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Section: New Results

New services and protocols

Participants : Mohammad Abdul Awal, Cédric Adjih, Khaldoun Al Agha, Tara Ali-Yahiya, Ichrak Amdouni, Emmanuel Baccelli, Youghourta Benfattoum, Lila Boukhatem, Lin Chen, Thomas Clausen, Walter Grote, Lana Iwaza, Philippe Jacquet, Nour Kadi, Foued Kribi, Saoucène Mahfoudh, Steven Martin, Sara Medlej, Amina Meraihi, Pascale Minet, Paul Mühlethaler, Simon Odou, Joseph Rahmé, Georges Rodolakis, Despina Triantafyllidou, Yasser Toor.

Executive summary

The user of a mobile network very quickly experience problems with quality of service: links fade, connectivity disrupts, delays accumulate.

In a wireless network, the set of neighbors which with one node can communicate depends on transmission range, and numerous factors, and in addition the transmission range is often lower than the interference range (the range within which a node prevents correct transmissions of other nodes). Thus bandwidth reservation, a crucial step of quality of service, is an important and difficult problem.

The services and protocols that need careful adaptation are

The connectivity continuity is the most important problem. Trivial in the wired world where a link failure is a rare event, it becomes problematic in the mobile world where link failure caused by mobility are frequent and normal. The first experiments of mobile ad hoc networks with regular internet protocols miserably failed simply because either the protocol was to slow to recover link failure, or when tuned appropriately was generating such a huge overhead that the network collapsed under its own weight. A new generation of routing protocols has arised that allow a suitable control of connectivity in mobile networks. Among them the Optimized Link State Routing combines the optmization of overhead for mobile networks and the full internet legacy. It naturally provides path redundancy which accelerate link failure recovery.

The most important lesson that must be retained is that most of these optimization become NP complete, which is a significant complication compared to their counterpart in the classical wired world. The reason for the NP-completeness is two-sided: on one side the co-interferences make impossible an optimization link by link, on the other side, the large dispersion of performance measurement makes simple heuristic ineffective. As an example, routing with respect to shortest delay average does not guarantee smallest probability of high delay.

Since the bandwidth is scarce, any multimedia application such as video streaming is resource demanding. For example a TV broadcast that uses a mesh network will rapidly exhaust the bandwidth if all connections are point to point. In this case multicast protocols that allows to gather all these point to point connections in a single flow is a need.

There are two classes of multicast protocols: the tree based protocols and the network coding protocols. In the first class the protocols take advantage of the relatively small size of the recipient node set. One can show equivalent results of Gupta and Kumar scaling properties but in the multicast plan when the ratio of recipient versus network size is a fundamental parameter. When this ratio tends to one the performance naturally worsen.

When the recipient set is the whole network, one can apply the network coding scheme with random packet combination. In network coding the packets are no longer isolated: relay nodes makes linear combination of packets and transmitted mixed packets. In theory the performance of network coding is better than isolated packet multicast. In practice network coding is simpler to operate does not need topology management such as spanning trees or Connectedc Dominating Set. The reason for this is highly non intuitive, as if packet superposition was acting like state superposition in quantum mechanic, leading to non expected results.

Quality of service has become the central requirement that users expect from a network. High throughput, service continuity are critical issue for multimedia application over the wireless internet where the bandwidth is more scarce than in the wired world. A significant issue in the ad-hoc domain is that of the integrity of the network itself. Routing protocols allow, according to their specifications, any node to participate in the network - the assumption being that all nodes are behaving well and welcome. If that assumption fails - then the network may be subject to malicious nodes, and the integrity of the network fails. An important security service over mobile networks is to ensure that the integrity of the network is preserved even when attacks are launched against the integrity of the network.

Scientific achievements

Optimized Link State Routing (OLSR)

The routing protocol OLSR is universally known in the mobile wireless community (more than 475,000 hits on Google). It has numerous implementations and is used in many wireless networks. It is a proactive protocol with full internet legacy which is based on partial topology information exchange, that non the less provide optimal path with additive metrics (such as BGP/OSPF). It is an experimental RFC within IETF and soon will become a full standard under the name OLSRv2.

Bandwidth reservation in mobile ad hoc networks

We have shown that the search of a good path for a new connection that does not destroy the quality of service of existing connections is an NP-hard problem. The result is independent on how the bandwidth nodes interfer as long they interfer at least on one hop. In this area, one contribution was the definition and testing of an efficient reservation algorithm bandwidth reservation, respecting wireless network constraints. A second contribution is more accurate computation of remaining link bandwidth by considering bandwidth on other links multiplied by the average packet retransmission on this link (inverse of packet successful transmission rate).

We have also proposed a solution called QoS-OLSR that enhances OLSR with Quality of Service support. This solution, taking radio interferences into account, ensures that QoS flows, if accepted by the admission control, will receive a bandwidth close to this requested. This solution has been implemented on the MANET/OLSR demonstrator of CELAR (MoD).

Multi-metric routing

Quality of service involves finding routes between two nodes in the network that satisfies a number of constraints. These constraints could be the requested bandwidth, the maximum delay, the minimum loss probability, the reliability of links, etc. This problem is NP-Complete because it combines additive metrics in the optimization problem. Hipercom proposed heuristics for finding routes that respect up to four metrics when calculating routes between source and destination. Another QoS issue is the creation of models that estimate the actual value of a metric. For example, computing the available bandwidth or the transfer delay on a link, etc. is very complex in a non-deterministic medium access such as Wi-Fi. To resolve this problem, we developed a model for estimating the available bandwidth in a wireless network. This model is based on considering interfering links in cliques, after which we provide the maximum capacity that could be deployed in a clique. We may still enhance the model by adding a scaling factor to the clique equations in order to become more accurate when compared to real measurements.

In particular we have investigated the metric based on packet delay distribution. Since propagation delays between routers are negligible, most delays occur in queueing and medium access control processing. Contrary to previous common belief there is no need of network synchronization. The objective is to proactively determine the delay in absence of packet data traffic. The estimate of delay distribution is done via analytical method. In order to keep control on quality of service flows we use source routing forwarding options.

End-to-end Optimizations, TCP

Transport Control Protocol is an old protocol that ensures end-to-end forwarding between source and destination. With multi-hop networks, TCP suffers from multiple problems such as delays introduced by the routing on wireless simple nodes. From the scalability point of view, changing TCP implies the modification of a billion of TCP/IP stacks that is not possible today. We have been exploring how to find the correct routing protocols in order to optimize the timers' calculation in TCP and increase bandwidth and fairness.

Multicasting in mobile ad hoc networks

The goal of multicast protocols is to allow the network to deliver the multicast information to interested users. The multicast protocol builds and maintains a structure that will provide routes to all nodes in the multicast group; hence, they will receive the information multicast in their group. Multicast protocols can be classified according to the following criteria:

Performance evaluation of multicast protocols

The HIPERCOM team-project has designed three multicast protocols:

We have performed extensive simulations on the INRIA cluster with NS2 to quantitatively study the behavior of each protocol in different scenarios and configurations. The quality of the multicast is evaluated by the delivery ratio. The overhead induced by the multicast protocol is given by the number of retransmissions per multicast packet. We have increased:

With these results, we can deduce the applicability domain of each multicast protocol studied: SMOLSR, MOLSR and MOST.

Theoretical upper bound

We have derived a theoretical upper bound of the multicast capacity in wireless network. This result is an extension of Gupta and Kumar result about unicast capacity in wireless network. It is shown that the multicast delivery allows an increase of capacity of the order of the square root of the size of the multicast group compared to the attainable capacity if only parallel unicast connections were used. We have also shown that the protocol MOST actually attains this upper bound.

Redeploying mobile wireless sensor networks with virtual forces

Wireless Sensor Networks should be self-organized to enhance the coverage after an initial random deployment. We consider a network of mobile sensors deployed randomly in a known surface S, for example sensors dropped from a helicopter or an airplane. The aim is to redeploy these sensors to fully cover the surface S. The network obtained must also be connected, while ensuring fault tolerance. In this context several solutions have been proposed. The solutions based on virtual forces, VFA, one of the most efficient algorithms proposed in the literature, have the advantage of faster convergence and less complexity.

In our study, we evaluate by simulation the behavior of existing approaches of VFA with respect to four performance criteria: coverage, connectivity, energy consumption and fault tolerance. This performance analysis shows the need for improvements to ensure the performance criteria previously described. This algorithm does not achieve full coverage and connectivity in some cases, even when the number of sensors is sufficient.

A first improvement gives rise to SerializedVFA. Simulations on several scenarios shown improvement in coverage and connectivity in relation to existing versions, and also the limitations of this solution in terms of energy consumed and fault tolerance.

In order to save sensors power for a higher network lifetime, we implemented LmaxSerializedVFA which exhibits good performance concerning the distance travelled by sensors to reach their final positions.

To ensure fault tolerance, we designed DthLmaxSerializedVFA. Simulations of this final version of VFA have shown that it guarantees the four performance criteria with the convergence of the algorithm in a number of iterations better than that achieved by existing versions. It proceeds by serializing and limiting nodes moves as well as increasing the robustness degree.

As a future work, we will propose a decentralized algorithm of virtual forces and make the necessary improvements to ensure our performance criteria.

Network coding

In traditional communication systems, nodes exchange data in packets, through relaying by intermediate nodes without modification of their content (routing). Seminal work from Ahlswede, Cai, Li and Yeung in has introduced the idea of network coding, whereby intermediate nodes are mixing information from different flows (different bits or different packets), for instance performing "exclusive or" between packets, before retransmitting them.

The Hipercom team is studying network coding specifically for MANET networks. It is mostly used as an efficient multicast/broadcast method (limiting the number of transmissions), and also as a reliable flooding mecanism. Hence, we are studying network coding in the context exclusively of energy-efficiency (and not capacity maximization).

We have proved theoretical results that extend previously obtained results. In another direction of work, we have designed a practical protocol to perform broadcast with network coding, DRAGONCAST which builds on these theoretical results. The main advantages of DRAGONCAST are its energy-efficiency and its simplicity. We analyzed it by simulations and simple models; it successfully illustres how (and how well) energy-efficient broadcast with a simple method could be performed with network coding.

Our research also aims to optimize the capacity of an ad hoc network by using network coding techniques. Our goal is to take advantage of the properties of ad hoc networks that use shared medium (radio interface) and also common forwarding nodes (MPR in OLSR for example), in order to increase their capacity.

Autoconfiguration

Thomas Clausen is co-chair of the IETF working group Autoconfiguration in MANET .

A preconditioning for all routing protocols, OLSR included, is that each node is identifiable through an unique identifier We have developed, and published, a simple auto-configuration mechanism for OLSR networks, aiming a solving the simple but common problem of one or more nodes emerging in an existing network. Our solution is simple, allowing nodes to acquire an address in two steps: first, acquiring a locally unique address from a neighbor node. Then, with that locally unique address and using the neighbor from which the address was acquired as proxy, obtaining a globally unique address.

Furthermore, autoconfiguration also addresses the problem of keeping the consistency of a nomadic network while changing frequently its attachment to the internet. A mobile network (MONET) is a specific network which has the ability to move as a unit while maintaining its connectivity to Internet. Examples of such networks are those deployed in public transportation systems (buses, trains, taxis, etc.) allowing travelers to exchange information and access to the global Internet. Our main research topic in this area concerns radio resource management during mobility. We proposed a resource reservation strategy which can be used by the MONET's mobile router to prepare the grouped handover of all the supported traffic flows. This strategy is based on the predictive movement of MONET networks and showed good results in terms of lost packets and handover dropping probability.

Security in OLSR

In ad hoc networks, security is a very important issue since routing nodes are anonymous. In this case, any node, could change its correct information, insert false information, take the identity of other nodes, etc. All the attacks are very easy because anybody could enter and exit the network and also the medium is wireless and open. Moreover, for the survival of a network, we need the willingness of the nodes in order to route packets to the final destination. If nodes do not cooperate correctly, the routing becomes inefficient. Our solution was to develop two different approaches, one based on intrusion detection that checks the incoherence in the routing protocols and then sends alerts to nodes in order to deactivate the intruders and the second is based on flow conservation that permits to check nodes that avoid forwarding. We introduced the latter property into QoS mechanisms, in order to introduce security as a metric in the routing protocol and to find reliable and secure links.

This issue is a hot issue in ad hoc networks since these networks are inherently open networks. We have reached the following results:

  1. we have designed two security mechanisms to counter most of the attacks when we assume that there is no compromized nodes in the network; the first one has been implemented on the MANET/OLSR demonstrator of CELAR (MoD).

  2. in presence of compromized nodes we have proposed mechanisms to detect compromised nodes or links and to remove such nodes or links in a numerous configurations of attacks.

A significant issue in the ad-hoc domain is that of the integrity of the network itself. Routing protocols allow, according to their specifications, any node to participate in the network - the assumption being that all nodes are behaving well and welcome. If that assumption fails - then the network may be subject to malicious nodes, and the integrity of the network fails.

An important security service over mobile networks is to ensure that the integrity of the network is preserved even when attacks are launched against the integrity of the network.

Hipercom@LIX has allied with TANC@LIX - a research group specialised in cryptography and security, which has developed strong security mechanisms yielding short cryptographic signatures which can be rapidly verified.

The goal of this Hipercom/TANC alliance is to develop secure OLSR networks, suitable for real-world deployments where network integrity is paramount.

This effort is supported by DIGITEO Labs.

OLSR with metrics

In practical networks, one property of many networks is that wireless transmissions may be done with the same equipment but with different parameters, such as modulations (with various payloads), transmission power, etc...This is true, for instance, for 802.11 networks, where different modulations are standardized.

However in the common OLSR routing protocol, this is not addressed, since the view is a binary view of links, which are considered either symmetrical (and then equivalent) or not usuable.

The question is how to take into account this ability to transmit in several manners, so that routing (with OLSR) is performed efficiently. We have proposed an extension of the OLSR routing protocol using metrics, that are well adapted to wireless networks with the characteristics of 802.11 networks.

Cross layer, sensor networks, energy efficiency

The diversity of the applications supported by wireless sensor networks explain the success of this type of network. These applications concern as various domains as environmental monitoring, wildlife protection, emergency rescue, home monitoring, target tracking, exploration mission in hostile environments... Sensor nodes are characterized by a small size, a low cost, an advanced communication technology, but also a limited amount of energy. This energy can be very expensive, difficult or even impossible to renew. That is why, energy efficient strategies are required in such networks in order to maximize network lifetime.

Solutions to maximize network lifetime can be classified into four categories:

Energy efficient routing

Energy efficiency is a key issue in wireless ad hoc and sensor networks. Energy efficient routing is a way to improve energy efficiency and prolong network lifetime. We have shown how to extend the standardized OLSR routing protocol, in order to make it energy efficient. We have first defined an energy model for multihop transmissions. The energy cost of a one-hop transmission is evaluated, taking into account the energy lost in transmitting, receiving, overhearing and interferences. We have then evaluated the energy cost of multihop transmissions. Because of radio interferences, the selection of a unicast path, between a source and a destination, ensuring that each node has sufficient residual energy is NP-hard (see Mans 2006).

The OLSR extension we propose, called EOLSR, selects the path minimizing the energy consumed in the end-to-end transmission of a flow packet and avoids nodes with low residual energy. To take into account residual node energy, the native selection of multipoint relays of OLSR is changed. It considers the weighted residual energy of the multipoint relay candidate and its 1-hop neighbors. The cost associated with a multipoint relay candidate represents the maximum transmission duration that can be sustained by this node. Each two-hop neighbor must be covered by the candidate of maximum cost. These new multipoint relays are called EMPRs. They are used to build energy efficient routes, whereas the native MPRs are used to optimize network flooding.

No additional message is required in EOLSR. In order to select the EMPRs, the Hello messages include the residual energy of the sending node and of its one-hop neighbors. In order to compute the energy cost of a flow, we need to know the number of nodes up to two-hop of the node considered, assuming that interferences are limited to two hops. Hence, the TC (Topology Control) messages include the number of nodes belonging to the interference area of the TC originator.

We show by simulation that EOLSR outperforms the solution that selects routes minimizing the end-to-end energy consumption, as well as the solution that builds routes based on node residual energy. We also compare EOLSR with a two-path source routing strategy: DL a two-path source routing with different links and DN a two path source routing with different nodes. As expected, native OLSR provides the smallest network lifetime. This shows that the selection of the shortest path is not sufficient to save energy. Concerning the two multipath source routing strategies, DN provides better results than DL. This is not surprising insofar as energy is dissipated per nodes and not per wireless link. Hence, DL that allows common nodes in the two paths can exhaust the energy of these common nodes more quickly. The main conclusion of these simulation runs is that EOLSR significantly outperforms DN and DL whatever the number of nodes. EOLSR prolonges the network lifetime of 50% compared with OLSR for a network of 200 nodes. This extensive performance evaluation allows us to conclude that EOLSR maximizes both network lifetime and the amount of data delivered.

We then show how we can improve the benefit of energy efficient routing using cross layering. Information provided by the MAC layer improves the reactivity of the routing protocol and the robustness of routes. Moreover, taking into account the specifities of some applications like data gathering allows the routing protocol to reduce its overhead by maintaining routes only to the sink nodes. We propose the strategic mode of EOLSR for that purpose.

The EOLSR protocol will be implemented in the OCARI project aiming at developing a wireless sensor communication module, based on IEEE 802.15.4 PHY layer and supporting EDDL and HART application layer and targeting applications in power generation industry and in warship construction and maintenance.

Nodes activity scheduling, real-time networking

In wireless ad hoc and sensor networks, an analysis of the node energy consumption distribution shows that the largest part is due to the time spent in the idle state. This result is at the origin of SERENA, an algorithm to SchEdule RoutEr Nodes Activity. SERENA allows router nodes to sleep, while ensuring end-to-end communication in the wireless network. The idea is to assign a color to each node, while using a small number of colors and ensuring that two nodes with the same color can transmit without interfering. This color is mapped into a slot in which the node can transmit its messages. Any node stays awake only during its slots and the slots assigned to its one-hop neighbors, it sleeps the remaining time. We propose a generic solution able to adapt to different application requirements: general or tree-based communications, broadcast, immediate acknowledgement of unicast transmissions... The impact of each additional requirement is evaluated by simulation. For instance, for general communications with immediate acknowledgement, two-hop coloring is no longer sufficient, three-hop coloring is required.

An originality of this work lies in taking into account real wireless propagation conditions. Unidirectionnal links, late node arrivals, appearance of new links and node mobility can create color conflicts. A cross-layering approach with the MAC layer is used to solve these conflicts. We also show how cross-layering with the application layer can improve the coloring performance for data gathering applications. In such applications, the freshness and time consistency of data collected from the sensors must be ensured. SERENA enables collected data to reach the sink in a single cycle, minimizing the end-to-end delays. This property is obtained by obliging a node to select a color higher than its parent. Hence, it will transmit before its parent that aggregates the data received from its children.

A performance evaluation allows us to compare SERENA coloring algorithm with existing ones such as Distributed Largest First, denoted DLF, both in terms of number of colors and complexity. SERENA and DLF use a similar number of colors, whereas the complexity of SERENA expressed in numbers of rounds is significantly lower. Moerover, it turns out that the number of colors used by SERENA depends (i) strongly on the network density and (ii) weakly on the number of nodes. We have also compared SERENA with TDMA-ASAP that does not support immediate acknowledgement of unicast transmissions. The immediate acknowledgement is very useful in a wireless environment prone to message losses.

Simulation results show that SERENA maximizes both network lifetime as well as the amount of data delivered to the application. Moreover SERENA improves efficiency in the the node energy consumption. The first benefit of SERENA is that less energy is lost in the idle state. Indeed, if a node has nothing to transmit and its one-hop neighbors are not transmitting, the node is sleeping. The second benefit is that SERENA contributes to significantly reduce the interference phenomenon that becomes negligible. Hence, SERENA considerably improves the energy efficiency of wireless ad hoc and sensor networks. Moreover, SERENA increases the utilization of network resources such as bandwidth by means of spatial reuse.

The SERENA protocol will be implemented in the OCARI project. A strong cooperation with the MAC layer enables an efficient time slot allocation and an early detection of color conflicts. This cooperation improves the performances of SERENA in a network where bandwidth and energy are limited.

In a real-time constrained network, some of the applications coexisting in the network require bounds on their worst case end-to-end response times and jitters to have a behavior compliant with their specifications (e.g. voice over IP, control-command applications, multimedia applications, distributed interactive games). To provide deterministic guarantees on these times, we developed an approach, called “trajectory approach”, based on flow scheduling. More precisely, assuming that flows are scheduled in each node according to fixed and/or dynamic priorities, our worst case analysis allow establishing upper bounds on the real-time constraints. These results address many applications. They enable to derive, for example, a simple admission control in charge of deciding whether a new flow can be accepted or not, by verifying that the new flow will not experiment a worst case response time greater than its end-to-end deadline and that the acceptance of this new flow will not compromise real-time guarantees given to the already accepted flows.

Optimized routing in low capacity sensor networks

Self-organization is considered as a key element in tomorrow's Internet architecture. A major challenge concerning the integration of self-organized networks in the Internet is the accomplishment of light weight network protocols in large ad hoc environments.

In this domain, Hipercom's activity with wireless sensor nodes in collaboration with the Freie Universitaet in Berlin explores various solutions, including extensions of OLSR (for example DHT-OLSR) using programmable sensor nodes co-designed by the Freie Universitaet, and provides one of the largest testbeds of this kind, to date.

Additionally, we're happy to make available a muOLSR for Scatterweb implementation.

Protocols for vehicular networks

We have achieved numerous studies and design of protocols for vehicular networks and more spefically for V2V (Vehicle-to-Vehicle) network.

First we have studied the channel occupancy induced by the OLSR proactive routing protocol used in a linear Vehicular Ad hoc Network (VANET). Unlike previous studies, which usually use simulations to evaluate the overhead, we have proposed a simple analytical model to carry out this evaluation. Moreover, we did not evaluate the total overhead induced by the routing protocol as is usually proposed, but, for a given node, the channel occupation induced by the routing protocol.

We have studied flooding techniques for safety applications in VANETs. The typical scenario is the diffusion of an alert message after a car crash in a platoon of vehicles. The packet is diffused with the pure flooding, the multipoint relay (MPR) diffusion of OLSR and a geographic aware protocol. For OLSR we have introduced a variant (Robust-MPR) to improve the reliability. Different realistic scenarios were considered and various parameters such as vehicle density, and background traffic load were scrutinized. We have shown that the Robust-MPR and the geographic aware protocol satisfy the requirements of the safety applications while using considerably less overhead than pure flooding.

We have shown that the geographic aware protocols can be improved for the diffusion of an alert message by using opportunistic routing. We have designed OB-VAN (Opportunistic Broadcast for VANets ) a new protocol that uses this idea. One of the novelty of this protocol is the use of an active signalling technique in the acknowledgement procedure to select the best relay taking advantage of the reception pattern of each message. We have studied OB-VAN in a linear VANET and have shown that it outperforms the flooding for the delay and the amount of overhead. However the delivery ratio of OB-VAN may be insufficient for safety applications. This remark has led to the design of R-OB-VAN which is a reliable variant of OB-VAN. With extensive simulations, we have shown that R-OB-VAN maintains a high delivery ratio even in the presence of packet loss due to shadowing.

We have studied the performance of the Aloha scheme in linear VANETs. This analysis assumes a SINR (Signal over Interference plus Noise Ratio) based model. In this model, we have derived the probability of a successful transmission between two vehicles at a distance of R meters. We have also computed the mean throughput according to Shannon's law. In these two models, we have optimized the two quantities directly linked to the achievable network throughput i.e., the mean packet progress and the density of transport.

Finally, we have studied the utilization of opportunistic routing and shown that this technique is also beneficial for point to point traffic. It decreases the delay and increases the throughput compared with shortest path first routing. Moreover, we have also shown that opportunistic routing for point to point traffic eases considerably the optimization of the MAC scheme e.g. the transmission probability for Aloha and the carrier sense threshold for CSMA.

Collaborations


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