Message%20In%20Message%20(MIM):%20A%20Case%20for%20Reordering%20Transmissions%20in%20Wireless%20Networks

1
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

2
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

3
Collision

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

Collision
R1
R2
SoI
Interference
4
Collisions affect Spatial Reuse

• MAC protocols designed to avoid collisions
• 802.11 physical carrier sensing
• RTS/CTS
• Greatly limits spatial reuse

5
PHY Capture KochutICNP04

• Interference may not always cause collisions
• Possible to decode SoI with higher SINR
• If SoI arrives within preamble of Interference

20 us
Preamble

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

6
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

• Capability exists in Atheros chipsets

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802.11, Capture, and MIM
d)
a)
b)
c)
Preamble
Successful Reception Yes/No (SINR Threshold
needed to avoid collision)
Yes (10 dB) Yes (10 dB) No
No
802.11
Yes (10 dB) Yes (10 dB) Yes
(10 dB) No
PHY Capture
Yes (10 dB) Yes (10 dB) Yes
(10 dB) Yes (20 dB)
MIM
Different thresholds based on frame ordering
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Link Ordering Matters
AP1 ? R1
Controller
AP2 ? R2
AP1
AP2
20 dB
AP1 ? R1
10 dB
R2
AP2 ? R2
R1
In general weaker transmissions must start first,
stronger receiver can recover signal with MIM
AP1 must start first Followed by staggered
transmission from AP2 Allows weaker link R1 to
lock on to signal at low SINR
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Measurements
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?

12
Optimality Analysis

• Integer Programming Formulation in CPLEX

Optimal benefits from MIM significant
13
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

Interference Relations
Rehearsal
Packet Queue
Scheduler
MIM Constraints
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

17
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
Controller
P31
P22
P32
AP2
AP3
AP1
P31
P13
P12
P21
R11
R21
P13
R31
R22
R32
R13
R12
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Schedule Execution
Controller
P31
P13
P32
P21
P22
P31
AP3
AP1
AP2
P13
P12
R11
R21
R31
R22
R32
R13
R12

• 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

21
Duke EWLAN Topologies

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

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Throughput Comparision
Higher gains with Shuffle
Gain with scheduling
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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

25
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
• Doesnt 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 later S1, S2 decodable
S1 not decodable
S2 first, S1, S2 decodable
RSS of S2
Both lost
Both S1 and S2 lost
S1 first, S1, S2 decodable
S1 Later S1, S2 decodable
S2 too weak to satisfy SNR
S2 not decodable
RSS of S1
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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

33
Idea to explore with SIC

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

-59 dBm
-60 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

-70 dBm
-60 dBm
Tx1 tx power 10mW
Tx2
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Limitations of Capture

• Capture does not help when
• SoI arrives after the preamble of interference
• i.e. Receiver locks on to interference

20 us
Preamble

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