ATM网络的可用比特率业务

本文介绍了可用比特率业务的业务特点，速率控制机制以及该业务存在的问题。     

ABR performance in presence of bursty TCP traffic

The available bit rate (ABR) service class is a solution for the integration of data traffic in asynchronous transfer mode networks. Many algorithms have been proposed to implement ABR services. The authors present simulation results showing poor performance by a common ABR algorithm when supporting TCP bursty traffic. As a solution to this problem, the authors propose time averaging of the parameters calculated by the ABR algorithms     

QoS and multipoint support for multimedia applications over the ATMABR service

Multimedia applications with stringent delay requirements are usually supported by the constant bit rate (CBR) or real-time variable bit rate (rt-VBR) service categories of ATM. This article shows that the available bit rate (ABR) service, which was originally designed for data, can also support multimedia applications under certain circumstances. Issues related to the design of proper ABR traffic management algorithms for such support are presented     

Intelligent Flow Control Technique of ABR Service in ATM Networks Based on Fuzzy Neural Networks

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Effective control of traffic flow in ATM networks using fuzzyexplicit rate marking (FERM)

We describe the fuzzy explicit fate marking (FERM) traffic flow control algorithm for a class of best effort service, known as available bit rate (ABR), proposed by the ATM Forum. FERM is an explicit rate marking scheme in which an explicit rate is calculated at the asynchronous transfer mode (ATM) switch and sent back to the ABR traffic sources encapsulated within resource management (RM) cells. The flow rate is calculated by the fuzzy congestion control (FCC) module by monitoring the average ABR queue length and its rate of change, then by using a set of linguistic rules. We use simulation to compare the steady-state and transient performance of FERM with EPRCA (a current favourite by the ATM Forum) in the presence of high priority variable bit rate (VBR) video and constant bit rate (CBR) in both a local-area network (LAN) and a wide-area network (WAN) environment. Our experiments show that FERM exhibits a robust behavior, even under extreme network loading conditions, and ensures fair share of the bandwidth for all virtual channels (VCs) regardless of the number of hops they traverse. Additionally, FERM controls congestion substantially better than EPRCA, offers faster transient response, leads to lower end-to-end delay and better network utilization     

Performance issues in public ABR service

The available bit rate (ABR) service attracted much attention during the negotiations leading to Traffic Management Specification Version 4.0, finalized by the ATM Forum. In thr ABR service, feedback flow control of the source rate is provided in response to the changing asynchronous transfer mode (ATM)-layer transfer characteristics. The reference behavior of the source end system, the destination end system, and the switch, as detailed in the specification, allows cooperative control among these systems. The performance of the public ABR service is discussed in connection with the evolution of ATM switches. Public networks with first-generation switches provide an ABR service with a limited peak cell rate (PCR), while those with second-generation switches can provide an ABR service with any PCR. In such networks, TCP-over-ABR works well. Point-to-multipoint ABR will be provided in advanced switches. A method is proposed for maintaining the throughput of point-to-multipoint ABR when the number of leaves is increased     

Evolution of controls for the available bit rate service

In the summer of 1993, the asynchronous transfer mode (ATM) community began a search for a mechanism to allocate bandwidth dynamically within an ATM network, while simultaneously preventing data loss. By the fall of 1994, the ATM Forum had introduced a new service category for this purpose, the available bit rate (ABR) service, and had selected a rate-based mechanism for its support. Simply put, the mechanism would consist of a stream of ABR resource management (RM) cells, generated by one end of the connection (the source) and looped back by the other (the destination), into which switches would encode the maximum rate at which the ABR source was to generate data. The ABR service is an addition to ATM that uses a feedback mechanism to control the transmission rates of traffic sources. Although the performance of the ABR service will depend critically on the quality of feedback sent by switches, the algorithms for determining this feedback are largely outside the scope of ATM standards and specifications. We describe how two of the properties implemented by the ABR source and destination, out-of-rate RM cells and a use-it-or-lose-it policy, were implicitly architectural decisions, and we explain how and why the ATM Forum ultimately revised these decisions. We then discuss how the ATM Forum addressed the issues of conformance testing, point-to-multipoint connections, and parameter negotiation for the ABR service. At the end, we reflect on the process by which the ABR specification was developed     

Picture quality optimization in ABR video services

In transmission of bursty traffic such as video and data, a network with a fixed capacity may have some unused bandwidth. The available bit-rate (ABR) scheme in ATM networks can make use of this spare channel capacity as a lower priority service. ABR has been developed initially for delay-tolerant services such as data. In this paper we demonstrate how with the help of a video transcoder delay-sensitive services such as video may be transmitted over ABR networks. The transcoder further compresses the video bit stream according to the rates dictated by the network with minimum degradation to the picture quality. We have optimised the video quality by combining the classical minimum cell rate (MCR) with the subjectively acceptable minimum quality (SAMQ). The performance of two different coding formats for ABR sources, namely the constant bit-rate (CBR) and variable bit-rate (VBR) video input to the transcoder are compared. We also present techniques for further reduction of congestion at the multiplexing switches to improve video quality and report on the impact of the forward and backward link delays on the ABR performance with a video transcoder.     

An update on ATM traffic management

The Traffic Management (TM) group at the ATM Forum has been fairly focused on developing a new service called guaranteed frame rate (GFR) service. This service is expected to be positioned as an intermediate alternative between the available bit rate (ABR) service, which uses a closed-loop feedback control framework, and the unspecified bit rate (UBR) service, which provides no assurances whatsoever. The motivation for GFR service comes from the perceived need for frame-based minimum rate guarantees. The article gives an update on the status of the GFR service definition, as well as other topics discussed in the ATM Forum     

WATMnet: a prototype wireless ATM system for multimedia personalcommunication

A prototype microcellular wireless asynchronous transfer mode network (WATMnet) capable of providing integrated multimedia communication services to mobile terminals is described in this paper. The experimental system's hardware consists of laptop computers (NEC Versa-M) with WATMnet interface cards, multiple VME/i960 processor-based WATMnet base stations, and a mobility-enhanced local-area ATM switch. The prototype wireless network interface cards operate at peak bit-rates up to 8 Mb/s, using low-power 2.4 GHz industrial, scientific, and medical (ISM)-band modems. Wireless network protocols at the portable terminal and base station interfaces support available bit rate (ABR), variable bit rate (VBR), and constant bit rate (CBR) transport services compatible with ATM using a dynamic time-division multiple-access/time-division duplex (TDMA/TDD) MAC protocol for channel sharing and data link control (DLC) protocol for error recovery. A custom wireless control protocol is also implemented between the portable and base units for support of radio link related functions such as user registration and handoff. All network entities including the portable, base and switch use a mobility-enhanced version of ATM (“Q.2931+”) signaling for switched virtual circuit (SVC) connection control functions, including handoff. In the first stage of the prototype, the application-level API is TCP/UP over ATM ABR service class using AAL5. Early experiments with the WATMnet prototype have been conducted to validate major protocol and software aspects, including DLC, wireless control, and mobility signaling for handoff, Selected network-based multimedia/video applications requiring moderate bit-rates (~0.5-1 Mb/s) in the ABR mode have been successfully demonstrated on the laptop PC     

QoS Performance Bounds and Efficient Connection Admission Control for Heterogeneous Services in Wireless Cellular Networks

Quality-of-Service (QoS) performance and connection admission control (CAC) for heterogeneous services in wireless multiple access networks are investigated. The heterogeneous services include constant bit rate (CBR), variable bit rate (VBR) and available bit rate (ABR) services. Multiple access control is handled by a polling-based scheme with non-preemptive priority. Tight delay variation (jitter) bounds for CBR connections and delay bounds for VBR connections are derived. A CAC scheme based on the derived bounds is developed. The CAC makes use of user mobility information to reserve an appropriate amount of system resources for potential handoff connections to achieve low handoff connection dropping rate (HCDR). Simulation results show that the proposed CAC scheme can achieve both low HCDR and high resource utilization.     

串行网络中ABR业务的流量控制和缓存设计

本文提出一种ABR业务速率在优化率和请求率之间的切换控制方法。在上述控制算法基础上,研究了保证信元在交换节点处无丢弃,同时输出带宽得到充分利用的交换节点的缓存和阈值设计问题。新的流量控制机制实现了源节点发送率和交换节点缓存的小抖动控制。     

ABR业务计费机制及带宽分配算法

本文详细分析了ATM网中ABR业务占用带宽的特点,评述了现有计费方案的缺点,在此基础上提出了一种新的ABR业务计费方案,适应于该方案的带宽分配公平准则,以及相应的带宽分配算法.仿真实验结果表明,本文方案不仅保持了静态计费机制原有的简便、易于实现的优点,而且很好地克服了其它方案适用范围小、公平性差等缺点.     

Allocating fair rates for available bit rate service in ATMnetworks

A central issue in congestion control for the available bit rate (ABR) service in ATM networks is the computation of the fair rate for every connection at each switching node in the network. The objective is to determine the fair rate for all the connections in a distributed network under dynamic changes in the absence of centralized knowledge about the network, and without the synchronization of different network components. The authors study the problem of fair rate allocation, specifying the requirements of a fair rate allocation algorithm, and provide a survey of various proposed fair rate allocation strategies in the context of the ABR service     

Design and evaluation of an efficient traffic control scheme for integrated voice,video, and data over asynchronous transfer mode networks: Explicit allowed rate algorithm

Asychronous transfer mode (ATM) networks are high‐speed networks with guaranteed quality of service. The main cause of congestion in ATM networks is over utilization of physical bandwidth. Unlike constant bit‐rate (CBR) traffic, the bandwidth reserved by variable bit‐rate (VBR) traffic is not fully utilized at all instances. Hence, this unused bandwidth is allocated to available bit‐rate (ABR) traffic. As the bandwidth used by VBR traffic changes, available bandwidth for ABR traffic varies; i.e., available bandwidth for ABR traffic is inversely proportional to the bandwidth used by the VBR traffic. Based on this fact, a rate‐based congestion control algorithm, Explicit Allowed Rate Algorithm (EARA), is presented in this paper. EARA is compared with Proportional Rate Control Algorithm (PRCA) and Explicit Rate Indication Congestion Avoidance Algorithm (ERICA), in both LAN and WAN environments. Simulations of all three algorithms are conducted under both congestion and fairness configurations with simultaneous generation of CBR, rt‐VBR, nrt‐VBR and ABR traffic. The results show that, with very small over‐head on the switch, EARA significantly decreases the required buffer space and improves the network throughput. Copyright © 1999 John Wiley & Sons, Ltd.     

Explicit rate allocation algorithm of generalised max-min fairnessfor ATM ABR services

A distributed explicit rate allocation algorithm based on the generalised max-min (GMM) fairness principle is proposed for the available bit rate (ABR) ATM services. Compared with a similar algorithm for GMM, it employs fewer fields in an RM cell and imposes lower requirements on the computational capability of the switches     

On max-min fair congestion control for multicast ABR service in ATM

In the ATM Forum activities, considerable efforts have focused on the congestion control of point-to-point available bit rate (ABR) service. We present a novel approach that extends existing point-to-point (unicast) congestion control protocols to a point-to-multipoint (multicast) environment. In particular, we establish a unified framework to derive a multicast congestion control protocol for an ABR service from a given rate-based unicast protocol. We generalize a known necessary and sufficient condition on the max-min fairness of unicast rate allocation for a multicast service. Using this condition, we show that the resulting multicast protocol derived using our framework preserves the fairness characteristics of the underlying unicast protocol. The practical significance of our approach is illustrated by extending a standard congestion control mechanism for an ABR service to a multicast environment. The performance of the resulting multicast protocol is examined using benchmark network configurations suggested by the traffic management subworking group at the ATM Forum, and simulation results are presented to substantiate our claims     

Definition and performance analysis of a simple, ABR-likecongestion control scheme for satellite ATM networks with guaranteedloss performance

We describe an ATM system architecture for satellite communications. The proposed architecture includes on-board switching, and supports the ATM traffic categories defined in previous specifications. In this framework, a critical issue is the control of congestion phenomena. In particular, the application of feedback-based control strategies to a satellite network is critical due to the peculiarities of such an environment: very large propagation delay, expensive transfer capacity, and limited on-board processing capability. The available bit rate (ABR) is the ATM service category handled according to a reactive congestion control (RCC). The focus of this paper is the definition of an RCC that is fully compatible with the standard ABR protocols, and that takes into account the constraints of the satellite environment. We also derive an analytical model that allows us to evaluate the performance of the proposed scheme and to dimension the system. The analytical model is validated with simulations     

An efficient feedback consolidation algorithm forpoint-to-multipoint ABR service in ATM network

In this letter, we propose an efficient feedback consolidation algorithm for a point-to-multipoint available bit rate (ABR) service in asynchronous transfer mode (ATM) networks. The proposed scheme can achieve fast transient response to the congestion status on a branch as well as effectively eliminate the consolidation noise     

Transporting compressed video over ATM networks with explicit-ratefeedback control

We propose a scheme for transmission of variable bit rate (VBR) compressed video for interactive applications using the explicit-rate congestion-control mechanisms proposed for the available bit rate (ABR) service in asynchronous transfer mode networks. Compressed video is inherently bursty, with rate fluctuations over both short and long time scales. This source behavior can be accommodated by the ABR service, since the explicit-rate scheme allows sources to request varying amounts of bandwidth over time. Moreover, when the bandwidth demand cannot be met, the network provides feedback indicating the bandwidth currently available to a connection. In our scheme, the video source rate is matched to the available bandwidth by modifying the quantization level used during compression. We use trace-driven simulations to examine how effective the enhanced explicit-rate scheme is in “rate matching” between the network and the source and the effect on end-to-end delay. We also look at the sensitivity of the proposed scheme to the estimates of the network round-trip times and to inaccuracies in the rate requests made by sources