On the Optimality of Cyclic Transmission in Teletext Systems

Teletext is a one-way information delivery system where pages of information are broadcast to all users in a continuous manner. System response time is an important consideration in the design of teletext systems. One factor contributing to response time is the order in which pages are transmitted. In this paper, we formulate the problem of determining the sequence of page transmissions as a Markovian decision process. Using this formulation we show that, from a response time point of view, a cyclic order of page transmissions is optimal. We also describe two algorithms for designing a teletext broadcast cycle.     

Response Time Performance of Videotex Systems

Videotex is an interactive information system which provides a variety of services to its users. Examples of such services are information retrieval, software distribution, transaction processing, and message handling. An important aspect of the quality of service experienced by a videotex user is the response time. We consider the use of mixed individual/broadcast delivery to enhance the response time performance. Broadcast delivery is attractive for information retrieval applications where several users may be requesting the same information page, and a single broadcast of this page will satisfy all requests simultaneously. Individual response, however, is required for transaction-oriented services and for the retrieval of confidential information. A queueing model is developed to study the performance of videotex systems under mixed delivery. Analytic results are derived for the mean response time. Numerical examples are presented to show the performance characteristics of mixed delivery, and how it can be used to enhance the response time performance without increasing the processing capacity of the system.     

Broadcast scheduling for information distribution

Broadcast data delivery is encountered in many applications where there is a need to disseminate information to a large user community in a wireless asymmetric communication environment. In this paper, we consider the problem of scheduling the data broadcast such that average response time experienced by the users is low. In a push-based system, where the users cannot place requests directly to the server and the broadcast schedule should be determined based solely on the access probabilities, we formulate a deterministic dynamic optimization problem, the solution of which provides the optimal broadcast schedule. Properties of the optimal solution are obtained and then we propose a suboptimal dynamic policy which achieves average response time close to the lower bound. The policy has low complexity, it is adaptive to changing access statistics, and is easily generalizable to multiple broadcast channels. In a pull-based system where the users may place requests about information items directly to the server, the scheduling can be based on the number of pending requests for each item. Suboptimal policies with good performance are obtained in this case as well. Finally, it is demonstrated by a numerical study that as the request generation rate increases, the achievable performance of the pull- and push-based systems becomes almost identical.     

A Cost-Efficient Scheduling Algorithm of On-Demand Broadcasts

In mobile wireless systems data on air can be accessed by a large number of mobile users. Many of these applications including wireless internets and traffic information systems are pull-based, that is, they respond to on-demand user requests. In this paper, we study the scheduling problems of on-demand broadcast environments. Traditionally, the response time of the requests has been used as a performance measure. In this paper we consider the performance as the average cost of request composed of three kinds of costs – access time cost, tuning time cost, and cost of handling failure request. Our main contribution is a self-adaptive scheduling algorithm named LDFC, which computes the delay cost of data item as the priority of broadcast. It costs less compared with some previous algorithms in this context, and shows good adaptability as well even in pure push-based broadcasts.     

Response time performance of a narrowband information broadcast system

This paper describes the architecture and performance analysis of a system using narrowband broadcasting channels for real-time information delivery. Broadcast delivery is attractive because a single transmission of a piece of information will satisfy all users waiting for that information. Therefore, the system may support an unlimited number of users with no effect on system response time. The main drawback of these types of systems compared with interactive systems is that a user must wait until the required information arrives. In order to reduce this waiting time, high data rate networks must be used if real-time information is to be broadcast. In this paper, a new method is proposed to enable inexpensive narrowband networks to be used.     

Efficient algorithms for scheduling data broadcast

With the increasing acceptance of wireless technology, mechanisms to efficiently transmit information to wireless clients are of interest. The environment under consideration is asymmetric in that the information server has much more bandwidth available, as compared to the clients. It has been proposed that in such systems the server should broadcast the information periodically. A broadcast schedule determines what is broadcast by the server and when. This paper makes the simple, yet useful, observation that the problem of broadcast scheduling is related to the problem of fair queueing. Based on this observation, we present a log‐time algorithm for scheduling broadcast, derived from an existing fair queueing algorithm. This algorithm significantly improves the time‐complexity over previously proposed broadcast scheduling algorithms. Modification of this algorithm for transmissions that are subject to errors is considered. Also, for environments where different users may be listening to different number of broadcast channels, we present an algorithm to coordinate broadcasts over different channels. Simulation results are presented for proposed algorithms.     

Real-Time Data Delivery Using Prediction Mechanism in Mobile Environments

Mobile data delivery is a critical issue in the mobile computing area. One of the most important problems is the efficient access to data. A proposed solution to this problem is the prefetching technique which consists in putting in reserving the information before the users need it. Low bandwidth, unreliable wireless links, and frequent disconnections of mobile environments make it difficult to satisfy the timing requirements of traditional strategies. This paper investigates broadcast scheduling strategies for push-based broadcast with timing constraints in the form of deadlines ,and proposes a prediction algorithm based on Kalman filter theory for this study. The proposed dissemination policy and adaptive bandwidth allocation scheme obtain sufficient conditions such that all the time-bounded traffic sources satisfy their timing constraints to provide various quality of service guarantees in the broadcast period. Our goal is to identify scheduling algorithms for broadcast systems that ensure requests meeting their deadlines. Our approach examines the performance of traditional real-time strategies and mobile broadcasting strategies, and demonstrates that traditional real-time algorithms do not always perform the best in mobile environments. The proposed design indeed achieves good performance in mobile environments.     

Response Time in a Teletext System: An Individual User's Perspective

Teletext is a one-way, broadcast-delivery information system. Pages of information are continuously broadcast to the users. User terminals monitor the broadcast stream and requested pages, once recognized, are captured and stored. Teletext systems possess many attractive features. Among them are simplicity of operation and insensitivity of system performance to user loads. Previous research has considered the issue of response time optimization in teletext systems. In those studies, response time was averaged over the entire user population. In this paper, we analyze an individual user's response time experience. Needless to say, this provides a better gauge of the quality of service being delivered. In addition, this perspective provides the appropriate framework for evaluating strategies that take advantage of user terminal storage capabilities to improve response time performance. To demonstrate this, one such strategy is developed and analyzed. Numerical examples that illustrate the use of the paper's results are also presented.     

Performance analysis of data scheduling algorithms for multi‐item requests in multi‐channel broadcast environments

Nowadays querying multiple‐dependent data items in a request is common in many advanced mobile applications, such as traffic information enquiry services. In addition, multi‐channel architectures are widely deployed in many data dissemination systems. In this paper, we extend a number of data productivity‐based scheduling algorithms and evaluate their performance in scheduling multi‐item requests in multi‐channel broadcast environments. We observe from the experimental results two performance problems that render these algorithms ineffective. Lastly, we discuss possible causes of these problems to give insights in the design of a better solution. Copyright © 2009 John Wiley & Sons, Ltd.     

Multicast traffic in input-queued switches: optimal scheduling and maximum throughput

The paper studies input-queued packet switches loaded with both unicast and multicast traffic. The packet switch architecture is assumed to comprise a switching fabric with multicast (and broadcast) capabilities, operating in a synchronous slotted fashion. Fixed-size data units, called cells, are transferred from each switch input to any set of outputs in one time slot, according to the decisions of the switch scheduler, that identifies at each time slot a set of nonconflicting cells, i.e., cells neither coming from the same input, nor directed to the same output. First, multicast traffic admissibility conditions are discussed, and a simple counterexample is presented, showing intrinsic performance losses of input-queued with respect to output-queued switch architectures. Second, the optimal scheduling discipline to transfer multicast packets from inputs to outputs is defined. This discipline is rather complex, requires a queuing architecture that probably is not implementable, and does not guarantee in-sequence delivery of data. However, from the definition of the optimal multicast scheduling discipline, the formal characterization of the sustainable multicast traffic region naturally follows. Then, several theorems showing intrinsic performance losses of input-queued with respect to output-queued switch architectures are proved. In particular, we prove that, when using per multicast flow FIFO queueing architectures, the internal speedup that guarantees 100% throughput under admissible traffic grows with the number of switch ports.     

Client-Side Caching Strategies and On-Demand Broadcast Algorithms for Real-Time Information Dispatch Systems

In this work, we propose a broadcast algorithm called Most Request Served (MRS) and its variants with caching strategies for on-time delivery of data in Real-Time Information Dispatch System. This family of algorithms consider request deadline, data object size and data popularity in making scheduling decisions. Although previous scheduling algorithms also base on some or all of these attributes to choose the most beneficial data to be broadcast, they did not consider the loss brought by their scheduling decisions. However, MRS considers both gain and loss in making a scheduling decision. We have performed a series of simulation experiments to compare the performance of various algorithms. Simulation results show that our proposed broadcast algorithm not only succeeds in providing good on-time delivery of data but at the same time provides 20% of improvement in response time over traditional scheduling algorithms like First-In-First-Out (FIFO) and Earliest-Deadline-First (EDF). Simulation results also show that our proposed caching strategy provides further improvement in terms of percentage of requests finished in time over traditional caching strategy like Least Recently Used (LRU).     

High Performance Data Broadcasting Systems

Data broadcasting as a means of efficient data dissemination is a key technology facilitating ubiquitous computing. For this reason, broadcast scheduling algorithms have received a lot of attention. However, all existing algorithms make the core assumption that the data items to be broadcast are immediately available in the transmitter's queue, ignoring the key role that the disk subsystem and the cache management play in the overall broadcast system performance. With this paper we contribute a comprehensive system's perspective towards the development of high performance broadcast systems, taking into account how broadcast scheduling, disk scheduling, and cache management algorithms affect the overall performance. We contribute novel techniques that ensure an efficient interplay between broadcast scheduling, cache management, and disk scheduling. We study comprehensively the performance of the broadcast server, as it consists of the broadcast scheduling, the disk scheduling, the cache management algorithms, and the transmitter. Our results show that the contributed algorithms yield considerably higher performance. Furthermore, one of our algorithms is shown to enjoy considerably higher performance, under all values of the problem and system parameters. A key contribution is the result that broadcast scheduling algorithms have only a small effect on the overall system performance, which necessitates the definition of different focal points for efforts towards high performance data broadcasting.     

Response Time in Data Broadcast Systems: Mean,Variance and Tradeoff

Data broadcast has been suggested as a promising method of information dissemination [2,33]. In such an environment, the information server cannot afford to serve the requests from a large population of users individually. Instead, the server uses a broadcast channel to deliver information to all users. A single transmission of a data item satisfies all pending requests for that item. The response time of a request depends on the broadcast time of the desired data item, which is scheduled by the server according to the overall demands for various data items. Therefore, the response time may vary in a large range. We argue that, in addition to mean response time, the variance of response time should also be taken into account by the broadcast scheduler. In this paper, we address the issue of variance optimization in regard to response time. Building on our previous research on mean response time optimization, we propose an algorithm which can minimize the variance of response time. Furthermore, we evaluate an algorithm that facilitates a tradeoff between the mean and variance of response time. Numerical examples that illustrate the performance of our algorithms are also presented.     

Simulation Models for Investigation of Multiuser Scheduling in MIMO Broadcast Channels

Spatial correlation is a result of insufficient antenna spacing among multiple antenna elements, while temporal correlation is caused by Doppler spread. This paper compares the effect of spatial and temporal correlation in order to investigate the performance of multiuser scheduling algorithms in multiple‐input multiple‐output (MIMO) broadcast channels. This comparison includes the effect on the ergodic capacity, on fairness among users, and on the sum‐rate capacity of a multiuser scheduling algorithm utilizing statistical channel state information in spatio‐temporally correlated MIMO broadcast channels. Numerical results demonstrate that temporal correlation is more meaningful than spatial correlation in view of the multiuser scheduling algorithm in MIMO broadcast channels. Indeed, the multiuser scheduling algorithm can reduce the effect of the Doppler spread if it exploits the information of temporal correlation appropriately. However, the effect of spatial correlation can be minimized if the antenna spacing is sufficient in rich scattering MIMO channels regardless of the multiuser scheduling algorithm used.     

On the optimality of multiantenna broadcast scheduling using zero-forcing beamforming

Although the capacity of multiple-input/multiple-output (MIMO) broadcast channels (BCs) can be achieved by dirty paper coding (DPC), it is difficult to implement in practical systems. This paper investigates if, for a large number of users, simpler schemes can achieve the same performance. Specifically, we show that a zero-forcing beamforming (ZFBF) strategy, while generally suboptimal, can achieve the same asymptotic sum capacity as that of DPC, as the number of users goes to infinity. In proving this asymptotic result, we provide an algorithm for determining which users should be active under ZFBF. These users are semiorthogonal to one another and can be grouped for simultaneous transmission to enhance the throughput of scheduling algorithms. Based on the user grouping, we propose and compare two fair scheduling schemes in round-robin ZFBF and proportional-fair ZFBF. We provide numerical results to confirm the optimality of ZFBF and to compare the performance of ZFBF and proposed fair scheduling schemes with that of various MIMO BC strategies.     

Scheduling data broadcast in asymmetric communication environments

With the increasing popularity of portable wireless computers, mechanisms to efficiently transmit information to wireless clients are of significant interest. The environment under consideration is asymmetric in that the information server has much more bandwidth available, as compared to the clients. In such environments, often it is not possible (or not desirable) for the clients to send explicit requests to the server. It has been proposed that in such systems the server should broadcast the data periodically. One challenge in implementing this solution is to determine the schedule for broadcasting the data, such that the wait encountered by the clients is minimized. A broadcast schedule determines what is broadcast by the server and when. In this paper, we present algorithms for determining broadcast schedules that minimize the wait time. Broadcast scheduling algorithms for environments subject to errors, and systems where different clients may listen to different number of broadcast channels are also considered. Performance evaluation results are presented to demonstrate that our algorithms perform well.     

Creating templates to achieve low delay in multi-carrier frame-based wireless data systems

We consider the problem of creating template-based schedules for multi-carrier frame-based wireless data systems. A template consists of an assignment of carriers to users over a fixed set of time slots. This schedule can then be repeated multiple times. Repeated template schedules require no continuous feedback of information (such as channel conditions), thereby relieving the signaling overhead. This setup is suitable for applications such as Wimax where users are typically static. Our aim is to assign carriers to users in such a way that the service per user is as smooth as possible. This in turn ensures that the users experience low delay. A number of elegant template scheduling algorithms exist for the single-carrier case. However, the case of multi-carrier systems where the channel rates can be different on different carriers has received much less attention. We present a general framework for studying the delay performance of a multi-carrier template. We then describe a number of deterministic and randomized scheduling algorithms for template creation and study their delay performance via analysis and simulation. We also show that the delay bounds can sometimes be improved by randomly shifting the schedule on each carrier and by scheduling in a hierarchical manner.     

Real time teletext broadcast system performance enhancement usingghost rows

Teletext is a one way broadcast system which provides a variety of services to its users. A one way broadcast system is attractive because a single transmission of a piece of information satisfies all potential users of that information. The main drawback of these types of systems is that a user must wait until the required information arrives. Therefore, the access time is an important consideration in the design of broadcast teletext systems. In addition, another important factor, information update delay, is very significant in real time broadcast teletext. The shortening of this delay is important for real time applications especially for systems broadcasting financial information. In this paper, we propose to use the ghost rows together with the storage capabilities of modern terminals to shorten both the access time and the update delay. A queueing model is developed to analyse the performance of this new system     

A non-competing hybrid optical burst switch architecture for QoS differentiation

In this paper, we present a new hybrid optical burst switch architecture (HOBS) that takes advantage of the pre-transmission idle time during lightpath establishment. In dynamic circuit switching (wavelength routing) networks, capacity is immediately hard-reserved upon the arrival of a setup message at a node, but it is used at least a round-trip time delay later. This waste of resources is significant in optical multi-gigabit networks and can be used to transmit traffic of a lower class of service in a non-competing way. The proposed hybrid OBS architecture, takes advantage of this idle time to transmit one-way optical bursts of a lower class of service, while high priority data explicitly requests and establishes end-to-end lightpaths. In the proposed scheme, the two control planes (two-way and one-way OBS reservation) are merged, in the sense that each SETUP message, used for the two-way lightpath establishment, is associated with one-way burst transmission and therefore it is modified to carry routing and overhead information for the one-way traffic as well. In this paper, we present the main architectural features of the proposed hybrid scheme and further we assess its performance by conducting simulation experiments on the NSF net backbone topology. The extensive network study revealed that the proposed hybrid architecture can achieve and sustain an adequate burst transmission rate with a finite worst case delay.     

Adaptive Dissemination of Data in Time-Critical Asymmetric Communication Environments

The proliferation of new data-intensive applications in asymmetric communication environments has led to an increasing interest in the development of push-based techniques, in which the information is broadcast to a large population of clients in order to achieve the most efficient use of the limited server and communication resources. It is important to note that quite often the data that is broadcast is time-critical in nature.Most of the related current research focuses on a pure push-based approach (Broadcast Disks model), where the transmission of data is done without allowing explicit requests from the users. More recently, some bidirectional models incorporating a low-capacity uplink channel have been proposed in order to increase the functionality of the Broadcast Disks model. However, the impact of integration of the uplink channel has been investigated using only static client profiles or ignoring the existence of time-sensitive data. None of the existing models integrates all the characteristics needed to perform effectively in a real-world, dynamic time-critical asymmetric communication environment.In this paper we present an adaptive data dissemination model and the associated on-line scheduling algorithms. These improve the functionality and performance of bidirectional broadcast models, maximizing the total number of satisfied users in asymmetric communication environments with dynamic client profiles and time requirements (e.g., mobile systems). This is achieved by means of dynamic adaptation of the broadcast program to the needs of the users, taking into account the bandwidth constraints inherent in asymmetric communication environments and the deadline requirements of the user requests. Performance is evaluated by simulation of a real-time asymmetric communication environment.