Vladimir M. Vishnevsky

A Simple Analytical Model for the Periodic Broadcasting in Vehicular Ad-Hoc Networks

Nowadays in Europe, USA, Japan and other countries many efforts are being made towards the development and ubiquitous deployment of active vehicular safety systems, which are based on car-to-car and car-to-infrastructure communications. International standard IEEE 802.11p defines low layers protocols for vehicular networks aimed to support Intelligent Transportation Systems (ITS) applications. Periodic broadcasting of short status messages (beacons) is foreseen as one of the key modes, which should be efficiently handled to provide active safety in vehicular networks. This paper presents a simple analytical method to compute the probability of successful message reception and mean message transmission delay in IEEE 802.11p vehicular ad-hoc network with periodic broadcasting of messages. We investigate the influence of beaconing period on the above performance metrics.

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.

1 LANs: Saturation Throughput in the Presence of Noise

IEEE 802.11 specifies a technology for wireless local area networks (LANs) and mobile networking. In this paper, we present an analytical method of estimating the saturation throughput of 802.11 wireless LAN in the presence of noise which distorts transmitted frames. Besides the Basic Access mechanism of the 802.11 MAC protocol, we study such optional tool as the RTS/CTS method, which allows reducing the influence of collisions. In addition to the throughput, our method allows estimating a probability of a packet rejection occurring when the number of packet transmission retries attains its limit. The obtained numerical results of investigating 802.11 LANs by this method are validated by simulation and show high estimation accuracy for any values of protocol parameters and bit error rates. These results also show that the method is an effective tool for tuning the protocol parameters.

IEEE 802.11 Wireless LAN: Saturation Throughput Analysis with Seizing Effect Consideration

The IEEE 802.11 network technology is the emerging standard for wireless LANs and mobile networking. The fundamental access mechanism in the IEEE 802.11 MAC protocol is the Distributed Coordination Function. In this paper, we present an analytical method of estimating the saturation throughput of 802.11 wireless LAN in the assumption of ideal channel conditions. The proposed method generalizes the existing 802.11 LAN models and advances them in order to take the Seizing Effect into consideration. This real-life effect consists in the following: the station that has just completed successfully its transmission has a better chance of winning in the competition and therefore of seizing the channel than other LAN stations. The saturation throughput of 802.11 wireless LANs is investigated by the developed method. The obtained numerical results are validated by simulation and lead to the change of the existing idea of the optimal access strategy in the saturation conditions.

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.

On a BMAP / G /1 Retrial System with Two Types of Search of Customers from the Orbit

A single server retrial queueing model, in which customers arrive according to a batch Markovian arrival process (BMAP), is considered. An arriving batch, finding server busy, enters an orbit. Otherwise one customer from the arriving batch enters for service immediately while the rest join the orbit. The customers from the orbit try to reach the server subsequently and the inter-retrial times are exponentially distributed. Additionally, at each service completion epoch, two different search mechanisms are switched-on. Thus, when the server is idle, a competition takes place between primary customers, the customers coming by retrial and the two types of searches. It is assumed that if the type II search reaches the service facility ahead of the rest, all customers in the orbit are taken for service simultaneously, while in the other two cases, only a single customer is qualified to enter the service. We assume that the service times of the four types of customers namely, primary, repeated and those by the two types of searches are arbitrarily distributed with different distributions. Steady state analysis of the model is performed.

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

Comparative study of 802.11 DCF and its modification in the presence of noise

IEEE 802.11 specifies a technology for wireless local area networks (LANs) and mobile networking. In this paper, we present an analytical method of estimating the saturation throughput of a 802.11 wireless LAN in the presence of noise, which distorts transmitted frames. With the Distributed Coordination Function (DCF) being the fundamental access mechanism in the IEEE 802.11 MAC protocol, sequential attempts to transfer by every station are separated by backoff intervals. Besides the standard backoff rule of the DCF, according to which the mean backoff interval is doubled after each failure in order to avoid collisions, we propose and study a modification of the backoff rule. This modification relies on the optional 802.11 tools to recognize a reason of a failure (collision or noise-induced distortion) and does not increase the mean backoff interval if a failure happens due to distortion by noise. In addition to the throughput, our method allows estimating a probability of a packet rejection occurring when the number of packet transmission retries attains its limit. The obtained numerical results of investigating 802.11 LANs by the developed method are validated by simulation and show high estimation accuracy for any values of protocol parameters and bit error rates. We adopt this method to tune the protocol parameters and to compare the proposed modification with the standard backoff rule.

Multicast QoS Support in IEEE 802.11 WLANs

Existing IEEE 802.11 spec, provides no error recovery mechanism (ARQ) for multicast traffic. ARQ absence in wireless networks unreliable by their nature leads to frequent packet losses, which is inappropriate for most of multimedia applications. In the paper, we propose new reliable multicast mechanism supporting multimedia QoS (packet loss ratio, delay and throughput), in IEEE 802.11 Wireless local Area Networks (WLANs). This mechanism is based on the concept of multiple ACK-leaders, i.e. multicast recipients responsible for data packets acknowledging. We develop an analytical model of the mechanism and use the model to study and to optimize the mechanism.

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.