846無線網路的概觀與802.11無線區域網路(Overview)

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1、IEEE 802.11 OverviewRong-Hong JanDept of Computer and Information ScienceNational Chiao Tung University802.11 Technologies:Past, Present and FutureApproved and near-term 802.11 Letter StandardslFaster: Standards-based Wi-Fi now delivers data rates up to 54 Mbps to Wi-Fi clients. (11a,11g)lBetter-per

2、forming: 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)lMore secure: Security enhancements have been developed to address a

3、ccess control and authentication and data privacy and integrity, driven by the market requirement for enterprise-level security in wireless LANs. (11i)lBroader applicability: Regulatory enhancements that broaden the applicability of 802.11 to other frequencies such as 4.9 GHz in Japan and other regu

4、latory domains. (11j)lPhysical Layer enhancements 11a,11b,11glQuality of Service (QoS) enhancements 11elSecurity enhancements 11i WPAlRegulatory enhancements 11d 11hFuture 802.11 Standardsl802.11j The 802.11j Task Group is working on Japanese regulatory extensions to allow for operation in the 4.9 G

5、Hz and 5GHz bands in Japan. This is expected to be approved in 2005.l802.11k 802.11k 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

6、can utilize radio environment information to make better decisions as to frequency use, transmit power levels, etc.l802.11n The 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 throu

7、ghput over an 802.11. Both physical and MAC layer changes are being considered, but backward compatibility is required.l802.11r The 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

8、 deployment of SIP-based Voice over Wi-Fi (VoWi-Fi) portable phones.l802.11s The 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

9、networks include interconnectivity for devices in the digital home, unwired campuses, and community area networks or hotzones.Standards Evolution of 802.11Standards Evolution of 802.11無線區域網路課程內容安排無線區域網路課程內容安排 -大綱大綱(I)l課程大綱課程大綱無線網路的概觀與無線網路的概觀與802.11無線區域網路無線區域網路(Overview)802.11的媒體擷取控制的媒體擷取控制(MAC)與訊框格式

10、與訊框格式802.11e : QoS of 802.11802.11的安全的安全 : WEP與與 802.11i802.11的管理機制的管理機制競爭與非競爭的媒體控制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安排無線區域網路課程內容安排 -大綱大綱(II) 實體層的簡介實體層的簡介 802.11f IAPP (Inter Access Points Protocol) 802.11 網路佈建與管理網路佈建與管理 網路分析與效能調整網路分析與效能調整 802.11隨意無線區域網路路由方法隨意無線區域網路路由方法無線網路的概觀與無線網路的概觀與802.11無線無線區域網路區域網路(Over

11、view)lWireless vs wiredlBluetooth, 802.11, GSM, GPRS, UTMLlWireless Mobility - Securityl802.11 serials Infrastructure Ad hocTypes of NetworkslIndependent networks (indep. basic service set, IBSS), also known as ad hoc networks.lInfrastructure networkslTwo advantages for infrastructure networks The m

12、obile stations need not to maintain neighbor relationships. Access points assist with stations attempting to save power.lIn an infrastructure network, stations must associate with an AP to obtain network services. (equivalent to plug in the network cable)lAn extended service set (ESS) is created by

13、chaining BSSs together with a backbone network.802.11的媒體擷取控制的媒體擷取控制(MAC)與訊框格式與訊框格式lMAC accessing modeslCSMA/CA & NAVlInter-frame spaceslDCF operation/ BackofflFragmentation & De-fragmentationlFrame FormatlRTS/CTS & Power Saving sequencel802.11 adapts Ethernet-style networking to radio li

14、nks. Ethernet CSMA/CD 802.11 CSMA/CAlChallenges for the MAC RF 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 ModeslDistributed coordination function (DCF): Ethernet-like. CSMA/CA. DCF may

15、use CTS/RTS to reduce the possibility of collisions.lPoint 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 infrastructure mode and not widely implemented.Contention-ba

16、sed access using the DCFContention-based access using the DCFContention-based access using the DCFCarrier-Sensing Functions and the Network Allocation VectorlTwo types of carrier-sensing functions: the physical carrier-sensing and virtual carrier-sensing functions.lThe virtual carrier-sensing is pro

17、vided by the Network Allocation Vector (NAV)lThe NAV is carried in the frame headers on the MAC frames (e.g., RTS, CTS, Beacon, Probe, Association, and so on)Frame FormatlFields are transmitted from left to right, and the most significant bits appear last.802.11e : QoS of 802.11lTraffic differentiat

18、ionlHybrid coordination function (HCF)lContention-based channel access EDCFlControlled channel accesslDirect link protocol (DLP)lBurst ACK802.11的安全的安全 : WEP，802.1X 與與 802.11ilWEP operationlRC4 encryption algorithmlEAP (Extensible Authentication Protocol)l802.1x: Network port authenticationl802.1x on

19、 wireless LANsSupplicantAuthenticator3:EAPOL-StartRadius4:Request/Identity5:Response/Identity5:Radius-Access-Request6:Radius-Access-Challenge7:Radius-Access-Request8:Radius-Access-Accept9:EAPOL-Key(WEP)8:EAP-Success7:EAP-Response6:EAP-RequestEAPOLRADIUS1:Association request2:Association response802.

20、11802.11的管理機制的管理機制lInfrastructure management Scanning：Passive & Active Authentication：Open system & Share-key Association Re-associationlIBSS managementlTime synchronizationPassive ScanninglPassive scanning saves battery power. In passive scanning, a station moves to each channel on the chan

21、nel list and waits for Beacon frames.Active ScanninglFor each channel in the channel list: Wait for either an indication of an incoming frame or ProbeDelay Timer to expire.nIf a frame is detected, the channel can be probed. Send Probe Request. Wait for min or max channel time. (Channel busy or not)n

22、Idle min channel time no networknBusy max channel timelProbe responses are unicast frames subject to ACK.lChoosing which BSS to join is an implementation-specific decision and may even involve user intervention. Power level, signal strength. Matching parameterslTimer synchronization is an important

23、task in the joining process.AssociationlAssociation is recordkeeping procedure that allows the distribution system to track the location of each mobile station.lAfter association, an AP must register the MS on the network so frames for the MS are delivered to the AP.lHow ? (AP may send a gratuitous

24、ARP.)l802.11 forbids associating with more than one AP.Associated procedurelAssociation req & rep are unicast frames. (ACK is required)Reassociated procedure競爭與非競爭的媒體控制機制競爭與非競爭的媒體控制機制lPCF vs DCFlPCF operationlPCF frame formatlPower saving mode in DCFlPower saving mode in PCFlThe PCF allows an 80

25、2.11 network to provide an enforced “fair” access to the medium. (Likes AP holding the token in a token-based MAC schemes)lPCF is an optional part of 802.11lPeriods of contention-free service arbitrated by the point coordinator alternate with the DCF-based services.A cycle Contention free +contentio

26、n periodAt the beginning 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實體層的簡介實體層的簡介lPhysical layer overviewlFrequency hopping GFSKlDirect sequence spectrum Chipping DPSK & QPSKlOFDM802.11f IA

27、PP (Inter Access Points Protocol)lIAPP security riskslIAPP protocol overview ADD MOVE CACHE-NotifylRADIUS protocol usagelIAPP frame formatIAPPlIAPP( Inter Access-Point Protocol ) is designed for the enforcement of unique association throughout a ESS ( Extended Service Set ) and for secure exchange o

28、f stations security context between current access point(AP) and new AP during handoff period. lBased on security level, communication session keys between APs are distributed by a RADIUS server. Proactive cachinglProactive caching is suggested to avoid long handoff delay caused by IAPP communicatio

29、n between two APs as well as AP and RADIUS server. lWith proactive caching, current access point distributes the security context of the mobile station to neighboring access points BEFORE the station actually handoffs. 802.11 網路佈建與管理網路佈建與管理lThe topology archetypelRoaming & MobilitylSpanning mult

30、iple locationslSecuritylProject planninglThe site surveylInstallation and the final rollout網路分析與效能調整網路分析與效能調整lNetwork analysislTuning radio managementlTuning power managementlTiming operationsTuning radio managementlBeacon interval Decreasing passive scanning more reliable & faster IncreasingnPo

31、wer-saving capability (listen, DTIM intervals)nThroughput802.11隨意無線區域網路路由隨意無線區域網路路由方法方法lMANETlRouting in MANETlTable-Driven Routing Protocols DSDV、CGSRlSource-Initiated On-Demand Routing Protocols DSR、TORA、ABR、SSR、ZRPOn-demand vs. Table-drivenqTable-Driven Routing Protocol:l proactive!l continuously

32、 evaluate the routesl attempt to maintain consistent, up-to-date routing informationwhen a route is needed, one may be ready immediatelyl when the network topology changesthe protocol responds by propagating updates throughout the network to maintain a consistent viewqSource-Initiated On-Demand Rout

33、ing Protocol:l reactive!l on-demand style: create routes only when it is desired by the source noderoute discovery: invoke a route-determination procedurethe procedure is terminated whena route has been foundno route is found after all route permutations are examined l longer delay: sometimes a rout

34、e may not be ready for use immediately when data packets come實驗介紹實驗介紹l實驗大綱實驗大綱 分析無線封包分析無線封包 - Ethereal 的使用的使用 擷取無線封包擷取無線封包 - Promiscuous Mode 無線網路的傳輸特性無線網路的傳輸特性 AP韌體的編譯與燒錄與編寫程式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快速執行自行編譯的程式與DHCP Server功功能的擴充能的擴充 無線網路與無線網路與802.1x與與RADIUS 架設架設Wireless LAN Hot Spot服務服務 隨意網路的遶徑實作隨意網路的遶徑實作