Photonics UK and Cyber Defense UK

Posted by Jon Crowcroft

Last couple of days I was in these two events


1.EPSRC Network of Networking 2 day workshop on Photoonics - see


Very interesting to see how coherent the UK's academic and industry photonics community are - they have a pretty clear roadmap for next 5 years and then some nice challenges - not a lot for CS (still) until they can do somethign cool in a) integration of optical links onto processors and b) build some more viable (in scale/integration/power terms) gates....but in terms of what they are doing for price/performance, they pretty much match Moore's law (terminating a 10GigE for 10 bucks is an amazing achievement!)


2. Rustat conference on UK Cybersecurity


This will almost certainly be blogged by Ross or someone else in the security group as they were there en masse. I chaired a session on UK skills and a couple of good outcomes were support from research counciles for more PhDs (whether this leads to money will remain to be seen) and


and the idea that CS graduates that end up on the Board as CIOs should make sure they have good business skills so they aren't looked down on by other board members as just a sort of uber "IT guy"...


Lots of very interesting corridor conversations. The UK gov budget in this space is 600M quid, so many SMEs scampering after it:) In general, we seem to be in ok shape (government policy doc on cybersecurity out soon, recent Chatam House report (can't find link right now) appareently less rosy, but still very useful. Expect to see more details here soon:


We're having a NATO work shop on this in 10 days at Wolfson in Cambridge...Rex Hughes there is coordinating it with the Cambridge Science and Policy group.

Finally, I suggested a Homeopathic remedy for cyberattacks might be to dilute the stuxnet virus say 10^11 times in some random bits (e.g windows vista kernel code) and add it to your site.


Oh yeah and can someone tell me just what does the ICTKTN do?? :)


IBM TCG Visit and Cambridge Networks Network

Posted by Jon Crowcroft

The last couple of days were busy - IBM visited en masse and their Technical consulting group of around 50 people showed up (in CMS) to talk about   various interesting topics - for me, the best one was a talk about financial service industry regulatory controls through risk data sharing (via a third party - a sort of nuclear test ban treaty assurance service) - very neat - lots of other good topics - Rolls Royce were also there - amusingly, IBM complimented Rolls on their reliable history (compared with the Software Industry) - i didn't feel it fair to mention the RB211 or the recent A380 shattered turbine:)


More locally crucual was the kickoff meeting of the Cambridge Networks Network - see for more info - the

This kickoff was to setup a cross group, grass roots movement to join up various people in systems biology, brain mapping, economics, eplidemiology (including plant sciences) and others to share common knowledge and methods/techniques for studying complex networked systems with interesting (e.g. emergent) phenomena - the kickoff was amibitious with talked from 5 people supposed to be 10 mins each (averaging 20 mins:)


some ideas i thought of while listening


1. weak ties (long links) in modular systems (social nets, the brain, the internet) serve the same purpose as random perturbations (like mutation) does in optimisation tools (like Genetic Algorithms or Simulated Annealing) - to get you out of local minima:- most GAs work by cross-over which implements parallel search in local areas of a fitness landscape (since similar genes share / cross over/breed and are succesful or not similarly) - I wonder if there is any literature on how graphs have a small (but non zero) fraction of "escape routes" from the highly interconnected/modular/cliqueish structure of a small world are slightyly more robust than purely hierarchical modular ones???


2nd thought was about epidemics (and economics) - the Vickers report on the banking sectore is basically quarantining domestic banks (building socieities) from the high risk (prostitution and drug user/gambling/casino) banking sector. on the other hand, sharing information problemly (see Efficient Markets) would also work (see IBM work above)


The difference is that a structural regulation is much easier to implement than a big bang transparent information regime. maybe we do one now, the other later - who knows?


The talk on Citrus Blight in Miami lemon trees was fun - reminded me plants (genetically) are a lot easier than animals (c.f. fluphone:)


The map of spread of the blight looked really like the map of the nuclear tests recently shown on youtube (see

for that (esp. for Anil:)


One nice name check was the work on neural structures and VLSI that showed Rent's Law applies to both - cute (but should we add weak ties to our multicore systems - one for Steve Furber maybe?)


Anyhow, this looks like a very good (young, active, enthusiastic, smart) initiative - they will be having a bi-weekly seminar series starting pretty soon - probably coordinated with the statslab's networking series....

(for people too young to recall, Rolls Royce actually went bankrupt in the 1970s trying to make carbon fiber turbine blades work - in the end, a government bailout fixed it, and they are ok - the problem they hit was the fibers in the original blades weren't knit in enough different directions - a prob,lem shared with the fiberglass bodywork o nthe Reliant Scimitar (and robin) which would shatter under fairly light impact into lots of dangeous shards. The solution is to sew 3 dimensions of fiber (much more expensve/complex, but immesnly strong, but also tunable for different flexibility in any given dimension) into the matrix - the recent A380 engine problem wasnt design, but manufacturing process...


Mobicom. Day 3

Posted by Narseo

3rd and final day... mainly about PHY/MAC layer and theory works

The day started with a Keynote by Farnan Jahanian (University of Michigan, NSF).  Jahanian talked about some opportunities behind cloud computing research. In his opinion, cloud computing can enable new solutions in fields such as health-care and also environmental issues. As an example, it can help to enforce a greener and more sustainable world and to predict natural disasters (e.g. the recent japanese tsunami) with the suport of a wider sensor network. His talk concluded with a discussion about some of the challenges regarding computer science research in the US (which seem to be endemic in other countries). He highlighted that despite the fact that the market demands more computer science graduates, few students are joining related programs at every level, including high school.

Session 7. MAC/PHY Advances.

No Time to Countdown: Migrating Backoff to the Frequency Domain, Souvik Sen and Romit Roy Choudhury (Duke University, USA); and Srihari Nelakuditi (University of South Carolina, USA)

Conventional WiFi networks perform channel contention in time domain. Such approach imposes a high channel wastage due to time back-off. Back2F is a new way of enabling channel contention in the frequency domain by considering OFDM subcarriers as randomised integer numbers (e.g. instead of picking up a randomised backoff length, they choose a randomly chosen subcarrier). This technique requires incorporating an additional listening antenna to allow WiFi APs to learn about the backoff value chosen by nearby access points and decide if their value is the smallest among all others generated by close-proximity APs. This knowledge is used individually by each AP to schedule transmissions after every round of contention. Nevertheless, by incorporating a second round of contention, the APs colliding in the first one will be able to compete again in addition to a few more APs. The performance evaluation was done on a real environment. The results show that the collision probability decreases considerable with Back2F with two contention rounds. Real time traffic such as Skype experiences a throughput gain but Back2F is more sensitive to channel fluctuation.

Harnessing Frequency Diversity in Multicarrier Wireless Networks, Apurv Bhartia, Yi-Chao Chen, Swati Rallapalli, and Lili Qiu (University of Texas at Austin, USA)

Wireless multicarrier communication systems are based on spreading data over multiple subcarriers but SNR varies in each subcarrier. In this presentation, the authors propose a join integration of three solutions to reduce the side-effects:

  1. Map symbols to subcarriers according to their importance.
  2. Effectively recover partially corrupted FEC groups and facilitate FEC decoding.
  3. MAC-layer FEC to offer different degrees of protection to the symbols according to their error rates at the PHY layer

Their simulation and testbed results corroborate that a joint combination of all those techniques can increase the throughput in the order of 1.6x to 6.6x.

Beamforming on Mobile Devices: A first Study, Hang Yu, Lin Zhong, Ashutosh Sabharwal, David Kao (Rice University, USA)

Wireless links present two invariants: spectrum is scarce while hardware is cheap. The fundamental waste in cellular base stations is because of the antenna design. Lin Zhong proposed passive directional antennas to minimize this issue. They used directional antennas to generate a very narrow beam with a larger spatial coverage. They have proved that this solution is practical despite small form factor of smartphone's antenna, resistent to nodes rotation (only 2-3 dB lost if compared to a static node), and does not affect the battery life of the handsets, specially in the uplink as the antenna's beam is narrower. This technique allows calculating the optimal number of antennas for efficiency. The system was evaluated both indoors and outdoors in stationary/mobile scenarios.  The results show that it is possible to save a lot of power in the client by bringing down the power consumption as the number of antennas increases with this technique.

SESSION 8. Physical Layer

FlexCast: Graceful Wireless Video Streaming, S T Aditya and Sachin Katti (Stanford University, USA)

This is a scheme to adapt video streaming to wireless communications. Mobile video traffic is growing exponentially and users' experience is very poor because of channel conditions. MPEG-4 estimates the quality over long timescales but channel conditions change rapidly thus it has an impact on the video quality. However, current video codecs are not equipped to handle such variations since they exhibit an all or nothing behavior. They propose that quality is proportional to instantaneous wireless quality, so a receiver can reconstruct a video encoded at a constant bit rate by taking into account information about the instantaneous network quality.

A Cross-Layer Design for Scalable Mobile Video, Szymon Jakubczak and Dina Katabi (Massachusetts Institute of Technology, USA)

One of the best papers in Mobicom'11. Mobile video is limited by the bandwidth available in cellular networks, and lack of robustness to changing channel conditions. As a result, video quality must be adapted to the channel conditions of different receivers. They propose a cross-layer design for video that addresses both limitations. In their opinion the problem is that the compression an error protection convert real-valued pixels to bits and as a consequence, they destroy the numerical properties of original pixels. In analog TV this was not a problem since there is a linear relationship between the transmitted values and the pixels so a small perturbation in the channel was also transformed on a small perturbation on the pixel value (however, this was not efficient as this did not compress data).

SoftCast is as efficient as digital TV whilst also compressing data linearly (note that current compression schemes are not linear so this is why the numerical properties are lost). SoftCast transforms the video in the frequency domain with a transform called 3D DCT. In the frequency domain, most temporal and spatial frequencies are zeros so the compression sends only the non-zero frequencies. As it is a linear transform, the output presents the same properties. They ended the presentation with a demo that demonstrated the real gains of SoftCast compared to MPEG-4 when the SNR of the channel drops.

Practical, Real-time Full Duplex Wireless, Mayank Jain, Jung II Choi, Tae Min Kim, Dinesh Bharadia, Kanna Srinivasan, Philip Levis andSachin Katti (Stanford University, USA); Prasun Sinha (Ohio State University, USA); and Siddharth Seth (Stanford University, USA)

This paper presents a full duplex radio design using signal inversion (based on a balanced/unbalanced (Balun) transformer)and adaptive cancellation. The state of the art in RF full-duplex solutions is based on techniques such as antenna cancellation and they present several limitations (e.g. manual tuning, channel-dependent). This new design supports wideband and high power systems without imposing any limitation on bandwidth or power. The authors also presented a full duplex medium access control (MAC) design and they evaluated the system using a testbed of 5 prototype full duplex nodes. The results look promising so... now it's the time to re-design the protocol stack!

Session 9. Theory

Understanding Stateful vs Stateless Communication Strategies for Ad hoc Networks, Victoria Manfredi and Mark Crovella (Boston University, USA); and Jim Kurose (University of Massachusetts Amherst, USA)

There are many communication strategies depending on the network properties. This paper explores adapting forwarding strategies that decides when/what state communication strategy should be used based on network unpredictability and network connectivity. Three network properties (connectivity, unpredictability, and resource contention) determine when state is useful. Data state is information about data packets, it is valuable when network is not well-connected whilst control-state is preferred when the network is well connected. Their analytic results (based on simulations on Haggle traces and DieselNet) show that routing is the right strategy for control state, DTN forwarding for data-state (e.g. Haggle Cambridge traces) and packet forwarding for those which are in the data and control state simultaneously (e.g. Haggle Infocom traces).

Optimal Gateway Selection in Multi-domain Wireless Networks: A Potential Game Perspective, Yang Song, H. Y. Wong, and Kang-Won Lee (IBM Research, USA)

This paper tries to leverage a coalition of networks with multiple domains with heterogeneous groups. They consider a coalition network where multiple groups are interconnected via wireless links. Gateway nodes are designated by each domain to achieve a network-wide interoperability.  The challenge is minimising the intra-domain cost and the sum of backbone cost. They used a game-perspective approach to solve this problem to analyse the equilibrium inefficiency. They consider that this solution can be also used in other applications such as power control, channel allocation, spectrum sharing or even content distribution.

Fundamental Relationship between Node Density and Delay in Wireless Ad Hoc Networks with Unreliable Links, Shizhen Zhao, Luoyi Fu, and Xinbing Wang (Shanghai JiaoTong University, China); and Qian Zhang (Hong Kong University of Science and Technology, China)

Maths, percolation theory ... quite complex to put into words

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Mobicom. Day 2

Posted by Kiran Rachuri

Day 2 of MobiCom 2011 started with my talk on SociableSense. Fourteen papers were presented over four sessions, including two best papers.

SESSION: Applications

SociableSense: Exploring the Trade-offs of Adaptive Sampling and Computation Offloading for Social Sensing, Kiran K. Rachuri, Cecilia Mascolo, Mirco Musolesi, and Peter J. Rentfrow (University of Cambridge, United Kingdom)

Our work. Details at:

Overlapping Communities in Dynamic Networks: Their Detection and how they can help Mobile Applications, Nam P. Nguyen, Thang N. Dinh, Sindhura Tokala, and My T. Thai (University of Florida, USA)

A better understanding of mobile networks in terms of overlapping communities, underlying structure, organisation helps in developing efficient applications such as routing in MANETs, worm containment, and sensor reprogramming in WSNs. So, the detection of network communities is important, however, they are large and dynamic, and overlapping communication.  Can community detection be performed in a quick and efficient way.

They propose a two phase limited input dependent framework to address this. Phase 1: basic communities detection (basic communities are dense parts of the networks). Phase 2: update network communities when changes are introduced, i.e., handle: adding a node/edge, and removing a node/edge.  The evaluation is based on MIT reality mining data.  They evaluate the proposed scheme with respect to two applications: routing in MANETs and worm containment.

Detecting Driver Phone Use Leveraging Car Speakers, Jie Yang and Simon Sdhom> (Stevens Institute of Technology, USA); Gayathri Chandrasekaranand Tam Vu (Rutgers University, USA); Hongbo Liu (Stevens Institute of Technology, USA);Nicolae Cecan (Rutgers University, USA); Yingying Chen (Stevens Institute of Technology, USA);Marco Gruteser and Richard P. Martin(Rutgers University, USA)

(Joint Best Paper Award)

80% of people talk on cell phone while driving. The consequences of this might be dangerous (18% accidents). They claim that hands-free devices do not help because of the effects in the cognitive load on the driver. Several mobile apps in the market trying to solve that. (zoom safer ïzup, cellsafety). Recent measures:

-hard blocking: jammers, blocking calls etc

-soft interaction: delay calls, route to voice mail, automatic reply

Current apps that actively prevent cell phone use in vehicle only detect the phone is in vehicle or not through: GPS, handover, signal strength, speedometer etc. None of them have capability to find whether phone is used by driver or passenger. They use an acoustic ranging approach to solve this problem.  They identify the position of the cell phone based on the car speakers and mobile phone, and based on speakers emitting different sounds at different times. Cell phone mic has wider range of frequency range: so beep frequency to outside user hearing range.  Evaluation shows that the accuracy of detection is over 90%.

I Am the Antenna: Accurate Outdoor AP Location Using Smartphones, Zengbin Zhang, Xia Zhou, Weile Zhang, Yuanyang Zhang, Gang Wang, Ben Y. Zhao, and Haitao Zheng (University of Calfornia at Santa Barbara, USA)

The density of APs in the environment is very high. How to find the location of an AP?  Conventional AP location methods:

- Directional antenna: Fast, very accurate but expensive

- Signal map: Simple but time consuming

- RSS gradient: Low accuracy, low measurement overhead but low accuracy

Their solution is based on the effect  of user orientation degree to an AP on RSS. The body of the user can affect the SNR (they observed around 13dBm difference). They also tested the generality of the effect with multiple phones, protocols, different users, and environments, and  RSS profiles all followed the same trend.

Evaluation is in a campus, with three scenarios. 1. Simple line of sight (no blocks) 2. complex line of sight (vehicles etc) 3. Non line of sight (line of sight is completely blocked). Metric: absolute angular error: detected direction - actual direction. results: error < 30 degree for 80% cases, in simple LOS (line of sight); error < 65 degree for 80% cases in Non LOS.

SESSION: Cellular Networks

Traffic-Driven Power Saving in Operational 3G Networks,  Chunyi Peng, Suk-Bok Lee, Songwu Lu, and Haiyun Luo (University of California at Los Angeles, USA)

Transmission power of Base Stations increases linearly with the traffic load. The cooling power keeps constant and its comparable to the transmission power. As a result, high energy is consumed energy even at zero traffic. Existing solutions do not address practical issues and they follow a theoretical analysis. In this work, they propose a traffic-driven approach that exploits traffic dynamics to turn off under-utilised BSs for system-wide energy efficiency. They claim that traffic is quite predictable in the base station. There’s a lot of potential to save energy in quite hours but also in peak hours. Their solution also tries to be compatible with current 3G standard/deployment. Issues addressed: Issue 1: how to satisfy location dependent coverage and capacity constraints. Issue 2: how to estimate traffic load ?

Solution: based on profiling: estimate traffic envelope via profiling and leverage near-term stability. The set of BS active in idle hours should be a subset of the ones in peak hours. Their condition is that they should not switch BSs more than once per day. Provide location-dependent capacity. Their estimation is a moving average with 24 daily intervals. However, frequent on/off switching is undesirable: takes several minutes. It should be based on traffic characteristics.

MOTA: Engineering an Operator Agnostic Mobile Service, Supratim Deb, Kanthi Nagaraj, and Vikram Srinivasan (Bell Labs Research, India)

Cellular coverage varies with respect to locations. Users may not be happy with a single service provider, and there is a case for users choosing services from multiple providers. Dual sim phones are already popular in asia. Users are using services based on the cost from the providers. Goal of this work: Ability for users to join the network of choice at will based on location, pricing, and applications.

Solution: to propose changing operator from the user-side. They consider several solutions: Option 1: Centralised approach making decisions but operators unlikely to share network planning information. Option 2: Users use signal strength from different base stations. This is insufficient and can result in poor user experience.

They propose MOTA in which a service aggregator is introduced: new intermediary between users and operator and is responsible for maintaining customer relationships and handles all control plane operations that cannot be handled by a single operator. The also use a Utility function that incorporates fairness. Evaluation is based on the data from one of the largest cellular operators in India.

Anonymization of Location Data Does Not Work: A Large-Scale Measurement Study, Hui Zang and Jean Bolot (Sprint Applied Research, USA)

Call Detail Records (CDR) keep a lot of information about the phone calls of the users and they can be linked to a location. They can be used for marketing, security, LBS, Mobility Modelling, however, privacy might be breached if such data is released. Traditional approaches to protect privacy of users is through anonymisation, however, this works shows that does not work. CDR contains: mobile id, time of call, call durations, start cell id, start sector id, end sector id, call direction, caller id. If mobile id and caller id are anonymised, can we detect the user. Its shown that with gender, zipcode, and birthdate, 87% of USA population can be identified.

Their dataset consists of more than 30 billion call records made by 25 million cell phone users across the USA. Their approach is to infer top N locations for each user and correlate this with publicly available information such as census data. They show that the top 1 location does not yield small anonymity sets, but top 2 and 3 locations do at the sector or cell-level granularity. They also provide possible solutions based on spatial and time domain approaches for publishing location data without compromising on privacy.

SESSION: Infrastructureless Networking.

Enhance & Explore: An Adaptive Algorithm to Maximize the Utility of Wireless Networks, Adel Aziz and Julien Herzen (École Polytechnique Fédérale de Lausanne, Switzerland); Ruben Merz (Deutsche Telekom Laboratories, Germany); Seva Shneer (Heriot-Watt University, UK); andPatrick Thiran (École Polytechnique Fédérale de Lausanne, Switzerland)

This work addresses the problem of providing efficiency and fairness in wireless networks. Their approach is based on maximising a utility function. They propose an algorithm called Enhance and Explore that maximises the utility function. The challenges in designing this scheme are: work on existing mac, non-network wide message passing, and wireless capacity is unknown a priory.

They consider two scenarios: WLAN setting: inter-flow problem and optimally allocate resources. Multi-hop setting: intra-flow problem and avoid congestion. They show analytically that the proposed algorithm converges to a point of optimal utility. Evaluation is through experiments in a testbed and simulations in ns-3.

Scoop: Decentralized and Opportunistic Multicasting of Information Streams, Dinan Gunawardena, Thomas Karagiannis, and Alexandre Proutiere (Microsoft Research Europe, UK); Elizeu Santos-Neto (University of British Columbia, Canada); and Milan Vojnovic (Microsoft Research Europe, UK)

This work aims at leveraging mobility for content delivery in networks of devices experiencing intermittent connectivity. Main challenge: routing / relaying strategies. Existing solutions include epidemic routing. Drawback of existing works are: simplifying assumptions on mobility, and interact contact times are exponentially distributed. This work proposes SCOOP that

  • maximizes some global system objective
  • accounts for storage and transmission costs
  • multi-point to multi-point communications
  • decentralized
  • model-free (allows general node mobility)

There is a necessity to propose a mobility model-free system. They used classic traces: UCSD, Infocom, DieelNet and SF Taxis.  They show that two hops are enough to reach a large percentage of nodes. They also show that the delays in paths between a source and a destination are positively correlated. They aim to identify the strategy optimally exploiting mobility and buffer constraints and relays. However, this is a hard problem. They use a sub-gradient algorithm to solve it efficiently. Evaluation is through numerical experiments. They compared SCOOP with an idealized version of R-OPT of RAPID algorithm (assumes full global knowledge). Performance with respect to delivery ratio is very close to R-OPT.

R3: Robust Replication Routing Wireless Networks with Diverse Connectivity, Xiaozheng Tie, Arun Venkataramani (University of Massachusetts Amherst, USA) and Aruna Balasubramanian (University of Washington).

Wireless routing protocols are designed for specific target environments, like well-connected meshes, intermittently connected MANETs. Problems with this is routing protocols are fragile, and perform poorly outside its target environment. Wireless networks exhibit spatio-temporal diversity, therefore, compartmentalized design is not efficient. Can we design a protocol that ensures a robust performance across networks.

They propose to use Replication routing. They present a model to quantify replication gain. Replication gain depends on the path delay distributions and not just expected value. They study the average replication gain with respect to number of paths using DieselNet-DTN and Haggle traces. They propose R3: a link state protocol that selects replication paths using the proposed model. The scheme also adapts the replication to load.

Evaluation is both on DieselNet DTN testbed and a Mesh testbed. Simulation validation is also performed  using DieselNet deployment. Compared with several protocols. Simulation based on haggle trace shows that R3 reduces delay by up to 60% and increases good put by up to 30% over SWITCH. Simulations on DieselNet-Hybrid shows that R3 improves median delay compared to SWITCH  by 2.1x.

Flooding-Resilient Broadcast Authentication for VANETs, Hsu-Chun Hsiao, Ahren Studer, Chen Chen, and Adrian Perrig (Carnegie Mellon University, USA); and Fan Bai, Bhargav Bellur, and Aravind Iyer (General Motors Research)

Each vehicle possess an On Board Unit (OBU), and broadcasts info for safety and convenience. This information has to be secured. IEEE 1069.2 standard suggests to use ECDSA signature for these messages, however, its expensive for verification and takes around 22ms to verify, and its difficult if many messages arrive in short time. Can we reduce this verification delay. Core idea of this work: entropy aware authentication.

They propose two methods: (1) FastAuth - exploits predictability of future messages. Uses hash to verify location updates instead of ECDSA . The result is 1 us instead of 22000 us in ideal case. (2) SelAuth - selective verification before forwarding. They also reduce the communication overhead. Evaluation is based on real vehicle traces (4 traces), each generated by driving a car along a 2 mile path for 2 hours. Results show that the signature generation is 20x faster and verification is 50x faster compared to ECDSA.

SESSION: Protocols.

E-MiLi: energy-Minimizing Idle Listening in Wireless Networks, Xinyu Zhang and Kang G. Shin (University of Michigan-Ann Arbor, USA)

(Joint Best Paper Award)

Wi-Fi is a popular means of wireless Internet connection. However, Wi-Fi is a main energy consumer in mobile devices, 14x higher than GSM on phone. This is due to cost of idle listening. Moreover, idle listening power is comparable to TX/RX power. Existing solutions are variants of PSM, but, is this good enough. No, this is due to carrier sensing time. To overcome this, they propose E-MiLI that reduces the power consumption of idle listening. They down-clock the radio in idle listening mode. Down-clocking by 1/4 saves power by 47.5%. The key challenge is how to decode a packet given that receiver sampling rate should be no less than senders clock rate to decode a packet. The solution proposed is to separate detection from decoding.They add a preamble to 802.11 packet that can be detected by low clock rates.

One issue with this is false triggering. Packets intended for one client may trigger all other clients and this is a waste of energy. The second problem is the energy overhead caused by large preambles. The solution is a minimum-cost address sharing to allow multiple nodes to be assigned the same address. Address allocated according to channel usage. There’s a delay caused by cold-rate switching too. To reduce this they use opportunistic downclocking. Evaluation is with respect to: Packet detection: software radio based experiments, Energy consumption: through Wi-Fi traces, and Simulations using ns-2. Results: When SNR is above 8dB, miss detection probability is almost zero. They achieved close to 40% energy saving.

Refactoring Content Overhearing to Improve Wireless Performance, Shan-Hsiang Shen, Aaron Gember, Ashok Anand, and Aditya Akella (University of Wisconsin-Madison, USA)

The main aim is to improve on wireless performance by leveraging overheard packets. Several techniques available currently, but,  none of these leverage duplicate data. This work takes a content based overhearing approach and suppresses duplicate data transmission. Ditto is first work that used content based overhearing approach,  but it works at the granularity of objects, and does not remove sub packet redundancy. Moreover, it only works for some applications. This work presents REfactor content overhearing:

(1) this scheme puts content overhearing at the network layer, and this results in savings across applications.  Transport layer approach (used in Ditto) ties data to application or object chunk. Network layer approach reduces redundancy across all flows. Transport approach also requires payload reassembly.

(2) this scheme identifies sub-packet redundancy. This saves transmission times. Ditto only works in 8 - 32kb object chunks, whereas the proposed scheme operates at a finer granularity. This results in savings from redundancy as small as 64 bytes. and this also results in leveraging any overhearing even a single packet.

Evaluation through test-bed experiments show 6 to 20% improvement in Goodput. Simulation results also show that 20% improvement is achieved in Goodput.

Distributed Spectrum Management and Relay Selection in Interference-Limited Cooperative Wireless Networks, Zhangyu Guan (Shandong University, P. R. China); Tommaso Melodia (State University of New York at Buffalo, USA); Donfeng Yuan (Shandong University, P. R. China); and Dimitris A. Pados (State University of New York at Buffalo, USA)

Emerging multimedia services require high data rates. This work aims to maximize the capacity of wireless networks by leveraging the frequency and spatial diversity. Frequency: by dynamic spectrum access, and this improves spectral efficiency. Spatial: by cooperative communication, and this enhances link connectivity. Problem: maximize sum utility (capacity, log-capacity) of multiple concurrent traffic sessions by jointly optimizing relay selection (whether to cooperate or not) and direct transmission. Problem formulated as mixed integer non-convex problem. This is NP hard. They propose a solution based on branch and bound that is able to find a globally optimum solution. Polynomial time  solution is not guaranteed but in practice it works well. Evaluation is based on simulations. Results show that the proposed schemes converge very fast. Centralized algorithm achieves at least 95% of the global optimum, and distributed schemes are very close to optimal.



Mobicom. Day 1

Posted by Narseo

Mobicom'11 is being held in the (always interesting) city of Las Vegas. In this first day, the talks were mainly about wireless technologies and different techniques to avoid congestions were proposed.

Plenary Session
Keynote Speaker: Rajit Gadh (Henry Samueli School of Engineering and Applied Science at UCLA)

Prof. Gadh talked about UCLA project “SmartGrid”, a topic which is gaining momentum in California.  This project is motivated by the fact that electricity comes from a grid that spread across a whole country and we are still using technology that has been deployed 100 years ago. The grid is rigid, fixed and large. In fact, Rajit Gadh thinks that there is a clear parallelism between data networks and power networks. Based on that observation, they aim to create a Smart Grid infrastructure with the following characteristics: self healing, active participation of consumers, capabilities to accommodate all the energy sources and storage options, eco-friendly, etc.. More information can be found in the project website.

SESSION 1. Enterprise Wireless
FLUID: Improving Throughputs in Entreprise Wireless LANs through Flexible Channelization, Shravan Rayanchu (University of Wisconsin-Madison, USA); Vivek Shrivastava (Nokia Research Center, Palo Alto); Suman Banerjee (University of Wisconsin-Madison, USA); and Ranveer Chandra (Microsoft, USA)

One of the problems in current 802.11 technologies is that channels width is fixed. However, many advantages arise by replacing fixed witch channels with flexible width ones. The goal of this paper is to build a model that can capture flexible channel conflicts, and then use this model to improve the overall throughput in a WLAN.

One of the problems in wireless channels is that depending on the interference, there are different approaches to avoid conflicts.  Nevertheless, the interference depends on the configuration of the channel. As an example, narrowing the width helps to reduce interference but they also tried to better understand the impact of the power levels.

They showed that given a SNR, it is possible that nodes can predict the delivery ratio for an specific channel width. As a result, the receiver can compute the SNR and predict the Delivery ratio as a function of the SNR autonomously. Given that, the problem of channel assignment and scheduling becomes into a flexible channel assignment and scheduling problem.

SmartVNC: An Effective Remote Computing Solution for Smartphones, Cheng-Lin Tsao, Sandeep Kakumanu, and Raghupathy Sivakumar (Georgia Tech University, USA)

In our opinion, this paper was a great example of how to improve the user experience with certain applications. In this case, they are trying to improve the UX of mobile VNC. This kind of service was designed for desktops and laptops so they do not take into account the nature of smartphones. The goal is allowing users to access a remote PC (in this case Windows) from a smart phone (Android) in a friendly way. They evaluated the UX of 22 users (experienced users, students between 20-30 y.o.) and 9 applications running on VNC. They defined different metrics such as the opinion score (the higher the complexity lesser the mean opinion score) and task effort (number of operations required for a task such as mouse clicks , key storekes etc). Given that, they correlated both metrics for those users running apps in VNC and the results showed that when the task effor is high, the UX is poorer.

They proposed aggregating repetitive sequences of operations in user activity to remove redundancy without being harmless. One of the main problems was that application macros (like in excel) are not completely application agnostic but they are extensible whilst others such as raw macros (e.g. autohotkey) are completely opposite.

They enabled Smart macros. For that, they record events and build macros and they enabled a tailored interface with collapsive overlays on the remote computing client, grouping macros by app, automatic zooming, etc.  For the applications they tested with those 22 users, they had a task effort reduction from 100 to 3 whilst the time to perform a task is also highly reduced. In the subjective evaluation, all the users showed their satisfaction with the new VNC. The talk was completed with a video recorded demo of the system.

FERMI: A FEmtocell Resource Management System for Interference Mitigation in OFDMA Networks, Mustafa Yasir Arslan (University of California Riverside, USA); Jongwon Yoon (University of Wisconsin-Madison, USA); Karthikeyan Sundaresan (NEC Laboratories America, USA); Srikanth V. Krishnamurthy (University of California Riverside, USA); Suman Banerjee (University of Wisconsin-Madison, USA); Mustafa Arslan

Femtocells: are small cellular base stations that use cable backhaul and they can extend the network coverage. In this scenario, interferences can be a problem but this problem differs from the ones that can be found in the WiFi literature. OFDMA (WiMax, LTE) uses sub-channels at the PHY and multiple users are scheduled in the same frame whilst WiFi uses OFDM (sequential units of symbols transmitted at an specific freq in time). Moreover, OFDMA presents a synchronous MAC (there's no carrier sensing like in WiFi). As a consequence, WiFi solutions cannot be applied to femtocells as interference leads to throughput loss and there are many clients coexisting in the same frame.

As a consequence, the solution must take into account both the time domain and the frequency domain. FERMI gathers load and interference related information. It operates at a coarse granularity (in the order of minutes) but this is not a drawback as interference does not change a lot in this time scale. Moreover, a per-frame solution is not feasible as the interference patterns change on each retransmission but aggregate interference and load change only at coarse time scales.

The system evaluation was done on a WiMax testbed and also on simulations. In both cases, they obtained a 50% throughput gain over pure sub-channel isolation solutions. The core results can be applicable to LTE as well.

SESSION 2. Wireless Access
WiFi-Nano: Reclaiming WiFi Efficiency through 800ns Slots, Eugenio Magistretti (Rice University, USA); Krishna Kant Chintalapudi (Microsoft Research, India);Bozidar Radunovic (Microsoft Research, U.K.); and Ramachandran Ramjee (Microsoft Research, India)

Wifi data rates have increased but throughput performance didn't see similar level of growth. Throughput is much lower than data-rate because of a high frame overhead. There’s a 45% overhead at 54Mbps but this overhead dominates at high bandwidth, around 80% in 300Mbps. This gets worst when multiple-links come at play.

This observation motivated WiFi nano, a technology that allows doubling the throughput of WiFi networks. Slot overhead can be reduced by 10x. Their solution proposes using nano slots to reduce slot duration to 9 microsec (that’s the standard one in 802.11a/n and it’s almost the minimum achievable). In addition, they exploit speculative preambles as preamble detection and transmission occur in parallel. As soon as the back-off expires, a node transmits the preamble but while transmitting preamble, it continues to detect incoming preambles even with self-interference. Their empirical results show that slightly longer preambles improve the throughput up to a 100% and frame aggregation can increase those figures even more. In fact, frame aggregation increases the efficiency as it grows from 17% to more than almost 80%.

XPRESS: A Cross-Layer Backpressure Architecture for Wireless Multi-Hop Networks, Rafael Laufer (University of California at Los Angeles, USA); Theodoros Salonidis; Henrik Lundgren and Pascal Leguyadec (Technicolor, Corporate Research Lab, France)

Multihop networks operate below capacity due to poor coordination across layers, and among transmitting nodes.  They propose using backpressure scheduling and cross-layering optimisations. At each slot, it selects optimal link set for transmission.  In their opinion, there are different challenges in multihop networks:

1- Time slots.
2- Link sets (e.g. knowing non-interfereng links)
3- Protocol overhead
4- Computation overhead
5- Link Scheduling.
6- Hardware constraints (e.g. memory limitations in wireless cards)

With XPRESS, all those challenges are addressed. XPRESS has two main components the MC (mesh controller) and the MAP (Mesh access point). MCs receive flow queues, computes schedule and disseminates schedule. On the other hand, MAP executes schedules and processes queues. The key challenge is computing the optimal schedule per slot. but this task takes a lot of time.

The MAP nodes use a x-layer protocol stack to compute the schedules. Apps running on the node go into the kernel who classifies the flows and allocates them on its own queue who is followed by a congestion controller. Then, the pipeline has a flow queue followed by a packet scheduller who puts into the proper link queue each packet. Somehow this reminds me of the work on Active Networks as they are dynamically change the behaviour of the network, in this case on a mesh-scenario. The proposed scheme achieves 63% and 128% gains over 802.11 24 Mbps and auto-rate schemes, respectively. They also performed an scalability evaluation.

CRMA: Collision-Resistant Multiple Access, Tianji Li, Mi Kyung Han, Apurva Bhartia, Lili Qiu, Eric Rozner, and Ying Zhang (University of Texas at Austin, USA); Brad Zarikoff (Hamilton Institute, Ireland)

FDMA, TDMA, FTDMA, CSMA are the traditional MAC protocols to avoid collisions. These techniques incur significant overhead so they move from collision avoidance to collision resistance based on a new encoding/decoding to allow mutliple signal to be transmitted.

In CRMA, every transmitter views the OFDM physical layer as multi orthogonal but sharable channels, and randomly selects a subset of the channels for transmission. When multiple transmissions overlap on a channel, these signals will naturally add up in the wireless medium.

In this system, ACKs are sent as data frames. However there’s a problem with misaligned collisions which are handled with cyclic prefixes (CP) so they force the collided symbols to fall in the same FFT window. On the other hand, overlapping transmissions are limited using exponential back-off.

The evaluation was done on a testbed experiment with CRMA on top of a default OFDM implementation in USRP. They also used Qualnet simulations to evaluate the efficiency of the networks.

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