Wireless ad hoc networks

Acknowledgement: Slides borrowed from Richard Y. Yang @ Yale

Infrastructure-based v.s. ad hoc

• Infrastructure-based networks

– Cellular network

– 802.11, access points

• Ad hoc networks• Ad hoc networks

– Mobile ad hoc networks

• Military applications, emergency rescue

– Mesh networks

• “Last mile” of the Internet. Provide high speed wireless

network.

Infrastructure v.s. ad hoc

infrastructuremode

APAP

AP wired networkAP: Access Point

3

ad-hoc (mesh) mode

Infrastructure-based v.s. ad hoc

• Infrastructure-based networks

– Deployment is costly. Structures are not flexible.

– Vulnerable to attacks.

• Ad hoc networks• Ad hoc networks

– Flexible, easy to deploy, cheaper.

– Robust and resilient to attacks/failures, self healing.

– Problem: many research problems to achieve high capacity.

Mesh networks

Multiple projects tested in Berlin, Germany, South africa, India.

DIY guide on wiki

Capacity of Wireless Networks

• The question we ask:

how much traffic can a wireless network carry,

assuming we can solve MAC issues perfectly?

• Why study capacity?• Why study capacity?

– learn the fundamental limits of wireless

networks

– separate the spatial reuse perspective and the

distributed synchronization (MAC) perspective

– gain insight for designing effective wireless

protocols6

Interference Model

• Transmission successful if there are no other

transmitters (transmit at the same freq and

code) within a distance (1+∆)r of the receiver,

where r is the distance from the sender to the

receiverreceiver

7

receiver

sender

r(1+∆)r

Derivation of Capacity for Arbitrary Networks

• Model

– domain is a disk of unit area

– there are n nodes in the domain

– the transmission rate is W bits/sec– the transmission rate is W bits/sec

8

Two Constraints

� transmissionsuccessful if there areno other transmitterswithin a distance

Interference constraint

� a single half-duplex transceiver at each node: � either transmits or

Radio interface constraint

9

within a distance (1+∆)r of the receiver

receiver

sender

r(1+∆)r

� either transmits or receives

� transmits to only one receiver

� receives from only one sender

Assumptions

• Optimal power assignment /transmission

range

• Optimal scheduling & multi-hop routing

• Node are static.• Node are static.

• Random source-destination pairs

• Consider asymptotic capacity when n-> infty

Capacity

• Capacity = λ

• Total bits transmitted by all nodes

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Transmission range

• Transmission range is big

– Interference constraints prevent simultaneous

transmissions

• Transmission range is small• Transmission range is small

– It takes a lot of hops to arrive at the destination

• Even at optimal configuration, the capacity is low.

• On average, each node has O(W/√n) bit-

meter/sec, which converges to 0 as n->infty

Capacity

• Capacity (in bit-meter) for n nodes is

• On average, each node has O(W/√n) bit-

nW

L∆

≤π

λ8

• On average, each node has O(W/√n) bit-

meter/sec

• When n-> infty, the capacity per node is 0.

Capacity upper bound

• An ad hoc network does not scale.

• To improve capacity

– Avoid multi-hop traffic

– Use multiple radio interfaces– Use multiple radio interfaces

– Reduce interference

– Use multiple channels

Improving Capacity: Change Traffic Pattern

• To make communications local

– node placement: change the demand patterns (thus L)

• e.g. base stations/access points with high-speed backhaul

– use mobility

15

FE

A

B CD

BS1 BS2

S

T

infrastructure

Improving Capacity: Reduce Radio Interface

Constraint

• Multiple radio interfaces/codes

16

1

m

Improving Capacity: Reduce Interference

Constraint

• Antenna design: steered/switched directional antennas

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• Non-interfering channels

A

D

CB A B

D

C

Improving capacity: exploiting

interference

• Network coding

A CBa b

A CBa⊕b a⊕b

• Hidden terminal problem

A CB

Wireless broadcast

C C CC

Hidden Terminal Scenario

R1 R2Src Dst

Hidden Terminal Scenario

C C CCR1 R2Src Dst

P1

Hidden Terminal Scenario

C C CCR1 R2Src Dst

P2 P1

1) Src and R2 transmit simultaneously

Hidden Terminal Scenario

C C CCR1 R2Src Dst

1) Src and R2 transmit simultaneously

2) R1 subtracts P1, which he relayed earlier to recover P2 that he wants

P1 P2

Hidden Terminal Scenario

C C CCR1 R2Src Dst

R2 and Src are hidden terminals

Simultaneous transmission � Collision

With analog network coding, Simultaneous transmission � Success!

P1 P2

Next

• Improving capacity using multiple channels in

802.11 mesh

• [SSCH]

Multi-Channel 802.11 Mesh

• Wireless LANs

– APs determine the channel

– Clients share the same channel as their associated APs

• 802.11 mesh networks

– Each node can choose operating 802.11 channels to – Each node can choose operating 802.11 channels to increase spatial reuse

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1

12

2

Multi-Channel 802.11 Mesh: Goal

Goal: Using right channels at right nodes at right time to improve network throughput

1

1

1

26

1

m

c

1

c

1

12

2

Interface Assignment Strategies

� Static assignment� an interface is assigned a fixedchannel

�Dynamic assignment� interface assignment changes

1

1

c

27

� interface assignment changes with time

�Hybrid interface assignment� some interfaces use static assignment, others use dynamic assignment

� To focus on the key issue: assume a singleinterface

Key Challenge

� Connectivity vs using multiple channels

A1 1

B C A1

B C

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Multiple channels not used Network is disconnected

1 11

D

2D

Additional constraints: intermediate relay

nodes need to share the same channel as

the upstream and downstream node

SSCH: Slotted Seeded Channel Hopping –

Overview

• A dynamic assignment algorithm

– divides the time into equal sized slots (e.g. 10 ms) and switches each radio across multiple orthogonal channels on the boundary of slots in a distributedmannermanner

• Main aspect of SSCH

– channel scheduling

• self-computation of tentative schedule

• communication of schedules

• synchronization with other nodes

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SSCH – Desired Properties

• No Logical Partition: Any two nodes in communication range will overlap on a channel with moderate frequency

• Synchronization: Allow nodes that need to • Synchronization: Allow nodes that need to communicate to synchronize

• De-synchronization: Infrequently overlap between nodes with no communication

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A1

2

B C

D

Channel Scheduling -Self-Computation

• Each node uses (channel, seed) pairs to represent its tentative schedule for the next slot

• Seed: [1 , number of channels -1] Initialized randomly• Focus on the simple case of using one pair• Update rule:

new channel = (old channel + seed) mod (number of channels)

new channel = (old channel + seed) mod (number of channels)

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1 0 2 1 0 2 1 0A: Seed = 2

0 1 2 0 1 2 0 1B: Seed = 1

Example: 3 channels, 2 seeds

Channel Scheduling – Logical Partition

� Are nodes guaranteed to overlap?

� same init channel, same seed (always overlap)

� same init channel, different seeds (overlap occasionally)

� different init channels, different seeds (overlap occasionally)

� special case: Nodes may never overlap if they have the same

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1 2 0 1 2 0 1 2A: Seed = 1

0 1 2 0 1 2 0 1B: Seed = 1

� special case: Nodes may never overlap if they have the same

seed but different channels

Channel Scheduling –

Solution to Logical Partition

• Parity slot

– every (number of channels) slots, add a parity slot

– in parity slot, the channel number is the seed

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A: Seed = 1

B: Seed = 1

1 2 0 1 2 0 1 211

0 1 2 0 1 2 0 11 1

Parity Slot Parity Slot

Channel Scheduling -

Communication of Schedules

• Each node broadcasts its tentative schedule

(represented by the pair) once per slot

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Channel Scheduling - Synchronization

• If node B needs to send data to node A, it adjusts its (channel, seed) pair to be the same as A.

1 1 1 1 1 1 1 11Seed

35

A

B

1 2 0 1 2 0 1 211

0 2 1 1 2 0 1 22 1

1 1 1 1 1 1 1 11Seed

Seed 2 2 2 1 1 1 1 12

Flow startsSync starts

upon the

parity slot

Channel Scheduling –

Channel Congestion

• It is likely various nodes will converge to the same (channel, seed) pair and communicate infrequently after that.

(1,2)(1,2)

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(1,2)

(1,2)

(1,2)(1,2)

(1,2)

Channel Scheduling – Solution to Channel

Congestion

• De-synchronization

• To identify channel congestion: compare the number of the synchronized nodes and the number of the synchronized nodes and the number of the nodes sending data. De-synchronize when the ratio >= 2

• To de-synchronize, simply choose a new (channel, seed) pair for each synchronized and non-sending nodes

37

Channel Scheduling –Synchronizing with Multiple Nodes

• Examples– a sender with multiple receivers

– a forwarding node in a multi-hop network

• Solution: Use multiple seeds per node– use one seed to synchronize with one node– use one seed to synchronize with one node

– add a parity slot every cycle ( = number of channels * number of seeds); the channel number of the parity slot is the first seed.

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2 2 1 0 110 2 2 1 00

Green slots are generated by seed 1

Yellow slots are generated by seed 2

1

Channel Scheduling –

Partial Synchronization

2 2 1 0 110 2 2 1 001A

Seed 1 2 1 1 2 1 1 222 1 12

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2 0 1 2 110 2 0 1 021B

2 1 2 2 2 2 2 222 2 22Seed

Flow starts

Partial Sync

Sync the second seed only

Evaluations of SSCH

• Simulate in QualNet

• 802.11a, 54Mbps, (used) 13 orthogonal channels

• Slot switch time = 80 µs• Slot switch time = 80 µs

• 4 seeds per node, slot duration = 10 ms

• UDP flows: CBR flows of 512 bytes sent every 50 µs (enough to saturate the channel)

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Evaluation – Throughput (UDP)

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Evaluation – Multi-hop Mobile Networks

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Conclusion

• Lots of research questions on multi-channel multi-

radio network

• Coordination of the nodes with distributed

algorithms

• Maximize throughput

• Often cross-layer optimization is considered.