Lossy Communication Research Articles

Fair testing

In this paper we present a solution to the long-standing problem of characterising the coarsest liveness-preserving pre-congruence with respect to a full (TCSP-inspired) process algebra. In fact, we present two distinct characterisations, which give rise to the same relation: an operational one based on a De Nicola–Hennessy-like testing modality which we call should-testing, and a denotational one based on a refined notion of failures. One of the distinguishing characteristics of the should-testing pre-congruence is that it abstracts from divergences in the same way as Milner’s observation congruence, and as a consequence is strictly coarser than observation congruence. In other words, should-testing has a built-in fairness assumption. This is in itself a property long sought-after; it is in notable contrast to the well-known must-testing of De Nicola and Hennessy (denotationally characterised by a combination of failures and divergences), which treats divergence as catastrophic and hence is incompatible with observation congruence. Due to these characteristics, should-testing supports modular reasoning and allows to use the proof techniques of observation congruence, but also supports additional laws and techniques. Moreover, we show decidability of should-testing (on the basis of the denotational characterisation). Finally, we demonstrate its advantages by the application to a number of examples, including a scheduling problem, a version of the Alternating Bit-protocol, and fair lossy communication channels.

Effects of compression parameters on the perceived quality of video stream over a lossy Internet protocol network

We analyze the connection between viewer-perceived quality and encoding schemes. The encoding schemes depend on transmission bit-rate, MPEG compression depth, frame size and frame rate in a constant bit-rate (CBR) video transmission of a MPEG-2 video sequence. The compressed video sequence is transmitted over a lossy communication network with quality of service (QoS) and a certain Internet (IP) loss model. On the end-user side, viewer-perceived quality depends on changes in the network conditions, the video compression, and the video content complexity. We demonstrate that, when jointly considering the impact of coding bit rate, packet loss, and video complexity, there is an optimal encoding scheme, which also depends on the video content. We use a set of subjective tests to demonstrate that this optimal encoding scheme maximizes the viewer-perceived quality.

New Hybrid Error Concealment for Digital Compressed Video

Transmission of a compressed video signal over a lossy communication network exposes the information to losses and errors, which leads to significant visible errors in the reconstructed frames at the decoder side. In this paper we present a new hybrid error concealment algorithm for compressed video sequences, based on temporal and spatial concealment methods. We describe spatial and temporal techniques for the recovery of lost blocks. In particular, we develop postprocessing techniques for the reconstruction of missing or damaged macroblocks. A new decision support tree is developed to efficiently choose the best appropriate error concealment method, according to the spatial and temporal characteristics of the sequence. The proposed algorithm is compared to three error concealment methods: spatial, temporal, and a previous hybrid approach using different noise levels. The results are evaluated using four quality measures. We show that our error concealment scheme outperforms all the other three methods for all the tested video sequences.

Method to recover lost Internet packets using (n, k, m) convolutional codes

AbstractA lost packet recovery method is proposed by using (n, k, m) convolutional codes. The amount of computation in encoding and decoding as well as the lost packet recovery capability are evaluated. In the proposed method, a packet with a length of q bits is considered as a symbol on GF(2q) for application of the convolutional codes. It is recognized that on the Internet the packet loss position can be identified. Then, decoding is applied to a lossy communications channel on the packet level. First, the procedures for coding and decoding are presented. Subsequently, the packet loss recovery characteristic in the proposed method is analyzed. The condition for recovery of all lost packets is discussed. In the proposed method, linear simultaneous equations can be developed from the generating matrix and the packet loss positions. If these equations have a unique solution, all packet losses can be recovered. Also, by means of the lost packet recovery simulation, the recovery capability and computational complexity of the proposed method are evaluated. Under the model in which packets are independently lost, the method is compared with the packet recovery method using (n, k) Reed–Solomon codes with identical n and k. Simulation shows that packet recovery by (n, k, m) convolutional codes is effective in the area where the packet loss rate is rather low. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 88(7): 1–13, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20155

Optimal Erasure Protection Assignment for Scalable Compressed Data with Small Channel Packets and Short Channel Codewords

We are concerned with the efficient transmission of scalable compressed data over lossy communication channels. Recent works have proposed several strategies for assigning optimal code redundancies to elements in a scalable data stream under the assumption that all elements are encoded onto a common group of network packets. When the size of the data to be encoded becomes large in comparison to the size of the network packets, such schemes require very long channel codes with high computational complexity. In networks with high loss, small packets are generally more desirable than long packets. This paper proposes a robust strategy for optimally assigning elements of the scalable data to clusters of packets, subject to constraints on packet size and code complexity. Given a packet cluster arrangement, the scheme then assigns optimal code redundancies to the source elements subject to a constraint on transmission length. Experimental results show that the proposed strategy can outperform previously proposed code redundancy assignment policies subject to the above-mentioned constraints, particularly at high channel loss rates.

Methods for Reducing the Amount of Computation in Modeling Lossy Communication Systems Based on Ignoring Low-Probability States

Ways for reducing the amount of computation when estimating capacity characteristics of models of lossy communication systems with product form representation of the stationary probability vector are considered. The algorithm of convolution is applied to the computation of the characteristics. The reduction in computation is due to both decreasing the number of convolutions required for the estimation of service characteristics of all the flows and decreasing the number of operations in performing a single convolution, which is achieved by ignoring low-probability states in the process of computation.

Generalized beam-splitting attack in quantum cryptography with dim coherent states

As it is very difficult to prepare a good approximation of one-photon states, practical quantum cryptography uses highly attenuated laser pulses which can well be represented by coherent states with average photon number below one photon. In such a case more than one photon may appear in some pulses. Thus an eavesdropper has a chance to split the signal and gain some information on the key without disturbing the transmission in a substantial way. In this paper we derive the number of bits an eavesdropper can gain by this sort of attack and the question of limits on the average number of photons in a pulse is discussed. It is assumed an eavesdropper may have detectors with 100% efficiency, she can store the `extracted' qubits, she can non-destructively measure the number of photons, and also she is able to perform `cascade' beam splitting resulting in extraction of just one photon. Besides, it is assumed that she can replace a lossy communication line by a lossless one.

TIME-DOMAIN MODELING OF FREQUENCY-DEPENDENT THREE-PHASE TRANSMISSION LINE RESISTANCE

Abstract Time-domain modelling of frequency-dependent line series resistance by a discrete lumped equivalent circuit is useful for simulation, either on a digital computer or on a transient network analyzer. A resistance model that is suggested by Wheeler (1) for resistance of a lossy coaxial communication cable with ideal square-root frequency-dependent losses is adapted to model resistance of a three-phase power line with earth return. The model is solved using the trapezoidal rule with damping, which is accurate and numerically stable even for large values of time step Δt, as used in simulation of power system transients. The model is simple, accurate, computationally fast, and it can be directly interfaced with an electromagnetic transient program.

Dynamic Programming of Optimum Flows in Lossy Communication Nets

This paper deals with assignments of optimum flows in lossy communication nets. In a lossy communication net, each edge <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</tex> has an edge efficiency factor <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\alpha_e (0 \leq \alpha_e \leq 1)</tex> as well as an edge capacity <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_e</tex> . If edge <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</tex> is an edge from node <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</tex> to node <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</tex> , then the flow <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\psi(e, x)</tex> entering into node <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</tex> is not only limited by the edge capacity <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(0 \leq \psi(e, x) \leq C_e)</tex> but also suffers a loss of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(1 - \alpha_e) \cdot \psi(e, x)</tex> in passing through <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</tex> , consequently, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\alpha_e \cdot \psi (e, x)</tex> emerges from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</tex> at node <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</tex> . A flow <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\psi</tex> with the sending flow value <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\hat{t}</tex> at the source and the receiving flow value <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\underbrace{t}</tex> at the sink is said to be optimum if there is no flow which has less sending flow value value than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</tex> while having the same receiving value <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\underbrace{t}</tex> . Necessary and sufficient conditions are described for the optimality of flows in Theorem 1 and Theorem 2. Based on these characterizations, dynamic programming of optimum flows in a lossy communication net is devised and demonstrated by an example.

Properties of Lossy Communication Nets

A communication net is considered in which the edges are lossy in the sense that flow through the edge is attenuated. Such a model describes many power or information transmission systems. The classical problem for such a net is that of maximizing the flow from a source to a sink. The solution to the problem for the lossless case is well known; the lossy case has been considered only recently by Fujisawa. In determining properties of the lossy communication nets, a saturated edge is defined as one in which the edge flow is equal to the capacity of that edge. The concept of the saturated cut set is introduced to obtain relationships that must be satisfied by the terminal capacities. If <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\bar{t}_{pq}</tex> is the source terminal capacity (maximum flow at the source vertex to receive maximum flow at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</tex> ) and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\underline{t}_{pq}</tex> is the sink terminal capacity (maximum flow that can be received at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</tex> when the maximum flow is sent from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</tex> ) then <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\underline{t}_{ij} \geq \underline{t}_{kj}</tex> or <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\overerline{t}_{ij} \geq \overline{t}_{ik}</tex> for any vertices <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i,j,</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</tex> . The property analogous to the S-submatrix property in the lossless case is also obtained. All results given reduce to known results when the loss of every edge becomes zero. This includes the maximum flow-minimum cut theory of Ford and Fulkerson, which is applicable only in the lossless case.