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1、IEEE 802.11 Overview,Rong-Hong JanDept of Computer and Information ScienceNational Chiao Tung University,802.11 Technologies:Past,Present and Future,Approved and near-term 802.11 Letter Standards,Faster:Standards-based Wi-Fi now delivers data rates up to 54 Mbps to Wi-Fi clients.(11a,11g)Better-perf
2、orming:The need for a better-performing MAC has been driven by demanding application such as voice and video.Quality of Service enhancements have modified the MAC to provide admission control and prioritized channel access.(11e),More secure:Security enhancements have been developed to address access
3、 control and authentication and data privacy and integrity,driven by the market requirement for enterprise-level security in wireless LANs.(11i)Broader applicability:Regulatory enhancements that broaden the applicability of 802.11 to other frequencies such as 4.9 GHz in Japan and other regulatory do
4、mains.(11j),Physical Layer enhancements11a,11b,11gQuality of Service(QoS)enhancements11eSecurity enhancements11iWPARegulatory enhancements11d11h,Future 802.11 Standards,802.11jThe 802.11j Task Group is working on Japanese regulatory extensions to allow for operation in the 4.9 GHz and 5GHz bands in
5、Japan.This is expected to be approved in 2005.802.11k802.11k is focused on standardizing the radio measurements that will allow uniform measurement of radio information across different manufacturer platforms.,By having standardized,repeatable measurements,system designers can utilize radio environm
6、ent information to make better decisions as to frequency use,transmit power levels,etc.802.11nThe 802.11n Task Group is focusing on creating a standard to further increase the net throughput of wireless networks.The goal is to achieve greater than 150 Mbps usable throughput over an 802.11.,Both phys
7、ical and MAC layer changes are being considered,but backward compatibility is required.802.11rThe 802.11r Task Group is working on reducing the handoff latency when client devices transition between access points or cells in an ESS.This standard will facilitate the deployment of SIP-based Voice over
8、 Wi-Fi(VoWi-Fi)portable phones.,802.11sThe 802.11s Task Group is working on an infrastructure mesh standard to allow 802.11 access points or cells from multiple manufacturers to self-configure into multi-hop wireless topologies.Example usage scenarios for mesh networks include interconnectivity for
9、devices in the digital home,unwired campuses,and community area networks or hotzones.,Standards Evolution of 802.11,Standards Evolution of 802.11,無線區域網路課程內容安排-大綱(I),課程大綱無線網路的概觀與802.11無線區域網路(Overview)802.11的媒體擷取控制(MAC)與訊框格式802.11e:QoS of 802.11802.11的安全:WEP與 802.11i802.11的管理機制競爭與非競爭的媒體控制機制,無線區域網路課程內容
10、安排-大綱(II),實體層的簡介802.11f IAPP(Inter Access Points Protocol)802.11 網路佈建與管理網路分析與效能調整802.11隨意無線區域網路路由方法,無線網路的概觀與802.11無線區域網路(Overview),Wireless vs wiredBluetooth,802.11,GSM,GPRS,UTMLWireless Mobility-Security802.11 serialsInfrastructureAd hoc,Types of Networks,Independent networks(indep.basic service se
11、t,IBSS),also known as ad hoc networks.Infrastructure networks,Two advantages for infrastructure networksThe mobile stations need not to maintain neighbor relationships.Access points assist with stations attempting to save power.In an infrastructure network,stations must associate with an AP to obtai
12、n network services.(equivalent to plug in the network cable)An extended service set(ESS)is created by chaining BSSs together with a backbone network.,802.11的媒體擷取控制(MAC)與訊框格式,MAC accessing modesCSMA/CA&NAVInter-frame spacesDCF operation/BackoffFragmentation&De-fragmentationFrame FormatRTS/CTS&Power S
13、aving sequence,802.11 adapts Ethernet-style networking to radio links.Ethernet CSMA/CD802.11 CSMA/CAChallenges for the MACRF link quality:802.11b uses unlicensed ISM bands as its radio link.It must assume that interference will exist and work around it.,ACK is required.,MAC Access Modes,Distributed
14、coordination function(DCF):Ethernet-like.CSMA/CA.DCF may use CTS/RTS to reduce the possibility of collisions.Point coordination function(PCF):Contention-free services.Special stations called point coordinators are used to ensure contention-free.The coordinators reside in AP.PCF is only for infrastru
15、cture mode and not widely implemented.,Contention-based access using the DCF,Contention-based access using the DCF,Contention-based access using the DCF,Carrier-Sensing Functions and the Network Allocation Vector,Two types of carrier-sensing functions:the physical carrier-sensing and virtual carrier
16、-sensing functions.The virtual carrier-sensing is provided by the Network Allocation Vector(NAV)The NAV is carried in the frame headers on the MAC frames(e.g.,RTS,CTS,Beacon,Probe,Association,and so on),Frame Format,Fields are transmitted from left to right,and the most significant bits appear last.
17、,802.11e:QoS of 802.11,Traffic differentiationHybrid coordination function(HCF)Contention-based channel access EDCFControlled channel accessDirect link protocol(DLP)Burst ACK,802.11的安全:WEP，802.1X 與 802.11i,WEP operationRC4 encryption algorithmEAP(Extensible Authentication Protocol)802.1x:Network por
18、t authentication802.1x on wireless LANs,Supplicant,Authenticator,3:EAPOL-Start,Radius,4:Request/Identity,5:Response/Identity,5:Radius-Access-Request,6:Radius-Access-Challenge,7:Radius-Access-Request,8:Radius-Access-Accept,9:EAPOL-Key(WEP),8:EAP-Success,7:EAP-Response,6:EAP-Request,EAPOL,RADIUS,1:Ass
19、ociation request,2:Association response,802.11,802.11的管理機制,Infrastructure managementScanning：Passive&ActiveAuthentication：Open system&Share-keyAssociationRe-associationIBSS managementTime synchronization,Passive Scanning,Passive scanning saves battery power.In passive scanning,a station moves to eac
20、h channel on the channel list and waits for Beacon frames.,Active Scanning,For each channel in the channel list:Wait for either an indication of an incoming frame or ProbeDelay Timer to expire.If a frame is detected,the channel can be probed.Send Probe Request.Wait for min or max channel time.(Chann
21、el busy or not)Idle min channel time no networkBusy max channel timeProbe responses are unicast frames subject to ACK.,Choosing which BSS to join is an implementation-specific decision and may even involve user intervention.Power level,signal strength.Matching parametersTimer synchronization is an i
22、mportant task in the joining process.,Association,Association is recordkeeping procedure that allows the distribution system to track the location of each mobile station.After association,an AP must register the MS on the network so frames for the MS are delivered to the AP.How?(AP may send a gratui
23、tous ARP.)802.11 forbids associating with more than one AP.,Associated procedure,Association req&rep are unicast frames.(ACK is required),Reassociated procedure,競爭與非競爭的媒體控制機制,PCF vs DCFPCF operationPCF frame formatPower saving mode in DCFPower saving mode in PCF,The PCF allows an 802.11 network to p
24、rovide an enforced“fair”access to the medium.(Likes AP holding the token in a token-based MAC schemes)PCF is an optional part of 802.11Periods of contention-free service arbitrated by the point coordinator alternate with the DCF-based services.,A cycle Contention free+contention periodAt the beginni
25、ng of the contention-free period,the AP transmits a Beacon frame and announces CFPMaxDuration.All stations set NAV to CFP-MD to lock out DCF-based access,實體層的簡介,Physical layer overviewFrequency hoppingGFSKDirect sequence spectrumChippingDPSK&QPSKOFDM,802.11f IAPP(Inter Access Points Protocol),IAPP s
26、ecurity risksIAPP protocol overviewADDMOVECACHE-NotifyRADIUS protocol usageIAPP frame format,IAPP,IAPP(Inter Access-Point Protocol)is designed for the enforcement of unique association throughout a ESS(Extended Service Set)and for secure exchange of stations security context between current access p
27、oint(AP)and new AP during handoff period.Based on security level,communication session keys between APs are distributed by a RADIUS server.,Proactive caching,Proactive caching is suggested to avoid long handoff delay caused by IAPP communication between two APs as well as AP and RADIUS server.With p
28、roactive caching,current access point distributes the security context of the mobile station to neighboring access points BEFORE the station actually handoffs.,802.11 網路佈建與管理,The topology archetypeRoaming&MobilitySpanning multiple locationsSecurityProject planningThe site surveyInstallation and the
29、final rollout,網路分析與效能調整,Network analysisTuning radio managementTuning power managementTiming operations,Tuning radio management,Beacon intervalDecreasing passive scanning more reliable&fasterIncreasingPower-saving capability(listen,DTIM intervals)Throughput,802.11隨意無線區域網路路由方法,MANETRouting in MANETTa
30、ble-Driven Routing ProtocolsDSDV、CGSRSource-Initiated On-Demand Routing ProtocolsDSR、TORA、ABR、SSR、ZRP,On-demand vs.Table-driven,Table-Driven Routing Protocol:proactive!continuously evaluate the routesattempt to maintain consistent,up-to-date routing informationwhen a route is needed,one may be ready
31、 immediatelywhen the network topology changesthe protocol responds by propagating updates throughout the network to maintain a consistent view,Source-Initiated On-Demand Routing Protocol:reactive!on-demand style:create routes only when it is desired by the source noderoute discovery:invoke a route-d
32、etermination procedurethe procedure is terminated whena route has been foundno route is found after all route permutations are examined longer delay:sometimes a route may not be ready for use immediately when data packets come,實驗介紹,實驗大綱分析無線封包-Ethereal 的使用擷取無線封包-Promiscuous Mode無線網路的傳輸特性AP韌體的編譯與燒錄與編寫程式快速執行自行編譯的程式與DHCP Server功能的擴充無線網路與802.1x與RADIUS架設Wireless LAN Hot Spot服務隨意網路的遶徑實作,
