Link Metrics Research Articles

Near optimal routing and capacity management for PWCE-based survivable WDM networks

Protected Working Capacity Envelope (PWCE) has been proposed to simplify resource management and traffic control for survivable WDM networks. In a PWCE-based network, part of the link capacity is reserved for accommodating working routes, and the remaining capacity is reserved for backup routes. The shortest path routing is applied in PWCE-based networks. An arrival call is accepted only when each link along the shortest path has a free working channel. Such a working path routing scheme greatly simplifies the call admission control process for dynamic traffic, and it is especially suitable for implementation in a distributed manner among network nodes. In this article, we investigate two protection strategies: Bundle Protection (BP) and Individual Protection (IDP). In BP, only one backup path can be used to protect a failure component, whereas multiple backup paths can be used in IDP. We formulate four mixed integer non-linear programming (MINLP) problems using BP and IDP strategies for single link and single node failure protection. Each model is designed to determine link metrics for shortest working path routing, working and backup channel assignments, and backup path planning. Our objective is to minimize call-blocking probability on the bottleneck link. Since these models are highly non-linear and non-convex, it is difficult to obtain exact global optimal solutions. We propose a Simulated Annealing-based Heuristic (SAH) algorithm to obtain near optimal solutions. This SAH adopts the concepts of simulated annealing as well as the bi-section technique to minimize call-blocking probabilities. To evaluate the performance, we made simulation comparisons between SAH and the unity link weight assignment scheme. The results indicate that SAH can greatly reduce call-blocking probabilities on benchmark and the randomly generated networks.

Actin Cross-link Assembly and Disassembly Mechanics for α-Actinin and Fascin

Self-assembly of complex structures is commonplace in biology but often poorly understood. In the case of the actin cytoskeleton, a great deal is known about the components that include higher order structures, such as lamellar meshes, filopodial bundles, and stress fibers. Each of these cytoskeletal structures contains actin filaments and cross-linking proteins, but the role of cross-linking proteins in the initial steps of structure formation has not been clearly elucidated. We employ an optical trapping assay to investigate the behaviors of two actin cross-linking proteins, fascin and alpha-actinin, during the first steps of structure assembly. Here, we show that these proteins have distinct binding characteristics that cause them to recognize and cross-link filaments that are arranged with specific geometries. alpha-Actinin is a promiscuous cross-linker, linking filaments over all angles. It retains this flexibility after cross-links are formed, maintaining a connection even when the link is rotated. Conversely, fascin is extremely selective, only cross-linking filaments in a parallel orientation. Surprisingly, bundles formed by either protein are extremely stable, persisting for over 0.5 h in a continuous wash. However, using fluorescence recovery after photobleaching and fluorescence decay experiments, we find that the stable fascin population can be rapidly competed away by free fascin. We present a simple avidity model for this cross-link dissociation behavior. Together, these results place constraints on how cytoskeletal structures assemble, organize, and disassemble in vivo.

Fading-aware reliable and energy efficient routing in wireless sensor networks

In this paper, we introduce two fading-aware reliability based routing algorithms for wireless sensor networks (WSNs) with lossy radio links. The proposed algorithms are able to find optimal multi-hop paths in polynomial complexity, over lossy links, which are modeled by using standard fading models (e.g. Rayleigh and Rice fading). These algorithms minimize the energy consumption and ensure reliable packet transmission to the base station ( BS) at the same time. A reliable path is defined in terms of successful packet transfer to the BS despite the lossy links. More precisely, the probability of correct reception of the packet at the BS must exceed a predefined threshold. The first algorithm minimizes the total energy consumption sending a packet over the selected path to the BS. On the other hand, the second algorithm selects a path which maximizes the minimum remaining energy on the node closest to exhaustion and, as a result, balances the energy consumption yielding high longevity. In both cases, reliable and energy efficient packet forwarding in WSN can be reduced to a constrained optimization problem. By using a specific link metrics, these problems can then be mapped into shortest path problems solved in polynomial time. Thus the obtained results ensure the selection of reliable paths which also guarantee minimum energy consumption in real time.

Multipath optimized link state routing for mobile ad hoc networks

Multipath routing protocols for Mobile Ad hoc NETwork (MANET) address the problem of scalability, security (confidentiality and integrity), lifetime of networks, instability of wireless transmissions, and their adaptation to applications. Our protocol, called MultiPath OLSR (MP-OLSR), is a multipath routing protocol based on OLSR [1]. The Multipath Dijkstra Algorithm is proposed to obtain multiple paths. The algorithm gains great flexibility and extensibility by employing different link metrics and cost functions. In addition, route recovery and loop detection are implemented in MP-OLSR in order to improve quality of service regarding OLSR. The backward compatibility with OLSR based on IP source routing is also studied. Simulation based on Qualnet simulator is performed in different scenarios. A testbed is also set up to validate the protocol in real world. The results reveal that MP-OLSR is suitable for mobile, large and dense networks with large traffic, and could satisfy critical multimedia applications with high on time constraints.

A Robust GA-based QoS Routing Algorithm for Solving Multi-constrained Path Problem

To support networked multimedia applications, it is important for a network to provide guaranteed quality-of-service (QoS). One way to provide such services is for the network to perform QoS routing, where the path taken must fulfill certain constraints. Multi-constrained path (MCP) problem refers to the problem of finding a path through a network subject to multiple additive constraints. It has been proven that this problem is NP-complete and finding an exact solution can be difficult. As such, various heuristics and approximation algorithms have been proposed to solve the MCP problem. However, the actual link metrics in a QoS-aware network is dynamic and may continuously change over time and since the path given by the routing algorithm is computed using the state information available to the router, which may or may not be up-to-date, it is possible that a feasible path returned by the algorithm may turn out to be no longer valid. This paper presents a GA-based QoS routing algorithm for solving the general k -constrained problem which has the capability to return multiple feasible paths in a single run. This makes the algorithm more robust in the case that the rate of change of state information in the network is higher than the rate of state information received by the router. Simulation results show that this algorithm consistently achieve higher feasibility ratio relative to existing well-known MCP routing algorithms when state information in the router lags behind the network.

Differentiated resilience services support in heterogeneous IP over wavelength division multiplexing networks

The authors develop an integrated scheme that supports differentiated resilience services (DiffResServ) for dynamic provisioning in heterogeneous IP-over- wavelength division multiplexing (WDM) networks. By integrated, the authors mean that the provisioning mechanism takes into account network topology, resource and resilience information in both the IP and WDM layers. By dynamic, the authors mean that the establishment process of connections and lightpaths is based on the fact that the arrivals and departures of IP traffic connections are not known in advance. By heterogenous, the authors mean that the state routing algorithms function on the fact that optical nodes in WDM network may support different switching capabilities. This has been achieved through our proposed scheme which operates based on a dual link metrics (bandwidth,delay)routing algorithm inter-operating with efficient connections and lightpaths rerouting algorithms. A genetic algorithm is introduced to operate based on the solution space generated by this scheme in order to minimise the total cost of network operation. The authors have conducted simulations, where the results show that this scheme enables IP-over-WDM networks to guarantee the quality-of-service and resilience required for heterogenous traffic classes with effective and efficient allocation of network resources.

Router Buffer Sizing for TCP Traffic and the Role of the Output/Input Capacity Ratio

The issue of router buffer sizing is still open and significant. Previous work either considers open-loop traffic or only analyzes persistent TCP flows. This paper differs in two ways. First, it considers the more realistic case of nonpersistent TCP flows with heavy-tailed size distribution. Second, instead of only looking at link metrics, it focuses on the impact of buffer sizing on TCP performance. Specifically, our goal is to find the buffer size that maximizes the average per-flow TCP throughput. Through a combination of testbed experiments, simulation, and analysis, we reach the following conclusions. The output/input capacity ratio at a network link largely determines the required buffer size. If that ratio is larger than 1, the loss rate drops exponentially with the buffer size and the optimal buffer size is close to 0. Otherwise, if the output/input capacity ratio is lower than 1, the loss rate follows a power-law reduction with the buffer size and significant buffering is needed, especially with TCP flows that are in congestion avoidance. Smaller transfers, which are mostly in slow-start, require significantly smaller buffers. We conclude by revisiting the ongoing debate on ldquosmall versus largerdquo buffers from a new perspective.

Designing Reliable IP Networks with an Access/Edge/Core Hierarchical Structure

In this paper, we tackle the design problem of reliable internet protocol (IP) networks with a three-level (access/edge/core) hierarchical structure. This problem consists in selecting the number of routers of each type to install at each level, locating each router, selecting the port types to install in each router, finding the access, edge and core networks, selecting the link types and finally, routing the traffic within the network. The routes are computed using the shortest paths using link metrics as the lengths of the links such as for the open shortest path first (OSPF) routing protocol; the most widely deployed intra domain routing protocol in IP networks. Moreover, a minimum information rate traffic parameter is guaranteed between each pair of access devices (routers and switches) for the normal state of the network (no failure) and for all single core router failure scenarios. A mathematical programming model is proposed for this problem and a tabu-based metaheuristic algorithm is proposed to find solutions within a reasonable amount of computational time. Finally, numerical results are presented and analyzed. The results show that for the test problems generated, the proposed approach found solutions, on average, 3,17% from a lower bound.

End-to-end performance aware association mechanism for wireless municipal mesh networks

In a wireless municipal mesh (muni mesh) network, a client station (STA) needs to associate with a mesh access point (MAP) for network access. Previous association mechanisms assume high-speed backhaul and only the access link being the bottleneck. This assumption holds for most WLANs, but in wireless mesh networks traffic could be bottlenecked either by the access link or by the bandwidth-limited wireless backhaul. In this paper, we propose a new joint MAP association mechanism for wireless muni mesh networks to improve STA’s end-to-end communication performance. A STA makes its association decision by jointly considering the quality of the access link between the STA and the associated MAP as well as the cost of the multi-hop path from the associated MAP to the Internet gateway. In addition, we design two new metrics, Contention Aware Expected Transmission Time (CAETT) and Load Aware Expected Transmission Time (LAETT), to measure the access link quality. The main strength of CAETT is incorporating the impact of 802.11 MAC layer contentions on the bandwidth sharing of multi-rate stations. LAETT further captures the real traffic load on the shared medium. In order to reduce the association delay, we use an analytical model to derive a hybrid measurement/estimation method to enable a station to quickly determine the cost of CAETT and LAETT. We conduct extensive simulations to evaluate the performance of the proposed joint MAP association mechanism. Especially within the joint association framework, we investigate the impact of various combinations of access link metrics (RSSI, PB, CAETT, LAETT) and backhaul routing metrics (hopcount, ETT, RALA) on the system performance. We show that the joint association mechanism can significantly improve the network performance in terms of throughput and delay by up to 100%. In particular, the joint association mechanism with LAETT as the access link metric and RALA as the routing metric outperforms other schemes and metrics.

Load aware traffic engineering for mesh networks

Wireless Mesh Network (WMN) is a multi-hop mesh network that consists of mesh routers and mesh clients, where mesh routers are static and form the backbone of the mesh network. The static nature of mesh nodes imposes requirements for designing routing metrics that support high-throughput and low packet delay. This article considers the problem of Interference-Load Aware routing in multi-channel Wireless Mesh Networks. We propose a new Interference-Load Aware routing metric, ILA, that finds paths with reduced inter-flow and intra-flow interference. The aim of this metric is to route the traffic through congestion free areas and balance the load amongst the network nodes. We incorporate this new metric in the well known AODV routing protocol and study the performance of ILA through simulations. We show that the proposed metric is able to adapt to changes in interfering traffic better than existing link metrics such as ETT, WCETT and MIC. We also demonstrate that our metric delivers high-throughput in multi-channel networks.

Avoiding Transient Loops During the Convergence of Link-State Routing Protocols

When using link-state protocols such as OSPF or IS-IS, forwarding loops can occur transiently when the routers adapt their forwarding tables as a response to a topological change. In this paper, we present a mechanism that lets the network converge to its optimal forwarding state without risking any transient loops and the related packet loss. The mechanism is based on an ordering of the updates of the forwarding tables of the routers. Our solution can be used in the case of a planned change in the state of a set of links and in the case of unpredictable changes when combined with a local protection scheme. The supported topology changes are link transitions from up to down, down to up, and updates of link metrics. Finally, we show by simulations that sub-second loop-free convergence is possible on a large Tier-1 ISP network.

Routing Optimization in IP Networks Utilizing Additive and Concave Link Metrics

Routing optimization provides network operators with a powerful method for traffic engineering. Its general objective is to distribute traffic flows evenly across available network resources in order to avoid network congestion and quality of service degradation. In this paper we consider routing optimization based on conventional routing protocols where packets are forwarded hop-by-hop in a destination-based manner. Unlike other work in this area, we consider routing protocols, which are able to take into account concave routing metrics in addition to additive ones. The concave link metric introduces an additional degree of freedom for routing optimization, thus, increasing its optimization potential. We present and evaluate a mixed-integer programming model, which works on these metrics. This model unifies the optimization for single-metric and dual-metric routing concepts and also includes the consideration of multipath routing. Furthermore, we propose a heuristic algorithm usable for larger network instances. Numerical results indicate that employment of both the dual-metric concept and multipath routing can achieve considerably better utilization results than default-configured single-metric routing. A significant finding is that metric-based routing optimization with two link metrics often comes close to the results obtainable by optimization of arbitrarily configurable routing.

Cooperative routing for distributed detection in large sensor networks

In this paper, the detection of a correlated Gaussian field using a large multi-hop sensor network is investigated. A cooperative routing strategy is proposed by introducing a new link metric that characterizes the detection error exponent. Derived from the Chernoff information and Schweppe's likelihood recursion, this link metric captures the contribution of a given link to the decay rate of error probability and has the form of the capacity of a Gaussian channel with the sender transmitting the innovation of its measurement. For one-dimensional Gauss-Markov fields, the link metric can be represented explicitly as a function of the link length. Cooperative routing is achieved using the Kalman data aggregation and shortest path routing. Numerical simulations show that cooperative routing can be significantly more energy efficient than noncooperative routing for the same detection performance

Effects of Functionalization, Catenation, and Variation of the Metal Oxide and Organic Linking Units on the Low-Pressure Hydrogen Adsorption Properties of Metal−Organic Frameworks

The dihydrogen adsorption isotherms of eight metal-organic frameworks (MOFs), measured at 77 K up to a pressure of 1 atm, have been examined for correlations with their structural features. All materials display approximately Type I isotherms with no hysteresis, and saturation was not reached for any of the materials under these conditions. Among the six isoreticular MOFs (IRMOFs) studied, the catenated materials exhibit the largest capacities on a molar basis, up to 9.8 H(2) per formula unit. The addition of functional groups (-Br, -NH(2), -C(2)H(4)-) to the phenylene links of IRMOF-1 (MOF-5), or their replacement with thieno[3,2-b]thiophene moieties in IRMOF-20, altered the adsorption behavior by a minor amount despite large variations in the pore volumes of the resulting materials. In contrast, replacement of the metal oxide units with those containing coordinatively unsaturated metal sites resulted in greater H(2) uptake. The enhanced affinities of these materials, MOF-74 and HKUST-1, were further demonstrated by calculation of the isosteric heats of adsorption, which were larger across much of the range of coverage examined, compared to those of representative IRMOFs. The results suggest that under low-loading conditions, the H(2) adsorption behavior of MOFs can be improved by imparting larger charge gradients on the metal oxide units and adjusting the link metrics to constrict the pore dimensions; however, a large pore volume is still a prerequisite feature.

Optimal path selection for minimizing the differential delay in Ethernet-over-SONET

We consider the problem of minimizing the differential delay in a virtually concatenated Ethernet-over-SONET (EoS) system by suitable path selection. In such a system, a service provider can dynamically add virtual channels to or drop them from a Virtually Concatenated Group (VCG). A new virtual channel can be added to the VCG provided that the differential delays between the new channel and the existing ones are within a certain limit that reflects the available memory buffer of the EoS system. We model the problem of finding such a virtual channel as a constrained path selection problem, where the delay of the required (feasible) path is constrained not only by an upper bound but also by a lower bound. We consider two cases: exactly known link delays and imprecisely known link delays. For the first case, we propose two algorithms for finding a feasible path. The first is based on a link metric that linearly combines the original link weight (the link delay) and its inverse. The theoretical properties of such a metric are studied and used to develop a highly efficient heuristic for path selection. The second algorithm is a “backward–forward” heuristic in which the nodes in the graph are prelabeled during the backward phase. The labels are then used in the forward phase to identify a feasible path. For the imprecise-link-state case, the problem is modeled as one of finding the most probable feasible path, where link weights are random variables. A “backward–forward” heuristic is proposed which again uses prelabeling of the graph in the backward direction followed by a forward search that attempts to minimize an objective function. Simulations are conducted to evaluate the performance of the proposed algorithms and to demonstrate the advantages of the probabilistic path selection approach over the classic trigger-based approach.

A Link Heterogeneity-Aware On-Demand Routing (LHAOR) Protocol Utilizing Local Update and RSSI Information

Many routing protocols have been proposed for mobile ad hoc networks. Among these protocols, the on-demand routing protocols are very attractive because they have low routing overhead. However, few of the existing on-demand routing protocols have considered the link heterogeneity, such as the different communication rate, different Packet Error Ratio (PER). As a result, the routes tend to have the shortest hop count and contain weak links, which usually provide low performance and are susceptible to breaks in the presence of mobility. In this paper, we analyze the existing on-demand routing protocols and propose a Link Heterogeneity Aware On-demand Routing (LHAOR) protocol, where the link quality and mobility are considered. Specifically, the Local Update (LU) is proposed and the link metric is inversely related with the Received Signal Strength Indicator (RSSI). By using the LU method and RSSI information, the routes adapt to the topology variation and link quality changes, and reach the local optimum quickly, which contains strong links and has a small metric. Simulation and experiment results show that our LHAOR protocol achieves much higher performance than the classical on-demand routing protocols.

Algorithmic aspects of capacity in wireless networks

This paper considers two inter-related questions: (i) Given a wireless ad-hoc network and a collection of source-destination pairs {( s i ,t i )}, what is the maximum throughput capacity of the network, i.e. the rate at which data from the sources to their corresponding destinations can be transferred in the network? (ii) Can network protocols be designed that jointly route the packets and schedule transmissions at rates close to the maximum throughput capacity? Much of the earlier work focused on random instances and proved analytical lower and upper bounds on the maximum throughput capacity. Here, in contrast, we consider arbitrary wireless networks. Further, we study the algorithmic aspects of the above questions: the goal is to design provably good algorithms for arbitrary instances. We develop analytical performance evaluation models and distributed algorithms for routing and scheduling which incorporate fairness, energy and dilation (path-length) requirements and provide a unified framework for utilizing the network close to its maximum throughput capacity.Motivated by certain popular wireless protocols used in practice, we also explore "shortest-path like" path selection strategies which maximize the network throughput. The theoretical results naturally suggest an interesting class of congestion aware link metrics which can be directly plugged into several existing routing protocols such as AODV, DSR, etc. We complement the theoretical analysis with extensive simulations. The results indicate that routes obtained using our congestion aware link metrics consistently yield higher throughput than hop-count based shortest path metrics.

Comparing Restoration Concepts Using Optimal Network Configurations with Integrated Hardware and Routing Decisions

The impact of link and path restoration on the cost of telecommunication networks is investigated. The surprising result is the following: the cost of an optimal network configuration is almost independent of the restoration concept if (i) the installation of network elements (ADMs, DXCs, or routers) and interface cards, (ii) link capacities, and (iii) working and restoration routings are simultaneously optimized. We present a mixed-integer programming model which integrates all these decisions. Using a branch-and-cut algorithm (with column generation to deal with all potential routing paths), we solve structurally different real-world problem instances and show that the cost of optimal solutions is almost independent of the used restoration concept. In addition, we optimize spare capacities for given shortest working paths which are predetermined with respect to different link metrics. In comparison to simultaneous optimization of working and restoration routings, it turns out that this approach does not allow to obtain predictably good results.

OSPF-based hybrid approach for scalable dissemination of QoS parameters

Current link-state routing protocols (e.g., OSPF) use flooding to disseminate link-state information throughout the network. Despite its simplicity and reliability, flooding incurs unnecessary communication and processing overheads in control plane since nodes may receive multiple copies of the same advertisement. These overheads become significant in protocols that support quality-of-service (QoS) routing, where links are associated with dynamic metrics (e.g., available bandwidth) that need to be advertised frequently. The overheads can be significantly reduced using tree-based broadcasting approaches. Although a number of such approaches have been proposed in the literature, they have not been used in real networks because of their complexity and/or unreliability. In this paper, we propose a hybrid link-state dissemination approach that combines the best features of flooding and tree-based broadcasting. Our approach is particularly suited for the dissemination of “dynamic” link metrics (e.g., available bandwidth), which are often used in QoS-based path selection and traffic engineering. In our approach, topological changes and first-time LSAs (link-state advertisements) are flooded, whereas refresh LSAs (the ones that provide updated information on the dynamic metrics) are sent using tree-based broadcasting. The broadcast trees in our approach are constructed dynamically during the flooding of the first-time LSA, without the need for the complex algorithms of previously proposed tree-based approaches. Two versions of the proposed scheme are provided; one being more suitable for quasi-static topologies (i.e., link failure rate is low) while the other is aimed at highly dynamic networks. We show how both versions can be integrated into the OSPF protocol. We further provide a working implementation of both versions, obtained after modifying Moy's OSPF source code [OSPF: Complete Implementation (with CD-ROM), Addison Wesley, Reading, MA, 2000]. We contrast the communications and processing overheads of our scheme with those of flooding and pure tree-based broadcasting, using both analysis and simulations. Our results indicate that the hybrid approach has a significantly lower overhead than flooding; yet it enjoys the simplicity, reliability, and fast convergence of flooding.

The connection between the research of a university and counts of links to its web pages: An investigation based upon a classification of the relationships of pages to the research of the host university

AbstractResults from recent advances in link metrics have demonstrated that the hyperlink structure of national university systems can be strongly related to the research productivity of the individual institutions. This paper uses a page categorization to show that restricting the metrics to subsets more closely related to the research of the host university can produce even stronger associations. A partial overlap was also found between the effects of applying advanced document models and separating page types, but the best results were achieved through a combination of the two.