Georgios S. Paschos

Asymptotic Laws for Joint Content Replication and Delivery in Wireless Networks

We investigate the scalability of multihop wireless communications, a major concern in networking, for the case that users access content replicated across the nodes. In contrast to the standard paradigm of randomly selected communicating pairs, content replication is efficient for certain regimes of file popularity, cache, and network size. Our study begins with the detailed joint content replication and delivery problem on a 2-D square grid, a hard combinatorial optimization. This is reduced to a simpler problem based on replication density, whose performance is of the same order as the original. Assuming a Zipf popularity law, and letting the size of content and network both go to infinity, we identify the scaling laws and regimes of the required link capacity, ranging from O(√N) down to O(1) .

Wireless caching: technical misconceptions and business barriers

Caching is a hot research topic and poised to develop into a key technology for the upcoming 5G wireless networks. However, the successful implementation of caching techniques crucially depends on joint research developments in different scientific domains such as networking, information theory, machine learning, and wireless communications. Moreover, there are business barriers related to the complex interactions between the involved stakeholders: users, cellular operators, and Internet content providers. In this article we discuss several technical misconceptions with the aim of uncovering enabling research directions for caching in wireless systems. Ultimately, we make a speculative stakeholder analysis for wireless caching in 5G.

Storage planning and replica assignment in content-centric publish/subscribe networks

Content-centric publish/subscribe networking is a flexible communication model that meets the requirements of the content distribution in the Internet, where information needs to be addressed by semantic attributes rather than origin and destination identities. In current implementations of publish/subscribe networks, messages are not stored and only active subscribers receive published messages. However, in a dynamic scenario, where users join the network at various instances, a user may be interested in content published before its subscription time. In this paper, we introduce a mechanism that enables storing in such networks, while maintaining the main principle of loose-coupled and asynchronous communication. Furthermore, we propose a new storage placement and replica assignment algorithm which differentiates classes of content based on their popularity and minimizes the clients response latency and the overall traffic of the network. We also present and compare two replica assignment alternatives and examine their performance when both the locality and the popularity of users request change. The performance of our proposed placement and replica assignment algorithm and the proposed storing mechanism is evaluated via simulations and insights are given for future work. The proposed mechanism is compared with mechanisms from the CDN (Content Delivery Networks) context and performs as close as 1-15% (depending on the conducted experiment) to a greedy (near optimal) approach installing up to 3 times less storage servers in the network and providing the necessary differentiation among the classes of the content.

Placing dynamic content in caches with small population

This paper addresses a fundamental limitation for the adoption of caching for wireless access networks due to small population sizes. This shortcoming is due to two main challenges: making timely estimates of varying content popularity and inferring popular content from small samples. We propose a framework which alleviates such limitations. To timely estimate varying popularity in a context of a single cache we propose an Age-Based Threshold (ABT) policy which caches all contents requested more times than a threshold N (τ), where τ is the content age. We show that ABT is asymptotically hit rate optimal in the many contents regime, which allows us to obtain the first characterization of the optimal performance of a caching system in a dynamic context. We then address small sample sizes focusing on L local caches and one global cache. On the one hand we show that the global cache learns L times faster by aggregating all requests from local caches, which improves hit rates. On the other hand, aggregation washes out local characteristics of correlated traffic which penalizes hit rate. This motivates coordination mechanisms which combine global learning of popularity scores in clusters and Least-Recently-Used (LRU) policy with prefetching.

Mobility Support Through Caching in Content-Based Publish/Subscribe Networks

In a publish/subscribe (pub/sub) network, message delivery is guaranteed for all connected subscribers at publish time. However, in a dynamic mobile scenario where users join and leave the network, it is important that content published at the time they are disconnected is still delivered when they reconnect from a different point. In this paper, we enhance the caching mechanisms in pub/sub networks to enable client mobility. We build our mobility support with minor changes in the caching scheme while preserving the main principles of loose coupled and asynchronous communication of the pub/sub communication model. We also present a new proactive mechanism to reduce the overhead of duplicate responses. The evaluation of our proposed scheme is performed via simulations and testbed measurements.

Asymptotic laws for content replication and delivery in wireless networks

A key consideration in novel communication paradigms in multihop wireless networks regards the scalability of the network. We investigate the case of nodes making random requests on content stored in multiple replicas over the wireless network. We show that, in contrast to the conventional paradigm of random communicating pairs, multihop communication is a sustainable scheme for certain values of file popularity, cache and network size. In particular, we formulate the joint problem of replication and routing and compute an order optimal solution. Assuming a Zipf file popularity distribution, we vary the number of files M in the system as a function of the nodes N, let both go to infinity and identify the scaling regimes of the required link capacity, from O(√N) down to O(1).

A new MAC protocol with pseudo-TDMA behavior for supporting quality of service in 802.11 wireless LANs

A new medium access control (MAC) protocol is proposed for quality-of-service (QoS) support in wireless local area networks (WLAN). The protocol is an alternative to the recent enhancement 802.11e. A new priority policy provides the system with better performance by simulating time division multiple access (TDMA) functionality. Collisions are reduced and starvation of low-priority classes is prevented by a distributed admission control algorithm. The model performance is found analytically extending previous work on this matter. The results show that a better organization of resources is achieved through this scheme. Throughput analysis is verified with OPNET simulations.

Scheduling with pairwise XORing of packets under statistical overhearing information and feedback

We study the problem of scheduling packets from several flows traversing a given node which can mix packets belonging to different flows. Practical wireless network coding solutions depend on knowledge of overhearing events which is obtained either by acknowledgments or statistically. In the latter case, the knowledge about each packet improves progressively with feedback from the transmissions. We propose a virtual network mechanism in order to characterize the throughput region of such a system for the case where we allow only pairwise XORing. We also provide the policy which achieves the stability region and compare it to simple heuristics. The derived policy is a modification of the standard backpressure policy, designed to take into account the fact that in the proposed virtual network the destination of a transmitted packet is known only probabilistically. We demonstrate simulation results according to which scheduling with statistical information can provide significant throughput benefits even for overhearing probabilities as small as 0.6.

Caching in Content-Based Publish/Subscribe Systems

In a publish/subscribe network, message delivery is guaranteed for all active subscribers at publish time. However, in a dynamic scenario where users join and leave the network, a user may be interested in content published before the subscription time. In this paper, we introduce mechanisms that enable caching in such networks, while maintaining the main principle of loose-coupled and asynchronous communication. Furthermore we investigate two caching policies; caching in all candidate brokers (basic caching) which yields high survivability and low delay and caching in leaf brokers (leaf caching) which maintains low overhead and querying complexity. The comparison is performed via simulations and testbed measurements and insights are given for future work.

Performance evaluation of IEEE 802.11e based on ON-OFF traffic model

We investigate the performance of the IEEE 802.11e while emphasizing on the end-to-end delay performance. In our MAC delay analysis, we are based on elementary conditional probabilities, avoiding the complex Markov Chains method. For a thoroughly study of the MAC delay, we take the Z-transform of the backoff duration in order to provide higher moments of the MAC delay distribution. The first moment corresponds to the mean MAC delay, while the second moment to the Standard Deviation of the MAC delay, which depicts the jitter. We also estimate the Probability Mass Function (PMF) of the MAC delay through the Lattice Poisson Algorithm. For the queuing delay, we consider a queue for each Access Category (AC) with one common server and an input process which is described by an ON-OFF model depicting the bursty nature of traffic. The service time follows the MAC delay distribution. Analyzing this queue, we provide results both for saturated and non-saturated channel condition. The end-to-end delay is estimated by the sum of queuing and MAC delay. The analytical results are validated through simulation.