Abstract—Wireless Community Networks (WCNs) are wide-area wireless networks whose nodes are owned and managed by volunteers. We focus on the provision of free Internet access to mobile users through WCN-controlled wireless LAN access points (APs). We rely on reciprocity: a person participates in the WCN and provides free Internet access to mobile users in order to enjoy the same benefit when mobile. Our reciprocity scheme is compatible with the distinctive structure of WCNs: it does not require registration with authorities, relying only on uncertified free identities (public-private key pairs). Users sign digital receipts when they consume service. The receipts form a receipt graph, which is used as input to a reciprocity algorithm that identifies contributing users using network flow techniques. Simulations show that this algorithm can sustain reciprocal cooperation. We have implemented our algorithm to run on common WCN equipment, namely the Linksys WRT54GS AP. I.

We present fast and simple fully...

We propose the first combinatorial solution to one of the most classic problems in combinatorial optimization: the generalized minimum cost flow problem (flow with losses and gains). Despite a rich history dating back to Kantorovich and Dantzig, until now, the only known way to solve the problem in polynomial-time was via general purpose linear programming techniques. Polynomial combinatorial algorithms were previously known only for the version of our problem without costs. We design the first such algorithms for the version with costs. Our algorithms also find provably good solutions faster than optimal ones, providing the first strongly polynomial approximation schemes for the problem. Our techniques extend to optimize linear programs with two variables per inequality. Polynomial combinatorial algorithms were previously developed for testing the feasibility of such linear programs. Until now, no such methods were known for the optimization version. 1 Introduction In the ...

Generalized network flow problems generalize normal network flow problems by specifying a flow multiplier (a) for each arc a. For every unit of flow entering the arc, (a) units of flow exit. Flow multipliers permit modelling transforming one type into another and modification of the amount of flow.

We consider the problem of scheduling n independent jobs on m unrelated parallel machines without preemption. Job i takes processing time pi j on machine j, and the total time used by a machine is the sum of the processing times for the jobs assigned to it. The objective is to minimize makespan. The best known approximation algorithms for this problem compute an optimum fractional solution and then use rounding techniques to get an integral 2-approximation. In this paper we present a combinatorial approximation algorithm that matches this approximation quality. It is much simpler than the previously known algorithms and its running time is better. This is the first time that a combinatorial algorithm always beats the interior point approach for this problem. Our algorithm is a generic minimum cost flow algorithm, without any complex enhancements, tailored to handle unsplittable flow. It pushes unsplittable jobs through a two-layered bipartite generalized network defined by the scheduling problem. In our analysis, we take advantage from addressing the approximation problem directly. In particular, we replace the classical technique of solving the LP-relaxation and rounding afterwards by a completely integral approach. We feel that this approach will be helpful also for other applications.

Abstract—In metropolitan areas, public infrastructures for high-speed wireless networking can be built through the private contributions of individual “microoperators” who use their Internet-connected Wireless LANs (WLANs) to forward foreign traffic from and to nearby low-mobility clients. We have designed a practical WLAN aggregation scheme that (1) assumes that microoperators are selfish and do not trust each other and uses a secure incentive technique to encourage their contribution; (2) protects the real-world identities of micro-operators and clients by relying only on disposable opaque identifiers (public keys); (3) is fully distributed, open to all, and does not rely on any authority to resolve disputes or to control membership; (4) is automated, using standard hardware and software we developed for some of the main available platforms (Linux-based WLAN access points and Windows Mobile-based cell phones).

We consider an optimization problem in probabilistic inference: Given n hypotheses H j , m possible observations O k , their conditional probabilities p k j , and a particular O k , select a possibly small subset of hypotheses excluding the true target only with some error probability e. After specifying the optimization goal we show that this problem can be solved through a linear program in mn variables that indicate the probabilities to discard a hypothesis given an observation. Moreover, we can compute optimal strategies where only O(m+n) of these variables get fractional values. The manageable size of the linear programs and the mostly deterministic shape of optimal strategies makes the method practicable. We interpret the dual variables as worst-case distributions of hypotheses, and we point out some counterintuitive nonmonotonic behaviour of the variables as a function of the error bound e. One of the open problems is the existence of a purely combinatorial algorithm that is faster than generic linear programming.

Abstract—In densely populated cities, Wi-Fi networks—private or otherwise—are ubiquitous. We focus on the provision of citywide broadband communication capability to mobile users through private Wi-Fi networks that are in range but belong to others. We form a club that relies on indirect reciprocity: Members participate in the club and provide free Wi-Fi access to other members in order to enjoy the same benefit when they are away from their own Wi-Fi network. Our club scheme does not require registration with an authority and does not rely on centrally issued club identities: Members create their own identities (public-private key pairs) and receive signed digital receipts when they provide Wi-Fi service to other members. These receipts form a distributed receipt graph, parts of which are used as input to an indirect reciprocity algorithm that classifies club members according to their contribution. We show that our algorithm can sustain cooperation within the club and is robust to attacks by free-riders. We implement and evaluate our proposed club algorithms on commodity Wi-Fi routers and dual-mode cellular/Wi-Fi phones. Because we anticipate that Wi-Fi telephony will be a popular club application, we present and evaluate a secure and decentralized architecture for citywide voice (and multimedia) communications that is compatible with our club both from an architectural as well as an incentives perspective.

Abstract—Wireless Community Networks (WCNs) are metropolitan-area networks whose nodes are owned and managed by volunteers. These networks can be used to build large scale public infrastructures for providing ubiquitous high-speed wireless broadband access through the private contributions of individual community members who use their hotspots to forward foreign traffic from and to nearby lowmobility clients. We have designed and developed a prototype aggregation scheme that (1) assumes that community members are selfish and do not trust each other and uses a secure incentive technique to encourage their contribution; (2) protects the real-world identities of community providers and clients by relying only on disposable opaque identifiers (public/private key pairs); (3) is fully distributed, open to all, and does not rely on any authority to resolve disputes or to control membership; (4) is automated, using standard hardware and software we developed for some of the main available platforms (Linux-based WLAN access points and Windows Mobile-based cell phones). Thus, it can easily complement 2G/3G cellular networks in metropolitan areas where some WCNs provide wide coverage.

ii For the first time in the history of telecommunications, private individuals can provide telecommunication services to their peers. This change was brought on by the emergence of low-cost Wireless Local Area Network (WLAN) technology. WLANs based on the IEEE 802.11 family of specifications are everywhere, from businesses and universities, to hotels and airports. Households with broadband connections also use WLANs for tetherless access to the Internet. WLANs usually cover greater areas than intended with their installation. However, a client that attempts to access a foreign WLAN is likely to fail: most WLANs are secured against outsiders. These WLANs, along with their backhaul connections, represent an underutilized resource. In metropolitan areas, the density of WLANs is high. The Internet access bandwidth they can offer is greater than what 2G cellular offers—even if the backhaul is a simple DSL connection. Newer cell phones are equipped with WLAN adapters. Thus, the stage is set for an alternative public cellular network, one that relies on