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1、IEEE 802.11 Overview,Rong-Hong Jan Dept of Computer and Information Science National 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) Bett
2、er-performing: 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 add
3、ress access 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 othe
4、r regulatory domains. (11j),Physical Layer enhancements 11a,11b,11g Quality of Service (QoS) enhancements 11e Security enhancements 11i WPA Regulatory enhancements 11d 11h,Future 802.11 Standards,802.11j The 802.11j Task Group is working on Japanese regulatory extensions to allow for operation in th
5、e 4.9 GHz and 5GHz bands in Japan. This is expected to be approved in 2005. 802.11k 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 des
6、igners can utilize radio environment information to make better decisions as to frequency use, transmit power levels, etc. 802.11n 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 usabl
7、e throughput over an 802.11.,Both physical and MAC layer changes are being considered, but backward compatibility is required. 802.11r 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 facilit
8、ate the deployment of SIP-based Voice over Wi-Fi (VoWi-Fi) portable phones.,802.11s 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 fo
9、r mesh networks include interconnectivity for 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 802.11 80
10、2.11的安全 : WEP與 802.11i 802.11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 wired Bluetooth, 802.11, GSM, GPRS, UTML Wireless Mobility - Security 802.11 serials Infrast
11、ructure Ad hoc,Types of Networks,Independent networks (indep. basic service set, IBSS), also known as ad hoc networks. Infrastructure networks,Two advantages for infrastructure networks The mobile stations need not to maintain neighbor relationships. Access points assist with stations attempting to
12、save power. In an infrastructure network, stations must associate with an AP to obtain 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 modes CSMA/CA & NAV Int
13、er-frame spaces DCF operation/ Backoff Fragmentation & De-fragmentation Frame Format RTS/CTS & Power Saving sequence,802.11 adapts Ethernet-style networking to radio links. Ethernet CSMA/CD 802.11 CSMA/CA Challenges for the MAC RF link quality: 802.11b uses unlicensed ISM bands as its radio link. It
14、 must assume that interference will exist and work around it.,ACK is required.,MAC Access Modes,Distributed 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
15、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-based access using the DCF,Contention-based access using the DCF,Contention-based access using the DCF,Carrier-Sensing Functions and
16、 the Network Allocation Vector,Two types of carrier-sensing functions: the physical carrier-sensing and virtual carrier-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,
17、 Probe, Association, and so on),Frame Format,Fields are transmitted from left to right, and the most significant bits appear last.,802.11e : QoS of 802.11,Traffic differentiation Hybrid coordination function (HCF) Contention-based channel access EDCF Controlled channel access Direct link protocol (D
18、LP) Burst ACK,802.11的安全 : WEP，802.1X 與 802.11i,WEP operation RC4 encryption algorithm EAP (Extensible Authentication Protocol) 802.1x: Network port authentication 802.1x on wireless LANs,Supplicant,Authenticator,3:EAPOL-Start,Radius,4:Request/Identity,5:Response/Identity,5:Radius-Access-Request,6:Ra
19、dius-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:Association request,2:Association response,802.11,802.11的管理機制,Infrastructure management Scanning：Passive & Active Authentication：Open system & Share-key Ass
20、ociation Re-association IBSS management Time synchronization,Passive Scanning,Passive scanning saves battery power. In passive scanning, a station moves to each channel on the channel list and waits for Beacon frames.,Active Scanning,For each channel in the channel list: Wait for either an indicatio
21、n 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. (Channel busy or not) Idle min channel time no network Busy max channel time Probe responses are unicast frames subject to ACK.,Choosing whic
22、h BSS to join is an implementation-specific decision and may even involve user intervention. Power level, signal strength. Matching parameters Timer synchronization is an important task in the joining process.,Association,Association is recordkeeping procedure that allows the distribution system to
23、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 gratuitous ARP.) 802.11 forbids associating with more than one AP.,Associated procedure,Association req & rep are unicast frames.
24、 (ACK is required),Reassociated procedure,競爭與非競爭的媒體控制機制,PCF vs DCF PCF operation PCF frame format Power saving mode in DCF Power saving mode in PCF,The PCF allows an 802.11 network to provide an enforced “fair” access to the medium. (Likes AP holding the token in a token-based MAC schemes) PCF is an
25、 optional part of 802.11 Periods of contention-free service arbitrated by the point coordinator alternate with the DCF-based services.,A cycle Contention free +contention period At the beginning of the contention-free period, the AP transmits a Beacon frame and announces CFPMaxDuration. All stations
26、 set NAV to CFP-MD to lock out DCF-based access,實體層的簡介,Physical layer overview Frequency hopping GFSK Direct sequence spectrum Chipping DPSK & QPSK OFDM,802.11f IAPP (Inter Access Points Protocol),IAPP security risks IAPP protocol overview ADD MOVE CACHE-Notify RADIUS protocol usage IAPP frame forma
27、t,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 point(AP) and new AP during handoff period. Based on security level, communication se
28、ssion 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 proactive caching, current access point distributes the security context of the m
29、obile station to neighboring access points BEFORE the station actually handoffs.,802.11 網路佈建與管理,The topology archetype Roaming & Mobility Spanning multiple locations Security Project planning The site survey Installation and the final rollout,網路分析與效能調整,Network analysis Tuning radio management Tuning
30、 power management Timing operations,Tuning radio management,Beacon interval Decreasing passive scanning more reliable & faster Increasing Power-saving capability (listen, DTIM intervals) Throughput,802.11隨意無線區域網路路由方法,MANET Routing in MANET Table-Driven Routing Protocols DSDV、CGSR Source-Initiated On
31、-Demand Routing Protocols DSR、TORA、ABR、SSR、ZRP,On-demand vs. Table-driven,Table-Driven Routing Protocol: proactive! continuously evaluate the routes attempt to maintain consistent, up-to-date routing information when a route is needed, one may be ready immediately when the network topology changes t
32、he 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 node route discovery: invoke a route-determination procedure the procedure is t
33、erminated when a route has been found no 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,