We propose an optimization model for single-cell LoRaWAN planning which computes the limit range of each spreading factor (SF) in order to maximize the minimum packet delivery ratio (PDR) of every node in the network. It allows to balance the opposite effects of attenuation and collision of the transmissions and guarantee fairness among the nodes. We show that our optimization framework improves the worst PDR of the nodes by more than 13 percentage points compared to usual SF boundaries based on SNR threshold. A study of the tradeoff between precision and resolution time of the model shows its effectiveness even with a small number of possible distance limits, and its scalability when the node density increases.

In this paper, we consider the problem of evaluating the capacity of a LoRaWAN cell. Previous analytical studies investigated LoRaWAN performance in terms of the Packet Delivery Ratio (PDR) given a number of devices around a gateway and its range. We improve the model for PDR by taking into consideration that the following two events are dependent: successful capture during a collision and successful frame decoding despite ambient noise. We consider a realistic traffic model in which all devices generate packets with the same inter-transmission times corresponding to the duty cycle limitation at the highest SF, regardless of the distance to the gateway. Based on the developed model, we optimize the Spreading Factor (SF) boundaries to even out PDR throughout the cell. We validate the analytical results with simulations, compare our model with previous work, and experimentally validate the hypothesis of Rayleigh fading for the LoRa channel. The important conclusion from our results is that a LoRa cell can handle a relatively large number of devices. We also show that there is practically no inter-SF interference (cross interference between transmissions with different SFs): interference from higher SFs comes from nodes located farther away, so they face greater attenuation and thus, they do not interfere with lower SF nodes.

We propose a theoretical framework for maximizing the LoRaWAN capacity in terms of the number of end nodes, when they all have the same traffic generation process. The model optimally allocates the spreading factor to the nodes so that attenuation and collisions are optimized. We use an accurate propagation model considering Rayleigh channel, and we take into account physical capture and imperfect SF orthogonality while guaranteeing a given transmission success probability to each served node in the network. Numerical results show the effectiveness of our SF allocation policy. Our framework also quantifies the maximum capacity of single cell networks and the gain induced by multiplying the gateways on the covered area. We finally evaluate the impact of physical capture and imperfect SF orthogonality on the SF allocation and network performances.

This paper investigates thermal covert channels in the context of the Internet-of-Things. More, precisely we demonstrate such a channel on BLE-enabled devices. We explain how the core design principles of BLE link-layer protocol, combined with well-timed intensive CPU calculations can be leveraged to mount a thermal covert channel between two devices.

Bluetooth Low Energy (BLE) has been designed as a power efficient protocol for small portable and autonomous devices, showing its efficiency for connecting these devices with smartphones to periodically and frequently exchange data, like heart rate or notifications. Additionally, BLE is present in almost every smartphone, turning it into perfect ubiquitous remote control for smart homes, buildings or cities. Nevertheless, there is still room to improve BLE performance for typical IoT use cases where battery lifetime should reach several years. In this paper we propose an extension to a model for evaluating BLE performance, latency and energy consumption, in order to provide realistic results based on various scenario conditions. In addition, we propose a parameter optimization of the BLE Neighbor Discovery process, in order to obtain the best performance possible depending on the constraints of specific use cases. Our results on two typical IoT test-cases show that advertiser battery lifetime can be increased up to ~89x (9.55 days to 2.32 years) for a first case, and ~281x (9.55 months to 7.36 years) for a second case.

Bluetooth Low Energy (BLE) has shown higher energy efficiency and robustness than other well known Wireless Sensor Network (WSN) protocols, making it a strong candidate for implementation in IoT scenarios. In addition, BLE is in almost every smartphone, turning it into perfect ubiquitous remote controls for smart homes, buildings or cities. In this paper we propose new operating modes for BLE to provide a much wider applicability range covering asynchronous, aperiodic and very low frequency scenarios while extending lifetime, without modification of the existing BLE specification. In order to estimate battery lifetime, we propose a coin cell battery model. We show how to efficiently adapt the protocol for different IoT scenarios based on their requirements, thus extending battery lifetime as much as possible while guaranteeing acceptable latency. Our results on two typical test-cases show that by using an optimized configuration and appropriate operating mode based on scenario requirements, lifetime can be increased up to ≈2× (4.1 to 7.3 years) for a BLE master in a first case, and ≈ 58× (3.05 months to 14.82 years) for a BLE slave in a second case. This shows BLE compatibility with a vast amount of IoT-related use cases.

In this paper, we describe the design and implementation of WiSH-WalT, a framework for controllable and reproducible LoRa testbeds. The implementation of the WiSHFUL unified interface provides the means for controlling and adapting the LoRa parameters to given conditions and low energy consumption by configuring transmission power, spreading factor, bandwidth, and error coding rate. WalT provides support for running reproducible experiments based on the deployment of docker images on a distributed set of nodes. Put together, the functionalities of WiSHFUL and WalT open new possibilities for deploying reproducible LoRa testbeds. We have used WiSH-WalT to experiment with adaptive configuration of the LoRa parameters and evaluated the communication performance of LoRa motes in a setup with several gateways.

In this paper, we propose IoTChain, a combination of the OSCAR architecture [1] and the ACE authorization framework [2] to provide an E2E solution for the secure authorized access to IoT resources. IoTChain consists of two components, an authorization blockchain based on the ACE framework and the OSCAR object security model, extended with a group key scheme. The blockchain provides a flexible and trustless way to handle authorization while OSCAR uses the public ledger to set up multicast groups for authorized clients. To evaluate the feasibility of our architecture, we have implemented the authorization blockchain on top of a private Ethereum network. We report on several experiments that assess the performance of different architecture components.

In this paper, we propose a fast predictive resynchronization scheme that allows nodes to rejoin a 802.15.4e TSCH network with which they were previously synchronized. The idea of the scheme comes from the investigation of the internal and external root causes of the clock drift between two nodes--we have identified the causes and proposed a means for its estimation. Based on the prediction of the drift between any pair of nodes, we are able to determine the instant at which a node needs to wake up after desynchronization, thus reducing its energy consumption and the rejoin latency. We compare the proposed scheme with other state-of-the-art proposals through experiments on different hardware platforms. The experimental results show that desynchronized nodes rejoin up to 34 times faster and consume less energy by a factor of 1000 compared to the other proposals.

The IEEE802.15.4e standard for low power wireless sensor networks defines a new mode called Time Slotted Channel Hopping (TSCH) as Medium Access Control (MAC). TSCH allows highly efficient deterministic time-frequency schedules that are built and maintained by the 6TiSCH operation sublayer (6top). In this paper, we propose a solution to limit the allocation of identical cells to co-located pair of nodes by distributed TSCH scheduling algorithms. It consists of making nodes able to overhear past cell negotiations exchanged in shared cells by their neighbors and prevent the nodes from reusing already assigned cells in future allocations. Our mechanism has been tested through simulations that show a significant improvement with respect to random scheduling algorithms.

The Internet of Things (IoT) requires a compact naming scheme, which can also bring significant advantages to service registration and discovery. We propose a novel approach, denoted as distributed naming service, which provides a new naming scheme as well as an efficient service discovery protocol for wireless sensor networks. It is based on three pillars: 1) Bloom filters, to create compact names from node descriptions; 2) message propagation strategies, to publish and discover information-not only names-within the network; and 3) distributed caches, to store names within the network. In this paper, we assume ContikiMAC at layer 2, IPv6 and Routing Protocol for Low-Power and Lossy Networks (RPL) at layer 3, and we present two particular UDP-based message propagation strategies that take advantage of the RPL protocol at layer 3. We evaluate the performance of the proposed solutions through Contiki/Cooja simulations and on a real testbed, using the open and large scale FIT IoT-LAB.

MAC address randomization is a common privacy protection measure deployed in major operating systems today. It is used to prevent user-tracking with probe requests that are transmitted during IEEE 802.11 network scans. We present an attack to defeat MAC address randomization through observation of the timings of the network scans with an off-the-shelf Wi-Fi interface. This attack relies on a signature based on inter-frame arrival times of probe requests, which is used to group together frames coming from the same device although they use distinct MAC addresses. We propose several distance metrics based on timing and use them together with an incremental learning algorithm in order to group frames. We show that these signatures are consistent over time and can be used as a pseudo-identifier to track devices. Our framework is able to correctly group frames using different MAC addresses but belonging to the same device in up to 75% of the cases. These results show that the timing of 802.11 probe frames can be abused to track individual devices and that address randomization alone is not always enough to protect users against tracking.

In this paper, we apply the Gilbert-Elliot model to analyze measurements on a large-scale wireless sensor testbed with nodes equipped with CC1101 and CC2420 radio chips. The model considers a packet loss process as a sequence of transitions between Good and Bad states. We analyze the Packet Reception Ratio (PRR) based on the probabilities derived from the Gilbert-Elliot model. We show that the probabilities and other parameters (run and loss lengths) can very well discriminate between the main categories of link quality such as good and weak links. Identification of link quality is crucial for routing protocols such as RPL and LOADng.

The Datagram Transport Layer Security (DTLS) protocol is the IETF standard for securing the Internet of Things. The Constrained Application Protocol, ZigBee IP, and Lightweight Machine-to-Machine (LWM2M) mandate its use for securing application traffic. There has been much debate in both the standardization and research communities on the applicability of DTLS to constrained environments. The main concerns are the communication overhead and latency of the DTLS handshake, and the memory footprint of a DTLS implementation. This paper provides a thorough performance evaluation of DTLS in different duty-cycled networks through real-world experimentation, emulation and analysis. In particular, we measure the duration of the DTLS handshake when using three duty cycling link-layer protocols: preamble-sampling, the IEEE 802.15.4 beacon-enabled mode and the IEEE 802.15.4e Time Slotted Channel Hopping mode. The reported results demonstrate surprisingly poor performance of DTLS in radio duty-cycled networks. Because a DTLS client and a server exchange more than 10 signaling packets, the DTLS handshake takes between a handful of seconds and several tens of seconds, with similar results for different duty cycling protocols. Moreover, because of their limited memory, typical constrained nodes can only maintain 3–5 simultaneous DTLS sessions, which highlights the need for using DTLS parsimoniously.

This paper presents GreenNet, an energy efficient and fully operational protocol stack for IP-enabled wireless sensor networks based on the IEEE 802.15.4 beacon-enabled mode. The stack runs on a hardware platform with photovoltaic cell energy harvesting developed by STMicroelectronics (STM) that can operate autonomously for long periods of time. GreenNet integrates several standard mechanisms and enhances existing protocols, which results in an operational platform with the performance beyond the current state of the art. In particular, it includes the IEEE 802.15.4 beacon-enabled medium access control (MAC) integrated with lightweight IP routing for achieving very low duty cycles. It offers an advanced discovery scheme that accelerates the process of joining the network and proposes an adaptation scheme for adjusting the duty cycle of harvested nodes to the available energy for increased performance. Finally, it supports security at two levels: a basic standard secure operation at the link layer and advanced scalable data payload security. This paper describes all techniques and mechanisms for saving energy and operating at very low duty cycles. It also provides an evaluation of the performance and energy consumption of GreenNet.

In this paper, we propose a new scheme that allows coupling beacon-enabled IEEE 802.15.4 with the RPL routing protocol while keeping full compliance with both standards. We provide a means for RPL to pass the routing information to Layer 2 before the 802.15.4 topology is created by encapsulating RPL DIO messages in beacon frames. The scheme takes advantage of 802.15.4 command frames to solicit RPL DIO messages. The effect of the command frames is to reset the Trickle timer that governs sending DIO messages. We provide a detailed analysis of the overhead incurred by the proposed scheme to understand topology construction costs. We have evaluated the scheme using Contiki and the instruction-level Cooja simulator and compared our results against the most common scheme used for dissemination of the upper-layer information in beacon-enabled PANs. The results show energy savings during the topology construction phase and in the steady state.

The Internet of Things requires a naming service that can also be beneficial for searching for services and applications. To avoid the traditional DNS approach in which a DNS server provides mapping of names to IP addresses in its domain, we propose a novel network service called DINAS (DIstributed NAming Service). It is based on three pillars: 1) Bloom filters for creating compact names from node descriptions, 2) distributed caches for storing names within the network, and 3) overlay routing strategies to publish and discover information - not only names - within the network. In this work, we assume ContikiMAC at Layer 2, 6LoWPAN at Layer 2.5, IPv6 and RPL (routing protocol) at Layer 3, and we present a particular UDP-based overlay routing strategy. We evaluate the proposal by Cooja simulations and compare its performance with a centralized naming service at the sink. The results show that DINAS outperforms the centralized solution.

Billions of smart, but constrained objects wirelessly connected to the global network require novel paradigms in network design. New protocol standards, tailored to constrained devices, have been designed taking into account requirements such as asynchronous application traffic, need for caching, and group communication. The existing connection-oriented security architecture is not able to keep up-first, in terms of the supported features, but also in terms of the scale and resulting latency on small constrained devices. In this paper, we propose an architecture that leverages the security concepts both from content-centric and traditional connection-oriented approaches. We rely on secure channels established by means of (D)TLS for key exchange, but we get rid of the notion of the “state” among communicating entities. We provide a mechanism to protect from replay attacks by coupling our scheme with the CoAP application protocol. Our object-based security architecture (OSCAR) intrinsically supports caching and multicast, and does not affect the radio duty-cycling operation of constrained objects. We evaluate OSCAR in two cases: 802.15.4 Low Power and Lossy Networks (LLN) and Machine-to-Machine (M2M) communication for two different hardware platforms and MAC layers on a real testbed and using the Cooja emulator. We show significant energy savings at constrained servers and reasonable delays. We also discuss the applicability of OSCAR to Smart City deployments.

The main idea of this paper is to rely only on analog channel sensing to provide integrated neighborhood maintenance and medium access control in a single mechanism for energy constrained low-power wireless sensor nodes. We propose Wake on Idle, a solution that can be implemented in the radio chip and provides these services without relying on the processor when the stable state is reached. No digital decoding is needed as we use pseudo random sequences to identify analog beacon positions in time and assess neighbors presence. Additionally, we use a code violation principle to provide medium access, ensuring that when a node wants to transmit data to a neighbor, the latter will be listening. We analyze this proposition, give some simulation results, and also provide extensive experimentation results based on an implementation running on two different hardware platforms and showing the concept to be valid, although some adaptation was needed to run on off-the-shelf radios. Even with this far from optimal initial implementation we can reach below 0.1% duty cycles for signaling periods on the order of minutes.

We report results of measurements on SensLAB, an indoor wireless sensor network testbed with a large number of nodes equipped with a CC1101 radio chip. We analyze RSSI and LQI to find the best way to discriminate good links from weak ones and in particular, to derive a metric that estimates the Packet Reception Ratio (PRR) the best. To obtain an estimator of PRR, we have fitted a Fermi-Dirac function to the scatter diagram of the average and standard variation of LQI. The function enables us to find PRR for a given level of LQI. We evaluate the estimator by computing PRR over a varying size window of transmissions and comparing with the estimator.

We propose MCCT (Multi-Channel Cluster Tree), a cluster-tree construction protocol for nodes in IEEE 802.15.4 beacon-enabled mode. By multiplexing transmissions across orthogonal channels, we reduce collisions between control and data frames, which leads to better packet delivery rate and fairness. We propose a method for constructing a cluster-tree suitable for minimizing beacon collisions. The protocol builds on a neighbor discovery procedure that uses a dedicated control channel while still sticking to the superframe structure of IEEE 802.15.4. We also specify a channel assignment and superframe scheduling method that takes into account channel diversity. We evaluate the proposed protocol through simulation and compare with other proposals: standard 802.15.4 and a representative of distributed solutions to the superframe scheduling problem — MeshMAC. The simulation results show that MCCT significantly improves packet delivery ratio, delay, and fairness. It also results in very good packet delivery ratio for increased network density.

Within IEEE 802.11 Wireless Local Area Networks (WLANs), client stations can move freely, but because of the short range of their Access Points (APs), they usually need to reassociate with different APs to continue to communicate. When changing APs, a client station starts a process known as a handoff that can take up to 2 seconds, which is too long for real-time applications such as Voice over IP (VoIP). Various solutions have been proposed to change or improve the client behaviour when doing a handoff. Previously, we proposed the idea of Virtual Access Points (VAP) implemented on APs in which a client station changes APs without disrupting its current communication. Based on this new concept, we have developed a solution called Multichannel Virtual Access Points (mVAP) to take advantage of APs operating on multiple channels. We have implemented mVAP using PACMAP, a tool for packet manipulation, and evaluated its performance. Our results show that mVAP is a new efficient technique for seamless handoffs without performance degradation.

Mobility management in WiFi networks is still an open issue today: there is no standard method defined, and client station mobility is handled either via proprietary protocols, or simply by re- association. However, managing mobility in an infrastructure network is utterly important for several reasons: controlling delay and jitter in communications, managing clients from the network, optimizing performance. We propose the concept of virtual access points to manage mobile station in infrastructure networks. In this scheme, stations are not aware that they move, and all the complexity is pushed back inside the network. It is then possible to control mobility from a global point of vue, to optimize network resources for mobile stations, hence providing a better quality of service. Finally, this scheme is compatible with existing clients without any hardware nor software modification.

Full-fledged software radio platforms are complex and expensive systems, focused on signal processing, and not very suitable for easy development and large scale experimentation. We propose a Multi-Processor System-on-Chip (MPSoC) prototyping platform targeting the support for flexible radio. This platform is fully customizable at every layer of the wireless networking stack, making it easy to prototype new protocols from the radio to the application layers. Our goal was threefold: design an efficient but cheap platform supporting flexible radio, provide support for a full system on the platform so that it can run autonomously, use "standard" components as much as possible and a modular design to ensure fast and simple development and testing to network developers. We rely on a highly modular Field-Programmable Gate Array (FPGA) based architecture. The practical results achieved so far show the effectiveness of the proposed solution in term of flexibility and cost.

We propose Binary Waypoint Routing, a novel geographical routing protocol for wireless mesh networks. Its idea is to learn and maintain source routes to a small number of nodes called binary waypoints that are placed in subspaces constructed as a result of binary space partitioning. A source node sends a packet to a waypoint for a given destination and intermediate nodes try to adapt the packet route by aiming at waypoints that are closer to the destination. Our simulation results show that the proposed scheme achieves high packet delivery rate with a traffic pattern similar to the Optimal Shortest Path Routing.

We propose a packet scheduling discipline called virtual flow queueing (VFQ) to improve the end-to-end performance of TCP connections over IEEE 802.11 WLANs. VFQ is based on an inter-layer approach: the IP layer schedules packets over the wireless link according to the information coming from both the transport and MAC layers. The scheduler computes virtual transmission times that take into account the channel time at the MAC layer needed to transfer both TCP and ACK segments. The scheduling discipline achieves fair allocation of wireless channel capacity to TCP connections in both directions (upload and download). VFQ also supports different weights like in the classical WFQ scheduling so that the access point can for instance give preference to download connections. In this paper, we describe the VFQ implementation in the Linux kernel and present the results of measurements that show very good performance behavior of VFQ compared to the FIFO scheduling discipline currently deployed in 802.11 access points.

In atypical deployment of IEEE 802.11 wireless LANs in the infrastructure mode, an access point acts as abridge between the wireless and the wired parts of the network. Under the current IEEE 802.11 Distributed Coordination Function (DCF) access method, which provides equal channel access probability to all devices in a cell, the access point cannot relay all the frames that it receives on the downlink. This causes significant unfairness between upload and download connections, long delays, and frame losses. This unfairness problem comes from the not-so-complex interaction of transport-layer protocols with the MAC-layer access method. The main problem is that the access point requires more transmission attempt probability than wireless stations for correct operation at the transport layer. In this paper, we propose to solve the unfairness problem in a simple elegant way at the MAC layer. We define the operation of an Asymmetric Access Point that benefits from a sufficient transmission capacity with respect to wireless stations so that the overall performance improves. The proposed method of operation is intrinsically adaptive so that when the access point does not need the increased capacity, it is used by wireless stations. We validate the proposed access method by simulation to compare it with other solutions based on IEEE 802.11e. Unlike many papers in this domain, which only validate MAC-layer modifications through simulation or analytical modeling, we provide measurement data gathered on an experimental prototype that uses wireless cards implementing the proposed method.

very selective A+

In this paper, we present an evaluation of chosen wireless LAN access methods involving stations with different bit error rates: n-1 stations in ideal transmission conditions (BER = 0) and 1 station with a given bit error rate (BER ne 0). The simulation results show that the IEEE 802.11 DCF and its modifications (Slow Decrease, AOB) are very sensitive to transmission errors, whereas Idle Sense provides good channel access fairness: the value of the contention window is almost the same regardless of transmission errors, so that the throughput difference between stations subject to different bit error rates corresponds only to the proportion of lost frames.

In this paper, we introduce and evaluate novel adaptive schemes for neighbor discovery in Bluetooth-enabled ad-hoc networks. In an ad-hoc peer-to-peer setting, neighbor search is a continuous, hence battery draining process. In order to save energy when the device is unlikely to encounter a neighbor, we adaptively choose parameter settings depending on a mobility context to decrease the expected power consumption of Bluetooth-enabled devices. For this purpose, we first determine the mean discovery time and power consumption values for In different Bluetooth parameter settings through a comprehensive exploration of the parameter space by means of simulation validated by experiments on real devices. The fastest average discovery time obtained is 0.2 s, while at an average discovery time of I s the power consumption is just 1.5 times that of the idle mode on our devices. We then introduce two adaptive algorithms for dynamically adjusting the Bluetooth parameters based on past perceived activity in the ad-hoc network. Both adaptive schemes for selecting the discovery mode are based only on locally-available information. We evaluate these algorithms in a node mobility simulation. Our adaptive algorithms reduce energy consumption by 50% and have up to 8% better performance over a static power-con serving scheme

In this paper, we address the issue of evaluating performance of wireless LANs in multicell scenarios. We try to understand the complex behavior of the DCF (distributed coordination function) access method defined in the IEEE 802.11 standard [1] and its modifications proposed for improving performance: slow decrease [2], asymptotically optimal backoff [3], and idle sense [4]. We analyze the influence of overlapping cells and large multicell environments on their performance. Our results show that the IEEE 802.11 DCF and its two modifications (slow decrease and AOB) exhibit important unfairness between stations close to the access point and those near the border of a neighbor cell. Idle Sense performs much better: it provides much better fairness than the IEEE 802.11 DCF and its modifications. It also obtains the highest throughput when stations adapt their bit rate to channel conditions.

We consider wireless LANs such as IEEE 802.11 operating in the unlicensed radio spectrum. While their nominal bit rates have increased considerably, the MAC layer remains practically unchanged despite much research effort spent on improving its performance. We observe that most proposals for tuning the access method focus on a single aspect and disregard others. Our objective is to define an access method optimized for throughput and fairness, able to dynamically adapt to physical channel conditions, to operate near optimum for a wide range of error rates, and to provide equal time shares when hosts use different bit rates.We propose a novel access method derived from 802.11 DCF [2] (Distributed Coordination Function) in which all hosts use similar values of the contention window CW to benefit from good short-term access fairness. We call our method Idle Sense, because each host observes the mean number of idle slots between transmission attempts to dynamically control its contention window. Unlike other proposals, Idle Sense enables each host to estimate its frame error rate, which can be used for switching to the right bit rate. We present simulations showing how the method leads to high throughput, low collision overhead, and low delay. The method also features fast reactivity and time-fair channel allocation.

We analyze in this paper the fairness of the 802.11 DCF (distributed coordination function) access method. Short-term fairness is an important property of a MAC layer for obtaining short delays. We show that contrary to the common wisdom, a 802.11 cell with two hosts does not exhibit short-term unfairness. We propose a new fairness index: the number of inter-transmissions that other hosts may perform between two transmissions of a given host. By means of this index, we analyze the fairness of 802.11 for the case of two hosts and derive the probability distribution of the number of inter-transmissions. Our results show that even on the short term time scale the 802.11 DCF access method provides fairness to competing hosts. When compared with slotted ALOHA, a multiple access randomized protocol with good fairness properties, 802.11 presents even better fairness. To validate the model, we compare the analytical results with experimental histograms obtained via simulations and measurements.

We consider the problem of interconnecting hosts in spontaneous edge networks composed of various types of wired or wireless physical and link layer technologies. We argue that this kind of networks requires a more sophisticated approach than standard IP forwarding: communication paths should be managed on a per flow basis, multiple paths need to be maintained to cope with link failures or changing topologies, and the interconnection architecture should provide a means for acquiring the information on destination reachability. To experiment with our approach, we have designed and implemented Lilith, a prototype of an interconnection node for spontaneous edge networks. We handle network dynamics by establishing MPLS (multi protocol label switching) label switched paths (LSP) on demand with a reactive ad hoc routing protocol. We present some measurements that show good performance with respect to the standard IP forwarding and important performance gains when multiple paths are used.

We propose to provide QoS differentiation at the IP level based on the DiffServ model over the standard distributed coordination function (DCF) method of 802.11. By constraining the output rate of each host in a cell via DiffServ scheduling and traffic shaping mechanisms, we can keep the 802.11 network in the state of nonsaturation so that the time-sensitive expedited forwarding (EF) class benefits from stable short delays. We analyze the performance of 802.11b to find how we should constrain the rate of DiffServ sources and present extensive measurements that show how the QoS differentiation is achieved.

The performance of the IEEE 802.11b wireless local area networks is analyzed. We have observed that when some mobile hosts use a lower bit rate than the others, the performance of all hosts is considerably degraded. Such a situation is a common case in wireless local area networks in which a host far away from an access point is subject to important signal fading and interference. To cope with this problem, the host changes its modulation type, which degrades its bit rate to some lower value. Typically, 802.11b products degrade the bit rate from 11 Mb/s to 5.5, 2, or 1 Mb/s when repeated unsuccessful frame transmissions are detected. In such a case, a host transmitting for example at 1 Mb/s reduces the throughput of all other hosts transmitting at 11 Mb/s to a low value below 1 Mb/s. The basic CSMA/CA channel access method is at the root of this anomaly: it guarantees an equal long term channel access probability to all hosts. When one host captures the channel for a long time because its bit rate is low, it penalizes other hosts that use the higher rate. We analyze the anomaly theoretically by deriving simple expressions for the useful throughput, validate them by means of simulation, and compare with several performance measurements.

International audience

We present an architecture for managing multiple media streams included in temporal HTML documents. The architecture provides generic services for negotiating media streams on servers, controlling their delivery, adapting their characteristics to varying network and host conditions, and synchronizing multiple remote streams. Integration of the streaming support within a synchronization architecture leads to the concept of QoP (quality of presentation) that characterizes global quality of document playback from remote servers.