MultiNet: Connecting to Multiple IEEE 802.11 Networks Using a Single Radio

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MultiNetConnecting 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)

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Motivation
A Sample Scenario
Company B guy at Company A
Company A employee
May not have more than one wireless card!
Infrastructure
Ad Hoc
Company A employee

• Other applications
• Gateway node of a wireless ad hoc network
• Extending the range of an infrastructure network

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Why is it so difficult?
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.
Why cant we use two wireless cards? Power, form
factor, and what if we want to connect to10
networks
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.
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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

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Our Approach MultiNet

• Idea
• Virtualize the wireless card, one instance per
  desired network.
• Multiplex the card across all the networks

Application is unaware of network switching

• 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

• 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

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

• Service
• Maintains synchronization with other MultiNet
  nodes
• Sends signals to the IM driver using IOCTLs.

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

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MultiNet Screen Snapshot
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Switching Is it feasible?
How long does it take to switch from one network
to another?
3.87 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.
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MultiNet Switching Networks
Problem 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

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

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MultiNet Buffering Packets

• Problem Statement
• Deliver packets sent by the MultiNet node when
  the network is not active
• 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)

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MultiNet Power Consumption
Traffic on the IS network (Surge trace)
TEST MACHINE
Traffic on the AH network (Real trace)
How much power is saved using MultiNet?
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MultiNet Power Consumption
Without Power Save MultiNet uses much less power
than the two radio scheme
With Power Save The PSM optimized MultiNet uses
much less power.
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Conclusions

• 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!