13893049
Pitfalls in the fluid modeling of RTT variations in window-based congestion control
Deterministic delay differential equation models, where the packet traffic is modeled as a fluid, are widely used to study congestion control algorithms in the Internet. In this paper, we point out some pitfalls in such fluid modeling of window flow control algorithms. Specifically, we argue that the modeling assumptions used to capture the variability in the RTT (due to queue length fluctuations) may play a critical role in our ability to design stable algorithms. We study two scenarios to illustrate the dramatic impact of RTT modeling. We first consider TCP-Reno with RED, and show that assuming that the RTT is a constant (when it is actually time-varying) leads to conservative parameter choices, i.e., the system continues to be stable even with variable RTT. On the other hand, for the recently proposed stabilized Vegas, we show the following result: while the network can be stabilized under the constant RTT assumption, there is no choice of parameters that would stabilize the system when the RTT variations are taken into account. Interestingly, such problems do not arise if the congestion-control mechanisms at the end-users are rate-based.
8071150
TCP-Illinois: A loss- and delay-based congestion control algorithm for high-speed networks
We introduce a new congestion control algorithm for high-speed networks, called TCP-Illinois. TCP-Illinois uses packet loss information to determine whether the window size should be increased or decreased, and uses queueing delay information to determine the amount of increment or decrement. TCP-Illinois achieves high throughput, allocates the network resource fairly, and is incentive compatible with standard TCP. We also build a new stochastic matrix model, capturing standard TCP and TCP-Illinois as special cases, and use this model to analyze their fairness properties for both synchronized and unsynchronized backoff behaviors. We finally perform simulations to demonstrate the performance of TCP-Illinois.
1315911
An Object Semantic Model of SOFL
SOFL (Structured-Object-based-Formal Language) is recently proposed to combine the advantages of formal methods, structured methods and object-oriented methodology into one method for software development. SOFL integrates modeling techniques from VDM (for specifying system component state) and data flow diagrams with Petri nets (for specifying system behaviours and interaction). To support the standardisation and the tool support for SOFL, a formal semantics of the language is desirable. As SOFL is under further development, the semantic representation of current SOFL needs to be easily reused and extended. In this paper, an object-oriented approach is taken to specify the semantics of SOFL and this approach is proved to be effective in providing a modular and reusable semantic representation. Object modeling techniques, such as inheritance is used to illustrate the common and the differences between various SOFL graphical constructs; object containment is used to capture the acyclic structure of SOFL hierarchical module decomposition.
7160160
QoS-UniFrame: a Petri net-based modeling approach to assure QoS requirements of distributed real-time and embedded systems
Assuring quality of service (QoS) requirements is critical when assembling a distributed real-time and embedded (DRE) system from a repository of existing components. This paper presents a two-level approach for assuring satisfaction of QoS requirements in the context of a reduced design space for DRE systems. A dynamic and parallel approach is introduced to prune off the infeasible design spaces at the first level. Evolutionary algorithms cooperating with a domain-specific scripting language then discard less probable design spaces using statistics. These techniques fulfill the collective objectives of pruning and assuring the design space at system assembly time.
9811887
A formal operational semantics for SOFL
SOFL (Structured-Object-Based-Formal Language) is a formal language and method for practical systems development. As a language, it integrates data flow diagrams, VDM-SL, and Petri nets to provide a graphical and textual formal notation for specification construction. Previous publications introducing SOFL are based on its informal semantics. We provide a formal operational semantics which gives a firm foundation for consistent understanding and application of SOFL in practical systems development. Development of the semantics, and application of SOFL to practical systems, have led to some changes in the original definitions. This formal semantics will serve as a firm foundation for building an effective support environment for SOFL.
10513438
Orientation-Selective aVLSI Spiking Neurons
We describe a programmable multi-chip VLSI neuronal system that can be used for exploring spike-based information processing models. The system consists of a silicon retina, a PIC microcontroller, and a transceiver chip whose integrate-and-fire neurons are connected in a soft winner-take-all architecture. The circuit on this multi-neuron chip approximates a cortical microcircuit. The neurons can be configured for different computational properties by the virtual connections of a selected set of pixels on the silicon retina. The virtual wiring between the different chips is effected by an event-driven communication protocol that uses asynchronous digital pulses, similar to spikes in a neuronal system. We used the multi-chip spike-based system to synthesize orientation-tuned neurons using both a feedforward model and a feedback model. The performance of our analog hardware spiking model matched the experimental observations and digital simulations of continuous-valued neurons. The multi-chip VLSI system has advantages over computer neuronal models in that it is real-time, and the computational time does not scale with the size of the neuronal network.
5842197
Configurable indexing and ranking for XML information retrieval
Indexing and ranking are two key factors for efficient and effective XML information retrieval. Inappropriate indexing may result in false negatives and false positives, and improper ranking may lead to low precisions. In this paper, we propose a configurable XML information retrieval system, in which users can configure appropriate index types for XML tags and text contents. Based on users' index configurations, the system transforms XML structures into a compact tree representation, Ctree, and indexes XML text contents. To support XML ranking, we propose the concepts of "weighted term frequency" and "inverted element frequency," where the weight of a term depends on its frequency and location within an XML element as well as its popularity among similar elements in an XML dataset. We evaluate the effectiveness of our system through extensive experiments on the INEX 03 dataset and 30 content and structure (CAS) topics. The experimental results reveal that our system has significantly high precision at low recall regions and achieves the highest average precision (0.3309) as compared with 38 official INEX 03 submissions using the strict evaluation metric.