Mart L. Molle

A nonstationary Poisson view of Internet traffic

Since the identification of long-range dependence in network traffic ten years ago, its consistent appearance across numerous measurement studies has largely discredited Poisson-based models. However, since that original data set was collected, both link speeds and the number of Internet-connected hosts have increased by more than three orders of magnitude. Thus, we now revisit the Poisson assumption, by studying a combination of historical traces and new measurements obtained from a major backbone link belonging to a Tier 1 ISP. We show that unlike the older data sets, current network traffic can be well represented by the Poisson model for sub-second time scales. At multisecond scales, we find a distinctive piecewise-linear nonstationarity, together with evidence of long-range dependence. Combining our observations across both time scales leads to a time-dependent Poisson characterization of network traffic that, when viewed across very long time scales, exhibits the observed long-range dependence. This traffic characterization reconciliates the seemingly contradicting observations of Poisson and long-memory traffic characteristics. It also seems to be in general agreement with recent theoretical models for large-scale traffic aggregation

Long-range dependence ten years of Internet traffic modeling

Self-similarity and scaling phenomena have dominated Internet traffic analysis for the past decade. With the identification of long-range dependence (LRD) in network traffic, the research community has undergone a mental shift from Poisson and memory-less processes to LRD and bursty processes. Despite its widespread use, though, LRD analysis is hindered by the difficulty of actually identifying dependence and estimating its parameters unambiguously. The authors outline LRD findings in network traffic and explore the current lack of accuracy and robustness in LRD estimation. In addition, they present recent evidence that packet arrivals appear to be in agreement with the Poisson assumption in the Internet core.

A user-friendly self-similarity analysis tool

The concepts of self-similarity, fractals, and long-range dependence (LRD) have revolutionized network modeling during the last decade. However, despite all the attention these concepts have received, they remain difficult to use by non-experts. This difficulty can be attributed to a relative complexity of the mathematical basis, the absence of a systematic approach to their application and the absence of publicly available software. In this paper, we introduce SELFIS, a comprehensive tool, to facilitate the evaluation of LRD by practitioners. Our goal is to create a stand-alone public tool that can become a reference point for the community. Our tool integrates most of the required functionality for an in-depth LRD analysis, including several LRD estimators. In addition, SELFIS includes a powerful approach to stress-test the existence of LRD, Using our tool, evidence are presented that the widely-used LRD estimators can provide misleading results. It is worth mentioning that 25 researchers have acquired SELFIS within a month of its release, which clearly demonstrates the need for such a tool.

A New Adaptive Channel Reservation Scheme for Handoff Calls in Wireless Cellular Networks

In wireless cellular networks, in order to ensure that ongoing calls are not dropped while the owner mobile stations roam among cells, handoff calls may be admitted with a higher priority as compared with new calls. Since the wireless bandwidth is scarce and therefore precious, efficient schemes which allow a high utilization of the wireless channel, while at the same time guarantee the QoS of handoff calls are needed. In this paper, we propose a new scheme that uses GPS measurements to determine when channel reservations are to be made. It works by sending channel reservation request for a possible handoff call to a neighboring cell not only based on the position and orientation of that calls mobile station, but also depends upon the relative motion of the mobile station with respect to that target cell. The scheme integrates threshold time and various features of prior schemes to minimize the effect of false reservations and to improve the channel utilization of the cellular system. Simulation results showthat our scheme performs better in almost all typical scenarios than prior schemes.

100Base-T/IEEE 802.12/packet switching

Three LAN technologies look set to satisfy the ever-increasing demand for LAN bandwidth. Two of these technologies are 100 Mb/s shared-medium LANs: 100Base-T (aka IEEE 802.3 Fast Ethernet) and IEEE 802.12 (aka 100VG-AnyLAN or 100VG). The third technology is packet switching, which is really an extension of existing LAN bridge technology, but offers excellent performance gains at very low cost. The authors describe the three technologies and provide a comparison between the two 100 Mb/s LANs. Also presented are results that compare the measured performance of 100 Mb/s shared-medium LANs with switched LANs.

Optimal distributed algorithm for minimum spanning trees revisited

In an earlier paper, Awerbuch presented an innovative distributed algorithm for solving minimum spanning tree problems that achieved optimal time and message complexity through the introduction of several advanced features. In this paper, we show that there are some cases where his algorithm can create cycles or fail to achieve optimal time complexity. We then show how to modify the algorithm to avoid these problems, and demonstrate both the correctness and optimality of the revised algorithm.

Comments on "Throughput Analysis for Persistent CSMA Systems"

We present a brief and simple analysis of asynchronous (unslotted) 1-Persistent CSMA protocols in the worst-case star topology. The purpose is two-fold. First, we give a simple approach based on an embedded Markov chain at the beginning of subbusy periods that greatly simplifies the analysis. And second, using the above approach we give the correct analysis for 1-Persistent CSMA with collision detection, since the only available analysis of the present model [1] is in error.

Scalable testbed for next-generation wireless networking technologies

This paper presents an overview of a new wireless network research testbed being developed by several wireless communications and networking research groups at UCLA. Its primary objective is to provide researchers at every layer of the protocol stack, from physical devices to transport protocols, a testbed to evaluate the impact of their technology on application level performance, using scalable and realistic operational scenarios. To achieve this objective, the testbed will use a collection of physical testbeds integrated with a scalable, multitool simulation framework.

Multiband Media Access Control in Impulse-Based UWB Ad Hoc Networks

We propose a MAC protocol for use in multihop wireless networks that deploy an underlying UWB (ultra wide band)-based physical layer. We consider a multiband approach to better utilize the available spectrum, where each transmitter sends longer pulses in one of many narrower frequency bands. The motivation comes from the observation that, in the absence of a sophisticated equalizer, the size of a slot for transmitting a UWB pulse is typically dictated by the delay spread of the channel. Therefore, using a wider frequency band to shorten the transmission time for each pulse does not increase the data rate in proportion to the available bandwidth. Our approach allows data transmissions to be contiguous and practically interference free, and, thus, highly efficient. For practicality, we ensure the conformance of our approach to FCC-imposed emission limits. We evaluate our approach via extensive simulations, and our results demonstrate the significant advantages of our approach over single-band solutions: the throughput increases significantly and the number of collisions decreases considerably. Finally, we analyze the behavior of our MAC protocol in a single-hop setting in terms of its efficiency in utilizing the multiple bands

On the Solution to QBD Processes with Finite State Space

Abstract In this article, we present a solution to a class of Quasi-Birth-and-Death processes with finite state space and show that the stationary probability vector has a matrix geometric representation. We show that such models have a level-dependent rate matrix. The corresponding rate matrix is given explicitly in terms of the model parameters. The resulting closed-form expression is proposed as a basis for efficient calculation of the stationary probabilities. The method proposed in this article can be applied to several queueing systems.