Mohammed N. Smadi

The need for access point power saving in solar powered WLAN mesh networks

Wireless LAN mesh networks are now being used to deploy Wi-Fi coverage in a wide variety of outdoor applications. In these types of networks, conventional WLAN mesh nodes must be operated using continuous electrical power connections. This requirement may often be very expensive, especially when the network includes expansive outdoor wireless coverage areas. An alternative is to operate some of the WLAN mesh nodes using an energy sustainable source such as solar or wind power. This eliminates the need for a fixed power connection, making the node truly tetherless and allowing more flexibility in node positioning. In this article we first review the background and recent activities in the area of energy sustainable WLAN mesh networks. These types of networks are provisioned geographically, in that the assigned resources are a function of the geographic region where the network is to be deployed. The theory behind this is briefly described using some sample North American locations. We then discuss the current shortcomings of IEEE 802.1 1 when used in these types of networks. IEEE 802.11 requires that the access point be continuously powered, and this requirement is a major barrier to deploying cost-effective sustainable energy networks in certain applications. Recent work is then reviewed that has begun to address the changes that would be required to the standard to better support these types of networks.

Interference Management in WLAN Mesh Networks Using Free-Space Optical Links

Frequency channels are assigned in wireless local-area network (WLAN) mesh networks subject to strict cochannel interference constraints. Since Wi-Fi may be freely used by other networks, added interference may eventually invalidate the original frequency assignment, making full link activation impossible. In this paper, we address this problem by selectively installing supplementary free-space optical (FSO) links when radio-frequency (RF) link performance has deteriorated. To minimize cost, the number of FSO links that are needed should be as small as possible. We first formulate the channel assignment problem with the objective of maximizing the number of simultaneous link activations while satisfying cumulative RF interference constraints. A proof is given for the NP-completeness of the joint frequency assignment and FSO link placement problem. We then propose an efficient heuristic to solve the channel assignment problem using a genetic algorithm. Results are then presented for various mesh networks which show that the proposed algorithm has good results compared with the computed bounds. The presented results show that the use of FSO links permits WLAN mesh network deployment in interference-prone situations.

Free-Space Optical Gateway Placement in Hybrid Wireless Mesh Networks

The capacity of wireless mesh networks (WMN) must usually be upgraded as usage demands evolve over time. This is normally done by adding gateways which serve to increase the backhaul capacity of the network. In this paper we consider adding capacity in this manner using free-space optical (FSO) backhaul links. To accomplish this, we formulate a joint clustering and gateway placement problem which includes the strong rate-distance dependence of practical FSO links. The formulation incorporates the positions of existing wireline gateways and minimizes the number of additional hybrid-FSO/RF gateways which are needed to satisfy the target capacity requirements. After showing the complexity of the problem, a solution that is motivated by genetic algorithms is proposed. The performance of our algorithm is then compared to an optimal solution generated via an integer linear program (ILP) for small WMNs. The proposed algorithm is then modified to allow for balancing the traffic load that is carried by each gateway in the WMN. Many scenarios are considered which demonstrate the value of using FSO backhaul links to obtain post-deployment capacity upgrades in response to changes in user traffic.

Error Recovery Service for the IEEE 802.11B Protocol

We develop a service that allows the current IEEE 802.11b MAC protocol to perform dynamic packet sizing and forward error correction. Our service, called ERSMAC, is designed to allow the deployment of the IEEE 802.11b protocol in industrial environments characterized by high BER and fast time variation. ERSMAC uses a maximum likelihood estimate of the BER to solve for the optimal packet size that maximizes the success probability of transmissions while minimizing the overhead cost. ERSMAC also implements an adaptive forward error correction scheme using Reed-Solomon code such that every retransmission attempt has a higher probability of success than the previous attempt due to its association with a stronger RS code. Finally, we show, through simulations, that ERSMAC outperforms the original unmodified IEEE 802.11b protocol in terms of average throughput, average delay and efficiency

QoS-Enabled Power Saving Access Points for IEEE 802.11e Networks

IEEE 802.11 is now being used in many situations where access point (AP) power saving would be highly desirable. Unfortunately, this is not possible since the existing standard requires that APs remain active at all times. In this paper, we present a framework for a power saving quality-of-service (QoS) enabled access point (PSQAP), intended for use in low power infrastructure applications. The proposed scheme introduces infrastructure based power saving while preserving the QoS requirements for delay and loss intolerant real-time applications. Using an extensive simulation study we have evaluated the integrated framework, which consists of a proposed energy efficient media access control (MAC) protocol and an adaptive connection admission control (CAC) scheme. Our results show that the power consumption at PSQAPs can be significantly reduced without violating any of the QoS requirements for real-time traffic streams. The effect on power saving at the stations is also reasonable and can be controlled via protocol specific parameters.

Dynamically Anchored Conferencing Handoff for Dual-Mode Cellular/WLAN Handsets

In this paper we consider vertical handoff for dualmode (DM) cellular/WLAN handsets. When the handset roams out of WLAN coverage, the DMs cellular interface is used to maintain the call by anchoring it through a PSTN gateway/PBX. Soft handoff can be achieved in this case if the gateway supports basic conference bridging, since a new leg of the call can be established to the conference bridge while the existing media stream path is active. Unfortunately this requires that all intraenterprise calls be routed through the gateway when the call is established. In this paper we consider conferenced dual-mode handoff and propose a much more scalable mechanism whereby active calls are handed off into the conference bridge just prior to the initiation of the vertical handoff. Results are presented which are taken from dual-mode handset simulations which characterize the scalability of the proposed mechanism.

Fast Client-Based Connection Recovery for Soft WLAN-to-Cellular Vertical Handoff

Wireless local area network (WLAN)-to-cellular vertical handoff (VHO) involves time-consuming procedures that may significantly disrupt real-time communication. When a soft handoff is anchored by an enterprise private branch exchange (PBX)/gateway, handoff execution can take several seconds due to the time required to establish the cellular-to-PBX call leg. This situation is compounded by the fact that WLAN coverage may sometimes be lost far sooner than a VHO can be triggered and executed. In this paper, we propose and investigate the use of a VHO support node (VHSN) which is attached locally to the wired LAN hotspot infrastructure. When WLAN coverage is lost, the VHSN improves soft-handoff performance by quickly intercepting and redirecting the media flow through the local cellular base station using its cellular end-station attachment. This action can quickly recover the connection before the new cellular call leg is established. Unlike proxy forwarding, wireless multihop, and other infrastructure-modification schemes, the VHSN does not extend wireless coverage or perform infrastructure-based packet forwarding. Instead, the VHSN functions as both an (Ethernet) LAN and a cellular end station. Results will be presented which show the performance improvements that are possible using the proposed mechanism.

A Measurement-Based Study of WLAN to Cellular Handover

Real-time vertical handover is an important capability for multimode WLAN/cellular handsets. In many cases however, seamless handover can be very difficult to achieve, since WLAN coverage may be lost long before a cellular call leg can be triggered and established. Worse-case handovers of this kind occur when mobile users walk from indoor building WLAN coverage to outdoors during voice connections. In this paper we report on a measurement-based study of WLAN-to-cellular handover. Our results are based on extensive IEEE 802.11 measurements that were made on the McMaster University campus during the summer of 2005. Our methodology involved traversing many indoor-to-outdoor paths for a large number of campus buildings and exits while monitoring multi-AP Wi-Fi coverage. The collected data was then processed to determine the probability of seamless handover using classical vertical handover algorithms. The results presented give important insights into the difficulty of this problem, and relate to issues such as Wi-Fi deployment type, handover triggering, and Wi-Fi link loss threshold. The results provided enable handset designers and WLAN administrators to better understand the sensitivity of vertical handover performance to these parameters and how they can be optimized

Dynamically anchored conferencing handoff for dual-mode cellular/WLAN handsets

In this paper we consider vertical handoff for enterprise-based dual-mode (DM) cellular/WLAN handsets. When the handset roams out of WLAN coverage, the DMs cellular interface is used to maintain the call by anchoring it through an enterprise PSTN gateway/PBX. Soft handoff can be achieved in this case if the gateway supports basic conference bridging, since a new leg of the call can be established to the conference bridge while the existing media stream path is active. Unfortunately this requires that all intra-enterprise calls be routed through the gateway when the call is established. In this paper we consider a SIP based architecture to perform conferenced dual-mode handoff and propose a much more scalable mechanism for short-delay environments, whereby active calls are handed off into the conference bridge prior to the initiation of the vertical handoff. Results are presented which are taken from a dual-mode handset testbed, from analytic models, and from simulations which characterize the scalability of the proposed mechanism.

Trigger Node Assisted WLAN to Cellular Vertical Handover

Dual-Mode handsets containing both WLAN and cellular interfaces are becoming increasingly common. When moving between WLAN and cellular radio coverage,the switching of an active session from one interface to another is referred to as Vertical Handover (VHO). To provide successful VHO the WLAN link has to be maintained until the cellular connection is established. Our previous measurements of the WLAN coverage inside a building shows that this is very difficult given the time constraints in a loosely coupled cellular/WLAN architecture. In this paper we extend our previous work and propose an explicit trigger node to assist early detection of a VHO when transitioning out of a building, which will give the handset enough time to connect to the cellular network before loosing its WLAN link. A trigger node is a very simple IEEE 802.11 device that merely transmits beacons and is placed at the building exits. It provides no coverage extension or relaying and does not communicate with any other entity. We also provide some guidelines for the deployment of an indoor WLAN so that conventional signal threshold based HO triggering can be used with acceptable success probability.