Federico Maguolo

Efficient collision detection for auto rate fallback algorithm

The physical rate adaptation in 802.11 is a deeply investigated, though still open issue. Since 802.11 uses the random access distributed coordination function (DCF) mechanism to access the medium, collisions can occur when two or more stations want to transmit data simultaneously. The challenge of rate adaptation schemes is to adapt the physical transmission rate based on channel-related losses, i.e. collisions should not influence the choice of the rate. In this paper we propose a new rate adaptation algorithm that behaves like auto rate fallback (ARF), but makes use of the RTS/CTS handshake, when necessary, to decide whether the physical transmission rate should be changed. Main advantages of this algorithm are its simple implementation and the good performance it attains in presence of collisions. We evaluate the performance of this new rate adaptation algorithm, comparing it with other well known algorithms, by using the new NS-3 simulator.

A new approach to simulating PHY, MAC and routing

In recent years, network simulation has become a very difficult task due to the proliferation and integration of wireless technologies. In this paper, we discuss the new challenges that have arisen regarding the simulation of the wireless channel and the PHY, MAC and Routing layers, argumenting why currently available network simulation tools such as ns2 and many of its recently proposed extensions do not address all these issues in a comprehensive and systematic fashion. We then present a novel framework designed to address these challenges. This framework has been developed as an extension of NS-Miracle, in order to have support in the definition and management of scenarios involving the use of multiple interfaces and radio technologies, and is made up of two components. The first component is the Miracle PHY and MAC framework, which provides support for the development of Channel, PHY and MAC modules, providing support for features currently lacking in most state-of-the-art simulators, while at the same time giving a strong emphasis on code modularity, interoperability and reusability. The second component is the Miracle Routing framework, which enables the integration of different routing schemes in a multi-tier architecture to provide support for the simulation of multi-technology and heterogeneous networks. We want to observe that, thanks to this framework, it is now possible to carefully simulate complex network architectures potentially at all the OSI layers, from the physical reception model to standard applications and system management schemes. This allows to have both a comprehensive view of all the networks interactions and its high level view, which plays an important role in many research investigation area, such as cognitive networking and cross-layer design.

GORA: Goodput Optimal Rate Adaptation for 802.11 Using Medium Status Estimation

Rate adaptation for 802.11 has been deeply investigated in the past, but the problem of achieving optimal rate adaptation with respect not only to channel-related errors but also to contention-related issues (i.e., collisions and variations in medium access times) is still unsolved. In this paper we address this issue by proposing (1) a practical definition of the medium status in a multi-user 802.11 scenario in terms of channel errors, MAC collisions and packet service times, and a method for its estimation based on measurements; (2) an analytical model of the goodput performance as a function of the Medium Status; (3) a rate adaptation algorithm, called goodput optimal rate adaptation (GORA), which is based on this model. Unlike other rate adaptation schemes proposed in literature, which require either modifications to the IEEE 802.11 standard or cooperation among nodes, GORA is totally stand-alone and standard compliant. In fact, the Medium Status Estimation used by GORA is obtained by using standard MAC counters that are commonly collected by commercial MAC drivers, and no explicit interactions with the other devices in the network is required. Therefore, GORA offers the advantage of being readily deployable on real devices. The performance of GORA is evaluated through NS2 simulations which reveal that, as expected, GORA outperforms other well- known rate adaptation algorithms in several scenarios and can be used as a new reference benchmark.

VoIP Communications in Wireless Ad-hoc Network with Gateways

In this paper, we investigate some of the issues that arise when mobile nodes engage voice connections with remote peers by using a wireless ad hoc network (MANET-cell) to access the distribution core network. We focus on a specific network scenario where all communications are directed to a special node, called gateway, which in turn bridges traffic to and from the core network. We first derive a mathematical expression to estimate the maximum number of sustainable voice sessions in a single-hop cell, with multi-rate terminals. Then, we assess the accuracy of the formula through ns2 simulations, which allow us to determine the voice capacity of the system also in presence of hidden terminals. Hence, we analyze the impact of multi-hop paths on the voice capacity of the system. From this analysis, we propose a new cost metric that we apply to a modified version of the OLSR routing algorithm, named HOLSR. This algorithm has proved to be able to preserve stability in some scenarios that were unstable under other routing algorithms.

Analysis of complexity for the message passing algorithm

Besides turbo codes, low-density parity-check (LDPC) codes form another class of Shannon limit-approaching codes. Their large coding gains are mostly due to the decoding algorithm, termed message passing algorithm (MPA) or sum-product algorithm (SPA).

APOS: Adaptive Parameters Optimization Scheme for Voice over IEEE 802.11g

In this paper we present APOS, a method for dynamically adapting the parameters of IEEE 802.11 g to the estimated system state, with the aim of enhancing the quality of a voice communication between a mobile station and a remote peer node. The system state is estimated based on a number of counters that are collected by the MAC layer of the mobile station, regarding the number of successful and unsuccessful transmission/reception events, channel busy periods and idle slots. These statistics are processed to estimate the collision probability and the signal to noise ratio at the receiver side. Hence, a mathematical model is used to get the expected end-to-end network performance in terms of throughput, delay and packet error rate, for different settings of some PHY and MAC parameters, such as the modulation/coding scheme and the retransmission limit. The setting that is estimated to maximize the quality of service for the end user is then selected. Unlike other optimization mechanisms proposed in literature, APOS is totally stand-alone and standard compliant. In fact, APOS makes use of local information that can be collected from the Network Interface Card, and no explicit interactions with the other devices in the network is required.

Transmission of 3D Scenes over Lossy Channels

This paper introduces a novel error correction scheme for the transmission of three-dimensional scenes over unreliable networks. We propose a novel Unequal Error Protection scheme for the transmission of depth and texture information that distributes a prefixed amount of redundancy among the various elements of the scene description in order to maximize the quality of the rendered views. This target is achieved exploiting also a new model for the estimation of the impact on the rendered views of the various geometry and texture packets which takes into account their relevance in the coded bitstream and the viewpoint required by the user. Experimental results show how the proposed scheme effectively enhances the quality of the rendered images in a typical depth-image-based rendering scenario as packets are progressively decoded/recovered by the receiver.