Schedulability criterion and performance analysis of coordinated schedulers

Inter-server coordinated scheduling is a mechanism for downstream nodes to increase or decrease a packet's priority according to the congestion incurred at upstream nodes. In this paper, we derive an end-to-end schedulability condition for a broad class of coordinated schedulers that includes Core-stateless Jitter Virtual Clock (CJVC) and Coordinated Earliest Deadline First (CEDF). In contrast to previous approaches, our technique purposely allows flows to violate their local priority indexes while still providing an end-to-end delay bound. We show that under a simple priority assignment scheme, coordinated schedulers can outperform WFQ schedulers, while replacing per-flow scheduling operations with a simple coordination rule. Finally, we illustrate the performance advantages of coordination through numerical examples and simulation experiments.     

Latency-rate servers: a general model for analysis of trafficscheduling algorithms

We develop a general model, called latency-rate servers (ℒℛ servers), for the analysis of traffic scheduling algorithms in broadband packet networks. The behavior of an ℒℛ server is determined by two parameters-the latency and the allocated rate. Several well-known scheduling algorithms, such as weighted fair queueing, virtualclock, self-clocked fair queueing, weighted round robin, and deficit round robin, belong to the class of ℒℛ servers. We derive tight upper bounds on the end-to-end delay, internal burstiness, and buffer requirements of individual sessions in an arbitrary network of ℒℛ servers in terms of the latencies of the individual schedulers in the network, when the session traffic is shaped by a token bucket. The theory of ℒℛ servers enables computation of tight upper bounds on end-to-end delay and buffer requirements in a heterogeneous network, where individual servers may support different scheduling architectures and under different traffic models     

Coordinated multihop scheduling: a framework for end-to-end services

In multihop networks, packet schedulers at downstream nodes have an opportunity to make up for excessive latencies due to congestion at upstream nodes. Similarly, when packets incur low delays at upstream nodes, downstream nodes can reduce priority and schedule other packets first. The goal of this paper is to define a framework for design and analysis of coordinated multihop scheduling (CMS) which exploits such internode coordination. We first provide a general CMS definition which enables us to classify a number of schedulers from the literature, including G-EDF, FIFO+, CEDF, and work-conserving CJVC as examples of CMS schedulers. We then develop a distributed theory of traffic envelopes which enables us to derive end-to-end statistical admission control conditions for CMS schedulers. We show that CMS schedulers are able to limit traffic distortion to within a narrow range resulting in improved end-to-end performance and more efficient resource utilization. Consequently, our technique exploits statistical resource sharing among flows, classes, and nodes, and our results provide the first statistical multinode multiclass admission control algorithm for networks of work conserving servers.     

iDSRT: Integrated Dynamic Soft Real-time Architecture for Critical Infrastructure Data Delivery over WLAN

The real-time control data delivery system of the Critical Infrastructure (i.e. SCADA—Supervisory Control and Data Acquisition system) is important because appropriate decisions cannot be made without having data delivered in a timely manner. Because these applications use multiple heterogeneous resources such as CPU, network bandwidth and storage, they call for an integrated and coordinated real-time scheduling across multiple resources to meet end-to-end deadlines. We present a design and implementation of iDSRT—an integrated dynamic soft real-time system to provide fine-grained end-to-end delay guarantees over WLAN. iDSRT takes the deadline partitioning approach: end-to-end deadlines are partitioned into multiple sub-deadlines for CPU scheduling and network scheduling. It integrates three important schedulers: task scheduler, packet scheduler and node scheduler to achieve global coordination. We validate iDSRT in Linux and evaluate it in an experimental SCADA test-bed. The results are promising and show that iDSRT can successfully achieve soft real-time guarantees in SCADA system with very low packet loss rate compared to available commodity best-effort systems.     

Performance Evaluation of Resource Reservation Policies for Rate-Controlled Earliest-Deadline-First Scheduling in Multi-Service Packet Networks

The paper addresses the issue of reserving resources at packet switches along the path of flows requiring a deterministic bound on end-to-end delay. The switches are assumed to schedule outgoing packets using the Rate-Controlled Earliest-Deadline-First (RC-EDF) scheduling discipline. EDF is known to be an optimal scheduling discipline for deterministic delay services in the single scheduler case. We propose a number of static and dynamic reservation policies for mapping the end-to-end delay requirement of a flow into local delay deadlines to be reserved at each scheduler. These policies are based on non-even resource reservation where the resources reserved depend on the capacities and loading at each node in the network. We define and prove the optimality of a certain non-even policy for the case of a single path network with homogenous static traffic. We present extensive simulation results for different scenarios which show that dynamic non-even resource reservation provides superior performance when compared to simple policies such as even dividing of end-to-end delay among the schedulers.     

Providing absolute differentiated services for real-time applications in static-priority scheduling networks

In this paper, we propose and analyze a methodology for providing absolute differentiated services for real-time applications. We develop a method that can be used to derive delay bounds without specific information on flow population. With this new method, we are able to successfully employ a utilization-based admission control approach for flow admission. This approach does not require explicit delay computation at admission time and, hence, is scalable to large systems. We assume the underlying network to use static-priority schedulers. We design and analyze several priority assignment algorithms and investigate their ability to achieve higher utilization bounds. Traditionally, schedulers in differentiated services networks assign priorities on a class-by-class basis, with the same priority for each class on each router. In this paper, we show that relaxing this requirement, that is, allowing different routers to assign different priorities to classes, achieves significantly higher utilization bounds.     

SRR: an O(1) time-complexity packet scheduler for flows in multiservice packet networks

We present a novel fair queueing scheme, which we call Smoothed Round Robin (SRR). Ordinary round-robin schedulers are well known for the burstiness of their scheduling output. In order to overcome this problem, SRR codes the weights of the flows into binary vectors to form a Weight Matrix and then uses a Weight Spread Sequence (WSS), which is specially designed to distribute the output more evenly, to schedule packets by scanning a Weight Matrix. By using the WSS and the Weight Matrix, SRR emulates the Generalized Processor Sharing (GPS) well. SRR possesses better short-term fairness and scheduling delay properties in comparison with various existing round-robin schedulers. At the same time, SRR preserves O(1) time complexity by avoiding the time-stamp maintenance employed in various fair queueing schedulers. Simulation and implementation experiments show that SRR provides good mean end-to-end delay for soft real-time services. SRR can be implemented in high-speed networks to provide quality of service due to its simplicity and low time complexity.     

Generalized guaranteed rate scheduling algorithms: a framework

In this paper, we define a class of generalized guaranteed rate (GR) scheduling algorithms that includes algorithms which allocate a variable rate to the packets of a flow. We define work-conserving generalized virtual clock, packet-by-packet generalized processor sharing, and self-clocked fair queueing scheduling algorithms that can allocate a variable rate to the packets of a flow. We also define scheduling algorithms suitable for servers where packet fragmentation may occur. We demonstrate that if a class of rate controllers is employed for a flow in conjunction with any scheduling algorithm in GR, then the resulting non-work-conserving algorithm also belongs to GR. This leads to the definition of several non-work-conserving algorithms. We then present a method for deriving the delay guarantee of a network of servers when: (1) different rates are allocated to packets of a flow at different servers along the path and the bottleneck server for each packet may be different, and (2) packet fragmentation and/or reassembly may occur. This delay guarantee enables a network to provide various service guarantees to flows conforming to any specification. We illustrate this by utilizing delay guarantee to derive delay bounds for flows conforming to leaky bucket, exponentially bounded burstiness, and flow specification. Our method for determining these bounds is valid in internetworks and leads to tighter results     

Rate-proportional servers: a design methodology for fair queueingalgorithms

Generalized processor sharing (GPS) has been considered as an ideal scheduling discipline based on its end-to-end delay bounds and fairness properties. Until recently, emulation of GPS in a packet server has been regarded as the ideal means of designing a packet-level scheduling algorithm to obtain low delay bounds and bounded unfairness. Strict emulation of GPS, as required in the weighted fair queueing (WFQ) scheduler, however, incurs a time-complexity of O(N) where N is the number of sessions sharing the link. Efforts in the past to simplify the implementation of WFQ, such as self-clocked fair queueing (SCFQ), have resulted in degrading its isolation properties, thus affecting the delay bound. We present a methodology for the design of scheduling algorithms that provide the same end-to-end delay bound as that of WFQ and bounded unfairness without the complexity of GPS emulation. The resulting class of algorithms, called rate-proportional servers (RPSs), are based on isolating scheduler properties that give rise to ideal delay and fairness behavior. Network designers can use this methodology to construct efficient fair-queueing algorithms, balancing their fairness with implementation complexity     

Downlink Wireless Packet Scheduling with Deadlines

Next-generation cellular wireless communication networks aim to provide a variety of quality-of-service (QoS)-sensitive packet-based services to downlink users. Included among these are real-time multimedia services, which have stringent delay requirements. Downlink packet scheduling at the base station plays a key role in efficiently allocating system resources to meet the desired level of QoS for various users. In this paper, we employ dynamic programming (DP) to study the design of a downlink packet scheduler capable of supporting real-time multimedia applications. Under well-justified modeling reductions, we extensively characterize structural properties of the optimal control associated with the DP problem. We leverage intuition gained from these properties to propose a heuristic scheduling policy, namely, Channel-Aware Earliest Due Date (CA-EDD), which is based on a "quasi- static" approach to scheduling. The per-time-slot implementation complexity of CA-EDD is only O(K) for a system with K downlink users. Experimental results show that CA-EDD delivers up to 50 percent of performance gains over benchmark schedulers. CA-EDD achieves these performance gains by using channel and deadline information in conjunction with application layer information (relative importance of packets) in a systematic and unified way for scheduling.     

Hardware implementation of fair queuing algorithms for asynchronoustransfer mode networks

Providing quality-of-service guarantees in both cell- and packet-based networks requires the use of a scheduling algorithm in the switches and network interfaces. These algorithms need to be implemented in hardware in a high-speed switch. The authors present a number of approaches to implement scheduling algorithms in hardware. They begin by presenting a general methodology for the design of timestamp-based fair queuing algorithms that provide the same bounds on end-to-end delay and fairness as those of weighted fair queuing, yet have efficient hardware implementations. Based on this general methodology, the authors describe two specific algorithms, frame-based fair queuing and starting potential-based fair queuing, and discuss illustrative implementations in hardware. These algorithms may be used in both cell switches and packet switches with variable-size packets. A methodology for combining a traffic shaper with this class of fair queuing schedulers is also presented for use in network interface devices, such as an ATM segmentation and reassembly device     

Adaptive proportional delay differentiated services:characterization and performance evaluation

We examine a proportional-delay model for Internet differentiated services. Under this model, an Internet service provider (ISP) can control the waiting-time "spacings" between different classes of traffic. Specifically, the ISP tries to ensure that the average waiting time of class i traffic relative to that of class i-1 traffic is kept at a constant specified ratio. If the waiting-time ratio of class i-1 to class i is greater than one, the ISP can legitimately charge users of class i traffic a higher tariff rate (compared to the rate for class i-1 traffic), since class i users consistently enjoy better performance than class i-1 users. To realize such proportional-delay differentiated services, we use the time-dependent priority scheduling algorithm. We formally characterize the feasible regions in which given delay ratios can be achieved. Moreover, a set of control parameters for obtaining the desired delay ratios can be determined by an efficient iterative algorithm. We also use an adaptive control algorithm to maintain the correctness of these parameters in response to changing system load. Experiments are carried out to illustrate the short-term, medium-term and long-term relative waiting-time performances for different service classes under Poisson, Pareto, MMPP and mixed traffic workloads. We also carry out experiments to evaluate the achieved end-to-end accumulative waiting times for different classes of traffic which traverse multiple hops under our service model     

Traffic scheduling in a photonic packet switching system with QoSguarantee

The main challenge in the design of future broadband networks is to efficiently support high-bandwidth multimedia services. Recent advances in the optical networking reveal that all optical networks offering multigigabit rate per wavelength may soon become economical as the underlying backbone in wide area networks, in which photonic switch plays a central role. Two issues are the essential in the design of photonic packet switching, the support of end-to-end virtual connections and the support of diverse quality-of-service (QoS) services. Existing work in wide-area optical networks has largely focused on the former, relatively less attention has been given to support heterogeneous traffic types and to satisfy the potentially different QoS requirements of different types of traffic. In this paper, we introduce a novel hierarchical scheduling framework to use in a class of photonic packet switching systems based on WDM, in which we separate the flow scheduling from the transmission scheduling. We show such separation is essential for achieving scalability such that large input-output ports can be accommodated, and also for offering flexibility in that optimal scheduling algorithms can be derived in different level that can be best tuned to the specific system requirements. The salient feature of the proposed scheduling mechanism is that it takes into account potentially different QoS requirements from different traffic flows. A number of interesting findings are observed from the results obtained by both analysis and simulation: (1) QoS requirements can be satisfied for both real-time and nonreal-time flows; (2) the impact Of the real-time traffic head-of-line (HoL) blocking on the system throughput can be effectively alleviated with the prevailing number of traffic flows. In addition, we investigate a variety of performance measures under different system configurations     

An adaptive cross-layer scheduler for improved QoS support of multiclass data services on wireless systems

With the rapid growth of the Internet and other Internet protocol-related applications, wireless systems have shifted toward supporting a wider variety of data services. This has placed an enormous strain on the already tight capacity of wireless systems. Many improvements have been made at the link layer in second- and third-generation wireless systems. However, further improvements in packet data admission, scheduling, and policing are necessary to maximize capacity and user satisfaction. Of the three, efficient scheduling has the greatest impact on increased system capacity. In addition, proper scheduling can greatly improve user satisfaction. We introduce an adaptive cross-layer packet scheduler, which outperforms popular packet schedulers with respect to both packet delay and user throughput.     

Virtual time reference system: a unifying scheduling framework forscalable support of guaranteed services

We propose and develop a novel virtual time reference system as a unifying scheduling framework to provide scalable support for guaranteed services. This virtual time reference system is designed as a conceptual framework upon which guaranteed services can be implemented in a scalable manner using the DiffServ paradigm. The key construct in the proposed virtual time reference system is the notion of packet virtual time stamps, whose computation is core stateless, i.e., no per-flow states are required for its computation. We lay the theoretical foundation for the definition and construction of packet virtual time stamps. We describe how per-hop behavior of a core router (or rather its scheduling mechanism) can be characterized via packet virtual time stamps, and based on this characterization establish end-to-end per-flow delay bounds. Consequently, we demonstrate that, in terms of its ability to support guaranteed services, the proposed virtual time reference system has the same expressive power and generality as the IntServ model. Furthermore, we show that the notion of packet virtual time stamps leads to the design of new core stateless scheduling algorithms, especially work-conserving ones. In addition, our framework does not exclude the use of existing scheduling algorithms such as stateful fair queuing algorithms to support guaranteed services     

Efficient network QoS provisioning based on per node trafficshaping

This paper addresses the problem of providing per-connection end-to-end delay guarantees in a high-speed network. We consider a network comprised of store-and-forward packet switches, in which a packet scheduler is available at each output link. We assume that the network is connection oriented and enforces some admission control which ensures that the source traffic conforms to specified traffic characteristics. We concentrate on the class of rate-controlled service (RCS) disciplines, in which traffic from each connection is reshaped at every hop, and develop end-to-end delay bounds for the general case where different reshapers are used at each hop. In addition, we establish that these bounds can also be achieved when the shapers at each hop have the same “minimal” envelope. The main disadvantage of this class of service discipline is that the end-to-end delay guarantees are obtained as the sum of the worst-case delays at each node, but we show that this problem can be alleviated through “proper” reshaping of the traffic. We illustrate the impact of this reshaping by demonstrating its use in designing RCS disciplines that outperform service disciplines that are based on generalized processor sharing (GPS). Furthermore, we show that we can restrict the space of “good” shapers to a family which is characterized by only one parameter. We also describe extensions to the service discipline that make it work conserving and as a result reduce the average end-to-end delays     

Improved MLWDF scheduler for LTE downlink transmission

In long-term evolution (LTE) downlink transmission, modified least weighted delay first (MLWDF) scheduler is a quality of service (QoS) aware scheduling scheme for real-time (RT) services. Nevertheless, MLWDF performs below optimal among the trade-off between strict delay and loss restraints of RT and non-RT traffic flows, respectively. This is further worsened with the implementation of hybrid automatic retransmission request (HARQ). As these restraints grow unabated with increasing number of user demands, the performance of MLWDF further reduces. In order to ameliorate this situation, there is a need to directly incorporate the variations in user demands and HARQ implementation as parameters to the MLWDF scheduler. In this work, an improvement to the MLWDF scheduler is proposed. The improvement entails adding two novel parameters that characterise user demand and HARQ implementation. The scheduler was tested using varying three classes of service in QoS class identifiers (QCIs) table standardised by Third Generation Partnership Project for LTE network to characterise different services. It was also tested on the basis of packet prioritisation. The proposed scheduler was simulated with LTE-SIM simulator and compared with the MLWDF and proportional fairness schedulers. In terms of delay, throughput and packet loss ratio; the proposed scheduler increased overall system performance.     

Hierarchical packet fair queueing algorithms

We propose to use the idealized hierarchical generalized processor sharing (H-GPS) model to simultaneously support guaranteed real-time, rate-adaptive best-effort, and controlled link-sharing services. We design hierarchical packet fair queueing (H-PFQ) algorithms to approximate H-GPS by using one-level variable-rate PFQ servers as basic building blocks. By computing the system virtual time and per packet virtual start/finish times in unit of bits instead of seconds, most of the PFQ algorithms in the literature can be properly defined as variable-rate servers. We develop techniques to analyze delay and fairness properties of variable-rate and hierarchical PFQ servers. We demonstrate that in order to provide tight delay bounds with an H-PFQ server, it is essential for the one-level PFQ servers to have small worst-case fair indices (WFI). We propose a new PFQ algorithm called WF ²Q+ that is the first to have all the following three properties: (1) providing the tightest delay bound among all PFQ algorithms; (2) having the smallest WFI among all PFQ algorithms; and (3) having a relatively low asymptotic complexity of O(log N). Simulation results are presented to evaluate the delay and link-sharing properties of H-WF²Q+, H-WFQ, H-SFQ, and H-SCFQ     

Delay-bounded packet scheduling of bursty traffic over wireless channels

In this paper, we study minimal power transmission of bursty sources over wireless channels with constraints on mean queuing delay. The power minimizing schedulers adapt power and rate of transmission based on the queue and channel state. We show that packet scheduling based on queue state can be used to trade queuing delay with transmission power, even on additive white Gaussian noise (AWGN) channels. Our extensive simulations show that small increases in average delay can lead to substantial savings in transmission power, thereby providing another avenue for mobile devices to save on battery power. We propose a low-complexity scheduler that has near-optimal performance. We also construct a variable-rate quadrature amplitude modulation (QAM)-based transmission scheme to show the benefits of the proposed formulation in a practical communication system. Power optimal schedulers with absolute packet delay constraints are also studied and their performance is evaluated via simulations.     

Routing Strategies to Minimize Packet Loss in an Optical Packet Switched Network with Recirculating FDL Buffers

The major goal of optical packet switching (OPS) is to match switching technology to the huge capacities provided by (D)WDM. We study optical packet switches with recirculating fiber delay line (FDL) buffers. Through simulation, we have assessed the logical performance of a single optical packet router (OPR), focusing on packet loss rate (PLR). By verifying that our scheduling algorithm does not alter the traffic profile characteristics from in- to output, we illustrate how the single node results can be used to assess network-wide performance. We use the capability of assessing end-to-end PLRs to develop network-wide routing algorithms designed to minimize the maximal PLR occurring in the network. In case studies on pan-European networks, we first compare two algorithm variants and thereafter we compare the PLR-based routing algorithm with both load balancing and shortest path routing. While load balancing achieves PLRs that are multiple orders of magnitude lower than shortest path routing, the PLR-based algorithm can reach PLRs up to two orders of magnitude better. The improvement in PLR comes at the price of only a small increase in used bandwidth (a few percent). Subsequently we show that the discussed PLR-based routing algorithm can be easily extended to multiple priorities. By introducing multiple priorities we can keep the loss rates for high priority traffic very low. However, it may lead to an increase of the obtained minimal max-PLR value for low priority traffic. But as we prove this increase to be limited, the cost of introducing multiple priorities is small.