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

3

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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