2578283
On the efficiency of fluid simulation of networks
Performance evaluation of computer networks through tratitional packet-level simulation is becoming increasingly difficult as networks grow in size along different dimensions. Due to its higher level of abstraction, fluid simulation is a promising approach for evaluating large-scale network models. In this paper we focus on evaluating and comparing the computaional effort required for fluid-and packet-level simulation. To measure the computaional effort required by a simulation approach, we introduce the concept of "simulation event rate", a measure that is both analytically tractable and adequate. We identify the fundamental factors that contribute to the simulation event rate in fluid- and packet-level simulations and provide an analytical characterization of the simulation event rate for specific network models. Among such factors, we identify the "ripple effect" as a significant contributor to the computational effort required by fluid simulation. We also show that the parameter space of a given network model can be divided into different regions where one simulation technique is more efficient than the other. In particular, we consider a realistic large-scale network and demonstrate how the computational effort depends on simulation parameters. Finally, we show that flow aggregation can effectively reduce the impact of the ripple effect and that the ripple effect has less impact when simulating the WFQ scheduling policy.
8584874
Throughput Optimization and Fair Bandwidth Allocation in Multi-Hop Wireless LANs
There is an inherent well-known conflict between fairness and throughput that arises in many networking scenarios. A number of researchers have studied this problem in the context of (single-hop) wireless local area networks (WLANs), where clients directly exchange traffic with access points (APs). More recently, researchers have proposed multi-hop extensions to WLANs where client traffic is forwarded via a series of client-client links. In this paper, we show that the objective of improving throughput without sacrificing fairness can be much better met in multi-hop WLANs. We decouple this objective into two separate but related problems. First, we need an algorithm to organize clients into a multi-hop structure such that fair bandwidth allocation within this structure leads to improved throughput. Second, we need algorithms for performing fair bandwidth allocation within the determined multi-hop structure. In this paper, we first design optimal fair bandwidth allocation algorithms for both max-min throughput fairness and max-min time fairness in multi-hop WLANs. Subsequently, we design an efficient algorithm to find desirable multi-hop structures. With slight modifications, our results in this paper can be generalized to other multi-hop wireless networks as well. Our proposed solutions seamlessly integrate with legacy devices and hence are incrementally deployable. Simulation results demonstrate that our solutions can effectively improve throughput (by up to 114% or more) as well as network coverage while preserving fairness.
800712
On the autocorrelation structure of TCP traffic
The statistical characteristics of network traffic--in particular the observation that it can exhibit long range dependence--have received considerable attention from the research community over the past few years. In addition, the recent claims that the TCP protocol can generate traffic with long range dependent behavior has also received much attention. Contrary to the latter claims, in this paper we show that the TCP protocol can generate traffic with correlation structures that spans only an analytically predictable finite range of time-scales. We identify and analyze separately the two mechanisms within TCP that are responsible for this scaling behavior: timeouts and congestion avoidance. We provide analytical models for both mechanisms that, under the proper loss probabilities, accurately predict the range in time-scales and the strength of the sustained correlation structure of the traffic sending rate of a single TCP source. We also analyze an existing comprehensive model of TCP that accounts for both mechanisms and show that TCP itself exhibits a predictable finite range of time-scales under which traffic presents sustained correlations. Our claims and results are derived from Markovian models that are supported by simulations. We note that traffic generated by TCP can be misinterpreted to have long range dependence, but that long range dependence is not possible due to inherent finite time-scales of the mechanisms of TCP.
1727263
A study of networks simulation efficiency: fluid simulation vs. packet-level simulation
Network performance evaluation through traditional packet-level simulation is becoming increasingly difficult as today's networks grow in scale along many dimensions. As a consequence, fluid simulation has been proposed to cope with the size and complexity of such systems. This study focuses on analyzing and comparing the relative efficiencies of fluid simulation and packet-level simulation for several network scenarios. We use the "simulation event" rate to measure the computational effort of the simulators and show that this measure is both adequate and accurate. For some scenarios, we derive analytical results for the simulation event rate and identify the major factors that contribute to the simulation event rate. Among these factors, the "ripple effect" is very important since it can significantly increase the fluid simulation event rate. For a tandem queueing system, we identify the boundary condition to establish regions where one simulation paradigm is more efficient than the other. Flow aggregation is considered as a technique to reduce the impact of the "ripple effect" in fluid simulation. We also show that WFQ scheduling discipline can limit the "ripple effect", making fluid simulation particularly well suited for WFQ models. Our results show that tradeoffs between parameters of a network model determines the most efficient simulation approach.
44632770
Modeling frame-level errors in GSM wireless channels
Abstract We compare four different approaches to modeling frame-level errors in GSM channels. One of these, the Markov-based Trace Analysis (MTA) model, was developed explicitly for this purpose. The next two, kth-order Markov models and hidden Markov models (HMMs), have been widely used to model loss in wired networks. All three of these have difficulty in modeling empirical GSM frame-level error traces. The MTA model and HMM predict frame-error rates substantially different from that measured from the trace, and all three models have difficulty in capturing the long term temporal correlation structure. We propose a fourth model, the extended On/Off model, which alternates between an On (error-free) and an Off (error-filled) state. The state holding times are taken from mixtures of geometric distributions. We show that this model, with mixtures of 4–7 geometric distributions captures first- and second-order statistics significantly better than the preceding three approaches.
2853015
On the capacity of hybrid wireless networks
This paper involves the study of the throughput capacity of hybrid wireless networks. A hybrid network is formed by placing a sparse network of base stations in an ad hoc network. These base stations are assumed to be connected by a high-bandwidth wired network and act as relays for wireless nodes. They are not data sources nor data receivers. Hybrid networks present a tradeoff between traditional cellular networks and pure ad hoc networks in that data may be forwarded in a multihop fashion or through the infrastructure. It has been shown that the capacity of a random ad hoc network does not scale well with the number of nodes in the system. In this work, we consider two different routing strategies and study the scaling behavior of the throughput capacity of a hybrid network. Analytical expressions of the throughput capacity are obtained. For a hybrid network of n nodes and m base stations, the results show that if m grows asymptotically slower than √n, the benefit of adding base stations on capacity is insignificant. However, if m grows faster than √n, the throughput capacity increases linearly with the number of base stations, providing an effective improvement over a pure ad hoc network. Therefore, in order to achieve nonnegligible capacity gain, the investment in the wired infrastructure should be high enough.
18992172
A practical packet reordering mechanism with flow granularity for parallelism exploiting in network processors
Network processors (NP) are usually designed to exploit packet level parallelism where system throughput is aggregated by multiple CPU cores. A serious problem in such a system is packet disordering (PD), which may deteriorate network performance greatly. To address the PD problem, a practical reordering mechanism is put forward in this paper. Different from the traditional reordering methods such as in ATM networks, it uses a central-controlled chain-based mechanism to implement packet reordering with flow granularity. We verify it in FPGA and carry out a series of experiments, where the results show that system throughput can be influenced greatly by traffic patterns. We also demonstrate that the reordering with flow granularity is quite requisite in NP, which can increase the system throughput to a great extent compared to the conventional method with global granularity.
62878
Single-hop probing asymptotics in available bandwidth estimation: sample-path analysis
In this paper, we take the sample-path approach in analyzing the asymptotic behavior of single-hop bandwidth estimation under bursty cross-traffic and show that these results are provably different from those observed under fluid models of prior work. This difference, which we call the probing bias, is one of the previously unknown factors that can cause measurement inaccuracies in available bandwidth estimation. We present an analytical formulation of "packet probing," based on which we derive several major properties of the probing bias. We then experimentally observe the <i>probing bias</i> and investigate its quantitative relationship to several deciding factors such as probing packet size, probing train length, and cross-traffic burstiness. Both our analytical and experimental results show that the probing bias vanishes as the packet-train length or packet size increases. The vanishing rate is decided by the burstiness of cross-traffic.
14245590
RED with optimized dynamic threshold deployment on shared buffer
Prior survey of RED algorithm deployment on multiqueue system with shared buffer was unfair and sensitive to congestion level by statically setting the parameters. In this paper, our goal is to deploy the Random Early Detection (RED) algorithm in routers on shared buffer and solve these problems. A novel buffer management scheme that dynamically adjusts the parameters of RED named RED-ODT is proposed. Simulations under uniform traffic load and nonuniform traffic load are given, the results of which ascertain and demonstrate the superiority of the proposed scheme in terms of low packet drop ratio, satisfying buffer utilization and fairness. Simulation also shows that RED-ODT is insensitive to congestion level.
14957159
Utility-Based Bandwidth Allocation for Triple-Play Services
It is expected that, in near future, the multi-class traffic previously in public switched telephone network (PSTN), cable television network and IP network will be multiplexed at backbone and carried by the converged next-generation network (NGN). A pragmatic challenge in facilitating NGN is how to schedule traffic and allocate bandwidth among the triple-play services, namely voice (VoIP), video (IPTV) and data. Different from traditional strict-priority based scheduling intensively used in industry, in this paper, we try to discuss this issue from the objective of network utility maximization (NUM). We first investigate the characteristics of most existing traffic classes and explicitly formulate their utilities to be the function of occupied bandwidth. After that, a novel scheduling scheme to achieve NUM is derived using Lagrange method with KKT conditions. Numerical results under two network scenarios are calculated. Both of them reveal the unique nature of this scheduling, compared with strict-priority scheduling, still highest priority is provided for VoIP traffic, however, no strict priority should be given to IPTV traffic since it will conflict with NUM objective. We hope our results will shed lights on the evolvement towards the converged network
14837083
A Transactional Approach to Parallel Data Warehouse Maintenance
Data Warehousing is becoming an increasingly important technology for information integration and data analysis. Given the dynamic nature of modern distributed environments, both source data and schema changes are likely to occur autonomously and even concurrently in different sources. We have thus developed a comprehensive solution approach, called TxnWrap, that successfully maintains the warehouse views under any type of concurrent source updates. In this work, we now overcome TxnWrap's restriction that the maintenance is processed one by one for each source update, since that limits the performance. To overcome this limitation, we exploit the transactional approach of TxnWrap to achieve parallel data warehouse maintenance. For this, we first identify the read/write conflicts among the different warehouse maintenance processes. We then propose a parallel maintenance scheduler (PMS) that generates legal schedules that resolve these conflicts. PMS has been implemented and incorporated into our TxnWrap system. The experimental results confirm that our parallel maintenance scheduler significantly improves the performance of data warehouse maintenance.