Minimum Average Delay Research Articles

Joint Routing and Scheduling in Mobile Aeronautical Ad Hoc Networks

In this paper, we formulate the joint Internet gateway allocation, routing, and scheduling problem in wireless ad hoc networks with the goal of minimizing the average packet delay in a space-time-division multiple-access (STDMA) network. We first propose a mathematical programming approach consisting of two steps: the minimization of the weighted hop count (mWHC) subject to scheduling constraints, followed by average delay minimization for the previously computed routes. Since the computational complexity of this approach is prohibitive for larger networks, we also formulate a genetic algorithm (GA) that can be applied to larger networks and mobile networks. We analyze the performance of both approaches by means of simulations and compare the solution that they provide to a simple hop-count-based routing and gateway selection solution. It is shown that the performance of the GA is comparable with the mathematical programming approach in terms of delay and packet delivery ratio (PDR) at lower complexity and is significantly better than the hop-count-based solution.

Cross‐layer resource allocation in orthogonal frequency multiple access systems based on channel distribution information

In this study, the authors propose a cross-layer resource allocation scheme for orthogonal frequency multiple access (OFDM) systems based on the subcarrier channel distribution information (CDI). Here, in contrast to most of the previous works, the authors consider practical discrete rate adaptation in which rate is adapted using a predefined set of modulation levels corresponding to fading regions. The OFDM system supports 'streaming traffic' that requires a minimum guaranteed average delay, as well as 'elastic traffic' with flexible rate requirements. The main objective is to maximise the total average transmission rate of elastic users, whereas the average delay is also guaranteed for streaming traffic, all subject to the maximum average transmission power constraint. The authors then propose an algorithm which finds suboptimal fading regions, as well as suboptimal subcarrier allocation based on dual decomposition method. In the proposed method, the solution of optimisation problem remains valid, until the channel distributions and/or the packet arrival and packet length distribution of the users are changed. This significantly reduces the computational complexity since there is no need to solve the optimisation problem for each channel side information (CSI) feedback. The proposed scheme also reduces the signalling overhead since corresponding to each user the CSI is required only for the assigned subcarriers. Particularly, the proposed method is suitable for cases in which wireless channel distributions is not changed, or can be simply estimated, for example, cases where a parametric distribution is fixed and only statistics such as average and variance is changed. Using simulations, the authors study the impact of number of fading regions and packet arrival rate on the achievable rate of elastic traffic.

Coordinated Optimization Model for Signal Timings of Full Continuous Flow Intersections

The continuous flow intersection (CFI) was invented by Francisco Mier to improve traffic operation in intersections that suffer from traffic problems caused by high numbers of left-turn movements. Although the CFI has been developed for 25 years and some built CFIs have achieved good results, it lacks a general model of signal timings and offsets. This paper (a) describes the main relationships needed to develop a CFI model; (b) builds a detailed model for optimization of phase sequence scheme, length of cycle, phase durations, and offsets between successive signals; and (c) compares the computing results of minimum cycle, capacity, and average delay of CFIs with those of conventional intersections. The results show that CFI has a clear advantage and a wide range of application. In the proposed model and algorithm, the location of the subintersection as a design parameter need not be input, but it is affected by offset and queue length. The solving process and algorithms of all critical features, such as average delay, are provided. The simplified solving process makes this model easy to apply; the computation is not onerous and can be made with the help of a simple procedure running on a personal computer.

Coordinated Optimization Model for Signal Timings of Full Continuous Flow Intersections

The continuous flow intersection (CFI) was invented by Francisco Mier to improve traffic operation in intersections that suffer from traffic problems caused by high numbers of left-turn movements. Although the CFI has been developed for 25 years and some built CFIs have achieved good results, it lacks a general model of signal timings and offsets. This paper (a) describes the main relationships needed to develop a CFI model; (b) builds a detailed model for optimization of phase sequence scheme, length of cycle, phase durations, and offsets between successive signals; and (c) compares the computing results of minimum cycle, capacity, and average delay of CFIs with those of conventional intersections. The results show that CFI has a clear advantage and a wide range of application. In the proposed model and algorithm, the location of the subintersection as a design parameter need not be input, but it is affected by offset and queue length. The solving process and algorithms of all critical features, such as average delay, are provided. The simplified solving process makes this model easy to apply; the computation is not onerous and can be made with the help of a simple procedure running on a personal computer.

Delay-constraint fair resource allocation scheme for an optical overlapped code-division multiple access-based optical network: a cross-layer approach

This study addresses the problem of resource allocation for a multi-class time-slotted optical overlapped code-division multiple access network. A delay-constraint fair resource allocation (DC-FRA) scheme is considered with the quality of service requirements on both physical layer signal-to-interference ratio and network layer average packet delay. A cross-layer approach in allocating the transmission power and rate for every class of users in the network is considered. The performances of the S-ALOHA, R-ALOHA and R3T medium access control protocols have been investigated in the DC-FRA scheme. It is shown that the R-ALOHA protocol has the highest throughput and the minimum average packet delay. On the other hand, although the DC-FRA allocates the largest bandwidth under the R-ALOHA protocol, it forces the users to transmit with relatively higher power in comparison with the power required for the S-ALOHA and the R3T protocols. In addition, the packets exhibit much smaller delay when using the DC-FRA, especially at low and moderate throughput for the three protocols.

Cluster Based Routing Protocol with Adaptive Scheduling for Mobility and Energy Awareness in Wireless Sensor Network

The technological advances in wireless communication, micro-electro-mechanical system (MEMS) technologies and digital electronics over the past few years have enabled the development of wireless sensor network (WSN). In some applications, the WSN nodes are dedicated to be mobile rather than static. This requirement poses new and interesting challenges for both medium access control (MAC) and routing protocols design. In this paper, we propose mobility and traffic adapted cluster based routing for mobile nodes (CBR-Mobile) protocol in WSN to support the mobility of sensor nodes in an energy-efficient manner while maintaining maximum delivery ratio and minimum average delay. The mobility and traffic adapted scheduling based MAC design enables the cluster heads in WSN to reuse the free or unused timeslots to support the mobility of sensor nodes. Each cluster head maintains two simple database tables to achieve this adaptation. The proposed CBR-Mobile protocol enables the mobile sensor nodes that disconnected with their cluster heads to rejoin the network through other cluster heads within a short time. The results show that it can achieve around 43% improvement on packet delivery ratio while simultaneously offering lower delay and energy consumption compare to LEACH-Mobile protocol.

Impact of source counter on routing performance in resource constrained DTNs

We study routing schemes for Disruption Tolerant Networks (DTNs) where transmission bandwidth is scarce. In such a setting, a key issue is how to schedule the transmission of packets under limited bandwidth to optimize performance. Such a scheduling consists of source control (i.e., source nodes choosing a routing scheme) and the local transmission scheduling performed by each node. Existing works typically focus on transmission scheduling and buffer management aspects, but due to theoretical and practical difficulties, only heuristics have been proposed. In this work, we explore an alternative way to improve DTN routing performance via source control. We first show through simulation that for spray-and-wait routing scheme where the source node specifies the maximum allowed number of copies of a packet in the network, there exists an optimal counter value that achieves the minimum network-wide average packet delivery delay. Then as a first step towards understanding multi-hop multi-copy DTN routing schemes such as spray-and-wait scheme, we perform modeling study of two-hop single-copy scheme and two-hop multi-copy scheme under various transmission scheduling schemes, via queuing network analysis and continuous time Markov chain model analysis. Our modeling analysis provides insights into the impact of source counter on routing performance and further suggests the existence of an optimal counter value. Relying on the insights gained via simulations and modeling studies, we propose an adaptive scheme where nodes adjust their counter values to achieve minimum packet delivery delay, in a distributed and asynchronous fashion. Simulations demonstrate the effectiveness of our scheme and suggest the potential of exploring this rich area for improving DTN routing performance.

Optimization of Road Section Crosswalk Signal Design

Abstract Signal settings of signalized urban road section crosswalks directly affect pedestrian safety and the vehicle delay of the road section. Based on the characteristics of pedestrians, this paper proposed the relationships between duration of pedestrian phase and the occurrence of pedestrian red light violation behaviors through the analysis of pedestrians and vehicles survey data collected from the road section crosswalk. It established a signal optimization model of the crosswalk, with the vehicle average delay minimization as an object and pedestrian green phase duration as a variable. The algorithm of the model was presented accordingly. Finally, with an empirical study, the paper illustrated that after optimizing the signal with the model, the average vehicle delay was reduced, meanwhile, the duration of pedestrian green phase was extended and the capacity of the crosswalk was improved.

Optimal Number of Active Users for Minimizing Average Data Delivery Delay in People-Centric Urban Sensing

We present a numerical analysis of the optimal number of active mobile users for minimizing average data delivery delay in intelligent people-centric urban sensing, in which context-aware mobile devices act as sensor-data carriers and sensor nodes act as data accumulators within CDMA cellular networks. In the analysis, we compute the optimal number of mobile users for different environmental conditions and then investigate the minimum average data delivery delay for this optimal number of mobile users.

A Comparative Analysis of Server Selection in Content Replication Networks

Server selection plays an essential role in content replication networks, such as peer-to-peer (P2P) and content delivery networks (CDNs). In this paper, we perform an analytical investigation of the strengths and weaknesses of existing server selection policies, based initially on an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i> /1 processor sharing (PS) queueing-theoretic model. We develop a theoretical benchmark to evaluate the performance of two general server selection policies, referred to as EQ_DELAY and EQ_LOAD, which characterize a wide range of existing server selection algorithms. We find that EQ_LOAD achieves an average delay always higher than or equal to that of EQ_DELAY. A key theoretical result of this paper is that in an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> -server system, the worst case ratio between the average delay of EQ_DELAY or EQ_LOAD and the minimal average delay (obtained from the benchmark) is precisely <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> . We constructively show how this worst case scenario can arise in highly heterogeneous systems. This result, when interpreted in the context of selfish routing, means that the price of anarchy in unbounded delay networks depends on the topology, and can potentially be very large. Our analytical findings are extended in asymptotic regimes to the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i> /1 first-come first-serve and multi-class <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i> /1-PS models and supported by simulations run for various arrival and service processes, scheduling disciplines, and workload exhibiting temporal locality. These results indicate that our analysis is applicable to realistic scenarios.

A Study of Tradeoffs in Scheduling Terminal-Area Operations

The terminal area surrounding an airport is an important component of the air transportation system, and efficient terminal-area schedules are essential for accommodating the projected increase in air traffic demand. Aircraft arrival schedules are subject to a variety of operational constraints, such as minimum separation for safety, required arrival time-windows, limited deviation from a first-come first-served sequence, and precedence constraints. There is also a range of objectives associated with multiple stakeholders that could be optimized in these schedules; the associated tradeoffs are evaluated in this paper. A dynamic programming algorithm for determining the minimum cost arrival schedule, given aircraft-dependent delay costs, is presented. The proposed approach makes it possible to determine various tradeoffs in terminal-area operations. A comparison of maximum throughput and minimum average delay schedules shows that the benefit from maximizing throughput could be at the expense of an increase in average delay, and that minimizing delay is the more advantageous of the two objectives in most cases. A comprehensive analysis of the tradeoffs between throughput and fuel costs and throughput and operating costs is conducted, accounting for both the cost of delay (as reported by the airlines) and the cost of speeding up when possible (from models of aircraft performance).

Efficient fair algorithms for message communication

A computer network serves distributed applications by communicating messages between their remote ends. Many such applications desire minimal delay for their messages. Beside this efficiency objective, allocation of the network capacity is also subject to the fairness constraint of not shutting off communication for any individual message. Processor Sharing (PS) is a de facto standard of fairness but provides significantly higher average delay than Shortest Remaining Processing Time (SRPT), which is an optimally efficient but unfair algorithm. In this paper, we explore efficient fair algorithms for message communication where fairness means that no message is delivered later than under PS. First, we introduce a slack system to characterize fair algorithms completely and develop efficient fair algorithms called Pessimistic Fair Sojourn Protocol (PFSP), Optimistic Fair Sojourn Protocol (OFSP), and Shortest Fair Sojourn (SFS). Then, we prove that a fair online algorithm does not assure minimal average delay attainable with fairness. Our analysis also reveals lower bounds on worst-case inefficiency of fair algorithms. We conduct extensive simulations for various distributions of message sizes and arrival times. During either temporary overload or steady-state operation, SFS and other newly proposed fair algorithms support SRPT-like efficiency and consistently provide much smaller average delay than PS.

Throughput-Optimal Scheduling with Low Average Delay for Cellular Broadcast Systems

While a number of scheduling policies achieve the maximum throughput region, the average delay minimization problem for cellular broadcast systems still awaits its complete solution. To this end, we introduce a scheduling policy which decomposes the cross-layer delay optimization problem into two subproblems: allocation of physical resources and user priority management. The first subproblem is translated into a weighted sum rate maximization problem that can be efficiently solved for different channel models. The solution of the second subproblem determines the weight factors in the maximization problem expressing the priorities of users. For the latter subproblem we present a so-called idle state prediction algorithm minimizing our relevant delay measure. Analytical and simulative tools are used to show that the introduced scheduling policy provides both optimal throughput and low delay performance.

Analysis of Hierarchical Diff-EDF Schedulability over Heterogeneous Real-Time Packet Networks

Packet networks are currently enabling the integration of traffic with a wide range of characteristics that extend from video traffic with stringent QoS requirements to the best-effort traffic requiring no guarantees. QoS guarantees can be provided in conventional packet networks by the use of proper packet scheduling algorithms. As a computer revolution, many scheduling algorithms have been proposed to provide different schemes of QoS guarantees with Earliest Deadline First (EDF) as the most popular one. With EDF scheduling, all flows receive the same miss rate regardless of their traffic characteristics and deadlines. This makes the standard EDF algorithm unsuitable for situations in which the different flows have different miss rate requirements since in order to meet all miss rate requirements it is necessary to limit admissions so as to satisfy the flow with the most stringent miss rate requirements. In this paper, we propose a new priority assignment scheduling algorithm, Hierarchal Diff-EDF (Differentiate Earliest Deadline First), which can meet the real-time needs of these applications while continuing to provide best effort service to non-real time traffic. The Hierarchal Diff-EDF features a feedback control mechanism that detects overload conditions and modifies packet priority assignments accordingly. To examine our proposed scheduler model, we introduced our attempt to provide an exact analytical solution. The attempt showed that the solution was apparently very complicated due to the high interdependence between the system queues' service. Hence, the use of simulation techniques seems inevitable. The simulation results showed that the Hierarchical Diff-EDF achieved the minimum packet average delay when compared with both EDF and Diff-EDF schedulers.

Short or Long—Which is Better?

Traffic signal timing would be a trivial undertaking if demand were constant and uniform. Once stochastic factors and demand fluctuation are taken into consideration, however, the optimization of signal timing becomes challenging, if not impossible, even for an isolated, fixed-time signal. To answer the question of whether a longer cycle—more than 150 s, for example—or a shorter one—less than 60 s, for example—is better under fluctuating demand conditions, a probabilistic approach is employed to study minimal average delay by the use of mathematical formulations and Monte Carlo simulations. The idea is to select a cycle length that is small enough to ensure low delay, and hence level of service, but can provide adequate capacity to handle most of the fluctuating demand conditions. A five-step framework is presented for carrying out the analyses, which are demonstrated with a hypothetical example. Subsequent sensitivity analyses, level-of-service assessment, and cycle failure rate estimation were conducted on the basis of random demand and are presented here. Conclusions of the study include the following: (a) introduction of a fluctuating demand level increases the average delay in general, (b) longer cycle lengths do not yield optimal delay results, and (c) with extremely short cycle lengths, delay is usually high because of a lack of capacity and hence frequent cycle failures are guaranteed. A major contribution of this study is a proposed framework for optimizing cycle length under stochastic inter- and intracycle demand levels based on the expectation function of delay. When deployed, this framework can aid traffic engineers in choosing the desirable cycle length for minimal delay or for any reasonable level-of-service requirements.

Optimal Power and Rate Control for Minimal Average Delay: The Single-User Case

Contemporary wireless systems combine aspects of network theory such as scheduling, throughput, and delay as well as information theory aspects such as capacity, coding, and power control. Design of such systems requires joint optimization of both network and physical layers. In this paper, we analyze a single-user communication system composed of a transmitter preceded by a queue used for retransmissions, Gaussian block-fading channel, and a receiver. The system average delay is optimized by using combined power/rate control under average power constraints. Dynamic programming is used for calculating the optimized policies using numerical analysis as well as analytic analysis for asymptotically large buffer size. Asymptotic results are obtained for all combinations of fixed or variable power and rate controls. The most important result extends the water-filling result for systems with average delay constraint

Load-Balanced Anycast with Minimal Average Delay

To achieve load-balance and fault-tolerance, a lot of current Internet applications will need a group of replicated servers spread all over the world. In IPv6, a new communication style, anycast provides the capability of routing packets to the nearest server. An enhanced Qos (Quality of Service) can be satisfied with such kind of computing paradigm. Web service, Distributed Database System and DNS are the most well-known examples. However, we still need further investigations before anycasting can be realized. The anycast routing scheme is one of the most important issues. In this paper, we propose a load-balanced anycast routing scheme based on the WRS (Weighted Random Selection) method. We suggest that the server capability should be propagated along with other fields in the routing tables. An anycast routing algorithm should take into account the network transmission capability as well as the server processing capability for the selection of a target server. Three weight determination strategies are given. We also develop a simple algorithm to calculate the weights of WRS to achieve optimization under both the heavy and the light system traffic environment. Our approach is locally optimized to minimize the average total delay and well balanced for the server load.

Prediction of minor stream delays at a limited priority freeway merge

This paper presents the development and application of a limited priority gap acceptance model to freeway merging. In the limited priority model, drivers in the major stream at a merge area may incur delay in restoring small headways to a larger, sustainable minimum headway between them and the vehicle in front. This allows minor stream drivers to accept smaller gaps. The headway distributions are assumed to be distributed according to Cowan’s M3 model, whose terms were calibrated for this system. Minor stream minimum follow-on time was calibrated, and a realistic range of the critical gap identified. An equation was developed for minimum average minor stream delay. A function was identified to model the relationship between minor stream average delay and degree of saturation. The shape parameter of this function was calibrated using simulated traffic flow data, under three different minor stream arrival pattern regimes. The model provides a useful means of comparing performance, through average minor stream delay, for varying minor and major stream flow rates and minor stream critical gap, under arrival patterns that differ due to traffic control upstream of the on-ramp. Minor stream delay is a particularly useful measure of effectiveness for uncongested freeway merging as it relates directly to the distance required to merge. Observations from the model developed provide physical evidence that minor stream drivers incur lesser delay, or have a better chance of merging quickly, when they arrive at constant intervals as is the case under constant departure ramp metering, than when they arrive in bunches downstream of a signalised intersection, or even a semi-bunched state downstream of an unsignalised intersection.

Traffic characteristics of Go‐back‐N ARQ scheme with selective repeat in intra‐block

AbstractThis paper discusses the traffic characteristics of the Go‐back‐N (GBN)_SR scheme, in which the receiver returns the acknowledge signal blockwise and selective repeat is performed within the retransmission block. First the strict expression for the throughput and the strict expression for the average transmission delay of the blockwise bulk arrival model are derived. Then, the throughput of the GBN_SR scheme and the average transmission delay of the blockwise bulk arrival model are examined by numerical calculation. The average transmission delay of the packetwise bulk arrival model is investigated by simulation. It is shown that the throughput is improved with increasing block size, and that the average transmission delay can be minimized by using the optimal block size, depending on the average packet error rate and the block arrival rate. It is also shown for the packetwise bulk arrival model that there exists a block size which minimizes the average transmission delay, and that the minimum average transmission delay is smaller than in the blockwise bulk arrival model. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(3): 9–16, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ ecja.10052

Enhanced Genetic Algorithm for Signal-Timing Optimization of Oversaturated Intersections

Enhancements were provided to a previously developed genetic algorithm (GA) for traffic signal optimization for oversaturated traffic conditions. A broader range of optimization strategies was provided to include modified delay minimization with a penalty function and throughput maximization. These were added to the initial delay minimization strategy and were further extended to cover all operating conditions. The enhanced program was evaluated at different intersection spacings. The optimization strategies were evaluated and compared with their counterpart from TRANSYT-7F, version 8.1. A microscopic stochastic simulation program, CORSIM, was used as the unbiased evaluator. Hypothesis testing indicated that the GA-based program with average delay minimization produced a superior signal-timing plan compared with those produced by other GA strategies and the TRANSYT-7F program in terms of queue time. It was also found from the experiments that TRANSYT-7F tended to select longer cycle lengths than the GA program to reduce random plus oversaturation delay.