cross-layer optimal decision policies for spatial diversity forwarding in wireless ad hoc networks
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Cross-layer Optimal Decision Policies for Spatial Diversity Forwarding in Wireless Ad Hoc Networks. Rensselaer Polytechnic Institute. Prof. Alhussein Abouzeid. Joint work with Jing Ai & Zhenzhen Ye. Jing Ai. Hussein Abouzeid. Zhenzhen Ye. (if we were here). Outline. - PowerPoint PPT PresentationTRANSCRIPT
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Cross-layer Optimal Decision Policies for Spatial Diversity Forwarding in
Wireless Ad Hoc Networks
Prof. Alhussein Abouzeid
Rensselaer Polytechnic Institute
Joint work with Jing Ai & Zhenzhen Ye
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Hussein Abouzeid
Jing Ai
(if we were here)
Zhenzhen Ye
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Outline
• Spatial Diversity Forwarding– Motivation
– Related work
• Problem formulation of Optimal Stopping Relaying
• Implementation of OSR
• Evaluations of OSR– Analytical results
– Simulation results
• Summary
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Spatial Diversity Forwarding: Motivation & Related Work
• Motivation: Exploiting spatial diversity inherent in multi-hop wireless networks– Observation: There naturally exist alternate next-hop relays with high
probability that one of them has favorable channel condition at any given instant!
• The key challenge is design of strategy for next-hop selection. Prior work can be classified into two classes:
1. FSR (First Stopping Relaying) [JD05, WZF04, etc.]– Selects the first relay that replies to the forwarding node. (Problem: though
minimum overheads incurred, the selected relay may be poor in quality.)2. LSR (Last Stopping Relaying) [SM05]
– Collecting CSI from all candidate relays and then selecting the best one. (Problem: though the selected relay might be (if CSI is not outdated) the best, it incurs the maximum delay.)
We propose OSR: Exploiting the range between these extreme cases.
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Problem formulation of optimal stopping relaying (OSR)
• Optimal stopping relaying: investigate and design spatial-diversity forwarding policies based on a formally defined stochastic decision framework.
• Mapping next-hop selection problem to a sequential optimal stopping problem– The Decision maker: the forwarding node which has a packet to be
forwarded– Every time step, the decision maker observes a random variable which
is the state (e.g. channel quality) of the next available candidate. It also computes the reward up to that point in time.
– Action: whether to “stop” at a candidate next-hop relay and forward the packet to it, or “continue” observing other candidates
– Policy: The goal of the problem is to derive an optimal policy (i.e. a rule for deciding which action to take at every decision instant) in order to maximize the expected reward.
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Problem formulation of OSR (cont.)• A “conceptual” decision making procedure of OSR
Channel Quality: 2
Channel Quality: 7
Channel Quality: 8
Channel Quality?
2
2 is low, continue!
Channel Quality?
7
7 is good enough, stop!
AB
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Problem formulation of OSR (cont.)
• Given: a forwarding node ns which intends to forward a packet toward its destination and a set of L candidate next-hop relays {n1, n2, . . . , nL} known at the routing layer, which can be characterized by L independent discrete random variables (rewards) {Θ1,Θ2, . . . ,ΘL}.
• Problem: what is the optimal policy at the forwarding node ns to select the next-hop relay to which the packet is to be forwarded so as to maximize the expected reward E{Θ}?
• Θ is defined as d*Rdata, a generalization of Information Efficiency (IE)
• Solution: the optimal policy is a threshold-based policy– easy to implement
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Implementation of OSR
• Physical layer: rayleigh fading channels– determined the channel quality statistics if known long-
term average
• MAC layer: an extended MAC anycast scheme based on IEEE 802.11– perform the “optimal stopping” decision-making procedure
• Network layer: Greedy geographic routing [BMSU99,BK00]– take charge of selecting a set of candidate next-hop relays
at a forwarding node
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October 11 2006IEEE MASS 2006A. A. Abouzeid
MAC layer anycast
• Motivation– transform “costly” sequential decision-making procedure at the
forwarding node to a relatively “cheap” parallel decision-making procedure on the relay side.
• MRTS-CTS dialogue– Multicast RTS (MRTS) carries the threshold-based policy– CTS comes from the relay selected by the optimal stopping policy
• Feedbacks collision resolution– it is possible that more than one candidate next-hop relay qualified to
relay the packet by performing the threshold-based policy issued by the forwarding node
– prioritized the responses of relays in an order pre-assigned by the forwarding node
– with CSMA, the response of a higher-priority relay can suppress the responses of lower-priority relays
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October 11 2006IEEE MASS 2006A. A. Abouzeid
MAC layer anycast (cont.)
• A sample timeline of the OSR scheme for three candidate next-hop relays
CTS1
NAV(MRTS)
NAV(CTS2)
MRTS
CTS2
DATA
ACK
Tx
Rx1
Rx2
Rx3
Others
CTS3
NAV(DATA)
SIFS
2SIFS
SIFS
SIFS
SIFS
Random back-offafter DIFS
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October 11 2006IEEE MASS 2006A. A. Abouzeid
OSR v.s. FSR: Analytical Results
• Scenario– X-axis represents the average
SNR– Y-axis represents the gain
OSR over FSR in terms of IE– L homogenous candidate
next-hop relays
• Main observation– FSR can not utilize more than
two candidate relays as efficiently as OSR, especially when channels quality is average
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October 11 2006IEEE MASS 2006A. A. Abouzeid
LSR v.s. OSR: Simulation Results (Qualnet)
• Comparison of end-to-end performance metrics in Qualnet [QUA]– Grid topology (8X8 grid), a single TCP flow
• throughput
– Random topology, multiple TCP flows (not included due to space limit)
– Static random topology, multiple UDP flows• packet delivery ratio• end-to-end delay• jitter
– Mobile topology, multiple UDP flows (not included due to space limit)
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Impact of Fading Velocity (vm) on Average FTP Throughput
• Note: M is a protocol parameter in geographic routing that specifies the maximum order of spatial diversity that can be utilized by a forwarding node • Observation: In OSR, larger M means better performance (which is what we want). Not true for LSR (due to overheads)
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Impact of Traffic Load on CBR flows
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October 11 2006IEEE MASS 2006A. A. Abouzeid
Summary & Future Work
• Formulated the next-hop relay selection problem as a sequential decision problem and derived the Optimal Stopping Relaying (OSR) policies for improving spatial diversity gain in wireless ad hoc networks
• Implement OSR in a realistic protocol stack• Both analytical and simulation results reveal that OSR
outperforms FSR and LSR in terms of IE/end-to-end performance metrics
• Enable nodes to learn the fading channel characteristics online instead of relying on a channel model
• Extend such a decision framework to flow/service differentiation schemes.
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October 11 2006IEEE MASS 2006A. A. Abouzeid
References
• [BK00] B. Karp and H T. Kung, “GPSR: greedy perimeter stateless routing for wireless networks,” in Proc. ACM/IEEE Mobicom 2000.
• [BMSU99] Prosenjit Bose, Pat Morin, Ivan Stojmenovic and Jorge Urrutia, “Routing with guaranteed delivery in ad hoc wireless networks,” in Proc. DIALM, 1999.
• [JD05] S. Jain and S. R. Das, “Exploiting path diversity in the link layer in wireless ad hoc networks,” in Proc. IEEE WoWMoM, 2005.
• [JZF04] J. Wang, H. Zhai and Y. Fang, “Reliable and efficient packet forwarding by utilizing path diversity in wireless ad hoc networks,” in Proc. IEEE Milcom, 2004.
• [QUA] “Qualnet 3.7 user’s guide.” http://www.scalable-networks.com/.
• [SM05] M. R. Souryal and N. Moayeri, “Channel-adaptive relaying in mobile ad hoc networks with fading,” in Proc. IEEE SECON, 2005.