Crowdsourcing enables the fine-grained characterization and performance evaluation of today׳s large-scale networks using the power of the masses and distributed intelligence. This paper presents SmartProbe, a system that assesses the bottleneck capacity of Internet paths using smartphones, from a mobile crowdsourcing perspective. With SmartProbe measurement activities are more bandwidth efficient compared to similar systems, and a larger number of users can be supported. An application based on SmartProbe is also presented: georeferenced measurements are mapped and used to compare the performance of mobile broadband operators in wide areas. Results from one year of operation are included.

In the last decade, many studies have used the Internet autonomous system (AS)-level topology to perform several analyses, from discovering its graph properties to assessing its impact on the effectiveness of worm-containment strategies. Yet, the Border Gateway Protocol (BGP) data used to reveal the topologies are far from complete. Our contribution is threefold. First, we analyze BGP data currently gathered by the most famous route collector projects, highlighting and explaining the causes of their incompleteness. We found that large areas of the Internet are not properly captured due to the geographical location of route collector feeders and due to BGP filters, such as export policies and decision processes. Second, we propose a methodology based on a new metric, named p2c-distance, which is able to: 1) identify the minimum number of ASs required to obtain an Internet AS-level topology that is closer to reality; and 2) identify a ranking list of these ASs to show that it is possible to obtain nonnegligible coverage improvements with a limited number of appropriately chosen feeding ASs. Third, we characterize the ASs that were found to be part of the solution of the above covering problems. We found that the route collectors are rarely connected to these ASs, thus highlighting that much effort is needed to devise a route collector infrastructure that ideally would be able to capture a complete view of the Internet.

We propose a model to analyze the decisions taken by an Autonomous System (AS) when joining the Internet. We first define a realistic model for the interconnection costs incurred and then we use this cost model to perform a game theoretic analysis of the decisions related to the creation of new links in the Internet. The proposed model doesn't fall into the standard category of routing games, hence we devise new tools to solve it by exploiting peculiar properties of our game. We prove analytically the existence of multiple equilibria for specific cases, and provide an algorithm to compute the stable ones. The analysis of the model's outcome highlights the existence of a Price of Anarchy (PoA) and a Price of Stability (PoS), originated by the non-cooperative behavior of the ASes, which optimize their cost function in a selfish and decentralized manner. We further observe the presence of competition between the facilities providing either transit or peering connectivity, caused by the cost differences between these two interconnection strategies.

Several works over the past few years have shown that the Internet AS-level topology is partially hidden from the current Internet measurement infrastructures. Most have focused on the incompleteness of the connectivity extracted from BGP data. A few have analysed the connectivity collected by traceroute measurement infrastructures showing the amount of connections introduced by traceroute campaigns. None, however, have investigated in detail the underlying rationale, i.e. the economic nature of the Internet. In this paper we fill this gap by analysing five traceroute infrastructures, found to be active in October 2013, with the p2c-distance metric, which is specifically designed to capture the complex economic dynamics that rule the Internet. We found that the traceroute infrastructures that currently run topology discovery measurements (Ark, DIMES and Portolan), together with BGP route collectors, are able to reveal the full connectivity of 23.50% of the Internet core ASes. This is a considerable improvement given that the BGP infrastructure alone is able to cover only 15.90% of the Internet core. This percentage could be increased up to 48.48% if the remaining two infrastructures (Dasu/Ono and RIPE Atlas) performed topology discovery campaigns. We also found that the placement of traceroute probes is not optimal from a topology discovery perspective, as it causes several probes to provide only redundant connectivity information. We show that the same number of traceroute probes optimally deployed, would be able to completely reveal the full AS connectivity of the Internet core.

As the Internet autonomous system (AS)-level topology grows over time, some of its structural properties remain unchanged. Such time-invariant properties are generally interesting because they tend to reflect some fundamental processes or constraints behind Internet growth. As has been shown before, the time-invariant structural properties of the Internet include some most basic ones, such as the degree distribution or clustering. Here, we add to this time-invariant list a nontrivial property - k-dense decomposition. This property is derived from a recursive form of edge multiplicity, defined as the number of triangles that share a given edge. We show that after proper normalization, the k-dense decomposition of the Internet has remained stable over the last decade, even though the Internet size has approximately doubled, and so has the k-density of its k-densest core. This core consists mostly of content providers peering at Internet eXchange Points, and it only loosely overlaps with the high-degree or high-rank AS core, consisting mostly of tier-1 transit providers. We thus show that high degrees and high k-densities reflect two different Internet-specific properties of ASs (transit versus content providers). As a consequence, even though degrees and k-densities of nodes are correlated, the relative fluctuations are strong, and related to that, random graphs with the same degree distribution or even degree correlations as in the Internet, do not reproduce its k-dense decomposition. Therefore an interesting open question is what Internet topology models or generators can fully explain or at least reproduce the k-dense properties of the Internet.

Autonomous Systems (AS) exist and co-exist in two parallel dimensions. In one dimension they are physical networks, whose interconnections are necessary to ensure global Internet reachabilty. In the other dimension, ASes are large well-known companies competing in the same industry. In this paper we bridge together these dimensions by investigating synchronous cross correlations of stock market data and AS-level topological properties. We find that geographically close companies offering similar services are driven by common economic factors. We also provide evidence on the existence and nature of factors governing AS global as well as local topological properties.

The DNS is one of the core protocols on which the Internet is built upon. Hidden behind higher-level protocols such as email and web, it carries valuable information that can be exploited for understanding trends and preferences of the Internet community. In this paper we propose novel methodologies for modelling DNS traffic that allow Internet domains, DNS resolvers and their interactions to be represented effectively by means of graphs. DNS traffic collected at “.it” ccTLD DNS domain servers has been used to validate this work on a large scale. We found highly-skewed, fat-tailed domain and resolver degree frequencies, obeying power laws at least in their tails. These findings shed light on the the scale-free nature of the DNS ecosystem, where a few domains and a few resolvers are responsible for most of the DNS activity.

Portolan is a crowdsourcing system, aimed at monitoring and measuring large-scale networks, that uses smartphones as mobile observation elements. Currently, Portolan is able to collect information about both wired and wireless networks, in particular it is used to obtain the graph of the Internet with unprecedented resolution and to associate performance indexes (received signal strength, maximum throughput) of cellular networks to geographic locations.

In this paper we investigate the structure of the Internet by exploiting an efficient algorithm for extracting k-dense communities from the Internet AS-level topology graph. The analyses showed that the most well-connected communities consist of a small number of ASs characterized by a high level of clusterization, although they tend to direct a lot of their connections to ASs outside the community. In addition these communities are mainly composed of ASs that participate at the Internet Exchange Points (IXPs) and have a worldwide geographical scope. Regarding k-max-dense ASs we found that they play a primary role in the Internet connectivity since they are involved in a huge number of Internet connections (42% of Internet connections). We also investigated the properties of three classes of k-max-dense ASs: Content Delivery Networks, Internet Backbone Providers and Tier-1s. Specifically, we showed that CDNs and IBPs heavily exploit IXPs by participating in many of them and connecting to many IXP participant ASs. On the other hand, we found that a high percentage of connections originated by Tier-1 ASs are likely to involve national ASs which do not participate at IXPs.

The analysis of real-world complex networks has been the focus of recent research. Detecting communities helps in uncovering their structural and functional organization. Valuable insight can be obtained by analyzing the dense, overlapping, and highly interwoven k-clique communities. However, their detection is challenging due to extensive memory requirements and execution time. In this paper, we present a novel, parallel k-clique community detection method, based on an innovative technique which enables connected components of a network to be obtained from those of its subnetworks. The novel method has an unbounded, user-configurable, and input-independent maximum degree of parallelism, and hence is able to make full use of computational resources. Theoretical tight upper bounds on its worst case time and space complexities are given as well. Experiments on real-world networks such as the Internet and the World Wide Web confirmed the almost optimal use of parallelism (i.e., a linear speedup). Comparisons with other state-of-the-art k-clique community detection methods show dramatic reductions in execution time and memory footprint. An open-source implementation of the method is also made publicly available.

In the last decade many studies have used the Internet AS-level topology to perform several analyses, from discovering its graph properties to assessing its impact on the effectiveness of worm-containment strategies. Yet, the BGP data typically used to reveal the topologies are far from being complete. Our contribution is three-fold. Firstly, we analyse BGP data currently gathered by RouteViews, RIS and PCH route collectors, and investigate the reasons for its incompleteness. We found that large areas of the Internet are not properly captured due to the geographic placement of the current route collector feeders and due to BGP filters, such as BGP export policies and BGP decision processes. Secondly, we propose a methodology to select the optimal number of ASes that should join a route collector project to obtain a view of the Internet AS level topology closer to reality. We applied this methodology to the global AS-level topology and to five regional AS-level topologies, highlighting that the particular characteristics of the Internet at a regional level cannot be ignored during this process. Thirdly, we provide a characterization of the ASes that we found to be part of at least one optimal solution set. By analysing these ASes we found that the current route collector infrastructure is rarely connected to them, highlighting that much more effort should be made in devising a route collector infrastructure that ideally would be able to capture a complete view of the Internet.

The structure of the Internet is still unknown even though it provides services for most of the world. Its current configuration is the result of complex economic interactions developed in the last 20 years among important carriers and ISPs. Although with only some success, in the last few years some research has tried to shed light on the economic relationships established among ASes. The typical approaches have two phases: in the first, data from BGP monitors is gathered to infer the Internet AS-level topology graph, while in the second phase, algorithms are instantiated on this graph to derive economic tags for all edges between nodes (i.e. ASes). This paper provides significant findings for both steps. Specifically, regarding the second step, a small set of transit-free ASes and the lifespan of any AS paths are the input to an algorithm that we have devised which assigns an economic tag to each AS connection. The quality of inferred tags is expressed by a weight which takes into consideration the lifespan of the connection it refers to, as well as the outcome of the so called two-way validation approach. Regarding the first step the paper reports another valuable contribution targeted at identifying and cleaning the presence of fake connections induced by typos.

In this paper, we deal with reliable and energy-efficient data delivery in sparse wireless sensor networks (WSNs) with multiple mobile sinks (MSs). This is a critical task, especially when MSs move randomly, as interactions with sensor nodes are unpredictable, typically of short duration and affected by message losses. In addition, multiple MSs can be simultaneously present in the sensor contact area making the minimum energy data delivery a complex optimization problem. To solve the above issues, in this paper we propose a novel protocol that efficiently combines erasure coding with an Automatic Repeat reQuest (ARQ) scheme. The key features of the proposed protocol are as follows: (i) the use of redundancy to cope efficiently with message losses in the multicast environment and (ii) the ability of adapting the level of redundancy based on feedbacks sent back by MSs through acks. We observed by simulation that our protocol outperforms an alternative protocol that relies only on an ARQ scheme, even when there is a single MS. We also validated our simulation results through a set of experimental measurements based on real sensor nodes. Our results show that the adoption of encoding techniques increases the lifetime of the sensor in the range (40–55%) compared with standard simple ARQ approaches when applied to WSNs with MSs.

The AS-level topology of the Internet has been quite a hot research topic in the last few years. However, only a small number of studies have been developed that give a structural interpretation of this graph. Such an interpretation is crucially important in order to test protocols and optimal routing algorithms, to design efficient networks, and for failure detection purposes. Moreover, most research does not highlight the role that IXPs have on the AS-level structure of the Internet, although their role is recognized as fundamental. The initial contribution of this study is an analysis of the most important AS-level topologies that are publicly found on the web and an analysis of the topology obtained when they are merged. We compiled structural information from this topology making considerable use of the k-core decomposition technique to delineate various particular classes of nodes. Next, we associated node properties with a reasonable modus operandi of the ASs on the Internet. The second contribution is a study of the impact that ASs connected to IXPs and BGP connections crossing IXPs have on the AS-level topology. To achieve this, we developed a procedure to gather reliable information related to IXPs and their participants.

Operating on a frequency band occupying several nonoverlapping channels, IEEE 802.11 is now widely used in Wireless Mesh Networks (WMNs). Many multichannel MAC protocols are proposed to improve the spatial reuse in the network under the assumption that the transmissions on nonoverlapping channels do not interfere with each other. Some joint routing and channel assignment algorithms are also designed to increase the network throughput based on the premise that we can switch between different channels freely. Although simulations show that great improvements on network throughput can be observed in both cases, two fundamental questions remain: 1) Can we really use multiple nonoverlapping channels freely in WMNs? 2) If we can, what will be the cost when we switch channels dynamically and frequently? In this paper, by conducting extensive experiments on our testbed, we attempt to answer these questions. We find that in spite of interference between both overlapping and nonoverlapping channels, we can still use multiple channels in mesh networks under certain conditions but with care. We also show that the channel switching cost is actually very significant in WMNs. We recommend not to switch the channels too frequently when designing the channel assignment algorithms, and those channel assignment algorithms selecting one channel for each packet are not really beneficial.

There is a vast literature on the throughput analysis of the IEEE 802.11 media access control (MAC) protocol. However, very little has been done on investigating the interplay between the collision avoidance mechanisms of the 802.11 MAC protocol and the dynamics of upper layer transport protocols. In this paper, we tackle this issue from an analytical, simulative, and experimental perspective. Specifically, we develop Markov chain models to compute the distribution of the number of active stations in an 802.11 wireless local area network (WLAN) when long-lived transmission control protocol (TCP) connections compete with finite-load user datagram protocol (UDP) flows. By embedding these distributions in the MAC protocol modeling, we derive approximate but accurate expressions of the TCP and UDP throughput. We validate the model accuracy through performance tests carried out in a real WLAN for a wide range of configurations. Our analytical model and the supporting experimental outcomes show that 1) the total TCP throughput is basically independent of the number of open TCP connections and the aggregate TCP traffic can be equivalently modeled as two saturated flows; and 2) in the saturated regime, n UDP flows obtain about n times the aggregate throughput achieved by the TCP flows, which is independent of the overall number of persistent TCP connections.

There are many different channels denned in the IEEE 802.11 standard. However, the performance of WiFi networks still greatly suffers from the interference between users, even if they are using different channels. In this paper, we conduct some theoretical analysis of the interference between two channels, which is further verified by experiments. We show that there is indeed serious interference between two non- overlapping channels if they are close to each other.

It is widely recognized that a prerequisite for the commercial penetration of the ad hoc networking technologies is the integration with existing wired/wireless infrastructure-based networks to provide an easy and transparent access to the Internet and its services. However, most of the existing solutions for enabling the interconnection between IPv4-based mobile ad hoc networks and the Internet are based on complex and inefficient mechanisms, as Mobile-IP and IP tunnelling. In this paper, we describe an alternative approach to build multi-hop and heterogeneous proactive ad hoc networks, which can be used as flexible and low-cost extensions of traditional wired local area networks (LANs). Our proposed architecture provides transparent global Internet connectivity and address autoconfiguration capabilities to mobile nodes without requiring configuration changes in the pre-existing wired LAN, and relying on basic layer-2 functionalities. We have prototyped the core components of this architecture for Optimized Link State Routing-based ad hoc networks and we have conducted several experiments comparing the throughput performance obtained using our scheme and a well-known alternative (network address translation) NAT-based solution. The experimental outcomes show that our proposed technique ensures higher perconnection throughputs than the NAT-based solution in the considered network scenarios.

The growing popularity of the 802.11 standard for building local wireless networks has generated an extensive literature on the performance modelling of its MAC protocol. However, most of the available studies focus on the throughput analysis in saturation conditions, while very little has been done on investigating the interactions between the 802.11 MAC protocol and closed-loop transport protocols such as TCP. This paper addresses this issue by developing an analytical model to compute the stationary probability distribution of the number of backlogged nodes in a WLAN in the presence of persistent TCP-controlled download and upload data transfers, and embedding the network backlog distribution in the MAC protocol modelling. A large set of experiments conducted in a real network validates the model correctness for a wide range of configurations. A particular emphasis is devoted to investigate and explain the TCP fairness characteristics. Our analytical model and the supporting experimental outcomes demonstrate that using default settings for the capacity of devices' output queues provides a fair allocation of channel bandwidth to the TCP connections, independently of the number of downstream and upstream flows. Furthermore, we show that the TCP total throughput does not degrade by increasing the number of wireless stations.

Several structured p2p systems exist in literature and many applications have been developed on top of them. To use these applications on top of different overlays without changing their implementations, a common API was proposed in. However, since that specification is meagre, current implementations of structured p2p systems have customized it reducing the portability of applications. In addition, in mobile environments, the possibility to exploit cross-layer interactions considerably improves overall performace. In fact, a cross-layer p2p system, called CrossROAD, has been recently designed to optimize structured overlays on MANETs. It directly interacts with a proactive routing protocol, and it can provide cross-layer information to upper-layer applications to further optimize their behavior. In this paper we propose a cross-layer extension of the common API pointing out advantages of the cross-layer approach even at the application layer

This paper proposes a distributed and topology-independent contention control mechanism for IEEE 802.11-based multi-hop ad hoc networks, which aims at ensuring per-station fairness without degrading the total network throughput, and without requiring any information exchange between the nodes. The proposed algorithm dynamically controls the frequency each station accesses the channel according to the station's channel share. The statistics of the stations' channel shares are updated locally in each node based only on the standard physical carrier sensing activity. To avoid throughput degradations while seeking fair allocation of bandwidth, we have also developed a credit-based scheme that operates in cooperation with the backoff procedure. The credits are a virtual currency earned by stations refraining from transmitting, and spent to reclaim backoff-free transmission opportunities. Simulations conducted in several well-known problematic topologies show that our scheme can achieve a better fairness than standard IEEE 802.11, while increasing the total network throughput

In recent years, the Internet has witnessed the introduction of many peer-to-peer systems designed to realize large-scale data sharing. These platforms exhibit interesting features like self-configuration, self-healing and complete decentralization, which make them appealing for employment in ad hoc environments as well. However, the impact of ad hoc dynamics on the performance of these protocols, and the different set of constraints which this emerging networking paradigm imposes, haven't been yet carefully evaluated. This paper investigates the performance of Gnutella, one of the most widely used peer-to-peer systems, when put through typical ad hoc conditions like node mobility, frequent network partitioning, etc.. We show that a straightforward implementation of the protocol is not satisfactory under the point of view of the produced overhead and the average overlay connectivity. Finally, we propose a cross-layer optimization of Gnutella, which enhances its performance up to the expectations and makes it more suitable to the degree of self-organization and self-healing required in ad hoc environments.

Abstract — Data transfer in ad hoc environments shows poor network performance due to frequent link breakages and route failures. Selfish and malicious nodes may further deteriorate nodes communication, having a strong impact on transport layer protocols such as TCP, which are highly sensitive to packet losses. Although these misbehaviors have similar effects on the network functioning (i.e. packets are dropped), they are separately addressed by the research community. This paper provides a comprehensive method to improve the performance and reliability (performability) of nodes commu-nication in presence of faults, selfish and malicious behavior. Specifically, we propose and evaluate a novel forwarding policy that is based on multi-path routing and considers nodes reliability and routes length in forwarding decisions. We investigate through simulations how this mechanism improves the performability of TCP data transfers. In particular, we show that the simultaneous use of multiple paths yields higher throughput and continuous network connectivity when compared to single path forwarding. This has been verified in case of both fault conditions and intentional nodes misbehavior. I.

In the near future more and more users will access Internet services by means of portable devices through wireless links. However, mobile computing is still strongly limited by the scarcity of energetic resources of portable devices. In This work we propose and evaluate an application-independent energy management policy for a Wi-Fi hotspot scenario. Unlike the IEEE 802.11 power saving mode, the proposed solution is able to adapt to the application traffic profile, thus saving a considerable amount of energy. For the same reason it is flexible, i.e., it exhibits good performance irrespectively of the specific network application, and even in the presence of concurrent applications. Experimental measurements performed on a prototype implementation with different traffic types have shown that our energy management policy is able to save up to 80% of the energy consumed in a legacy architecture, without a significant degradation on the QoS perceived by the user.

In this paper we investigate the performance of mica2 and mica2dot Berkeley motes by means of an extensive experimental analysis. This study is aimed at analyzing the main elements that characterize the performance of a sensor network, e.g., power consumption in different operating conditions, impact of weather conditions, interference between neighboring nodes, etc. Even if the analysis is related to a specific technology it provides some general useful information. Specifically, we found that the transmission range of mote sensor nodes decreases significantly in the presence of fog or rain. We also investigated the interference between neighboring nodes and, based on the experimental results, we propose a channel model for mote sensor nodes. This model is very similar to the channel model of IEEE 802.11 networks.

In this paper we investigate the performance of IEEE 802.11b ad hoc networks by means of an experimental study. This analysis reveals several aspects that are usually neglected in simulation studies. Firstly, since different transmission rates are used for control and data frames, different transmission ranges and carrier-sensing ranges may exist at the same time in the network. In addition, the transmission ranges are in practice much shorter than usually assumed in simulation analysis, not constant but highly variable (even in the same session) and depends on several factors (i.e., mobile height, interference condition, etc.). Finally, exploiting our performance measurements, we present a channel model for an 802.11 network that indicates virtual carrier sensing is generally not necessary and the RTS/CTS mechanism only introduces additional overhead.

IEEE 802.11 is the standard for wireless local area networks (WLANs) promoted by the Institute of Electrical and Electronics Engineers. Wireless technologies in the LAN environment are becoming increasingly important and the IEEE 802.11 is the most mature technology to date. Previous works have pointed out that the standard protocol can be very inefficient and that an appropriate tuning of its congestion control mechanism (i.e., the backoff algorithm) can drive the IEEE 802.11 protocol close to its optimal behavior. To perform this tuning, a station must have exact knowledge of the network contention level; unfortunately, in a real case, a station cannot have exact knowledge of the network contention level (i.e., number of active stations and length of the message transmitted on the channel), but it, at most, can estimate it. We present and evaluate a distributed mechanism for contention control in IEEE 802.11 wireless LANs. Our mechanism, named asymptotically optimal backoff (AOB), dynamically adapts the backoff window size to the current network contention level and guarantees that an IEEE 802.11 WLAN asymptotically achieves its optimal channel utilization. The AOB mechanism measures the network contention level by using two simple estimates: the slot utilization and the average size of transmitted frames. These estimates are simple and can be obtained by exploiting information that is already available in the standard protocol. AOB can be used to extend the standard 802.11 access mechanism without requiring any additional hardware. The performance of the IEEE 802.11 protocol, with and without the AOB mechanism, is investigated through simulation. Simulation results indicate that our mechanism is very effective, robust, and has traffic differentiation potentialities.

We propose a scheduling strategy for radio resources management when transmitting Internet traffic over third-generation systems. More precisely, we consider the UMTS terrestrial radio access network (UTRAN) time division duplex (TDD) mode standardized by ETSI. UTRAN TDD uses a hybrid solution of code and time division multiple access, called TD-CDMA. We present a fair and efficient scheduling algorithm that adapts its behavior to traffic and interference conditions. Specifically, our scheduling algorithm is able to manage the radio resources taking into account both traffic fluctuations, in the uplink and downlink direction, and variations in system interference. The goal of our scheduler is data-throughput maximization for an efficient utilization of available radio resources. The performance of our scheduling algorithm is evaluated via simulation.

We address the throughput analysis of high-speed IEEE 802.11b WLANs from both an analytical and a simulative perspective. Specifically, we derive the throughput formula for the RTS/CTS access method of the p-persistent IEEE 802.11b MAC protocol. The accuracy of the proposed model is exhaustively validated via simulation results. By exploiting our formulas, we derive the theoretical upper bound for the throughput performance of the IEEE 802.11b protocol. Our analytical and simulative results indicated that the RTS/CTS mechanism produces very limited advantages in the standard IEEE 802.11 networks with respect to the basic access when no hidden stations are present. Finally, we extend a distributed backoff-tuning strategy firstly proposed for the basic access method, and we validate its effectiveness to approach closely the throughput limit of the IEEE 802.11b protocol.

Wireless technologies in the LAN environment are becoming increasingly important. The IEEE 802.11 is the most mature technology for wireless local area networks (WLANs). The limited bandwidth and the finite battery power of mobile computers represent one of the greatest limitations of current WLANs. In this paper, we deeply investigate the efficiency and the energy consumption of MAC protocols that can be described with a p-persistent CSMA model. As already shown in the literature, the IEEE 802.11 protocol performance can be studied using a p-persistent CSMA model (Cali et al. 2000). For this class of protocols, in the paper, we define an analytical framework to study the theoretical performance bounds from the throughput and the energy consumption standpoint. Specifically, we derive the p values (i.e., the average size of the contention window in the IEEE 802.11 protocol (Cali et al.)) that maximizes the throughput, p/sub opt//sup C/, and minimizes the energy consumption, p/sub opt//sup E/. By providing analytical closed formulas for the optimal p values, we discuss the trade-off between efficiency and energy consumption. Specifically, we show that power saving and throughput maximization can be jointly achieved. Our analytical formulas indicate that the optimal p values depend on the network configuration, i.e., number of active stations and length of the messages transmitted on the channel. As network configurations dynamically change, the optimal p values must be dynamically updated. In this paper, we propose and evaluate a simple but effective feedback-based distributed algorithm for tuning the p parameter to the optimal values, i.e., p/sub opt//sup E/ and p/sub opt//sup C/. The performance of the p-persistent IEEE 802.11 protocol, enhanced with our algorithm, is extensively investigated by simulation. Our results indicate that the enhanced p-persistent IEEE 802.11 protocol is very close to the theoretical bounds both in steady-state and in transient conditions.

Mobile IPv4 is the standard protocol for managing the user mobility in the Internet. This protocol provides transparent support of mobile hosts, allows system scalability and doesn’t require changes in the fixed portion of the network. Unfortunately, it introduces the so-called triangle routing which is not an optimal one. Simulative study have shown that triangle routing causes a throughput degradation for the applications that use TCP protocol. For this reason we propose a new architecture named DNS-based as it exploits the Domain Name System (DNS) extended with functionalities for updating the DNS on-line. The DNS-based architecture removes the inefficiencies of the triangle routing and keeps the nice properties of the Mobile IPv4 solution, i.e. transparency and scalability.

In WLANs, the medium access control (MAC) protocol is the main element that determines the efficiency of sharing the limited communication bandwidth of the wireless channel. The fraction of channel bandwidth used by successfully transmitted messages gives a good indication of the protocol efficiency, and its maximum value is referred to as protocol capacity. In a previous paper we have derived the theoretical limit of the IEEE 802.11 MAC protocol capacity. In addition, we showed that if a station has an exact knowledge of the network status, it is possible to tune its backoff algorithm to achieve a protocol capacity very close to its theoretical bound. Unfortunately, in a real case, a station does not have an exact knowledge of the network and load configurations (i.e., number of active stations and length of the message transmitted on the channel) but it can only estimate it. In this work we analytically study the performance of the IEEE 802.11 protocol with a dynamically tuned backoff based on the estimation of the network status. Results obtained indicate that under stationary traffic and network configurations (i.e., constant average message length and fixed number of active stations), the capacity of the enhanced protocol approaches the theoretical limits in all the configurations analyzed. In addition, by exploiting the analytical model, we investigate the protocol performance in transient conditions (i.e., when the number of active stations sharply changes).

In wireless LANs (WLANs), the medium access control (MAC) protocol is the main element that determines the efficiency in sharing the limited communication bandwidth of the wireless channel. In this paper we focus on the efficiency of the IEEE 802.11 standard for WLANs. Specifically, we analytically derive the average size of the contention window that maximizes the throughput, hereafter theoretical throughput limit, and we show that: 1) depending on the network configuration, the standard can operate very far from the theoretical throughput limit; and 2) an appropriate tuning of the backoff algorithm can drive the IEEE 802.11 protocol close to the theoretical throughput limit. Hence we propose a distributed algorithm that enables each station to tune its backoff algorithm at run-time. The performances of the IEEE 802.11 protocol, enhanced with our algorithm, are extensively investigated by simulation. Specifically, we investigate the sensitiveness of our algorithm to some network configuration parameters (number of active stations, presence of hidden terminals). Our results indicate that the capacity of the enhanced protocol is very close to the theoretical upper bound in all the configurations analyzed.

In WLAN the medium access control (MAC) protocol is the main element for determining the efficiency in sharing the limited communication bandwidth of the wireless channel. This paper focuses on the efficiency of the IEEE 802.11 standard for wireless LANs. Specifically, we derive an analytical formula for the protocol capacity. From this analysis we found (i) the theoretical upper bound of the IEEE 802.11 protocol capacity; (ii) that the standard can operate very far from the theoretical limits depending on the network configuration; (iii) that an appropriate tuning of the backoff algorithm can drive the IEEE 802.11 protocol close to its theoretical limits. Hence we propose a distributed algorithm which enables each station to tune its backoff algorithm at run-time. The performances of the IEEE 802.11 protocol, enhanced with our algorithm, are investigated via simulation. The results indicate that the enhanced protocol is very close to the maximum theoretical efficiency.

Variable bit-rate (VBR) video is expected to become increasingly important with the large scale deployment of broadband-integrated services networks (B-ISDNs). While the modeling of VBR video sources has received significant attention, there is no widely accepted model which lends itself to mathematical analysis. Furthermore, new video compression standards are emerging, such as the MPEG family. A detailed statistical analysis of a long sample of a movie encoded with an MPEG-1 algorithm highlights that traffic generated by an MPEG-1 encoder has both a high degree of burstiness (peak/average ratio greater than five) and a strong long-term correlation (positive correlation lasts for ten/twenty minutes). In this paper, we propose and analyze a Markov model which captures both the burstiness and the temporal dependencies of MPEG traffic. Furthermore, we show that our model is a flexible traffic generator which is suitable for studying statistical multiplexing issues. Specifically, we study both the multiplexing of a VBR video source with low priority traffic and the multiplexing of several VBR video sources. The results obtained show that the long-term correlation significantly affects the tail of the delay distribution even for lightly loaded networks. The presence of a long range dependence in the MPEG traffic significantly reduces the gain in terms of network utilization which can be achieved with statistical multiplexing of i.i.d. sources.

A simulation-based analysis of the integration of VBR video and voice traffic in a cyclic-reservation multiple-access (CRMA) network is reported. The video sources are characterized via a trace of the output of a real codec. As VBR video applications can tolerate extremely low packet loss rates (>

The authors present a novel approach to distributed queue dual bus (DQDB) modeling. The innovative aspect lies in the representation of the slot-occupancy-pattern process on the forward bus. In the literature, this process has been modeled as a Bernoulli process, implying that the status (empty/busy) of consecutive slots is independent. Results presented, however, show that even at light loads the interdependence between slots is significant. The model captures some of this interdependence by using a discrete-time first-order Markov process for the slot-occupancy-pattern process on the forward bus. A simplified DQDB network is identified for which a closed formula is derived for the joint probability density function for the slot occupancy pattern of two consecutive slots. The accuracy of the results was investigated with simulation.>

A distributed queue dual bus media access control (DQDB MAC) protocol state machine is described. The DQDB mathematical models and the tools used for their solution are introduced. The workload characterization is shown, and fairness metrics and performance indexes are defined. An analysis in underload and overload conditions is described, and the influence of the DQDB internal mechanisms is discussed. The asymptotic analysis is reported. DQDB behavior at higher speeds is analyzed.>

The authors describe and analyze a novel MAC (media access control) protocol called distributed-control polling (DCP), which has been designed to bring together the most interesting features of distributed-control MAC protocols (e.g. distributed queue dual bus or DQDB) and centralized token-passing MAC protocols (e.g. FASNET, FDDI or fiber distributed data interface, and Express-ent). From the fully distributed MAC protocols. DCP acquires the ability to guarantee both a complete utilization of the medium capacity and an access delay of only a few slots at light loads. From the centralized token-passing MAC protocols, DCP inherits a more predictable and fair behavior at heavy loads. The basic ideas of the proposal are: a cycle for acquiring transmission rights, and a balancing function between reservations and empty slots. It is shown that DCP guarantees a complete utilization of the medium capacity and that its behavior at light loads is close to that of DQDB, while at heavy loads it approaches a polling system with limited service and zero reply interval.>

An extended simulation analysis of the asynchronous part of the distributed queue dual bus (DQDB) MAC protocol is reported. DQDB is analyzed in asymptotic conditions, i.e. bandwidth sharing among network nodes when each node is trying to seize all of the medium capacity. DQDB fairness and performance is investigated under a more realistic scenario. The results show that DQDB without the bandwidth balancing mechanism (BBM) is unfair and that the unfairness is dependent on the medium capacity and the bus length. When the BBM is enabled, DQDB fairness and performance, in the steady state, improve significantly. However, DQDB takes some time to achieve the steady state, during which time the node quality of service is degraded.>

A novel media access control (MAC) protocol named distributed-control polling (DCP) is described and analyzed. It has been designed to bring together the most interesting features of distributed-control MAC protocols and centralized token-passing MAC protocols. From the fully distributed MAC protocols, DCP acquires the capability to guarantee both a complete utilization of the medium capacity and an access delay of only a few slots at light loads. From the centralized token-passing MAC protocols, DCP inherits a more predictable and fair behavior at heavy loads. The basic ideas of the proposal are: a cycle for acquiring transmission rights and a balancing function between reservations and empty slots. The analysis shows that DCP inherits a more predictable and fair behavior at heavy loads. The analysis shows that, at heavy loads, DCP approaches a polling system with G-limited service and zero reply interval. At light loads DCP behavior is close to that of the distributed queue dual bus protocol.>