Post on 16-Jan-2016
description
SSCH: Slotted Seeded Channel Hoppingfor Capacity Improvement
in Ad Hoc Networks
Victor Bahl (Microsoft Research)Ranveer Chandra (Cornell University)
John Dunagan (Microsoft Research)
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SERIAL ETHERNET
SERIAL ETHERNET
SERIAL ETHERNET
Motivation: Improving CapacityTraffic on orthogonal channels do not interferee.g. Channels 1, 6 and 11 for IEEE 802.11b
Channel 11
Channel 1
Channel 6
Example: An IEEE 802.11b network with 3 Access Points
Can we get the benefits of multiple channels in ad hoc networks?
Channel 6
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Channel Hopping: Prior Work• Using multiple radios:
– DCA (ISPAN’00): a control and a data channel
– MUP (Broadnets’04): multiple data channels
Consumes more power, expensive
• Using non-commodity radios:– HRMA (Infocom’99): 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) (Mobihoc’04)
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Channel Hopping: MMACMMAC Basic idea:
Periodically rendezvous on a fixed channel to decide the next channel
Issues
• Packets to multiple destinations high delays
• Control channel congestion
• Does not handle broadcasts
Channel 1
Channel 6
Channel 11
Data DataControl DataControl
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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
3 channelsE.g. for 802.11bCh 1 maps to 0Ch 6 maps to 1
Ch 11 maps to 2
1 0 2 1 0 2 1 0
0 1 2 0 1 2 0 1
New Channel = (Old Channel + seed) mod (Number of Channels)seed is from 1 to (Number of Channels - 1)
Seed = 2
Seed = 1
(1 + 2) mod 3 = 0
(0 + 1) mod 3 = 1
A
B
• 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)
3 channels1 0 2 1 0 2 1 0
0 1 2 1 0 2 1 0
Seed
Seed
Follow A: Change next (channel, seed) to (2, 2)
A
B
2 2 2 2 2 2 2 2
1 2 2 2 2 2 2 2
2
2
1
Stale (channel, seed) info simply results in delayed syncing
B wants to start a flow with A
2
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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:
3 channels
Seed = 2
Seed = 2
Nodes are off by a slot Nodes will not overlap
1 0 2 1 0 2 1 0
1 0 2 1 0 2 12
A
B
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SSCH: Parity Slots
3 channels
Seed = 1
Seed = 1
Every (Number of Channels+1) slot is a Parity Slot
In the parity slot, the channel number is the seed
Parity Slot Parity Slot
Guarantee:
If nodes change their seeds only after the parity slot,
then they will overlap
0 1 2
0 12 2
2
0 1 1 1
111 0A
B
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SSCH: Partial SynchronizationSyncing 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 1 0 110 2 2 11 00
(1 + 1) mod 3 = 2
(2 + 2) mod 3 = 1
Parity Slot= seed 1
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Illustration of the SSCH Protocol
2 2 1 0 110 2 2 11 00Node A
2 0 1 2 120 2 2 11 00Node B
Seeds
B wants to start a flow with A
Complete Sync
(sync 1st seed)
Seeds (1, 2)
Channels: (1, 2)
Partial Sync
(only 2nd seed)
Seeds: (2, 2)
Channels: (2, 1)
1 2 1 2 1 2 1 2 1 2 1 2
Seeds 2 1 2 2 2 2 1 2 1 2 1 2
Suppose each node has 2 seeds, and hops through 3 channels.
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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
2 1 0 10
0 1 2 20
B’s broadcast
Node A
Node B
Seeds
Seeds
1 2 1 2
2 2 2 2
SSCH Approach
Rebroadcast the packet over ‘X’ consecutive slots
a greater number of nodes receive the broadcast
B’s broadcast in SSCH
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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 4*13+1 = 53 slots,– 53 slots is: 53*10 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 ThroughputPer-Flow throughput for disjoint flows
0
2
4
6
8
10
12
14
0 5 10 15
# Flows
Th
rou
gh
pu
t (i
n M
bp
s)
IEEE 802.11a
SSCH
SSCH significantly outperforms single channel IEEE 802.11a
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SSCH Handles Broadcasts
0
0.1
0.2
0.3
0.4
2 3 4 5 6 7 8 9
# Broadcasts
Ro
ute
Dis
cove
ry T
ime
(in s
ec)
0
1
2
3
4
5
6
7
2 3 4 5 6 7 8 9
# Broadcasts
Ave
rag
e R
ou
te L
eng
th
(# h
op
s)
10 Flows in a 100 node network using DSR
For DSR, 6 broadcasts works well (also true for AODV)
Average discovery time for IEEE 802.11a
Average route length for IEEE 802.11a
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SSCH in Multihop Mobile Networks
0
0.5
1
1.5
2
2.5
0.2 0.4 0.6 0.8 1
Speed (in m/s)
Flo
w T
hrou
ghpu
t (in
Mb
ps)
Random waypoint mobility: Speeds min: 0.01 m/s max: rand(0.2, 1) m/s
0
1
2
3
4
5
0.2 0.4 0.6 0.8 1
Speed (in m/s)
Ave
rag
e R
ou
te L
eng
th
(# h
op
s)
Average flow throughput for IEEE 802.11a
Average route length for IEEE 802.11a
SSCH achieves much better throughputalthough 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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