Komunikasi Data dan Jaringan Komputer (Bagian 3)

Komunikasi Data dan Jaringan Komputer(Bagian 3)Dr. Tb. Maulana [email protected]://staffsite.gunadarma.ac.id/mkusumaInternet dan Jaringan Komputer

Magister Manajemen Sistem Informasi

LAN GenerationFirstCarrier Sense Multiple Access (CSMA) / Collision Detection (CD) and Token RingTerminal to host and client serverModerate data ratesSecondFiber Distributed Data Interface (FDDI)BackboneHigh performance workstationsThirdAsynchronous Transfer Mode (ATM) Aggregate throughput and real time support for multimedia applications


Third Generation LANSupport for multiple guaranteed classes of serviceLive video may need 2MbpsFile transfer can use background classScalable throughputBoth aggregate and per hostFacilitate LAN / WAN internetworking


LAN Applications (1)Personal computer LANsLow costLimited data rateBack end networks and storage area networksInterconnecting large systems (mainframes and large storage devices)High data rateHigh speed interfaceDistributed accessLimited distanceLimited number of devices


LAN Applications (2)High speed office networksDesktop image processingHigh capacity local storageBackbone LANsInterconnect low speed local LANsReliabilityCapacityCost


LAN ArchitectureProtocol architectureTopologiesMedia Access Control (MAC)Logical Link Control (LLC)


Protocol ArchitectureLower layers of OSI modelIEEE 802 reference modelPhysicalLLCMAC


IEEE 802 v OSI


802 Layers - PhysicalEncoding/decodingPreamble generation/removalBit transmission/receptionTransmission medium and topology


802 Layers -Logical Link ControlInterface to higher levelsFlow and error control


802 Layers -Media Access ControlAssembly of data into frame with address and error detection fieldsDisassembly of frameAddress recognitionError detectionGovern access to transmission mediumNot found in traditional layer 2 data link controlFor the same LLC, several MAC options may be available


LAN Protocols in Context


TopologiesTreeBusSpecial case of treeOne trunk, no branchesRingStar


LAN Topologies


Bus and TreeMultipoint mediumTransmission propagates throughout medium Heard by all stationsNeed to identify target stationEach station has unique addressFull duplex connection between station and tapAllows for transmission and receptionNeed to regulate transmissionTo avoid collisionsTo avoid hoggingData in small blocks - framesTerminator absorbs frames at end of medium


Frame Transmission - Bus LAN


Ring TopologyRepeaters joined by point to point links in closed loopReceive data on one link and retransmit on anotherLinks unidirectionalStations attach to repeatersData in framesCirculate past all stationsDestination recognizes address and copies frameFrame circulates back to source where it is removedMedia access control determines when station can insert frame


Frame TransmissionRing LAN


Star TopologyEach station connected directly to central nodeUsually via two point to point linksCentral node can broadcastPhysical star, logical busOnly one station can transmit at a timeCentral node can act as frame switch


Media Access ControlWhereCentralGreater controlSimple access logic at stationAvoids problems of co-ordinationSingle point of failurePotential bottleneckDistributedHowSynchronousSpecific capacity dedicated to connectionAsynchronousIn response to demand


Asynchronous SystemsRound robinGood if many stations have data to transmit over extended periodReservationGood for stream trafficContentionGood for bursty trafficAll stations contend for timeDistributedSimple to implementEfficient under moderate loadTend to collapse under heavy load


Logical Link Control Transmission of link level PDUs between two stationsMust support multiaccess, shared mediumRelieved of some link access details by MAC layerAddressing involves specifying source and destination LLC usersReferred to as service access points (SAP)Typically higher level protocol


Bus LANSignal balancingSignal must be strong enough to meet receivers minimum signal strength requirementsGive adequate signal to noise rationNot so strong that it overloads transmitterMust satisfy these for all combinations of sending and receiving station on busUsual to divide network into small segmentsLink segments with amplifies or repeaters


Transmission MediaTwisted pairNot practical in shared bus at higher data ratesBaseband coaxial cableUsed by EthernetBroadband coaxial cableIncluded in 802.3 specification but no longer madeOptical fiberExpensiveDifficulty with availabilityNot usedFew new installationsReplaced by star based twisted pair and optical fiber


Baseband Coaxial CableUses digital signalingManchester or Differential Manchester encodingEntire frequency spectrum of cable usedSingle channel on cableBi-directionalFew kilometer rangeEthernet (basis for 802.3) at 10Mbps50 ohm cable


10Base5Ethernet and 802.3 originally used 0.4 inch diameter cable at 10MbpsMax cable length 500mDistance between taps a multiple of 2.5mEnsures that reflections from taps do not add in phaseMax 100 taps10Base5


10Base2Cheaper0.25 inch cableMore flexibleEasier to bring to workstationCheaper electronicsGreater attenuationLower noise resistanceFewer taps (30)Shorter distance (185m)


RepeatersTransmits in both directionsJoins two segments of cableNo bufferingNo logical isolation of segmentsIf two stations on different segments send at the same time, packets will collideOnly one path of segments and repeaters between any two stations


Baseband Configuration


Ring LANEach repeater connects to two others via unidirectional transmission linksSingle closed pathData transferred bit by bit from one repeater to the nextRepeater regenerates and retransmits each bitRepeater performs data insertion, data reception, data removalRepeater acts as attachment pointPacket removed by transmitter after one trip round ring


Ring Repeater States


Listen State FunctionsScan passing bit stream for patternsAddress of attached stationToken permission to transmitCopy incoming bit and send to attached stationWhilst forwarding each bitModify bit as it passese.g. to indicate a packet has been copied (ACK)


Transmit State FunctionsStation has dataRepeater has permissionMay receive incoming bitsIf ring bit length shorter than packetPass back to station for checking (ACK)May be more than one packet on ringBuffer for retransmission later


Bypass StateSignals propagate past repeater with no delay (other than propagation delay)Partial solution to reliability problem (see later)Improved performance


Star LANUse unshielded twisted pair wire (telephone)Minimal installation costMay already be an installed baseAll locations in building covered by existing installationAttach to a central active hubTwo linksTransmit and receiveHub repeats incoming signal on all outgoing linesLink lengths limited to about 100mFiber optic - up to 500mLogical bus - with collisions


Hubs and SwitchesShared medium hubCentral hubHub retransmits incoming signal to all outgoing linesOnly one station can transmit at a timeWith a 10Mbps LAN, total capacity is 10MbpsSwitched LAN hubHub acts as switchIncoming frame switches to appropriate outgoing lineUnused lines can also be used to switch other trafficWith two pairs of lines in use, overall capacity is now 20Mbps


Switched HubsNo change to software or hardware of devicesEach device has dedicated capacityScales wellStore and forward switchAccept input, buffer it briefly, then outputCut through switchTake advantage of the destination address being at the start of the frameBegin repeating incoming frame onto output line as soon as address recognizedMay propagate some bad frames


Hubs and Switches


Wireless LANWireless LANs are growing in popularity because they eliminate cabling and facilitate network access from a variety of locations. The most common wireless networking standard is IEEE 802.11, often called Wireless Ethernet or Wireless LAN.Broadband wireless (IEEE 802.16) is now growing in popularity


Wireless CommunicationsIn wireless communications signals travel through space instead of through a physical cable.Two general types of wireless communications are:Radio transmissionInfrared transmission


Why Wireless LAN?MobilityFlexibilityHard to wire areasReduced cost of wireless systemsImproved performance of wireless systems


Types of Wireless LANsIEEE 802.11aIEEE 802.11bIEEE 802.11gInfrared Bluetooth


IEEE 802.11bTwo forms of the IEEE 802.11b standard currently exist:

Direct Sequence Spread Spectrum (DSSS) systems transmit signals through a wide range of frequencies simultaneously. The signal is divided into many different parts and sent on different frequencies simultaneously. Data rate: ~ 11Mbps.Frequency Hopping Spread Spectrum (FHSS) divides the frequency band into a series of channels and then use each frequency in turn. FHSS changes its frequency channel about every half a second, using a pseudorandom sequence.


FHSS is more secure, but is only capable of data rates of 1 or 2 Mbps.IEEE 802.11a is another Wireless LAN standard developed around the same time as 802.11b. It operates in the 5 GHz band and is capable of data rates of up to 54 Mbps.IEEE 802.11g combines the best of both 802.11a and 802.11b. 802.11g supports bandwidth up to 54 Mbps, and it uses the 2.4 Ghz frequency for greater range. 802.11g is backwards compatible with 802.11b, meaning that 802.11g access points will work with 802.11b wireless network adapters and vice versa.


IEEE 802.11a vs 802.11b vs 802.11g802.11a is the most expensive. It fits predominately in the business market, whereas 802.11b better serves the home market. 802.11a supports bandwidth up to 54 Mbps and signals in a regulated 5 GHz range. Compared to 802.11b, this higher frequency limits the range of 802.11a. The higher frequency also means 802.11a signals have more difficulty penetrating walls and other obstructions.


Although slower than 802.11a, the range of 802.11b is about 7 times greater than that of 802.11a.Because 802.11a and 802.11b utilize different frequencies, the two technologies are incompatible with each other. Some vendors offer hybrid 802.11a/b network gear, but these products simply implement the two standards side by side. 802.11g offers the best of both worlds and allow for greater flexibility.


IEEE LAN StandardProsCons802.11afastest maximum speed; supports more simultaneous users; less signal interference from other devices highest cost; shorter range signal that is more easily obstructed

802.11blowest cost; signal range is best and is not easily obstructed slowest maximum speed; supports fewer simultaneous users; appliances may interfere on the unregulated frequency band


IEEE Lan StandardProsCons802.11gfastest maximum speed; supports more simultaneous users; signal range is best and is not easily obstructed costs more than 802.11b; appliances may interfere on the unregulated signal frequency


Wireless LAN ApplicationsLAN ExtensionCross building interconnectionNomadic accessAd hoc networks


LAN ExtensionBuildings with large open areasManufacturing plantsWarehousesHistorical buildingsSmall officesMay be mixed with fixed wiring system


Single Cell Wireless LAN


Multi Cell Wireless LAN


Cross Building InterconnectionPoint to point wireless link between buildingsTypically connecting bridges or routersUsed where cable connection not possiblee.g. across a street


Nomadic AccessMobile data terminale.g. laptopTransfer of data from laptop to serverCampus or cluster of buildings


Ad Hoc NetworkingPeer to peerTemporarye.g. conference


Wireless LAN Configurations


Wireless LAN RequirementsThroughputNumber of nodesConnection to backboneService areaBattery power consumptionTransmission robustness and securityCollocated network operationLicense free operationHand-off / roamingDynamic configuration


Wireless LAN TechnologyInfrared (IR) LANsSpread spectrum LANsNarrow band microwave


Wireless LAN standard IEEE 802.11The IEEE 802.11 standard for wireless LANs was finalized in 1997.The standard defines three different physical layer specifications - 2 are radio frequency-based and one is infrared-based:Direct Sequence Spread Spectrum Frequency-hopping spectrumInfrared


Wireless LAN ComponentsThe smallest building block of a wireless LAN is called the Basic Service Set (BSS).BSS is a number of stations executing the same MAC protocol and using the same shared medium.A BSS may be isolated or connected to a backbone distribution system via an access point.The distribution system is usually a wired backbone LAN.


Wireless LAN Components (contd)


Wireless LAN Components (contd)Signals from wireless computers are transmitted via built-in antennas on the NIC to the nearest access point, which serves as a wireless repeater.Because of the ease of access, security is a potential problem.The IEEE 802.11 has specified a data link security protocol called Wired Equivalent Privacy (WEP), which is designed to make the security of WLAN as good as that of wired LANs.


Medium Access ControlThe MAC protocol used in 802.11 LANs is called Distributed Foundation Wireless MAC (DFWMAC).This protocol provides a distributed access control mechanism with an optional centralized control built in.A distributed access mechanism distributes the decision to transmit over all the nodes, using a carrier sense mechanism, like CSMA/CD.


Medium Access Control (contd)A centralized access mechanism involve regulation of transmission by a centralized manager. It is particularly useful for time-sensitive or high priority data.The MAC layer is divided into 2 sub-layers:The lower layer is called the Distributed Coordination Function (DCF), operates similar to CSMA/CD. Used for ordinary traffic.


Medium Access Control (contd)The upper layer is called the Point Coordination Function (PCF). PCF is a centralized MAC algorithm used for contention-free service. All implementations must support DCF, but PCF is optional.When DCF is employed, 802.11 uses a protocol called CSMA/CD to regulate access to the medium.


Wireless LANs use CSMA/CA .Like CSMA/CD, stations listen before they transmit and if the line is free, they transmit.Detecting collisions is more difficult in wireless networks, so wireless LANs try to avoid collisions to a greater extent than traditional Ethernet.Two different WLAN MAC techniques are now in use: the Physical Carrier Sense Method and the Virtual Carrier Sense Method.Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA )


Physical Carrier Sense MethodIn the physical carrier sense method, a node that wants to send first listens to make sure that the transmitting node has finished, then waits a period of time longer.Each frame is sent using the Stop and Wait ARQ, so by waiting, the listening node can detect that the sending node has finished and can then begin sending its transmission. With Wireless LAN, ACK / NAK signals are sent a short time after a frame is received, while stations wishing to send a frame wait a somewhat longer time, ensuring that no collision will occur.


Virtual Carrier Sense MethodWhen a computer on a Wireless LAN is near the transmission limits of the AP at one end and another computer is near the transmission limits at the other end of the APs range, both computers may be able to transmit to the AP, but can not detect each others signals. This is known as the hidden node problem. When it occurs, the physical carrier sense method will not work.


Virtual Carrier Sense Method (contd)The virtual carrier sense method solves this problem by having a transmitting station first send a request to send (RTS) signal to the AP.If the AP responds with a clear to send (CTS) signal, the computer wishing to send a frame can then begin transmitting.


Infrared Wireless LANInfrared WLAN is less flexible than IEEE 802.11 WLANs because, as with TV remote controls that are also infrared based, they require line of sight to work. Infrared Hubs and NICs are usually mounted in fixed positions to ensure they will hit their targets. The main advantage of infrared WLAN is reduced wiring.A new version, called diffuse infrared, operates without a direct line of sight by bouncing the infrared signal off of walls, but is only able to operate within a single room and at distances of only about 50-75 feet.


Infrared Wireless LAN (contd)


BluetoothBluetooth is a 1 Mbps wireless standard developed for piconets, small personal or home networks. It may soon be standardized as IEEE 802.15.Bluetooth is designed to facilitate networking of different hand-held and mobile devices. For example:linking a wireless mouse to a computer, a telephone headset to a base unit, or a Palm handheld computer to your car to lock or unlock the door.3-in-1 phone conceptautomatic synchronizer : automatically synchronizes a users desktop PC, mobile PC and mobile phone.


Bluetooth was designed to operate within a very small area (up to 30 feet). May be extended.Devices are small and cheap.A Bluetooth network consists of no more than eight devices, but can be linked to other piconets to from larger networks.Although Bluetooth uses the same 2.4 GHz band as Wireless LANs, it is not compatible with the IEEE 802.11 standard and so cannot be used in locations that use the Wireless LANs.


Ethernet (CSMA/CD)Carriers Sense Multiple Access with Collision DetectionXerox - EthernetIEEE 802.3


IEEE802.3 Medium Access ControlRandom Access Stations access medium randomly ContentionStations content for time on medium


CSMAPropagation time is much less than transmission timeAll stations know that a transmission has started almost immediatelyFirst listen for clear medium (carrier sense)If medium idle, transmitIf two stations start at the same instant, collisionWait reasonable time (round trip plus ACK contention)No ACK then retransmitMax utilization depends on propagation time (medium length) and frame lengthLonger frame and shorter propagation gives better utilization


If Busy?If medium is idle, transmitIf busy, listen for idle then transmit immediatelyIf two stations are waiting, collision


CSMA/CDWith CSMA, collision occupies medium for duration of transmissionStations listen whilst transmitting

If medium idle, transmitIf busy, listen for idle, then transmitIf collision detected, jam then cease transmissionAfter jam, wait random time then start againBinary exponential back off


CSMA/CDOperation


Collision DetectionOn baseband bus, collision produces much higher signal voltage than signalCollision detected if cable signal greater than single station signalSignal attenuated over distanceLimit distance to 500m (10Base5) or 200m (10Base2)For twisted pair (star-topology) activity on more than one port is collisionSpecial collision presence signal


Gigabit Ethernet Configuration


Gigabit Ethernet - DifferencesCarrier extensionAt least 4096 bit-times long (512 for 10/100)Frame bursting


Gigabit Ethernet - Physical1000Base-SXShort wavelength, multimode fiber1000Base-LXLong wavelength, Multi or single mode fiber1000Base-CXCopper jumpers

Token Ring (802.5)MAC protocolSmall frame (token) circulates when idleStation waits for tokenChanges one bit in token to make it SOF for data frameAppend rest of data frameFrame makes round trip and is absorbed by transmitting stationStation then inserts new token when transmission has finished and leading edge of returning frame arrivesUnder light loads, some inefficiencyUnder heavy loads, round robin


Token Ring Operation


Priority Scheme


Dedicated Token RingCentral hubActs as switchFull duplex point to point linkConcentrator acts as frame level repeaterNo token passing


FDDI100MbpsLAN and MAN applicationsToken Ring


FDDI Operation


ATM LANsAsynchronous Transfer ModeVirtual paths and virtual channelsPreconfigured or switchedGateway to ATM WANBackbone ATM switchSingle ATM switch or local network of ATM switchesWorkgroup ATMEnd systems connected directly to ATM switchMixed system


Example ATM LAN


ATM LAN HUB


CompatibilityInteraction between end system on ATM and end system on legacy LANInteraction between stations on legacy LANs of same typeInteraction between stations on legacy LANs of different types


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Komunikasi Data dan Jaringan Komputer (Bagian 3)