Wireless Data Networking Research: From Concept to Practice Songwu Lu UCLA

Drivers for Wireless Networking Research Transport Layer Network Layer Link Layer New Services, Architectures, Requirements New Wireless Communications Technology Top Down Bottom Up

Key Driver: Wireless Communications Many examples of them: –Sector Antenna, antenna arrays, Smart antennas –Adaptive modulation, MIMO, OFDM, UWB,.. –Cognitive radio, software radio, spectrum sharing, channel management –Multiple radios, device heterogeneity –… many orthogonal dimensions –RF spectrum, antenna, data processing, … main goal: improve performance in terms of spectral efficiency Challenge: How to exploit these new PHY communication capabilities in the protocols?

Root Cause of Problems two largely disconnected communities speak different terminologies –wireless communications: Symbols, signals probabilistic terms: –information theoretic bounds –confidence factor on symbol reception, … –wireless networking Packets, bits deterministic terms –Correct/wrong binary reception

Root Cause of Problems (2) Two largely disconnected communities different methodologies –wireless communications solid theoretic foundation on information theory a set of well known assumptions: noises, interferences, etc. Theory Design-->Analysis-->prototype in chips-- >experiments –wireless networking mostly on heuristics network setting ad hoc: no agreed benchmarks/base settings Heuristic Design-->Simulations--Network Prototype-- >Experiments

Perspective From Wireless Networking We are not on the drivers seat so far –communication has driven the technology so far –we are followers No need to be sad –still plenty of space the direct communication almost NEVER works in reality at the 1st place! –other brothers also facing similar situations sometime Internet: PC/hardware industry Cellular: mobile phones

Research Life Cycle in Traditional Wireless Networking Researcher 1.wait for new radio communication tech. to come to life 2.be the 1st to design networking solution to it 3.not so lucky? understand the problem better check other aspects/components in the system 4.apply the set of tricks in your bag 5.claim credit/declare failure Experiments!!!! Positive success: insights learned Negative failure: lessons learned

Two Design Guidelines 2 most popular design principles used in the research community 1.Adaptation high-dimension dynamics 2.Coordination coherent system

Bag of Tricks in Adaptation Model-referenced design –Ideal model to capture expected behaviors under idealized situation e.g., error-free, static settings –Track the reference model under realistic conditions/scenarios Mobility, wireless channel dynamics, … Opportunistic design approach –Make each perform under peak conditions –Exploit the system population –Leverage system diversity Multiple receivers, multiple devices, multiple applications/flows, …

Bag of Tricks in Coordination Cross-Layer design –not integrated design cross layers –information sharing, informed decision at other layers –… Coordination via indirection –Adaptation-aware proxy provides indirection: act as converter/translator

Illustration Case: Rate Adaptation in Wi-Fi Problem: Adapt transmission rate to channel quality Sender Receiver 54Mbps Signal is good Signal becomes weaker 12Mbps The a/b/g/n standards allow for multiple rates based on adaptive modulation –802.11b: 4 rate options (1,2,5.5,11Mbps) –802.11a: 8 options (6,9,12,18,24,36,48,54) –802.11g: 12 options (11a set + 11b set) unspecified by the IEEE standard

As the Lucky, 1st Guy Driver: adaptive modulation Good news: SNR based feedback not there! Opportunity: packet-level information available Solution: –Hypothesis: packet loss indicates channel quality change –Tricks: Decrease transmission rate upon severe packet loss 10 consecutive successes increase rate

Rules For Not So Lucky? Understand the problem better –if a problem is not better understood, it is probably best not to provide a new solution at all –no rush for quick solutions incremental improvement is #1 enemy in research! –do not improve on flawed design!! adding gas into fire

Experiments to Discover (No) Problems ARFAARFSampleRateFixed Rate UDP Goodput (Mbps) SenderReceiver Hidden Station Case: packet collision scenario? The sender performs worse with Rate Adaptation!

The sender should not decrease the rate upon collision losses –Decreasing rate increases collisions ! Find Root Cause Severe loss Decrease Tx rate Increase Tx time Increase collision Prob. Fail to handle hidden-station!

Solution? Straightforward idea: RTS/CTS more thinking: make RTS/CTS adaptive –reduce overhead –infer collision levels Performance: ~80% throughput gain Software Hardware PHY feedback MAC Adaptive RTS Loss Estimation Rate Selection send RTS Option RRAA

Now MIMO Case? Driver: pre-n MIMO Good/Bad News: SNR feedback to some extent –more direct & timely information on channel quality? –Loss-based design obsolete?

SNR vs Rate vs Throughput SNR vs rate vs thruput are non-monotonic in fine grain main trend can still be correct RF Chamber experiments

Solution in MiRA using SNR pre-selects a range of rates –determine a rate window [minRate, maxRate]. Loss-based best rate choice within the window –play old tricks using loss-based design

Experiments on Static Clients: UDP Gains in blue arrows refer to MiRA vs. Atheros RA

Static Clients Scenario: TCP Gains in blue arrows refer to MiRA vs. Atheros RA

Broader View on Well-Known Areas look at other systems component the design works with illustrative example: Network Coding –hot topics –several papers on top conferences, from MIT, Microsoft Research, … –what can I do?

Network Coding in Reality: Wi-Fi Nets Multicast/broadcast (a XOR 6Mbps Base rate without RA Used in COPE, Wi-Fi broadcast NC is worse ! Xmit time w/o NC 2L/54 + 2L/24 Xmit time with NC L/54 +L/24+L/6 Conclusion: NC works but loses without any RA! Alice Bob a b a XOR b 54Mbps 24Mbps Native NC base rate) May NOT gain at all !

NC Gain May Vanish Simple multicast RA solution: multicast = min (rate_receiver) NC gain reduces NC: 25% (4 tx ->3 tx) In the literature Actual gain (11a): 5% NC tx time: 2*L/6+L/54 No NC: 2*L/54+2*L/ b: 1/24 (11M&1M) Root cause: NC cannot exploit rate diversity! Alice Bob a b a XOR b 54Mbps 6Mbps NC gain optimal rate) may reduce in rate diversity case!

My View on New Frontiers no need to get squeezed in crowded traditional areas bag of tricks grow much slower! problem space is wild wide west!

Wireless Networking on a Chip 1000s of cores Systems on a Chip wired interconnect: latency, physical wiring constraints High-speed wireless shortcuts

Composable Wireless Networking composable & modular from radio to networking Radios become dynamically loadable modules –no clear separation of multi-radios –Software Defined Radios platforms

Green Wireless Infrastructure infrastructure is power hungry –asymmetric design in cellular network more base stations –from radio communication, to signaling, to higher layers lots of energy-saving side –no on the infrastructure

Resilience-Oriented Design mostly performance driven for wireless networking so far resilience as the 1st principle –not as patches –learn the success from the Internet still early to have a nice try

Still Unhappy? Looking Up New requirements –Security, privacy, robustness/dependability, distributed management New applications and services –MMS, P2P image/video sharing, IP TV streaming, … –(Location-based, context-aware, personalized, pervasive) services

My View on Pervasive Cloud Computing Data stored in the Cloud Data follows you & your devices Data accessible anywhere Data can be shared with others music preferences maps news contacts messages mailing lists photo s calendar phone numbers investments Anytime, Anywhere, Any device Data Service

Research Sub-areas 1.Data Center Networking: Improving the Cloud Infrastructure 2.New Services For Mobile Devices –Security: Virus detection –Location-based Service, –social networking,… 3.Better Access for the Client 1.Improving Wi-Fi, 3G+, … for user access 2.Opportunistic Client-Client Service

Final Words Life can be good or bad in wireless networking research –It is more about your choice You are part of inventing the artifact for wireless networking

Acknowledgments most real work is done by the real heroes in projects: –Students: Innaois Yannis, Suk-Bok Lee, Starsky Wong, Hao Yang, Haiyun Luo,… –Colleagues: Lixia Zhang, Mario Gerla, Chuanxiong Guo, Jacky Shen, Yongguang Zhang, Shugong Xu, …