Tor Skeie

Downlink Spectrum Sharing for Cognitive Radio Femtocell Networks

Femtocell is envisioned as a highly promising solution for indoor wireless communications. The spectrum allocated to femtocells is traditionally from the same licensed spectrum bands of macrocells. In this case, the capacity of femtocell networks is highly limited due to the finite number of licensed spectrum bands and also the interference with macrocells and other femtocells. In this paper, we propose a radically new communication paradigm by incorporating cognitive radio in femtocell networks. The cognitive radio enabled femtocells are able to access spectrum bands not only from macrocells but also from other licensed systems (e.g. TV systems) provided the interference from femtocells to the existing systems is not harmful. It results in more channel opportunities for femtocells. Thus, the co-channel interference in femtocells can be greatly reduced and the network capacity can be significantly improved. Because of the difference from other traditional wireless networks, we argue the traditional spectrum sharing schemes such as coloring methods are not efficient to femtocell networks especially for dense deployment scenarios. We formulate the downlink spectrum sharing problem in cognitive radio femtocell networks, and employ decomposition theories to solve the problem. Simulation results indicate that cognitive radio enabled femtocells could achieve much higher capacity than the femtocell networks which do not employ agile spectrum access. Simulation results also show that our proposed scheme without any iteration can achieve almost twice of the average capacity by coloring method when the number of available channels is less than five. Moreover, our proposed scheme can converge very fast with a typical value of only five iterations, and it can achieve around two percent extra average capacity than the fixed power control scheme.

Timeliness of real-time IP communication in switched industrial Ethernet networks

Through several giant evolutionary steps, Ethernet has become an almost ubiquitous technology for communication. Being versatile enough to be employed in new and various fields of application, it is now making inroads in factories. However, automated systems are different from many other applications of Ethernet,first and foremost because they require the network technology to deliver real-time performance. In the present study, a number of critical aspects of Ethernet, usually referred to as an Industrial Ethernet, are examined. More specifically, there is a focus on the application-to-application delay and jitter characteristics of such networks, when using Internet protocols such as UDP and TCP. It is demonstrated how important it is to take control of the latency in the station nodes, since the main communication delays occur inside the nodes, and different solutions are presented for controlling these delays. In particular, a priority-based protocol stack is assessed. The results show that real-time, Ethernet-based IP communication is now adequate even for demanding automated applications. In this paper, substation automation (power distribution) is used as an example of a demanding automation system.

Segment-based routing: an efficient fault-tolerant routing algorithm for meshes and tori

Computers get faster every year, but the demand for computing resources seems to grow at an even faster rate. Depending on the problem domain, this demand for more power can be satisfied by either, massively parallel computers, or clusters of computers. Common for both approaches is the dependence on high performance interconnect networks such as Myrinet, Infiniband, or 10 Gigabit Ethernet. While high throughput and low latency are key features of interconnection networks, the issue of fault-tolerance is now becoming increasingly important. As the number of network components grows so does the probability for failure, thus it becomes important to also consider the fault-tolerance mechanism of interconnection networks. The main challenge then lies in combining performance and fault-tolerance, while still keeping cost and complexity low. This paper proposes a new deterministic routing methodology for tori and meshes, which achieves high performance without the use of virtual channels. Furthermore, it is topology agnostic in nature, meaning it can handle any topology derived from any combination of faults when combined with static reconfiguration. The algorithm, referred to as segment-based routing (SR), works by partitioning a topology into subnets, and subnets into segments. This allows us to place bidirectional turn restrictions locally within a segment. As segments are independent, we gain the freedom to place turn restrictions within a segment independently from other segments. This results in a larger degree of freedom when placing turn restrictions compared to other routing strategies. In this paper a way to compute segment-based routing tables is presented and applied to meshes and tori. Evaluation results show that SR increases performance by a factor of 1.8 over FX and up*/down* routing

Layered routing in irregular networks

Freedom from deadlock is a key issue in cut-through, wormhole, and store and forward networks, and such freedom is usually obtained through careful design of the routing algorithm. Most existing deadlock-free routing methods for irregular topologies do, however, impose severe limitations on the available routing paths. We present a method called layered routing, which gives rise to a series of routing algorithms, some of which perform considerably better than previous ones. Our method groups virtual channels into network layers and to each layer it assigns a limited set of source/destination address pairs. This separation of traffic yields a significant increase in routing efficiency. We show how the method can be used to improve the performance of irregular networks, both through load balancing and by guaranteeing shortest-path routing. The method is simple to implement, and its application does not require any features in the switches other than the existence of a modest number of virtual channels. The performance of the approach is evaluated through extensive experiments within three classes of technologies. These experiments reveal a need for virtual channels as well as an improvement in throughput for each technology class.

A routing methodology for achieving fault tolerance in direct networks

Massively parallel computing systems are being built with thousands of nodes. The interconnection network plays a key role for the performance of such systems. However, the high number of components significantly increases the probability of failure. Additionally, failures in the interconnection network may isolate a large fraction of the machine. It is therefore critical to provide an efficient fault-tolerant mechanism to keep the system running, even in the presence of faults. This paper presents a new fault-tolerant routing methodology that does not degrade performance in the absence of faults and tolerates a reasonably large number of faults without disabling any healthy node. In order to avoid faults, for some source-destination pairs, packets are first sent to an intermediate node and then from this node to the destination node. Fully adaptive routing is used along both subpaths. The methodology assumes a static fault model and the use of a checkpoint/restart mechanism. However, there are scenarios where the faults cannot be avoided solely by using an intermediate node. Thus, we also provide some extensions to the methodology. Specifically, we propose disabling adaptive routing and/or using misrouting on a per-packet basis. We also propose the use of more than one intermediate node for some paths. The proposed fault-tolerant routing methodology is extensively evaluated in terms of fault tolerance, complexity, and performance.

A Survey and Evaluation of Topology-Agnostic Deterministic Routing Algorithms

Most standard cluster interconnect technologies are flexible with respect to network topology. This has spawned a substantial amount of research on topology-agnostic routing algorithms, which make no assumption about the network structure, thus providing the flexibility needed to route on irregular networks. Actually, such an irregularity should be often interpreted as minor modifications of some regular interconnection pattern, such as those induced by faults. In fact, topology-agnostic routing algorithms are also becoming increasingly useful for networks on chip (NoCs), where faults may make the preferred 2D mesh topology irregular. Existing topology-agnostic routing algorithms were developed for varying purposes, giving them different and not always comparable properties. Details are scattered among many papers, each with distinct conditions, making comparison difficult. This paper presents a comprehensive overview of the known topology-agnostic routing algorithms. We classify these algorithms by their most important properties, and evaluate them consistently. This provides significant insight into the algorithms and their appropriateness for different on- and off-chip environments.

Layered shortest path (LASH) routing in irregular system area networks

In recent years we have seen a growing interest in irregular network topologies for cluster interconnects. One problem related to such topologies is that the combination of shortest path and deadlock free routing is difficult. As a result of this the existing solutions for routing in irregular networks either guarantee shortest paths relative to some constraint (like up*/down*), or have to resort to deadlock recovery through non-minimal escape channels. In this paper we propose a method that guarantees shortest path routing and in-order delivery, and that uses virtual channels for deadlock avoidance. We present a theoretical upper bound on the number of virtual channels needed, and through extensive empirical testing we demonstrate that the actual number of virtual channels is very low even for large networks.

An Efficient Fault-Tolerant Routing Methodology for Meshes and Tori

In this paper we present a methodology to design fault-tolerant routing algorithms for regular direct interconnection networks. It supports fully adaptive routing, does not degrade performance in the absence of faults, and supports a reasonably large number of faults without significantly degrading performance. The methodology is mainly based on the selection of an intermediate node (if needed) for each source-destination pair. Packets are adaptively routed to the intermediate node and, at this node, without being ejected, they are adaptively forwarded to their destinations. In order to allow deadlock-free minimal adaptive routing, the methodology requires only one additional virtual channel (for a total of three), even for tori. Evaluation results for a 4 x 4 x 4 torus network show that the methodology is 5-fault tolerant. Indeed, for up to 14 link failures, the percentage of fault combinations supported is higher than 99.96%. Additionally, network throughput degrades by less than 10% when injecting three random link faults without disabling any node. In contrast, a mechanism similar to the one proposed in the BlueGene/L, that disables some network planes, would strongly degrade network throughput by 79%.

Energy minimization approach for optimal cooperative spectrum sensing in sensor-aided cognitive radio networks

In a sensor-aided cognitive radio network, collaborating battery-powered sensors are deployed to aid the network in cooperative spectrum sensing. These sensors consume energy for spectrum sensing and therefore deplete their life-time, thus we study the key issue in minimizing the sensing energy consumed by such group of collaborating sensors. The IEEE P802.22 standard specifies spectrum sensing accuracy by the detection and false alarm probabilities, hence we address the energy minimization problem under this detection accuracy constraint. Firstly, we derive the bounds for the number of sensors to simultaneously guarantee the thresholds for high detection probability and low false alarm probability. With these bounds, we then formulate the optimization problem to find the optimal sensing interval and the optimal number of sensor that minimize the energy consumption. Thirdly, the approximated analytical solutions are derived to solve the optimization accurately and efficiently in polynomial time. Finally, numerical results show that the minimized energy is significantly lower than the energy consumed by a group of randomly selected sensors. The mean absolute error of the approximated optimal sensing interval compared with the exact value is less than 4% and 8% under good and bad SNR conditions, respectively. The approximated optimal number of sensors is shown to be very close to the exact number.

First experiences with congestion control in InfiniBand hardware

In lossless interconnection networks congestion control (CC) can be an effective mechanism to achieve high performance and good utilization of network resources. Without CC, congestion in one node may grow into a congestion tree that can degrade the performance severely. This degradation can affect not only contributors to the congestion, but also throttles innocent traffic flows in the network. The InfiniBand standard describes CC functionality for detecting and resolving congestion. The InfiniBand CC concept is rich in the way that it specifies a set of parameters that can be tuned in order to achieve effective CC. There is, however, limited experience with the InfiniBand CC mechanism. To the best of our knowledge, only a few simulation studies exist. Recently, InfiniBand CC has been implemented in hardware, and in this paper we present the first experiences with such equipment. We show that the implemented InfiniBand CC mechanism effectively resolves congestion and improves fairness by solving the parking lot problem, if the CC parameters are appropriately set. By conducting extensive testing on a selection of the CC parameters, we have explored the parameter space and found a subset of parameter values that leads to efficient CC for our test scenarios. Furthermore, we show that the InfiniBand CC increases the performance of the well known HPC Challenge benchmark in a congested network.