Antonis Panagakis

On the Effects of Cooperation in DTNs

In a delay tolerant network (DTN) the nodes may behave autonomously deciding on their own whether to implement or not the rules of a routing algorithm. In this paper, the effects of node cooperation (or lack of it) are explored for three well-known routing algorithms proposed for DTNs with respect to the message delivery delay and the transmission overhead incurred until message delivery or the termination of the message spreading process. The results show that the sensitivity of the algorithms to the cooperation degree can be high, to the point of making them inferior to algorithms they typically outperform under a fully cooperative environment. Finally, it is demonstrated how a simple mechanism that incorporates the cooperation degree can help improve effectiveness.

Study of two-hop message spreading in DTNs

In this paper, a delay tolerant network environment is considered where the source is in full control of the two-hop spreading mechanism by setting key parameters such as the number of copies allowed to be spread in the network and the delay bound of the messages. The analysis allows for a differentiation between the source of the message and the intermediate nodes (in terms of e.g. transmission power or speed). Analytical expressions for the cumulative distribution function (cdf) of the delivery delay and the induced overhead are extracted, taking into account the fact that the source node may continue spreading copies after the message delivery. In addition, a fairly accurate approximate expression for the cdf of the delivery delay is also derived and validated through simulations.

The Impact of Replacement Granularity on Video Caching

In this paper the idea that large objects, such as video files, should not be cached or replaced in their entirety, but rather be partitioned in chunks and replacement decisions be applied at the chunk level is examined. It is shown, that a higher byte hit ratio (BHR) can be achieved through partial replacement. The price paid for the improved BHR performance is that the replacement algorithm, e.g. LRU, takes a longer time to induce the steady state BHR. It is demonstrated that this problem could be addressed by a hybrid caching scheme that employs variable sized chunks; the use of small chunks leads to the maximization of BHR in periods of stable video popularity, while large chunks are used when extreme popularity changes occur to assist the fast convergence to the new steady state BHR.

Optimal call admission control on a single link with a GPS scheduler

The problem of call admission control (CAC) is considered for leaky bucket constrained sessions with deterministic service guarantees (zero loss and finite delay bound) served by a generalized processor sharing scheduler at a single node in the presence of best effort traffic. Based on an optimization process, a CAC algorithm capable of determining the (unique) optimal solution is derived. The derived algorithm is also applicable, under a slight modification, in a system where the best effort traffic is absent and is capable of guaranteeing that if it does not find a solution to the CAC problem, then a solution does not exist. The numerical results indicate that the CAC algorithm can achieve a significant improvement on bandwidth utilization as compared to a (deterministic) effective bandwidth-based CAC scheme.

Approximate analysis of LRU in the case of short term correlations

One of the most widely considered cache replacement policies is least recently used (LRU) based on which many other policies have been developed. LRU has been studied analytically in the literature under the assumption that the object requests are independent. However, such an assumption does not seem to be in agreement with recent studies of Web-traces, which indicate the existence of short term correlations among the requests. This paper introduces an approximate analysis that fairly accurately predicts the hit ratio of the LRU policy in the case of short term correlations. The approximation approach is based on the relation between the working set model and LRU, while the request generation process is assumed to follow a recently proposed model for Web-traces, which captures short term correlations among the requests. The accuracy of the introduced approximate analysis is validated for synthetic as well as real Web-traces.

On the performance of two-hop message spreading in DTNs

In this paper, a Delay Tolerant Network environment is considered where the source is in full control of the two-hop spreading mechanism by setting key parameters such as the number of copies allowed to be spread in the network and the delay bound of the messages. The introduced analysis allows for a differentiation between the source of the message and the intermediate nodes (in terms of e.g. transmission power, speed or cooperation degree). Analytical expressions for the cumulative distribution function (cdf) of the delivery delay and the induced overhead are extracted, taking into account the fact that the source node may continue spreading copies after the message delivery. In addition, a fairly accurate approximate expression for the cdf of the delivery delay is also derived and validated through simulations.

Study of the Impact of Replacement Granularity and Associated Strategies on Video Caching

Abstract Partial caching of large media objects such as video files has been proposed recently as the caching of entire objects can easily exhaust the storage resources of a proxy server. In this paper the idea of segmenting video files into chunks and applying replacement decisions at the chunk level rather than on entire videos is examined. It is shown that a higher byte hit ratio (BHR) can be achieved by appropriately adjusting the replacement granularity. The price paid for the improved BHR performance is that the replacement algorithm takes a longer time to converge to the steady state BHR. For the segmentation of video into chunks two methods are presented. The Fixed Chunk Size segmentation scheme that is rather simple and reveals the basic trade-off between byte hit ratio (BHR) and responsiveness to changes of popularity; the Variable Chunk Size segmentation scheme that uses the request frequencies to dynamically adjust the size of the chunk and is shown to be capable of combining a small response time with high BHR. Moreover, a variation of the fixed chunk size segmentation scheme is presented, which is shown to improve its performance by switching between different chunk sizes. Video segmentation is also considered as a mechanism to provide for caching differentiation based on access costs. By employing access cost dependent chunk sizes an overall access cost reduction is demonstrated.

Optimal Call Admission Control under Generalized Processor Sharing Scheduling

In this paper the problem of Call Admission Control is considered for leaky bucket constrained sessions with deterministic service guarantees (zero loss and finite delay bound), served by a Generalized Processor Sharing scheduler at a single node in the presence of best effort traffic. Based on an optimization process a CAC algorithm capable of determining the (unique) optimal solution is derived. The derived algorithm is applicable, under slight modification, in a system where the best effort traffic is absent and capable of guaranteeing that a solution to the CAC problem does not exist, if not found. The provided numerical results indicate that the presented algorithm can achieve, under certain conditions, a significant improvement on bandwidth utilization compared to a (deterministic) effective bandwidth based CAC scheme.

Panel 1 report: autonomicity versus complexity

The first panel in WAC2005 focused on the relation between autonomicity and complexity. It is widely believed that autonomicity is a principle that can reduce complexity, but there is also concern that autonomicity itself is complexity-producing. Autonomicity promotes all self-* attributes of a system and naturally distributes responsibilities and costs, but it can also bring the system close to a state of anarchy (modern Greek interpretation of autonomous) if not properly handled. It appears that the overall system complexity may increase, but it is distributed and shared (hence, it is potentially easier to manage), in a similar way in which Integrated Circuits encapsulate the increased complexity and hide it from the bigger system. In addition to reducing complexity in the above sense, autonomicity can also help design truly adaptable, self-tuning and all-weather near-optimal systems, something not possible under traditional system design that are difficult to cope with the combined fine-tuning of a very large number of parameters.

Study of the Capacity of Multihop Cellular Networks

Recently, the application of the peer to peer networking paradigm (typical for an ad hoc network) has been proposed for wireless local area networks (WLANs), instead of the traditional cellular networking paradigm. In this paper the performance of a WLAN employing the peer to peer networking paradigm is studied via simulations; the results indicate that the direct application of the peer to peer networking paradigm in a WLAN leads to a substantially decreased throughput for the traffic directed to the Access Point (AP). The study also reveals that the cumulative receiving throughput of nodes located at the periphery of relatively small circular areas around the AP is substantially higher. Thus, the capacity of the multihop cellular network may be enhanced by employing the peer to peer paradigm only outside a circular area around the AP and the cellular paradigm inside this circular area. Examples are provided of environments where the aforementioned idea of distributing the traditional AP functionality to a set of nodes at the periphery of a circular area around the AP can be effectively applied.