Heterogeneity in MultiHop Wireless Networks Nitin H Vaidya

Heterogeneity in Multi-Hop Wireless Networks Nitin H. Vaidya University of Illinois at Urbana-Champaign www. crhc. uiuc. edu/~nhv © 2003 Vaidya 1

Summary g g Heterogeneity is essential Heterogeneity is beneficial Research Agenda g Develop protocols that exploit the heterogeneity g Develop mechanisms to better evaluate wireless systems Proof by example … 2

Heterogeneity g Many dimensions of heterogeneity: i. Architecture i. Physical capability of hosts i. Higher layers 3

Architecture Multi-hop wireless networks g Pure ad hoc networks g Hybrid networks 4

Pure Ad Hoc Networks g g No “infrastructure” All communication over (one or more) wireless hops B C A D E Z Ad hoc connectivity X Y 5

Hybrid Networks g Infrastructure + Ad hoc connectivity AP 1 infrastructure B C A AP 2 D E Z Ad hoc connectivity X Y 6

Hybrid Networks g Infrastructure may include wireless relays AP 1 P infrastructure AP 2 R B C A D E Z Ad hoc connectivity X Y 7

Hybrid Networks g Heterogeneity i. Some hosts connected to a backbone, most are not i. Access points may have more processing capacity, energy AP 1 infrastructure B C A AP 2 D E Z Ad hoc connectivity X Y 8

Hybrid Networks g Heterogeneous wireless technologies AP 1 infrastructure AP 2 Type 1 (3 G) B A Type 2 (802. 11) X C D E Z Y 9

Hybrid Networks g Heterogeneity is essential i. Pure ad hoc or pure infrastructure networks inadequate for many environments g Heterogeneity is beneficial … 10

Benefit over Pure Ad Hoc Networks g Infrastructure provides a frame of reference i. Can assign approximate locations to the mobiles – Provide location-aware services – Reduce route discovery overhead AP 0 AP 1 B R 1 A AP 2 AP 3 D R 2 R 3 A 11

Benefit over Pure Ad Hoc Networks g Infrastructure can reduce diameter of the network i. Lower delay i. Potentially greater per-flow throughput AP 1 P infrastructure AP 2 R B C A D E Z Ad hoc connectivity X Y 12

Infrastructure Facilitates New Trade-Offs (hypothetical curves) overhead connectivity 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 g Unique IP addresses need to be assigned to hosts in a network g DHCP used in traditional networks g Difficult to use DHCP in pure ad hoc networks g But Can also be deployed on the infrastructure in a hybrid network 14

Infrastructure Helps in Resource Allocation Address Assignment g Impossible to detect address duplication in networks that can get partitioned • Unbounded delays cause difficulty g Clusters of hosts may partition from the infrastructure, rejoin, over time g 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 g Achievable despite unbounded delay, but incurs overhead g 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 g Ad hoc routing increases the “reach” of the infrastructure g Connectivity can be traded with overhead i. Example: Limit “ad hoc-ness” to K hops 18

Hybrid Networks: Research Issues How to implement infrastructure? g How to deploy relays/access points? g What functionality should be given to relays and access points? • Density, distribution • Should they cooperate? With each other? With mobiles? g Are relays an optimization or necessary components? g Should the spectrum be divided between the infrastructure and ad hoc components? 19

Hybrid Networks: Research Issues How to design protocols? g How to trade “complexity” with “performance” ? g 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? g 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 g Many dimensions of heterogeneity: i. Architecture i. Physical capability of hosts – – Antennas Topology control mechanisms Processing capability Energy availability i. Higher layers 21

Antenna Capabilities g “Fixed beam” antennas prevalent on mobile devices g “Movable beam” antennas likely to become more prevalent over time i. Omnidirectional antennas i. Switched, steered, adaptive, smart … – Can form narrow beamforms, which may be changed over time i. Re-configurable antennas – Beamforms can be changed over time by reconfiguring the antenna g Different devices may incorporate different antennas 22

Antenna Heterogeneity All antennas are not made equal g Beamforms: Only directional, or omni too? g Timescale: Can beams be “moved” at packet timescales? g Single beam or multiple beams? g Variations with time? 23

Antenna Capabilities g Protocols designed for omnidirectional (fixed beam) antennas inadequate with movable beam antennas g State of the art: MAC Protocols designed for specific antenna capabilities g Need “antenna-adaptive” MAC and routing protocols that allow for antenna heterogeneity 24

Antenna Heterogeneity g Heterogeneity is essential i. Enforcing homogeneity will limit benefits from antenna improvements g Heterogeneity is beneficial i. Devices can employ best antennas that they can “afford” – Device constraints: energy, processing, size, weight, $$ – Access points may use more capable antenna than mobiles i. 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 ? g Need to develop suitable antenna abstractions that span a range of antenna designs g 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 g 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 g Benchmarks: Need benchmarks for comparison of different protocols • State of the art: Toy benchmarks, almost no real data (for evaluating multi-hop wireless networks) g Simulations • Commonly used simulation models are poor • Need better physical layer models accessible to protocol community 29

Evaluation of Wireless Protocols g Experimentation: i“Full scale” experiments not always practical i. Need mechanisms to build and experimentally evaluate “scaled models” of the network – – Physical dimensions Mobility Number of hosts Traffic density i. How to “scale down” the network, and still maintain essential behaviors? 30

Conclusions 31

Conclusions g Heterogeneity essential, and beneficial g Heterogeneity Complexity ? g Need protocols that can exploit heterogeneity g Need approaches for realistic comparative evaluation of protocols i. Not necessarily i. Thinking about heterogeneity useful in arriving at better abstractions 32

Thanks! 33