Traffic characterization algorithms for VBR video in multimedia networks

A network that offers deterministic, i.e., worst case, quality-of-service guarantees to variable-bit-rate (VBR) video must provide a resource reservation mechanism that allocates bandwidth, buffer space, and other resources for each video stream. Such a resource reservation scheme must be carefully designed, otherwise network resources are wasted. A key component for the design of a resource reservation scheme is the traffic characterization method that specifies the traffic arrivals on a video stream. The traffic characterization should accurately describe the actual arrivals, so that a large number of streams can be supported; but it must also map directly into efficient traffic-policing mechanisms that monitor arrivals on each stream. In this study, we present a fast and accurate traffic characterization method for stored VBR video in networks with a deterministic service. We use this approximation to obtain a traffic characterization that can be efficiently policed by a small number of leaky buckets. We present a case study where we apply our characterization method to networks that employ a dynamic resource reservation scheme with renegotiation. We use traces from a set of 25–30-min MPEG sequences to evaluate our method against other characterization schemes from the literature.     

Deterministic delay bounds for VBR video in packet-switchingnetworks: fundamental limits and practical trade-offs

Compressed digital video is one of the most important traffic types in future integrated services networks. However, a network service that supports delay-sensitive video imposes many problems since compressed video sources are variable bit rate (VBR) with a high degree of burstiness. In this paper, we consider a network service that can provide deterministic guarantees on the minimum throughput and the maximum delay of VBR video traffic. A common belief is that due to the burstiness of VBR traffic, such a service will not be efficient and will necessarily result in low network utilization. We investigate the fundamental limits and trade-offs in providing deterministic performance guarantees to video and use a set of 10 to 30 min. long MPEG-compressed video traces for evaluation. Contrary to conventional wisdom, we are able to show that, in many cases, a deterministic service can be provided to video traffic while maintaining a reasonable level of network utilization. We first consider an ideal network environment that employs the most accurate deterministic, time-invariant video traffic characterizations, the optimal earliest-deadline-first packet schedulers, and exact admission control conditions. The utilization achievable in this situation provides the fundamental limits of a deterministic service. We then investigate the utilization limits in a network environment that takes into account practical constraints, such as the need for simple and efficient policing mechanisms, packet scheduling algorithms, and admission control tests     

A Network Calculus With Effective Bandwidth

This paper establishes a link between two principal tools for the analysis of network traffic, namely, effective bandwidth and network calculus. It is shown that a general version of effective bandwidth can be expressed within the framework of a probabilistic version of the network calculus, where both arrivals and service are specified in terms of probabilistic bounds. By formulating well-known effective bandwidth expressions in terms of probabilistic envelope functions, the developed network calculus can be applied to a wide range of traffic types, including traffic that has self-similar characteristics. As applications, probabilistic lower bounds are presented on the service given by three different scheduling algorithms: static priority, earliest deadline first, and generalized processor sharing. Numerical examples show the impact of specific traffic models and scheduling algorithms on the multiplexing gain in a network.     

Priority queue schedulers with approximate sorting inoutput-buffered switches

All recently proposed packet-scheduling algorithms for output-buffered switches that support quality-of-service (QoS) transmit packets in some priority order, e.g., according to deadlines, virtual finishing times, eligibility times, or other time stamps that are associated with a packet. Since maintaining a sorted priority queue introduces significant overhead, much emphasis on QoS scheduler design is put on methods to simplify the task of maintaining a priority queue. In this paper, we consider an approach that attempts to approximate a sorted priority queue at an output-buffered switch. The goal is to trade off less accurate sorting for lower computational overhead. Specifically, this paper presents a scheduler that approximates the sorted queue of an earliest-deadline-first (EDF) scheduler. The approximate scheduler is implemented using a set of prioritized first-in/first-out (FIFO) queues that are periodically relabeled. The scheduler can be efficiently implemented with a fixed number of pointer manipulations, thus enabling an implementation in hardware. Necessary and sufficient conditions for the worst-case delays of the scheduler with approximate sorting are presented. Numerical examples, including traces based on MPEG video, demonstrate that in realistic scenarios, scheduling with approximate sorting is a viable option     

Dual bus MAN's with multiple-priority traffic

A protocol with strictly preemptive priorities that does not admit low-priority traffic if the load from high-priority traffic exceeds the capacity of the transmission channel in a MAN is presented. The protocol guarantees fairness for transmissions at the highest priority level. By introducing a general characterization of bandwidth allocation schemes for dual bus networks, existing priority mechanisms can be categorized according to the provided quality of service. The unique existence of a bandwidth allocation scheme for multiple priority traffic is shown with a full utilization of the channel capacity, with a fair distribution of bandwidth respective to traffic from a particular priority level, and with preemptive priorities. The performance of the presented protocol is compared to existing proposals for multiple priority mechanisms. It is shown that adopting the new protocol results in shorter access delays for high-priority transmissions. The protocol allows the stations of the network to react quickly to load changes. It is shown that the effectiveness of the priority scheme, compared to priority schemes using the bandwidth-balancing mechanism, is less dependent on increasing the transmission speed of the network     

Tradeoffs in Designing Networks with End-to-End Statistical QoS Guarantees

Recent research on statistical multiplexing has provided many new insights into the achievable multiplexing gain in QoS networks. However, usually, these results are stated in terms of the gain experienced at a single switch, and evaluating the statistical multiplexing gain in a general network remains a difficult challenge. In this paper we describe two distinct network designs for statistical end-to-end delay guarantees, referred to as class-level aggregation and path-level aggregation. These designs illustrate an inherent trade-off between attainable statistical multiplexing gain and the ability to support delay guarantees. The key characteristic of both designs is that they do not require, and instead, intentionally avoid, consideration of the correlation between flows at multiplexing points inside the network. Numerical examples are presented for a comparison of the statistical multiplexing gain achievable by the two designs. The class-level aggregation design is shown to yield very high achievable link utilizations while simultaneously achieving desired statistical guarantees on delay.     

A priority scheme for the IEEE 802.14 MAC protocol for hybridfiber-coax networks

In order to support quality-of-service (QoS) for real-time data communications such as voice, video and interactive services, multiaccess networks must provide an effective priority mechanism. The context of this work is the IEEE 802.14 standard for hybrid fiber coaxial (HFC) networks which has a shared upstream channel for transmissions from stations to the headend. This work presents a multilevel priority collision resolution scheme, which separates and resolves collisions between stations in a priority order, thereby, achieving the capability for preemptive priorities. We present a set of simulation scenarios which show the robustness and efficiency of the scheme, such as its ability to isolate higher priority traffic from lower priorities and to provide quick access to high-priority requests. In March 1998, a framework for handling priorities in the collision resolution process, which adopts a semantics similar to the semantics of our scheme, was included in the 802.14 standard     

Statistical service assurances for traffic scheduling algorithms

Network services for the most demanding advanced networked applications which require absolute, per-flow service assurances can be deterministic or statistical. By exploiting the statistical properties of traffic, statistical assurances can extract more capacity from a network than deterministic assurances. We consider statistical service assurances for traffic scheduling algorithms. We present functions, so-called effective envelopes, which are, with high certainty, upper bounds of multiplexed traffic. Effective envelopes can be used to obtain bounds on the amount of traffic on a link that can be provisioned with statistical service assurances. We show that our bounds can be applied to a variety of traffic scheduling algorithms. In fact, one can reuse existing admission control functions for scheduling algorithms with deterministic assurances. We present numerical examples which compare the number of flows with statistical assurances that can be admitted with our effective envelope approach to those achieved with existing methods     

Influence of phase transformations on the mechanical properties of high-strength austenitic Fe-Mn-Cr steel

The influence of Cr and N additions on the mechanical properties of a Fe-Mn-C steel was investigated. The chemical composition was found to have a pronounced effect on the strain-hardening behavior, due to the strain-induced sequence of the γ → ▓ → α′ martensitic transformations. It was found that Cr and N suppress this transformation sequence. At Cr levels higher than 7.5 mass pct, no α′ martensite was formed, which led to a pronounced improvement of the ductility. The differences in transformation behavior can be attributed to the change in the intrinsic stacking-facult energy (ISFE): in the compositional range studied, Cr and N additions cause an increase of the ISFE.