Efficiency-driven selection of bandwidth request mechanism in broadband wireless access networks

To avoid collisions in WiMAX networks, the connections in Subscriber Stations (SSs) use a request–grant process to acquire transmission resources from the Base Station (BS). In accordance with the IEEE 802.16 standard, the request–grant process is accomplished using either a unicast polling method or a contention request method. In WiMAX systems, the number of bandwidth-request (BR) slots per frame is limited. Thus, to enhance the network performance, the BR slots must be used in the most efficient manner possible. In practical WiMAX systems, the offered network load varies over time, and thus the strict use of either the unicast polling method or the contention request method results in a poor utilization efficiency of the BR slots. Accordingly, the present study proposes a scheme designated as Efficiency-Driven Selection of Bandwidth Request (EDSBR), in which the request–grant mechanism is adjusted dynamically on a frame-by-frame basis in accordance with the network conditions. The performance of the proposed scheme is evaluated by simulations. The results show that EDSBR achieves a more efficient utilization of the BR slots than the unicast polling scheme or the contention request scheme, and therefore yields an improved network performance.     

On tandem blocking queues with a common retrial queue

We consider systems of tandem blocking queues having a common retrial queue. The model represents dynamics of short TCP transfers in the Internet. Analytical results are available only for a specific example with two queues in tandem. We propose approximation procedures involving simple analytic expressions, based on mean value analysis (MVA) and on fixed point approach (FPA). The mean sojourn time of a job in the system and the mean number of visits to the orbit queue are estimated by the MVA which needs as an input the fractions of blocked jobs in the primary queues. The fractions of blocked jobs are estimated by FPA. Using a benchmark example of the system with two primary queues, we conclude that the approximation works well in the light traffic regime. We note that our approach becomes exact if the blocking probabilities are fixed. Finally, we consider two optimization problems regarding minimizing mean total sojourn time of a job in the system: (i) finding the best order of queues and (ii) allocating a given capacity among the primary queues.     

An adaptive random access strategy for multi-channel relaying networks

In this paper, an adaptive random access strategy is presented for multi-channel relaying networks to address the issue of random access of the non-real-time (NRT) services. In the proposed scheme, NRT services access the base station (BS) by first accessing the nearest relay node (RN). When collision occurs, for the sake of fast and efficient access, the user will begin a frequency domain backoff rather than randomly retry in time domain. A remarkable feature of this scheme is that the RN will adaptively determine the maximum allowed frequency backoff window at each access period. This is achieved according to the new arrival rate as well as the number of available access channels. Moreover, to alleviate the interference caused by sub-channel reuse among RNs, a fractional frequency reuse scheme is also considered. The analysis and numerical results demonstrate that our scheme achieves higher throughput, lower collision probability and lower access delay than conventional slotted Aloha as well as the scheme without frequency backoff window adaptation.     

Dynamic frame length channel assignment in wireless multihop ad hoc networks

A good channel assignment scheme in a multihop ad hoc network should not only guarantee successful data transmissions without collisions but also enhance the channel spatial reuse to maximize the system throughput. From the channel assignment schemes in time division multiple access (TDMA) slot assignment protocols developed in previous studies, we have found that these protocols do not have a convenient frame length shrink scheme after the expansion of the frame length. As the network size grows, the frame length expands quickly, particularly when we set the frame length as a power of two. A very long frame may result in poor channel utilization when it contains many unused slots. In this paper, we propose a dynamic frame length expansion and recovery method called dynamic frame length channel assignment (DFLCA). This strategy is designed to make better use of the available channels by taking advantage of the spatial reuse concept. In DFLCA, the increase in the spatial reuse is achieved by adding certain amount of control overhead. We show that the bandwidth saved due to the channel spatial reuse is larger than the additional bandwidth spent on the control overhead.     

WiMAX系统中的一种最优化准入控制方法

WiMAX正逐渐成为"最后一公里"无线宽带接入的主要解决方案,在WiMAX系统中,准入控制(AC)起着很重要的作用。目前关于AC策略最优化的研究主要考虑服务提供商的利益,即收益最大化,而很少考虑用户的要求。AC策略在收益最大化的基础上增加了关系用户利益的两个约束条件,即阻断率和带宽利用率。实验结果表明,该方法在收益、阻断率、带宽利用率三方面都有良好的性能,兼顾了服务提供商和用户的利益。     

A data collection protocol for real-time sensor applications

The nature of many sensor applications as well as continuously changing sensor data often imposes real-time requirements on wireless sensor network protocols. Due to numerous design constraints, such as limited bandwidth, memory and energy of sensor platforms, and packet collisions that can potentially lead to an unbounded number of retransmissions, timeliness techniques designed for real-time systems and real-time databases cannot be applied directly to wireless sensor networks. Our objective is to design a protocol for sensor applications that require periodic collection of raw data reports from the entire network in a timely manner. We formulate the problem as a graph coloring problem. We then present TIGRA (Timely Sensor Data Collection using Distributed Graph Coloring) — a distributed heuristic for graph coloring that takes into account application semantics and special characteristics of sensor networks. TIGRA ensures that no interference occurs and spatial channel reuse is maximized by assigning a specific time slot for each node. Although the end-to-end delay incurred by sensor data collection largely depends on a specific topology, platform, and application, TIGRA provides a transmission schedule that guarantees a deterministic delay on sensor data collection.     

Capacity estimation algorithm for simultaneous support of multi-class traffic services in Mobile WiMAX

Mobile profile of WiMAX technology, based on IEEE 802.16e standard, is due to support multi-application services, while benefiting a wide range of implementation flexibilities. The current document presents a simple capacity estimation methodology for WiMAX systems. In this paper, the term capacity concerns with the maximum number of simultaneous multi-service users that each specific Mobile WiMAX access point can jointly support. A dynamic PHY+MAC overhead removal method is formulated to achieve an improved system goodput with respect to users distribution and multi-burst construction strategy. A service delivery model is presented that investigates the QoS requirements of the services to be supported, along with elaborating an application profile as a consistent input for capacitating and dimensioning studies. The minimum resource consumption calculation for the joint-application users is derived at each given time. An incremental algorithm compares the optimal available resources with minimum service demand for each number of users to arrive at maximum system capacity. Using the proposed algorithm, different simulation scenarios are studied based on the most used WiMAX implementation parameters in practice. The simulation results prove the significant roll of overhead calculation in performance evaluation studies. Furthermore, these results can be used for network planning and dimensioning purposes, as well as providing reference measures for scheduling performance analysis and detailed simulations.     

Simulation model reuse: definitions, benefits and obstacles

The term ‘simulation model reuse’ can be taken to mean various things from the reuse of small portions of code, through component reuse, to the reuse of complete models. On a more abstract level, component design, model design and modelling knowledge are prime candidates for reuse. The reuse of simulation models is especially appealing, based on the intuitive argument that it should reduce the time and cost for model development. In a discussion with four simulation modelling experts, however, a number of issues were raised that mean these benefits may not be obtainable. These issues include the motivation to develop reusable models, the validity and credibility of models to be reused, and the cost and time for familiarisation. An alternative simulation methodology was proposed, that may lend itself better to model reuse.     

Dynamic hybrid automatic repeat request (DHARQ) for WiMAX – Mobile multihop relay using adaptive power control

Wireless multihop network is currently attracting much attention as a new wireless broadband access technology due to numerous benefits. This work proposes a power control scheme for WiMAX multihop relay system. In contrast to existing power control and optimization approaches, our proposed method uses an adaptive Channel Quality Measurement for a relay station to reduce interferences to other mobile station (MS) or relay station (RS) within the same cell and hence increase the number of hops per link and consequently maximize the spatial reuse. The proposed power control is applied to a new dynamic HARQ algorithm for adaptive channel quality enhancement. Simulation results have indicated that the proposed approach achieves superior BER/PER performance enhancement in comparison to previous related works.     

Flow level performance analysis of a multi-service system supporting elastic and adaptive services

We consider a multi-rate loss system where two types of non-peak allocated traffic flows receive service. Both elastic and adaptive flows are associated with a peak- and a minimum bandwidth requirement and they tolerate bandwidth compression while in service. The holding time of elastic flows depends on their received throughput, while the holding time of the adaptive flows is insensitive to that. Unfortunately, while this system is Markovian under quite non-restrictive assumptions (that are often used in the literature), it is not reversible. We propose a method whereby the approximation of this system by a reversible system is possible. We derive recursive formulas for determining the occupancy distribution and the mean number of flows in the system. By using a continuous approximation of the discrete state space, we also derive an explicit formula for the average throughputs that is independent of the size of the state space. The recursive formulas and the continuous approximation together provide a powerful tool for the performance analysis of this quite general system in the sense that they allow the calculation of the blocking probabilities and the mean throughputs in medium and large systems as well.     

e-MAC:一种面向Ad Hoc网络的高吞吐量MAC协议

解决ad hoc网络中隐藏节点问题、暴露节点问题的最终目的是减少节点间的冲突,提高网络空间复用率,从而提高网络吞吐量.现有MAC协议在解决隐藏节点问题时着重于彻底消除网络中的隐藏节点,忽略了网络空间复用率,即使能够彻底解决隐藏节点问题,也不能有效提高网络吞吐量.同样,现有协议在解决暴露节点问题时着重于如何允许暴露节点并行发送数据,忽略了暴露节点接收数据的问题,也影响了网络空间复用率.提出了一种高效的MAC协议e-MAC,协议采用两种方法提高网络空间复用率:首先,协议中接收节点根据接收到发送节点的信号强度动态调整忙音发射功率,使忙音恰好覆盖所有的隐藏节点,在彻底解决隐藏节点问题的同时,提高网络空间复用率;其次,隐藏节点接收到RTS消息后,通过判断RTS消息信号强度与信道中干涉信号的强度之比来决定是否接收数据,允许满足信噪比要求的接收节点接收数据,进一步提高网络空间复用率.仿真结果验证了协议的有效性,在任意拓扑结构下,e-MAC协议的平均吞吐量比DUCHA(dual channel access)协议高87%.     

An embedded fuzzy expert system for adaptive WFQ scheduling of IEEE 802.16 networks

WiMAX is a futuristic technology which provides simultaneous support for web, video, and voice applications. WiMAX networks are best suitable to real time traffic however the quantity of non real time and best effort traffic cannot be neglected. Distribution of resources in such heterogeneous applications is therefore a challenging task. There are many schedulers available for WiMAX but adaptive and adequate schedulers are still in growing stage of development. This paper introduces a novel method using which a system is developed based on concepts of fuzzy logic to schedule traffic in WiMAX networks. The proposed fuzzy expert system simplifies fair allocation of resources to real as well as non real time traffic. The implementation is based on changing the weights of the queues serving real and non- real time traffic adaptively. New weights will be calculated for each bandwidth request made to base station and these weights will in turn decide amount of bandwidth allocated to different traffic classes. The weights are calculated based on three parameters that are amount of real time and non real time traffic in queues, change in throughput requirement for non real time flows and latency requirement of real time input data. Results obtained by virtue of simulations justify the significance of the proposed method.     

UMTS与WiMAX之间的网络切换技术研究

通用移动通信系统（UMTS）为移动用户提供了较广阔的覆盖范围,而无线广域网WiMAX能在局部热点地区提供较高的接入带宽。两种网络的显著优势能够相互结合,为处于异构网络覆盖的地区提供无缝连接。在分析WiMAX与UMTS 的融合结构和切换的基础上,提出了一种以接收信号强度（RSS）、迟滞电平和迟滞时间的结合作为切换判决基准的切换策略,并且在判决过程中,对实时与非实时业务进行了区分。仿真结果表明,该切换决策能有效减少“乒乓效应”和时延。     

Advanced hashing schemes for packet forwarding using set associative memory architectures

Building a high performance IP packet forwarding (PF) engine remains a challenge due to increasingly stringent throughput requirements and the growing size of IP forwarding tables. The router has to match the incoming packet’s IP address against all entries in the forwarding table. The matching process has to be done at increasingly higher wire speed; hence, scalability and low power consumption are critical for PF engines.Various hash table based schemes have been considered for use in PF engines. Set associative memory can be used for hardware implementations of hash tables with the property that each bucket of a hash table can be searched in a single memory cycle. However, the classic hashing downsides, such as collisions and worst case memory access time have to be dealt with. While open addressing hash tables, in general, provide good average case search performance, their memory utilization and worst case performance can degrade quickly due to collisions (that lead to bucket overflows).The two standard solutions to the overflow problem are either to use predefined probing (e.g., linear or quadratic probing) or to use multiple hash functions. This work presents two new simple hash schemes that extend both aforementioned solutions to tackle the overflow problem efficiently. The first scheme is a hash probing scheme that is called Content-based HAsh Probing (CHAP). As the name suggests, CHAP, based on the content of the hash table, avoids the classical side effects of predefined hash probing methods (i.e., primary and secondary clustering phenomena) and at the same time reduces the overflow. The second scheme, called Progressive Hashing (PH), is a general multiple hash scheme that reduces the overflow as well. The basic idea of PH is to split the prefixes into groups where each group is assigned one hash function, then reuse some hash functions in a progressive fashion to reduce the overflow. Both schemes are amenable to high-performance hardware implementations with low overflow and constant worst-case memory access time. We show by experimenting with real IP lookup tables and synthetic traces that both schemes outperform other existing hashing schemes.     

Fast Penetration Volume for Rigid Bodies

Handling collisions among a large number of bodies can be a performance bottleneck in video games and many other real‐time applications. We present a new framework for detecting and resolving collisions using the penetration volume as an interpenetration measure. Given two non‐convex polyhedral bodies, a new sampling paradigm locates their near‐contact configurations in advance, and stores associated contact information in a compact database. At runtime, we retrieve a given configuration's nearest neighbors. By taking advantage of the penetration volume's continuity, cheap geometric methods can use the neighbors to estimate contact information as well as a translational gradient. This results in an extremely fast, geometry‐independent, and trivially parallelizable computation, which constitutes the first global volume‐based collision resolution. When processing multiple collisions simultaneously on a 4‐core processor, the average running cost is as low as 5 μs. Furthermore, no additional proximity or contact‐regions queries are required. These results are orders of magnitude faster than previous penetration volume approaches.     

On collisions with unlimited energies in the vicinity of Kerr and Schwarzschild black hole horizons

Two-particle collisions close to the horizon of a rotating non-extremal Kerr black hole and a Schwarzschild black hole are analyzed. For the case of multiple collisions, it is shown that high energy in the center-of-mass frame occurs due to a great relative velocity of two particles and a large Lorentz factor. We analyze the dependence of the relative velocity on the distance to the horizon and calculate the motion time from the point in the accretion disc to the point of scattering with large energy as well as the time of back motion to the Earth. It is shown that they have a reasonable order.     

An efficient scheduling algorithm for radio resource reuse in IEEE 802.16j multi-hop relay networks

The IEEE 802.16j standard for WiMAX introduced the concept of relay station in order to increase the service area and decrease the deployment cost of the network. In this paper, we have proposed an efficient scheduling scheme for IEEE 802.16j networks, which maximizes the number of connections that are scheduled in a particular time slot. The proposed scheme schedules the connections based on their priority, which is decided by the quality of service (QoS) requirement of the connections. A selected connection can be scheduled in the current timeslot only if it is not having interference with any of the already scheduled connections. Our algorithm considers scheduling of lower priority connections even if any higher priority connection is not scheduled due to interference. Thus, the delay for lower priority connections is reduced without increasing the delay for higher priority connections. In addition, our algorithm achieves higher frame utilization and higher system throughput by reducing the length of the schedule.     

A performance study of uplink scheduling algorithms in point-to-multipoint WiMAX networks

The IEEE 802.16 standard defines the specifications for medium access control (MAC) and physical (PHY) layers of WiMAX networks. A critical part of the MAC layer specification is packet scheduling, which resolves contention for bandwidth and determines the transmission order of users. Evaluating the performance packet scheduling algorithms is of utmost importance towards realizing large-scale WiMAX deployment. In this paper, we conduct a comprehensive performance study of scheduling algorithms in point-to-multipoint mode of OFDM-based WiMAX networks. We first make a classification of WiMAX scheduling algorithms, then simulate a representative number of algorithms in each class taking into account that vital characteristics of the IEEE 802.16 MAC layer and OFDM physical layer. We evaluate the algorithms with respect to their abilities to support multiple classes of service, providing quality of service (QoS) guarantees, fairness amongst service classes and bandwidth utilization. To the best of our knowledge, no such comprehensive performance study has been reported in the literature. Simulation results indicate that none of the current algorithms is capable of effectively supporting all WiMAX classes of service. We demonstrate that an efficient, fair and robust scheduler for WiMAX is still an open research area. We conclude our study by making recommendations that can be used by WiMax protocol designers.     

A new blocking problem from Java-based schedulers

We consider a form of blocking, which is typical in client–server systems including those implemented under the Enterprise JavaBean (EJB) specification. The novel feature is that tasks must wait for one of a number of parallel queues to clear its outstanding work. Thus, blocking time is the minimum of sojourn times at the parallel queues. Under certain simplifying assumptions, we solve this model for the probability distribution of blocking time and obtain a simple formula for its mean value. We then use this result in an aggregate server model of a larger queueing network in which further non-standard techniques are included to represent this form of blocking. We compare our approximate results against simulation data, obtaining good agreement for both system throughput and queue length probability distributions at equilibrium.     

A case study comparing defect profiles of a reused framework and of applications reusing it

The benefits of software reuse have been studied for many years. Several previous studies have observed that reused software has a lower defect density than newly built software. However, few studies have investigated empirically the reasons for this phenomenon. To date, we have only the common sense observation that as software is reused over time, the fixed defects will accumulate and will result in high-quality software. This paper reports on an industrial case study in a large Norwegian Oil and Gas company, involving a reused Java class framework and two applications that use that framework. We analyzed all trouble reports from the use of the framework and the applications according to the Orthogonal Defect Classification (ODC), followed by a qualitative Root Cause Analysis (RCA). The results reveal that the framework has a much lower defect density in total than one application and a slightly higher defect density than the other. In addition, the defect densities of the most severe defects of the reused framework are similar to those of the applications that are reusing it. The results of the ODC and RCA analyses reveal that systematic reuse (i.e. clearly defined and stable requirements, better design, hesitance to change, and solid testing) lead to lower defect densities of the functional-type defects in the reused framework than in applications that are reusing it. However, the different “nature” of the framework and the applications (e.g. interaction with other software, number and complexity of business logic, and functionality of the software) may confound the causal relationship between systematic reuse and the lower defect density of the reused software. Using the results of the study as a basis, we present an improved overall cause–effect model between systematic reuse and lower defect density that will facilitate further studies and implementations of software reuse.