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Message In Message (MIM):A Case for Reordering Transmissions

in Wireless Networks

Naveen Santhapuri, Srihari Nelakuditi University of South Carolina

Justin Manweiler, Souvik Sen, Romit Roy Choudhury, Kamesh Munagala

Duke University

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Outline

Motivation Understanding MIM How transmission order affects spatial

reuse?

Contributions Validate benefits of ordering Design MIM aware scheduling framework

Future work on MIM and SIC

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Collision

Signal of Interest (SoI) successful When SINR is substantially higher And, SoI arrives earlier than Interference Else, collision

CollisionCollision

SoI

Interference

R2R1

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Collisions affect Spatial Reuse

MAC protocols designed to avoid collisions 802.11 physical carrier sensing RTS/CTS

Greatly limits spatial reuse

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PHY Capture [Kochut:ICNP04]

Interference may not always cause collisions

Possible to decode SoI with higher SINR If SoI arrives within preamble of Interference

Interference

Preamble

Signal of Interest

Preamble time small (20 us in 802.11a) Benefits small as well

20 us

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Message In Message (MIM)

While receiving a message Receiver looks for new message preamble Stronger Message extracted while receiving

ongoing Message Requires higher SINR than when SOI arrives earlier

Preamble

Signal of Interest

Interference

Capability exists in Atheros chipsets

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802.11, Capture, and MIM

Preamble

SoI

Interference

SoI

Interference

SoI

Interference

SoI

Interference

802.11:

PHY Capture:

MIM:

a) b) c) d)

Yes (10 dB) Yes (10 dB) No No

Yes (10 dB) Yes (10 dB) Yes (10 dB) No

Yes (10 dB) Yes (10 dB) Yes (10 dB) Yes (20 dB)

Successful Reception Yes/No (SINR Threshold needed to avoid collision)

Different thresholds based on frame ordering

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AP1 must start first

Followed by staggered transmission from AP2

Allows weaker link R1 to lock on to signal at low SINR

AP1 must start first

Followed by staggered transmission from AP2

Allows weaker link R1 to lock on to signal at low SINR

In general weaker transmissions must start first,

stronger receiver can recover signal with MIM

In general weaker transmissions must start first,

stronger receiver can recover signal with MIM

Link Ordering Matters

R1R2

10 dB

20 dB Data

Data

AP1 R1

AP2 R2

Controller

AP1 AP2

Data

Data

AP1 R1

AP2 R2

Measurements

Rx

Tx Interferer 1 2 3 4 5

Order doesn’t affect delivery

Order affects delivery ratio

Observe that 802.11 does not enforce the order and

thus fails to exploit concurrency

Observe that 802.11 does not enforce the order and

thus fails to exploit concurrency

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MIM Capable vs MIM Aware

MIM Capable Network card can recover MIM Current MAC does not exploit MIM Appropriate ordering happens by chance

MIM Aware MAC MAC layer harnesses MIM capability Enforces appropriate ordering of

transmissions

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Goal

Design MIM aware scheduling that reorders transmissions for improving concurrency

Research Questions Does MIM awareness yield significant

benefits? What is the bound on improvement? How to effect the appropriate order?

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Optimality Analysis

Integer Programming Formulation in CPLEX

Optimal benefits from MIM significant

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MIM Aware MAC

Shuffle Centralized MIM-aware scheduling protocol For Enterprise Wireless LANs (EWLAN) Why EWLAN?

Increasingly popular architecture Realizes potential of MIM

Controller

AP1 AP2 AP3

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Shuffle: Assumptions

Dominant downward traffic

Powerful controller, Gigabit Ethernet Low latency for scheduling/communication

Additive Interference Total = sum of individual interferences

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Shuffle: Components

Rehearsal: Measuring interference relations

Packet Scheduler Use rehearsal and MIM-

constraints Output transmission

schedules (ordered)

Schedule Executer

RehearsalInterference

Relations

MIM Constraints

Packet Queue

Scheduler

Ordered Transmissions

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Measuring Interference Relationships

Periodic Rehearsals APs transmit probes at base rate Each client replies with RSSI values APs too record RSSI values from clients Controller derives interference map

Opportunistic rehearsal Piggyback RSSI values in data transmissions Continually refine interference map

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MIM-Aware Scheduler Objective

Maximize concurrency Avoid starvation

With MIM, conflicts are asymmetric Conflict graph methods unsuitable Optimal link scheduling is NP-hard

Least conflict greedy heuristic Score links based on asymmetric conflicts Links that prevent other links assigned higher score Compute link order based on ‘lower score first’

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Batch Selection & Dispatch

P31

P22

P31

P13

P12

R11

R12R13

R21

R22

R31R32

P13

P32

AP1 AP2 AP3

Controller

P21

Batch

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Schedule Execution

P31

P22

P31

P13

P12

R11

R12R13

R21

R22

R31R32

P13 P32

AP1 AP2 AP3

ControllerP21

APs transmit at specified time DATA Staggering order: AP1-AP3-AP2

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Evaluation

Qualnet simulations Throughput and the effect of Fading

Parameters 802.11a physical model with MIM PLCP: 20 us Fading: Ricean, varying K factor Wired backbone: 1 Gbps ethernet Controller processing latency: 50 us

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Duke EWLAN Topologies

Client, AP placement traces used to derive topologies (topo1, topo2, etc.)

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Throughput Comparision

Gain with scheduling

Higher gains with Shuffle

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Increasing AP Density

AP density yields higher benefit from Shuffle

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Impact of Channel Fading

Better throughput gain at lower (Ricean) fading

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Related Work

Location aware 802.11: Infocom 05 Takes advantage of MIM but not ordering

CMAP: NSDI08 Partially benefits from MIM capable hardware

Speculative Scheduling for EWLANs: Mobicom 07

Doesn’t consider MIM

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Looking Forward

MIM helps recover if SoI is stronger

What if SoI is weaker than interference? MIM cannot help Successive Interference Cancellation

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Successive Interference Cancellation

SIC can be used to recover weaker SoI First extract stronger frame Subtract it from the combination Recover weaker frame from residue

Feasibility depends on Strengths of SoI and Interference

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Interplay of MIM and SIC

Ordering helps SIC too If Interference (I) moderately stronger than

SoI Initiate I first to take advantage of MIM Decode I

If I much stronger than SoI Initiate SoI before I SoI characterized better for decoding

later

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Characterization & Cancellation

S2 fir

st, S

1, S

2 de

coda

ble B

oth

S1 an

d S2

lost

S2 laterS1, S2 decodable

RSS of S1

RS

S o

f S

2

S1 LaterS1, S2 decodable

S1 fir

st, S

1, S

2 de

coda

ble

S2 not decodable

S1

not

deco

dabl

e

Both

lost

S2 too weak to satisfy SNR

Rx

Tx1

Tx2

S1 S2

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Future Work

Shuffle Implementing and deploying on a test-bed Integrating upload traffic Comparing with other schemes

SIC vs MIM Explore Characterization vs. Cancellation Advantage of reordering transmissions

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Thank you

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Coping with Fading Loss

Immediate corrective rehearsal Controller identifies links suspected of fading Schedules a packet batch only for these APs

This is a partial rehearsal Packets are transmitted in serial order

APs and clients unaware, send Data and ACKs Controller updates Interference map from ACK

RSSIs

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Idea to explore with SIC

Power Control to enable SIC Suppose SINR threshold is 10dB SINR is 1 dB

-60 dBm -59 dBm

Tx1: tx power = 100mW

Tx2

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Idea to explore with SIC

Power control to enable SIC Suppose SINR threshold is 10dB SINR is 10dB after Tx1 reduced transmit

power

-60 dBm -70 dBm

Tx1: tx power = 10mW

Tx2

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20 us

Limitations of Capture

Capture does not help when SoI arrives after the preamble of

interference i.e. Receiver locks on to interference Preamble

Signal of Interest

Interference

Preamble time small (20 us in 802.11a) Benefits small as well