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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)
Objectives (cont’d.)
• In this chapter you will (cont’d.): – 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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Figure 4-1, Coaxial, twisted pair, and fiber-optic cable are guided media
Transmission Medium (cont’d.)
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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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Table 4-1, EIA/TIA twisted pair cable categories
Copper Wire: Coaxial and Twisted Pair (cont’d.)
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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Table 4-2, Wireless technologies
Unguided Media: Wireless Technologies (cont’d.)
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Figure 4-2, Wireless technologies
Unguided Media: Wireless Technologies (cont’d.)
Unguided Media: Wireless Technologies (cont’d.)
• Difference between low-pitched sound and a high-pitched 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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Table 4-4, Timing diagram for a communication protocol
Protocols (cont’d.)
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
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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Figure 4-3, How the OSI model processes data
ISO OSI Reference Model (cont’d.)
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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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Figure 4-4, Example of a WAN configuration
Network Types (cont’d.)
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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 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 – 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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Table 4-5, Bandwidths of LAN technologies
LAN Communication Technologies (cont’d.)
Network Communication Devices
• LANs, WANs, and WLANs can be connected to form larger, more complex WANs
• Devices used to created connectivity – 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
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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Copy editor: “3Kbps”-should this be “30Kbps”?
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Figure 4-6, Frequency modulation, amplitude modulation, and phase modulation
Switched Networks (cont’d.)
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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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Table 4-6, High-speed WAN connections
Switched Networks (cont’d.)
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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.)
• Digital subscriber line (DSL) – 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
• 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.)
• 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