Throughput analysis of the IEEE 802.4 priority scheme

The IEEE 802.4 token bus standard defines an optional priority scheme to handle multiple classes of data. It allocates the channel bandwidth among different priority classes of messages by a set of timers at each station. An analytical model for the priority scheme is presented. The model relates the throughput of each priority class of messages to the traffic intensities of different classes, the target rotation times, and the high-priority token holding time. The network is assumed to be symmetric with respect to its parameters and the traffic distribution among nodes. Simulation results are used to evaluate the accuracy of the model. The model provides means of evaluating the network throughput and can be used to determine the time values to meet the throughput requirements of different classes of traffic     

The IEEE token bus-A performance bound on GM MAP

All versions of the General Motors manufacturing automation protocol (GM MAP) specify that MAP's lower-layer services are to be provided by the IEEE 802.4 token bus. An understanding of 802.4 and its performance aspects is therefore a prerequisite to predicting the performance of MAP. It is shown how total bus capacity is divided among data throughput, token traffic, and propagation delays. The relative contributions of access delay and queuing delay to total message delivery time are discussed. The effects on message delivery times of message size and the number of active stations are also reported. As the token cycle time increases beyond the target rotation time for each of the asynchronous access classes, service to the lower-priority classes is curtailed; a formula that can be used to identity the offered load at which the transition from normal to curtailed service begins is presented     

Throughput analysis of a timer controlled token passing protocolunder heavy load

An analysis is presented of the throughput performance of some recently proposed token-passing local area network (LAN) protocols under a heavy load assumption. The IEEE 802.4 token bus standard which uses a cycle-dependent timing mechanism to control station access to the channel, is studied. It is shown that the cycle-dependent timing mechanism limits the token circulation times properly. Moreover, priority among different access classes is being implemented. Analytic results for computations of throughputs and cycle lengths are derived. These results provide insights into the cycle-dependent timing mechanism and also guidelines for design. Using the results derived, examples concerning the throughput behavior under different heavy load configurations, the sensitivity to parameters, system optimization with delay and bandwidth constraints, and comparison with the other integrated access schemes are studied     

Prioritized-virtual-time CSMA: head-of-the-line priority classeswithout added overhead

The prioritized-virtual-time carrier sense multiple access (PVT-CSMA) protocol is a media access algorithm for cable- and radio-based local area networks that incorporates message-based priority classes. PVT-CSMA implements the head-of-the-line (HOL) priority queuing discipline, in which higher priority messages are always transmitted in preference to lower priority messages, and messages are transmitted first-come-first-served within each priority class. Expressions are obtained for both throughput as a function of offered load and mean delay as a function of throughput for asynchronous (unslotted) PVT-CSMA, which are easily solvable for any number of priority classes and for class-dependent message lengths, arrival rates, retransmission rates, etc. Specialization of the delay analysis to the single class case yields a substantial improvement over previous results for asynchronous virtual-time CSMA. Comparisons are made between this analysis and simulations of PVT-CSMA, and with simulations of Tobagi's P-CSMA (1982) protocol     

Performance of Single Access Classes on the IEEE 802.4 Token Bus

The IEEE 802.4 token bus defines both synchronous and asynchronous message access classes. Their performance is compared for networks implementing a single message class. Throughput bounds are derived from the access class timer. It is shown that for some configurations, the asynchronous class yields lower observable message delays. Mean observable delay is minimized by allowing each medium access controller unrestricted service.     

Performance and parameter region for real-time use in IEEE 802.4token bus network

The upper and lower bounds for the mean cycle time and the mean message transmission time of class six and class four in the IEEE 802.4 token bus network, within which the minimum utilization constraint of class four is guaranteed, are derived. Stability conditions for the token bus network are also derived. These bounds and stability conditions are represented in terms of the high-priority token hold time, the token rotation time, the arrival rate, the total number of stations, etc. A parameter-tuning algorithm in a partially symmetric token bus network with two classes is suggested. This algorithm maximizes the utilization of class four for a given high-priority token hold time and at the same time meets the constraints of the stability condition of the network, the real-time constraint, and the minimum utilization of class four     

Performance of token schemes supporting delay-constrained prioritytraffic streams

A symmetric priority-based token network is considered. Messages are divided into two priority classes. High-priority messages are assumed to require tight delay constraints. As a result, each station is allowed to establish, at any time, at most a single real-time high-priority access concentration. High-priority messages are guaranteed access onto the channel within a prescribed limited period. In turn, regular priority messages are only served when the system determines, through the repetitive use of circulating tokens (as used by the IEEE 802.5 token-ring-type protocol), that no high-priority messages are currently waiting in the system. Two token schemes employing different service disciplines are used to provide network access. Exact and approximate mean delay formulas for both message classes are derived. Numerical results are then exhibited to illustrate the network performance under various traffic conditions     

Performance of the timed token scheme in MAP

Two new analytic approximations are given for the mean delay in the timed token bus network specified in MAP (manufacturing automation protocol). Each station on the network has two kinds of timer for controlling the maximum amount of time for data transfer before giving up the token. High-priority stations would set large values in the token holding timer (THT) and transmit until the timer expires. Low-priority stations compare the elapsed time since the last token arrival (measured cycle time) to the token rotation timer (TRT) setting, and transmit only for the time left in the timer. The first approximation, based on Fuhrmann's bound for the limited-k service discipline, is for the symmetric case where all the stations have the same traffic load and the identical THT or TRT settings. The approximation has been compared to simulation results, and found to be quite accurate for a representation factory network, with THT or TRT set at two and five times the average total walk time. The second approximation is for the asymmetric case where there is one saturated low-priority (limited-TRT) station, and the rest of the stations in the network have high priority with limited-THT service. These two formulas will be useful in the planning and operation of MAP networks by providing guidance in the choice of timer settings for meeting different priority service requirements     

The token grid network

An overwhelming majority of local and metropolitan area network products (LANs and MANs) are based upon linear topologies such as buses and rings. Such networks are economical for high speed operation since the station interfaces are simple and require very little transit buffering. However because of their linear structure, the total throughput is restricted by the transmission rate of the media access channels. In the paper, a token grid network is introduced where media access is performed over a two-dimensional mesh. In the resulting system, each station is two-connected and has the same transmission hardware and small station latency as in a dual token ring. In the token grid however, the total system throughput may be many factors larger than that which is possible in a dual token ring. In a large √N×√N network, the uniform load capacity is approximately √N/2 times that of an N station dual token ring. In addition, the token grid can take advantage of communities-of-interest amongst the stations. It is possible to implement the system in such a way as to achieve robust operation in the presence of station and link failures     

Modeling and performance analysis of a symmetric fast-circuitswitched robust-WDM LAN with the AR/LTP protocol

Robust wavelength division multiplexing (WDM) is a technique to implement WDM local area networks (LAN's) in the presence of laser wavelength drifts. A medium access control (MAC) protocol is used in conjunction with a wavelength-tracking receiver to tolerate the variations of transmission wavelengths. Among the proposed medium access schemes, the aperiodic reservation (AR) scheme with token-passing based control channel gives the best performance. An AR protocol with a lenient token passing policy (AR/LTP) is thus presented. An analytical model is developed to design Robust-WDM AR/LTP LAN's and predict their performance characteristics. The model can be used to evaluate the variation of waiting time and throughput for load and network parameters such as the arrival rate, number of nodes, number of channels and timing parameters. It also addresses the issues related to traffic loss, channel-blocking, token rotation time, network span, and the effect of device parameters     

Lookahead Network

This paper formalizes and extends a tree-based local area network, called the lookahead network, proposed by Lipovski, Goyal, and Malek. The leaf nodes in the network are workstations, and the nonleaf nodes consist of pure combinational logic used in carry lookahead circuits of binary adders. Any of the ring or bus access protocols (token passing, contention, etc.) can be used on this network. The first objective of this paper is to give a formal definition of the lookahead logic and show that it can be operated in either a bus or a ring mode. The advantage of using the lookahead logic is that it makes the network operation fail-soft, and since this logic is recursively defined, it simplifies network installation, expansion, and partitioning procedures. The second objective of this paper is to define a new round-robin access protocol on the lookahead network, which has much better throughputdelay characteristics than that of a token ring when the number of active nodes on the network is increased and/or the transmission speed is increased, and/or the average packet size is decreased. Moreover, this new protocol can implement a message-based priority scheme, which makes it useful in integrated voice and data networks.     

Performance comparison of two input access methods for a multicastswitch

A comparison of the performance of two input access mechanisms for multicast switching is presented. The first of these-a cyclic priority input access method-is a derivative of the ring token reservation method which eliminates the unfairness of the ordinary ring token reservation. It has the advantage of being relatively simple and easy to implement. A second approach employs a neural network to resolve output port conflict. While more difficult to implement, it has a delay-throughput performance advantage over the cyclic priority approach. The primary performance measurements are the switch throughput and the packet delay. A key assumption is that all copies of the same packet must be switched in the same slot     

基于令牌总线网络的RS 485应用

应用RS 485物理层标准,构建总线型的控制网络,采用令牌方式实现总线的共享访问,以适应控制网络实时性要求。在进行令牌总线协议设计时,从RS 485特点出发,以特殊的设计来简化协议,提高协议的效率。为了保证网络的可靠性,在协议中采用了有效的错误处理方法,即保证数据传输的正确性,又减小了协议的控制开销,以简单、方便、快捷、高效的方式实现控制网络的介质访问管理,给RS 485网络提供一种较好的连网模型。     

Mixed time-constrained and non-time-constrained communications inlocal area networks

It is well known that some time-token medium access protocols for local area networks (LANs) like the IEEE 802.4 token bus and the FDDI token ring can guarantee the medium access delay for time-constrained packets. However, a problem which has been largely overlooked is how these protocols can be made to provide a maximum throughput for nontime-constrained packets while guaranteeing the delay bound of time-constrained packets. The authors first show how the parameters of the IEEE 802.4 token bus and the FDDI token ring can be set to solve the above problem. Then, they design a new timer mechanism for the timed-token protocols which provides the highest guaranteed throughput of nontime-constrained packets among a set of medium access protocols called the token passing protocol, to which most of the existing non-contention LAN protocols belong. They present numerical examples to compare different protocols, all of which have shown the superiority of the proposed protocol to the others     

QoS-based remote control of networked control systems via Profibus token passing protocol

This paper focuses on a quality-of-service (QoS)-based remote control scheme for networked control systems via the Profibus token passing protocol. Typically, token passing experiences random network delay due to uncertainties in token circulation, but the protocol has in-built upper and lower bounds of network delay. Thus, to ensure the control performance of networked control systems via the Profibus token passing protocol, the network delay should be maintained below the allowable delay level. As the network delay is affected by protocol parameters, such as target rotation time, we present here an algorithm for selection of target rotation time using a genetic algorithm to ensure QoS of control information. We also discuss the performance of the QoS-based remote control scheme under conditions of controlled network delay. To evaluate its feasibility, a networked control system for a feedback control system using a servo motor was implemented on a Profibus-FMS network.     

Performance Evaluation of Wireless Controller Area Network (WCAN) Using Token Frame Scheme

In this paper, a proposed new wireless protocol so-called wireless controller area network is introduced. WCAN is an adaptation of its wired cousin, controller area network protocol. The proposed WCAN uses token frame scheme in providing channel access to nodes in the system. This token frame method follows the example used in wireless token ring protocol which is a wireless network protocol that reduces the number of retransmissions as a result of collisions. This scheme based on CAN protocol allows nodes to share a common broadcast channel by taking turns in transmitting upon receiving the token frame that circulates around the network for a specified amount of time. The token frame allows nodes to access the network one at a time, giving ‘fair’ chance to all nodes instead of competing against one another. This method provides high throughput in a bounded latency environment. The proposed WCAN protocol has been developed and simulated by means of QualNet simulator. The performances of this proposed protocol are evaluated from the perspective of throughput, end-to-end delay and packet delivery ratio, and are compared against the IEEE 802.11 protocol. Simulation results show that the proposed WCAN outperforms IEEE 802.11 based protocol by 62.5 % in terms of throughput with increasing network size. Also, it shows an improvement of 6 % compared to IEEE 802.11 standard at a higher data interval rate.     

Proof that Timing Requirements of the FDDI Token Ring Protocol are Satisfied

The fiber distributed data interface (FDDI) is an ANSI draft proposed standard for a 100 Mbit/s fiber-optic token ring. The FDDI timed token access protocol provides dynamic adjustment of the load offered to the ring, with the goal of maintaining a specified token rotation time and of providing a guaranteed upper bound on time between successive arrivals of the token at a station. FDDI also provides automatic recovery when errors occur. The bound on time between successive token arrivals is guaranteed only if the token rotates quickly enough to satisfy timer requirements in each station when all ring resources are functioning properly. Otherwise, recovery would be initiated unnecessarily. The purpose of this paper is to prove that FDDI timing requirements are satisfied, i.e., the token rotates quickly enough to prevent initiation of recovery unless there is failure of a physical resource or unless the network management entity within a station initiates the recovery process.     

Comparative performance of voice/data local area networks

Using simulation, a network-independent framework compares the performance of contention-based Ethernet and two contention-free round-robin schemes, namely Expressnet and the IEEE 802.4 token bus. Two priority mechanisms for voice/data traffic on round-robin networks are studied: the alternating-rounds mechanism of the Expressnet, and the token rotation timer mechanism of the token bus, which restricts access rights based on the time taken for a token to make one round. It is shown that the deterministic schemes almost always perform better than the contention-based scheme. Design issues such as the choice of minimum voice packet length, priority parameters, and voice encoding rate are investigated. An important aspect that is noted is the accurate characterization of performance over a wide region of the design space of voice/data networks     

不对称令牌总线在分布式控制系统中应用的性能分析与参数设置

建立了实时信息与非实时信息到达模型，并分析了完全不对称令牌总线网络用于控制系统时的性能，主要包括令牌循环时间和网络稳定条件。给出了参数(包括高优先级令牌持有时间和低优先级目标令牌循环时间)设置时应满足的条件。最后给出了收发缓冲器容量的设置方法。     

Approximate performance analysis of real-time traffic over heavilyloaded networks with timed token protocols

This paper studies timed token protocols with respect to real-time packet traffic in local area networks (LANs), such as FDDI and token bus, employed in distributed control systems. Typically, in such systems, three classes of packet traffic are encountered. The first class consists of packets cyclically generated by data acquisition tasks. The second traffic class is represented by packets generated in a random manner by control tasks and sporadic events. Finally, the third traffic class represents nonreal-time packet streams such as, for example, file transfers. To evaluate protocol performance, three performance measures are taken into account with respect to randomly generated real-time traffic: the mean waiting time, the blocking probability, and the probability that accepted packets will wait for service no longer than a specified time limit. In order to determine the last performance measure, a two-moment approximation of the waiting time distribution is applied. All three performance measures are evaluated at the beginning of the heavy network load region. Two examples of numerical calculations compared with computer simulations done for FDDI-II and token bus networks are given