SSCH: Slotted Seeded Channel Hopping for Capacity Improvement in Ad Hoc Networks

1
SSCH Slotted Seeded Channel Hoppingfor Capacity
Improvement in Ad Hoc Networks

• Victor Bahl (Microsoft Research)
• Ranveer Chandra (Cornell University)
• John Dunagan (Microsoft Research)

2
Motivation Improving Capacity
Traffic on orthogonal channels do not
interfere e.g. Channels 1, 6 and 11 for IEEE
802.11b
Can we get the benefits of multiple channels in
ad hoc networks?
Example An IEEE 802.11b network with 3 Access
Points
Channel 1
Channel 6
Channel 6
Channel 11
3
Channel Hopping Prior Work

• Using multiple radios
• DCA (ISPAN00) a control and a data channel
• MUP (Broadnets04) multiple data channels
• Consumes more power, expensive
• Using non-commodity radios
• HRMA (Infocom99) high speed FHSS networks
• Nasipuri et al, Jain et al listen on many
  channels
• Expensive, not easily available
• Using a single commodity radio
• Multi-channel MAC (MMAC) (Mobihoc04)

4
Channel Hopping MMAC
MMAC Basic idea Periodically rendezvous on a
fixed channel to decide the next channel
Channel 1
Channel 6
Channel 11
Data
Data
Control
Data
Control

• Issues
• Packets to multiple destinations ? high delays
• Control channel congestion
• Does not handle broadcasts

5
Our Contributions

• SSCH a new channel hopping protocol that
• Increases network capacity using multiple
  channels
• Overcomes limitations of dedicated control
  channel
• No control channel congestion
• Handles multiple destinations without high delays
• Handles broadcasts for MANET routing

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Outline of the Talk

• Problem Overview
• Related Work
• SSCH The Main Idea
• SSCH A Few Details
• Performance of SSCH
• Conclusion

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SSCH Slots and Seeds
Divide time into slots switch channels at
beginning of a slot
New Channel (Old Channel seed) mod (Number of
Channels) seed is from 1 to (Number of Channels -
1)

(1 2) mod 3 0
Seed 2
3 channels E.g. for 802.11b Ch 1 maps to 0 Ch 6
maps to 1 Ch 11 maps to 2
A
0
2
1
0
2
0
1
1
B
Seed 1
0
1
2
0
1
2
0
1
(0 1) mod 3 1

• Enables bandwidth utilization across all
  channels
• Does not need control channel rendezvous

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SSCH Syncing Seeds

• Each node broadcasts (channel, seed) once every
  slot
• If B has to send packets to A, it adjusts its
  (channel, seed)

Seed
2
2
2
2
2
2
2
2
2
A
0
2
1
0
2
0
1
1
2
3 channels
B wants to start a flow with A
B
0
1
2
1
0
2
1
2
0
2
Seed
1
2
2
2
2
2
2
1
Stale (channel, seed) info simply results in
delayed syncing
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Outline of the Talk

• Problem Overview
• Related Work
• SSCH The Main Idea
• SSCH A Few Details
• Parity Slots Ensuring overlap
• Partial Sync Sending to multiple destinations
• Handling broadcasts
• Performance of SSCH
• Conclusion

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Nodes might not overlap!
If seeds are same and channels are different in a
slot
Seed 2
0
2
1
0
2
0
1
1
A
3 channels
B
Seed 2
1
1
2
0
2
1
0
2
Nodes are off by a slot ? Nodes will not overlap
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SSCH Parity Slots
Every (Number of Channels1) slot is a Parity
Slot In the parity slot, the channel number is
the seed
Seed 1
A
0
1
2
2
1
1
1
0
3 channels
B
Seed 1
0
1
2
2
0
1
1
1
Parity Slot
Parity Slot
Guarantee If nodes change their seeds only after
the parity slot, then they will overlap
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SSCH Partial Synchronization

• Syncing to multiple nodes, e.g., A sends packets
  to B C
• Each node has multiple seeds
• Each seed can be synced to a different node
• Parity Slot Still Works
• Parity slot (Number of Channels)(Number of
  Seeds) 1
• In parity slot, channel is the first seed
• First seed can be changed only at parity slot

If the number of channels is 3, and a node has 2
seeds 1 and 2
(2 2) mod 3 1
2
2
1
0
1
1
0
2
2
1
1
0
0
Parity Slot seed 1
(1 1) mod 3 2
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Illustration of the SSCH Protocol
Suppose each node has 2 seeds, and hops through 3
channels.
Seeds
1
2
1
2
1
2
1
2
1
2
1
2
2
2
1
0
1
1
0
2
2
1
1
0
0
Node A
B wants to start a flow with A
2
0
1
2
1
2
0
2
2
1
1
0
0
Node B
Seeds
2
1
2
2
2
2
1
2
1
2
1
2
Complete Sync (sync 1st seed) Seeds (1,
2) Channels (1, 2)
Partial Sync (only 2nd seed) Seeds (2,
2) Channels (2, 1)
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SSCH Handling Broadcasts
A single broadcast attempt will not work with
SSCH since packets are not received by neighbors
on other channels
Seeds
1
2
1
2
2
1
0
1
0
Node A
Bs broadcast
Bs broadcast in SSCH
0
1
2
2
0
Node B
Seeds
2
2
2
2
SSCH Approach Rebroadcast the packet over X
consecutive slots ? a greater number of nodes
receive the broadcast
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Outline of the Talk

• Problem Overview
• Related Work
• SSCH The Main Idea
• SSCH A Few Details
• Performance of SSCH
• Improvement in throughput
• Handling broadcast packets
• Performance in multi-hop mobile networks
• Conclusions

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Simulation Environment

• QualNet simulator
• IEEE 802.11a at 54 Mbps, 13 channels
• Slot Time of 10 ms and 4 seeds per node
• a parity slot comes after 4131 53 slots,
• 53 slots is 5310 ms 530 ms
• Channel Switch Time 80 µs
• Chipset specs Maxim04,
• EE literature J. Solid State Circuits 03
• CBR flows of 512 byte packets per 50 µs

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SSCH Stationary Throughput
Per-Flow throughput for disjoint flows
SSCH
IEEE 802.11a
SSCH significantly outperforms single channel
IEEE 802.11a
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SSCH Handles Broadcasts
10 Flows in a 100 node network using DSR
Average route length for IEEE 802.11a
Average discovery time for IEEE 802.11a
For DSR, 6 broadcasts works well (also true for
AODV)
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SSCH in Multihop Mobile Networks
Random waypoint mobility Speeds min 0.01 m/s
max rand(0.2, 1) m/s
Average route length for IEEE 802.11a
Average flow throughput for IEEE 802.11a
SSCH achieves much better throughput although it
forces DSR to discover slightly longer routes
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Conclusions

• SSCH is a new channel hopping protocol that
• Improves capacity using a single radio
• Does not require a dedicated control channel
• Works in multi-hop mobile networks
• Handles broadcasts
• Supports multiple destinations (partial sync)

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Future Work

• Analyze TCP performance over SSCH
• Study interoperability with non-SSCH nodes
• Study interaction with 802.11 auto-rate
• Implement and deploy SSCH (MultiNet)

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