保证QoS的IQ交换机调度算法

输入队列 ( IQ)交换机在采用虚输出队列 ( VOQ)技术基础上 ,能够提供低成本的高速交换机 ,但在一般调度算法下 ,IQ交换机缺乏保证 Qo S的能力。本文在 Birkhoff和 Von Neumann研究的基础上运用随机过程理论和网络计算理论提出一种预留带宽的调度算法 ,并分析了相关的延迟上界和 VOQ需要的内存情况。     

保证QoS的IQ交换机调度算法

输入队列(IQ)交换机在采用虚输出队列(VOQ)技术基础上，能够提供低成本的高速交换机，但在一般调度算法下，IQ交换机缺乏保证QoS的能力。本文在Birkhoff和Von Neumann研究的基础上运用随机过程理论和网络计算理论提出一种预留带宽的调度算法，并分析了相关的延迟上界和VOQ需要的内存情况。     

一种新型的负载均衡-交叉点缓冲交换结构

针对混合输入-交叉点队列(CICQ)交换结构受限于"流控通信延时"、"需要2倍内部加速仿真输出队列(OQ)交换"以及单纯交叉点缓冲(CQ)存在"非均衡流量模式下吞吐量性能不足"等问题,本文提出一种新型的"负载均衡交叉点缓冲交换结构".采用固定模式时隙轮转匹配进行负载均衡处理,将到达输入端口的非均衡流量转化为近似均衡流量并且平均分配到同一输出端口对应的交叉缓冲中,从而可以利用较小的交叉点缓冲来模拟输出队列调度,简化调度过程并且提高吞吐量.理论分析证明了这种新结构的稳定性以及模拟输出队列交换的能力.同时仿真表明,采用该交换结构可以在不需要内部加速的条件下获得相当于输出队列交换的性能,并且有效地解决了交叉点缓冲队列非均衡流量性能不足的问题.     

带VOQ的输入队列交换网络中的分组调度算法研究

交换技术已经成为高速路由器的核心技术。本文基于目前高速交换技术所采用的主要体系结构,带有虚拟输出缓冲队列(VOQ)的输入队列交换结构,分析已经存在的各种调度算法的性能,并设计基于遗传算法的调度策略,提供IP数据网络的QoS对吞吐量和抖动的保障。     

基于SRAM的核心路由器交换矩阵输入端口设计

交换矩阵是核心路由器的重要组成部分,为了避免来自不同输入端口的信元同时发往同一个输出端口,需要在输入端口设置缓冲区,即输入排队交换结构。基于静态随机存储器完成了交换矩阵输入端口虚拟输出队列（VOQ）的设计,该设计可以降低核心路由器交换芯片的面积,提高输入端口缓冲区信元的响应速率,并通过DE-115开发板完成对设计的验证。     

CICQ交换结构中共享缓存的研究与应用

针对带缓存的的交叉开关(Combined Input and Crosspoint Buffered Queuing,CICQ)交换结构在多端口多优先级而引起的存储规模和面积开销增大的问题,提出了一种基于共享缓存思想的CICQ交换结构。它使用虚拟输入排队(VOQ)的方式,支持多优先级流控,并通过动态分配虚拟输出队列的方式设计队列管理结构,从而降低复杂度,减少对缓存区容量的需求。通过对各模块进行VHDL代码设计和使用Modelsim进行仿真,结果表明该结构能够实现交换结构数据的缓存。     

星载Clos网络的全分布式容错调度算法

针对星载交换结构受空间辐射影响造成的可靠性严重下降问题,该文提出了一种支持全分布式调度的三级Clos网络及其全分布式容错(Fully Distributed Fault Tolerant, FDFT)调度算法,以提高星载交换结构在交叉点故障下的容错能力。该Clos网络的中间级和输出级采用联合输入交叉点队列,以支持Clos网络和交换单元内部的全分布式调度。FDFT采用一种分布式故障检测算法获得交叉点故障信息。基于对交叉点故障影响范围的分析,FDFT在输入级采用一种容错信元分发算法,实现无故障路径的负载均衡。理论分析证明,当任一输入/输出级交换单元故障个数不超过(m-n)或所有中间级交换单元故障个数不超过(m-n)时,其中m, n分别为输入级交换单元输入、输出端口数,FDFT能够达到100%吞吐率。仿真结果进一步验证,故障随机发生情况下,FDFT能够抵抗比故障任意发生情况下更多的故障,且在不同的业务场景下具有良好的吞吐率和时延性能。     

带VOQ的输入队列交换网络中的分组调度算法研究

交换技术已经成为高速路由器的核心技术。本文基于目前高速交换技术所采用的主要体系结构，带有虚拟输出缓冲队列（VOQ）的输入队列交换结构，分析已经存在的各种调度算法的性能，并设计基于遗传算法的调度策略，提供IP数据网络的QoS对时延抖动的保障。     

基于队列长度和时延约束的异步OPS权重调度

分组调度在异步光分组交换(OPS)节点中起着重要作用,一种智能的调度算法能减小分组的丢失率,增加节点的吞吐量,从而提高服务质量.文章提出了一种基于队列长度和时延约束的解决队列中分组"饿死"和时延问题的权重调度算法.算法通过计算队列长度和分组在队列中等待时间确定调度的权重值,以减小分组在核心节点处的资源竞争和等待时延,调度器计算每个输入端口的N×K个虚拟输出队列的权重值并调度最大权重值队列到可用的输出信道上.分析和仿真结果表明所提的权重调度算法具有高吞吐量和低分组丢失率特点.     

一种用于分组调度的遗传模拟退火算法

分组调度已成为高速IP路由器中的关键技术之一。文章基于目前高速路由交换技术所采用的主体结构,带有虚拟输出队列（Virtual-output—Queue,VOQ）的输入队列交换结构,提出了一种遗传模拟退火算法,并将该算法应用于分组调度问题的求解之中。通过遗传模拟退火算法和传统遗传算法的仿真结果可以看出,遗传模拟退火算法具有良好的鲁棒性和收敛性。     

An evolution to crossbar switches with virtual output queuing and buffered cross points

Input queued (IQ) switch architectures with virtual output queues (VOQ) scale up to very high speeds and have been a subject of intense research in the past decade. VOQ IQ switches require switch matrix scheduling algorithms to match input ports to out ports. In this tutorial article, we present an overview of switch matrix scheduling for VOQ IQ switches with crossbar switch fabrics. We then describe what we believe will be the next generation of high-speed crossbar switches: the evolution of IQ switches to combined input and crossbar queued (CICQ) switches. With the continued increase in density of VLSI, sufficient buffering at crossbar cross points for one cell or packet has become feasible to implement. We show how CICQ switches have simple schedulers and result in lower delay than IQ switches. Both IQ and CICQ switches have unstable regions. We show how a threshold and bursting technique can feasibly achieve stability. We also show how CICQ switches are better suited (than IQ switches) for switching of variable-length packets such as IP packets. Many challenges remain in IQ and CICQ switches. In particular, the inclusion of QoS scheduling methods that are currently only suitable for output queued switches is a major open problem.     

Asymptotic Performance Limits of Switches With Buffered Crossbars Supporting Multicast Traffic

Input queued (IQ) switches exploiting buffered crossbars (CICQ switches) are widely considered very promising architectures that outperform IQ switches with bufferless switching fabrics both in terms of architectural scalability and performance. Indeed the problem of scheduling packets for transfer through the switching fabric is significantly simplified by the presence of internal buffers in the crossbar, which makes possible the adoption of efficient, simple and fully distributed scheduling algorithms. This paper studies the throughput performance of CICQ switches supporting multicast traffic, showing that, similarly to IQ architectures, also CICQ switches with arbitrarily large number of ports may suffer of significant throughput degradation under ldquopathologicalrdquo multicast traffic patterns. Despite the asymptotic nature of these results, the authors believe that they can contribute to a deeper understanding of the behavior of CICQ architectures supporting multicast traffic.     

On the stability of local scheduling policies in networks of packet switches with input queues

A significant research effort has been devoted to the design of simple and efficient scheduling policies for input queued (IQ) and combined input-output queued (CIOQ) packet switches. As a result, a number of switch control algorithms have been proposed. Among these, scheduling policies based on maximum weight matching (MWM) were identified as optimal, in the sense that they were proved to achieve 100% throughput under any admissible arrival process satisfying the strong law of large number. On the contrary, it has been shown that the usual MWM policies fail to guarantee 100% throughput in networks of interconnected IQ/CIOQ switches. Hence, new policies suited for networks of interconnected switches were proposed and proved to achieve 100% throughput. All of these new policies require coordination and cooperation among different switches. We identify scheduling policies that require no coordination among switches (and are, thus, said to be local), and that guarantee 100% throughput in a network of IQ/CIOQ switches. The only assumptions on the input traffic pattern are that it is stationary, satisfies the strong law of large numbers and does not oversubscribe any link in the network.     

Captured-frame eligibility and round-robin matching for input-queued packet switches

A variety of matching schemes for input-queued (IQ) switches that deliver high throughput under traffic with uniform distributions has been proposed. However, there is a need of matching schemes that provide high throughput under several admissible traffic patterns, including those with nonuniform distributions, while keeping implementation complexity low. In this letter, first, we introduce the captured frame concept for matching schemes in IQ switches. Second, we propose a round-robin based matching scheme, uFORM, which uses the proposed concept for cell matching eligibility. We show via simulation that our matching scheme delivers high throughput under several nonuniform traffic patterns, and retains the high performance under uniform traffic that round-robin matching schemes are known to offer.     

An Efficient Packet Scheduling Algorithm With Deadline Guarantees for Input-Queued Switches

Input-queued (IQ) switches overcome the scalability problem suffered by output-queued switches. In order to provide differential quality of services (QoS), we need to efficiently schedule a set of incoming packets so that every packet can be transferred to its destined output port before its deadline. If no such a schedule exists, we wish to find one that allows a maximum number of packets to meet their deadlines. Recently, this problem has been proved to be NP-complete if three or more distinct deadlines (classes) are present in the set. In this paper, we propose a novel algorithm named Flow-based Iterative Packet Scheduling (FIPS) for this scheduling problem. A key component in FIPS is a non-trivial algorithm that solves the problem for the case where two classes are present in the packet set. By repeatedly applying the algorithm for two classes, we solve the general case of an arbitrary number of classes more efficiently. Applying FIPS to a frame-based model effectively achieves differential QoS provision in IQ switches. Using simulations, we have compared FIPS performance with five well-known existing heuristic algorithms including Earliest-Deadline-First (EDF), Minimum-Laxity-First (MLF) and their variants. The simulation results demonstrate that our new algorithm solves the deadline guaranteed packet scheduling problem with a much higher success rate and a much lower packet drop ratio than all other algorithms     

一种公平输入排队调度算法

输入排队交换结构以其良好的可扩展性被越来越多的高速交换机和路由器所采用。当前的调度算法大都以牺牲公平性来换取最大的吞吐量。但随着对QoS支持的要求增强,适用于输入排队交换结构的高效、公平的调度算法成为迫切需要解决的问题。该文提出了一种具有公平性保证的基于虚服务量的公平调度算法。理论分析和计算机仿真都表明算法在信元时延和公平性方面都能提供较好的保证。算法还具有与iSLIP相同的较低通信开销,以及和iLQF相同的算法复杂度。因此,算法具有较好的实用性。     

Cell Switching Versus Packet Switching in Input-Queued Switches

Input Queued (IQ) switches have been well studied in the past two decades by researchers. The main problem concerning IQ switches is scheduling the switching fabric in order to transfer packets from input ports to output ports. Scheduling is relatively easier when all packets are of the same size. However, in practice, packets are of variable length. In the current implementation of switches, variable length packets are segmented into fixed length packets—also knowns as cells—for the purpose of scheduling. However, such cell-based switching comes with some significant disadvantages: (a) loss of bandwidth due to the existence of incomplete cells; and (b) additional overhead of segmentation of packets and re-assembly of cells. This is a strong motivation to study packet-based scheduling, i.e., scheduling the transfer of packets without segmenting them. The problem of packet scheduling was first considered by Marsan They showed that under any admissible Bernoulli IID (independent and identically distributed) arrival traffic, a simple modification of the Maximum Weight Matching (MWM) algorithm achieves 100% throughput. In this paper, we first show that no work-conserving (i.e., maximal) packet-based algorithm is stable for arbitrary admissible arrival processes. Thus, the results of Marsan are strongly dependent on the arrival distribution. Next, we propose a new class of “waiting” algorithms. We show that the “waiting”-MWM algorithm is stable for any admissible traffic using the fluid limit technique. We would like to note that the algorithms presented in this paper are distribution independent or universal. The algorithms and proof methods of this paper may be useful in the context of other scheduling problems.     

Compensation of IQ mismatch in optical PDM–OFDM coherent receivers

The performance enhancements based on Gram–Schmidt orthogonalization procedure (GSOP) for compensating IQ mismatch in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems are investigated. We analytically explore IQ mismatch in optical OFDM systems and investigate the impacts of phase and amplitude IQ mismatch on required optical signal-to-noise ratio (OSNR) for the different values of data mapping and polarization multiplexing. The impacts of analog-to-digital converter (ADC) resolution and the number of samples in GSOP are also evaluated. The results show that the GSOP operation efficiently compensate IQ mismatch induced performance degradations regardless of the amount of IQ phase mismatch, density of data mapping, and polarization multiplexing.     

MIMO OFDM Receivers for Systems With IQ Imbalances

Orthogonal frequency division multiplexing (OFDM) is a widely recognized modulation scheme for high data rate communications. However, the implementation of OFDM-based systems suffers from in-phase and quadrature-phase (IQ) imbalances in the front-end analog processing. Such imbalances are caused by the analog processing of the received radio frequency (RF) signal, and they cannot be efficiently or entirely eliminated in the analog domain. The resulting IQ distortion limits the achievable operating SNR at the receiver and, consequently, the achievable data rates. The issue of IQ imbalances is even more severe at higher SNR and higher carrier frequencies. In this paper, the effect of IQ imbalances on multi-input multioutput (MIMO) OFDM systems is studied, and a framework for combating such distortions through digital signal processing is developed. An input–output relation governing MIMO OFDM systems is derived. The framework is used to design receiver algorithms with compensation for IQ imbalances. It is shown that the complexity of the system at the receiver grows from dimension$(n_Rtimes n_T)$for ideal IQ branches to$(2n_Rtimes 2n_T)$in the presence of IQ imbalances. However, by exploiting the structure of space-time block codes along with the distortion models, one can obtain efficient receivers that are robust to IQ imbalances. Simulation results show significant improvement in the achievable BER of the proposed MIMO receivers for space-time block-coded OFDM systems in the presence of IQ imbalances.     

Effect of IQ Mismatch Compensation in an Optical Coherent OFDM Receiver

The performance enhancement using the Gram–Schmidt orthogonalization procedure (GSOP) for compensating transmitter (Tx) and receiver (Rx) in-phase/quadrature (IQ) mismatch in the coherent optical orthogonal frequency-division-multiplexing system is investigated. The analytic result containing frequency offset is presented to describe the different behavior of Tx and Rx IQ mismatch. Bit-error-rate curves measured at back-to-back and 1040-km transmission over single-mode fiber indicate that the GSOP is more efficient to compensate of Rx IQ mismatch than Tx IQ mismatch.