Transcript PPT_692430031

The Challenge of Wi-Fi
(Wireless fidelity) Roaming
報告者:周士倫
Outline
 Technical roaming issues
 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 complementarycode-keying (CCK)
 802.11a,802.11g use orthogonalfrequency-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 WiFi
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
1.
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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 callprocessing 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 accesspoint 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 WiFi 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.”