MultiNet: Connecting to Multiple IEEE 802.11 Networks Using a Single Radio Ranveer Chandra, Cornell University joint work with: Victor Bahl (MSR) and Pradeep.

MultiNet:Connecting to Multiple IEEE 802.11 Networks

Using a Single Radio

Ranveer Chandra, Cornell University

joint work with:

Victor Bahl (MSR) and Pradeep Bahl (Windows Networking)

MotivationA Sample Scenario

Company ‘B’ guy at Company ‘A’

Ad Hoc

Infrastructure

Company ‘A’ employee

Company ‘A’ employee

May not have more than one wireless

card!

Other applications:

• Gateway node of a wireless ad hoc network

• Extending the range of an infrastructure network

Why is it so difficult?

Why can’t we use two wireless cards?Power, form factor, and what if we want to connect to10 networks

How is it so different from Ethernet?Wireless networks require association and authentication to communicate on a network. You can only be connected to one network at a time.

OK.. Why not just connect to the network on which you want to send the packet?The association and authentication steps take time. Moreover, even if this time were reduced, a new scheme is required to maintain the state of each network, to simultaneously support connections on multiple networks.

Our Problem

Provide a mechanism for users to connect to multiple networks, such that:- The overhead is transparent to the user- The performance is acceptable on each network- The performance does not degrade with the number of switching nodes- It works with commercial wireless cards.

A set of suggestions for improving performance without breaking the 802.11 protocol

The virtual instances:• Maintain the network state, such as the IP address, and the SSID of the network• Present an always active network interface to the upper layers

Our Approach: MultiNetIdea: • Virtualize the wireless card, one instance per desired network. • Multiplex the card across all the networks

Intermediate layer between IP and MAC: • Maintains a virtual instance of the wireless card per network• Multiplexes across networks, activates one virtual instance at a time. • Buffers packets for inactive networks

Application is unaware of network switching

Implementation on Windows XPCombination of NDIS IM driver and a user level service

Drivero MultiNet Miniport Driver (MMD): Miniport instances. One per network, and IP sees each miniport instance as a different network card.o MultiNet Protocol Driver (MPD): Manages miniports, switches card and buffers packets.

Serviceo Maintains synchronization with other MultiNet nodeso Sends signals to the IM driver using IOCTLs.

MultiNet: Screen Snapshot

Switching: Is it feasible?How long does it take to switch from one network to another?

12.2 12.4 12.6 12.8 13 13.2 13.4 13.6 13.8 14 14.2 14.4 14.6 14.8 15 15.2 15.4 15.6 15.8 16 16.2

Time in Seconds

802.11 Ack

802.11 Beacon

802.11 CTS

802.11 Probe Req

802.11 Probe Resp

802.11 RTS

TCP FTP Data

Steps 1,2 ~ 70 msDelay! ~ 3.8 seconds

Reduced by 3.7 seconds3.87 sec to go from IS to AH

network

Reduced it to 170 ms by trapping media disconnects

Conclusion: Although feasible, switching still takes considerable time. MultiNet should be designed to handle large switching delays, and should perform better when this delay is reduced.

MultiNet: Switching NetworksProblem Statement: When does the card switch from one network to another?

The solution should:• allow users to specify priorities among the connected networks• give more time to networks that have seen more traffic• not starve networks with low traffic

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Time (in seconds)

Ac

tiv

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Pe

rio

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in m

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-130

-110

-90

-70

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-10

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Tra

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(in

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)

Ad hoc Infrastructure TrafficAH TrafficIS

One approach:See traffic over a window of‘x’ time slots, and give time proportional to the numberof packets seen on the network.Plusses:- better utilization of network time- avoids need for zero configurationMinuses:- Might not work with multihop networks

MultiNet: Buffering PacketsProblem Statement:

(i) Deliver packets sent by the MultiNet node when the network is not active(ii) Deliver packets sent to the MultiNet node when the node is not active

B

A

MultiNet Node in ad hoc mode

Proposed Architecture: • Access points store switching state and buffer packets for Multinet Nodes

(Analogous to Power Save Mode of IEEE 802.11 for IS networks)

• Nodes in ad hoc networks also store the same information for MultiNet nodes in the ad hoc network

(Works for a single hop ad hoc network)

MultiNet: Power Consumption

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yte

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Traffic on the IS network(Surge trace)

Traffic on the AH network(Real trace)

TEST MACHINE

How much power is saved using MultiNet?

MultiNet: Power Consumption

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nerg

y (In

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les)

Two Radio PS MultiNet No PS MultiNet PS

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Two Radios MultiNet

Without Power SaveMultiNet uses much less power

than the two radio scheme

With Power SaveThe PSM optimized MultiNet uses

much less power.

Future Work• Test MultiNet with more than 2 networks • Improving performance through tighter coupling with newer drivers

and/or MAC changes • Better bandwidth sharing with legacy non-MultiNet nodes • Improve TCP Performance over MultiNet• Make code available for download

Contributions of this paper• Proposes a new virtualization architecture for wireless cards.• Relaxes a physical constraint of the number of wireless cards that

can be used by an application.• Actually builds and shows the feasibility of such a system!

Conclusions