Patrick Stuedi

ASAP: an adaptive QoS protocol for mobile ad hoc networks

With the increasing widespread use of wireless technologies, the need arises for QoS provisioning mechanisms for multimedia applications in wireless networks. However, to support QoS in mobile ad hoc networks (MANETs) is more challenging than in fixed and last-hop wireless access networks. QoS protocols designed for fixed network, e.g. RSVP, are not applicable in MANETs, because they cannot cope with MANETs highly dynamic topology. Existing QoS frameworks explicitly built for MANETs are not as flexible and efficient as needed. In this paper, we propose a new QoS framework for MANETs-adaptive reservation and preallocation protocol (ASAP). By using two signaling messages, ASAP provides fast and efficient QoS support while maintaining adaptation flexibility and minimizing wasted reservations. Simulation of this framework using AODV C. Perkins, E. Royer (1999) illustrates the features and performance of ASAP, and demonstrates the design concepts.

Connectivity in the presence of shadowing in 802.11 ad hoc networks

Connectivity is an important property for QoS support in mobile ad hoc networks (MANETs). Recently, there has been a big effort in exploring the critical transmission range (CTR) analytically, based on different network models. While most of these studies rely on a geometric model and come up with asymptotic bounds, their significance regarding finite 802.11 based MANETs is unclear. In this paper, we investigate connectivity in MANETs from a layered perspective. We first point out how the transmission range affects the end-to-end connection probability in a log-normal shadowing model and compare the results to theoretical bounds and measurements in the path loss model. We then show how connectivity issues behave in 802.11 and IP based networks if the fading effect increases. The paper concludes with an analytical model for the link probability in log-normal shadowing environments as a function of the number of nodes, network area, transmission range, path loss exponent and shadowing deviation.

Understanding Radio Irregularity in Wireless Networks

In an effort to better understand connectivity and capacity in wireless networks, the log-normal shadowing radio propagation model is used to capture radio irregularities and obstacles in the transmission path. Existing results indicate that log-normal shadowing results in higher connectivity and interference levels as shadowing (i.e., the radio irregularity) increases. In this paper we demonstrate that such a behavior is mainly caused by an unnatural bias of the log-normal shadowing radio propagation model that results in a larger transmission range as shadowing increases. To avoid this effect, we analyze connectivity and interference under log-normal shadowing using a normalization that compensates for the enlarged radio transmission range. Our analysis shows that log-normal shadowing still improves the connectivity of a wireless network and even reduces interference. We explain this behavior by studying in detail what network parameters are affected by shadowing. Our results indicate that, when it comes to connectivity and interference, an analysis based on a circular transmission range leads to worst case results.

Transparent heterogeneous mobile ad hoc networks

Most research on mobile ad hoc networks assumes a medium access (MAC) protocol similar to 802.11. Nevertheless, such networks can be built on top of any medium access scheme. An example are Bluetooth multi-hop networks (so called scatternets). In practice, many devices support several interfaces although the protocol stack used in each interface tends to be interface specific at the lower layers. This limits the ability of a device to switch back and forth between networks as need and opportunity dictates. As a step towards providing a more flexible handover infrastructure, in this paper we address the issue of integrating heterogeneous, mobile ad-hoc networks that use different MAC layer protocols. The goal is to provide an end-to-end communication abstraction that hides heterogeneity. In the paper, we discuss different possible designs with regard to application transparency, performance and mobility. Using these ideas, we describe an IP based heterogeneous mobile ad hoc testbed using Bluetooth and IEEE 802.11 that implements a virtual interface approach as the end-to-end abstraction. Furthermore, we also provide a detailed evaluation of the system in terms of throughput and handover-time.

SIPHoc: efficient SIP middleware for ad hoc networks

Mobile Ad Hoc Networks (MANETs) offer a flexible way to connect mobile devices to build complex infrastructures. A key issue in MANETs is session set up and management since, unlike in conventional networks, there is no centralized component to provide such a service. Yet, session set up is necessary to provide any form of communication beyond unreliable, single message communication. In this paper we describe SIPHoc, a middleware infrastructure for session set up and management in MANETs. SIPHoc provides the same interface as the SIP standard but its implementation is fully decentralized. Moreover, SIP session establishment to and from the Internet is possible as soon as a single node in the MANET has Internet access. The paper presents the architecture and implementation of SIPHoc and evaluates its performance. The experiments show that SIPHoc is message efficient and provides a low dial-to-ring delay. SIPHoc allows SIP based applications to be used in MANETs without modification. In the paper, this is demonstrated by showing how SIPHoc supports VoIP conversations within a MANET and between the MANET and end-points on the Internet.

Computing throughput capacity for realistic wireless multihop networks

Capacity is an important property for QoS support in Mobile Ad Hoc Networks (MANETs) and has been extensively studied. However, most approaches rely on simplified models (isotropic radio propagation, unidirectional links, perfect scheduling, perfect routing, etc.) and either provide asymptotic bounds or are based on integer linear programming solvers. In this paper we present a probabilistic approach to capacity calculation by linking the normalized throughput of a communication pair in an ad hoc network to the connection probability of the two nodes in a so called schedule graph GT(N,E). The effective throughput of a random network is modelled as a random variable and expected values of it are computed using Monte-Carlo methods. A schedule graph GT(N,E) for a given network directly emerges from the physical properties of the network, like node distribution, radio propagation or channel assignment. The modularity of the approach allows for capacity analysis under more realistic network models. In the paper throughput capacity is computed for various forms of network configurations and the results are compared to simulation results obtained with ns-2.

Log-normal shadowing meets SINR: A numerical study of Capacity in Wireless Networks

The capacity of wireless multi-hop networks has been studied extensively in recent years. Most existing work tackles the problem from an asymptotic perspective and assumes a simplified physical layer model, as e.g., the protocol interference model or the path loss radio propagation. Real life network planning, provisioning and deployment can only be done with more precise statements about capacity in finite networks. With this in mind, we adopt a numerical approach based on Monte-Carlo methods to study capacity under various interference and radio propagation models, including the physical interference model and log-normal shadowing radio propagation. Our results indicate that, depending on the interference model, capacity may experience a three phase transition related to the connectivity of the network. We further show that throughput capacity increases in the presence of randomized radio propagation, even above the critical node density. Our analysis of the numerical data illustrates that log-normal shadowing creates more interference, but decreases the total amount of transmissions to be scheduled.

Modeling and computing throughput capacity of wireless multihop networks

Capacity is an important property for QoS support in Mobile Ad Hoc Networks (MANETs) and has been extensively studied. However, most approaches rely on simplified models (e.g., protocol interference, unidirectional links, perfect scheduling or perfect routing) and either provide asymptotic bounds or are based on integer linear programming solvers. In this paper, we present a probabilistic approach to capacity calculation by linking the normalized throughput of a communicating pair in an ad hoc network to the connection probability of the two nodes in a so-called schedule graphGT(N,E). The effective throughput of a random network is modeled as a random variable and its expected value is computed using Monte-Carlo methods. A schedule graph GT(N,E) for a given network is directly derived from the physical properties of the network like node distribution, radio propagation and channel assignment. The modularity of the approach leads to a capacity analysis under more realistic network models. In the paper, throughput capacity is computed for various forms of network configurations and the results are compared to simulation results obtained with ns-2.

SymPhone: design and implementation of a VoIP peer for Symbian mobile phones using Bluetooth and SIP

VoIP is born from the growing Internet infrastructure, which has over the years seen significant improvements in both bandwidth and end-to-end latency. In this paper, we explore making VoIP available on a mobile phone. For that purpose, we propose an architecture and describe the various components involved. Data entering and leaving the mobile phone is encapsulated in a wireless Bluetooth connection. The bridge to the Internet is provided by a linux Bluetooth access point. The system is compatible with current VoIP standards using RTP and SIP for data and signaling transmission, respectively. It has been tested to work in combination with several known softphones. Both RTP and SIP stack are built on top of a special IP emulation layer on the mobile phone, which has been developed to facilitate the migration of the application onto devices having a direct IP binding available for Bluetooth such as BNEP (Bluetooth Network Encapsulation Protocol). Apart from presenting design and implementation details, the paper also provides measurement results with regard to delay and session setup time and discusses certain limitations of the application evolving from restrictions imposed by the mobile phones programming platform.

Recall and Precision in Distributed Bandwidth Allocation

Recent work on QoS in Mobile Ad Hoc Networks (MANETs) has shown that bandwidth needs to be reserved in a distributed manner by also including interfering nodes. However, in practice, QoS protocols have difficulties to locally determine the set of nodes that actually interfere with a given transmission. To solve this problem, it is not uncommon to consider all nodes within a distance of k hops (k-neighbors) as interfering nodes. In this paper we use Monte-Carlo methods to study the correlation between the k-neighborhood of a node and the set of interfering nodes. We compute expected values for both reservation recall -the fraction of interfering k-neighbors to all interferers -and reservation precision -the fraction of interfering k-neighbors to all k-neighbors. The two metrics reflect the quality of a reservation and the loss of resources due to over-reservation. We further investigate the impact of different physical layer properties (e.g., fading) and network settings (e.g., network density) on the quality of the reservations. Our results indicate that existing reservation techniques to ensure QoS are inadequate and that new techniques are needed to efficiently implement QoS in MANETs.