This paper presents the design, implementation and deployment of GreenApps, a ground-up platform that enables off-grid, near off-line and highly available cellular applications in rural contexts. The GreenApps platform enables rapid development and deployment of almost-always available applications that can be executed locally on top of open-sourced cellular base stations. The GreenApps platform has been deployed in two rural regions in Kumawu, Ghana and Pearl Lagoon, Nicaragua and supports different community-centric applications.

Recent industry trends suggest a new phenomenon in the mobile market: mobile virtual network operators or MVNOs that operate on top of existing cellular infrastructures. While MVNOs have shown significant growth in the US and elsewhere in the past two years and have been successful in attracting customers, there is anecdotal evidence that users are concerned about cellular performance when choosing MVNOs over traditional cellular operators. In this paper, we present the first systematic measurement study to shed light on this emerging phenomenon. We study the performance of 3 key applications: web access, video streaming and voice, in 2 popular MVNO families (a total of 8 carriers) in the US, where each MVNO family consists of a major base carrier and 3 MVNOs running on top of it. We observe that some MVNOs do indeed exhibit significant performance degradation and that there are key differences between the two MVNO families.

In this paper we develop a new MAC protocol for improving network throughput and hence, channel utilization in wireless LANs that can support very high PHY layer data rates (>; 1Gbps). We call our new MAC protocol Busy Tone Assisted Fine-Grained Channel Access (btFICA). btFICA is based upon the framework of a prominent state-of-the-art PHY/MAC scheme for high data rate WLANs, called Fine-Grained Channel Access (FICA). While the rationale behind the FICA scheme appears effective for enhancing channel utilization in high data rate WLANs, a recent study shows that problems, such as deafness, muteness and a form of hidden terminal problem, can easily arise with the FICA MAC protocol. These problems can degrade the network performance, if left unaddressed. This motivates us to develop our btFICA MAC protocol that uses an additional busy tone antenna. btFICA comprehensively solves all of the three problems faced by the FICA MAC protocol, while maintaining the beneficial aspects of the original FICA scheme. Finally, we show via extensive simulations, that btFICA significantly outperforms the original FICA scheme and 802.11 DCF, in different network topologies and traffic scenarios, in terms of channel utilization, per-user-throughput and fairness.

Recently wireless radios and hardware technologies have been developed that allow operation on very wide channels and that can support very high physical layer data rates (1Gbps and up). However, it is proven that the conventional 802.11 CSMA/CA MAC protocol causes a drastic under-utilization of such high-speed channels. In order to improve channel utilization, a new scheme, called, Fine-Grained Channel Access (FICA), has been recently put forward. While the FICA approach appears more promising than the other proposed schemes for high data rate WLANs, it has not been studied extensively before. Hence, in this paper, we focus on the FICA MAC protocol, and we study this protocol thoroughly in different traffic scenarios and network topologies. We identify, for the first time, some of the serious problems that can arise with the FICA MAC protocol. We call these problems deafness, muteness and a form of hidden terminal problem. We quantify the impact of these problems on the performance of FICA via extensive simulations. Our results show that under some typical scenarios, FICA can perform even worse than the conventional 802.11 DCF, in terms of channel utilization, per-user-throughput or fairness. The insights obtained in this paper motivates the need for addressing FICA's problems and paves the path for future development of better new MAC protocols for high data rate WLANs.

The IEEE 802.11 standard statically channelizes the wide spectrum into smaller channels of equal width, and classically, each access point is assigned one of these channels to operate on. While an intelligent channel assignment for interfering APs can provide better network performance than just any arbitrary channel assignment, still recent work shows that even far better performance can be achieved if the fixed 802.11 channels are not used and instead the channel width and central frequency for each AP is adapted, based upon the traffic load at each AP in the network. However, the existing work for dynamic distribution of spectrum amongst APs, provides non-overlapping channels to interfering APs, which can cause loss of spectral reuse opportunities. In this paper, we propose a new dynamic spectrum distribution technique in infrastructured wireless LANs, that exploits spectrum reuse opportunities and allows overlap between channels provided to interfering APs. Our technique achieves two goals: 1) max-min fairness amongst flows in the network and 2) high network throughput. Finally, we evaluate the performance of our technique, via simulations, and present its superiority when compared to single-channel 802.11-like DCF, classic fixed channelization and the state-of-the-art dynamic spectrum distribution technique for WLANs.