Setting target rotation times in an IEEE token bus network

The IEEE standard 802.4 token bus protocol requires each network station to implement a synchronous (highest priority) message class, and permits a station to implement three lower priority classes: urgent asynchronous, normal asynchronous, and time available. Each of the lower three priorities (called access classes) is assigned a target token rotation time that limits the amount of time that a station can use to service lower priority traffic. A formulation of the problem is presented in which messages are transmitted from an access class as long as network throughput remains below a user-specified threshold. Formulas are derived that transform this priority scheme, based on network throughput limits, into the proper target rotation time settings that the token bus protocol actually requires. The analytical model is compared with a computer simulation of the token bus protocol and shows close agreement     

Selection of timed token protocol parameters to guarantee messagedeadlines

Networks that use the timed token protocol (such as the 100 Mbit/s FDDI network) are well suited for real-time applications because they guarantee, to each node, an average bandwidth and a bounded access time to the communication network. This guarantee is necessary but not sufficient for the timely delivery of deadline-constrained messages; protocol parameters must be carefully selected to ensure that these messages meet their deadlines. This paper addresses the issue of selecting the protocol parameters TTRT (target token rotation time) and the synchronous capacities assigned to each node. The objective is to guarantee that each synchronous message is transmitted before its deadline. An upper bound is derived on the worst case achievable utilization (WCAU) of any parameter selection scheme. The WCAU of a scheme is defined as the maximum utilization U such that the scheme guarantees all synchronous messages as long as their utilization is less than U. An algorithm for selecting the above parameters is proposed, The algorithm is shown to have a WCAU that is very close to the upper bound     

An optimal synchronous bandwidth allocation scheme for guaranteeingsynchronous message deadlines with the timed-token MAC protocol

This paper investigates the inherent timing properties of the timed-token medium access control (MAC) protocol necessary to guarantee synchronous message deadlines in a timed token ring network such as, fiber distributed data interface (FDDI), where the timed-token MAC protocol is employed. As a result, an exact upper bound, tighter than previously published, on the elapse time between any number of successive token arrivals at a particular node has been derived. Based on the exact protocol timing property, an optimal synchronous bandwidth allocation (SBA) scheme named enhanced MCA (EMCA) for guaranteeing synchronous messages with deadlines equal to periods in length is proposed. This scheme is an enhancement on the previously published MCA scheme     

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     

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     

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。     

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     

FDDI: current issues and future plans

Fiber Distributed Data Interface (FDDI), a set of standards developed by the American National Standards Institute (ANSI) X3T9.5 Task Group, is reviewed. The timed token access method, used to share the medium among stations in this 100-Mbit/s local area network (LAN), differs from the traditional token access method in that the time for the token to walk around the ring is accurately measured by each station and used to determine the usability of the token. FDDI-II, which provides support for isochronous service in addition to the asynchronous and synchronous service provided by FDDI, the media-dependent physical layer (PMD) standard called low-cost fiber PMD (LCF-PMD), and the implementation of FDDI on the synchronous optical network (SONET) are discussed     

一种多信道的分簇式短波令牌协议

传统的短波令牌协议( HFTP)调度方式单一,在没有数据传送时,信道资源会被各节点间的令牌传递所占用,且令牌在通信质量较差时易丢失。基于此,提出一种多信道的分簇式短波令牌协议( CHFTP),通过以通信质量评估为标准的分簇算法和基于预约的动态令牌调度,减小了令牌丢失的概率和令牌传递、处理的开销,并给出了仿真分析。仿真结果表明,该协议的端到端平均时延和网络吞吐量明显优于短波令牌协议, CHFTP 的平均时延最多可减少75%,网络吞吐量最多可增加66.7%,适合在短波通信网络中使用。     

Comments on `Performance of single access classes on the IEEE 802.4token bus' by M.A. Colvin and A.C. Weaver

In the above correspondence (ibid., vol.COM-34, no.12, p.1253-6, Dec. 1986), throughput bounds are derived for the IEEE 802-4 token passing scheme, in the presence of a single class of messages. It does not consider the fact that even after the token holdtimer expires, a node is allowed to complete its message transmission. Here, the fact is considered, and the upper bounds on throughput are modified     

Performance analysis of type 3 LLC in industrial 802.5 nets

Some performance results obtained by simulating the behavior of a local area network based on the IEEE 802.5 standard are presented. The simulation model has been designed to evaluate the mean response time observed by the user of the LLC (logical link control) service, instead of measuring the turnaround time experienced by the medium access control user; i.e. the performance index is the average delay between the message generation and the receipt of the corresponding acknowledgement. Type-3 LLC service is analyzed since it appears particularly attractive in industrial LANs concerned with real-time activities and adopting reduced protocol profiles. The effect of having different priority classes for the messages in the network system has been studied. Some results that demonstrate the existence of a practical upper bound to the maximum value of the token holding time have been obtained     

Message Delay Analysis for Polling and Token Multiple-Access Schemes for Local Communication Networks

Two efficient polling (token) schemes, which provide multiaccess coordination for local area networks, are described, analyzed, and compared. The two multiple-access polling protocols described here utilize gated and exhaustive disciplines in ordering the transmission of the messages buffered at each terminal. The delay-throughput performance behavior of polling schemes operating under the above mentioned disciplines is derived. Network terminals are modeled as independent sources, which generate messages in accordance with a renewal process. A queueing theoretic approach is employed in deriving the actual message delay. behavior of both schemes. Several comparisons are presented. Interesting results are obtained when we compare the average message waiting times in the case when the network traffic is completely balanced with the ease when this same traffic is mostly due to a single node. The latter case would arise, for example, m networks employing gateways.     

Effect of nodes reordering on the schedulability of real-timemessages in timed token networks

This letter addresses the effect of different ordering patterns of network nodes on the ability to schedule real-time messages in a timed token network where the timed token medium access control protocol is employed. It is found that for any given setting of network parameters, a set of real-time message streams which is unschedulable under one particular nodes-ordering may become schedulable under another different nodes-ordering. Some guidelines for avoiding a possible misjudgement in the schedulability due to inappropriately ordered nodes are discussed     

Design and performance comparison of multiple-token based MAC protocols for optical burst switched ring networks

Optical burst switching (OBS) has been proposed as a new optical switching paradigm for the next generation Internet due to its flexibility and feasibility compared to OCS and OPS. Moreover, serving as a backbone that interconnects a number of access networks, OBS ring topologies have been a good choice for solving the current metro gap problem between core network and access network owning to its simplicity and scalability. In this paper, we provide an insight into the OBS ring network that consists of nodes using TT–TR (Tunable Transmitter–Tunable Receiver). The node architectures with TT–TR may make efficient use of network resources even though traffic pattern, such as IP traffic with self-similarity dynamically change, and can support good expandability. However, all nodes share the limited network resources. This may result in contention such as wavelength contention and transceiver contention leading to burst loss. In order to use the shared network resources fairly and efficiently as well as reducing the resource contention, we focus on the design of medium access control (MAC) protocols based on multiple tokens. Each token is allocated to one wavelength to denote the accessibility of that wavelength, i.e., once the token is captured, the corresponding wavelength can be used to transmit a burst. As tokens hold the key for using wavelengths to transmit bursts, token management including the token release time is crucial in the proposed MAC protocols. Thus, two kinds of multiple-token based MAC protocols with different token release times are proposed: token release after transmitting burst (TRTB) and token release after transmitting control header (TRTC). Each of them is classified into two schemes called TRTB/TRR and TRTB/RCA and correspondingly TRTC/TRR and TRTC/ RCA. RCA stands for receive collision avoidance. The target is to increase the performance while reducing the processing overhead at each node. The performance of the TRTB and TRTC protocols are evaluated and compared in terms of queuing delay, burst loss rate, and channel utilization by OPNET simulation. The effects of various design parameters are also investigated through simulation in order to evaluate their scalability. In all the proposed schemes, tokens are just used to denote the accessibility of each wavelength. Finally, as an alternative, we also propose a new scheme based on the TRTC protocol called TRTC/CAT (collision avoidance by tokens) to avoid contention by using tokens.

Performance analysis of FDDI network under frequent bidding requirements

A bidding scheme is described for the fibre distributed data interface (FDDI). An analysis is presented for the throughput performance of an FDDI network under the assumption of heavy load, which allows the target token rotation time (TTRT) to be bid for and adjusted frequently as and when the access time requirements of synchronous traffic change. The authors' results show that better throughput performance is achievable under the new bidding scheme. It is also observed that although re-bidding is desirable, escalating and uncontrolled bidding intensity may incur undue overheads that results in unacceptable throughput degradation.<>     

Encounter based fuzzy logic routing in delay tolerant networks

Delay tolerant networks (DTNs) are a newest class of networks that have the ability to provide connectivity to areas that are yet to be served by conventional networks. Routing in DTN is a tough task because nodes have no prior information about the partitioned network and transfer opportunities between peer nodes are limited. A node in a DTN delivers messages to the destination using the store and forward strategy. Messages are transmitted to multiple intermediate relay nodes encountered in order to increase the opportunity for the message to reach the destination. Encounter duration is the time period in which a pair or more mobile nodes move into the communication range of each other and hence are able to transfer messages between them. Since the node movements are arbitrary, the encounter duration is unpredictable. This research work proposes a novel encounter based fuzzy logic routing (EFLR) scheme to maximize message delivery with reduced overhead. The fuzzy based utility computation is used for finding a better node to forward messages as well as to drop messages from buffer. Simulation results reveal that EFLR performs better than other existing DTN routing protocols.

     

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     

基于深度学习方法的句子及语素边界划分研究

针对哈萨克语的句子、单词及语素边界检测问题,文中提出了一种基于深度学习的边界检测方法：CNN-TSS模型。通过将边界检测问题视为序列标注任务,将句子、单词及语素的边界检测合并为一种任务完成。通过对CNN-TSS模型选取最优超参数,对不同语言进行了测试。实验结果表明,该模型在不使用额外特征的情况下,在性能上超过了基于传统方法的边界检测系统。     

A Highly Reliable LAN Protocol

As a research project for NASA's Langley Research Center, we developed a variation on MIL-STD-1553B (the military standard for avionics buses) whose goal was increased fault tolerance. The resulting protocol, called implicit token passing (ITP), replaces an explicit token with brief "soundoff" messages from all nodes participating on the LAN. Since every node participates on every "token cycle," bus silence is an error indication and initiates recovery action. By encoding state information in the headers of the transmitted words, nodes are continuously aware of the global state of the network. This local knowledge of the global network state allows the system to continue operation in spite of nodal failures. A station which fails but then recovers can quickly assess the global network state and then safely rejoin the active nodes. ITP features high throughput and bounded message delay, and achieves high reliability through tolerance of failed nodes and automatic resynchronization when failed nodes are revived. The protocol is ideally suited for a bus topology and fiber optic media.     

Approximate analysis of asymmetric single-service prioritized tokenpassing systems

A LAN or MAN (local or metropolitan area network) using token passing to allow stations to access the transmission medium is studied. Ibe and Cheng (1989) recently presented an approximate derivation of message delays for an asymmetric single-service token passing with only one type of message. In the present work, this analysis is extended to an operation with nonpreemptive priority queuing. The number of message priority levels and the walktime vary from one station to another. Moreover, the assumed Poisson message arrival process and the message transmission time vary, not only from one station to another, but also from one class to another. The performance, as measured by the mean delay for any message class at any station, is derived. The simulation results show excellent agreement with the analytical results, even under heavy loading. The results confirm earlier findings that the performance at a station is influenced by its position relative to heavily loaded stations