SSCH Slotted Seeded Channel Hopping for Capacity Improvement

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

SSCH: Slotted Seeded Channel Hopping for Capacity Improvement in Ad Hoc Networks Victor Bahl (Microsoft Research) Ranveer Chandra (Cornell University) John Dunagan (Microsoft Research) presented by Zheng Zeng

Motivation: Improving Capacity Traffic on orthogonal channels do not interfere e. g. Channels 1,

Motivation: Improving Capacity Traffic on orthogonal channels do not interfere e. g. Channels 1, 6 and 11 for IEEE 802. 11 b Example: An IEEE 802. 11 b network with 3 Access Points Can we get the benefits of multiple channels in ad hoc networks? Channel 1 Channel 6 Channel 11 2

Channel Hopping: Prior Work • Using multiple radios: – DCA (ISPAN’ 00): a control

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) 3

Our Contributions SSCH: a new channel hopping protocol that – Increases network capacity using

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 4

SSCH Overview • • Distributed link-layer protocol Utilize frequency diversity (orthogonal channels) Only a

SSCH Overview • • Distributed link-layer protocol Utilize frequency diversity (orthogonal channels) Only a single radio per node Use unmodified IEEE 802. 11 protocol (including RTS/CTS) when not switching channels • No logical partition 5

Outline of the Talk • • • Problem Overview Related Work SSCH: The Main

Outline of the Talk • • • Problem Overview Related Work SSCH: The Main Idea SSCH: A Few Details Performance of SSCH Conclusion 6

SSCH: basic concept • Slot: the time spent on a single channel (10 ms)

SSCH: basic concept • Slot: the time spent on a single channel (10 ms) • Channel schedule: the list of channels to switch in subsequent slots and the time at which it plans to make each switch. • Packet schedule: packet transmission attempts by each node within a slot. • (channel, seed) pair 7

SSCH: Slots and Seeds Divide time into slots: switch channels at beginning of a

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 A Seed = 2 B Seed = 1 1 0 2 1 0 0 1 2 0 1 3 channels E. g. for 802. 11 b Ch 1 maps to 0 Ch 6 maps to 1 Ch 11 maps to 2 (0 + 1) mod 3 = 1 • Enables bandwidth utilization across all channels • Does not need control channel 8

SSCH: Syncing Seeds • Each node broadcasts (channel, seed) once every slot • If

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 A 2 2 2 2 2 1 0 3 channels B wants to start a flow with A B Seed 2 0 1 2 1 0 1 1 2 2 2 2 Follow A: Change next (channel, seed) to (2, 2) Stale (channel, seed) info simply results in delayed syncing 9

Outline of the Talk • • Problem Overview Related Work SSCH: The Main Idea

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 10

Nodes might not overlap! If seeds are same and channels are different in a

Nodes might not overlap! If seeds are same and channels are different in a slot: A Seed = 2 1 0 3 channels B Seed = 2 2 1 0 2 1 Nodes are off by a slot Nodes will not overlap 11

SSCH: Parity Slots Every (Number of Channels+1) slot is a Parity Slot In the

SSCH: Parity Slots Every (Number of Channels+1) slot is a Parity Slot In the parity slot, the channel number is the seed A Seed = 1 1 2 1 0 3 channels B Seed = 1 0 1 1 2 Parity Slot Guarantee: If nodes change their seeds only after the parity slot, then they will overlap 12

SSCH: Partial Synchronization Syncing to multiple nodes, e. g. , A sends packets to

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 1 2 2 (1 + 1) mod 3 = 2 1 0 0 1 Parity Slot = seed 1 1 2 2 1 0 0 13

Illustration of the SSCH Protocol Suppose each node has 2 seeds, and hops through

Illustration of the SSCH Protocol Suppose each node has 2 seeds, and hops through 3 channels. Seeds 1 Node A 1 2 2 1 1 0 2 0 1 1 1 2 2 1 1 0 2 0 1 2 2 1 0 0 1 2 1 2 B wants to start a flow with A Node B 1 Seeds 2 2 0 1 2 2 Partial Sync (only 2 nd seed) Seeds: (2, 2) Channels: (2, 1) 2 Complete Sync (sync 1 st seed) Seeds (1, 2) Channels: (1, 2) 14

SSCH: Handling Broadcasts A single broadcast attempt will not work with SSCH since packets

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 Node A 2 1 0 0 1 B’s broadcast in SSCH Node B Seeds 0 1 2 0 2 2 2 SSCH Approach Rebroadcast the packet over ‘X’ consecutive slots a greater number of nodes receive the broadcast 15

Outline of the Talk • • • Problem Overview Related Work SSCH: The Main

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 16

Simulation Environment Qual. Net simulator: • IEEE 802. 11 a at 54 Mbps, 13

Simulation Environment Qual. Net simulator: • IEEE 802. 11 a 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 [Maxim 04], – EE literature [J. Solid State Circuits 03] • CBR flows of 512 byte packets per 50 µs 17

SSCH: Stationary Throughput Per-Flow throughput for disjoint flows SSCH IEEE 802. 11 a SSCH

SSCH: Stationary Throughput Per-Flow throughput for disjoint flows SSCH IEEE 802. 11 a SSCH significantly outperforms single channel IEEE 802. 11 a 18

SSCH Handles Broadcasts 10 Flows in a 100 node network using DSR Average route

SSCH Handles Broadcasts 10 Flows in a 100 node network using DSR Average route length for IEEE 802. 11 a Average discovery time for IEEE 802. 11 a For DSR, 6 broadcasts works well (also true for AODV) 19

SSCH in Multihop Mobile Networks Random waypoint mobility: Speeds min: 0. 01 m/s max:

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. 11 a Average flow throughput for IEEE 802. 11 a SSCH achieves much better throughput although it forces DSR to discover slightly longer routes 20

Conclusions SSCH is a new channel hopping protocol that: • Improves capacity using a

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) 21

Future Work • • Analyze TCP performance over SSCH Study interoperability with non-SSCH nodes

Future Work • • Analyze TCP performance over SSCH Study interoperability with non-SSCH nodes Study interaction with 802. 11 auto-rate Implement and deploy SSCH (Multi. Net) 22