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MultiNet ： Connecting to Multiple IEEE 802.11 Networks Using a Single Wireless Card

MultiNet ： Connecting to Multiple IEEE 802.11 Networks Using a Single Wireless Card. Ranveer Chandra Department of Computer Science Cornell University. By 徐志賢. Paramvir Bahl Microsoft Research One Microsoft Way. Pradeep Bahl Microsoft Corporation One Microsoft Way. INFOCOM 2004.

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MultiNet ： Connecting to Multiple IEEE 802.11 Networks Using a Single Wireless Card

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  1. MultiNet：Connecting to Multiple IEEE 802.11 Networks Using a Single Wireless Card Ranveer Chandra Department of Computer Science Cornell University By 徐志賢 Paramvir Bahl Microsoft Research One Microsoft Way Pradeep Bahl Microsoft Corporation One Microsoft Way INFOCOM 2004

  2. Outline • Introduction • Background & Related Work • The MultiNet Approach • Implementation • System Evaluation • Discussion • Comments

  3. Introduction • MultiNet • A new virtualization architecture that abstracts a single WLAN card to appear as multiple virtual cards to the user. • Simultaneously connected to physically different wireless networks. • Achieved by “switching” between networks and activating the corresponding stack

  4. Introduction (cont’) • MultiNet Advantages • Allows application and protocol like TCP/IP to work without changes • Save up to 50% of the energy consumed over nodes with 2 cards while providing similar functionality • Increase connectivity for end users • Connect to ad hoc network while staying on a IS network • Bridging between infrastructure and ad hoc networks • A gateway node • Increased range of wireless network • Border nodes can act as relays for authorized nodes that are outside the range of AP

  5. Virtual Machine

  6. Virtual Machine (cont’)

  7. Outline • Introduction • Background & Related Work • The MultiNet Approach • Implementation • System Evaluation • Discussion • Comments

  8. Background • Limitations in Existing Systems • The card can NOT interact with nodes in another network if the nodes are operating on a different frequency channel • As Practical concerned, most available WLAN cards trigger a firmware reset each time the mode is changed • Infrastructure <-> ad hoc

  9. Background (cont’) • Next Generation of IEEE 802.11 WLAN cards • On the concern of cost, venders (AMD, Realtek) are minimizing the functionality of code residing in the cards • Native WiFi card • Implement just the basic time-critical MAC functions • Leave their control and configuration to OS • Allow OS to maintain state and don’t undergo a firmware reset on changing the mode

  10. Related Work • This topic has not been studied before in the context of WLANs • Bluetooth networks • node  piconet  scatternets • Use a frequency hopping scheme for communication between multiple nodes • Knowing the correct hopping sequence of 2 networks and hops fast enough  simultaneously connected to the 2 networks

  11. Outline • Introduction • Background & Related Work • The MultiNet Approach • The virtualization architecture • Buffer protocol • Switching Algorithm • Synchronization Protocol • Implementation • System Evaluation • Discussion • Comments

  12. The MultiNet Approach • Virtualization：An abstraction of multiple wireless networks as different always active virtual adapter over a single WLAN card • An adaptive network hopping scheme where a card gets a time slot, called the Activity Period, for each network on particular channel • The sum of the active period over all connected networks  Switching Cycle (SC) • The authors limit the scope to study of single-hop network • Infrastructure <=> ad hoc

  13. The Virtualization Architecture • Implemented by the MultiNet Protocol Driver • Responsible for switching the wireless card across different networks and buffering packets for networks that are currently inactive

  14. Buffering Protocol • Packets sent from the MultiNet card • buffering packets for networks that are currently inactive • 最多delay • Tj activity period of network j • δj  the time taken to switch to network j

  15. Buffering Protocol (cont’) • Packets sent to the MultiNet card • 送給某inactive adapter的packet lost!! • 因此要送packet給MultiNet node的nodes要為其做buffer • A card send packet to <all node in ad hoc network> or <AP in IS network> to inform its “temporarily unavailability” before switching out the network

  16. AP buffer packets for nodes which enter Power Saving Mode • 4-tuple in the state table： • Card’s address, • SSID, • the time when the card switched from the SSID, • when it is expected to switch back

  17. Switching Algorithm • Strategies： • Fixed Priority：使用者依需求自訂 • Adaptive Schemes：依node上的traffic來決定 • If MultiNet switch across N networks, and network i has seen Pi packets in its last Activity Period ATPi , then the node stays in network j for an ATPj= • Adaptive Buffer：pkts buffered • Adaptive Traffic：pkts sent+rcved Utilization across all network Utilization of network j Total amount of time the node is active across all networks

  18. Switching Algorithm (cont’) • Switching on IEEE 802.11 network card • Listen Interval(進入PSM的時間長度), Activity Period on IS network • 都是AP Beacon Period的倍數 • 前者≦後者 • Allow MultiNet node to sleep to PSM if • In Infrastructure network • No packets to send

  19. Synchronization Protocol • Effect on IS and Ad Hoc networks • AP永遠留在同一個network中no problem • 對ad hoc網路來說，如果有兩個node一個switch進來，一個剛好switch出去無法互傳 • A Distributed Switching Algorithm • Synchronize留在同一個network中的時間 • Notation： • ATPi , • SC：constant, known by al nodes • TEATPi ：在ATPi中已過時間 • Leader ：在某個network中MAC最大的node

  20. Synchronization Protocol (cont’) • Algorithm Description：進入network j時， • Send announcement <ATPi , TEATPi > • Listen 2*SC for announcements • 將自己的<ATPj , TEATPj >調成與Leader一樣 • 如果經過2*SC還沒聽到比自己MAC大的，表示自己是Leader • 聽到比自己MAC小的，在接下來的時間內(ATPj - TEATPj)再送出一個announcement • 接著每一個SC開頭都送announcement

  21. Synchronization Protocol (cont’) • Discussion： • 有可能兩個不同的Leader的時間重疊，則想要同時加入該2個network的node將不會成功 • 2 solution：該不幸的node • 負責協調將會對其他node帶來影響 • 加入其中一個network，剩下的時間再switch入另外一個(且不送announcement)如果兩個network時間完全重疊就不行

  22. Outline • Introduction • Background & Related Work • The MultiNet Approach • Implementation • System Evaluation • Discussion • Comments

  23. Implementation • 在WinXP上實作MultiNet • Network Driver Interface Specification (NDIS) • An intermediate layer between the network device driver and IP • 上層直接call NDIS的API來用，不用擔心不同網卡廠商有不同implement • Implement MultiNet as a combination of an NDIS intermediate driver and a service. • Service：有buffering/switching的logic，下指令給driver • Driver：實作buffering/switching機制

  24. MultiNet Driver • Miniport driver： • 依照某些rule寫的話，就可以在”任何平台”上使用 • 直接管理NIC, 提供interface for higher level-driver • MPD expose a virtual adapter for each network • Manage card state, switch, buffer, • 將收到的packet, 送給目前active的adapter • MMD maintains the states for each virtual adapter • SSID, operational mode of the network, • Handle query and set operation meant for underlying wireless adapter

  25. MultiNet Service • Implemented at user level • Buffering and switching logic • Interact with other MultiNet node • Responsible for signaling the switch time to MPD • Broadcast for its temporarily unavailability before MPD switch to another network • 其他MultiNet node的service收到後，會通知MultiNet driver開始buffer packet • 為了簡化coding難度，所以做在user level

  26. Buffering Protocol under Infrastructure mode • 成功使用Native WiFi card實作發出PSM req.給AP，使AP開始做buffering • 一般網卡 • 沒有提供API給Windows呼叫進入Power Save Mode，且廠商也有其driver的所有權 • 因此使用類似ad hoc的方式，在其他在同一AP下的node，持續觀察ongoing session，如果該node switch到其他network中為它buffer • 但如果將來要擴大scale，這樣是不可行的 • 不過也可以利用在AP上面使用Native WiFi card，為它作buffer

  27. Outline • Introduction • Background & Related Work • The MultiNet Approach • Implementation • System Evaluation • Discussion • Comments

  28. Test Configuration • MultiNet was deployed on 2 laptops • Windows XP • Compaq Evo N600c (Built in IEEE 802.11b card), HP Omnibool 6000 • 各加一塊PCMCIA無線網卡 • Cisco 340 Series, Orinoco Gold Wireless card • 也有幾塊AMD和Realtek的Native WiFi cards • 4台802.11b APs • Cisco 340 Series, EZConnect 2656, DLink DI-614, Native WiFi APs

  29. Switching Delay • Legacy WLAN card perform entire association every time they switched to a network • Investigations revealed that this delay is caused by media disconnect and media connect notifications to the IP stack

  30. Switching Delay (cont’) • IP stack damps the notifications for a few seconds to protect itself and its client from spurious signals. • MultiNet採用MPD將這些notifications攔截起來的方法 • 結果有相當好的效果(ex. 3.9s ->170ms ) • 如果使用Native WiFi card來做的話更好。 • Ex. 170ms -> 25ms

  31. Switching Strategies • 某MultiNet Node (using legacy WLAN card) 分享一個47MB的檔案

  32. MultiNet versus Multiple Radios • Simulation environment • 甲公司會議室中，有乙公司的人(P乙)來開會。 • P乙想要與會議室中其他人分享3個presentation files，並且持續與其他人使用通訊軟體通話。 • Ad Hoc network • 同時間甲公司的人想要使用Internet。 • Infrastructure network • 使用75%IS 25%AH strategy

  33. 使用“Surge”來產生web browsing的request

  34. 1.分享presentation檔案分別在1st, 38th, 75th minute 2.使用Ethereal抓MSN chat的資料流量來用

  35. Without Power Saving Mode

  36. With Power Saving Mode

  37. Maximum Connectivity in MultiNet • Use Fixed Priority switching strategy • 每一個network的priority都一樣

  38. Outline • Introduction • Background & Related Work • The MultiNet Approach • Implementation • System Evaluation • Discussion • Comments

  39. Reducing the Switching Overhead • Main cause of switching overhead is the 802.11 protocol • Card believes that it has disassociated from previous network • Card don’t store the state for more than 1 network in the firmware • Many card vendor force a firmware reset when changing mode • Native WiFi card!!!! • Allowing upper layer to control associations, switching delay can be reduced to 1ms!! • Only requesting a synchronization beacon

  40. Network Port Based Authentication • 802.1X做在Wireless Zero Configuration (WZC)，可是MultiNet卻需要把WZC關掉。 • 在每次switch到需802.1X的網路時，不應每次都重新跑一次其authentication protocol • 建議也是利用MPD紀錄state來達到

  41. Can MultiNet be done in the Firmware? • Yes • 可是buffering牽涉到memory的使用，應該交由OS來管理比較好 • 而且網卡成本會提高

  42. Future Research • Switching的方式對TCP有很大的影響。 • Currently exploring ways improve it. • Scaling in multi-hop networks

  43. Outline • Introduction • Background & Related Work • The MultiNet Approach • Implementation • System Evaluation • Discussion • Comments

  44. Comments • 如果可以同時與2個以上的AP溝通，事先的address configuration以及authentication/authorization 的確有可能達成seamless handoff!!! • 開發hierarchical架構時，VM ware有可能將整個hierarchy放在一台電腦上面跑 • Using host-only + bridge mode virtual Ethernet card以及設好routing tables • CPU要夠力，memory要夠多

  45. Other Reference • http://www.usenix.org/publications/library/proceedings/usenix01/sugerman/sugerman_html/node1.html • http://taiwan.cnet.com/enterprise/technology/0,2000062852,20025224,00.htm

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