Bogdan Pavković
email: pavkovic@imag.fr

Fabrice Theoleyre
email: theoleyre@unistra.fr

Andrzej Duda
email: andrzej.duda@imag.fr

Multipath Opportunistic RPL Routing over IEEE 802.15.4

Keywords: C.2.1 [Computer-Communication Networks]: [Network Architecture and Design -Wireless Communication] Experimentation, Algorithms, Performance Wireless Sensor Networks, RPL, QoS, ieee 802.15.4, opportunistic routing, multi-path

We consider the problem of running RPL on top of the ieee 802.15.4 MAC layer-the two layers operate over two different structures, a directed acyclic graph in the case of RPL and a cluster-tree for ieee 802.15.4. We propose to adapt the cluster-tree of ieee 802.15.4 so that it can efficiently work coupled with rpl. Nodes in our modified cluster-tree can associate with several parent nodes by taking advantage of an adequate organization of superframes at the MAC layer. Building on this modified MAC layer, we define an opportunistic forwarding scheme that extends rpl with the possibility of forwarding packets over multiple paths. Instead of always using a preferred parent, a node opportunistically forwards packets through other parents as long as their routes towards the sink are better. We take advantage of the opportunistic forwarding to support higherpriority delay-sensitive alarms that need to arrive in sink before a given deadline along with low-intensity monitoring data considered as best-effort. We compare our opportunistic version of RPL to its basic version through detailed simulations in terms of packet delivery ratio, incurred delay, and overhead.

INTRODUCTION

We consider Wireless Sensor Networks (WSN) supporting IP connectivity and running over low-duty ieee 802. 15.4 wireless links. Such networks will lead to the development of the future Internet of Things and enable large deployments of sensors in various domains (smart homes, smart cities, smart grids, environmental sensing, critical infrastructure surveillance, etc.).

IP connectivity in sensor networks mainly relies on two IETF standards: 6LoWPAN [START_REF] Montenegro | Transmission of IPv6 Packets over IEEE[END_REF] and RPL (Routing Protocol for Low power and Lossy Networks) [START_REF] Thubert (ed | RPL: IPv6 Routing Protocol for Low Power and Lossy Networks[END_REF]. 6LoWPAN enables IPv6 networking over low-power wireless networks thus bridging the gap between 802.15.4 and IP in a simple way-it defines a header compression and a fragmentation mechanism for IPv6 to run over IEEE 802.15.4 radio links.

RPL is a Distance Vector routing protocol that builds a DODAG (Destination Oriented Directed Acyclic Graph) anchored at a border router of a sensor network. Each node selects at most three parent nodes. It considers the best parent node (in the sense of some metric) as a preferred one and uses it for packet forwarding to the border router. Alternate parents provide backup routes to the border router making the network robust to unexpected changes in radio connectivity. However, the network structure built by RPL is different from the topology required in multi-hop networks with the 802.15.4 MAC layer-a cluster-tree. This latter topology allows a hierarchical organization in which a node can only select and associate with just one parent node. So, the main problem of running RPL on top of the 802.15.4 MAC layer is to make the two different structures work together.

In this paper, we propose to adapt the cluster-tree of ieee 802. [START_REF] Fraboulet | Worldsens: Development and Prototyping Tools for Application Specific Wireless Sensors Networks[END_REF].4 so that it can efficiently work coupled with rpl. Nodes in our modified cluster-tree can associate with several parent nodes by taking advantage of an adequate organization of superframes at the MAC layer. Building on this modified MAC layer, we define an opportunistic forwarding scheme that extends rpl with the possibility of forwarding packets over multiple paths. Instead of always using a preferred parent, a node opportunistically forwards packets through other parents as long as their routes towards the sink are better. We take advantage of the opportunistic forwarding to support higher-priority delay-sensitive alarms that need to arrive in sink before a given deadline along with low-intensity monitoring data considered as best-effort. We compare our opportunistic version of RPL to its basic version through detailed simulations in terms of packet delivery ratio, incurred delay, and overhead. Our scheme results in improved packet delivery, shorter delays, while keeping almost the same overhead.

RELATED WORK

We briefly review here the work related to IEEE 802.15.4 and RPL.

2.1 IEEE 802.15.4 ieee 802.15.4 defines PHY and MAC layers for wireless sensor networks [3]. In the non-beacon mode, it implements a classical CSMA-CA approach in which a receiver periodically initiates communication (the period is implementation dependent) and the PAN (wireless Personal Area Network) coordinator has to remain awake to receive frames from its children and cannot save energy. This feature increases the overhead and has a significant impact on the end-to-end delay.

ieee 802.15.4 also offers the beacon-enabled mode in which a coordinator periodically sends a beacon to delimit its superframe (cf. Figure 1). The Contention Access Period (CAP) follows the beacon: all associated nodes may send a packet according to a slotted CSMA-CA. The superframe finishes after the Contention Free Period (CFP): nodes reserve dedicated Guaranteed Time Slots (GTS). In the beacon mode, nodes wake-up just before a beacon transmitted by the coordinator to save energy-a node may sleep during the inactive period if BO > SO. We consider the beacon mode in this paper, because it leads to better energy savings. ieee 802.15.4 requires a cluster-tree topology anchored at the PAN coordinator for multihop operation [START_REF] Cuomo | Cluster-Tree Characterization[END_REF]. Once a node associates with a coordinator, it begins to periodically send beacons to maintain its own superframe and control the access to the channel of the associated nodes. Thus, different nodes may have overlapping superframes, which may create collisions between beacons and data frames. To alleviate this problem, two main approaches exist in the literature:

• Beacon-Only Period (BOP): at the beginning of each superframe, nodes reserve a period of several slots with each slot able to contain a beacon [START_REF][END_REF] (cf. Figure 2). Interfering coordinators should choose different BOP slots. This method only solves collisions between beacons: it is only suitable for low-intensity traffic.

• Superframe Scheduling: nodes implement a distributed scheduling algorithm so that interfering coordinators maintain non-overlapping superframes [6][7]. Although such a collision-free scheduling is more complex, it results in better capacity.

An experimental comparison of both techniques [START_REF] Villaverde | Experimental Evaluation of Beacon Scheduling Mechanisms for Multihop IEEE 802.15.4 Wireless Sensor Networks[END_REF] showed that Superframe Scheduling outperforms BOP in terms of the number of delivered packets: if two interfering coordinators do not have overlapping superframes, the number of collisions between data packets and between beacons is reduced. Nevertheless, its parameters (BO) should be carefully set to avoid excessive battery consumption.

Routing in multihop wireless networks

RPL has recently emerged as the standard for routing in wireless sensor networks. It defines a framework for constructing a routing topology and efficiently forwarding packets while saving energy.

The topology is based on a Destination-Oriented Directed Acyclic Graph (DODAG) anchored in the sink (the PAN coordinator in the ieee 802.15.4 jargon). This kind of structure is particularly suitable for convergecast traffic: the sink is the destination for all data packets. Each node maintains its rank towards the sink and includes its value in periodical DIO (DODAG Information Object) messages. This rank represents more or less its depth in the DODAG.

To avoid loops, an objective function determining the rank is defined so that it monotonically decreases toward the sink. A simple way proposed by ROLL for computing the rank is to closely track ETX (Expected Transmission Count). This metric estimates the average number of transmissions required to send a data packet to a neighbor. By summing up the ETX along the path toward the sink, a node may obtain the cumulative ETX, strictly decreasing and consequently forbidding loops. RPL uses trickle timers to adapt the overhead incurred by the maintenance of a DODAG to the topology stability-when it is stable, nodes increase the period for sending routing advertisements.

RPL is currently under extensive implementation and experimentation work so little results are published so far. Since nodes in a wireless sensor network are prone to failures, RPL builds a DODAG with several paths towards the sink so that each node maintains several parents: if the preferred one fails, the node can switch to another one. The existence of several paths may improve resilience and performance [START_REF] Ganesan | Highly-Resilient, Energy-Efficient Multipath Routing in Wireless Sensor Networks[END_REF], help to guarantee QoS [START_REF] Huang | Multiconstrained QoS Multipath Routing in Wireless Sensor Networks[END_REF], or allow to deal with real-time traffic [START_REF] Yahya | An Energy Efficient and QoS Aware Multipath Routing Protocol for Wireless Sensor Networks[END_REF].

Instead of providing backup routes, opportunistic routing dynamically chooses the next hop on a per packet basis [START_REF] Liu | Opportunistic Routing for Wireless Ad Hoc and Sensor Networks: Present and Future Directions[END_REF]. Since the transmission conditions are highly time-varying, a node may estimate the link quality and the best next hop just before the transmission.

To the best of our knowledge, no work so far considered the joint rpl and ieee 802.15.4 operation.

MULTIPATH OPPORTUNISTIC RPL

OVER IEEE 802.15.4

Our objective is to integrate RPL and ieee 802.15.4 to enable QoS multipath routing and improve packet delivery before a deadline, while minimizing overhead and energy consumption. We assume that there are two types of traffic in the network:

1. low-intensity monitoring data that can be considered as best-effort;

2. higher-priority delay-sensitive alarms that need to arrive in sink before a given deadline.

We want to provide support for such service differentiation over RPL and multi-hop wireless network by taking advantage of multiple paths.

The first problem to consider is the tree structure of ieee 802.15.4: in the current standard, a node can only associate with one parent to avoid loops. Indeed, all the nodes at a given depth in a tree maintain their superframe simultaneously. To take advantage of several parents in the DODAG structure, we need to allow a node to associate with several parents in the ieee 802.15.4 structure (we consider this problem below in Section 3.1). Then, we propose an extension to RPL for using several parents in an opportunistic way-as soon as a node can send a frame to a parent in a synchronized way, it will do it, which increases the probability of packet delivery and reduces delay (cf. Section 3.2).

Support for Multiple Paths in IEEE 802.15.4

A node joining an ieee 802.15.4 cluster-tree needs to associate with a neighbor coordinator. It may implement an active scan method: it sends a solicitation to retrieve the information of the cluster-tree from listening neighbors. This method only works when nodes never sleep, which is not the case of the applications we are targeting. In a passive scan, a node waits for beacons from a neighbor coordinator that define a superframe even if it is not yet associated with the cluster-tree. Then, an association-request / ack / datarequest / association-reply / ack exchange is required to finalize the association. Thus, a node can associate with one parent during a given superframe. In the original ieee 802.15.4, an incoming superframe directly follows an outgoing superframe (i.e. the superframes of a node and its children are immediately consecutive). To avoid loops, a node should only associate with a parent closer to the PAN coordinator. Thus, there is no way of maintaining several parents in the original ieee 802.15.4.

To implement a DAG structure maintaining several parents, we have to appropriately schedule superframes: a node should be able to receive beacons from different neighbors during non-overlapping superframes. We assume that all the nodes have the same BO and SO values (duration and interspacing between superframes). Consequently, a schedule of superframes corresponds to a TDMA assignment with timeslots of a fixed length. We propose to extend the algorithm proposed by Muthukumaran et al. [START_REF] Panneer Selvan Muthukumaran | MeshMAC: Enabling Mesh Networking over IEEE802.15.4 Through Distributed Beacon Scheduling[END_REF] with a random assignment of slots to interfering nodes in a given neighborhood.

Detailed Schedule Construction

We assume that only 1-hop and 2-hop neighbors may interfere. To avoid collisions, we have to schedule superframes so that no pair of interfering coordinators use the same slot. The number of these slots depends on the BO and SO pa-rameters: BO is the repetition period of superframes during which we can place at most N slots superframes:

N slots = 2 BO-SO (1)
We propose a simple algorithm that each node executes to find a valid slot to transmit its superframes:

1. Each node retrieves the slots used by its 2-neighbors (i.e. interfering coordinators). A node considers only nodes with at least one child since other coordinators maintain an empty superframe and do not create collisions for data frames;

2. A node randomly selects one free slot.

While Muthukumaran et al. schedule the slots to be immediately consecutive, we rather chose a random assignment approach for the following reasons:

• it presents a trade-off to reduce the delay for both upload and download directions (Muthukumaran et al.

focus only on the upload case);

• it accelerates convergence: two interfering nodes have little chance to choose the same slot when they have the same parent.

The resulting structure has an interesting property: a node can access the channel during several superframes coordinated by different parent neighbors, which provides the basis for multipath forwarding supported by RPL.

Distributed version

To obtain the list of interfering nodes, each node maintains a neighborhood table. It piggybacks in its beacons the list of neighbors and the slots they have already chosen. Thus, a node has just to select one free slot to avoid collisions. This procedure can be safely integrated in the association procedure of ieee 802.15.4: a new coordinator chooses its slot just after having received its association-reply scheduling immediately its future superframes.

As long as a node does not have children, it may safely change its slot without impacting the MAC performance: there is no domino effect. Besides, colliding beacons often avoid children to associate with interfering coordinators. In other words, a coordinator without any child means perhaps that its slot collides with another one. Thus, a coordinator reapplies the assignment rule to choose its slot at the end of each of its superframes, except for an association-request during the current superframe (i.e. at least one child is associated).

Example

Figure 3 illustrates the principle of slot assignment. We assume here for the sake of simplicity that the interfering range is twice the radio range so that B and E cannot choose the same superframe. If we consider that each superframe slot is a different color, we face a classical graph coloring problem. The reader can notice that the number of slots in this example should be at least 4 since B, C, D, and E interfere with each other.

Let us focus on node E assuming that all other coordinators have already chosen a slot for their superframes. E will collect hellos from C and D, and will be able to construct the list of its 2-neighbors (interfering nodes). Finally, E has just to randomly select one slot not yet present in this list. In our case, it will select slot 0 since it is the only one remaining free.

Deadline Oriented Opportunistic RPL over IEEE 802.15.4

As we aim at service differentiation of best-effort and timesensitive traffic, we propose to adapt RPL so that it can take into account delay and packet delivery before a deadline. We only consider convergecast (multipoint-to-point) traffic.

We consider three classes of service:

min-delay: critical packets for which we need to minimize the end-to-end delay without concerns for energy consumption deadline: alarm packets to deliver before deadline D while minimizing energy consumption best effort: packets that do not require any guarantee, but their forwarding needs to take into account energy consumption.

Notation

We adopt the following notation:

• N : a node that forwards a packet

• deadline(p): deadline associated with packet p

• t: current time

• d(N ): hop distance between node N and the sink

• slot(t): current superframe slot (node N has just received a beacon in this slot so it will be able to forward it)

• slot(N H): slot used by the superframe of node N H

• P DR bcn (N H): beacon packet delivery ratio for neighbor N H

• P DR data (N H): data packet delivery ratio over the link to neighbor N H;

• ttx data+ACK : time needed for data and acknowledgement frame transmissions

• queue(N H): queue of packets scheduled for transmission during the superframe of N H

• ST EP : a constant to extend the local time budget if no parent can satisfy it at the first attempt

Taking into account deadlines with RPL

When a node generates a packet, it assigns a deadline according to the class of service it belongs to. Nodes maintain a queue of packets ordered by their deadlines. When a node has a packet to forward, it waits for a successful beacon reception from one of its parents. Then, it needs to decide to transmit its packets during the current superframe or later if another parent offers better performance (e.g. smaller energy consumption, better reliability).

The node extracts the first packet from its queue: if the deadline is elapsed, the packet is simply dropped and the next packet is extracted. Then, it must find the next hop that guarantees the deadline. It assumes the time before the deadline can be uniformly shared among the nodes in the route. Thus, the transmission has to meet the local time budget constraint:

budget = deadline(p) -t d(N ) (2)
When a packet is at node N , the delay before the packet is correctly transmitted to the next hop N H depends on:

1. the delay until the superframe of NH starts while taking into account the average number of superframes to wait in case of beacon losses:

D sf rame = SD * | slot(N H) -slot(t) | + BI * max 0, 1 P DR bcn (N H) -1 (3)
where SD denotes the superframe duration while BI represents the time separating two beacons. For the currently received beacon, this delay is zero, since the node can immediately try to send the packet.

2. the average delay until N H correctly receives the packet, it is estimated via the packet probability delivery ratio:

Dtx = ttx data+ACK P DR data (N H) (4)
Finally, nodes need to satisfy the following deadline constraint:

budget ≥ D sf rame + Dtx (5)
As several candidate next hop nodes may satisfy the deadline constraint, the protocol will choose the best one based on the cumulative Expected Transmission Count (ETX) routing metric: it represents the cumulative number of packet transmissions required to reach the sink, also used as the node rank in the DODAG. It should be noted that per link ETX is simply calculated as inverse value of measured P DR data (N H) (link to neighbor N H). A node chooses among all the possible next hops the node with the lowest cumulative ETX so packets will experience the smallest delay.

If a node cannot satisfy the budget constraint, it reconsiders all the parents for an increased time budget hoping that the packet will benefit from shorter delay further in the network. We gradually extend the budget up to two times the initial value given by Eq. 2 ( ST EP and relax parameters in Algorithm 1)

PERFORMANCE EVALUATION

We have compared our opportunistic version of RPL to its basic version in terms of packet delivery ratio, incurred delay, and overhead through detailed simulations. Both protocols take advantage of the 802.15.4 superframe scheduling so that even the basic version of rpl can dynamically adapt the choice of the preferred parent node in function of performance.

For the sake of simplicity, we implemented a centralized coloring solution to assign slots for each superframe as described in Section 3.1.1. However, we may use any scheduling algorithm creating a Directed Acyclic Graph such that the distributed version described in Section 3.1.

Simulation setup

We have used the WSNet/Worldsens event-driven simulator for large scale wireless sensor networks [START_REF] Fraboulet | Worldsens: Development and Prototyping Tools for Application Specific Wireless Sensors Networks[END_REF]. We have ported the Contiki RPL implementation [START_REF] Ko | ContikiRPL and TinyRPL: Happy Together[END_REF] to WS-Net. We used the ieee 802.15.4 implementation in baconenabled mode [START_REF] Abddedaim | Implementation of a WSNet Module to Simulate the IEEE 802.15.4 Beacon-Enabled Mode in Multihop Topologies[END_REF]. We have additionally implemented the superframe scheduling mechanism.

The simulations have considered 10 different topologies and randomly deployed up to 256 nodes in a square area 400 x 400 m. To make the simulations as close as possible to the reality, we have not adopted the Unit Disk Graph assumptions commonly used in the literature, but rather the Rayleigh propagation model and the parameters of the ieee 802.15.4 radio.

We have only considered low intensity traffic with the av- erage interval between data of 7.5 minutes. We have empirically established this value to avoid performance degradation of ieee 802.15.4 under heavy traffic. We divide the traffic into three classes with the respective proportions: 70%-20%-10% (best-effort, min-delay, deadline). We vary the SO parameter from 3 to 5 and choose the BO parameter so that the number of superframe slots is sufficient to avoid colliding superframes (Eq. 1). We run a simulation for each of the topologies during 50.000 s and average the results over multiple runs to obtain 95% confidence intervals. Table 1 summarizes the important simulation parameters. Both protocols generate the same fixed amount of application data packets and none of them is destroyed before the end of the simulation. We can notice that our opportunistic solution results in a slightly greater number of transmitted packets (9%). This increase may come from better performance: since less packets are dropped, this mechanically results in more transmissions at the MAC layer. A larger overhead also comes from the forwarding rule: if the deadline is short, the node will privilege the forwarding delay compared to minimizing the number of transmissions (i.e. ETX). This aggressive decision would privilege short deadlines, but also negatively impacts the number of transmissions.

Result analysis

Figure 4(b) presents the packet delivery ratio for all packet types. As soon as the deadline becomes more critical, the fact that we use alternative parents results in a higher PDR. Less packets get dropped because of the short packet deadline, since the fact that we have missed a beacon from our preferred parent could still be caught up with the reception of beacons from some alternative parents. In the same situation, the basic rpl version needs to wait for the whole inter-beacon period (BI) and thus there is a risk that the short packet deadline incurs more packet drops. We have considered above the performance of the tested protocols from the global point of view. Let us analyze performance with respect to the QoS delay requirements of min-delay and deadline data packet types. We can notice a similar behavior as previously for both types of traffic when it comes to PDR (Figures 5(a) and 6(a)) and for the experienced delay (Figures 5(b) and 6(b)). If we consider delay, it is clear that our opportunistic scheme exhibits much shorter delay than the basic rpl version due to the possibility of interchangeably using alternative parents. With respect to the PDR performance, our opportunistic scheme presents a real gain when we deal with packets with harsh deadline constraints.

Finally, we can notice an interesting property of our opportunistic approach that directly comes from its forwarding policy even if we do not show it in the numerical results. Our opportunistic scheme spreads traffic over parents by not only selecting the preferred parent as the next hop, but the alternative ones as well. In realistic scenarios with a lim-ited battery capacity and limited queue lengths, this may appear as the primary concern since it would increase the overall network lifetime and prevent packet drops due to full queues. We plan to include this kind of realistic constraints in the future work.

CONCLUSION AND PERSPECTIVES

We have presented a scheme allowing the coexistence of two structures in emerging IP enabled wireless sensor networks: rpl routing and ieee 802.15.4 MAC. We have modified the cluster-tree operation of ieee 802.15.4 to support RPL DODAG so that nodes are able to follow multiple parents and use them for traffic forwarding when needed. Our simple opportunistic routing scheme benefits from an interesting feature: traffic is spread more uniformly over all possible parents instead of going through the preferred one. It results in improving the network lifetime and optimizing fairness to avoid quick energy depletion. Our solution achieves slightly better results with respect to end-to-end packet reliability (PDR) and delay while keeping almost the same amount of generated traffic.

In the future, we plan to validate the proposed scheme on an experimental testbed and take into account additional constraints of limited packet buffers. We also want to explore how reliability can improve with the use of more advanced multi-path routing schemes or by adapting the parameters of ieee 802.15.4 to accommodate higher traffic loads.

Figure 1 :

 1 Figure 1: The superframe structure in ieee 802.15.4

Figure 2 :

 2 Figure 2: Beacon-Only-Period superframe organization

Figure 3 :

 3 Figure 3: Scheduling of 802.15.4 superframes

  = 360s RPL Basic deadline = 360s RPL Opportunistic deadline = 180s RPL Basic deadline = 180s (b) PDR for all packet types

Figure 4 :

 4 Figure 4: General comparison of basic and opportunistic RPL

Figure 4 (

 4 Figure4(a) presents the total number of transmitted packets for the basic and opportunistic rpl. We measure the raw number of packets transmitted by the MAC layer, i.e. a data frame transmitted for the first time or retransmitted after a failure. The ieee 802.15.4 MAC layer drops a frame when the number of retransmissions exceeds 3 or the number of Clear Chanel Assessments exceeds 4. Packet are also dropped if the deadline is missed. At the end of the simulation, these values are summed up for all generated packets.

Figure 5 :

 5 Figure 5: Comparison of basic and opportunistic RPL, deadline traffic type

  = 360s RPL Basic deadline = 360s RPL Opportunistic deadline = 180s RPL Basic deadline = 180s (b) delay

Figure 6 :

 6 Figure 6: Comparison of basic and opportunistic RPL, min-delay traffic type

  2.

	Algorithm 1: Does a packet has to be transmitted in the
	current superframe?
	1: src ← waitBeacon();
	2: nexthopcandidate ← ∅
	3: if empty(queue) or end(superframe, src) then
	4:	return false
	5: else
	6:	repeat
	7:	p ← getFirstPacket(queue);
	8:	if (p.deadline ≤ t) then
	9:	DropPacket(p);
	10:	end if
	11:	until (p.deadline > t)
	12:	budget ← computeHopBudget(p, src.hops + 1);
	13:	relax ← 0;
	14:	while (nexthopcandidate = ∅) and (relax < 2 * bud-
		get) do
	15:	for neigh NH do
	16:	D sf rame		←	computeDelaySuper-
		frame(NH.sframe, NH.pdr);
	17:	Dtx	←	computeExpectedTransmission-
		Time(NH.pdr);
	18:	if (budget + relax > D sf rame + Dtx) then
	19:	nexthopcandidate ← nexthopcandidate +
		{N H}
	20:	end if
	21:	end for
	22:	relax += budget * STEP;
	23:	end while
	24:	if (src = getBestETX(nexthopcandidate)) then
	25:	return true
	26:	else
	27:	return false
	28:	end if
	29: end if

Table 1 :

 1 Simulation parameters

	Simulated area	400m x 400m
	Number of nodes	up to 256
	Traffic type, rate	periodic, 1/7.5 minutes
	Simulation duration	50000 s
	SO	3, 4, 5
	Deadline	360s, 180s

ACKNOWLEDGEMENT

Authors would like to thank Nazim Abdeddaim for providing his source code implementation of the ieee 802.15.4 protocol. His comments were of great help as well while adapting the code to work with rpl.

This work was partially supported by the French National Research Agency (ANR) project ARESA2 under contract ARESA2 ANR-09-VERS-017 and the Competitive Clusters Minalogic and System@tic under contracts FUI SensCity and SAMEE.


Pierre Bataille
email: pierre.bataille@unil.ch

What happened to the queens ? Normaliens' career styles at the end of the XXe century in France

The ENS within the French field of "grandes écoles" : an ambiguous position Three Queens at the crossroad of three fields

The Ecoles Normales Supérieures (ENS) are part of the more prestigious French "grandes écoles". Like the Ecole Polytechnique, the ENS have been created after the French Revolution. Like the others "grandes écoles" -and unlike French universities -admission to these institutions is subject to success in a competitive examination (a concours). This system of selection is suppose to select on "meritocratic" basis the best of the student to open them the gate of the national elite. The initial purpose of the ENS was to select and train future teachers and future administrators of the French school system. The more renowned, the ENS of Ulm, was definitely created in 1808 and, initially, used to train future teachers for the secondary education. The two other ones, the ENS of Saint-Cloud et Fontenay and the ENS of Cachan, were created more recently, between the end of the 19th century and the beginning of the first world war. These two "little sisters" of the ENS of Ulm, were also originally created to provide teachers for the relatively less prestigious sectors of the French school system (the "primary" education and the "technical" education).

But, as Diane Rubenstein noted (Rubenstein, 1990, p. 6), during the XIXe century until the first half of the XXe century, if the manifest function of the ENS was to train future teachers, the "latent" function of the ENS seems have been to provide future eminent agents of the intellectual and the academic field. For instance, even if the normaliens (i.e the graduates of the ENS) have been more and more numerous to choose an academic career since the creation of the first ENS, they have been formally constrained to succeed at at least one of the examinations for being teacher (the agregation or the capes) until 1962 for the normaliens of Ulm -and until 1985 for the normaliens of the others ENS. The tension between this two social function of the ENS ( i.e selecting and training future teachers or being a breeding ground of future intellectuals or scientists) has been well documented by several academic work, for the ENS of Ulm [START_REF] Bourdieu | Homo Academicus[END_REF][START_REF] Bourdieu | The state nobility : Elite schools in the field of power[END_REF][START_REF] Sirinelli | Génération intellectuelle : khâgneux et normaliens dans l'entre-deux-guerres[END_REF][START_REF] Sirinelli | École normale supérieure : Le livre du bicentenaire[END_REF] like for the others ones, especially the one of Fontenay and Saint-Cloud [START_REF] Barbé | Des Normaliens[END_REF].

The ENS of Ulm are thus often describe as "the queen" of academic system (Rubenstein, 1990, p. 6) when the ones of Fontenay et Saint Cloud are seen as the "coronation" of the meritocratic system, who give the opportunity to the best student to become teacher or manager of the educational system [START_REF] Barbé | Des Normaliens[END_REF]. Looking at the wide range of careers followed by the normaliens (from secondary teacher to President), one of the specificity of the three ENS within the field of grandes ecoles is also to produce a less specialized elite than many other institutions like the Ecole Polytechnique or the Ecole nationale d'administration (ENA) or the Haute école de commerce (HEC). In this sense, using the words of G. Pompidou, the ENS are more an "open gate" for several pathway (including elite pathway) than an a back-room of political or economical power. More precisely and from a relational analysis point of view, one can say that within the French context, the ENS are traditionally situated at the crossroad of three field : the academic field ; the intellectual field and the field of power. FFollowing these remarks, we assume that analyzing the career pathway of ENS graduates is a heuristic way to analyze the evolution of the interactions between these three field within the French context.

After the 1960's : The ENS at the crossroad of their own history

Maybe because they symbolize the excellence à la française and because they have been an incubator of many important public personalities during the decades after the WWII (like Sartre, Foucault, Derrida or Bourdieu), the ENS have been at the core of several researches until the end of the 80's. Nevertheless, one must note that since the 80's and the works of Bourdieu [START_REF] Bourdieu | The state nobility : Elite schools in the field of power[END_REF], there has been a relative "overshadowing" of the ENS within the scientific production. Indeed, since twenty years, the most studied "grandes écoles" have been undoubtedly the ENA or Sciences-Po Paris [START_REF] Bauer | L'ENA est-elle une business school ? Etudes sociologiques sur les Enarques devenus cadres d'entreprise de 1960 à[END_REF][START_REF] Bellier | Les élèves entre eux : une tribu, un corps, un esprit ?[END_REF][START_REF] Biland | La fabrique de la pensée d'État[END_REF][START_REF] Eymeri | La fabrique des énarques[END_REF][START_REF] Oger | Le façonnage des élites de la République[END_REF][START_REF] Pasquali | Passer les frontières sociales : Comment les "filières d"élite' entrouvrent leurs portes[END_REF] and, more recently, HEC and the others French business schools [START_REF] Abraham | Du souci scolaire au sérieux managérial, ou comment devenir un "HEC[END_REF][START_REF] Blanchard | From "Ecoles Supérieures de Commerce" to "Management Schools" : transformations and continuity in French business schools[END_REF][START_REF] Blanchard | Les écoles supérieures de commerce, du début des années 1960 au début des années 1990[END_REF][START_REF] Lambert | Le comblement inachevé des écarts sociaux. Trajectoire et devenir professionnels des élèves boursiers d'HEC et de l'ESSEC[END_REF][START_REF] Lazuech | Le processus d'internationalisation des grFsocialandes écoles françaises[END_REF]. Some researches have also been done around the engineers school, specially on the feminization of these institutions [START_REF] Marry | Les femmes ingénieurs, une révolution respectueuse[END_REF].

How interpret this lack of interest -compared to the relative abundance of research on these institutions twenty years ago ? Following J.-C. Passeron, one can argue that sociological production is always embedded in a particular socio-historical context [START_REF] Passeron | Sociological reasoning[END_REF]. Then, one of the main hypotheses could be that this lack of sociological interest is correlative to a weakening of the position of these institutions within the field of grandes écoles in France. Indeed the 80's then the 90's and the 00's were decades where the ENA, Sciences-Po and the biggest French business school (as HEC) did occupy a more and more central place in the public debate and in the recruitment of future elite. This trends have been already shown by P. Bourdieu at the end of the 70's (Bourdieu, 1998, pp. 197-214). Nevertheless, the example of the dynamic of introduction of "positive discrimination" policies by most of the grandes écoles since the end of the 90's is one of the best illustration of the dominant position of Sciences-Po and the most prestigious business schools reached within the field of the grandes écoles -and corelatively, within the field of power in France. Indeed, since the end of the 90's, the requirements inequalities of the grandes écoles -and especially the ways to struggle against these inequalities through the promotion of positive discrimination policies -has been at the core of public debate on grandes écoles [START_REF] Pasquali | Des boursiers dans les grandes écoles ? Contretemps.net[END_REF][START_REF] Van Zanten | L'ouverture sociale des grandes écoles : diversification des élites ou renouveau des politiques publiques d'éducation ?[END_REF]. Sciences-Po and the ESSEC (one of the most prestigious French business school) has been the two first grandes écoles who did implement these policies. Such policies have been thereafter adopted and imitated by most of the other grandes écoles during the 00's -notably some of the ENS since 2010. Reading this generalization from a neo-institutionalist theoretical framework, these two institutions appear undoubtedly as the leaders to follow within this particular social space [START_REF] Buisson-Fenet | Réputation, mimétisme et concurrence : Ce que "l'ouverture sociale" fait aux grandes écoles[END_REF]. After have been the matrix of the recruitment of future French elite (and been imitated by the ENA or the business schools [START_REF] Bourdieu | The state nobility : Elite schools in the field of power[END_REF]), the ENS didn't seems to be anymore one of the leading institutions of the field.

Such review of disposable scientific literature may lead to the conclusion that the queens are not as powerful as they used to be. Nevertheless, there is no analysis of the concrete implication of these changes on the ENS and their graduates. Indeed, has we already mention it, very little is known on this population since the 80's and the only sources of sociological information on the recruitment and the professional career of ENS' graduates are exploratory researches [START_REF] Baudelot | Que deviennent les normaliens à la sortie de l'école ?[END_REF][START_REF] Pavis | Les anciens élèves des sections économiques de l'ENSET/ENS Cachan : quel devenir professionnel ?[END_REF].

Following these remarks, this paper will follow a twofold purpose. Our first aim will be to document what the ENS became since the 1960's. We will thus try to define their position within the contemporary field of the grandes écoles through the analysis of the professional careers of the normaliens. As the ENS are also at the crossroad of three field (the academic field, the intellectual field and the field of power), our second aim will be to analyse, through the case of the ENS, part of the changes that affect these three field during the last three decades. Our main research question will be the following ones : Are the ENS still a possible path to the field of power ? Under which condition ENS graduates can reach dominant position within the French field of power ? What does these conditions indicate about the interaction between the three fields in which the ENS are committed within the French context ?

Conference paper

Data & Methods

Data and sample

In our analyses, we use two dataset.

The first one (D1) is based on the cross exploitation of the yearbooks of the three ENS for the Classes of 1960 to 1995 and of a national database that record all the thesis defended in France between 1970 and2000 (DOCTHESE). Within the 159675 thesis registered in DOCTHESE, we find 4618 normaliens (1703 from Ulm-Sevres, 1600 from Saint-Cloud & Fontenay and 1255 from Cachan). Even if informations compiled in this base are relatively poor (only the class, the sex, the disciplinary field of specialization), it give us some of the major trends within the three ENS concerning the normaliens' higher education achievement during this period.

The second one (D2) is based on the exploitation of a questionnaire survey (mail and letter) sent to the 1600 people of ENS of Fontenay & Saint Cloud Classes of 1981[START_REF] Bataille | Quand le genre rattrape l'excellence. Une comparaison des parcours de formation et d'insertion des normaliens et normaliennes[END_REF], between February 2006and March 2013. The choice of these particular subgoup of ENS was at the beginning due to practical reasons. In 2005 we has the opportunity to lead a survey amongst these ENS alumni on the behalf of the direction. Because studying elites is made difficult because of the access to sources [START_REF] Joly | Les élites politiques : regard croisé sur le cas français[END_REF], such context are needed to launch an empirical research -especially when the researcher isn't an alumnus of the institution, like us. Nevertheless the choice of these particular ENS is also relevant for more scientific reasons. The ENS of Saint-Cloud and Fontenay-aux-Roses are situated on an intermediary position in term of prestige and recruitment between the one of Ulm (the oldest and the most prestigious ENS) and the one of Cachan (the more recent of the three ENS). Because of this median position, they are an interesting case to analyze the multiple facets of the ENS. Further more, one must say that according to P. Bourdieu analysis, differences between the three ENS in term of recruitment and looking at the career of their alumni became more and more thin since the 1970's and the 1980's. [START_REF] Bourdieu | The state nobility : Elite schools in the field of power[END_REF].

Two major reasons shaped the choice of the Classes. The first one is an historical one. 1981 and 1987 present two major changes in the story of these institutions. 1981 is the introduction of the coeducation at the ENS of Saint-Cloud and Fontenay. The main incidence of this institutional change has been the increasing of the under-representation of women in the scientific pathway. 1987 is the year when scientific and humanities students of the ENS where divided in two subgroup, each one into one specific school. One (humanities) stay staged in Paris (ENS de Saint-Cloud-Fontenay), the other (scientific) migrated to Lyon. In this « new » scientific school, alumni recruited after 1987 have been more than before directed to academic and applied research. So the period between 1981 and 1987 did seem to be relatively homogenous on the institutional level.

Closer to our research questions, we secondly decided to focus on the alumni of 1981-87, in order to analyze their life course paths up until 'occupational maturity' (Erikson and Goldthorpe 1992) and analyze what do they become 20 years after they did successfully access to the ENS.

418 alumni forward us our questionnaire. The response rate could seem relatively low (30%), but this can be nuanced on two perspectives. This response rate is similar to other survey on similar population, like in the case of the survey on Harvard graduates from the 80's conducted by Claudia Goldin and Lawrence F. Katz [START_REF] Goldin | Transitions : Career and family life cycles of the educational elite[END_REF], where the rate was 34%. Moreover, we compute the differences between our respondent sub-population and the whole population of the alumni from the class of 1981-1987 with three control variables : the sex of the respondent, their curriculum (science or humanities), their social background and their class. According to these three control variables, using a multivariate analysis, there is no significant differences at the level of 10% between our respondent sub-population and the whole population of the alumni from the class of 1981-1987.

Data collected give broad information on the family (grandparents, parents and siblings) and on the educational background of the alumni. They also give information on the type of educational achievement they reached after they accessed the ENS, their professional career (first and actual job), their marital status, the professional and educational status of their partners and the children they may have.

Methods & Variables

As mentioned, data from D1 has been used to draw the trends in terms of doctoral achievement amongst all ENS alumni between 1960 and 1990.

Data from D2 has been used in a more relational approach. As P. Bourdieu analysed, field's effects are generally transposed at each level of analysis within all the institutions committed in the same field (Bourdieu, 1998, pp. 142-152). To shine a light on the balance of power in which ENS are committed, we tried to build Conference paper and analyse the space of the "careers' styles" of the normaliens. To do so, we did an MCA with two set of variables relevant to analyse these different kinds of "careers' styles"

MCA analyze and variables

To sum up, there is two main goal to our MCA : drawing the space of the "careers' style" of the normaliens on the one hand ; looking how theses different "careers style" are located within the social space of the ENS alumni on the other hand. According to these goals, our variable can be subdivided in two groups : the "achievement variables" and the "background variables". All of them are categorical.

To compute the MCA, we used the "achievement" variable as "active variable". Once the space of these "carreers' style" drawn, we used the "background variable" as inactive and illustrative variable, in order to locate where are situated graduates within this space according to these variables (for more info, see Appendix).

"Achievement variables"

Our "achievement variables" can be grouped as follow : the educational achievement variables and the professional achievement variable.

The main educational achievement variables are the level of education reached by the alumni and the fact that they successfully completed a secondary teacher recruitment competition (especially the Agregation). Note that the ENS (and the other grandes écoles) lead their alumni to the level of Master. After that, they are free to continue to get a PhD. We add a third one that concerns the additional training that alumni followed in one other grandes écoles. Potentially open to all of the alumni, this possibility was chosen by a few of them. The other grandes écoles chosen where most of the time Sciences-Po or the ENA for the humanities alumni and the Ecole Polytechnique for the scientist. All of these educational achievement variables are dichotomous : obtain a PhD or not for the first one, obtain an Agregation or not, obtain an additional training or not. We also introduce two variables for the class of the alumni and the disciplinary field of specialization. Even these variable are not strictly link with "achievement", these variables give information on the educational trajectories of the graduated.

The main "professional achievement" variable is the professional status reached by the alumni. To evaluate the prestige associate to the professional status reached, we also introduce a variable on the work place of our interviewees. Indeed, the biggest cities -and above all Paris -are in France the main centers of "cultural values" (Bourdieu, 1984, p. 363). And the proximity to such centre is a huge factor of distinction among French social space. So we use the status of the city where they work (categorized by the fact of being a place near from Paris and being a more or less small place in term on inhabitant) as an indicator of the prestige.

"Background Variables"

The "background variables" are the sex and a generational indicator of the familial cultural capital.

As several authors already noted, operationalizing the bourdieusian concept of « cultural capital » is always difficult [START_REF] Lareau | Cultural capital : Allusions, gaps and glissandos in recent theoretical developments[END_REF]. Following the work of P. Bourdieu, part of the effectiveness and performance of the cultural capital depends on time and familiarity that people benefit during its acquisition [START_REF] Bourdieu | The forms of capital[END_REF]. This familiarity distinguishes the "title" and the "quartier" of cultural nobility [START_REF] Bourdieu | Distinction : A social critique of the judgement of taste[END_REF], so the ways of acquisition of the cultural capital (school or family) and its character more ore less innate (i.e. more ore less powerful). From this idea, we decide to build an indicator of the oldness of the familial cultural capital based on the professional and educational achievement of the father and the grandfathers, following the typology already used in similar case in research on French educational elites [START_REF] Ferrand | L'excellence scolaire : une affaire de famille[END_REF][START_REF] Bras | Les origines d'une promotion de Polytechniciens[END_REF]. We use a three-letter code (e.g. : aaa) in which the first letter represent the professional status of the father, the second one the professional status of the maternal grandfather and the third one, the one of the paternal grandfather (see Figure 1). We didn't take into account the professional status of the women (the mother and the two grandmother), as Ferrand et al. did (1999), because it didn't change the typology obtained with the professional status of the men. We noted "a" when the father was being part of the low class and the low middle class (manual worker either agricultural or industrial, craftsmen or small traders, employees, teacher at the primary level) when the respondent acceded to the ENS. We noted "A" when the father was being part of the upper classes (professional occupation, manager, teacher at the secondary or the tertiary level) when the respondent acceded to the ENS. We did the same for both maternal and paternal -The "Well Endowed" families, where the father and one of the grandfather belonged to the upper class (33%). -The " Upward " families, where the father belonged to the upper class, but not any grandfathers (30%). -The "Catching Up" families, where one of the grandfather belonged to upper class, but not the father (8%). -The "Poorly Endowed" families, where neither the father nor the grandfathers belonged to the upper class (26%).

Conference paper

Result Toward a specialization and an homogenization of the ENS

With our first dataset (D1), we can draw some of the trends among all the normaliens population between 1966 to 1995. The Figure 2 present the results of a linear regression of the rate of PhD within each generation of ENS graduates for each ENS during this period, with a confidence interval of 90%.

As it appears on this first graph, there is a progressive "specialization" of the ENS. During the period, the three ENS seem became more and more a launching pad for an academic career. At the end of the period, there is around 50% of the graduates who defended a PhD among the Ulm alumni or the Saint-Cloud/Fontenay alumni. So, in a sense, since the 1960's, the ENS seem have become more than before the "Queens" of academic system.

The second important thing represented here is the progressive "homogenization" of the ENS -and especially the progressive "homogenization" of the ENS At the end of the period, there is no more significant differences in therm of future PhD production among Ulm and Saint-Cloud/Fontenay.

The Figure 3 to the field of power -but for a really small group of the graduates.

The four normaliens' career styles

Now we will more deeply analyse the educational and social characteristics that seems linked to each of these professional achievement. Figure 5 represent the results of our MCA compute with data. Every "achievement variable" have been used as "active variable". Only the variables who contributed more than the average to the orientation of the axes are plotted. The variables in italic are the ones who contribute to the second dimension (Dim 2). The others are the ones who mostly contribute to the orientation of the first dimension (Dim 1). Because they concerned less than 10% of our population, some of the modalities of our "achievement variables" were turned into inactive modalities. We plotted these inactive modalities in grey, because they are helpful for the interpretation of the two axes.

Figure 5 show that the first axe (Dim 1) differentiates the status reached according to the global volume of prestige they represent. At the bottom of the graph, we find normaliens who are often secondary teachers, without a PhD and who work in relatively small city (less than 100 000 inhabitants). At the top of graph we find the ones who pursued an academic career, who are professor or lecturer in an universities and who defended a PhD.

The second axe (Dim 2) differentiates the positions reached according to different kinds of "style" of prestige associate with their status. On the right, we see status that are more related to "academic" prestige, like owning an Agregation or being a CPGE teacher -ie two signs of recognition among the most based on "academic" criteria. On the left, we find people who "deviate" from academic pathway to reach prestigious position, but according other criteria than the academic ones. Here we find alumni who followed an additional curriculum in one other parisians grande école, who work in Paris or near Paris -and who are senior civil servant, manager or independent intellectual worker. Within this left sector of the graph we also find people who didn't get an Agregation and who are more likely to work outside of France.

We can also note that this second dimension distinguishes the careers according to the disciplinary field. People who followed a curriculum in "Modern humanities" (sociology, geography or modern literature) are more likely to be located on the left of the graph when people who followed a curriculum in physic are more likely to be located on the right. This distinction reflects the differences of job opportunities according to the field of specialization of graduates. Indeed, positions of CPGE teachers are many in sciences but quite rare in literature or humanities [START_REF] Baudelot | Les classes préparatoires aux grandes écoles : évolutions sur vingt-cinq ans (Les dossiers évaluations et statistiques n°146[END_REF]. It also reflects differences in terms of recruitment between "sciences" and "literature" Division within the ENS, as we will analyse in the next section.

To sum up, among our population, there is four main careers style. At first, there is the normaliens who follow the "highway to academic success" to the top of the academic hierarchy. Secondly, there is the ones who follow a "secondary roads". Such roads are "secondary" in term of stages of education but also in term of prestige. Indeed, they also more likely to work away from Paris, in relatively small city. Thirdly, there is the ones who follow the "highway to academic success" but who stop halfway. They are less likely to reach a PhD, but they has been able to turn their educational success into academic capital -and they has been able to reach

Dim1 Dim2

Activ.

Inact.

Figure 5: The space of normaliens' carreer styles -Source D2 the elite teacher group of the CPGE teachers. Fourthly, there is the ones who have "deviated" from the normal normarliens' career. For those normaliens on one of the "deviated" roads, their successfully passage through an ENS selection has less likely turn into academic capital.

But it has been nevertheless turn into an admission ticket to dominant social positions outside academiaincluding, for some of them, dominant positions within the field of power.

Reconversion strategies and social reproduction

Now, let see how those careers styles are distributed among ENS alumni. To visualize and analyse such distribution, we plot in our plan the cloud of individuals some "concentration ellipses", in order to "summarize" where the sub-could of individuals are located (Roux & Rouanet, 2010, p. 69). To illustrate this analytic and visualization strategy, we plot the Figure 6, that represent the cloud of individuals and two ellipses of concentration, regarding the "general" disciplinary field of specialization (literature and humanities VS sciences). Dim 1 (57.1%) q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q L q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q S Figure 6: Cloud of individuals and discipline -Source D2

First of all, one must note that each individual is plotted by a grey point. The more individuals occupy a similar position within the space, the more the point is black. We can see that there is more dark points at the top of the graph than anywhere else. This is one other illustration of the concentration of the ENS graduate within the academic field. Then, this first example illustrates the differences between scientists and non-scientists in term of professional career. The first ones seem to stay more close to the academic field, when the second ones are more likely to "deviate" from the academics roads for reaching professional positions within other fields. This trend is mainly due to the differences in term of career opportunities, as we already mentioned it. For scientists, between the universities and the many public institutions dedicated to scientific research (the Centre National de la Recherche Scientifique, the Institut National de la Santé et de la Recherche Medicale, the Commissariat à l'Energie Atomique or the Institut National de Recherche Agronomique for instance), the probabilities to find a job within academic are good. For the non-scientists, this is less the case, and they are highly dependant of the demand of the universities in term of pedagogic supervision. In such context, one can do the hypothesis that "deviate" is a strategy to get better chances to reach a dominant position -and avoid their only other option, the "secondary roads". But these differences are not only due to the lack of academic career opportunities within non-scientists. It also reflect different strategies of reconversion of the Dim 1 (57.1%) q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q Female q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q Male Figure 7: Cloud of individuals and gender -Source D2 educational capital linked to the social background of ENS alumni.

First of all, we have to say that, looking at gender (Figure 7), there is no big differences within the space of careers style. Women seems just a little more likely to follow a "secondary road". Indeed, as we analysed in other articles [START_REF] Bataille | Intégrer une Ecole normale supérieure[END_REF][START_REF] Bataille | Quand le genre rattrape l'excellence. Une comparaison des parcours de formation et d'insertion des normaliens et normaliennes[END_REF], the biggest difference in term of careers between men and women is concentrated in the access of the top of academic field (i.e being a Professor/Research Director or not). In the space drawn by the MCA, there is no big differences between the Lecturer/Research associate and the higher ranks of academic hierarchy. This could be one of the reason that no differences appears here between the sub-cloud of men and the sub-cloud of women. Social background of the individuals is more relevant in our case, and especially the differences between alumni situated at either end of our familial typology (the "Well endowed" families and the "Poorly endowed" families). Figure 8 show the two sub-clouds of individuals according to these two modalities of the "social background" variable.

The two sub-clouds are partly overlapping. The zone of overlapping is mainly situated at the top of the graph. The sub-cloud of "well endowed" people is nevertheless more situated at the top of the graph. It moreover include a little more the zone at the left of the graphie the zone were "deviated" careers are over represented. The Figure 6 suggests two main comments. Dim 1 (57.1%) q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q Poorly end. q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q Well end. Firstly, even at this high level of educational achievement, there still is a trends to social reproduction. As we see, graduates from a "poorly endowed" social background are more likely to take a "secondary road".

Conference paper

Then, the graph also show that the "deviated" career style is more likely to be a career style of well endowed graduates. From this point of view, the orientation to a "deviated" career path can be interpreted as a "strategy of reconversion" (Saint-Martin (de), 2011). In this case, the ENS diploma seems has been used as an instrument of reproduction of a dominant social position.

The results from a non-ordinal multinomial logistic regression that uses the kind of professional achievement as dependant variable confirms those analysis (Figure 9).

With a control for all educational achievement variables used in the MCA, the disciplinary field, the class, the sex and the workplace, the fact of being from a well endowed social background is 6 times more widespread among "deviated" graduate than the ones who have followed the "highway to academic success" ("HAS"). It also show that the normaliennes are also more numerous among the graduates who have become secondary teacher than among the ones who have become academics.

Discussion and conclusion

Our main goal was to analyse if the ENS is still a possible pathway to reach the field of power. As our results show it, it's still the case for a really small group of graduates. But, above all, this kind of "career" style is strongly linked with the social background of the graduates. This result is one illustration of how the social uses of the higher education is shaped by social background, as many researches in sociology have already shown it [START_REF] Van Zanten | Choisir son école : stratégies familiales et médiations locales[END_REF]. It show that even when they reach the same (prestigious) educational achievement, the strategy to make the better use of this educational capital remain strongly linked to social background. In term of analysis of the interaction between the different fields in which the ENS are committed, our result deserve two main comments. Firstly, our result show that the ENS are more than ever a "launching pad" for an academic career. Nevertheless, our result also show that between 1960 and 1980 more and more ENS graduates have defended a PhD. So if the ENS remain a major asset for an academic career, it has to be linked with a PhD, when an normalien title was an asset per se before the end of the 1960's [START_REF] Bourdieu | Homo Academicus[END_REF]. This trends may be an indicator of the raise of the influence of scientific based criteria within the academic field -in France like in many other country during this period [START_REF] Gingras | L'emprise du champ scientifique sur le champ universitaire et ses effets[END_REF].

In other words, this trends could be an illustration of a change within the ruling of academic field in France -and an illustration of a relatively stronger autonomy of this field. Secondly, our results can be mobilized to understand the social logics that rule the French field of power and its access. The ENS are in the French context the main pole of production and reproduction of the intellectual values of "freedom" and "disinterest" when the ENA and moreover Sciences Po or HEC are institutions more dedicated to the promotion of economic and applied knowledge [START_REF] Bourdieu | The state nobility : Elite schools in the field of power[END_REF]. As we see, if some of the normaliens can reach position within the field of power, it's because they add to their ENS curricula a curricula in the ENA or Polytechnique.

It may indicate the lack of influence of the kind of value linked to the normalien status within the ruling of the field of power.

Conference paper

Appendix

Conference paper

Figure 1 :

 1 Figure 1: Family types -Source D2

Figure 2 :

 2 Figure 2: Rate of PhD among ENS alumni (linear regression) -Source : D1

Figure 3 :Figure 4 :

 34 Figure 3: Professional achievement of Saint-Cloud/Fontenay Alumni (1981 -1987) -Source D2

Figure 8 :

 8 Figure 8: Cloud of individuals and social background -Source D2

Figure 10 :Figure 11 :

 1011 Figure 10: Achievement variables -Source D2

University of Lausanne, NCCR LIVES, Switzerland

Conference paper