Section: New Results
New services and protocols
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.
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).
Routing based on packet delay distribution in multihop ad hoc network
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.
Multicast services in mobile ad hoc networks
We have designed the Multicast extension for the Optimized Link State Routing protocol (MOLSR). MOLSR is in charge of building a multicast structure in order to route multicast traffic in an ad-hoc network. MOLSR is based on natural radio broadcast. This multicast protocol has also been implemented on the MANET/OLSR demonstrator of CELAR (MoD). A new version of the multicast protocol called MOST uses unicast traffic.
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.
We have just started the work in this area. We have proven optimality of network coding in 1D and 2D dense network and have designed a practical protocol, DRAGONCAST, that fits these properties. We propose a simple protocol for network coding. We describe how the packets flowing from the one source evolves in waves , and by studying the propagation of the waves, we are able to quantify the performance of the simple protocol. We analyze the performance of the protocol in an unit graph wireless network model in an 1 dimension model, with an asymptotic model and show that it is asymptotically close to optimal flooding obtained via MPR flooding. In 2 dimension model, we conjecture similar performance, simulations show that the protocol outperforms the MPR flooding (which is no longer optimal).
The simple network coding protocol is interesting since it does not need sophisticated network management such as neighborhood management and two-hop monitoring as it is demanded with MPR flooding. Nevertheless the asymptotic performance are obtained with rather large dimensions, which lead to non negligible information extraction cost from rather large linear combinations.
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.
Security in OLSR
This issue is a hot issue in ad hoc networks since these networks are inherently open networks. We have reached the following results:
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).
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.
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:
Topology control: These strategies adjust the transmission power of wireless nodes to spare energy;
Reduction of the volume of information transferred: These strategies aggregate data with or without clustering, optimize network flooding, tune the periodicity of information refreshment;
Nodes activity scheduling: as the sleeping state is the radio state consuming the least energy, these strategies make nodes sleep in order to spare energy, while ensuring network and application functions. Large benefits are expected.
Energy efficient routing: Such strategies notice that a multihop transmission is energy consuming and reducing the energy spent in the transmission of a packet from its source to its destination would increase network lifetime. Moreover, avoiding nodes with a low residual energy would also contribute to prolong network lifetime. Avoiding nodes that already have a high traffic load would reduce medium access contention, collisions if the medium access type is CSMA-CA and then spare energy lost in useless transmissions.
For low cost device, OLSR must be reduced and integrated with medium access control and application. This cross layering is an important starting activity which allows experiments on large scale. 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. The OLSR extension we propose, called OLSRE , selects the path minimizing the energy consumed in the end-to-end transmission of a flow packet and avoids nodes with low residual energy. An extensive performance evaluation allows us to conclude that OLSRE maximizes both network lifetime and the amount of data delivered.
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. It is a localized and decentralized algorithm assigning time slots to nodes. Any node stays awake only during its slots and the slots assigned to its neighbors, it sleeps the remaining time. SERENA is based on distributed and localized two-hop coloring. The node's color is then mapped in time slot. Thus, each node is ensured to get at least one time slot, it also gets additional time slots proportionally to its traffic rate. Such a solution adapts to varying traffic rates and supports late node arrivals. 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 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.
Many contractual collaborations:
MoD (QoS, security, interconnection between the OLSR and OSPF routing domains),
Hitachi (Vehicular applications, OLSRv2),
OCARI project (QoS, cross layer, energy efficiency),
SARAH project (QoS, localization),
Com2react (vehicular applications, multicast),
STIC INRIA-Tunisian Universities: the team of Prof. Leila Saidane at ENSI (QoS and routing).