Yusuke Sakumoto

Performance Evaluation of Broadcast Communication Protocol DSCF (Directional Store-Carry-Forward) for VANETs with Two-Dimensional Road Model

In this paper, we reveal several characteristics of a broadcast communication protocol called DSCF (Directional-Store-Carry-Forward). In recent years, realization of VANETs (Vehicular Ad-hoc Networks) has been demanded for delivering information to vehicles at a low cost. In VANETs, a vehicle may or may not find other vehicles within its radio communication range because of vehicles’ mobility, which results in frequently intermittent vehicle-to-vehicle radio communications. DTN (Delay Tolerant Networking) broadcast communication mechanisms can continue communication against intermittent vehicle-to-vehicle radio communications. Our previous work, we have proposed a DTN broadcast communication mechanism called DSCF. The effectiveness of DSCF in simple one-dimensional road models has been extensively studied. However, characteristics of DSCF in a realistic environment have not been investigated. In this paper, we therefore quantitatively examine several characteristics of the broadcast communication with DSCF in two-dimensional road models through simulation. In particular, four major characteristics of a broadcast communication in VANETs are investigated: dissemination speed, uniformity, reliability, and efficiency. Through extensive simulations, we show that DSCF performs quite effectively in terms of dissemination speed, uniformity, reliability, and efficiency unless the vehicle density is too low.

Design and Implementation of Flow-Level Simulator for Performance Evaluation of Large Scale Networks

In this paper, we propose a flow-level simulator called FSIM (Fluid-based SIMulator) for performance evaluation of large-scale networks, and verify its effectiveness using our FSIM implementation. The notable features of our flow-level simulator FSIM are its accuracy and fast simulation execution compared with conventional flow-level simulators. For improving simulation accuracy, our flow-level simulator FSIM utilizes accurate fluid-flow models. For accelerating simulation execution speed, our flow-level simulator FSIM adopts an adaptive numerical computation algorithm for ordinary differential equations. Another notable feature of our flow-level simulator FSIM is its compatibility with the existing network performance analysis tool. In this paper, through several experiments using our FSIM implementation, we evaluate the effectiveness of our flow-level simulator FSIM in terms of simulation speed, accuracy and memory consumption. Consequently, we show that our flow-level simulator FSIM outperforms a conventional flow-level simulator; i.e., it realizes approximately 100% faster simulation with higher accuracy and less memory consumption than a conventional flow-level simulator.

On XCP Stability in a Heterogeneous Network

In this paper, we analyze stability of XCP (explicit control protocol) in a network with heterogeneous XCP flows (i.e., XCP flows with different propagation delays). Specifically, we model a network with heterogeneous XCP flows using fluid-flow approximation. We then derive the conditions that XCP control parameters should satisfy for stable XCP operation. Furthermore, through several numerical examples and simulation results, we quantitatively investigate effect of system parameters and XCP control parameters on stability of the XCP protocol. Our findings include: (1) when XCP flows are heterogeneous, XCP operates more stably than the case when XCP flows are homogeneous, (2) conversely, when variation in propagation delays of XCP flows are very large, operation of XCP becomes less stable, and (3) output link bandwidth of an XCP router is independent of stability of the XCP protocol.

On Convergence Rate of Autonomous Decentralized Structure Formation Technology for Clustering in Ad Hoc Networks

Hierarchization by clustering is effective for scalable routing control in an ad hoc network. Due to a constraint from structural features of ad hoc networks, clustering in ad hoc networks should be autonomous decentralized algorithm based on local information. Bio-inspired approach gives a solution of such autonomous decentralized clustering mechanism and its well-known mechanism is based on reaction-diffusion equations. Apart from this, we have already proposed the autonomous decentralized clustering mechanism for ad hoc networks based on two concepts of renormalization transformation and back-diffusion potential. One of most important requirements to the clustering mechanism is faster convergence speed of clustering to adapt to dynamic environments. In this paper, we compare convergence speed of both mechanisms.

Performance Evaluation of Epidemic Broadcast with Directional Antennas in Vehicular Ad-Hoc Networks

In this paper, we extensively investigate the effectiveness of an epidemic broadcast with directional antennas on two-dimensional road model. In VANETs, realization of broadcast communication from a vehicle to other vehicles solely using a vehicle-to-vehicle radio communication channel is desired. In our previous work, we have proposed DSCF (Directional Store-Carry-Forward), which is an epidemic broadcast with two directional antennas. DSCF is a simple epidemic broadcast but it intentionally utilizes two directional antennas for directed message delivery for alleviating the broadcast storm problem. In this paper, we extensively investigate the effectiveness of DSCF on two-dimensional road a model in terms of reach ability, dissemination speed, uniformity, and efficiency. The performance of DSCF is thoroughly compared with that of P-BCAST, which is one of the simplest epidemic broadcasts with an omni directional antenna. In theory, P-BCAST achieves the optimal reach ability, dissemination speed, and uniformity while compromising the worst efficiency. Our findings include that DSCF can achieve satisfactory reach ability and dissemination speed comparable to those of P-BCAST with significantly better efficiency, which implies favorable characteristics of directional antennas for epidemic broadcasts in VANETs.

Design and Implementation of Flow-Level Simulator for a Network with Heterogeneous Flows

In this paper, we present design and implementation of a flow-level network simulator called FSIM+ (Fluid-based network SIMulator plus) for a network with heterogeneous flows transferred with different transport protocols: TCP,XCP, and UDP. In recent years, flow-level network simulators,which can perform significantly faster simulation than conventional packet-level network simulators, have been receiving attention. However, existing flow-level network simulators can only simulate persistent TCP flows. In this paper, by utilizing fluid models for several transport protocols, a flow-level network simulator FSIM+ for a network with heterogeneous flows is designed. Through several experiments using our FSIM+ implementation, we evaluate its effectiveness quantitatively. Both accuracy and effectiveness of FSIM+ are evaluated by comparing with the popular packet-level network simulator ns-2. Our findings include that our FSIM+ implementation achieves (1) less than 6% simulation errors in steady-state performance metrics, (2) approximately 10 times faster simulation, and (3) more than 20 times memory efficiency.

An autonomous decentralized control for indirectly controlling system performance variable in large-scale and wide-area network

In this paper, we propose a novel autonomous decentralized control (ADC) for indirectly controlling a system performance variable, while not measuring the variable. In a large-scale and wide-area network, each node cannot gather information from the whole network, and has to control all over the network by collaborating with other nodes according to information in its local area. Some important problems (e.g., resource allocation) in a network are often formulated by a system performance variable as a function of system information including all node states. To tackle such a problem by an ADC, we design a node action to indirectly control the probability distribution of a system performance variable by only using local information on the basis of Markov Chain Monte Carlo. We then investigate the effectiveness of the node action through the analysis based on statistical mechanics. Moreover, we apply our ADC to design a traffic-aware virtual machine placement control with load balancing in a data center network. Simulations confirm that our control yields the performance desired.

On the Impact of Scale-Free Structure on End-to-End TCP Performance

In this paper, we analyze end-to-end TCP performance (i.e., Sending rate, packet loss probability, and roundtrip time) of TCP flows on a network with scale-free structure. Using a numerical example of our analysis, we show that the scale-free structure contributes to improve the end-to-end TCP performance, and that such performance improvement is caused by reduction in the average path length and also reduction of traffic intensity at a bottleneck link.

Transmission Rate Control Utilizing Chaotic Nature of Coupled Oscillators

The TCP global synchronization problem is a phenomenon that packet losses or window control actions of different flows are synchronized. It causes degradation of the utilization of network bandwidth. RED is a well known approach to avoid this problem. However, discard of packets occurs in some probability even if they are in low-rate flow, and it might be unfair. In this paper, we propose new transmission rate control by using chaotic nature appearing in coupled oscillators. Our control can avoid the synchronization problem despite no use of random numbers and improve the network utilization.

Fluid-Based Analysis of TCP Flows in a Scale-Free Network

In this paper, we derive distributions of throughput, round-trip time, and packet loss probability of TCP flows in a scale-free network. Different from non scale-free networks such as ER (Erdos and Renyi) random graph, the connecting probability between nodes is dependent on their node degrees in scale-free networks. Hence, scale-free networks are generally not easy to analyze. Fekete et al. have derived the average throughput of TCP flows in a scale-free network by limiting network topologies to a class of scale-free trees (i.e., Barabasi Albert) tree). Not only the average throughput of TCP flows but also distributions of their performance metrics are important for design and control of communication networks. In this paper, we therefore derive distributions of throughput, round-trip time, and packet loss probability of TCP flows in a scale-free network. Similarly to Feketes approach, we use the BA tree as the network topology. Dynamics of TCP flows and routers are modeled with fluid-flow models. By utilizing known analytical results (i.e., distributions of link betweenness and path length in BA trees), we derive distributions of throughput, round-trip time, and packet loss probability of TCP flows in steady state.