Wired/Wireless Internet Communications

It is well known that one of the critical sources of TCP poor performance in multihop ad hoc networks lies in the TCP window mechanism that controls the amount of traffic sent into the network. In this paper, we propose a novel cross layer solution called “TCP Contention Control” that dynamically adjusts the amount of outstanding data in the network based on the level of contention experienced by packets as well as the throughput achieved by connections. Our simulation results show TCP Contention Control can drastically improve TCP performance over 802.11 multihop ad hoc networks.

We propose a combined transport – medium access control (MAC) layer scheme to provide

relative

service differentiation to Transmission Control Protocol (TCP) flows over a geostationary (GEO) bandwidth on demand (BoD) satellite networks. Our approach involves the joint configuration of TCP-Performance Enhancing Proxy (TCP-PEP) agents at the transport layer and the scheduling algorithm controlling the resource allocation at the MAC layer. The scheme is independent of the TCP variant used in the network. Extensive simulation results show that the two mechanisms exhibit complementary behavior in achieving the desired differentiation throughout the traffic load space: the TCP-PEP agents control differentiation at low system utilization, whereas the MAC scheduler becomes the dominant differentiation factor at high load.

Current standard defines two variants of TCP NewReno: the Slow-but-Steady and Impatient. While the behavior of various TCP implementations has been extensively studied over the last years, little attention has been paid to performance analysis of different variants of TCP NewReno. In this paper, we first develop an analytical model of the Impatient variant, which being combined with the earlier proposed model of the Slow-but-Steady variant gives a comprehensive analytical model of TCP NewReno throughput. We then make an analytical comparison of the Impatient and Slow-but-Steady throughputs in the presence of correlated losses. We show that, although neither of the two variants is optimal, the Impatient variant provides the same throughput as the Slow-but-Steady one in a wide range of network conditions and significantly outperforms it in case of large windows and multiple packet drops. This could be of special interest for networks with large bandwidth and long delay.

This paper studies the effect of path delay on SCTP performance. It focuses on the SCTP fast retransmit algorithm and demonstrates that the performance in the current retransmission strategy will degrade acutely when the secondary path delay is less than the primary path delay at a certain level. The performance degradation is due to the disordered SACKs and constant congestion window size during the fast retransmit phase. Some modifications aimed at these problems are proposed and evaluated. This paper also identifies that the cause of the performance degradation in SCTP is a result of the single path configuration oriented design of the current fast retransmit algorithm. Several fast retransmission strategies are evaluated for different path delay and bandwidth configurations.

In mobile IPv6 network, the handoff latency for mobile node challenges service quality of real-time stream media applications. This paper proposes an improved handover scheme for stream media applications, which switch data-stream to foreign network link predictively by session-level handover between mobile and correspondent node. And to reduce the duration of dual-stream transmission in network, a time-compensation mechanism is used. The scheme works at session-level independently, and requires no modification to current standards and protocols. Simulation results show that the scheme effectively improves the handoff latency and packet-losses for stream media applications.

This paper analyses security mechanisms in wireless networks, and designs for secure and efficient protocol for a handover reusing the key materials generated before the handover phase. In order to be sure that our handover protocol does not have any security flaws, we formally validated it using automatic protocol analyzer AVIPSA.

Seamless handover over heterogeneous mobile environments is a major requirement to the success of the next generation of networks. However, seamless movement requires the control of the quality level and connectivity of communication sessions with no perceived service degradation to the users. This seamless characteristic is equally important for communication sessions encompassing only one or multiple receivers, being the latter called multi-user sessions. This paper presents a solution to allow seamless mobility for multi-user sessions over heterogeneous networks with mobile receivers and static senders. The proposed solution integrates end-to-end

Quality of Service

(QoS) mapping, QoS adaptation and connectivity control with seamless mobility support. The latter is achieved by using buffers in the mobile nodes and caches in the access-routers together with mobility prediction and context transfer schemes. Simulations present the efficiency of this proposal to setup ongoing sessions and its impact in reducing packet losses during movement.

It is sometimes believed that a “broadband access” network, providing ample transmission capacity to residential environments, is enough so as to allow a flawless delivery of advanced services. However, the provisioning of a combination of multiple services with guaranteed quality up to the end-user terminal requires a carefully designed architecture incorporating the appropriated Quality of Service (QoS) concepts throughout the data path. And this path includes the Residential Gateway (RGW) as the last hop towards the home network. This paper describes the different experiences performed with the RGW prototype developed within the framework of the European IST research project MUSE. Special emphasis will be made on the QoS capabilities of the RGW as well as on authentication and auto-configuration features.

In this study, we analyze security and quality of service (QoS) issues in cluster-based wireless sensor networks (WSN). Taking spatial resolution as the main QoS metric and limiting the security definition to data integrity and authentication, we present a control strategy to maintain desired QoS and security levels during the entire operation of a cluster-based sensor network. Besides, our proposed strategy considers some other WSN QoS attributes such as coverage, packet collision and system lifetime. It provides sufficient coverage by statistical means and minimizes packet loss due to collisions by employment of a slotted MAC (media access control) scheme. It also tries to maximize the operational lifetime of the sensor network by a power conserving scheme which make all sensors participate equally. In this study, we also determine the best tradeoff between security and spatial resolution.

Differentiated Services (DiffServ) are seen as the technology to support Quality of Service (QoS) in IP networks in a scalable manner by allowing traffic aggregation within the engineered traffic classes. In DiffServ domains, admission control additionally needs to be employed in order to control the amount of traffic into the engineered traffic classes so as to prevent overloads that can lead to QoS violations. In this paper we present an admission control scheme for inter-domain real-time traffic originating from DiffServ stub domains; that is real-time traffic originating from end-users connected to a DiffServ stub domain towards destinations outside the geographical scope of that domain. By means of simulations we show that our scheme performs well and that it compares favorably against other schemes found in the literature.

The P-SHIM6 architecture provides ISP independence to IPv6 sites without compromising scalability. This architecture is based on a middle-box, the P-SHIM6, which manages the SHIM6 protocol exchange on behalf of the nodes of a site, which are configured with provider independent addresses. Incoming and outgoing packets are processed by the P-SHIM6 box, which can assign different locators to a given communication, either when it is started, or dynamically after the communication has been established. As a consequence, changes required for provider portability are minimized, and fine-grained Traffic Engineering can be enforced at the P-SHIM6 box, in addition to the fault tolerance support provided by SHIM6.

This paper proposes a novel traffic engineering framework able to automatically provide near-optimal OSPF routing configurations for QoS constrained scenarios. Within this purpose, this work defines a mathematical model able to measure the QoS compliance in a class-based networking domain. Based on such model, the NP-hard optimization problem of OSPF weight setting is faced resorting to Evolutionary Algorithms. The presented results show that, independently of other QoS aware mechanisms that might be in place, the proposed framework is able to improve the QoS level of a given domain only taking into account the direct influence of the routing component of the network. The devised optimization tool is able to optimize OSPF weight configurations in scenarios either considering a single level of link weights or using multiple levels of weights (one for each class) in multi-topology routing scenarios.

The explosive growth of VoIP traffic poses a potential challenge to the stability of the Internet that, up to now, has been guaranteed by the TCP congestion control. In this paper, we investigate how Skype behaves in the presence of time-varying available bandwidth in order to discover if some sort of congestion control mechanism is implemented at the application layer to match the network available bandwidth and cope with congestion. We have found that Skype flows are somewhat elastic, i.e. they employ some sort of congestion control when sharing the bandwidth with unresponsive flows, but are inelastic in the presence of classic TCP responsive flows, which provokes extreme unfair use of the available bandwidth in this case. Finally, we have found that when more Skype calls are established on the same link, they are not able to adapt their sending rate to correctly match the available bandwidth, which would confirm the risk of network congestion collapse.

We propose a layered quality adaptation scheme for video streams to smooth the short-term oscillations induced by

Additive Increase Multiplicative Decrease

(

AIMD

) mechanisms, and eventually refine the perceptual video quality. The layered scheme utilizes receiver buffering, adapting the video quality along with long-term variations in the available bandwidth. The allocation of a new layer is based on explicit criteria that consider the available bandwidth, as well as the amount of buffering at the receiver. Consequently, the adaptation mechanism prevents wasteful layer changes that have an adverse effect on user-perceived quality. In the sequel, we concentrate on the interactions of the layered approach with

Scalable Streaming Video Protocol

(

SSVP

). Exploiting performance measures related to the perceived quality of rate-adaptive video streams, we quantify the combination of SSVP rate control and receiver-buffered layered adaptation.

This paper describes network level simulation results for VoIP and Best Effort traffic running over HSDPA. The main focus involves an analysis of how the system is able to deal with user mobility, and in particular handover. Both a handover threshold level as well as a handover setup delay have been included in the simulation model, and a discard timer is used to remove outdated packets from the Node B queue.

Link adaptation and H-ARQ (Hybrid-Automatic Repeat reQuest) functionalities are also included in the simulator. An infinite multi-cell environment has been modeled and the users are assumed to move at vehicular speeds. A performance analysis will show the end-to-end QoS. The handover threshold, delay and discard times are also varied in order to investigate their impact on the performance.

Since Integrated Services architecture is not scalable, it seems the only solutions for Quality of Service (QoS) architecture in the Internet are Differentiated Services (DiffServ) or its variations. It is generally understood that networks with DiffServ architectures can guarantee the end-to-end delay for packets of the highest priority class, only in lightly utilized networks. We show that, in networks without loops, the delay bounds for highest priority packets exist regardless of the level of network utilization with DiffServ architecture. These bounds are quadratically proportional to the maximum hop counts in heavily utilized networks; and are linearly proportional to the maximum hop counts in lightly utilized networks. We argue that, based on the analysis of these delay bounds in realistic situations, DiffServ architecture is able to support real time applications even in large networks. Considering that loop-free networks, especially the Ethernet networks, are being adopted more than ever for access networks and for provider networks as well, this conclusion is quite encouraging. Throughout the paper we use Latency-Rate (

$\mathcal{LR}$

) server model, with which it has been proved that FIFO and Strict Priority schedulers are

$\mathcal{LR}$

servers to each

flows

in certain conditions.

It is well known that the presence of nodes using a low data transmit rate has a disproportionate impact on the performance of an IEEE 802.11 WLAN. ORP is an opportunistic relay protocol that allows nodes to increase their effective transmit rate by replacing a low data rate transmission with a two-hop sequence of shorter range, higher data rate transmissions, using an intermediate node as a relay. ORP differs from existing protocols in discovering relays experimentally, by optimistically making frames available for relaying. Relays identify themselves as suitable relays by forwarding these frames. This approach has several advantages compared with previously proposed relay protocols: Most importantly, ORP does not rely on observations of received signal strength to infer the availability of relay nodes and transmit rates. We present analytic and simulation results showing that ORP improves the throughput by up to 40% in a saturated IEEE 802.11b network.

This paper presents analysis of the performance on the IEEE 802.11b in the presence of multiple IEEE 802.15.4 interferences. To analyze the performance, packet error rate (PER) and throughput are used as performance metrics. The PER is computed by the bit error rate (BER), the collision time and the number of IEEE 802.15.4 interferers in the presence of in-band of the IEEE 802.11b. The throughput of the IEEE 802.11b under multiple IEEE 802.15.4 interferences is obtained from the total IEEE 802.11b packet length received during a specified time. Analytic results of the performance of IEEE 802.11b under multiple IEEE 802.15.4 interferences are verified by simulation results. These results can support coexistence criteria for the IEEE 802.11b and the IEEE 802.15.4.

First order metrics (throughput, average delay) of the IEEE 802.11 DCF MAC protocol have been extensively analyzed. The service process of the same protocol has not received the same attention, although it is known that it might be bursty. We develop a simple and accurate Markov model that allows a complete characterization of the service process of an 802.11 BSS. A major result of simulations is that correlation between consecutive service times is negligible, hence the service process can be safely described as a renewal process. The analytic model highlights that service burstiness lies essentially in the doubling of the contention window after a collision up to very large values. The trade-off between average throughput and service burstiness is obtained from the model.

Providing security services for wireless sensor networks plays a vital role in secure network operation especially when sensor networks are deployed in hostile areas. In order to pave the way for these mechanisms, cryptographic keys must be agreed on by communicating nodes. Unfortunately, due to resource constraints, the key agreement problem in wireless sensor networks becomes quite intricate. To deal with this problem, many public-key unrelated proposals have been proposed so far. One prominent branch of these proposals is based on random key pre-distribution. Inspired by this trend, in this paper, we propose a new random key pre-distribution scheme that is comparable to Du et al.’s scheme [2] in terms of network resiliency and memory usage. On the other hand, our later analysis shows that our scheme outperforms Du et al.’s scheme in terms of computational and communication overhead.

In this paper we present the distributed event localization and tracking algorithm DELTA that solely depends on light measurements. Based on this information and the positions of the sensors, DELTA is able to track a moving person equipped with a flashlight by dynamically building groups and electing well located nodes as group leaders. Moreover, DELTA supports object localization. The gathered data is sent to a monitoring entity in a fixed network which can apply pattern recognition techniques to determine the legitimacy of the moving person. DELTA enables object tracking with minimal constraints on both sensor hardware and the moving object. We show the feasibility of the algorithm running on the limited hardware of an existing sensor platform.

In this paper, we introduce a cross-layer diversity framework for multi-air interface wireless communication devices. As an initial step, we focus on devices, of the cellular phones type, that have both long-range relatively low-rate communication air-interface (e.g., GSM) and short-range high-rate communication air-interface (e.g., IEEE802.11). The devices can cooperate through the energy-efficient high-rate interface to improve the performance of the long-range interface. Within this framework we propose a distributed signal-combining technique that accounts for the limited bandwidth of the short-range communication:

Threshold Maximum Ratio Combining

. We analytically derive the probability distribution function of the signal to noise ratio (SNR) of the combined signals as a function of the number of involved devices and show that significant improvement of the SNR is achievable which translates into a reduction of the overall system outage probability.

With the rapid improvement of wireless networking technologies, current mobile devices can be potentially equipped with multiple interfaces to access different kinds of wireless networks. Thus, there have been many efforts to provide a seamless roaming between heterogeneous networks. However, most previous studies do not address how to select the best possible interface in terms of energy consumption. Therefore, in this paper, we propose to take advantage of accurate location positioning via WWAN interface, so that Mobile Nodes (MNs) obtain the information of hotspot range and completely turn off WLAN interface during idle state, leading to save energy consumption. Simulation results show that our mechanism outperforms existing approaches in terms of energy consumption and signaling overhead.

In this paper we propose the use of non-persistent CSMA as an anti-collision procedure for RFID active tags. Current proposals for both passive and active tags are based on the framed slotted ALOHA protocol, which does not scale well requiring additional procedures for frame length adaptation. However, active RFID devices already include carrier sense capabilities with no additional cost and, thus, CSMA may be employed seamlessly. Nevertheless, selecting the contention micro-slots of CSMA in the classical way (

i.e.

, with a uniform distribution and an exponential back-off algorithm) does not result in an efficient identification process, as we will demonstrate. Fortunately, better choices can be found. Recently, an optimal distribution for the selection of micro-slots for event-driven sensor networks has been computed, as well as a practical implementation: the Sift distribution. In this work we propose the application of the quasi-optimal Sift distribution along with CSMA for active tag identification. By means of an analytical study, we evaluate the average time needed for identification with this mechanism and compare it with the current ISO 18000-7 and EPC “Gen 2” standard. The results reveal that the Sift-based non-persistent CSMA outperforms both of them. Moreover, it also scales much better, without the need for further adaptation mechanisms.

In this paper we present an extension to the OLSR unicast routing protocol to support multicast routing in mobile ad hoc networks. The proposed protocol is based on Multicast Overlay Spanning Trees (MOST). The main benefits of this approach are twofold. Firstly, it implies that only nodes interested in taking part in the multicast communication need to participate in the protocol operation, which is transparent to other OLSR nodes. In addition, the MOST approach scaling properties achieve the theoretical performance bounds concerning the capacity of multicast communication in massive ad hoc networks. We perform simulations of the MOST protocol under the ns-2 simulator to compare with the theoretical results, and we present a fully working implementation for real network environments.

Mobile Ad Hoc networks (MANETs) are susceptible to having their effective operation compromised by a variety of security attacks. Nodes may misbehave either because they are malicious and deliberately wish to disrupt the network, or because they are selfish and wish to conserve their own limited resources such as power, or for other reasons. In this paper, we present a mechanism that enables the detection of nodes that exhibit packet forwarding misbehavior. We present evaluation results that demonstrate the operation of our algorithm in mobile ad hoc environments and show that it effectively detects nodes that drop a significant fraction of packets.

Vehicular ad-hoc networks (VANETs) offer a large number of new potential applications without relying on significant infrastructure. Many of these applications benefit from multi-hop relaying of information, thus requiring a routing protocol. Characteristics unique to VANETs (such as high mobility and the need for geographical addressing) make many conven tional ad hoc routing protocols unsuitable. Also, some envisioned applica tions have end-to-end QoS requirements. In this paper we propose a new multicast routing protocol specifically designed for VANETs. Its purpose is to provide a routing service for a future reliable transport protocol. We eval uate its performance using realistic network and traffic models. It is shown that it is possible to implement a reliable multicast routing protocol for VANETs.

The IEEE 802.11 standard defines a medium access control protocol for wireless local area networks (WLANs). In typical WLANs, access points (APs) are connected with one another in a wired manner. As a way to loosen this restriction wireless mesh networks (WMNs), which are rather independent of a backbone infrastructure, is emerging. Due to their inherent characteristics unlike ad hoc networks, it is unreasonable that the ad hoc network routing protocols are directly applied for the WMNs without any modification. The tree-based routing protocol (TRP) defined in the IEEE 802.11s is one of the routing protocols used in WMNs. The TRP, however, forwards data on the only one active path. In this paper, we propose a flexible tree-based routing protocol with a mesh relaying node (TRP-MRN) in WMNs. The TRP-MRN provides a concurrent multi-path establishment algorithm. We analyze the end-to-end delay of the TRP and the TRP-MRN and show that the TRP-MRN can effectively reduce the end-to-end delay and provide alternative routes via simulations.

This paper deals with a simple way of reducing the multiple access interference (MAI) introduced by symbol timing misalignment in an OFDMA uplink system. To alleviate the MAI, we provide a new OFDMA transmitter equipping with a simple symbol repetition coupled with a cyclic time shift. It is found that this scheme can provide the MAI-robust reception without decreasing the bandwidth efficiency when the system is fully loaded.

Under dynamic time division duplexing (D-TDD) architecture in OFDM cellular systems, the inter-BS and inter-MS interference is inevitable during the cross time slot (CTS) period, and this interference seriously degrades the wireless access system performance. To mitigate such interference, we propose a region and time partitioning D-TDD architecture for OFDM systems. Each time slot in the CTS period is split into several minislots, and then each cell is divided into as many regions as the number of minislots per time slot. We then assign each minislot only to the users in its corresponding region. On top of such architecture, which inherently separates interfering entities farther from each other, we design a robust time slot allocation scheme considering the channel status of each mobile station. By computer simulation, it has been verified that the proposed scheme outperforms conventional time slot allocation methods in both the outage probability and the bandwidth efficiency.

Cognitive radio (CR) has been proposed as an effective technology for flexible use of the radio spectrum. The interference among primary users and CR users, however, becomes a critical problem when they are operating nearby using adjacent frequency channels with different transmission powers. In this paper, a CR-based bandwidth sharing architecture is proposed, which can effectively suppress adjacent channel interference (ACI) between them. This new approach is characterized by the adaptive frequency spreading, and the adaptive time spreading with power control for the subcarriers near the borders of the CR user’s spectrum. The frequency spreading scheme provides with extra power gain against ACI, whereas the time spreading with power control guarantees the minimal interference to primary users from CR users. By computer simulation, it has been proved that the proposed system outperforms the conventional OFDM systems in both throughput and BER performance.

This paper presents a CDMA closed-loop power control (CLPC) using channel prediction in time-varying mobile fading channels. The proposed CLPC that relies on least square (LS) or least mean square (LMS) based autoregressive (AR) low-rate prediction provides a particularly robust performance for large loop delays, especially in the presence of channel estimation errors. As the mobility increases the proposed CLPC reduces mean-square error (MSE) further than conventional CLPCs. The Monte-Carlo simulation results demonstrate the superiority of the proposed CLPC over conventional nonpredictive and predictive CLPCs.