1349074
Static Guard Analysis in Timed Automata Verification
By definition Timed Automata have an infinite state-space, thus for verification purposes, an exact finite abstraction is required. We propose a location-based finite zone abstraction, which computes an abstraction based on the relevant guards for a particular state of the model (as opposed to all guards).We show that the location-based zone abstraction is sound and complete with respect to location reachability; that it generalises active-clock reduction, in the sense that an inactive clock has no relevant guards at all; that it enlarges the class of timed automata, that can be verified. We generalise the new abstraction to the case of networks of timed automata, and experimentally demonstrate a potentially exponential speedup compared to the usual abstraction.
4574615
As Cheap as Possible: Efficient Cost-Optimal Reachability for Priced Timed Automata
In this paper we present an algorithm for efficiently computing optimal cost of reaching a goal state in the model of Linearly Priced Timed Automata (LPTA). The central contribution of this paper is a priced extension of so-called zones. This, together with a notion of facets of a zone, allows the entire machinery for symbolic reachability for timed automata in terms of zones to be lifted to cost-optimal reachability using priced zones. We report on experiments with a cost-optimizing extension of Uppaal on a number of examples.
14315022
Scaling up Uppaal Automatic Verification of Real-Time Systems Using Compositionality and Abstraction
To combat the state-explosion problem in automatic verification, we present a method for scaling up the real-time verification tool UPPAAL by complementing it with methods for abstraction and compositionality. We identify a notion of timed ready simulation which we show is a sound condition for preservation of safety properties between real-time systems, and in addition is a precongruence with respect to parallel composition. Thus, it supports both abstraction and compositionality. We furthermore present a method for automatically testing for the existence of a timed ready simulation between real-time systems using the UPPAAL tool.
8987497
Efficient Timed Reachability Analysis Using Clock Difference Diagrams
One of the major problems in applying automatic verification tools to industrial-size systems is the excessive amount of memory required during the state-space exploration of a model. In the setting of real-time, this problem of state-explosion requires extra attention as information must be kept not only on the discrete control structure but also on the values of continuous clock variables. 
 
In this paper, we exploit Clock Difference Diagrams, CDD's, a BDD-like data-structure for representing and effectively manipulating certain nonconvex subsets of the Euclidean space, notably those encountered during verification of timed automata. 
 
A version of the real-time verification tool Uppaal using CDD's as a compact data-structure for storing explored symbolic states has been implemented. Our experimental results demonstrate significant spacesavings: for eight industrial examples, the savings are in average 42% with moderate increase in runtime. 
 
We further report on how the symbolic state-space exploration itself may be carried out using CDD's.
3252817
Optimal Strategies in Priced Timed Game Automata
Priced timed (game) automata extend timed (game) automata with costs on both locations and transitions. In this paper we focus on reachability priced timed game automata and prove that the optimal cost for winning such a game is computable under conditions concerning the non-zenoness of cost. Under stronger conditions (strictness of constraints) we prove that in case an optimal strategy exists, we can compute a state-based winning optimal strategy.
12983032
Practical verification of embedded software
Using a new verification algorithm called the compositional backward technique, the authors demonstrate that they can exhaustively verify even the largest industrial applications-comprising more than 1,000 components-in a few minutes on a standard PC.
27033090
Exponentially Decreasing Number of Operations in Balanced Trees
While many tree-like structures have been proven to support amortized constant number of operations after updates, considerably fewer structures have been proven to support the more general exponentially decreasing number of operations with respect to distance from the update. In addition, all existing proofs of exponentially decreasing operations are tailor-made for specific structures. We provide the first formalization of conditions under which amortized constant number of operations imply exponentially decreasing number of operations. Since our proof is constructive, we obtain the constants involved immediately. Moreover, we develop a number of techniques to improve these constants.
19914316
Extending the Accommodating Function
The applicability of the accommodating function, a relatively new measure for the quality of on-line algorithms, is extended. If a limited amount n of some resource is available, the accommodating function A(?) is the competitive ratio when input sequences are restricted to those for which the amount ?n of resources suffices for an optimal off-line algorithm. The accommodating function was originally used only for ? ? 1. We focus on ? < 1, observe that the function now appears interesting for a greater variety of problems, and use it to make new distinctions between known algorithms and to find new ones.
26883600
Distributed Redundancy or Cluster Solution? An Experimental Evaluation of Two Approaches for Dependable Mobile Internet Services
Third generation mobile networks are offering the user access to Internet services. The Session Initiation Protocol (SIP) is being deployed for establishing, modifying, and terminating those multimedia sessions. Mobile operators put high requirements on their infrastructure, in particular – and in focus of this paper – on availability and reliability of call control. One approach to minimize the impact of server failures is to implement redundant servers and to replicate the state between them in a timely manner. In this paper, we study two concepts for such a fault-tolerant architecture in their application to the highly relevant use-case of SIP call control. The first approach is implemented as a distributed set of servers gathered in a so-called pool, with fail-over functionality assigned to the pool access protocols. The second is a cluster-based solution that normally implies that the servers are confined in a local network, but on the other hand the latter solution is completely transparent to clients accessing the service deployed in the cluster. To evaluate these two approaches, both were implemented in an experimental testbed mimicking SIP call control scenarios in 3rd generation mobile networks. An approach for measurement of various dependability and performance parameters in this experimental setting is developed and concluded with a set of preliminary results.
14238837
Dynamic TCP acknowledgment in the LogP model
When messages, which are to be sent point-to-point in a network, become available at irregular intervals, a decision must be made each time a new message becomes available as to whether it should be sent immediately or if it is better to wait for more messages and send them all together. Because of physical properties of the networks, a certain minimum amount of time must elapse in between the transmission of two packets. Thus, whereas waiting delays the transmission of the current data, sending immediately may delay the transmission of the next data to become available even more. We propose a new quality measure and derive optimal deterministic and randomized algorithms for this on-line problem.
125126786
Relaxed multi-way trees with group updates
Data structures with relaxed balance differ from standard structures in that rebalancing can be delayed and interspersed with updates. This gives extra flexibility in both sequential and parallel, applications. We study the version of multi-way trees called (a, b)-trees (which includes B-trees) with the operations insertion, deletion, and group insertion. The latter has applications in for instance document databases, WWW search engines, and differential indexing. We prove that we obtain the optimal asymptotic rebalancing complexities of amortized constant time for insertion and deletion and amortized logarithmic time in the size of the group for group insertion. These results hold even for the relaxed version. This is an improvement over the existing results in the most interesting cases.