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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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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


Outline

  • Introduction

  • Background & Related Work

  • The MultiNet Approach

  • Implementation

  • System Evaluation

  • Discussion

  • Comments


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


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


Virtual Machine


Virtual Machine (cont’)


Outline

  • Introduction

  • Background & Related Work

  • The MultiNet Approach

  • Implementation

  • System Evaluation

  • Discussion

  • Comments


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


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


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


Outline

  • Introduction

  • Background & Related Work

  • The MultiNet Approach

    • The virtualization architecture

    • Buffer protocol

    • Switching Algorithm

    • Synchronization Protocol

  • Implementation

  • System Evaluation

  • Discussion

  • Comments


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


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


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


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


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


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


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


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


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


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


Outline

  • Introduction

  • Background & Related Work

  • The MultiNet Approach

  • Implementation

  • System Evaluation

  • Discussion

  • Comments


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


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


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


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


Outline

  • Introduction

  • Background & Related Work

  • The MultiNet Approach

  • Implementation

  • System Evaluation

  • Discussion

  • Comments


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


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


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


Switching Strategies

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


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


使用“Surge”來產生web browsing的request


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


  • Without Power Saving Mode


  • With Power Saving Mode


Maximum Connectivity in MultiNet

  • Use Fixed Priority switching strategy

    • 每一個network的priority都一樣


Outline

  • Introduction

  • Background & Related Work

  • The MultiNet Approach

  • Implementation

  • System Evaluation

  • Discussion

  • Comments


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


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


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


Future Research

  • Switching的方式對TCP有很大的影響。

    • Currently exploring ways improve it.

  • Scaling in multi-hop networks


Outline

  • Introduction

  • Background & Related Work

  • The MultiNet Approach

  • Implementation

  • System Evaluation

  • Discussion

  • Comments


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


  • 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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