Catálogo de publicaciones - libros

Currently, IEEE 802.16a wireless MAN supports the contention based OFDMA-CDMA ranging subsystem for ranging operation (Initial Ranging, Periodic Ranging, Bandwidth Request)[1]. This system uses essentially the slotted ALOHA protocol. However, the number of code-slots/frame for random access transmission tends to be much greater than one and a simple Markov chain analysis may not be numerically feasible. At the same time, the frame size in number of code-slots may be dynamically adjusted based on the traffic load. In order to evaluate delay-throughput performance and stability measure of the random access protocol, we first examine the possible traffic load to be carried throughput the ranging subchannel. We the present performance analysis and numerical examples.

In the high performance radio access networks, the number of random access channels can be used for mobile stations to transmit their bandwidth requests in contention mode via multiple random access. In this paper a collision reduced random access scheme based on -art split algorithm in the centralized medium access control protocol is presented. In this method the splitting algorithm is used in two fold. On one hand the whole random access channels are exclusively separated into two parts; one is for initial contention where only initial access mobile stations can be served and the other is for retransmit contention where the mobile stations whose initial random access was not successful can join for their retransmission. On the other hand the -ary split algorithm applied in the retransmit contention area to resolve the collisions. By doing so the proposed scheme achieves considerably greater performance in terms of maximum throughput, mean access delay, and delay jitter, which is one of the important criteria for real-time traffic. Through numerical examples and computer simulations the effect of the various parameters of the algorithm, initial number of random access report(s) and the split size , on the system performance is examined.

We propose simple and computationally efficient wireless channel modeling algorithm. For this purpose we adopt the special case of the algorithm initially proposed in [1] and show that its complexity significantly decreases when the time-series is covariance stationary binary in nature. We show that for such time-series the solution of the inverse eigenvalue problem returns unique transition probability matrix of the modulating Markov chain that is capable to match statistical properties of empirical frame error processes. Our model explicitly takes into account autocorrelational and distributional properties of empirical data. We validate our model against empirical frame error traces of IEEE 802.11b wireless access technology operating in DCF mode over spread spectrum at 2Mbps and 5.5 Mbps bit rates. We also made available the C code of the model as well as pre-compiled binaries for Linux and Windows operating systems at http://www.cs.tut.fi/~moltchan.

In this paper we address the problem of establishing a cost efficient multicast tree among a group of stationary nodes in a multi-hop wireless network. The flooding of broadcast discovery messages is a major limitation to the scalability of most ad hoc protocols. To avoid massive flooding, we limit the reach of broadcast discovery messages, and consider the case were joining nodes can only learn limited information about the multicast group topology from neighbors within a fixed number of hops. We propose two algorithms that satisfy this constraint. We analyze the worst case cost of the established trees and prove that the first algorithm builds a minimal cost spanning tree, while the second builds a sub-optimal tree with a worst-case approximation ratio of O(log n/loglog n). The advantage of the second algorithm is that the communication requirement for a node to join the multicast tree is smaller. We simulate and compare the proposed algorithms. Finally, we discuss the implementation issues and scenarios for using each one of them. We also describe our secure multicast application that builds on top of the proposed protocols.

The widespread use of Mobile Ad Hoc Networks (MANETs) in many fields of applications has led to the continuous development of routing protocols which can perform well when deployed under different scenarios, as well as offer better Quality of Service (QoS) support. In this paper, we focus on how link stability is utilized as a metric to provide accurate feedback on the network. We then introduce two mechanisms – L-REQ (Limited forwarding of Route Requests) and L-REP (Limited initiation of Route Replies by intermediate nodes) – which make use of link stability to dynamically adapt the characteristic behaviour of existing protocols to achieve better network performance. The two adaptive schemes are then applied to the Ad Hoc On Demand Distance Vector (AODV) routing protocol as proof of concept.

In the Mobile Ad Hoc Network (MANET) paradigm, multipath routing protocols were initially proposed due to QoS needs, since they do not require initiation of the route discovery process after each link disconnection. Moreover, research on MANET routing security has shown that multipath routing provides increased resilience against security attacks of collaborating malicious nodes. Towards this direction, several secure multipath routing protocols have been recently proposed in the literature, which indeed provide such increased security protection for critical applications. However, embedding security mechanisms always imposes extra burden to the route discovery process. In this paper, we evaluate the performance of the existing secure multipath routing protocols for MANET through extensive simulations in various traffic scenarios.

In this paper, the packet error rate (PER) of IEEE 802.15.4 low rate wireless personal area network (WPAN) under the interference of IEEE 802.11b wireless local area network (WLAN) is analyzed. The PER is obtained from the bit error rate (BER) and the collision time. The BER is obtained from signal to noise and interference ratio. The power spectral density of the IEEE 802.11b is considered in order to determine in-band interference power of the IEEE 802.11b to the IEEE 802.15.4. The simulation results are shown to validate the numerical analysis.

Most existing WLAN access mechanisms cannot provide QoS assurances. Even those that are QoS aware can only provide relative service differentiation. Based on EY-NPMA, the HIPERLAN Medium Access Control algorithm, we propose a dynamic priority medium access scheme to provide time-bounded services. By approximating an ideal Earliest Deadline First (EDF) scheduler, the proposed scheme can offer delay and delay jitter assurances while achieving high medium utilization. Furthermore, we compare our scheme with a mechanism that enhances the IEEE 802.11 MAC protocol with QoS support. Simulation studies document and confirm the positive characteristics of the proposed mechanism.

A rudimentary approach to mitigate interference issues in license-exempt 802.16 systems is presented. This approach operates by permitting each (BS), and associated (SSs) to remain inactive for a specified fraction of the time. Other systems can then transmit with a reduced likelihood of interference. A simulator was developed to determine how this system performs. The results show that the throughput of the system is very sensitive to the fraction of time each BS is active; the system throughput is maximised when each BS is active less than 40% of the time for the scenarios studied. The results demonstrate a discrepancy between uplink and downlink throughput which can be attributed to the greater amount of overheads in the uplink. Finally, the results show that broadcast information being transmitted periodically at full power has a significant detrimental impact on the system.

In this paper we propose an approach to increase TCP’s fairness in multihop wireless networks, using ECN as a congestion signalling mechanism. The novel idea we introduce is that the marking probability at a wireless node is a function of the aggregate utilization in the node’s neighborhood, which is determined by the sum of the receiving rates of all nodes within its collision domain. A node’s received rate can be communicated to neighboring nodes by piggy-backing it on control packets, such as CTS and RTS messages, or data packets. Simulation results demonstrate that our approach can improve TCP’s fairness in a multihop wireless network compared to drop tail queueing, while achieving the same aggregate throughput. Moreover, the proposed approach yields smaller average packet delay and delay jitter compared to drop tail queueing.