1 message in message (mim): a case for reordering transmissions in wireless networks naveen...
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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
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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