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Chapter 8-10
Local Area Networks(LANs)
Comparison 4e and 5eCh 7, 4e Ch 8, 5e Ch 10, ForouzanCh 8, 4e Ch 14, 5e Ch 13, ForouzanCh 9, 4e Ch 13, 5e Ch 14, ForouzanCh 10, 4e Ch 15, 5e Ch 14, Forouzan
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Classification TerminologyNetwork technologies classified into three
broad categoriesLocal Area Network (LAN)Metropolitan Area Network (MAN)Wide Area Network (WAN)
LAN and WAN most widely deployed
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The Local Area Network (LAN)
Engineering classificationExtremely popular (most networks are LANs)Many LAN technologies exist
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Key Features of a LANHigh throughputRelatively low costLimited to short distanceOften rely on shared media
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Scientific Justification ForLocal Area Networks
A computer is more likely to communicate with computers that are nearby than with computers that are distant
Known as the locality principle
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TopologyMathematical termRoughly interpreted as “geometry for curved
surfaces”
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Network TopologySpecifies general “shape” of a networkHandful of broad categoriesOften applied to LANPrimarily refers to interconnectionsHides details of actual devices
Fully connected mesh topology (for five devices)
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Bus Topology
Shared medium forms main interconnectEach computer has a connection to the
medium
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Ring Topology
No central facilityConnections go directly from one computer
to another
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Star Topology
Central component of network known as hubEach computer has separate connection to
hub
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Example Bus Network: Ethernet
Most popular LANWidely usedIEEE standard 802.3Several generations
Same frame formatDifferent data ratesDifferent wiring schemes
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Shared Medium in a LANShared medium used for all transmissionsOnly one station transmits at any timeStations “take turns” using mediumMedia Access Control (MAC) policy ensures
fairness
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Illustration of Ethernet Transmission
Only one station transmits at any timeSignal propagates across entire cableAll stations receive transmissionCSMA/CD media access scheme
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CSMA/CD ParadigmMultiple Access (MA)
Multiple computers attach to shared mediaEach uses same access algorithm
Carrier Sense (CS)Wait until medium idleBegin to transmit frame
Simultaneous transmission possible
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CSMA/CD Paradigm(continued)
Two simultaneous transmissionsInterfere with one anotherCalled collision
CSMA plus Collision Detection (CD)Listen to medium during transmissionDetect whether another station’s signal interferesBack off from interference and try again
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Backoff After CollisionWhen collision occurs
Wait random time t1, 0 < t1 < dUse CSMA and try again
If second collision occursWait random time t2, 0 < t2 < 2*d
Double range for each successive collisionCalled exponential backoff
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Media Access on a Wireless Net
Limited rangeNot all stations receive all transmissionsCannot use CSMA/CD
Example in diagramMaximum transmission distance is dStations 1 and 3 do not receive each other’s
transmissions
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CSMA/CAUsed on wireless networksBoth sides send small message followed by data
transmission“X is about to send to Y”“Y is about to receive from X”Data from sent from X to Y
Purpose: inform all stations in range of X or Y before transmission
Known as Collision Avoidance (CA)
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Identifying a DestinationAll stations on shared-media LAN receive all
transmissionsTo allow sender to specify destination
Each station assigned unique numberKnown as station’s addressEach frame contains address of intended
recipient
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Assignment of physical address
The stations may get their address in different ways:
StaticConfigurableDynamic
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Ethernet AddressingStandardized by IEEEEach station assigned by unique 48-bit
addresse.g. 00:30:65:52:2E:96 in hexadecimal form
Address assigned when network interface card (NIC) manufactured (In most cases)
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Ethernet Address Recognition
Each frame contains destination addressAll stations receive a transmissionStation discards any frame addresses to
another stationImportant: interface hardware, not software,
checks address
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Possible DestinationsPacket can be sent to:
Single destination (unicast)All stations on network (broadcast)Subset of stations (multicast)
Address used to distinguish
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Advantages of Address Alternatives
UnicastEfficient for interaction between two computers
BroadcastEfficient for transmitting to all computers
MulticastEfficient for transmitting to a subset of
computers
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Broadcast on EthernetAll 1s address specifies broadcast
(FF:FF:FF:FF:FF:FF in hexcode)Sender
Places broadcast address in frameTransmits one copy on shared networkAll stations receive copy
Receiver always accepts frame that contains this address
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MulticastHalf of addresses reserved for multicastNetwork interface card
Always accepts unicast and broadcastCan accept zero or more multicast addresses
SoftwareDetermines multicast address to acceptInforms network interface card
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Promiscuous ModeDesigned for testing / debuggingAllows interface to accept all packetsAvailable on most interface hardware
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Identifying Frame Contents
Integer type field tells recipient the type of data being carried
Two possibilitiesSelf-identifying or explicit type (hardware record
type)Implicit type (application sending data must
handle type)
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Conceptual Frame Format
HeaderContains address and type informationLayout fixed
PayloadContains data being sent
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Illustration Of Ethernet Frame
Sender placesSender’s address in sourceRecipient’s address in destinationType of data in frame typeCyclic redundancy check in CRC
Figure 14.3 Minimum and maximum length
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Example Ethernet Types
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When Network HardwareDoes Not Include Types
Sending and receiving computers must agreeTo only send one type of dataTo put type information in first few octets of
payloadMost systems need type information
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Illustration of TypeInformation Added to Data
In practiceType information small compared to data carriedFormat of type information standardized
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A Standard For Type Information
Defined by IEEEUsed when hardware does not include type fieldCalled LLC / SNAP header
Logical Link ControlSubNetwork Attachment Point
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Demultiplexing On TypeNetwork interface hardware
Receives copy of each transmitted frameExamines address and either discards or acceptsPasses accepted frame to system software
Network device softwareExamines frame typePasses frame to correct software module
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Network AnalyzerDevice used for testing and maintenanceListens in promiscuous modeProduces
Summaries (e.g., % of broadcast frames)Specific items (e.g., frames from a given address)
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Bus Topology
Any user with a Network Analyzer can read all packets!
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Star Topology
Star Topology and Bus Topology are equal fom security point!
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Ethernet WiringThree schemes
Correspond to three generationsAll use same frame format
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Original Ethernet Wiring
Used heavy coaxial cableFormal name 10Base5Called thicknet
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Second Generation Ethernet Wiring
Used thinner coaxial cableFormal name 10Base2Called thinnet
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Modern Ethernet Wiring
Uses a hubFormal name 10Base-TCalled twisted pair Ethernet
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Ethernet Wiring In An Office
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A Note About Ethernet Topology
ApparentlyOriginal Ethernet used bus topologyModern Ethernet uses star topology
In fact, modern Ethernet isPhysical starLogical busCalled star-shaped bus
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Higher Speed EthernetsFast Ethernet
Operates at 100 MbpsFormally 100Base-TTwo wiring standards10/100 Ethernet devices available
Gigabit EthernetOperates at 1000 Mbps (1 Gbps)Slightly more expensive
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Another LAN Using Bus Topology
LocalTalkDeveloped by
Apple Corp.1984
Simple to use
Slow by current standards
(230,4 kbps)
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Illustration Of LocalTalk
Transceiver required per stationTransceiver terminates cable
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Ring Topology
Second most popular LAN topologyBits flow in single directionSeveral technologies exist
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Token PassingUsed with ring topologyGuarantees fair accessToken
Special (reserved) messageSmall (a few bits)
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Token Passing ParadigmStation
Waits for the token to arriveTransmits one packet around ringTransmits token around ring
When no station has data to sendToken circulates continuously
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Token Passing Ring Transmission
Station waits for token before sendingSignal travels around entire ringSender receives its own transmission
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Strengths of Token Ring Approach
Easy detection ofBroken ringHardware failuresInterference
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Weaknesses of Token Ring Approach
Broken wire disables entire ringPoint-to-point wiring
Awkward in office environmentDifficult to add / move stations
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Token Passing Ring Technologies
ProNet-10Operated at 10 Mbps
IBM Token RingOriginally operated at 4 MbpsLater version operated at 16 Mbps
Fiber Distributed Data Interconnect (FDDI)Operated at 100 Mbps
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FDDI TerminologyFDDI
Uses optical fibersHigh reliabilityImmune to interference
CDDIFDDI over copperSame frame formatSame data rateLess noise immunity
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FDDI Hub TechnologyPart of FDDI standardStations attach to hubSame frame format and data rate as FDDICalled star-shaped ring
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FDDI Failure RecoveryUses two ringsAutomatic failure recoveryTerminology
Dual-attachedCounter rotatingSelf healing
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Illustration of FDDIFailure Recovery
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Another Example of aPhysical Star Topology
Asynchronous Transfer Mode (ATM)Designed by telephone companiesIntended to accommodate
VoiceVideoData
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ATM
Building block known as ATM switchEach station connects to switchSwitches can be interconnected
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Details of ATM Connection
Full-duplex connectionsTwo fibers required
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ATM CharacteristicsHigh data rates (e.g. 155 Mbps)Fixed size packets
Called cellsImportant for voice
Cell size is 53 octets48 octets of data5 octets of header
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SummaryLocal Area Networks
Designed for short distanceUse shared mediaMany technologies exist
Topology refers to general shapeBusRingStar
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Summary (continued)
AddressUnique number assigned to stationPut in frame headerRecognized by hardware
Address formsUnicastBroadcastMulticast
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Summary (continued)
Type informationDescribes data in frameSet by senderExamined by receiver
Frame formatHeader contains address and type informationPayload contains data being sent
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Summary (continued)
LAN technologiesEthernet (bus)IBM Token RingFDDI (ring)ATM (star)
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Summary (continued)
Wiring and topologyCan distinguish
Logical topologyPhysical topology (wiring)
Hub allows Star-shaped busStar-shaped ring
Figure 14.1 Three generations of Ethernet
Figure 14.5 Unicast and multicast addresses
Figure 14.6 Physical layer
Figure 14.7 PLS
Figure 14.8 AUI
Figure 14.9 MAU (transceiver)
Figure 14.10 Categories of traditional Ethernet
Figure 14.11 Connection of a station to the medium using 10Base5
Figure 14.12 Connection of stations to the medium using 10Base2
Figure 14.13 Connection of stations to the medium using 10Base-T
Figure 14.14 Connection of stations to the medium using 10Base-FL
Figure 14.15 Sharing bandwidth
Figure 14.16 A network with and without a bridge
Figure 14.17 Collision domains in a nonbridged and bridged network
Figure 14.18 Switched Ethernet
Figure 14.19 Full-duplex switched Ethernet
14.2 Fast Ethernet14.2 Fast Ethernet
MAC Sublayer
Physical Layer
Physical Layer Implementation
Figure 14.20 Fast Ethernet physical layer
Figure 14.21 MII
Figure 14.22 Fast Ethernet implementations
Figure 14.23 100Base-TX implementation
Figure 14.24 Encoding and decoding in 100Base-TX
Figure 14.25 100Base-FX implementation
Figure 14.26 Encoding and decoding in 100Base-FX
Figure 14.27 100Base-T4 implementation
Figure 14.28 Using four wires in 100Base-T4
14.3 Gigabit Ethernet14.3 Gigabit Ethernet
MAC Sublayer
Physical Layer
Physical Layer Implementation
Figure 14.29 Physical layer in Gigabit Ethernet
Figure 14.30 Gigabit Ethernet implementations
Figure 14.31 1000Base-X implementation
Figure 14.32 Encoding in 1000Base-X
Figure 14.33 1000Base-T implementation