Presence Of Channel Failures Research Articles

Dynamic Spectrum Reservation for CR Networks in the Presence of Channel Failures: Channel Allocation and Reliability Analysis

Providing channel access opportunities for new service requests and guaranteeing continuous connections for ongoing flows until service completion are two challenges for service provisioning in wireless networks. Channel failures, which are typically caused by hardware and software failures or/and by intrinsic instability in radio transmissions, can easily result in network performance degradation. In cognitive radio networks (CRNs), secondary transmissions are inherently vulnerable to connection breaks due to licensed users’ arrivals as well as channel failures. To explore the advantages of channel reservation on performance improvement in error-prone channels, we propose and analyze a dynamic channel reservation (DCR) algorithm and a dynamic spectrum access (DSA) scheme with three access privilege variations. The key idea of the DCR algorithm is to reserve a dynamically adjustable number of channels for the interrupted services to maintain service retainability for ongoing users or to enhance channel availability for new users. Furthermore, the DCR algorithm is embedded in the DSA scheme enabling spectrum access of primary and secondary users with different access privileges based on access flexibility for licensed shared access. The performance of such a CRN in the presence of homogeneous and heterogeneous channel failures is investigated considering different channel failure and repair rates.

Distribution of the Number of Retransmissions of Bounded Documents

Retransmission-based failure recovery represents a primary approach in existing communication networks that guarantees data delivery in the presence of channel failures. Recent work has shown that, when data sizes have infinite support, retransmissions can cause long (-tailed) delays even if all traffic and network characteristics are light-tailed. In this paper we investigate the practically important case of bounded data units 0 ≤ Lb ≤ b under the condition that the hazard functions of the distributions of data sizes and channel statistics are proportional. To this end, we provide an explicit and uniform characterization of the entire body of the retransmission distribution ℙ[Nb > n] in both n and b. Our main discovery is that this distribution can be represented as the product of a power law and gamma distribution. This rigorous approximation clearly demonstrates the coupling of a power law distribution, dominating the main body, and the gamma distribution, determining the exponential tail. Our results are validated via simulation experiments and can be useful for designing retransmission-based systems with the required performance characteristics. From a broader perspective, this study applies to any other system, e.g. computing, where restart mechanisms are employed after a job processing failure.

Distribution of the Number of Retransmissions of Bounded Documents

Retransmission-based failure recovery represents a primary approach in existing communication networks that guarantees data delivery in the presence of channel failures. Recent work has shown that, when data sizes have infinite support, retransmissions can cause long (-tailed) delays even if all traffic and network characteristics are light-tailed. In this paper we investigate the practically important case of bounded data units 0 ≤ L b ≤ b under the condition that the hazard functions of the distributions of data sizes and channel statistics are proportional. To this end, we provide an explicit and uniform characterization of the entire body of the retransmission distribution ℙ[N b > n] in both n and b. Our main discovery is that this distribution can be represented as the product of a power law and gamma distribution. This rigorous approximation clearly demonstrates the coupling of a power law distribution, dominating the main body, and the gamma distribution, determining the exponential tail. Our results are validated via simulation experiments and can be useful for designing retransmission-based systems with the required performance characteristics. From a broader perspective, this study applies to any other system, e.g. computing, where restart mechanisms are employed after a job processing failure.

Uniform approximation of the distribution for the number of retransmissions of bounded documents

Retransmission-based failure recovery represents a primary approach in existing communication networks, on all protocol layers, that guarantees data delivery in the presence of channel failures. Contrary to the traditional belief that the number of retransmissions is geometrically distributed, a new phenomenon was discovered recently, which shows that retransmissions can cause long (-tailed) delays and instabilities even if all traffic and network characteristics are light-tailed, e.g., exponential or Gaussian. Since the preceding finding holds under the assumption that data sizes have infinite support, in this paper we investigate the practically important case of bounded data units 0≤ L b ≤ b. To this end, we provide an explicit and uniform characterization of the entire body of the retransmission distribution Pr[N b > n] in both n and b . This rigorous approximation clearly demonstrates the previously observed transition from power law distributions in the main body to exponential tails. The accuracy of our approximation is validated with a number of simulation experiments. Furthermore, the results highlight the importance of wisely determining the size of data units in order to accommodate the performance needs in retransmission-based systems. From a broader perspective, this study applies to any other system, e.g., computing, where restart mechanisms are employed after a job processing failure.

Multiple description image and video coding for wireless channels

We consider the problem of efficient image/video transmission over wireless channels. Such a problem involves good compression rates and effectiveness in presence of channel failures. In this work, we use the multiple description coding (MDC) techniques, based on Wavelet Transforms, that have been shown to be powerful against channel failures. We propose a bit allocation procedure that dispatches the source redundancy between the different channels when compressing to a target bit rate with a bounded side distortion. In this way, we develop an MDC scheme well adapted to channel noise. Then, we extend this bit allocation to video. This extension uses the 3D scan-based DWT of Parisot et al. (MMSP, IEEE, New York, 2001); that involves scan-based MDC with rate or quality control. In this work we propose to adapt redundancy between the descriptors in function of channel model and state (BER). Furthermore, the channels can have time-varying states. We consider BSC channels, Gaussian channels, and UMTS channels. The method can be efficiently extended to other channels when the channel model that matches the channel behavior is known. We evaluate the performance of the proposed MDC image/video coder for two descriptions. In a first group of simulations, we compare our application with some different MDC techniques presented by Vaishampayan (IEEE Trans. Inform. Theory 39 (3) (1993) 821) and Servetto et al. (ICIP, IEEE, Chicago, USA, 1998; IEEE Trans. Image Process. 9 (5) (2000) 813–826). In a second group of simulations, we evaluate the robustness of the proposed MDC when submitted to different kinds of channels noise.