Ahmed Akl
email: aakl@laas.fr

Thierry Gayraud
email: gayraud@laas.fr

Pascal Berthou
email: berthou@laas.fr

A Metric for Evaluating Density Level of Wireless Sensor Networks

Keywords:

The term "Dense networks" is usually used to identify networks having large number of nodes within a small area while affecting network performance. Starting from this concept, different networking solutions were proposed. However, it seems to us that this view is very abstract and needs to be further investigated. Knowing the network density level can be of great importance in determining the network parameters. In this paper, we firstly examine the effect of network size over its performance, then introduce a metric to determine if a network is considered as a "dense network" or not. Finally, we applied our metric on experimental results published in the CRAWDAD dataset site to verify our metric.

INTRODUCTION

Wireless Sensor Networks (WSNs) are a special type of adhoc networks where its simplest form consists of a set of sensors for data capturing, usually environmental data (i.e., temperature), and sending them to a base station through a heterogeneous network, which may contain base stations, different types of sensors, actuators, and processing elements [START_REF] Dressler | Self-Organization in Sensor and Actor Networks[END_REF]. The nodes (i.e., sensors) are characterized by being limited in resources (i.e., power, storage, processing, etc...), and small in size. They are powered by tiny batteries where the battery power is usually directly proportional to its size. Such power constraint has a great effect over the node activities. The major power consuming activities are mobility and communication. However, power consumption due to mobility is not a must; a WSN can be static or the nodes attached to a moving object, so that no power is consumed through mobility. On the contrary, power consumption due to communication is a must to perform the required task. A single hop communication can be more power consuming than a multiple hop communication [START_REF] Zhao | Understanding Packet Delivery Performance In Dense Wireless Sensor Networks[END_REF] because more transmission power is required.

Apparently, a dense network can raise different problems to MAC layer such as overhearing, communication grouping, over-provisioning, and neighbor state [START_REF] Halkes | Crankshaft : An Energy-Efficient MAC-Protocol for Dense Wireless Sensor Networks[END_REF]. Moreover, the negative effect propagates to the Network [START_REF] Silva | Magnetworks : how mobility impacts the design of Mobile Networks[END_REF], [START_REF] Liarokapis | DibA : An Adaptive Broadcasting Scheme in Mobile Ad Hoc Networks[END_REF] and Application [START_REF] Cho | Density-adaptive network reprogramming protocol for wireless sensor networks[END_REF], [START_REF] Haghpanahi | Implementing Information Paths in a Dense Wireless Sensor Network[END_REF] layers where it can't be over-passed.

Section II gives an overview on dense networks, showing the factors affecting density, and how density is usually calculated. In section III, we discuss our proposed metric.

Section IV shows the simulation held by NS2 simulator. Section V shows the usage of the proposed metric to analyze a real experimentation results. Finally, we introduce the conclusion and future work.

II. DENSE NETWORKS: AN OVERVIEW

The term Dense Network is usually used in the ad-hoc networking context to represent high concentration of nodes within certain area. There are alternative terms (i.e., massively dense networks, and very large networks), other than "Dense networks", used in the context, but the term "Dense Networks" is the most used one. However, this term can be misleading because it gives the impression that the population around all nodes is high, although it may be true for only some of them [START_REF] Toumpis | A survey of recent results on wireless networks based on analogies with physics[END_REF].

In a dense ad-hoc network, too many communication links are sometimes detrimental for energy consumption, network throughput, and quality of service. In spite of the imposed difficulties, some WSN techniques prefer the usage of a dense network to obtain better performance since it encourages the cooperation between sensors.

A. Factors affecting density

Different factors have influence over the network density; some of them are controllable while the others are not. However, it is difficult to consider some factors as being absolutely controllable or uncontrollable (i.e., Mobility). The main factors are mobility, obstacles, transmission range and deployment scheme.

B. Calculating network density

Toumpis [START_REF] Toumpis | A survey of recent results on wireless networks based on analogies with physics[END_REF] surveyed different wireless networks solutions based on analogies with physics. He noted that, most of these solutions are based on the assumption that the network has high node density. However, few attempts were done to find a measure for density. Bulusu [START_REF] Bulusu | Scalable Coordination for Wireless Sensor Networks : Self-Configuring Localization Systems[END_REF] proposed an equation to calculate the network density and many other publications had used it [START_REF] Wu | Wireless Ad Hoc Networks (Personal-Area, Local-Area, and the Sensory-Area Networks[END_REF], [START_REF] Akyildiz | A Survey on Sensor Networks[END_REF]. He said that Network Density (µ) can be roughly calculated as

( ) A R N 2 π μ =
; Where N is the number of nodes in area A, and R is the radio transmission range for a 978-1-4577-2028-4/11/$26.00 ©2011 IEEE disk communication model. A more precise equation was proposed by Toumpis [START_REF] Bulusu | Scalable Coordination for Wireless Sensor Networks : Self-Configuring Localization Systems[END_REF] who defined the node density as ( ) ( )

A A N r d A 0 lim ⎯→ ⎯ =
, where density is measured as the number of nodes per m 2 .

These equations are derived from the idea that network density is the amount of certain quantity (i.e., number of nodes) within certain area, and this quantity is only dependent on the node transmission range. However, it seems to us that such assumption is very abstract, since there are other factors (i.e., mobility, obstacles, etc...), which are not considered in the equation and can affect the number of connected nodes. For example, a node can have 5 neighbors because they are located within its transmission range even if some of them are not really connected due to their short transmission range.

III. A METRIC FOR EVALUATING NETWORK DENSITY

Although different solutions are based on the assumption of having a dense network, there are still some questions that need further investigations; Does the number of nodes per unit area is the only required measure to identify a dense network?

According to the studied use cases, we can deduce that: [START_REF] Dressler | Self-Organization in Sensor and Actor Networks[END_REF] The number of nodes within the transmission range is not an enough measure for network density; (2) The minimum number of nodes to achieve a dense network depends on different factors in addition to the number of neighboring nodes and (3) it may be necessary to have different degrees of density not only dense and non-dense.

For these reasons, we propose the usage of Effective Density (ED) as an indication of the density level. The ED of a node is the ratio between the number of single hop connected nodes (N), and the node Throughput (th), where ED=N/th. In other words, we can consider ED as a measure to find out how much each additional node is going to participate in changing the Throughput When more nodes enter the transmission range of a node, the node connectivity increases causing th to increase, so that the enhancement of Throughput decreases the effect of increasing neighbors over the effective density. In other words, the increase of Throughput suppresses the effect of increasing N.

IV. SIMULATION RESULTS

The objective of this simulation is, firstly, to study the effect of increasing the number of nodes over the receiver Throughput, secondly, to show the Effective Density and how it is changed with respect to nodes number, and throughput.

The configuration of the simulated scenario is as follows; the network terrain is 500m x 500m with the Tx and Rx nodes located at (0, 0) and (500, 500), respectively. Node transmission range is adjusted, so that the Tx and Rx are not directly connected, but they are connected through a group of single hop connected neighbors. The simulation is repeated 9 times where 10 randomly distributed nodes are added to the network each time. CBR connection of 0.3Mb is used to inject data traffic to the network, and throughput was calculated at the application Layer level. The simulation duration is 600sec.

Fig. 1 shows the change of Effective Density with respect to the number of nodes. It is noticeable that the rate of change had increased after adding 40 nodes. This means that the network became crowded, so each additional node causes more collisions and Throughput degradation becomes obvious. However, there is still one more question, what is the value after which the network is considered dense. In fact, there is not a discrete value; each protocol and application should determine its own threshold values. For example, an application can consider the previous network as non-dense until it has 40 nodes, then semi-dense until 80 nodes, and finally highly dense for more than 80 nodes. On the other hand, another application can consider it as non-dense till 80 nodes and highly dense after that.

V. EXPERIMENTAL ANALYSIS CRAWDAD [START_REF] Gray | CRAWDAD data set dartmouth/outdoor (v. 2006-11-06)[END_REF] provides rich datasets originating from different real experimentations in various networking domains; one of them was conducted by Liu [START_REF] Liu | Empirical Validation of Wireless Models in Simulations of Ad Hoc Routing Protocols[END_REF]. He used a test bed to have a practical outdoor comparison between different ad-hoc protocols. The experimentation was held in an area of 225mx365m where 40 laptops were moving randomly. They were divided into groups, of 10 laptops each, where each group was randomly distributed into one of the 4 zones in the terrain. The moving pattern is as follows; each device randomly chooses a location in a different zone and moves straight toward it, then the process is repeated after reaching the destination. GPS location data and traffic data are located for each device We used the dataset generated by Liu [START_REF] Liu | Empirical Validation of Wireless Models in Simulations of Ad Hoc Routing Protocols[END_REF] to show the effect of changing the number of neighboring nodes over Throughput. As nodes move, the number of direct connected neighbors changes, and is calculated through the SIN values, so that inactive nodes are excluded from the calculations. Fig. 2 shows the values of Effective Density (ED), Throughput, and number of nodes (N) as they change with time. It is noticeable that Throughput has the same pattern as the change of number of nodes. This is because almost all network parameters are kept constant and it is the number of neighboring nodes that change due to mobility. In Zone1, ED started at its maximum, because N was high and Throughput was very low. This means that the large number of nodes didn't enhance Throughput. However, ED decreased dramatically with decreasing N because Throughput didn't change too much. This indicates that at this period, the current distribution of high number of nodes didn't increase connectivity, but it affected Throughput negatively; this can be noticed in the trivial change of Throughput when compared to the large drop in number of nodes. Consequently, ED decreased quickly as N decreases.

In Zone2, the rate of change of nodes is almost the same as the rate of change of Throughput, so there were no great changes in the ED. This means that when ED is constant, any change in N is accompanied with the same proportional change in throughput. In other words, the new injected nodes are affecting the network positively and causing an increase in Throughput.

Zone3 has the same effect as Zone2, but in the opposite sense. ED is almost constant due to, approximately, the same decreasing rate of both N and Throughput We deduce that ED can be used as a measure for the effect injected or leaving nodes over performance in terms of Throughput.

VI. CONCLUSION AND FUTURE WORK

The network density is an important parameter for network description. However, we believe that the current definition of network density only represents the number of neighboring nodes within the node transmission range, and doesn't represent its effect over the network. In other words, the network density calculation, which is presented in the literature, is not an enough metric to judge the network state as being dense or non-dense since it doesn't consider the network performance. Thus, we propose the usage of Effective Density as a new measure, which allows us to study the dynamic effect of the neighbor's number. Moreover, it allows us to divide the network into zones of different densities where each zone can behave according to the influence of its current population.

We conducted a simulation as a proof of the concept where we showed how the Effective Density is influenced by the changing number of node's neighbors and its Throughput. Then, we showed the metric applicability over a data set extracted from a real experimentation.

Our future work is to integrate our metric within a protocol that uses network density as its control parameter to show how our metric can enhance the protocol behavior

Fig. 1 :

 1 Fig. 1: Effective Density


Ahmed Akl

Thierry Gayraud

An investigation of self-organization in ad-hoc networks

Keywords: Wireless sensor networks, self-organization, selfconfiguration, Ad-hoc networks

come

I. INTRODUCTION

According to Marriam-Webster dictionary[1], the term "Ad hoc" means " formed or used for specific or immediate problems or needs (i.e. ad hoc solutions)". This definition can show the sense of the term "Ad-hoc networks".

Ad-hoc networks are wireless networks where nodes can communicate wirelessly with each other without the need for a fixed infrastructure. This is the most distinguishing feature that differentiates between ad-hoc networks and traditional wireless networks (i.e. LAN, cellular networks). There is no centralized control; nodes are autonomous they can take their own actions depending on network's situation. In other words, they are responsible for determining the way they communicate, organizing themselves, responding to changes that happen to the network due to external or internal factors, etc…

In addition to features inherited from ordinary wireless networks, ad-hoc networks have certain features which are specific to ad-hoc networking. Ad-hoc networks can be composed of heterogeneous nodes; most of these nodes are in continuous mobility. The importance of ad-hoc 1 CNRS; LAAS; 7 avenue du Colonel Roche, F-31077 Toulouse, France 2 Université de Toulouse; UPS, INSA, ONP, ISAE; LAAS; F-31077 Toulouse, France networking concept is greatly recognised when the nodes are deployed over a large area where a single hop communication is not possible. This situation introduces a challenge to find a technique that provides an appropriate multi-hop routing. Nodes can join or leave the network independently without causing the network to fail.

During the recent years, great developments in electronics and wireless communication allowed researchers to implement miniature sensing devices with wireless communication capabilities. This modern technology satisfies the needs for special type of applications where Wireless Sensor Networks (WSN) can play an important role. This type of networks is useful for applications where rapid deployment is required in areas that lack the appropriate infrastructure to setup the network. It is suitable for environmental measurements, communications in disaster areas, and numerous commercial, personal, and military applications.

WSN is a special type of ad-hoc networks where nodes are smart sensors with scarce resources. They are small in size, have limited computational power, short range communication capabilities, low energy, limited and storage capacity, and usually numerous in number. They differ from the ordinary ad-hoc network's nodes in that they are usually homogenous nodes unless different phenomena are going to be sensed. Many challenges are facing WSN; the main challenges related to WSN implementation are energy conservation, low quality communication, and scalability. Self-organization can help in solving these problems or in the best case to minimize their drawbacks.

Having a quick look on old surveys can give a good image about the great developments that took place in this field. In 1986, T.G.Robertazzi [START_REF] Robertazzi | Self-organizing Communication Networks[END_REF] showed the new rising adhoc network technology. At that time they were interested in how to setup the connectivity between nodes, transmission scheduling, self-organization, etc… the concepts of traditional wireless networks were still in mind; using local hubs, backbone networks, gateway were thought to be part of the solution. Nowadays, it is totally different; Ad-hoc networking has its own concepts which proved to be practical and reliable. Although connectivity and transmission scheduling are still important issues, but it became the issue of their performance and reliability. Last but not least, more topics were introduced such as security, and QoS.

An investigation of self-organization in wireless sensor networks

Ahmed Akl 12 , Thierry Gayraud 12 , and Pascal Berthou 12 {aakl, gayraud, berthou}@laas.fr II. SELF-ORGANIZATION VS SELF-CONFIGURATION Self-organization is not a man made concept. Kevin L. Mills [START_REF] Mills | A brief survey of self-organization in wireless sensor networks[END_REF] showed that it is a natural phenomena that exists in different natural systems. Most of the artificial selforganization techniques were inspired from the natural ones. For example, some anti virus programming concepts were derived from the natural immune system. Natural systems are full of self-organizing mechanisms and concepts that can solve different WSN issues.

The terms self-organization and self-configuration are used interchangeably in the WSN domain to express changes in the current network status to cope with certain environmental change or to enhance network's and/or node's performance [START_REF] Ibriq | Cluster-Based Routing in Wireless Sensor Networks : Issues and Challenges[END_REF]. The term self-organization is used more frequently. However, some contributions considered a difference between the two terms [START_REF] Lu | FISCO : a Fully Integrated Scheme of self-Configuration and self-Organization for WSN[END_REF] to emphasize certain ideas, but there is still a need for a general definition to specifically specify the differences between the two terms. In this section we will try to highlight the differences and propose a clear definition for both of them, so that they can be used unambiguously.

According to Marriam-Webster dictionary[1], "Organization" is derived from the verb "Organize". It has different meanings; those whom we may be interested in are as follows:

• To form into a coherent unity or functioning whole. • To set up an administrative structure.

• To persuade to associate in an organization.

• To arrange by systematic planning and united effort.

• To arrange elements into a whole of interdependent parts

From the above meanings we can deduce that the verb "Organize" means to arrange different independent entities into a single unity to cooperate together for performing a certain task. Applying the same meaning on the WSN domain, we can define self-organization as "the changes that the node does in its behaviour to cooperate with its neighbours in the network to perform a certain task or achieve a certain goal".

On the other hand, "Configure" was defined as "to set up for operation especially in a particular way" [1]. Applying the same meaning on WSN domain, we can define selfconfiguration as "the changes that the node makes in its parameters to perform certain task".

To sum up, we can say that a node may perform selfconfiguration actions to achieve self-organization that helps the node to have certain behaviour.

For example, if there is an environmental change that causes frames to collide frequently, then each node must be self-organized to overcome this problem in order to minimize power loses. To achieve this behaviour, the node starts to configure its MAC protocol to control the number of sent frames. In this case, we can say that self-configuration had lead to self-organization.

In other situations, self-organization can be achieved without self-configuration. If we considered the case when a node detects a weakness in the received signal due to moving in a certain direction, then it starts to change its direction to keep the signal. This happens without setting up any internal changes, so its behaviour (i.e. self-organization) was changed without any changes in its internal parameters (i.e. self-configuration). This assumption is greatly dependent on the level of abstraction when considering selfconfiguring parameters. In other words, do we consider changes in the values that cause alteration in direction as being changes in configuration or not.

III. SELF-ORGANIZATION ARCHITECTURAL REQUIREMENTS

In WSN domain the term self-organization is tightly coupled with routing protocols, so the term self-organizing may be confused with the term routing. Routing is the action of relaying data from one node to another through a communication path. Routing decision may be centralized or decentralised depending on the network architecture. Selforganization is the action of reorienting network nodes. This reorientation can be due to a change in location, hardware or software configuration to change the role that a node plays in the network. The decision of self-organizing is taken by the node itself according to the surrounding environment.

Self-organization is of great importance to manage and save the node's scarce resources. Power consumption can be greatly reduced when transmission range is efficiently managed. In addition, node's location can dramatically affect the choice of an efficient transmission range. Also, it has an influence on network traffic where it can reduce congestion in locations with high nodes density.

An important issue in WSN is that system requirements usually conflict with its physical limitations. A WSN is usually used for gathering and sending data. At the same time, WSN suffers from limited resources such as power source and hardware limitations [START_REF] Li | A Low Power Consumption Implementation for WSN Nodes in Lumber Drying Kiln[END_REF], and limited computational capabilities that needs specific application structure [START_REF] Kuorilehto | A Survey of Application Distribution[END_REF]. Further more, WSN are used in different applications ranging from civil [START_REF] Stankovic | Wireless Sensor Networks for In-Home Healthcare : Potential and Challenges[END_REF] and environmental [START_REF] Behnke | Wireless Sensor Networks for Life Science Automation[END_REF] applications to military [START_REF] Bokareva | Wireless Sensor Networks for Battlefield Surveillance[END_REF] ones. This wide range of applications needs different system's architectural requirement. It is of a great importance to try to define a general basic architecture and try to find out its requirements. Different application's requirements can be added to this generic architecture to obtain the desired outcome. In this section, we are interested in showing general structural requirements for MAC, network, transport, and application layers that make them suitable for the WSN environment.

A. MAC Layer

According to the search done by Nait-Abdesselam [START_REF] Nait-Abdesselam | O-MAC : An Organized Energy-Aware MAC Protocol for Wireless Sensor Networks[END_REF], it was stated that MAC protocols can be categorized into two groups, Time Division Multiplexing Access (TDMA), and Carrier Sense Multiple Access (CSMA). The former is basically a technique that allows nodes to share a communication media by synchronising them to share available time slots. Its scheduling nature helps to reduce power losses due collisions. However, there still exists the non ignorable synchronisation overhead. The later is dependent on an agreement between the sender and receiver to share the media instead of waiting for time slots. However, there are losses due to collision, idle listening, and overhearing.

Areas with high density of nodes suffer from congestion due to sharing of same communication medium. Further more, recent improvements in hardware raised the importance of finding appropriate MAC protocols. Ilker Demirkol [START_REF] Demirkol | MAC Protocols for Wireless Sensor Networks : A Survey[END_REF] introduced a survey for different MAC protocols where critical WSN properties for the design of MAC protocols were highlighted. They were also investigated to show their points of weakness and strength. It was mentioned that communication patterns can be categorized into four patterns, broadcast, convergecast, local gossip, and multicast. The differences between these patterns were discussed. Different attributes of a good MAC protocol were proposed. These attributes include energy efficiency, scalability, and adaptability to network changes. In spite of their importance, other attributes can be considered as secondary attributes when compared with the main goals of a MAC protocol. These attributes can be latency, throughput, and bandwidth utilization.

B. Network layer

The way data are routed between nodes is of great importance. An efficient routing or placement algorithm can save a lot of lost energy. In a WSN environment, nodes are usually deployed randomly causing unintended node distribution. In such situation, it is usually impossible to establish a single hop communication link; this makes it very important to find out a way to transfer packets through multi-hop path(s) efficiently and accurately. This can minimize the energy lost due to packet losses, and sending packets in inappropriate or obsolete paths.

Most of the routing protocols -proposed for the wireless networks' domain-do not consider limited resources constrain that exists in WSN environment. In addition, traditional addressing scheme doesn't work well in such dynamic environment [START_REF] Al-Karaki | Routing Techniques in Wireless Sensor Networks : A Survey[END_REF]. A well defined routing protocol can solve these issues, but it is impossible to have a single general design, so it is of great importance to identify different categories of protocols. Al-Karaki [START_REF] Al-Karaki | Routing Techniques in Wireless Sensor Networks : A Survey[END_REF] proposed a good categorization of WSN routing protocols through different points of view according to network structure, and protocol operation. According to the former view, they can be categorized as flat, hierarchical, and location based routing. According to the latter, they can be categorized as negotiation, multi-path, query, QoS, and coherent based routing. In addition, Yazeed Al-Obaisat [START_REF] Al-Obaisat | On Wireless Sensor Networks : Architectures , Protocols , Applications , and Management[END_REF] showed that protocols can also be categorized according to routing discovery; they can be divided into proactive, reactive and hybrid protocols.

Different surveys agreed on common network layer challenges [START_REF] Al-Karaki | Routing Techniques in Wireless Sensor Networks : A Survey[END_REF][14][15] [START_REF] Akkaya | A survey on routing protocols for wireless sensor networks[END_REF]. Node placement, sensors are deployed in either a deterministic or random way. In either case, the network must keep a link of communication between nodes. Energy saving, routing protocols must try to increase the network's life time by adopting an efficient algorithm to minimize energy required for routing. Scalability, node sensors are usually deployed in numerous numbers, the routing protocol must be able to handle extra nodes joining the network and preserve the required performance even if some nodes left the network (i.e. due to power depletion). Mobility, depending on the application, nodes can be either dynamic or static. These issues must be considered during the protocol design.

In addition, the role that the node plays in the network must be considered during the design. In a heterogeneous network environment specialized nodes can exist. A sensing node may be only able to communicate with a router node where there is no need to communicate with neighbouring sensing nodes. Router nodes can communicate with each other to relay data between different groups of sensing nodes. A sink node can be used to collect data sent by sensing nodes. On the other hand, a homogeneous network can exist where each node can play different roles; it can sense as well as route data.

C. Transport layer

On the other hand, transport protocols have there own attributes of interest. Chonggang Wang [START_REF] Wang | A Survey of Transport Protocols for Wireless Sensor Networks[END_REF] submitted a survey on transport protocols for WSN. Challenges in using transport protocols in sensor environment such as energyefficiency, quality of service, reliability, and congestion control were mentioned. These challenges (which are due to the especial characteristics of WSN) raises the need to design either new transport protocols or to adapt the existing ones because ordinary transport protocols such as UDP or TCP don't behave well in WSN [START_REF] Wangl | Issues of Transport Control Protocols for Wireless Sensor Networks[END_REF] [START_REF] Kosanovic | Implementation of TCP / IP Protocols in Wireless Sensor Networks[END_REF]. A guideline for transport protocol design was proposed [START_REF] Wang | A Survey of Transport Protocols for Wireless Sensor Networks[END_REF]. This guideline discuses common performance metrics (i.e. reliability, QoS, and fairness), and required functions of transport protocols (i.e. congestion control, and loss recovery) with respect to WSN environment. Chonggang Wangl [START_REF] Wangl | Issues of Transport Control Protocols for Wireless Sensor Networks[END_REF] presented different design issues for transport layer in WSN. In [START_REF] Wang | A Survey of Transport Protocols for Wireless Sensor Networks[END_REF] and [START_REF] Wangl | Issues of Transport Control Protocols for Wireless Sensor Networks[END_REF] different types of transport protocols dedicated for WSN were discussed.

WSN are not an isolated network; for certain applications it is necessary to be connected to other types of networks such as internet or a database. Most of these networks, if not all, are dependent on the TCP protocol. Unfortunately, TCP doesn't behave well in WSN. So, if this type of connection is really needed, then adapting the transport layer should be considered to be able to establish this type of connection. Chonggang Wangl [START_REF] Wangl | Issues of Transport Control Protocols for Wireless Sensor Networks[END_REF] presented the disadvantages of using TCP and UDP, with respect to a WSN environment, which were originally designed for wired, or in the best cases for wireless networks. Mirko R. Kosanovic [START_REF] Kosanovic | Implementation of TCP / IP Protocols in Wireless Sensor Networks[END_REF] proposed a solution to overcome this type of problems in order to connect WSN to other TCP networks.

D. Application layer

From the first sight, it seems that self-organization is only dependent on the lower communication layers. In fact, a WSN application layer can have a considerable impact on self-organization and node's performance. The basic software level is the Operating System (OS). It is the responsibility of the OS to manage the node's resources such as, memory, CPU, and communication capabilities. Requirements for such OS were introduced by Yao minghai [START_REF] Xiao-Xiao | Study and Transplant of Operating System for Wireless Sensor Network Node[END_REF].

When the node's structure is too complex, the applications may need a solution to hide this complexity for easier development process. In this case, intermediate software is required to handle the situation; this type of software is called middleware. Middleware was presented in a survey by Miao-Miao Wang [START_REF] Wang | Middleware for Wireless Sensor Networks : A Survey[END_REF], where different issues were discussed including a study for the topic to determine the challenges, the services required, and provide a reference model for determining the required functionalities and services. In addition, he showed the current work related to this topic, as well as proposing a way to organize the relations between the middleware features to give a better understanding for the issue.

Although different programming models were introduced for the networking domain, but programming for WSN applications needs extra attention due to the different constrains that exist in WSN environment. Ryo Sugihara [START_REF] Sugihara | Programming Models for Sensor Networks : A Survey[END_REF], introduced a comprehensive survey for representing the programming requirements, showing the challenges that arises due to limited resources, and introducing available programming models of WSN.

Another issue that shows the importance of application layer for self-organization is software update. When an environmental change occurs, such as the existence of new type of nodes, extra phenomena to be measured, and a change in the node's task; a software update may be vital for the node to cope with these changes. S.Brown [START_REF] Brown | Updating Software in Wireless Sensor Networks : A Survey[END_REF], introduced a survey for software update in WSN. It showed different issues such as the effect of update on performance, security, and energy saving, as well as known research categories for software update.

IV. SELF-ORGANIZATION TECHNIQUES

In many applications especially those interested in measuring natural phenomena or rescue missions, sensor nodes are randomly spread in a large geographical area. This situation leads to the need for an organizing technique to reorganize the nodes to efficiently utilize them.

Recently, different techniques were investigated to show the importance and effectiveness of self-organization. In this part, we will introduce techniques related to topological and power conservation self-organization.

A. Topological self-organization

WSN is a special type of networks where not all topologies can be efficiently implemented. Star, mesh, and cluster hierarchy are the most chosen topologies for WSN. Akhilesh Shrestha [START_REF] Shrestha | A Performance Comparison of Different Topologies for Wireless Sensor Networks[END_REF] introduced a comparison between the three topologies. It was shown that mesh topology takes the highest organizing time. Star topology takes the lowest time and is the least complex. In the cluster approach, organization time is dependent on the time taken for clusters initiation. It was stated that for star topology, adding more nodes has a great impact on network's performance. This is due to the increased overhead on the base node, and the increased congestion in areas of high nodes' density. Further more, most nodes have no direct connection to the base node; this increases the network overhead and creates a scalability problem. On the other hand, mesh networks determines routes depending on link quality between neighbours. Theoretically it can be extended to any size, but due to performance limitations it is suitable for medium sized networks; while faces scalability problems with large networks or networks of high density areas. In contrast, nodes in a cluster based network neighbouring are managed by their cluster head node. This allows a rapid and periodic reformation of the network hierarchy in cases of node(s) failure, node addition, and changes due external factors (i.e. environmental changes).

In some applications, it is required to disperse a huge number of sensing nodes in a large area. This may lead to a different nodes' densities distribution within the working area. TheinLai Wong [START_REF] Wong | A Self-organizing Technique for Sensor Placement in Wireless Micro-Sensor Networks[END_REF] proposed an algorithm for reorganizing the initial random nodes distribution to enhance the coverage of the sensing network while minimizing energy consumption by minimizing the amount of movements depending on local information only. In this technique a concept of virtual forces was applied to reorganize the nodes. If the distance between two nodes is less than certain limit, then there will be a repulsive force between them. If the distance is more than certain limit, then there will be an attraction force between them. The node will not move if the distance is equal to the threshold. This mechanism will introduce unneeded movements that will consume more power, so a random "Back-off" delay is initiated at each node at the beginning to minimize this effect. This process is repeated until the network settles.

Sungyun Park [START_REF] Park | Optimized Self Organized Sensor Networks[END_REF] work was based on the clustering concept. The idea is to minimize the number of network's clusters where nodes are participating. Each cluster has a single cluster head, and nodes are only communicating with their cluster head. A clustering algorithm was introduced to merge overlapping clusters and remove redundant cluster heads. A caching strategy was suggested to enhance the algorithm's performance. The algorithm was compared with two other clustering algorithms, Algorithm for Cluster Establishment (ACE), and Self Organizing Sensor network algorithm (SOS). The proposed optimization algorithm showed better results and more stability than the other two algorithms.

B. Power management self-organization

The sensing nodes have a limited energy supply, so lowpower operation is a must. The lifetime of a sensor network is generally limited to the battery lifetime of sensor nodes. Energy consumption occurs in sensing, data processing, and communication. In this part we are concerned with power savings due communication self-organization. Apart of node movements, communication is the most intensive power consuming activity.

Sooksan Panichpapiboon [START_REF] Panichpapiboon | Optimal Transmit Power in Wireless Sensor Networks[END_REF] stated that high power transmissions decreases the node's life time as well as introducing unnecessary interference. It is one of the main goals of the network to preserve connectivity. Using low transmission power may cause simultaneous disconnections in the networks, while using excessive power will increase interference due to sharing of same radio channel. The author considered QoS, with respect to Bit Error Rate (BER), to indicate the presence or absence of a communication link. Generally speaking, it is usually assumed that a communication link is present between two nodes if they are within the transmission range of each other; according to the QoS criteria this may not be true, because a link between two nodes may not satisfy the required QoS constrain. The optimal transmission power was evaluated analytically for a regular topology, and using simulation for random topology. The proposed optimal transmitting power is dependent on the used MAC and routing protocols, but the same concept can be applied to other protocols.

The O-MAC protocol was introduced by Farid Nait-Abdesselam [START_REF] Nait-Abdesselam | O-MAC : An Organized Energy-Aware MAC Protocol for Wireless Sensor Networks[END_REF]. It is specifically designed for WSN. It was inspired from the S-MAC protocol. The aim of the proposed protocol is to minimize power losses due to collisions, overhearing, and idle listening. This leads to a prolonged life time of the network. It is based on CSMA technique to minimize collisions. Nodes within the transmission range who are not concerned with the current transmission can enter a sleeping mode to save their power. Their neighbors are informed with their status in order not to send unnecessary data. The protocol was tested with an OPNET simulation and its performance was compared with S-MAC and IEEE802.11 protocols. The comparison was made for three topologies with respect to the metrics, energy consumption, volume of data collected at the sink(s) node, and the ratio of the data collection to the energy consumption (to indicates the energy efficiency factor). The results showed that O-MAC has a good performance when compared with the other two protocols.

Some WSN localization systems are optimized for indoor applications. Most of these systems provide static anchor nodes and localizing mobile nodes fixed on the object to be observed. This structure helps in estimating the location with high precision. Marcel Baunach [START_REF] Baunach | A Method for Self-Organizing Communication in WSN Based Localization Systems : HashSlot[END_REF] introduced a method adapted for localized environments called "HashSlot". This method is suitable for environments where the working area is well defined. It is a TDMA (Time Division Multiplexing Access) technique that depends on the location of anchor nodes within a grid field to dynamically compute transmission slots. These slots are computed by each anchor node without any interaction or communication with other anchor nodes. This method provides a collision free communication environment between one hop multiple resources and single destination. This method was verified using the "Snow Bat" test bed which uses Ultrasonic sensors for distance measurement with an accuracy of 4mm. It is a reliable system to measure 2D/3D tracking of mobile objects.

V. CONCLUSION Natural systems had proved to be good competent, more reliable, and fault tolerant. These pre-tested natural systems give confidence in acquiring good results when inspiring techniques derived from them. One of their most interesting features is self-organization.

In WSN domain, self-organization and self-configuration are two different terms which are usually used interchangeably. We thoroughly identified them so that they can be used more precisely in the context.

One of the current features of WSN is that solutions tend to be application dependent; leading to different design concepts and approaches. We believe that, although each network layer can have a sole effect on self-organization, a better performance can be achieved if the global view of all layers were considered, so we showed the role of each network layer to acquire self-organization in order to achieve better understanding as well as being able to evaluate different approaches.

VI. FUTURE NEEDS

WSN is a new domain that faces a lot of challenges, and can be used in numerous types of applications which are not completely revealed till now. One of these domains is time critical applications where time of data transfer is a great issue, and they need to be zero tolerant for data loss. This type of applications needs to be considered by researchers.

Moreover, some applications need certain level of fault tolerance and reliability. Current WSN designs are mainly concerned with connectivity and power saving without paying attention for reliability issues.

The aim of most WSN design is to achieve certain goal(s) depending on the required application. This way of design lacks a standard measure of performance. Determining general metrics for performance can be a new challenge for researchers.

Some traditional concepts such as distributed computing can attract the attention for usage in WSN environment. However, such concepts must be retailored to suit this domain


Monica Saucedo
email: monica.saucedo@inserm.fr

Catherine Deneux-Tharaux

Marie-Hélène Bouvier-Colle

Understanding regional differences in maternal mortality: A national casecontrol study in France

Keywords: case-control study, maternal mortality, regional disparities, substandard care. 44

Understanding regional differences in maternal mortality: a national case-control study in France.

Main body of text Introduction

Maternal mortality (MM) remains the principal indicator of maternal health, a simultaneous marker of the quality of and access to care [START_REF] Atrash | Maternal mortality in developed countries: not just a concern of the past[END_REF][START_REF] Berg | Preventability of pregnancy-related deaths: results of a state-wide review[END_REF][START_REF] Graham | Now or never: the case for measuring maternal mortality[END_REF][START_REF] Bouvier-Colle | Confidential enquiries and medical expert committees: a method for evaluating healthcare. The case of Obstetrics[END_REF]. The last report from the French National Confidential Enquiry into Maternal Deaths (ENCMM) covered the 2001-2006 period and showed a global maternal mortality ratio (MMR) of 9.6 deaths per 100,000 live births [START_REF]Report of the National Expert Committee on Maternal Mortality (CNEMM)[END_REF].

Although similar to MMRs in other high-resource countries with an enhanced surveillance system [START_REF] Cantwell | Saving Mothers' Lives: Reviewing maternal deaths to make motherhood safer: 2006-2008. The Eighth Report of the Confidential Enquiries into Maternal Deaths in the United Kingdom[END_REF][START_REF] Schutte | Rise in maternal mortality in the Netherlands[END_REF], this ratio can probably be reduced still further. One approach to this goal is to identify the subgroups of women at increased risk and develop preventive strategies for them.

Results in several countries show an association between some individual characteristics, such as advanced age or foreign nationality or geographic origin, and a higher risk of maternal death [START_REF] Philibert | Can excess maternal mortality among women of foreign nationality be explained by suboptimal obstetric care?[END_REF][START_REF] Stirbu | Differences in avoidable mortality between migrants and the native Dutch in The Netherlands[END_REF][START_REF] Temmerman | Delayed childbearing and maternal mortality[END_REF]. The implications of these findings in terms of prevention nonetheless remain unclear.

Geographic disparities in MM within the same country are potentially informative but have been studied less [START_REF] Saucedo | Regional disparities in maternal mortality in France: specificities of Ile-de-France region and French overseas departments, 2001-2006[END_REF][START_REF] Sullivan | Maternal-fetal medicine specialist density is inversely associated with maternal mortality ratios[END_REF]. Of the 27 administrative French regions, risk of MM is reported to be highest in the Ile-de-France region (Paris and its suburbs) and in the overseas districts (DOM: French Guyana, Reunion, Guadeloupe and Martinique) than in the rest of continental France, and this excess risk persists after standardization for maternal age [START_REF] Saucedo | Regional disparities in maternal mortality in France: specificities of Ile-de-France region and French overseas departments, 2001-2006[END_REF]. These 2 regions account for 42% of the maternal deaths in France (28% in Ile-de-France and 14% in the DOM), although they account for only 26% of live births (22% in Ile-de-France and 4% in the DOM) [Figure 1]. The reasons for this excess MM remain controversial. Such disparities can result from differences in the women's characteristics but also from heterogeneity in the organization and quality of care.

Although policies governing the organization of health care and clinical guidelines are defined at a national level in France, they are implemented regionally. Understanding the mechanisms of geographic disparities may help to design customized regional policies focused on specific subgroups and/or modes of health-service organization.

Our objectives were to test the hypothesis that the risk of postpartum maternal death in France remains significantly higher in the Ile-de-France region and the DOM, taking the individual characteristics of women into account and to determine if the quality of care received by the women who died differed according to their region of delivery.

Methods

This study used a case-control design, with both cases and controls selected from recent nationwide surveys.

Population

Cases: Women who died were selected from the ENCMM [START_REF]Report of the National Expert Committee on Maternal Mortality (CNEMM)[END_REF], conducted in France since 1996 and specifically from those who died during the 2001-2006 period. This permanent survey system follows the International Classification of Diseases in defining maternal death (ICD-10th revision) [START_REF]ICD 10: international statistical classification of diseases and related health problems. 10th revision[END_REF] as "the death of a woman while pregnant or within 42 days of termination of pregnancy, irrespective of the duration and site of the pregnancy, from any cause related to or aggravated by pregnancy or its management but not from accidental or incidental causes." Deaths with any mention of pregnancy or birth or puerperium, or for which the pregnancy tick box is marked, on review of the death certificate's content are selected by the national center of statistics for medical causes of death (CépiDc) and reported to the ENCMM. A team of assessors (an obstetrician and an anaesthetist) conducts a confidential enquiry of each death that occurred in the context of a current or recent pregnancy. The assessors use a standardized detailed medical questionnaire to collect the relevant clinical information related to the woman and her death through interviews and a review of hospital records and autopsy reports. Given the non-participation of some local clinicians, the confidential enquiry is completed and deaths fully documented in only three quarters of maternal deaths identified by the ENCMM. Deaths are then anonymously reviewed by the National Committee of Experts [START_REF]Report of the National Expert Committee on Maternal Mortality (CNEMM)[END_REF], who make a unanimous determination about the underlying cause of death (whether the death is a maternal death, either direct or indirect, according to the ICD definition), its avoidability (certainly, perhaps, or cannot be determined) and the reasons for avoidability (one or more of these reasons: delay in treatment, missed diagnosis, inadequate or insufficient treatment, medical error or patient negligence), and the global quality of medical and obstetric care (not optimal, optimal or cannot be determined) [START_REF]Report of the National Expert Committee on Maternal Mortality (CNEMM)[END_REF]. The surveillance system identified 463 maternal deaths for the 6-year study period considered here. Specifically, this study included the postpartum maternal deaths from that period associated with birth at a gestational age of 22 weeks or more (still-or live births), for consistency with the definition of the controls (see below). To avoid possible referral bias, we excluded women who died outside their region of residence. The study population of women who died during the postpartum period therefore included 328 cases [figure 2]. Controls: The control women came from the 2003 French National Perinatal Survey (NPS), a national representative sample of births in France (n=15,108). NPS are repeated crosssectional studies intended to monitor trends in perinatal health indicators and medical practices. They cover all births (live births and stillbirths) occurring during 1 week in France if they are at a gestational age of 22 weeks or more or weighing at least 500 g. The precise methodology of the 2003 survey has been described elsewhere [START_REF] Blondel | Enquête nationale périnatale 2003. Situation en 2003 et évolution depuis 1998[END_REF][START_REF] Blondel | Trends in perinatal health in metropolitan France between 1995 and 2003: results from the National Perinatal Surveys[END_REF]. Data were collected through interviews with the mother and from medical records. The comparison group for our

Study variables

The primary predictor variable of interest was the region -the region of delivery for the controls and the region of death for the cases. Regions were classified in three groups: the DOM, Ile-de-France, and other continental regions as the reference group. The following socio-demographic variables examined as potential confounders were the mother's age, nationality, work status and marital status. These data were collected from the interviews of control subjects and from the death certificates for the cases.

We collected the following obstetric characteristics for cases and controls: parity, mode of delivery, multiple birth, and variables considered as markers of preexisting morbidity, i.e., hypertensive disorders during pregnancy, hospitalization during pregnancy, induction of labor, emergency cesarean delivery, and preterm delivery. These data came from the medical records for the controls and from the confidential enquiry for the cases reviewed by the National Committee of Experts. Because the proportion of missing data for these clinical variables was so high in the maternal deaths from the DOM (50%, n=21), they were only analyzed for women from continental France.

Analyses

To test the hypothesis of an excess risk of postpartum maternal death among women from Ilede-France and the DOM, we used different multivariate logistic regression models. A first model was adjusted for the relevant socio-demographic characteristics in all women and crude and adjusted odds ratios associated with region were assessed, overall and for cause-specific postpartum MM. A second logistic regression analysis included socio-demographic and relevant obstetric characteristics and was conducted only in women from IDF and the rest of continental France; among obstetrics characteristics, hypertension during pregnancy and induced labor were not included in the multivariate model because of significantly different missing-value rates for cases between regions. The same analysis was conducted in the subgroup of women from continental France with a singleton term delivery (gestational age ≥37 weeks), to eliminate possible residual confounding related to pre-existing morbidity

The last part of the analysis was restricted to the women who died. In this group, we compared quality of care received, avoidability of death and reasons for avoidability, according to geographic region. Again, because of the proportion of missing data for women who died in the DOM, this analysis was limited to the cases from continental France.

Proportions were compared with chi-square tests or, when appropriate, Fisher's exact tests.

The level of statistical significance was .05. Statistical analysis was performed with STATA 10 software (StataCorp., LP, College Station, TX, USA).

Results

Characteristics of cases and controls are shown in Table 1. They differed significantly for the distribution of geographic region: there were more women in the DOM among the cases than controls (12.8% compared with 4.1%). Women in Ile-de-France were also overrepresented among the women who died (29.9% compared with 21.3%, p < 0.001). Compared with controls, cases were significantly older and more often of foreign nationality. Among cases from continental France, clinical information obtained through the enquiry was available for 74.8% (N=214). Compared with controls, cases were more likely to be multiparous, to have been hospitalized during pregnancy and to have had a hypertensive disorder during pregnancy. The proportions of induced labor, emergency cesarean deliveries, and preterm deliveries were all significantly higher among cases than controls (Table 1).

The risk of postpartum maternal death was 4 times higher for women from the DOM and 1.8 times higher for those from Ile-de-France, compared with the rest of continental France. After taking age and nationality into account, the adjusted odds ratio (aOR) was 3.5 (95% CI: 2.4, 5.0) for the DOM and 1.6 (95% CI: 1.2, 2.0) for Ile-de-France (Table 2).

Figure 3 shows the distribution of causes of death among the cases, according to region.

Haemorrhage was the most important cause of MM in both the DOM (40.5%) and Ile-de-France (34.7%), whereas indirect causes were the leading cause in rest of continental France (34.0%). Further analysis of the risk for cause-specific MM associated with region showed, after adjustment for age and nationality, that the risk of mortality from all main causes of direct maternal death was significantly higher in Ile-de-France and the DOM (Table 2). The risk of death from hypertensive disorders and haemorrhage was 5.6 and 6.5 times higher, respectively, in the DOM and 2.7 and 2.3 times higher in Ile-de-France, compared with women in the rest of continental France (Table 2). The risk of maternal death from indirect obstetric causes did not differ significantly by regions.

After adjustment for socio-demographic and obstetric factors (parity, hospitalization during pregnancy and emergency cesarean), women in Ile-de-France had a higher risk of postpartum maternal death (aOR 1.8; 95% CI: 1.3, 2.6) than women in other continental regions (Table 3). The analysis by specific cause of death showed that the risk of postpartum death from haemorrhage was higher in Ile-de-France than in the rest of continental France (aOR 2.2, 95% CI: 1.2, 4.0). After excluding multiple and preterm deliveries, we repeated this analysis and obtained similar results (Table 3).

The National Expert Committee concluded that among the women who died (all causes included) women in the Ile-de-France received non-optimal care (64.8%) more often than those from the rest of continental France (43.4%, p =.01). Similarly, maternal deaths were avoidable more often in Ile-de-France (45.1%) than elsewhere in continental France (35.0%), although this difference was not statistically significant (Table 4).

Moreover, among the avoidable deaths, the reasons differed between Ile-de-France and the rest of continental France. Avoidability was related to "delay in treatment'' more often in Ilede-France (37.5%) than in the other continental regions (26.5%). "Inadequate or insufficient treatment" was the least frequent reason for avoidable maternal deaths in Ile-de-France (6.3%) and the leading reason (28.6%) elsewhere (Table 4). However, these differences were not statistically significant.

Discussion

The risk of postpartum maternal death is clearly higher for women in Ile-de-France and in the DOM (French Guyana, Guadeloupe, Martinique and Ile de la Reunion) than in the other regions of continental France. The excess MM in these regions was especially high for direct obstetric causes, that is, haemorrhages, pregnancy-related hypertension and amniotic fluid embolisms, and, for the DOM, thromboembolisms as well. In addition, we observed differences in the quality of care for the women who died between Ile-de-France and the rest of continental France; unfortunately this analysis could not be performed in the DOM.

These results, suggesting mechanisms of MM that have not been explored until now, must nonetheless be considered cautiously in view of the study's limitations.

The number of maternal deaths is small and generally limits our statistical power. This is one of the reasons that we chose a geographic division into three broad areas. These areas are not homogeneous in terms of demographic, geographic and economic characteristics. Ile de France, the highly urbanized region around the capital, and the DOM located in tropical and subtropical areas, each have a specific profile. The other regions of continental France are diverse, to the point that combining them creates a sort of national average. Nonetheless, the legislation and regulations, especially related to health and health care, are common to the entire country. Clinical information could not be collected for 25% of the potentially postpartum maternal deaths identified by the ENCMM, because the local clinicians did not participate. The only available data were thus those on the death certificate. These deaths therefore could not be included in the analyses involving either the women's obstetric characteristics or the quality of their care. Nonetheless, this would induce bias only if the deaths that could not be investigated differed in nature from the deaths for which information could be collected, or if they were distributed differently between Ile-de-France and the rest of continental France. The distribution by region of uninvestigated cases did not differ from that of the cases that were studied. Moreover, the women's age, and nationality did not differ between the 2 groups, nor did the distribution of the causes of death (results not shown).

Accordingly, the cases studied provide an acceptable sample that accurately reflects all the maternal deaths.

The limited number of individual covariables, in particular socio-economic characteristics, included in the analysis is also a limitation. Nonetheless two important known risk factorsage, which is a primordial factor in terms of risk of death, and nationality -could be considered for all the women [START_REF] Philibert | Can excess maternal mortality among women of foreign nationality be explained by suboptimal obstetric care?[END_REF][START_REF] Stirbu | Differences in avoidable mortality between migrants and the native Dutch in The Netherlands[END_REF][START_REF] Temmerman | Delayed childbearing and maternal mortality[END_REF][START_REF] Callaghan | Pregnancy-related mortality among women aged 35 years and older, United States, 1991-1997[END_REF].

The clinical characteristics (parity, hospitalization during pregnancy, and emergency cesarean) are not especially refined, but they can be considered as a proxy for the mother's health status, during pregnancy and, to some extent, at delivery. Residual confounding cannot be excluded, in particular educational level, income [START_REF] Maine | How do socioeconomic factors affect disparities in maternal mortality?[END_REF], obesity [START_REF] Cevik | Morbid obesity: a risk factor for maternal mortality[END_REF], or inadequacy of prenatal care [START_REF] Harper | Pregnancyrelated death and health care services[END_REF]. The regional environment, in particular the socio-economic context, such as the deprivation index [START_REF] Rey | Ecological association between a deprivation index and mortality in France over the period 1997 -2001: variations with spatial scale, degree of urbanicity, age, gender and cause of death[END_REF], working and commuting conditions, especially transportation to the different health-care facilities where women might be seen according to their health status, have not been studied because this type of information was not available. A different study protocol would be required to take them into account.

We will discuss Ile-de-France and the DOM separately.

The maternal age in Ile-de-France, higher on average than in the rest of France did not explain the excess MM -older women being at higher risk of dying [START_REF] Temmerman | Delayed childbearing and maternal mortality[END_REF][START_REF] Callaghan | Pregnancy-related mortality among women aged 35 years and older, United States, 1991-1997[END_REF][START_REF] Breart | Delayed childbearing[END_REF]-nor did the higher proportion of women from foreign countries, principally sub-Saharan Africa [START_REF] Philibert | Can excess maternal mortality among women of foreign nationality be explained by suboptimal obstetric care?[END_REF][START_REF] Ibison | Maternal mortality in England and Wales 1970-1985: an analysis by country of birth[END_REF], in this region. The persistence of excess postpartum mortality, after adjustment for relevant clinical characteristics, suggests that this is not explained by the prevalence of obstetrical complications; in addition, though the attractive effect of Ile-de-France medicalization does exist, our study population included only women who gave birth in their region of residence to exclude referral bias.

The heterogeneity between regions in the quality of care provided by the healthcare system is another explanation of the regional variations in MM. This hypothesis is especially interesting in that the causes of death for which there is a significant excess risk in Ile de France are direct obstetric causes, in particular postpartum haemorrhages and complications of pregnancy-related hypertension.

The experts' judgment about the quality of care, based on a meticulous reconstruction of each maternal death, shows that suboptimal care was more frequent in Ile-de-France than in the other regions of continental France. This result might seem paradoxical, given the high density in this region of specialized centers offering a very high level of care and especially the significantly higher proportion of level 3 maternity units [START_REF] Blondel | Enquête nationale périnatale 2003. Situation en 2003 et évolution depuis 1998[END_REF]. It appears to contradict the results of a US study that showed that the density of such specialized centers was significantly and inversely associated with the MM rate [START_REF] Sullivan | Maternal-fetal medicine specialist density is inversely associated with maternal mortality ratios[END_REF]. This result must not be immediately interpreted as a demonstration of poor performance by the obstetric care system, it must be considered only as a warning signal of possibly inadequate care.

For a more complete judgment, we would need to know how all severe complications were handled by the system. Only a prospective population-based study of severe maternal morbidity can provide such a judgment. The data from our study about the reasons for suboptimal care provide markers extremely useful in designing such a study. The fact that "delay in intervention" and "missed diagnosis" were more frequent among the maternal deaths considered avoidable in Ile-de-France, where the highest excess risk is for death from complications of hypertension and then from postpartum haemorrhages suggests that a detailed study of the following factors would be useful: the role of interhospital transfers, flaws in the continuity of care, potential work overloads or inadequate staffing, or both, as well as the possibility that patients may be negligent in seeking care or complying with prescriptions or other doctors' orders. Delays in care may be especially important for these causes, for which serious complications could be either prevented or treated more rapidly [START_REF] Driessen | Postpartum haemorrhage resulting from uterine atony after vaginal delivery: factors associated with severity[END_REF].

Insufficient data from the DOM prevented us from advancing far in the analysis of excess maternal deaths, whether related to the women's clinical characteristics or the quality of care.

It is still more regrettable that we were unable to study these aspects for the DOM, for the women in these districts are more often multiparous, have fewer prenatal visits and are hospitalized more often during pregnancy [START_REF] Blondel | Enquête nationale périnatale 2003. Situation en 2003 et évolution depuis 1998[END_REF]. Nonetheless, this first result that maternal mortality excess in the DOM is not explained by maternal age or nationality attracts attention to this population and will help to develop studies focused more directly on the local determinants. Such studies are all the more necessary in that our results are consistent with the results of other studies of reproductive health, which show a poorer health status globally throughout the DOM [START_REF] Bazely | État de santé, offre de soins dans les départements d'Outre-Mer (Guadeloupe, Guyane, Mar tinique, Réunion)[END_REF].

Conclusion

Regional differences in maternal mortality in France are not explained by individual characteristics in this study. Although we cannot exclude the implication of socio-economic factors that were incompletely characterized, this analysis suggests that disparities exist in the

  case-control study thus included women who participated in the 2003 National Perinatal Survey (because it fell in the middle of the 2001-2006 time window for case inclusion) who delivered in their region of residence, for consistency with the definition of the cases. The control sample therefore included 14,878 women [figure 2].

  The ENCMM is funded in part by the Institute for Health Surveillance (InVS) and byINSERM.The 2003 NPS was funded in part by the General Health Directorate (Ministry of Health).

  403DOM, overseas districts; OR, odds ratio; CI, confidence interval.

404a

  Logistic model including maternal age and nationality 405 b Complications of anaesthesia, infections and other complications directly related to pregnancy

Figure 1 .

 1 Figure 1. Map of study sites and corresponding numbers of live births (LB), France 2001-2006.

Figure 2 Selection

 2 Figure 2Selection of cases and controls.

Figure 3 .

 3 Figure 3. Distribution of causes of postpartum maternal deaths, percentage by region.

10.-List of tablesTable 1 .

 1 Distribution of characteristics of women and deliveries among cases and controls.

		Cases	Controls
		(n=328)	(n=14878)	p value a
	Region	328	14878	<0.001
	Ile-de-France	29.9	21.3
	Overseas districts (DOM)	12.8	4.1
	Rest of continental France	57.3	74.6
	Age	328	14687	<0.001
	<25	11.6	19.2
	25-34	47.3	64.7
	35+	41.2	16.1
	Nationality	328	14469	<0.001
	French	80.8	88.0
	Foreign	19.2	12.0
	Marital status	328	14423	0.4
	Married	50.0	52.4
	Not married	50.0	47.6
	Work status	286	14212	0.8
	Yes	60.8	60.1
	No	39.2	39.9
	Only continental France b	(n=214) c	(n=14269)
	Parity	182	14050	<0. 001
	0	21.9	43.3
	1-3	63.2	53.8
	More than 3	14.8	2.9
	Hospitalization during pregnancy	188	13825	<0.001
	No	68.6	81.4
	Yes	31.4	18.6
	Hypertensive disorder during pregnancy	187	14112	0.001
	No	90.4	95.4
	Yes	9.6	4.6
	Induced labor	195	14234	<0.001
	No	38.5	67.8
	Yes	61.5	32.1
	Mode of delivery	209	14230	<0.001
	Vaginal	38.2	80.4
	Caesarean	60.8	19.6
	Emergency caesarean	208	14010	<0.001
	No	47.1	90.2
	Yes	52.9	9.8
	Preterm delivery	198	14204	<0.001
	No	67.2	93.6
	Yes (less than 37 wk)	32.8	6.4
	Multiple birth	211	14269	0.03
	No	96.2	98.2
	Yes	3.8	1.8

a For chi2 test.

b Ile-de-France and the rest of continental regions, c Only postpartum maternal deaths reviewed by the National Committee of Experts

Table 2 .

 2 Overall and cause-specific postpartum maternal mortality associated with region, crude and adjusted odds ratios (controlling for socio-401 demographic factors). 402 Data for the columns for cases and controls are numbers.

	Causes of death	Region	Cases Controls Crude OR 95% CI adjusted OR a 95% CI
	All causes	DOM	42	609	4.1	2.9 -5.7	3.5	2.4 -5.0
		Ile-de-France	98	3166	1.8	1.4 -2.3	1.6	1.2 -2.0
		Rest of continental France	188	11103	1		1
	Haemorrhage	DOM	17		7.6	4.2 -13.4	6.5	3.6 -11.6
		Ile-de-France	34		2.9	1.8-4.6	2.3	1.4 -3.7
		Rest of continental France	41		1		1
	Amniotic fluid embolism	DOM	5		3.1	1.2 -8.1	2.8	1.1 -7.3
		Ile-de-France	21		2.5	1.4 -4.6	2.1	1.2 -3.8
		Rest of continental France	28		1		1
	Thromboembolism	DOM	4		3.8	1.3 -11.3	3.3	1.1-9.8
		Ile-de-France	4		0.7	0.3 -2.2	0.6	0.2 -1.9
		Rest of continental France	20		1		1
	Hypertensive disorders	DOM	4		6.6	2.1 -20.9	5.6	1.7 -17.7
		Ile-de-France	9		2.9	1.2 -6.9	2.7	1.1 -6.8
		Rest of continental France	11		1		1
	Other direct causes b	DOM	8		6.1	2.7 -13.6	5.2	2.3-11.8
		Ile-de-France	16		2.3	1.2 -4.4	1.9	1.0 -3.7
		Rest of continental France	24		1		1
	Indirect causes	DOM	4		1.1	0.4 -3.1	1.0	0.4 -2.8
		Ile-de-France	14		0.8	0.4 -1.4	0.8	0.4 -1.3
		Rest of continental France	64		1		1

Table 4 .

 4 Expert judgment about the quality of care and avoidability of postpartum maternal deaths according to region.

	Ile-de-France	Rest of continental France	p value

a Certainly avoidable or perhaps b Fisher's exact test Data are %, unless otherwise specified.

provision of care and flaws in the organization of the healthcare system. The hypotheses that the application of national clinical guidelines may differ from region to region or that the resources are used or mobilized differently should be explored.

4.-Acknowledgements

We thank the medical assessors of the national confidential enquiry into maternal deaths who collected all the information about maternal deaths; the members of the National Expert Committee on Maternal Mortality; the coordinators of the national perinatal survey of Epidemiological research unit on perinatal health and women's health (INSERM U953) and the Directorate of Research, Studies, Evaluation and Statistics (Ministry of Health).

5.-Disclosure of Interests

The authors have no potential conflicts of interest to disclose.

7.-Details of ethics approval

The Confidential Enquiry into Maternal Deaths (ENCMM) and the National Perinatal Survey (NPS) were approved by The Commission nationale de l'informatique et des libertés (National Data Protection Authority). Table 3. Overall and cause-specific postpartum maternal mortality associated with region in continental France, crude and adjusted odds ratios (controlling for socio-demographic and obstetric factors).

Cases Controls

Causes of death

Rest of continental


Insight of past hydrological conditions in

Holocene fluvial sediments in the Loire River (France) through signatures of stables isotopes Philippe Négrel, Wolfram Kloppmann

Insight into past hydrological conditions through signatures of stables isotopes in

Holocene Loire River fluvial sediments (France)

Philippe Négrel

Wolfram Kloppmann BRGM, Orléans, France

Multi-proxy indices (δ 18 O, δ 13 C, granulometry, mineralogy) in the sediments from a channel infill in the Middle Loire alluvial plain are used to highlight some aspects of the basin evolution over the period from 0 to 10000 yr. BP. The Loire River in central France is 1010 km long and drains an area of 117,800 km 2 . The catchment is composed of plutonic rocks (500 -300 Ma), a large volcanic area in the upstream section and carbonate deposits (200 -6 Ma old) in the central part. During the Late-glacial and Holocene, the lacustrine carbonate substratum in the Middle Loire alluvial plain was incised by the Loire River and numerous oxbows and channels related to meander migration have been identified. We investigated a channel fill that spans part of the Boreal, the Atlantic, the Subboreal and probably a large part of the Subatlantic through sediment coring.

The channel fill consists mainly of fine clayey sediments deposited during flooding of the river; there is a total absence of coarse clastics and sandy materials in the 5m core excepted in the lower part. The distribution of particle sizes shows an up-core decrease with a coarser fraction occurring in the first 2/3 th of the record. Semi-quantitative XRD analysis of the sediment showed that quartz, K-feldspar, plagioclase and calcite are the main mineral components and that the <2µm fraction is dominated by illite-smectite and kaolinite. The distribution of the clays highlights an up-core increase of the illite-smectite fraction.

The historical record of isotope ratio variations together with the distribution of particle sizes allows to constrain the evolution of the river dynamics. The strong decrease of the δ 13 C of carbonates in the upper part of the record has been ascribed to the progressive closure of the meander and thus the control of the C isotope signature by biological activity in a local environment. Furthermore, we can postulate that the variations in the carbonate δ18O are related to the respective contribution of river water and groundwater, being less dependent of the meander evolution than carbon isotope signatures. The isotope record of the river dynamics also agrees with the variations in the clay-mineralogy.


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Lead isotopes in sediments of the Loire River (France): natural versus anthropogenic origin

Philippe Négrel Emmanuelle Petelet-Giraud BRGM, Orléans, France

Sediments along the Loire River (central France) were investigated by means of lead isotopes determined on the labile sediment fraction, or acid-extractable matter (AEM). The combination of trace elements and lead isotopes allows deciphering the origin of the elements (i.e. natural or anthropogenic) and their history, both in the sediment and soil from two small watersheds (one draining basalt, the other one granite-gneiss gneiss in the upper part of the catchment), in present-day suspended matter in Loire River water, and in sediment from the Loire estuary.

Fe-Mn oxides act as the main carrier phase of the elements in the AEM extracted by cold HBr, Th and Pb concentrations were determined by ICP-MS and lead isotopes by ICP-MS-MC Neptune. Thorium displays mostly insoluble behaviour in hydrosystems, but the small amount of dissolved Th shows a strong tendency to be adsorbed onto oxyhydroxides. Therefore, Mn and Th (as well as other trace elements) correlate well in AEM, the correlations of Mn, and Pb with Th as a typical indicator of crustal weathering points to their derivation from the silicate basement of the upstream part of the catchment. Crustal weathering, as confirmed by the Pb-isotope ratios for most sample points, is the main natural source of lead in the upstream part of the Loire River, as well as that transported in the middle part of the basin and up to the estuary. Among anthropogenic sources, the use of Pb-isotopic compositions shows an influence of agricultural lead-derived inputs and a large input from lead as a gasoline additive, particularly in the estuary due to harbour activities and for one downstream tributary river that is strongly marked by this type of lead input.

  Lead isotopes in sediments of the Loire River (France): natural versus anthropogenic origin Philippe Négrel, Emmanuelle Petelet-Giraud To cite this version: Philippe Négrel, Emmanuelle Petelet-Giraud. Lead isotopes in sediments of the Loire River (France): natural versus anthropogenic origin. EGU General Assembly 2012, Apr 2012, Vienna, Austria. pp.XX. hal-00661120