Source-destination Node Pairs Research Articles

On the Sumrate of Amplify-and-Forward Relay Networks with Multiple Source-Destination Pairs

The paper considers a network scenario in which multiple source-destination node pairs need to communicate simultaneously. Each source and destination is equipped with one antenna and the communication is assisted by a multi-antenna relay operating in Amplify-and-Forward fashion. The communication occurs in two slots; in the first slot, the sources transmit simultaneously, and in the second slot, the relay retransmits the signals which were received by its antennas during the first slot, after linearly processing them via a Zeroforcing Beamforming (ZFBF) matrix. Two different designs for the ZFBF matrix are proposed. The first design allocates the relay power so that all data streams have the same useful power. The second design adjusts the relay weights for all S-D pairs in a way that maximizes the sumrate. It is shown analytically that, when the source or relay power is high, the proposed sumrate maximization method has the same ergodic sumrate as the cut-set bound with two identical slots and a ZF precoder in the broadcast hop or a ZF equalizer in the multiple access hop, or maintains a constant gap from the cut-set bound with a ZF precoder in the broadcast hop. Although initially perfect channel state information (CSI) is assumed available at the relay, an upper bound of the ergodic sumrate loss caused by relay CSI errors is also derived.

Relay Precoder Optimization in MIMO-Relay Networks With Imperfect CSI

In this paper, we consider robust joint designs of relay precoder and destination receive filters in a nonregenerative multiple-input multiple-output (MIMO) relay network. The network consists of multiple source-destination node pairs assisted by a MIMO-relay node. The channel state information (CSI) available at the relay node is assumed to be imperfect. We consider robust designs for two models of CSI error. The first model is a stochastic error (SE) model, where the probability distribution of the CSI error is Gaussian. This model is applicable when the imperfect CSI is mainly due to errors in channel estimation. For this model, we propose robust minimum sum mean square error (SMSE), MSE-balancing, and relay transmit power minimizing precoder designs. The next model for the CSI error is a norm-bounded error (NBE) model, where the CSI error can be specified by an uncertainty set. This model is applicable when the CSI error is dominated by quantization errors. In this case, we adopt a worst-case design approach. For this model, we propose a robust precoder design that minimizes total relay transmit power under constraints on MSEs at the destination nodes. We show that the proposed robust design problems can be reformulated as convex optimization problems that can be solved efficiently using interior-point methods. We demonstrate the robust performance of the proposed design through simulations.

Performance Analysis of a Bi-Objective Model for Routing with Protection in WDM Networks

The operation of wavelength division multiplexing (WDM) networks involves not only the establishment of lightpaths, defining the sequence of optical fibres and thewavelength in each fibre for traffic flow, but also a fault management scheme in order to avoid the huge loss of data that can result from a single link failure. Dedicated path protection, which establishes two end-to-end disjoint routes between the source–destination node pair, is an effective scheme to preserve customers’ connections. This paper reviews a bicriteria model for dedicated path protection, that obtains a topological path pair of node-disjoint routes for each lightpath request in a WDM network, developed by the authors. An extensive performance analysis of the bicriteria model is then presented, comparing the performance metrics obtained with the monocriterion models using the same objective functions, in four different reference networks commonly used in literature.

Route Availability Model for Inter-working Multi-hop Wireless Networks

Route Availability Model for Inter-working Multi-hop Wireless NetworksIn order to link a source-destination node pair in inter-working multi-hop wireless networks, links or routes must first be available. It is only after establishing the availability of links and routes between nodes that factors which affect connectivity e.g. interference can be considered. Connectivity in multi-hop wireless networks has been studied. However, the studies focused on network connectivity in ad-hoc networks. Since the next generation of wireless networks will be inter-working, an understanding of connectivity as it applies to such networks is needed. Specifically, this paper emphasizes that an analysis of route connectivity rather than network connectivity is needed for inter-working multi-hop wireless networks. With a focus on route connectivity, a route availability model for inter-working multi-hop wireless networks is presented.

Link reliability based hybrid routing for tactical mobile ad hoc network

Tactical mobile ad hoc network (MANET) is a collection of mobile nodes forming a temporary network, without the aid of pre-established network infrastructure. The routing protocol has a crucial impact on the network performance in battlefields. Link reliability based hybrid routing (LRHR) is proposed, which is a novel hybrid routing protocol, for tactical MANET. Contrary to the traditional single path routing strategy, multiple paths are established between a pair of source-destination nodes. In the hybrid routing strategy, the rate of topological change provides a natural mechanism for switching dynamically between table-driven and on-demand routing. The simulation results indicate that the performances of the protocol in packet delivery ratio, routing overhead, and average end-to-end delay are better than the conventional routing protocol.

Allocating point-to-point connections over point-to-multipoint optical channels

Optical channels are currently able to carry 10 Gbit/s and even 40 Gbit/s traffic flows. However, it is not usual to have such amounts of traffic between any pair of client nodes. This article proposes using point-to-multipoint optical channels for the allocation of point-to-point connections in transparent wavelength-routed optical networks. Specifically, when an optical connection between a source-destination node pair has to be established, the optical signal is also sent to some adjacent nodes by introducing passive optical splitters; in this way a light-tree is built. Then, the already established point-to-multipoint optical channel can be used to groom further point-to-point connections between the same source node and each of the other nodes composing the light-tree. The benefits of this strategy are 2-fold: first, the reduction of optical transmission equipments allowing cost savings with respect to the traditional typical point-to-point approach and, second, the optimization of the optical channels utilization meeting in such a way Traffic Engineering objectives. The merits of proposed approach are evaluated by simulation.

Finding a Path Subject to Many Additive QoS Constraints

A fundamental problem in quality-of-service (QoS) routing is to find a path between a source-destination node pair that satisfies two or more end-to-end QoS constraints. We model this problem using a graph with n vertices and m edges with K additive QoS parameters associated with each edge, for any constant Kges2. This problem is known to be NP-hard. Fully polynomial time approximation schemes (FPTAS) for the case of K=2 have been reported in the literature. We concentrate on the general case and make the following contributions. 1) We present a very simple (Km+nlogn) time K-approximation algorithm that can be used in hop-by-hop routing protocols. 2) We present an FPTAS for one optimization version of the QoS routing problem with a time complexity of O(m(n/epsi) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K-1</sup> ). 3) We present an FPTAS for another optimization version of the QoS routing problem with a time complexity of O(nlogn+m(H/epsi) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K-1</sup> ) when there exists an H-hop path satisfying all QoS constraints. When K is reduced to 2, our results compare favorably with existing algorithms. The results of this paper hold for both directed and undirected graphs. For ease of presentation, undirected graph is used

On the partial path protection scheme for WDM optical networks and polynomial time computability of primary and secondary paths

As a generalization of the traditional path protection (PP) scheme in WDM networks where a backup path is needed for each active path, the partial path protection (PPP) scheme uses a collection of backup paths to protect an active path, where each backup path in the collection protects one or more links on the active path such that every link on the active path is protected by one of the backup paths. While there is no known polynomial time algorithm for computing an active path and a corresponding backup path using the PP scheme for a given source destination node pair, we show that an active path and a corresponding collection of backup paths using the PPP scheme can be computed in polynomial time, whenever they exist, under each of the following four network models: (a) dedicated protection in WDM networks without wavelength converters; (b) shared protection in WDM networks without wavelength converters; (c) dedicated protection in WDM networks with wavelength converters; and (d) shared protection in WDM networks with wavelength converters. This is achieved by proving that that for any given source $s$ and destination $d$ in the network, if one candidate active path connecting $s$ and $d$ is protectable using PPP, then any candidate active path connecting $s$ and $d$ is also protectable using PPP. It is known that the existence of PP implies the existence of PPP while the reverse is not true. We demonstrate a similar result in the case of segmented path protection. This fundamental property of the PPP scheme is of great importance in the context of achieving further research advances in the area of protection and restoration of WDM networks.

Providing deterministic quality of service in slotted optical networks

This paper proposes an efficient framework for deterministic service guarantees in slot-based optical networks. The framework uses the combination of a control plane and a data plane to solve the complex problem of capacity allocation, slot-matching and traffic scheduling. The control plane implements admission control and capacity allocation to source-destination node pairs and the data plane handles traffic aggregation, buffering, and scheduling. We propose an efficient algorithm in the control plane for slot collision-resolution. In the data plane, we present a comprehensive aggregation and scheduling mechanism that realizes Service Curves assurance. We use the Time-Domain Wavelength Interleaved Network (TWIN) architecture for the proof of concept and conduct extensive simulations to assess the performance of the algorithm and scheduling mechanism.

Too Much Mobility Limits the Capacity of Wireless Ad Hoc Networks

We show that for highly mobile ad hoc networks, the benefits of mobility are overshadowed by the cost of mobility in terms of the increased channel uncertainty and network homogeneity. We assume a block-fading channel model with jointly isotropic fading. We allow relays which can transmit and receive simultaneously. Under fairly general assumptions for the users' channel fades and additive noise distributions we show that increasing the number of transmit antennas M at any node beyond the channel coherence time T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (measured in units of channel uses) does not affect the capacity region of the ad hoc network. For a fast-fading (coherence time T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> lesM) homogeneous network, we determine the exact capacity region of the ad hoc network for any partition of the nodes into source, destination, and relay nodes. The optimal strategy is such that only one pair of source-destination nodes is active at a time while all the other nodes are inactive. There is no benefit from relaying and at high signal-to-noise ratio (SNR) the total throughput grows at most double-logarithmically with the number of nodes. Even for the case of slow fading, where the channel variations are slow enough that the receiver can track the channel perfectly, the inability of the transmitter to track the network topology limits the total throughput growth rate to no more than logarithmic in the number of nodes. Spatial correlation is shown to enhance the capacity region of the Rayleigh-fading ad hoc network

Dynamic routing and wavelength assignment in optical networks by means of genetic algorithms

We propose a novel genetic algorithm for solving the dynamic routing and wavelength assignment (DRWA) problem in wavelength-routed optical networks. The algorithm not only obtains low call blocking probability, but it also employs a very short computation time. Moreover, it is capable of providing fairness among connections, that is, to offer approximately the same quality of service (in terms of blocking probability) for all source-destination node pairs. Since requirements on optical network availability are highly severe, we also propose an extension of the algorithm to provide fault-tolerance capability at the optical layer. It is achieved by means of protection, where each optical connection request is provided with a pair of lightpaths (a primary and a backup lightpath). Again, the genetic algorithm proves to be highly efficient, in this case, at performing routing and wavelength assignment of pairs of lightpaths.

Heuristic for the maximum disjoint paths problem in wavelength-routed networks with shared-risk link groups [Invited

We tackle the problem of finding the maximum number of link-disjoint paths between a pair of source-destination nodes in wavelength-routed networks, subject to the shared-risk link group (SRLG) constraints. We propose a simple heuristic algorithm whose main idea is to iteratively find the path with the minimum total weight of all the SRLGs through which it passes until no further path can be found, where the weight of each SRLG is basically proportional to its size, and those SRLGs appearing in the minimum cut between the source-destination nodes are assigned higher weights. Extensive simulation results show that the proposed method, with a complexity comparable to that of the simplest conventional k-shortest-path algorithm, steadily outperforms the best existing method.

More on the Efficiency of Interval Routing

Interval routing is a space-efficient routing method for computer networks. The method is said to be optimal if it can generate optimal routing paths for any source-destination node pair. A path is optimal if it is a shortest path between the two nodes involved. A seminal result in the area, however, has pointed out that ‘the interval routing algorithm cannot be optimal in networks with arbitrary topology’. The statement is correct but the lower bound on the longest routing path that was derived is not. We give the counterproof in this paper and the corrected bound.

A dynamic routing concept for ATM-based satellite personal communication networks

Satellite systems are going to build a part of the future personal communications infrastructure. The first-generation candidates for satellite personal communication networks (S-PCN) will rely on low Earth orbit (LEO) and medium Earth orbit (MEO) constellations. A noticeable trend in this field is toward broadband services and the use of ATM. For LEO satellite systems employing intersatellite links (ISLs), this paper proposes an overall networking concept that introduces the strengths of ATM to their operation. The core of the paper is the design of a new routing scheme for the periodically time-variant ISL subnetwork, discrete-time dynamic virtual topology routing (DT-DVTR), and its ATM implementation. DT-DVTR works completely off line, i.e., prior to the operational phase of the system. In a first step, a virtual topology is set up for all successive time intervals of the system period, providing instantaneous sets of alternative paths between all source-destination node pairs. In the second step, path sequences over a series of time interval are chosen from that according to certain optimization procedures. An ATM-based implementation of DT-DVTR in LEO satellite ISL networks is presented with some emphasis on the optimization alternatives, and the performance in terms of delay jitter is evaluated for an example ISL topology.

An Almost Regular Fault Tolerant Network with Arbitrary Number of Nodes

A new family of almost regular interconnection topology is proposed that can be defined for an arbitrary number of nodes. The proposed network can be very efficiently used to design communication architecture for distributed systems with an arbitrary number of computing nodes. The design involves a novel generalization of the concept of group based star graphs. The proposed topology is shown to be incrementally extensible in steps of 1 and optimally fault tolerant, and its diameter is sublogarithmic in the number of nodes. We have also developed a simple routing algorithm for any source destination pair of nodes.

Delay and Throughput Evaluation of Switching Methods in Computer Communication Networks

Message, packet and line switching in computer communication networks are analyzed by a queueing model. Message transmission delay time and network throughput between a source-destination node pair are obtained as a function of various parameters including message length, traffic arriving at the network, and the number of switching nodes existing between the nodes. A criterion to determine the most suitable switching method under the given conditions is offered. Also, the maximum length of a packet in packet switching is discussed.

Distribution of end-to-end delay in message-switched networks

An open queueing network model is used to derive the distribution of end-to-end delay in a message-switched network. It is shown that under fixed routing, the end-to-end delay of messages belonging to a particular source-destination node pair is given by a sum of independent and exponentially distributed random variables. The generalization of this basic result to random routing and to messages belonging to a group of a source-destination pairs is also considered. Numerical examples based on a hypothetical network are presented.