Data rate adaptation (RA) schemes are the key means by which WLAN adapters adjust their operation to the variable quality of wireless channels. The IEEE 802.11 standard does not specify any RA preferences allowing for a competition in performance among vendors, thus numerous proprietary solutions coexist. While the RA schemes implemented in individual user terminals are unknown to the AP of a hotspot, it is well known that the way how individual stations adapt their rates strongly influences the performance of the whole WLAN cell. Therefore, the knowledge of the scheme applied by each station may be useful for the radio resource management in complex networks (e.g., HetNets or dense WLAN deployments in enterprise networks). In this paper, we present a novel approach to estimate the features of the RA schemes implemented in individual stations and demonstrate its efficiency using both simulated WLAN configurations as well as measurements.

Offloading data traffic to IEEE 802.11 hotspots is one of the key techniques with which mobile operators aim at dealing with the increasing traffic demand of their users in cellular networks. Usually, these offloads are simply conducted once an end-device has WLAN connectivity. This paper identifies gains if the decisions for accommodating certain traffic flows in WLAN consider the characteristics of this technology, i.e., the suitability of traffic being offloaded in terms of occupation of the channel and MAC overhead as result of contention, interference, and fluctuating channels. The contribution of this work is twofold: A performance evaluation compares the novel scheme with common received-signal-strength-based decisions as well as with simpler randomized choices for offloading particular traffic flows. Our scheme shows substantial improvements over a broad range of factors. Lastly, the results identify operational points of the WLAN cell allowing operators to dynamically choose the mix of the offloaded traffic according to their requirements.

Future wireless mobile devices will have to support a variety of heterogeneous access technologies while having a limited number of transceiver chains. One of the most challenging tasks remains how to support wireless access in one of the technologies, while preparing a “smooth transition” to another, especially to technologies with a lengthy network entry such as WiMAX. This work proposes an efficient way to organize a WiMAX network entry process while continuing a communication session with stringent QoS requirements via WLAN. We advocate the prioritization of the strict WiMAX timing over the contention-based WLAN channel access. Our results show the applicability of this approach over a wide range of mobile WiMAX parameters and identify bounds on the WiMAX downlink load for which the WiFi communication does not suffer quality distortions.

This paper presents a detailed performance evaluation of a novel stochastic network discovery approach denoted as opportunistic scanning. Therein, a station only pauses its communication for an extremely short time interval to scan for other technologies / systems. The selected scanning duration is short enough not to be noticeable by higher layer applications with strict QoS constraints.We apply this concept to 802.11-based networks using power saving as one possible signaling protocol to pause the communication between the scanning station and its interlocutor. Within this paper, we present a detailed simulation-based performance analysis of the opportunistic scanning approach showing its effects on real-time services such as VoIP streams. Our results show that opportunistic scanning behaves sturdy against background traffic. This novel approach enables successful network discovery within a 850-milisecond time span while guaranteeing QoS constraints of such a higher layer real-time application.

This paper presents the concept and preliminary performance evaluation of a stochastic network discovery approach named opportunistic scanning. Therein, a station only pauses its communication for an extremely short time interval to scan for other technologies / systems. The selected scanning duration is short enough not to be noticeable by higher layer applications. We place this concept into the 802.11 mesh network context and evaluate 802.11 power saving as one possible signaling protocol used to pause the communication between the scanning station and its interlocutor. We herein derive the theoretical performance limits of opportunistic scanning in combination with 802.11 power save and present first results classifying the time required to find a neighboring technology / system at a given probability.

The IEEE 802.11 working group currently integrates the option of mesh networking into its standard for WLANs. In addition to WLAN meshes, future wireless networks will be expanded by multiple heterogeneous access technologies such as UMTS and WiMax. These technologies differ greatly regarding medium access scheme, network capacity, QoS support, choice of data rates, and other various parameters such as power consumption and AAA aspects. Even though current research focuses on different network access selection and handover strategies maximizing the utilization for such a heterogeneous network, it is still an open issue how to select ldquoinefficientrdquo mobiles as vertical handover candidates. This work presents a novel scheme for the selection of handover candidates which is exemplarily discussed for an IEEE 802.11 WLAN mesh. After exploring the design rationale of the decision metric, simulation studies show the impact of single and multiple handovers on remaining users in the cell.

In wireless local area networks often a station can potentially associate with more than one access point. Therefore, a relevant question is which access point to select "best" from a list of candidate ones. In IEEE 802.11, the user simply associates to the access point with the strongest received signal strength. However, this may result in a significant load imbalance between several access points, as some accommodate a large number of stations while others are lightly loaded or even idle. Moreover, the multi-rate flexibility provided by several IEEE 802.11 variants can cause low bit rate stations to negatively affect high bit rate ones and consequently degrade the overall network throughput. This paper investigates the various aspects of "best" access point selection for IEEE 802.11 systems. In detail, we first derive a decision metric the selection can be based on. Using this metric we propose two new selection mechanisms which are decentralized in the sense that the decision is performed by each station, given appropriate status information of each access point. In fact, only few bytes of status information have to be added to the beacon and probe response frames which does not impose significant overhead. In addition, we show that our mechanism improves station quality of service and better utilizes network resources compared to the conventional one implemented today in IEEE 802.11 devices

Power efficiency is one of the most critical factors for wireless sensor networks. In this paper, we present a family of receiver-initiated pseudo-asynchronous rendezvous schemes designed specifically to reduce sensor nodes' power consumption. We analyze their performances in terms of power efficiency and latency, under different static and dynamic system parameters. In addition, we verify our analysis by extensive network simulations. Based on the results, our proposed scheme demonstrates superior performance compared to previous proposed schemes

Opportunistic routing protocols have been proposed as efficient methods to exploit the high node densities in sensor networks to mitigate the effect of varying channel conditions and non-availability of nodes that power down periodically. They work by integrating the network and data link layers so that they can take a joint decision as to the next hop forwarding node based on its availability and suitability as a forwarder. This cross-layer integration makes it harder to optimize the protocol due to the dependencies among the different components of the protocol stack. In this paper, we provide a framework to model opportunistic routing that breaks up the functionality into three separate components and simplifies analysis. The framework is used to model two variants of opportunistic routing and is shown to match well with simulation results. In addition, using the model for performance analysis yields important guidelines for the future design of such protocols.