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WiFi Overview - Wireless@ICTP

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<strong>WiFi</strong> <strong>Overview</strong>Abdus Salam ICTP, February 2005Radio Based Computer Networking forResearch and Trainingin Developing CountriesErmanno PietrosemoliLatin American Networking School(Fundación EsLaRed) – ULAMérida Venezuela www.eslared.org.ve2/14/2005 Pietrosemoli 1


<strong>WiFi</strong> <strong>Overview</strong>Agenda• 802.11 Standards• 802.11 Terminology• DSSS Channel Allocation• Medium Access Control• Power Considerations• Scanning• Configuration• Interference2/14/2005 Pietrosemoli 2


Wireless Data Transmission flavors• Packet Radio over VHF or HF• Wireless Local Area Networks (WLAN)• Wireless Local Loop (WLL, LMDS)• Free Space Optics• Satellite Transmission2/14/2005 Pietrosemoli 3


Wi-Fi Technology <strong>Overview</strong>• Wireless networks where borne as LANs,but for developing countries’ applicationsthey are more useful as MANs or evenWANs• The enormous success of this technologyhas led to a dramatic price reduction of theradios +modem, from $750 in 1992 to $30in 2004, while transmission speed hasincreased up to 108 Mbps on the same 20MHz channel2/14/2005 Pietrosemoli 4


Wi-Fi Technology <strong>Overview</strong>: Standards• IEEE 802.111 and 2 Mbps, Frequency Hopping, DSSS (915 or2400 MHz ) or IR, Ratified in 1977• IEEE 802.11 a up to 54 Mbps, 5 GHz, OFDM• IEEE 802.11 b up to 11 Mbps, 2.4 GHz, DSSSBoth ratified in 1999• IEEE 802.11g up to 54 Mbps, 2.4 GHz, OFDM,downward compatible with 802.11b, Ratified in20032/14/2005 Pietrosemoli 5


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Elements of a TransmissionSystem•Transmitter•Connecting cable or waveguide•Antennas•Receiver•Power Supply, Grounding andLightning Protection2/14/2005 Pietrosemoli 8


System ConfigurationSub StationPoint to PointSub StationPoint to PointBase StationPoint to MultipoinAccess PointSub StationPoint to Point


IEEE 802 .11 TerminologyStation (STA) Architecture:• Device that contains IEEE 802.11conformant MAC and PHYinterface to the wireless medium,but does not provide access to adistribution system• Most often end-stations availablein terminals (work-stations,laptops etc.)RadioRadioHardwareHardwarePC-CardPC-CardHardwareHardware802.11 frame formatWMAC controller withWMAC controller withStation FirmwareStation Firmware(WNIC-STA)(WNIC-STA)802.3 frame formatDriverDriverSoftwareSoftware(STADr)(STADr)PlatformPlatformComputerComputerEthernet V2.0 / 802.3frame formatProtocol StackProtocol Stack


IEEE 802 .11 TerminologyAccess-Point (AP) Architecture:• Device that contains IEEE 802.11conformant MAC and PHYinterface to the wireless medium,and provide access to adistribution system for associatedstations• Most often infra-structureproducts that connect to wiredbackbonesRadioRadioHardwareHardwarePC-CardPC-CardHardwareHardware802.11 frame formatWMAC controller withWMAC controller withStation FirmwareStation Firmware(WNIC-STA)(WNIC-STA)802.3 frame formatDriverDriverSoftwareSoftware(STADr)(STADr)PlatformPlatformComputerComputerEthernet V2.0 / 802.3frame formatProtocol StackProtocol Stack


IEEE 802 .11 TerminologyBSS• A set of stations controlled by a single“Coordination Function” (the logical function thatdetermines when a station can transmit orreceive)• Similar to a “cell” in mobile phone terminology• A BSS can have an Access-Point (both instandalone networks and in building-wideconfigurations), or can run without and Access-Point (in standalone networks only)• Diameter of the cell is app. twice the coveragedistancebetween two wireless stations


IEEE 802 .11 TerminologyIndependent Basic Service Set (IBSS):• A Basic Service Set (BSS) which forms a selfcontainednetwork in which no access to aDistribution System is available• A BSS without an Access-Point• One of the stations in the IBSS can be configuredto “initiate” the network and assume theCoordination Function• Diameter of the cell determined by coveragedistance between two wireless stations


Independent Basic Service Set(IBSS)IBSS


IEEE 802 .11 TerminologyExtended Service Set (ESS):• A set of one or more Basic Service Setsinterconnected by a Distribution System (DS)• Traffic always flows via Access-PointDistribution System (DS):• A system to interconnect a set of Basic Service SetsIntegrated; A single Access-Point in astandalone networkWired; Using cable to interconnect theAccess-PointsWireless; Using wireless to interconnect theAccess-Points


Extended Service Set (ESS)BSS’s and wireless Distribution System (DS)BSSDistributionSystemBSS


IEEE 802 .11 TerminologyService Set Identifier (SSID):• “Network name”• 32 octets long• One network (ESS or IBSS) has one SSID


IEEE 802 .11 TerminologyBasic Service Set Identifier (BSSID)• “cell identifier”• 6 octets long (MAC address format)• One BSS has one SSID• Value of BSSID is the same as the MAC addressof the radio in the Access-Point


MAC Management Frames• Beacon Timestamp, Beacon Interval, Capabilities, SSID,Supported Rates, parameters Traffic Indication Map• Probe SSID, Capabilities, Supported Rates• Probe Response Timestamp, Beacon Interval, Capabilities, SSID,Supported Rates, parameters same for Beacon except for TIM


MAC Management Frames(cont’d)• Association Request Capability, Listen Interval, SSID, SupportedRates• Association Response Capability, Status Code, Station ID, SupportedRates• Re-association Request Capability, Listen Interval, SSID, SupportedRates, Current AP Address• Re-association Response Capability, Status Code, Station ID, SupportedRates


Channel Overlapping2/14/2005 Pietrosemoli 24


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IEEE 802.11 Medium Access Control LogicReady to transmit a framemedium free?NoWait IFSYesStill free?YesTransmit FrameNoWait for the end ofcurrent tr.Wait IFSStill free?NoYesExponential BackoffTransmit Frame2/14/2005 Pietrosemoli 27


Operational processesInter-Frame SpacingFree access when mediumis free longer than DIFSDIFSContention WindowDIFSPIFSBusy MediumSIFSBackoff-WindowNext FrameSlot timeDefer AccessSelect Slot and Decrement Backoff as long as medium is idle.• Inter frame spacing required for MAC protocol traffic SIFS = Short interframe space PIFS = PCF interframe space DIFS = DCF interframe space• Back-off timer expressed in terms of number of timeslots2/14/2005 Pietrosemoli 28


Operational processesData Frames and their ACKDIFSSrcDataSIFSDestOtherAckDIFSContention WindowNext MPDUDefer AccessBackoff after Defer• Acknowledgment are to arrive within the SIFS• The DCF interframe space is observed beforemedium is considered free for use2/14/2005 Pietrosemoli 29


Channel ReservationSendingClientAccess PointReceivingClientRequest to send (RTS)dataClear to send (CTS)Acknowledgment (ACK)2/14/2005 Pietrosemoli 30


802.11b spectral maskTransmitSpectrumMask0 dBrUnfilteredSinx/x-30 dBr-50 dBrfc -22 MHzfc -11 MHzfcfc +11 MHzfc +22 Mhz2/14/2005 Pietrosemoli 31


Control Frames• Request to send (RTS)• Clear to send (CTS)• Acknowledgement (ACK)• Power-Save Poll (PS Poll)• Contention-Free End (CF End)• CF End + CF Ack2/14/2005 Pietrosemoli 32


Management Frames• Association request frame• Association response frame• Reassociation request frame• Reassociation response frame• Probe request frame• Probe response frame• Beacon frame• ATIM frame• Disassociation frame• Authentication frame• Deauthentication frame2/14/2005 Pietrosemoli 33


LLCContention-free ServiceContention ServiceMacLayerPointCoordinationFunction (PCF)Distributed Coordination Function(DCF)2.4 GHzfrequencyhoppingspreadspectrum1 Mbps2 Mbps2.4 GHzdirectsequencespreadspectrum1 Mbps2 Mbps5 GHzorthogonalFDM6,9.12.18,24,3648,54 Mbps2.4 GHzdirectsequencespreadspectrum5.5 Mbps11 Mbps2.4 GHzDSS54 Mbps5.5 Mbps11 MbpsIEEE 802.11 IEEE 802.11a IEEE 802.11bIEEE 802.11g2/14/2005 Pietrosemoli 34


Frames spacing intervals for DSSSShort Interframe Spacing(SIFS) 10 µsPoint Coordination Function InterframeSpace(PIFS) 30 µsDistributed Coordination FunctionInterframe Space(DIFS) 50 µs2/14/2005 Pietrosemoli 35


Fragmentation2/14/2005 Pietrosemoli 36


Dynamic transmission rate2/14/2005 Pietrosemoli 37


Traffic Indication Map (TIM)The TIM is used an as indicator of which sleeping stationshave packets queued at the access point. This informationis passed in each beacon to all associated stations. Whilesleeping, synchronized stations power up their receivers,listen for the beacon, check the TIM to see if they arelisted, then, if they are not listed, they power down theirreceivers and continue sleeping..2/14/2005 Pietrosemoli 38


Operational processesAssociation• To establish relationship with Access-Point• Stations scan frequency band to and select Access-Point with best communications quality Active Scan (sending a “Probe request” on specific channelsand assess response) Passive Scan (assessing communications quality frombeacon message)• Access-Point maintains list of associate stations inMAC FW Record station capability (data-rate) To allow inter-BSS relay• Station’s MAC address is also maintained in bridgelearn table associated with the port it is located on


2/14/2005 Pietrosemoli 40


Network Name (SSID)Configuration ParametersBasic parameters (Station)• ASCII string to identify the network that the stationwants to connect to.Station Name (SSID)• ASCII string to provide a user friendly stationidentification, when used in diagnostic purposes (inWindows systems: equal to “computer name”)Type of Operation• To identify the kind of network that the station willbe part of Network centered around APs IBSS (peer-to-peer network)


Configuration ParametersAdvanced parameters (Station)MAC Address• Physical address of the card: Universal; factory installed (default) Local; user-defined (6 Hexadecimal characters)Distance between APs• To specify the coverage of a “cell” in terms of thedistance between the Access-Points Large Medium Small


Configuration ParametersAdvanced parameters (Station)Interference Robustness• Check box to enable/disable data-rate fallback delaymechanismto allow improved performance in presence ofmicrowave ovens or other interference signalsRTS/CTS Medium Reservation• Check box to enable/disable the RTS/CTS handshake.Card Power Management• Check box to enable/disable Power Management


Configuration ParametersEncryption parameters (Station)Enable Encryption• To enable/disable EncryptionEncryption keys• Four fields to store up to four different encryption keys• Entries take up to 5 ASCII or 10 hexa-decimal values (whenusing 64 WEP)Encryption key index• Index identifying which of the four keys is the active one


Configuration ParametersBasic parameters (AP)Network Name (SSID)• ASCII string to identify the network that the Access-Point is part of (similar to Domain-ID in WaveLANpre-IEEE). Only available in “Access Point” mode.Frequency (channel)• To indicate the frequency channel that the AP-500/1000 will use for its “cell”. The channel isselected from the set that is allowed in the regulatorydomain.


Interference RobustnessConfiguration ParametersAdvanced parameters (AP)• Check box to enable/disable data-rate fallback delaymechanismto allow improved performance in presence ofInterferenceDTIM• Power Management related parameter to specify the timingof the delivery of multicast traffic to stations that haveindicated to receive multicast messages while under powermanagement.Example: DTIM=1 means multicast traffic when it arrives at the AP ispassed through after every beacon DTIM=3 means multicast traffic is passed through after every3rd beacon message


Closed System (AP)Configuration ParametersSecurity parameters• To enable rejection of association requests fromstations with Network Name set to “ANY”Enable Encryption• To enable/disable EncryptionEncryption keys• Four fields to store up to four different encryptionkeysEncryption key index• Index identifying which of the four keys is the activeone


Medium ReservationConfiguration ParametersAdvanced parameters• To enable/disable the RTS/CTS handshake. Threshold value 0-2346 (value=2347 disables MediumReservation)Distance between APs• To specify the coverage of a “cell” in terms of thedistance between the Access-Points Large Medium SmallMulticast Rate• To specify data-rate used for transmitting Multicastframes


Interference2/14/2005 Pietrosemoli 49


Interference2/14/2005 Pietrosemoli 50


Channel 11Channel 32422 GHzChannel 62437Channel1124622/14/2005 Pietrosemoli 51


Proxim Mp11.aPower over EthernetMax 36 W, Typ. 7.5WTurbo mode gives 108 Mbpsrate adaptive to 6 MbpsMax Out 18 dBmBuilt in attenuator to control int.Spanning tree protocol includedRadius support


OFDM2/14/2005 Pietrosemoli 53


Supported Channels for 802.11a2/14/2005 Pietrosemoli 54


Enterprise Gateway2/14/2005 Pietrosemoli 55


Common options that most wirelessresidential gateways include are:• Point-to-Point Protocol over Ethernet (PPPoE)• Network Address Translation (NAT)• Port Address Translation (PAT)• Ethernet switching• Virtual Servers• Print Serving• Fail-over routing• Virtual Private Networks (VPNs)• Dynamic Host Configuration Protocol (DHCP) Server and Client• Configurable Firewall2/14/2005 Pietrosemoli 56


Enterprise Gateway FeaturesEnterprise wireless gateways do have features, suchas Role-Based Access Control (RBAC), that are notfound in any access points. RBAC allows anadministrator to assign a certain level of wirelessnetwork access to a particular job position in thecompany. If the person doing that job is replaced, thenew person automatically gains the same networkrights as the replaced person. Having the ability tolimit a wireless user's access to corporate resources,as part of the "role", can be a useful security feature.2/14/2005 Pietrosemoli 57


Enterprise Gateway FeaturesClass of service is typically supported, and anadministrator can assign levels of service to aparticular user or role. For example, a guestaccount might be able to use only 500 kbpson the wireless network whereas anadministrator might be allowed 2 Mbpsconnectivity.2/14/2005 Pietrosemoli 58


Configuration and Managementof EGEnterprise wireless gateways are installed in the maindata path on the wired LAN segment just past theaccess point(s)They are configured through console ports using telnet,internal HTTP or HTTPS servers, etc.Centralized management of only a few devices is onebig advantage of using enterprise wireless gateways.An administrator, from a single console, can easilymanage a large wireless deployment using only a fewcentral devices instead of a very large number ofaccess points.2/14/2005 Pietrosemoli 59


Configuration and Managementof EWGEnterprise wireless gateways are normally upgradedthrough use of TFTP in the same fashion as manyswitches and routers on the market today.Configuration backups can often be automated sothat the administrator won't have to spend additionalmanagement time backing up or recovering from lostconfiguration files. Enterprise wireless gateways aremostly manufactured as rack-mountable 1U or 2Udevices that can fit into your existing data centerdesign.2/14/2005 Pietrosemoli 60


PoweroverdistanceGtGrPtTxAtArRxL= 32.4 +20 log(d/km)+20 log(f/MHz)dBmFree Space LossPrThreshold2/14/2005 Pietrosemoli 61km


Power LimitsPtMP links have a central point of connection and two ormore non-central connection points. PtMP links aretypically configured in a star topology. The centralconnection point may or may not have an omnidirectionalantenna It is important to note that when anomnidirectional antenna is used, the FCC automaticallyconsiders the link a PtMP link.Regarding the setup of a PtMP link, the FCC limits theEIRP to 4 Watts in both the 2.4 GHz ISM band and upper5 GHz UNII band. The power limit set for the intentionalradiator (the device transmitting the RF signal) in each ofthese bands is 1 Watt. If the transmitting wireless LANdevices are adjustable with respect to their output power,then the system can be customized to the needs of theuser.2/14/2005 Pietrosemoli 62


Power LimitsSuppose a radio transmitting at 1 Watt (+30 dBm) isconnected directly to a 12 dBi omnidirectional antenna.The total output power at the antenna is about 16 Watts,which is well above the 4 Watt limit. The FCC stipulatesthat for each 3 dBi above the antenna's initial 6 dBi ofgain, the power at the intentional radiator must bereduced by 3 dB below the initial +30 dBm. For theexample, since the antenna gain is 12 dBi, the power atthe intentional radiator must be reduced by 6 dB. Thisreduction will result in an intentional radiator power of+24 dBm (30 dBm – 6 dB), or 250 mW and an EIRP of36 dBm (24 dBm + 12 dBi), or 4 Watts. The power at theintentional radiator must never be more than 1 Watt andthe EIRP must never be above 4 Watts for a PtMPconnection.2/14/2005 Pietrosemoli 63


Power Limits2/14/2005 Pietrosemoli 64


Power Limits2/14/2005 Pietrosemoli 65


Power Limits2/14/2005 Pietrosemoli 66


Power Limits2/14/2005 Pietrosemoli 67


IEEE 802.11g802.11g provides the same maximum speed of 802.11a, coupledwith backwards compatibility for 802.11b devices. Thisbackwards compatibility makes upgrading wireless LANs simpleand inexpensive.IEEE 802.11g specifies operation in the 2.4 GHz ISM band. Toachieve the higher data rates found in 802.11a, 802.11gcompliant devices utilize Orthogonal Frequency DivisionMultiplexing (OFDM) modulation technology. These devices canautomatically switch to QPSK modulation in order tocommunicate with the slower 802.11b- and 802.11- compatibledevices. There is no reason to keep purchasing 802.11b onlydevices nowadays, since for all practical purposes 802.11g is asuperset of b, offering higher speed and some multipath inmunity2/14/2005 Pietrosemoli 68


Wireless Ethernet Compatibility AllianceThe Wireless Ethernet Compatibility Alliance (WECA)promotes and tests for wireless LAN interoperability of802.11b devices and 802.11a devices. WECA’smission is to certify interoperability of Wi-Fi (IEEE802.11) products and to promote Wi-Fi as the globalwireless LAN standard across all market segments.As an administrator, you must resolve conflicts amongwireless LAN devices that result from interference,incompatibility, or other problems.2/14/2005 Pietrosemoli 69


Wireless Ethernet Compatibility Alliance2/14/2005 Pietrosemoli 70


Supported Rates802.11b compliant device supports 11, 5.5, 2, & 1Mbps.802.11g can extend the capabilities to 54 Mbps as does802.11a.Some vendors offer “enhancements” over the standardsthat reach 108 Mbps and even 150 Mbps, but this oftenincreases the interference problem2/14/2005 Pietrosemoli 71


Thanks for your attention2/14/2005 Pietrosemoli 72

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