Alexander A. Safonov

Study of Beaconing in Multihop Wireless PAN with Distributed Control

Distributed media access control (MAC) architecture has many merits to make it a favorable candidate for high-data-rate wireless personal area networks (WPANs) with physical layer (PHY) based on ultrawideband (UWB) technology. This paper focuses on the WiMedia MAC, which is the first distributed MAC for WPANs, approved as a standard. In such a MAC, all devices transmit their beacons to provide timing reference and to broadcast control and reservation information, that is, to maintain device synchronization. We investigate problems related to beacon collisions, which occur when multiple devices join a piconet almost at the same time. To join a piconet or to resolve a collision, a device chooses a slot for its beacon randomly within some window. We show that an improper slot choice scheme leads to repeated collisions, increasing the time overhead to achieve device synchronization. It also leads devices into trouble in joining and collision resolution, even into deadlocks, when devices have no chance to escape repeated collisions. We develop an analytical model to evaluate the performance of various slot choice schemes for multihop WPANs. The model is employed to compare the efficiency of these choice schemes and to optimize their parameters to achieve the best performance.

Scalable blind search and broadcasting over Distributed Hash Tables

Typical blind search algorithms in P2P networks generate a significant amount of duplicate query messages in order to increase the success rate. We present a novel framework, named Recursive Partitioning Search (RPS), for blind search over structured peer-to-peer (P2P) networks, by which the query message duplication can be avoided. Two realizations of the framework for Chord and Pastry are presented. By simulation, we compare success rate, lookup delay and overlay network load of RPS with various well-known blind search algorithms, and illustrate RPS being a superior blind search algorithm running over DHTs. The algorithm guarantees that with high probability the lookup delay to visit every peer is of O(logN) hops, comparable to the delay of the exact-match search over the DHTs, which is proved for two example DHTs, Chord and Pastry in the paper. RPS is a simple and intuitive method for blind search over DHTs compared to other complex approaches like those building sophisticated index structures or requiring analysis of the words in the stored documents, yet a lot more efficient than known simple methods like Flooding and Random Walk. With RPS, every node in the overlay network is visited not more than once by design. These characteristics qualify the Recursive Partitioning Search over DHT as an efficient broadcasting algorithm. We investigate RPS scalability and propose a formula to choose an appropriate Time-to-Live (TTL) parameter value to maintain the balance between high success rate and reasonable network load. Active peer churn degrades the performance of RPS as a broadcasting mechanism proportionally to the churn rate. But the success rate of blind search using RPS may be affected negligibly if proper replications exist as in most P2P file sharing networks.

Scalable Blind Search and Broadcasting in Peer-to-Peer Networks

Typical blind search algorithms in P2P networks generate a significant amount of duplicate query messages to increase the success rate. We present a framework, named recursive partitioning search (RPS), for blind search over structured peer-to-peer (P2P) networks, by which the query message duplication can be avoided. Two realizations of the framework for Chord and Pastry are presented. With simulation model, we compare success rate, lookup delay and overlay network load of RPS and various well-known blind search algorithms, and show RPS is the superior blind search algorithm running over a DHT. The algorithm guarantees that with high probability the lookup delay to visit every peer is of O(log N) hops, comparable to the delay of the exact match search over the DHTs. With RPS, every node in the overlay network is visited not more than once by design. These characteristics qualify the recursive partitioning search over DHT as an efficient broadcasting algorithm. We investigate RPS scalability and propose the way to choose an appropriate time-to-live (TTL) parameter value to maintain the balance between high success rate and reasonable network load

Analytical model of IEEE 802.11s MCCAbased streaming in the presence of noise

In the paper, we present an analytical study of multimedia stream transmission with the reservation-based channel access method called MCCA in IEEE 802.11s mesh networks. Various aspects of MCCA have been already studied under the assumption that the reservation guarantees successful transmission, no retries are needed and the period of reserved times is equal to the inter-arrival time of the input stream. However, recent papers reveal the fact that two-hop advertisement of reservations, as adopted in MCCA, fails to completely defeat the interference affecting transmissions in addition to random noise. To keep the packet loss ratio (PLR) acceptable for a stream, period of reserved times may be shortened to allow packet retries. However, a packet of a stream is usually discarded when the packet delay reaches its threshold, making its contribution to the PLR. Also, additional reservations is a burden. In this paper, we propose an analytical model to find the maximal period of reserved times to keep the packet loss ratio and delay below thresholds, given the input stream bit rate and packet error rate.

Multiple metrics in MANET with end-to-end QoS support for unicast and multicast traffic

The paper proposes an approach of using multiple metrics in a wireless multihop network, when one of the metrics called optimizable reflects consuming network resources, and other metrics called restrictive reflect traffic QoS requirements. Compared to popular Hop Count and Air Time Link metrics, a set of metrics is proposed, increasing the network capacity measured as the number of unicast voice calls with tolerable quality. The metrics are further used in a proposed multicast tree construction algorithm.

Starvation effect study in IEEE 802.11 mesh networks

Initially, IEEE 802.11 standard was not designed for multihop networks, but its market success gave it a powerful incentive, and nowadays IEEE 802.11 Working Group is preparing various amendments and looking for new application scenarios to ensure future success in the market. In this paper, we analyze the performance of an IEEE 802.11s mesh network and show that the effect of starvation caused by hidden terminals interference makes the distribution of channel capacity between links very much disproportional and unpredictable. Some links may be suppressed completely by other links and their throughput is close to zero. The effect of starvation was discovered by simulations in many papers. We investigate the roots of this phenomenon in a mesh network with an analytical model. Also, this paper reports the results of an outdoor testbed which was deployed to find out the scale of the starvation problem in a real network with Voice-over-IP, Video-over-IP and FTP traffic. While the results for VoIP may be considered as satisfactory, FTP traffic suffers dramatically from interference. So, at the end of the paper, we outline possible solutions to address starvation problem in mesh networks.

Beaconing for MDA Support in IEEE 802.11s Mesh Networks

The main purpose of beaconing in the legacy IEEE 802.11 networks is maintaining stations clock synchronization. Recently appeared 11s Amendment specifying mesh networking makes beacons responsible for supporting the mesh deterministic access method (MDA) designed for delay-sensitive multimedia applications. In this paper, we study the legacy beaconing scheme and show its inability to support the MDA in a typical mesh network use case. We propose a P-persistent beaconing approach to improve the MDA support and analyze its efficiency. The simulation results show that applying the P-persistent approach successfully maintains and protects the channel reservations obtained via MDA.

Synchronization and beaconing in IEEE 802.11s mesh networks

Synchronization and beaconing attract much attention with the appearance of IEEE 802.11s Amendment draft specifying mesh networking based on IEEE 802.11 concepts. This paper discusses the drawbacks of using unsynchronized beaconing and focuses on beaconing of synchronized mesh points which adopt beaconing algorithm for IBSS networks with some modifications. We provide an analytical model of the beaconing process and prove its accuracy with simulation results.

Analysis of the joint use of the proactive and reactive methods of the topology information dissemination in ad-hoc wireless networks

A number of routing protocols for wireless ad-hoc (self-organizing) networks selects routes according to the topology information obtained by the station. Hence, the method of these protocols used to disseminate topology information strongly affects the network performance. An unsuccessful choice of the method can lead to a large amount of traffic and the information obsolescence. These factors diminish the network performance and make it impossible to meet quality of service requirements in transmitting realtime multimedia data, which is a topical problem. The routing protocol combining the proactive and reactive methods of the topology information dissemination has been proposed. It is demonstrated that this protocol is efficient in a wide range of scenarios applied to the exchange of real-time multimedia data.

A method to estimate efficiency of the connection control mechanisms in wireless self-organizing networks

An original method to estimate efficiency of the algorithms for opening and closing links in the wireless self-organizing networks (mesh networks) was proposed. It can be used to develop a mathematical model of the algorithm standardized in the mesh network specification IEEE 802.11s. The proposed method was shown to be sufficiently general and applicable to developing the models of other link management algorithms.