The Challenge of Wi-Fi (Wireless fidelity) Roaming 報告者:周士倫
Outline Technical roaming issues ROAMING BETWEEN WI-FI AND CELLULAR Implementation incompatibility Handoff standards Security and authentication ROAMING BETWEEN WI-FI AND CELLULAR BUSINESS-RELATED ROAMING ISSUES TODAY’S LIMITED APPROACHES Aggregators Hotspot networks
A Quick Look At Wi-Fi Technologies Most Wi-Fi hotspots currently use IEEE 802.11b technology It’s cheap to deploy Client-side devices are widely available IEEE 802.11b use complementary-code-keying (CCK) 802.11a,802.11g use orthogonal-frequency-division-multiplexing(OFDM)
Introduction The Wi-Fi technology faces a critical challenge to its continued popularity and success: Seamless roaming between different networks is difficult for various technical and business reasons
Introduction Roaming is difficult in part because the technologies used in different systems don’t always work together. service providers’ billing and authentication systems sometimes aren’t compatible Security is another concern
Introduction Finally, carriers want to figure out ways that Wi-Fi users can roam from their networks to cellular networks for example, let users begin working at an airport on a laptop via a Wi-Fi hotspot and then continue via a cellular network until they get to their office and can switch to a traditional LAN.
TECHNICAL ROAMING ISSUES – 1.Implementation incompatibility IEEE 802.11 standards are loose enough that vendors have been able to develop incompatible implementations even with compatible equipment, there are no standards for the methodology used in handing off transmissions between networks
TECHNICAL ROAMING ISSUES – 2.Handoff standards The IEEE 802 Handoff Executive Committee Study Group is working on developing a common-handoff-frame-work standard that will be useful to Wi-Fi roaming If a certain quality of service is being maintained for a user, user does not necessarily want to hand off to a base station that cannot also support that QoS
TECHNICAL ROAMING ISSUES – 3.Security and authentication WEP is designed to give wireless LANs (WLANs) similar protection to that offered by wired networks WPA, though, is only a stopgap measure while the IEEE works on the proposed 802.11i standard, which would include the stronger Advanced Encryption Standard algorithm Authorization to make sure people trying to sign on to a network are supposed to have access to a hotspot is also an issue for Wi-Fi
Handoff ,Security and authentication 802.11f:提供了無線網路使用者漫遊(Roaming)時,透過不同無線接取點(Access Point)連線的資料交換機制,其中定義了IAPP(Inter Access Point Protocol) 機制,讓使用者在切換無線接取點時,可以讓新的AP與舊的AP交換使用者的連線資料,以便於維持原有的連線,而不會導致連線中斷,影響使用者無線網路傳輸 802.11i: 定義了在802.11a/b/g MAC(Media Access Control) Layer的安全機制,主要以EAP-802.1x做為認證的方式,AES與TKIP(Temporal Key Integrity Protocol)作為加密的方法,當然目前跟802.11f,一樣都沒有最後定案的版本,不過在可預見的未來裡,802.11i肯定會成為無線網路安全方面最為重要的通訊協定架構.
802.11f和802.11i示意圖
Handoff
Handoff 因為在目前的網路架構IPv4下並不支援可攜式IP位址(Mobile IP)，當資訊設備跨越了不同服務網路時，送出去的訊號無法被其他電腦所辨識，造成斷訊。這個問題將在下一代的網路架構IPv6獲得改善
Handoff 目前可以利用DHCP的架構，將所有上網的資訊設備採用DHCP模式取得網路IP位址，可暫時解決部分跨越服務網路的問題
MN在原網路收到來自HA廣播之Agent Advertisement信息，得知所在網路為原網路及HA位址。 MN移至其他網路，同時收到FA廣播之Agent Advertisement信息，得知已移至其他網路，同時得知FA位址。 MN透過FA轉送註冊信息給HA，並告知HA其拜訪網路之CoA(MN在漫遊至其他網路之暫時網路位址) 。 HA廣播Proxy ARP信息至原網路所有節點，告知目前MN的封包需交由HA轉送。 CN傳送至原網路的封包將路由至HA，HA查表得知MN之CoA透過通道機制(Tunneling)將封包包裝後再送至FA。 FA收到後，解通道封包後，將原封包轉送至MN。 MN送至外部之封包可以直接遞送，若拜訪網路有作封包過濾(Packet Filtering)，則可以透過FA轉送至HA再行傳送到CN。 MN返回原網路，傳送解除註冊動作，封包路由回原MN。
安全性 目前WLAN的安全是利用WEP加解密的方式來達到安全需求，不過這樣的方式由於WEP加密上的缺失,導致容易被惡意使用者入侵破解。例如駭客只要在辦公大樓外面，架設天線，並進行加解密的動作，就容易侵入公司內部網路. 在無線網路安全沒有進一步的保障前,最好的方式就是可以透過WLAN加上VPN的安全機制
VPN的加密機制可以補足WLAN所缺乏的安全問題,例如可以在公司內部架設一個VPN Server,提供PPTP或是IPSec的機制,讓使用者在無線網路傳輸時,可以先透過PPTP簽入內部主機後再連結上網路資源,如此一來在無線部分傳輸的資料就可以透過加密的方式來傳輸了 不過,用VPN 來解決無線網路安全問題並不是一個最佳的方案,不過在目前更佳的無線網路安全標準尚未建立以前,有些廠商則採用這樣的架構作為暫時的系統解決方案
認證 在計費問題上，一般像是網路隨插即用的設備會提供簡單的帳號管理以及計費機制，這很適合於小範圍的公用空間上網，像咖啡館、旅社 若是提供大範圍的無線網路服務時，這要的機制顯然不足，因此必須搭配其他系統作為計費以及身份認證的需求，像是RADIUS伺服器
撥號接入用戶遠程驗證服務協議（RADIUS） 票據結算中心將用戶資訊發往該用戶的開戶ISP，開戶ISP對用戶進行驗證和計費，並向為該用戶提供接入服務的ISP支付一定的費用，最後，該用戶就可以連接到其開戶ISP並透過他連接到自己的公司網路 無線ISP間共用用戶資訊的技術相對來說簡單而直接，而如何協調無線ISP間的利益分配，並制定相應標準則略顯複雜。 WISPr的發言人 Homan認為，收費系統是最關鍵的部分。為此，WISPr對RADIUS協議進行了擴充，增添了一些新的協議屬性，如用戶名、在線時間、發出與接收字節數等等。此外，透過標識用戶所處位置的地域代碼，該協議還能向用戶提供某特定站點的服務
RADIUS架構圖
ROAMING BETWEEN WI-FI AND CELLULAR Motorola、Avaya、及Proxim三家公司宣佈合作計畫，期盼能解決Wi-Fi漫遊的問題。此合作計畫將基於VoIP技術來整合蜂巢式(cellular)行動通訊技術及無線區域網路技術，目的在於讓Wi-Fi熱點也能提供語音的服務 Motorola will develop the dual-mode phones, create the mobility management server that provides the handoff between Wi-Fi and cellular networks, and implement system design tools Avaya will integrate its IP telephony technology and call-processing software to support mobile capabilities Proxim will provide the Wi-Fi infrastructure, QoS software, and centralized management systems to facilitate network handoffs
BUSINESS-RELATED ROAMING ISSUES Some companies charge per-use, per-minute, or subscription fees for using their hotspots. Other individuals and groups deploy free hotspots as a public service For roaming to succeed, carriers must resolve differences in their business models and their billing and authentication mechanisms
TODAY’S LIMITED APPROACHES – Aggregators Small wireless ISP (WISP) aggregators offer a single account that provides access to networks of many partners Aggregators don’t build new hot-spots but instead sign up existing access-point operators
想要達到的目標
Bad news 位於紐約的Joltage公司成為最新的犧牲品。這家公司成立於去年年初(2002)，它的創始人希望成為“規劃外頻譜領域的世界頭號運營商”，同時向患有寬頻飢渴症的消費者提供無線網路服務提供商(wISP)的選擇。它的業務計劃聽起來確實可行。Joltage公司免費向所有擁有電腦、乙太網路連接和一些廉價網路設備的用戶提供軟體。這些人的電腦將成為全國性無線網路的節點，在理論上讓公司免於花費巨額成本建設自己的網路。用戶每小時支付1.99美元就可享受無線網路，而這筆收入將由Joltage公司和“熱點”托管方平均分配。 鑒于這項計劃十分具有創新意義，許多行業觀察家預測Joltage公司(還有其他幾家Wi-Fi技術發起公司，如Boingo無線公司)將成為新時代的先驅，並預測小型ISP將奪走大型無線和有線ISP的客戶。但不幸的是，甚至連Joltage公司相對較低的價格仍然不夠便宜。由於用戶太少，Joltage公司董事長Andrew Weinreich發出一封電子郵件，稱公司將停止運營。
TODAY’S LIMITED APPROACHES – Hotspot networks Large companies are also trying to form nationwide hotspot networks. For example, US wireless service providers T-Mobile USA and Wayport have their own extensive Wi-Fi networks, largely in airports, coffee shops, and convention centers. Several leading computer and telecommunications companies including AT&T, IBM, and Intel have formed Cometa Networks
Conclusion many companies currently don’t offer hotspots to make a profit. Instead, they frequently offer hotspots to enhance other services they offer and to attract customers customers won’t pay a high usage fee for hotspots, especially when so many free ones exist national Wi-Fi coverage would be a key factor for the technology’s success, as has been the case with cellular phone services. BUT “It’s not here yet for the average consumer.”