含优先级机制的ATM Mux中的信元丢失率分析

含丢失优先级机制的ATM Mux中的信元丢失率分析是ATM网络流量控制研究中的一个重要问题.本文用两状态的MMDP近似实际输入过程,并采用流体流技术对不同优先级信元在ATM Mux中的丢失率进行了分析,得到了各类信元的丢失率与缓冲容量之间关系的解析式.模拟文验表明该方法足够精确,完全可以满足实际应用的需要.     

Estimates of loss probabilities for delay-sensitive traffic in ATMnetworks

Simple estimates of loss probabilities for heterogeneous delay-sensitive traffic in ATM networks are presented. Cells of different connections can have different loss priorities. Numerical results show that the estimates, which are derived from the bufferless fluid flow model, are close to the actual cell loss probabilities     

An ATM queue manager handling multiple delay and loss priorities

The asynchronous transfer mode (ATM) technique has been widely accepted as a flexible and effective scheme to transport various traffic over the future broadband network. To fully utilize network resources while still providing satisfactory quality of service (QOS) to all network users, prioritizing the user's traffic according to their service requirements becomes necessary. During call setup or service provisioning, each service can be assigned a service class determined by a delay priority and a loss priority. A queue manager in ATM network nodes will schedule ATM cells departing and discarding sequence based on their delay and loss priorities. Most queue management schemes proposed so far only consider either one of these two priority types. The queue manager handles multiple delay and loss priorities simultaneously. Moreover, a cell discarding strategy, called push-out, that allows the buffer to be completely shared by all service classes, has been adopted in the queue manager. We propose a practical architecture to implement the queue manager by using available VLSI sequencer chips     

An efficient solution method for Markov models of ATM links withloss priorities

Algorithms for solving for the cell loss rates in an asynchronous transfer mode (ATM) network using cell loss priorities are presented. With the loss priority scheme, cells of low-priority classes are accepted only if the instantaneous buffer queue length at the cell arrival epoch is below a given threshold. The input is modeled by Markov-modulated Bernoulli processes. The effect of the loss priority scheme on data, voice, and video traffic is investigated     

Packet reassembly during cell loss

Asynchronous transfer mode (ATM) is a packet switched data transport system based on short, fixed length cells. Each cell carries a virtual channel indicator (VCI) and virtual path indicator (VPI) in its header. Essential to the services offered by the ATM networks is the ATM adaptation layer (AAL), an ITU-TSS defined layer that adapts the cell-based ATM physical layer to packet, datagram, or bit-stream-oriented higher layers. Failure modes causing cell loss along a virtual connection are examined, and the ways AALs cope are analyzed. The sources of cell loss and their effects on AAL3/4 or AAL5 type of service are described. The usefulness of the ability of AAL3/4 to pass fragments of corrupted data up to higher layer protocols is discussed, and the implementation of selective cell discarding within switching nodes is considered, and the limitations imposed by each AAL are examined     

Neural network congestion controller in prioritised ATM switch

A neural network (NN) scheme is proposed for congestion control in an ATM switch with time priorities. It is shown that in a prioritised switch it is necessary to monitor the buffer to be controlled as well as buffers with higher priorities. Furthermore, it is shown that the NN scheme in a time prioritised switch gives lower cell loss and delay when compared to the conventional binary scheme     

Interaction of call blocking and cell loss in an ATM network

The introduction of broadband ATM networks and services will raise new operational issues. An area of significant challenge to the network operators will be network dimensioning. ATM networks differ from today's STM networks in that they will have to provide acceptable performance with respect to not only call blocking, but also cell loss and cell delay. Consequently, it is important to understand the interaction of call blocking and cell loss/delay in ATM networks. The paper presents a method of exploiting the interaction of call blocking and cell loss performance in order to efficiently operate the ATM networks. Specifically, the paper demonstrates that when the network is dimensioned or engineered to meet an appropriate call blocking objective, the cell loss performance perceived by the accepted calls can be significantly better than the cell loss objective set for the connection admission control (CAC) to admit or deny a call     

ATM network design and optimization: a multirate loss networkframework

Asynchronous transfer mode (ATM) network design and optimization at the call-level may be formulated in the framework of multirate, circuit-switched, loss networks with effective bandwidth encapsulating cell-level behavior. Each service supported on the (wide area, broadband) ATM network is characterized by a rate or bandwidth requirement. Future networks will be characterized by links with very large capacities in circuits and by many rates. Various asymptotic results are given to reduce the attendant complexity of numerical calculations. A central element is a uniform asymptotic approximation (UAA) for link analyses. Moreover, a unified hybrid approach is given which allows asymptotic and nonasymptotic methods of calculations to be used cooperatively. Network loss probabilities are obtained by solving fixed-point equations. A canonical problem of route and logical network design is considered. An optimization procedure is proposed, which is guided by gradients obtained by solving a system of equations for implied costs. A novel application of the EM algorithm gives an efficient technique for calculating implied costs with changing traffic conditions. Finally, we report numerical results obtained by the software package TALISMAN, which incorporates the theoretical results. The network considered has eight nodes, 20 links, six services, and as many as 160 routes     

Policing mechanism and cell loss priority control on voice cells in ATM networks using fuzzy logic

The authors present a fuzzy logic-based implementation of the policing mechanism (PM) and cell loss priority control (CLPC) functions on voice cells in asynchronous transfer mode (ATM) networks. Specifically, a redesigned model of fuzzy leaky bucket (FLB) is presented which serves as an alternative PM on the voice sources in an ATM network. As a continuation of the implementation of the FLB in the ATM network, three models of ATM switches with different algorithms are constructed, to perform CLPC on the voice cells, as well as to enhance the network throughput. Simulation results show that FLB is a better PM than the conventional leaky bucket (LB) in terms of cell loss probability and throughput while the mean transfer delay remains unchanged. It is also shown that the model of ATM switch with fuzzy token generator is the most balanced CLPC scheme as compared with the other two models.     

Statistical multiplexing with loss priorities in rate-basedcongestion control of high-speed networks

Statistical multiplexing with loss priorities is a central element in ATM-based B-ISDN. Cell priorities arise from the marking schemes employed by the access regulators to identify excess cells, which are dropped during periods of congestion, Also, in real time applications, such as hierarchically coded voice and video, cells are assigned priorities which correspond to their importance to service quality, so that when congestion occurs only the least important are dropped. The authors present a stochastic fluid model of statistical multiplexing with loss priorities. Each Markov modulated fluid source generates streams of different priorities. The burstiness of each stream and the correlation between the priority streams are captured in the mode. The loss priority is implemented by selectively discarding cells of certain priority classes when the buffer content exceeds a corresponding threshold. To handle high dimensional source models, the authors develop an algebraic theory for the efficient computation of the spectrum of the statistical multiplexing system, which generalizes previous results for on-off sources. It is shown that to obtain the solution of the statistical multiplexing problem with J priority classes, J different 1-class problems need to be solved, together with a system of linear equations which describe the behavior of the stationary distribution at the thresholds. The numerical results demonstrate the manner in which i) the threshold level controls the tradeoff between delay of higher priority cells and the loss probability of lower priority cells, and ii) the buffer size controls the loss probability of higher priority cells     

Estimating cell loss rates in high-speed networks with leaky bucket controlled sources

This paper deals with high-speed networks, such as the ATM network, over which data is transferred in cells or packets. Results based on asymptotic analysis of stochastic fluid flow models imply that these networks would have to be run at very low utilization to satisfy the stringent performance requirement of very low cell loss rates. Recent papers have quantified that asymptotic approximations can be quite bad with even a modest number of users sending traffic into the network. In this paper it is shown that two more assumptions in the asymptotic analysis may lead to significant overestimation of the cell loss rates. The more significant of these is the use of the underlying exponential ON durations if sources are policed. The second and less significant is the assumption of an infinite buffer. A new approximate nalytic method is then developed for estimating the cell loss rate with sources policed by a leaky bucket type mechanism. This method focuses directly on the problem of measuring cell losses in the presence of many sources simultaneously bursting in a time period small enough to cause buffer overflow. It turns out to be highly accurate even with some rather drastic simplifying assumptions.     

Cell loss analysis and design trade-offs of nonblocking ATMswitches with nonuniform traffic

In practical ATM switch design, a proper dimensioning of buffer sizes and a cost effective selection of speed-up factor should be considered to guarantee a specified cell loss requirement for a given traffic. Although a larger speed-up factor provides better throughput for the switch, increasing the speed-up factor involves greater complexity and cost. Hence, it may not be cost effective to increase the speed-up factor for 100% throughput. Moreover, with a given buffer budget, an increase in the speed-up factor beyond a certain value only adds to the cell loss. The paper addresses design trade-offs existing between finite input/output buffer sizes and speed-up factor in a nonblocking ATM switch. Another important issue is the adverse effect on cell loss performance caused by nonuniform traffic (different traffic intensity and unevenly distributed routing). The paper analyzes cell loss performance of ATM switches with nonuniform traffic, and examines the effect of each nonuniform traffic parameter. The authors also provide an algorithm for effective buffer sharing that alleviates the performance degradation caused by traffic nonuniformity     

AAL层抗信元丢失技术

ＡＴＭ网络中传输视频信号存在的信元丢失会严重影响接收图象的质量，可以从视频编解码端与ＡＴＭ适配层采取抗信元丢失技术以减少该影响。不同ＡＡＬ有不同的抗信元丢失原理。本文分别对用ＡＡＬ１，ＡＡＬ２，ＡＡＬ５适配ＭＰＥＧ－２视频信号时在ＡＡＬ层采取的抗信元丢失技术进行了分析。     

A novel approach to estimating the cell loss probability in an ATMmultiplexer loaded with homogeneous on-off sources

Estimating the cell loss probability in an ATM multiplexer is one of the most important problems concerning congestion control and bandwidth management in an ATM-based BISDN. We propose a new approach to estimating the cell loss probability in an ATM multiplexer. We use the Markov modulated deterministic process (MMDP) to approximate the actual arrival process and then model the ATM multiplexer as an MMDP/D/1/K queueing system. Using queueing analysis, we derive a formula for the cell loss probability expressed in terms of the limiting probabilities of a Markov chain. We propose two approximation methods based on the results of the analysis. The actual arrival process is approximated by an (M+1)-state MMDP in the first method and by a two-state MMDP in the second. The major advantages of both methods are simplicity, computational efficiency, and numerical stability. The most attractive feature of the second method is that the cell loss probability can be expressed in closed form. Numerical and simulation results show that the first method is sufficiently accurate for all cases in which burst-level congestion is the main contributing factor to cell loss, while the closed-form formula is sufficiently accurate for applications where the average burst length is large (such as large file transfers, image retrievals, etc.)     

Accurate approximation of cell loss probability for self-similartraffic in ATM networks

Recently, self-similar (or fractal) stochastic processes have been regarded as more accurate models of certain categories of traffic (e.g. LAN traffic) which will be transported in ATM networks. The authors propose a method for estimating the cell loss probability in an ATM multiplexer fed by a self-similar arrival process. The packet arrivals are generated by a fractional Brownian motion process and the service process is deterministic. The approach is based on theory for large deviations, and simulation tests show that it is more accurate than the existing approximation result     

Design and architecture of a new space priority mechanism for ATM network nodes

This paper presents the architecture of a new space priority mechanism intended to control cell loss in ATM switches. Our mechanism is a new generic concept called: the multiple pushout. It is based on the utilization of both AAL and ATM features and on a particular definition of the priority bit. Whenever one cell of a message overflows the buffer of an ATM switch, the algorithm causes the switch to discard other cells of the message (including later arrivals). Such discarding frees buffer spaces for cells of other messages that have a chance of arriving at their destination intact. Our objective is to emphasize that in case of overload, with most of proposed mechanisms, cells are discarded without any semantic information about the type of cells. Therefore, at the destination, all the fragments of the corrupted messages will be discarded anyway. Finally, we present simulation results comparing cell loss rates and message loss rates of several space priority mechanisms.     

Improved loss calculations at an ATM multiplexer

In this paper we develop a simple and accurate analytical technique to determine the loss probability at an access node to an asynchronous transfer mode (ATM) network. This is an important problem from the point of view of admission control and network design. The arrival processes we analyze are the Markov-modulated Poisson process (MMPP) and the Markov-modulated fluid (MMF) process. These arrival processes have been shown to model various traffic types, such as voice, video, and still images, that are expected to be transmitted by ATM networks. Our hybrid analytical technique combines results from large buffer theories and quasi-stationary approaches to analyze the loss probability of a finite-buffer queue being fed by Markov-modulated sources such as the MMPP and MMF. Our technique is shown to be valid for both heterogeneous and homogeneous sources. We also show that capacity allocation based on the popular effective-bandwidth scheme can lead to considerable under-utilization of the network and that allocating bandwidth based on our model can improve the utilization significantly. We provide numerical results for different types of traffic and validate our model via simulations     

Dynamic queue length thresholds for multiple loss priorities

Buffer management schemes are needed in shared-memory packet switches to regulate the sharing of memory among different output port queues and among traffic classes with different loss priorities. Earlier, we proposed a single-priority scheme called dynamic threshold (DT), in which the maximum permissible queue length is proportional to the unused buffering in the switch. A queue whose length equals or exceeds the current threshold value may accept no new arrivals. We propose, analyze and simulate several ways of incorporating loss priorities into the DT scheme. The analysis models sources as deterministic fluids. We determine how each scheme allocates buffers among the competing ports and loss priority classes under overload conditions. We also note how this buffer allocation induces an allocation of bandwidth among the loss priority classes at each port. We find that minor variations in the DT control law can produce dramatically different resource allocations. Based on this study, we recommend the scheme we call OWA, which gives some buffers and bandwidth to every priority class at every port. Scheme OWA has tunable parameters, which we give rules of thumb for setting. Another scheme, called AWA, is also a good choice. It has an allocation philosophy more akin to strict priority and hence is not tunable     

Analysis of a priority cell discarding method for ATM networks

One of the most important problems that must be solved in ATM networks is the priority cell discarding (PCD). A two priorities cell discarding method is suggested and analyzed under conditions of discrete time cell arrival. By introducing the operator, an iterative algorithm that can accelerate the speed of convergence when calculating the cell loss probability is suggested. The effectiveness of the PCD in ATM networks is proved by the numeric results.This work was supported in part by the National Key Laboratory of ISN.     

信元调度中的优先级排队研究

ATM交换结构通常应具备优先级控制功能,以适应宽带业务的多样性。优先级控制功能是指可以按各类业务的优先级高低来控制服务质量。当不同优先级的信元在争抢资源时,应该保证优先级高的信元先得到服务,优先级低的信元在缓冲器中等待。实际情况是缓冲区的长度是有限的,丢包现象不可避免。但是如何让丢包对于该结构性能影响不大,比较可行的办法便是支持优先级。