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Heterogeneity in Multi-Hop Wireless Networks Nitin H. Vaidya University of Illinois at Urbana-Champaign www.crhc.uiuc.edu/~nhv © 2003 Vaidya 1 Summary Heterogeneity is essential Heterogeneity is beneficial Research Agenda Develop protocols that exploit the heterogeneity Develop mechanisms to better evaluate wireless systems Proof by example … 2 Heterogeneity Many dimensions of heterogeneity: Architecture Physical capability of hosts Higher layers 3 Architecture Multi-hop wireless networks Pure ad hoc networks Hybrid networks 4 Pure Ad Hoc Networks No “infrastructure” All communication over (one or more) wireless hops B C A D E Z Ad hoc connectivity X Y 5 Hybrid Networks Infrastructure + Ad hoc connectivity infrastructure AP1 AP2 B C A D E Z Ad hoc connectivity X Y 6 Hybrid Networks Infrastructure may include wireless relays infrastructure AP1 P AP2 R B C A D E Z Ad hoc connectivity X Y 7 Hybrid Networks Heterogeneity Some hosts connected to a backbone, most are not Access points may have more processing capacity, energy infrastructure AP1 AP2 B C A D E Z Ad hoc connectivity X Y 8 Hybrid Networks Heterogeneous wireless technologies infrastructure AP1 AP2 Type 1 (3G) B A Type 2 (802.11) X C D E Z Y 9 Hybrid Networks Heterogeneity is essential Pure ad hoc or pure infrastructure networks inadequate for many environments Heterogeneity is beneficial … 10 Benefit over Pure Ad Hoc Networks Infrastructure provides a frame of reference Can assign approximate locations to the mobiles – Provide location-aware services – Reduce route discovery overhead AP0 AP1 B R1 A AP2 AP3 D R2 R3 A 11 Benefit over Pure Ad Hoc Networks Infrastructure can reduce diameter of the network Lower delay Potentially greater per-flow throughput infrastructure AP1 P AP2 R B C A D E Z Ad hoc connectivity X Y 12 Infrastructure Facilitates New Trade-Offs (hypothetical curves) Poor Man’s Ad Hoc Network Ad hoc-ness = K User density distribution affects the trade-off 13 Infrastructure Helps in Resource Allocation Address Assignment Unique IP addresses need to be assigned to hosts in a network DHCP used in traditional networks Difficult to use DHCP in pure ad hoc networks But Can also be deployed on the infrastructure in a hybrid network 14 Infrastructure Helps in Resource Allocation Address Assignment Impossible to detect address duplication in networks that can get partitioned • Unbounded delays cause difficulty Clusters of hosts may partition from the infrastructure, rejoin, over time Need a mechanism to assign unique addresses despite partitions • Impossible with unbounded message delays 15 If a problem cannot be solved Change the problem 16 Weak Duplicate Address Detection Packets from a given host to a given address should be routed to the same destination, despite duplication of the address Achievable despite unbounded delay, but incurs overhead Infrastructure to the rescue: Use weak DAD only for nodes partitioned from the infrastructure Can this extend to other resource allocation problems? 17 Benefit over Pure Infrastructure Networks Ad hoc routing increases the “reach” of the infrastructure Connectivity can be traded with overhead Example: Limit “ad hoc-ness” to K hops 18 Hybrid Networks: Research Issues How to implement infrastructure? How to deploy relays/access points? What functionality should be given to relays and access points? • Density, distribution • Should they cooperate? With each other? With mobiles? Are relays an optimization or necessary components? Should the spectrum be divided between the infrastructure and ad hoc components? 19 Hybrid Networks: Research Issues How to design protocols? How to trade “complexity” with “performance” ? How to design protocols that maximize “performance” for a given complexity? • Parameterize ad hoc-ness ? • Power control: How should the heterogeneity affect power control? • MAC: Should the infrastructure do more work? • Routing: Reduce overhead using infrastructure • Transport: How to approach theoretical capacity bounds? • How to deal with potentially unbounded delays? The answers to the above questions are inter-dependent • Power control, MAC, routing, transport protocols affect each other’s behavior • Cross-layer design needed 20 Heterogeneity Many dimensions of heterogeneity: Architecture Physical capability of hosts – – – – Antennas Topology control mechanisms Processing capability Energy availability Higher layers 21 Antenna Capabilities “Fixed beam” antennas prevalent on mobile devices Omnidirectional antennas “Movable beam” antennas likely to become more prevalent over time Switched, steered, adaptive, smart … – Can form narrow beamforms, which may be changed over time Re-configurable antennas – Beamforms can be changed over time by reconfiguring the antenna Different devices may incorporate different antennas 22 Antenna Heterogeneity All antennas are not made equal Beamforms: Only directional, or omni too? Timescale: Can beams be “moved” at packet timescales? Single beam or multiple beams? Variations with time? 23 Antenna Capabilities Protocols designed for omnidirectional (fixed beam) antennas inadequate with movable beam antennas State of the art: MAC Protocols designed for specific antenna capabilities Need “antenna-adaptive” MAC and routing protocols that allow for antenna heterogeneity 24 Antenna Heterogeneity Heterogeneity is essential Enforcing homogeneity will limit benefits from antenna improvements Heterogeneity is beneficial Devices can employ best antennas that they can “afford” – Device constraints: energy, processing, size, weight, $$ – Access points may use more capable antenna than mobiles Antenna-adaptive protocols allow separation of the antenna as a “layer” in the protocol stack 25 Antenna Heterogeneity: Research Challenge How to design “antenna-adaptive” protocols ? Need to develop suitable antenna abstractions that span a range of antenna designs Forces us to think about essential characteristics of antennas – Example: Variability of beam patterns a more fundamental property than directionality 26 Evaluation of Wireless Networks 27 Capacity Capacity analysis: • Capacity results useful to determine the gap between actual performance and the best case scenario • Significant progress in recent years • Need further work to model heterogeneous environments 28 Evaluation of Wireless Protocols Benchmarks: Need benchmarks for comparison of different protocols • State of the art: Toy benchmarks, almost no real data (for evaluating multi-hop wireless networks) Simulations • Commonly used simulation models are poor • Need better physical layer models accessible to protocol community 29 Evaluation of Wireless Protocols Experimentation: “Full scale” experiments not always practical Need mechanisms to build and experimentally evaluate “scaled models” of the network – – – – Physical dimensions Mobility Number of hosts Traffic density How to “scale down” the network, and still maintain essential behaviors? 30 Conclusions 31 Conclusions Heterogeneity essential, and beneficial Heterogeneity Complexity ? Not necessarily Thinking about heterogeneity useful in arriving at better abstractions Need protocols that can exploit heterogeneity Need approaches for realistic comparative evaluation of protocols 32 Thanks! 33