Marco Tacca

Cooperative and Reliable ARQ Protocols for Energy Harvesting Wireless Sensor Nodes

One class of wireless sensor networks makes use of sensor nodes that recharge their batteries by harvesting energy from the surrounding environment. Being continuously recharged, the battery does not need to be replaced regularly and the sensor node is maintenance-free. A key module in such sensor network solutions is the data link automatic repeat request (ARQ) protocol, which must be designed to reliably deliver sensor nodes data at the minimum energy cost. With this objective in mind, two ARQ protocol classes are compared. In one class, each sensor node operates individually. In the other, the concept of cooperative communications is adopted, whereby neighboring sensor nodes help each other during the retransmission process. It is shown that the use of cooperative ARQ protocols in energy harvesting sensor networks enables sensor nodes to balance their energy consumption to match their own battery recharge rate. In turn, a balanced energy consumption-to-recharge rate ratio has the potential to improve the network throughput. Both classes of ARQ protocols are analyzed and compared. Estimated throughput gains are discussed under various network scenarios.

Survivable networks based on optimal routing and WDM self-healing rings

The design of survivable all-optical networks based on self-healing WDM rings (SHR/WDM) to provide 100% protection from any single link failure requires the joint solution of three sub-problems. These are the ring cover of the mesh topology (the RC sub-problem), the routing of working lightpaths between node pairs to support traffic demands (the WL sub-problem) and the selection of the SHR/WDM spare wavelengths for the protection of every link traffic (the SW subproblem). This paper presents an integer linear programming (ILP) formulation of the problem of minimizing the total wavelength mileage (/spl lambda/-miles) required to support a set of given traffic demands in a given network topology using SHR/WDM employing 1:N line protection mechanism (the WRL problem). This formulation allows to jointly and optimally solve the three subproblems, and yields up to 15% reduction of the total /spl lambda/-miles required by existing solutions that separately resolve the sub-problems. A simplified sub-optimal solution of the WRL problem is also provided, that yields results few percent worse than the optimal solution and that is tractable for networks whose size is on the order of the pan-European network, i.e., 19 nodes.

Shared path protection with differentiated reliability

The authors (Fumagalli and Tacca (2001)) introduced the concept of differentiated reliability (DiR) applied to dedicated path protection (DPP) switching in wavelength division multiplexing (WDM) rings. By means of the DiR concept, a network can be designed to provide multiple degrees of reliability and efficiently satisfy the user-specific requirements, yet minimizing the network total cost. This paper extends the DiR concept to the case of shared path protection (SPP) switching in arbitrary (mesh) topology, the so called SPP-DiR. A time efficient algorithm is proposed to determine the primary and backup path of each demand in both conventional SPP and SPP-DiR (WDM) networks. When compared to DPP, results obtained for the pan-European network by using the proposed algorithm indicate cost reductions of about 16% when SPP is applied, and up to 34% when SPP-DiR is applied.

Differentiated reliability (DiR) in WDM rings without wavelength converters

The concept of differentiated reliability, (DiR) was recently introduced by the authors to provide multiple reliability degrees (or classes) at the same network layer using a common protection mechanism, e.g., path switching. According to the DiR concept, each connection at the layer under consideration is guaranteed a minimum reliability degree, defined as the maximum failure probability allowed for that connection. The reliability degree chosen for a given connection is thus determined by the application requirements, and not by the actual network topology, design constraints, robustness of the network components, and span of the connection. In the paper the DiR concept is applied to designing the wavelength division multiplexing (WDM) layer of a ring network in which wavelength conversion is not available. To solve the routing and wavelength assignment problem at the WDM layer an efficient algorithm is proposed that resorts to reusable protection wavelengths while guaranteeing the required reliability degree of each connection. Lower bounds on the network bandwidth required by two approaches-respectively based on non-reusable and reusable protection wavelengths-reveal interesting properties of the DiR concept and the proposed algorithm.

Regenerator placement with guaranteed connectivity in optical networks

The problem of minimizing the number of optical nodes with signal regeneration capability can be constrained to guarantee a desired degree of end-to-end connectivity in the all-optical transport network. The problem can be formulated using a k-connected, k-dominating node set, which is a known approach in mobile ad hoc wireless networks. This paper presents a preliminary study aimed at establishing whether efficient centralized solutions to this problem in optical networking ought to be investigated to improve the decentralized solutions already available for wireless networks.

IEEE 802.11b cooperative protocols: a performance study

This paper investigates the use of cooperative communications in the context of IEEE 802.11b to combat radio signal degradation. The performance gain of both an existing cooperative protocol and the one proposed in the paper is discussed. It is quantitatively shown how much the two cooperative protocols increase throughput, lower delivery latency, and extend transmission span, when compared to the conventional IEEE 802.11b protocol. These features may help improve connectivity and network performance in ad hoc applications.

Optimal Design of Optical Ring Networks with Differentiated Reliability (DiR)

Current optical networks typically offer two degrees of service reliability: full protection in presence of a single fault in the network, and no protection at all. This situation reflects the historical duality that has its roots in the once divided telephone and data environment. The circuit oriented service required protection, i.e., provisioning of readily available spare resources to replace working resources in case of a fault. The datagram oriented service relied upon restoration, i.e., dynamic search for and reallocation of affected resources via such actions as routing table updates. The current development trend, however, is gradually driving the design of networks towards a unified solution that will jointly support traditional voice and data services as well as a variety of novel multimedia applications. The growing importance of concepts, such Quality of Service (QoS) and Differentiated Services that provide varying levels of service performance in the same network evidences this trend. Consistently with this pattern, the novel concept of Differentiated Reliability (DiR) is formally introduced in the paper and applied to provide multiple reliability degrees (classes) in the same network layer using a common protection mechanism, i.e., path switching. According to the DiR concept, each connection in the layer under consideration is guaranteed a minimum reliability degree, defined as the Maximum Failure Probability allowed for that connection. The reliability degree chosen for a given connection is thus determined by the application requirements, and not by the actual network topology, design constraints, robustness of the network components, and span of the connection. An efficient algorithm is proposed to design the Wavelength Division Multiplexing (WDM) layer of a DiR ring.

Optimal design of survivable mesh networks based on line switched WDM self-healing rings

Network survivability provided at the optical layer is a desirable feature in modern high-speed networks. For example, the wavelength division multiplexed (WDM) self-healing ring (or SHR/WDM) provides a simple and fast optically transparent protection mechanism against any single fault in the ring. Multiple self-healing rings may be deployed to design a survivable optical mesh network by superposing a set of rings on the arbitrary topology. However, the optimum design of such a network requires the joint solution of three subproblems: the ring cover of the arbitrary topology (the RC subproblem); the routing of the working lightpaths between end node pairs to carry the offered traffic demands (the WL subproblem); and the provisioning of the SHR/WDM spare wavelengths to protect every line that carries working lightpaths (the SW subproblem). The complexity of the problem is exacerbated when software and hardware requirements pose additional design constraints on the optimization process.The paper presents an approach to optimizing the design of a network with arbitrary topology protected by multiple SHRs/WDM. Three design constraints are taken into account, namely, the maximum number of rings acceptable on the same line, the maximum number of rings acceptable at the same node, and the maximum ring size. The first objective is to minimize the total wavelength mileage (working and protection) required in the given topology to carry a set of traffic demands. The exact definition of the problem is given based on an integer linear programming (ILP) formulation that takes into account the design subproblems and constraints and assumes ubiquitous wavelength conversion availability. To circumvent the computational complexity of the exact problem formulation, a suboptimal solution is proposed based on an efficient pruning of the solution space. By jointly solving the three design subproblems, it is numerically demonstrated that the proposed optimization technique yields up to 12% reduction of the total wavelength mileage when compared to solutions obtained by sequentially and independently solving the subproblems. The second objective is to reduce the number of wavelength converters required in the solution produced by the ILP formulation. Two approaches are proposed in this case that trade the required wavelength mileage for the number of wavelength converters.

The multi-hop multi-rate wavelength division multiplexing ring

Transparency of the optical layer offers the possibility to design a network that operates at varying transmission bit rates. While variable bit rate interfaces are being tested and will soon provide the possibility to optimally select the transmission rate for each optical channel, the potential advantages of relying upon multiple transmission rates in the optical network have yet to be fully explored. In this paper, we define the concept of multi-hop and multi-rate (M&M for short) network in which the tributary signal is transmitted over a concatenation of optical channels, with each optical channel operating at its own transmission rate. The optimal rate of each optical channel is determined by a number of factors including the end nodes interface, amount of multiplexed traffic and cost of the network components. The potential advantages provided by the M&M network when compared to first generation optical networks (i.e., SONET/SDH), to single- and multi-hop (constant bit rate) optical networks, are discussed in general and demonstrated numerically in a WDM ring. Presented results show that the network cost reduction achieved by the M&M design is a function of the cost ratio between the optical bandwidth (wavelengths) and the optical terminals.

Perceptual based voice multi-hop transmission over wireless sensor networks

Multimedia applications over wireless sensor networks (WSNs) are rapidly gaining interest by the research community in order to develop new and mission critical services such as environmental video monitoring and emergency speech calls. In this work we analyze the possibility of sending voice using a network of wireless tiny motes with the final goal of enhancing speech quality by protecting the most perceptually important packets. We first evaluate the speech quality for a modified version of the ITU-T G.711 standard implemented to fit the particular selected hardware. Hence, we propose a lowcomplexity measure to evaluate the perceptual importance of speech packets. When performing single-hop experimental data collection, we apply packet redundancy (protection) by using a cooperative mote (relay) which retransmits speech packets that are perceptually important to protect them against potential transmission losses. Collected experimental results are then used to assess multi-hop performance, showing that the combination of the selected hardware and the proposed perceptual marking algorithm achieves good speech quality levels, according to the MOS scale, while reducing the percentage of protected packets by 40% when compared to random protection.