth (24 channels) results in
faster connection
• Very expensive
• Dedicated line called T1
• T3 line consists of 28 T1 lines
– Fiber-optic cables
• Optical carrier (OC) lines faster than T3
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Switched Networks (cont’d.)
Table 4-6, High-speed WAN connections
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Switched Networks (cont’d.)
• Multiple access
– FDM (frequency-division multiplexing)
• Divides bandwidth among subscribers
• Channel is sustained for duration of session
• Wasteful use of resources
– TDM (time-division multiplexing)
• Divides bandwidth based on time
• Achieves effect speeds greater than FDM
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Switched Networks (cont’d.)
Figure 4-7, FDM and TDM
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Switched Networks (cont’d.)
• Digital subscriber line (DSL)
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Combines FDM and TDM
Divides bandwidth into 247 channels
Allocates 4 KHz for voice, remainder for data
Modem used to place voice communication into the
correct frequency band
– Speeds range from 256 Kbps to 1.5 Mbps
– Download speeds differ from upload speeds
– Subscriber located less than 18,000 feet from station
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Cable Modems
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CATV coax cable carries hundreds of channels
Channels allocated 6 MHz bandwidth
Transmit speeds up to 42 Mbps
Connects Ethernet cable to modem
Uses TDM technology to vary upload and download
speeds
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Wireless Technologies
• Offered by cell phone providers
• Technologies
– EDGE, EVDO, and 3G
– May become the standard method of wireless
networking
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Satellite Technologies
• Long distance wireless technology
• Provides high-speed access to users in remote
locations
• Dish is used to receive television signals adapted for
data transmission
• Becoming an affordable alternative
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One Last Thought
• Networks are integral to computers and computing
• Computer scientists extensively interact with
networks
• Network concepts form a foundation for further study
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Summary
• Networks link computers around the world
• Networks are extensions of system bus
• Transmission media, set of protocols, and network
devices create connectivity
• Metrics for rating media: bandwidth, signal-to-noise
ratio, bit error rate, and attenuation
• Transmission media may be guided or unguided
• Guided media: copper wire (coax and twisted pair)
and fiber-optic cables
• Unguided media: air or space (wireless)
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Summary (cont’d.)
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Protocols: set of rules for communication
Standards: TCP/IP and seven-layered OSI model
Network types: LANs, WLANs, WANs, MANs
LAN topologies: ring, star, bus
LAN technologies: Ethernet, token ring, FDDI, ATM
Network devices: NIC, repeater, hub, switch, bridge,
gateway, router, firewall
• Switched networks: convert analog to digital using
FM, AM, PM, compression, rearranged transitions
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Summary (cont’d.)
• DSL: combines FDM and TDM to boost copper wire
signals to 1.5 Mbps
• Cable modems: coax cables transmit at 1.5 to 42
Mbps
• Satellite technologies: long distance wireless
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