Molle Christelle
email: christelle.molle@sophia.inria.fr

Mascotte

Marie-Emilie Voge
email: marie-emilie.voge@lifl.fr

Effects of the Acknowledgment Traffic on the Capacity of Wireless Mesh Networks

Since the emergence of ubiquitous computing, evaluating wireless network performances has become one of the major economic issues. Among the existing performance indicators, the network capacity, defined as the maximal amount of flow carried by a topology during a fixed time period, is essential. Some cross-layer characteristics have to be taken into account in order to optimally allocate the common resources. In this article, a comparative study is done between interference consequences in the two following models: (i) usual IEEE 802.11 MAC layer with acknowledgments at each hop, and (ii) block acknowledgments reported at the transport layer that can be included in the IEEE 802.16 standard. Cross-layer properties are modeled in a linear programming formulation that is solved using the column generation process. We quantify the gain in capacity induced by the move of the MAC acknowledgments into the transport layer, and show the better load distribution obtained in the network with the second model.

I. INTRODUCTION

In the past few years, wireless mesh networks (WMNs) have emerged as a key technology to provide ubiquitous highspeed services. They are a cost-effective solution that offers good reliability, with a simpler infrastructure than cellular networks, and better coverage than hot spots [START_REF] Akildiz | Wireless mesh networks : a survey[END_REF]. In urban areas, they evolve into the next generation of ubiquitous networks, providing better services for a large variety of applications like video, sound, voice. . . Laying competitive WMNs is a necessary condition to the development of these new applications.

In WMNs, nodes are comprised of mesh routers and mesh clients. The routers automatically establish and maintain mesh connectivity among themselves, forming a fixed infrastructure that is dynamically self-organized and self-configured. Mobile mesh clients, moving on a potentially unlimited area, access services like Internet through special routers called mesh gateways as depicted in Figure 1.

A very important requirement for the network performance evaluation is to maximize its capacity, i.e. the throughput offered to each flow. Theoretical network capacity has been investigated by numerous studies [START_REF] Gupta | The capacity of wireless networks[END_REF], [START_REF] Jun | The nominal capacity of wireless mesh networks[END_REF], [START_REF] Du | Centralized scheduling and channel assignment in multi-channel single-transceiver wimax mesh network[END_REF], [START_REF] Djukic | Performance comparison of 802.16 centralized scheduling algorithms[END_REF]. One of the major issue in wireless networking is the capacity reduction when the network size, i.e. the number of nodes, increases. This is mainly due to interferences arising between radio transmissions between neighboring nodes [START_REF] Kodialam | On the capacity region of multi-radio multi-channel wireless mesh networks[END_REF], [START_REF] Jain | Impact of interference on multi-hop wireless network performance[END_REF]. Limiting these interferences is therefore necessary, either at the equipment level (using directional antennas, orthogonal codes or

Internet backbone g a t e w a y g a t e w a y g a t e w a y m e s h r o u t e r s b a c k h a u l m e s h c l i e n t s

Fig. 1. Mesh routers and gateways form a fixed infrastructure providing Internet access to mobile mesh clients. frequencies), or at the protocol level like in 802.11 when nodes listen to the radio channel or 802.16 in mesh mode in which a base station allows the channel access.

In this paper, we evaluate the performances of a periodic synchronous single-channel wireless mesh network in which communications are scheduled in order to avoid simultaneous contending transmissions. Our work is based on the round weighting problem introduced by [START_REF] Klasing | On the complexity of bandwidth allocation in radio networks with steady traffic demands[END_REF] in which each set of compatible transmissions, i.e. a round, is assigned a weight corresponding to its activation duration during a time period. The objective is to minimize the sum of the round weights, i.e. the length of the time period. In a steady state of the network, the round weighting corresponds to a link schedule that allows routers-to-gateways routing establishment. Traffic requirements from every router can therefore be sent on these routes at maximum rate.

This work is based on a first formulation of the joint routing and scheduling problem for wireless mesh networks [START_REF] Molle | An optimization framework for the joint routing and scheduling in wireless mesh networks[END_REF]. The authors considered a binary symmetric interference model in which acknowledgment traffic is integrated into the MAC layer like in the 802.11 Wifi standard with a channel access based on CSMA/CA. The main contribution of this paper is to measure and analyse the gain in capacity induced by another interference model modeling a different acknowledgment policy. The second model introduced corresponds to the move of the acknowledgments to the transport layer.

The rest of the paper is organized as follows. We first describe the network model in the next section. In Section IV, we present the linear formulation to model the cross-layer network characteristics and obtain the theoretical optimal capacity. Then in Section V, we analyse the results obtained by doing a comparative study that evaluates the effects of the acknowledgment traffic on the network capacity and load. We quantify this gain depending on the network topology.

II. RELATED WORK Solutions to maximize the capacity of wireless multi-hop networks have been proposed in the literature. In [START_REF] Du | Centralized scheduling and channel assignment in multi-channel single-transceiver wimax mesh network[END_REF], authors present a multi-channel scheduling over a routing tree for 802.16 based wireless mesh networks.

In [START_REF] Djukic | Performance comparison of 802.16 centralized scheduling algorithms[END_REF], authors look at the complementary problem of finding a link schedule during a time period that minimizes the maximum delay induced by the re-emission of packets by the forwarding nodes.

In optimization problems for wireless mesh networks in the literature, the common interference model chosen is the binary distance two interference model presented in the next section, in which acknowledgments are modeled at each hop like in the 802.11 standard [START_REF] Kodialam | On the capacity region of multi-radio multi-channel wireless mesh networks[END_REF], [START_REF] Jain | Impact of interference on multi-hop wireless network performance[END_REF], [START_REF] Molle | An optimization framework for the joint routing and scheduling in wireless mesh networks[END_REF].

Our study is based on the problem formulation proposed and studied in [START_REF] Molle | An optimization framework for the joint routing and scheduling in wireless mesh networks[END_REF] where the authors' objective was to maximize the minimum flow throughput that can be achieved by appropriately configuring the network in terms of the sets of links to activate, the flow routes, and the link activation schedule.

In this paper, we specifically quantify the effects of the move of the acknowledgments from the MAC layer to the transport layer on the wireless mesh network capacity. The traffic load in each router is also investigated in order to estimate the amount of flow each mesh router has to forward.

III. NETWORK MODEL

We consider the fixed infrastructure of a wireless mesh network forming a wireless backhaul (Fig. 1). This backhaul is comprised of mesh routers (MRs) interconnected by wireless links and integrated with Internet through special nodes called mesh gateways (MGs). We globally call mesh points (MPs) the union of MRs and MGs. Each MR has a traffic requirement corresponding to the aggregated traffic demand of the mesh clients connected to it. These requests have to be sent from the MRs to Internet, i.e. the MGs, through multi-hop paths.

Mesh points in the WMN all use omnidirectional antennas and have the same transmission power. We therefore model the network as a directed graph G = (V, E), where V is the set of MPs, and E is the set of possible radio transmissions between any pair of nodes in the network based on the transmission range of each MP.

We study two different protocols for wireless networks that have the same technical and traffic settings but differ in their interference model.

A. Technical model

We consider periodic, scheduled WMNs in which all nodes have only one wireless interface that operates on the same frequency band (or channel).

In order to avoid contentions, communications are periodically scheduled. A time period is divided into slots of equal length and each slot is then assigned to a round, i.e. a set of pairwise non-interfering transmissions. A link can be selected in different rounds, and thus it can be activated during several slots of a period.

B. Traffic model

At each time period, a mesh router v k sends d k units of traffic in the network to Internet through the gateways. Given a link scheduling during one period, routing one unit of traffic up to the gateways can need several periods.

In the two different interference models described in the next subsection, acknowledgments are considered as traffic flow going on the link opposite to the data transmission. We set the acknowledgment traffic as a percentage α from the total data traffic of a link. Communication time alloted for this acknowledgment traffic in the network can be used either (i) at the MAC layer, or (ii) at the transport layer. One can note that the second model enforces a symmetric routing for the acknowledgment traffic in the transport layer.

C. Interference models

In both models, we first assume the half-duplex property: a node cannot send and receive traffic simultaneously. Moreover, we assume that each node has an interference range that is slightly greater than its transmission range. Therefore, two neighboring nodes cannot transmit simultaneously since they are within interference range of each other. Similarly, a node cannot receive a transmission if its neighbors are transmitting at the same time.

The difference between the two models resides in the traffic exchanges occurring between two nodes during a time slot of the period.

(i) Symmetric model: In the first case, acknowledgments are included into a one hop communication and are controlled at the MAC layer like in the 802.11 standard.

A node sending traffic must then be able to receive the acknowledgment without interferences from other transmissions. Similarly, a node must acknowledge the received traffic without interfering with other receptions. Thus, during a time slot, a communicating node is both a sender and a receiver. This is translated into a symmetric distance-2 interference model as depicted into a grid topology in Figure 2. (ii) Asymmetric model: When acknowledgments disappear from the MAC layer, a sending node does not need to be a receiver anymore: it does not wait for immediate acknowledgments at this level. Similarly, a receiving node is only a receiver. As depicted in Figure 3, two neighboring nodes can now send traffic simultaneously to two different receivers. Acknowledgments have now to be scheduled at the transport layer like traffic flow.

We therefore call this model the asymmetric distance-1 interference model.

IV. OPTIMIZATION OF THE NETWORK CAPACITY

We use the following capacity measure: for a set of traffic requests, we compute the minimum time period length needed to route all the requests from the routers to the gateways through multi-hop paths. We allow a fractional multi-path routing from one mesh router to the gateways. Indeed, we would like to consider not only a router-to-gateway oriented traffic, but also return traffic as well as transverse traffic between two routers. Fractional routing allows, for a given bandwidth allocation, to route more traffic leading to a capacity increase in comparison to a more constrained mono-routing.

A. Problem formulation

Given the network graph G = (V, E), a subset AP ⊂ V of MGs, and a set of requests K, the problem consists in finding a routing such that all the requests d k ∈ K from router v k to the gateways are satisfied in a minimum period of time. This means that we need to compute routes whose links can be activated during a time period of minimum length. This is equivalent of maximizing the flow rate given traffic requirements: if λ units of flow are sent from a router to a gateway in T units of time, then the flow rate is λ/T in steady state.

In the following linear program, f p ∈ R + is the flow variable over path p ∈ P , where P is the set of all possible paths in the graph G. The integer variable a r corresponds to the number of time slots assigned to round r ∈ R, where R ⊆ P(E) with P(E) containing the set of transmissions in the network that are pairwise non-interfering. P k is the set of paths from source node v k . min r∈R a r

(1)

p∈P k f p = d k , ∀k ∈ K (2) p∈P,p e f p + α p∈P,p xe f p r e a r , ∀e ∈ E (3)
Objective (1) seeks to minimize the sum of the round activation durations. Routing the traffic requests in a minimum period of time is equivalent of maximizing the per-node flow rate as explained above. Constraints (2) are the demand satisfaction constraints: each MR v k has a traffic requirement d k that has to be routed to the set of MGs over a set of paths contained in P k . Constraints (3) link the routing and the scheduling and can be compared to capacity constraints: flow carried by a link is limited by its time activation during the period. But, if a link e carries some flow, then the acknowledgment traffic has to be carried by the link x e depending on the interference model considered. Actually, x e = e in the symmetric model since the activation of e includes both the traffic and acknowledgment sending. In the asymmetric model, x e becomes e, the opposite link of e, because we have to schedule the acknowledgments flow on e during the time period.

B. Resolution method

The model is then solved using the column generation process [START_REF] Lubbecke | Selected topics in column generation[END_REF], that is a prominent technique to cope with largescale linear programs with an exponential set of variables. Its efficiency on routing problems has been shown in [START_REF] Molle | An optimization framework for the joint routing and scheduling in wireless mesh networks[END_REF], [START_REF] Zhang | Joint routing and scheduling in multi-radio multi-channel multi-hop wireless networks[END_REF], [START_REF] Johansson | Cross-layer optimization of wireless networks using nonlinear column generation[END_REF]. It is a primal-dual process that solves the continuous relaxation of our problem that corresponds to the Round Weighting Problem [START_REF] Klasing | On the complexity of bandwidth allocation in radio networks with steady traffic demands[END_REF].

The sets P and R of possible paths and rounds have an exponential size. Column generation allows to limit the set of generated variables needed for the computation of the optimal solution. More precisely, the process runs as follows:

(i) We start with restricted sets of paths and rounds allowing a feasible solution of the problem: R O = {{e}, ∀e ∈ E}, and P 0 = {shortest paths from each router v ∈ V \ AP to a gateway ∈ AP}. (ii) We then solve program (1) -(3) and obtain primal and dual optimal solutions for this current set of variables. (iii) Given the dual reduced costs, we seek for new paths and rounds violating the following constraints of the dual program:

α e∈p y e x k , ∀k ∈ K, p ∈ P k (4) 1 e∈r y e , ∀r ∈ R (5)
We compute the new variables using auxiliary linear programs. (iv) We add these paths and rounds to the current set of variables and repeat (ii)-(iii). The separation theorem claims that solving again the program (1)-( 3) produces a better solution [START_REF] Grotschel | The ellipsoid method and its consequences in combinatorial optimization[END_REF]. (v) When no such paths or rounds exist, then duality theory claims that both the primal and the dual are optimized. One can notice that special QoS requirements can be easily added in the column generation process, like limiting the number of hops in the paths, or getting more fairness by selecting specific links in the rounds.

V. SIMULATIONS

The column generation process has been implemented in Java programming language using the Mascopt graph library developed by team members [START_REF]Mascopt (mascotte optmization) project[END_REF]. Linear programs have been solved using Ilog Cplex solver.

Tests have been realized on regular grid topologies (Manhattan-like topologies), as well as on random graphs in which a fixed number of nodes has been deployed on a rectangular plane following a Poisson law. Then, a fixed transmission power constructs the transmission area of each node. If a node is within the transmission range of another node, then a link is constructed between them.

The percentage α of capacity reserved for the acknowledgment traffic varies from 1 to 10%. Traffic requests have been generated either uniformly with value 1, or randomly with values between 1 and 20 for each router. Several gateway placements have been computed, either regular for grids (in the center and/or the corners), or random in more general topologies.

A. Gain in capacity

The asymmetric interference model constructs rounds containing more edges (the mean size increases by 50%) since a transmission will not block all its 2-neighborhood (see Table I). Therefore the capacity obtained with this model is greater than the one with the symmetric model.

As depicted in figure 4(a), the gain in capacity for grid topologies is always greater than 20%, and can be close to 50% depending on gateway distribution and network size. One can notice that, when the grid has one gateway placed in the center, the ratio of the capacities from the two models is constant with the acknowledgment size (α). Similarly, the variation is slow with 4 gateways situated at the corners of the grid. This phenomenon comes from the bottleneck region located around each gateway that restricts the optimal solution of our problem [START_REF] Molle | An optimization framework for the joint routing and scheduling in wireless mesh networks[END_REF]. A regular gateway placement also allows a fair distribution of the traffic flow between the gateways. In these regular topologies with special connectivity properties, links are activated enough to schedule the acknowledgment traffic in the reverse links.

In random networks, this phenomenon disappears. As depicted in Figure 4(b), the gain in capacity is growing with α, the size of the the acknowledgment traffic. It stays lower than the gain in the grid topologies, i.e. between 0 and 20%, since the node distribution and the gateway placement is completely random.

B. Network load distribution

We also investigated the network load within the two models. We call load of a mesh router the amount of flow received from other routers that it has to forward. Simulations point out that the asymmetric model gives a better load distribution. The mean load is really close between the two models, but we have remarked that there is a more important variation between the nodes with the symmetric model. As for an example, Figures 6(a) and 6(b) shows the load distribution into a random network of 50 nodes. One can remark that there is more nodes with an important amount of data to forward in the symmetric model (6 nodes have more than 10 units of flow to forward whereas in the asymmetric model they are only 3).

Figure 5 depicts the percentage of leaves in the network. We call leaf a mesh router that sends only its flow traffic, without forwarding any other flow. Following the definition, leaves have load 0. One can see that, for each topology, the curve with the asymmetric model is always below the one with the symmetric model. The asymmetric model decreases the number of leaves for every topology we have tested, and for all size of acknowledgment traffic.

Since rounds are less constrained in the asymmetric model, i.e. they can contain more links, the solution allows the use of more paths than in the symmetric model (10% more in the asymmetric model), leading to a better load distribution in the network (see Table I).

VI. CONCLUSION

Our results confirm that the couple (MAC layer, topology) has a strong impact on the wireless mesh network capacity. In particular, removing the acknowledgments from the MAC layer and adding them into the transport layer allows to increase the network capacity thanks to an interference limitation. This growth is greater than 20% for regular gridlike topologies and gateway placement.

Acknowledgments at the transport layer gives also a better load distribution in the network. The amount of traffic flow to forward is balanced between the mesh routers, leading to the use of more paths in the asymmetric interference model.

However, the move of the MAC acknowledgments could induce important delays in the packet delivery in case of transmission failures. This delay would seriously degrade the QoS offered to the clients. A study of the effects of the acknowledgment traffic on the delay is a topic for further research. We would like to do this analysis through simulations using the WSNet simulator [START_REF]Wsnet simulator[END_REF]. Optimizing the transmission schedule during a time period, as well as evaluating the size of the maximum queue in each mesh router are also interesting prospects we want to go into.

Fig. 4 .

 4 Fig. 4. Capacity gain induced by the move of acknowledgments from the MAC layer to the transport layer.

Fig. 5 .

 5 Fig.5. The network load is more spread among the routers in the asymmetric model since there is less nodes without flow traffic to forward.

Fig. 6 .

 6 Fig. 6. Network load distribution in a random network of 50 nodes.

  Symmetric distance-2 interference model in a grid network. Activation of link[START_REF]Wsnet simulator[END_REF] 16) blocks all communications in the 2-neighborhood of the link since node 16 sends an ACK to node 15 instantly.

	1	2		4	5	6
	7	8	9	10	11	12
	13	14	15	16	17	18
	19	20	21	22	23	24
	25	26	27	28	29	30
	Fig. 2.

TABLE I THE

 I MEAN SIZE OF THE ROUNDS AND THE NUMBER OF PATHS IN THE OPTIMAL SOLUTIONS ARE INCREASED WITH THE ASYMMETRIC MODEL.

	Topology	Symmetric model	Asymmetric model
		Mean round size	# paths Mean round size	# paths
	3x3 grid -center	1.33	8	2.03	8
	3x3 grid -corners	1.57	6	3	9
	7x7 grid -corners	9.3	61	13.96	73
	10x10 grid -10 gtws	19.04	158	27.78	173
	10 nodes -1 gtw	1	9	1.43	12
	50 nodes -1 gtw	4.69	55	6.74	61
	100 nodes -2 gtws	7.76	106	11.5	116

Capacity gain for grids vs Size of ack

		1.5
	OPT(Asym) / OPT(Sym)	1.2 1.3 1.4 1.1	3x3 -corners 5x5 -corners		5x5 -cent. + corn. 3x3 -center
			7x7 -corners		7x7 -center
			7x7 -cent. + corn.	5x5 -center
		1
		0	0.02	0.04	0.06	0.08	0.1
				#ACK
			(a) Grid topologies.
		Capacity gain for random topo vs Size of ack 1.5 50N -5gtw 10N -1gtw 50N -1gtw
			50N -7gtw	50N -4gtw	20N -4gtw
	OPT(Asym) / OPT(Sym)	1.1 1.2 1.3 1.4	20N -1gtw 50N -2gtw	20N -2gtw 20N -3gtw	20N -5gtw 10N -2gtw
		1
		0	0.02	0.04	0.06	0.08	0.1
				#ACK
			(b) Random topologies.

ACKNOWLEDMENT

C. Molle PhD is funded by DGA, France. This work has been partially supported by ARC CARMA and European project IST/FET AEOLUS.


N Fourrier

VIRTUAL REALITY BEYOND DESIGN REVIEWS IN SHIPBUILDING: THE NEED FOR INDUSTRY-TAILORED IMMERSIVE DATA INTERACTION

. However, many conditions must be met for VR to be widely adopted in the shipbuilding industry. It primarily must be seamlessly integrated into current workflows. The shipbuilding industry will likely first need to tame its data with efficient models accessible throughout the ship's lifecycle. However, some productivity issues with VR have not yet been identified clearly. While working to develop a VR design tool for ship outfitting, it became clear to us that there is a need for industrydriven development of efficient data interaction and manipulation tools in VR. As an example of such new manipulation tools, we are currently developing a new way to input text in VR that is efficient and easy-to-learn through handwriting. The evaluation of this system highlights the need for more controlled and systematic testing of VR tools in the shipbuilding industry to justify investments and adoption.

1.

INTRODUCTION

The popularity of virtual reality (VR) in industry is at an all-time high. Even in shipbuilding, industrials and academics investigate the usefulness of VR, pushed by growing demands from many shipyards around the world. At ICCAS, the topic of mixed reality (MR) has been tackled regularly for at least a dozen years [START_REF] Lödding | Virtual Reality Supported Assembly Planning in the Shipbuilding Industry[END_REF][START_REF] Martin | Virtual reality: tool or toy for shipbuilding?[END_REF][START_REF] Cassar | Integrating virtual reality software into the early stages of ship design[END_REF]. From marketing to visualization, the range of VR potential applications is as vast as the range of operations related to shipbuilding.

In the consumer market, immersion and social interaction are the main reasons for VR to be used. In an industrial setting, immersion can provide better visualization of 3D models and VR's social aspects may be used to design efficient collaborative tools. As such, there is no unique way to use VR and its potential is demonstrated in many fields: sales, design reviews, training and more. However, integrating new and disruptive tools into existing workflows is technologically complex, time-consuming and expensive. The first step, which has already received major contributions, is to define properly all the conditions for a successful integration of VR into current industrial processes.

Design and conception are two core industrial processes and they both require manipulating data and not only visualizing it. Paradoxically, the industry only uses VR for visualization purposes. The usage of VR as a data manipulation tool remains mostly unexplored today. Indeed, this use case requires massive changes in the way industries handle their data. This kind of transformation is already happening though, hidden behind terms like digitalization and Industry 4.0. Using VR as a design tool can then become a new incentive to accelerate the transition towards standardized and easily consumable data. This would make it possible to develop more efficient design tools that can leverage all the advantages VR has to offer.

CONDITIONS FOR LARGE-SCALE INDUSTRY ADOPTION

There are many barriers to large-scale adoption of virtual and augmented reality (AR) systems in an industrial setting, and especially in shipbuilding. Defining them properly is necessary as a first step towards better implementations of VR tools.

COMMUNICATION BETWEEN VR AND CAD APPS

Integrating new tools into established processes requires bidirectional communication and data transfer with existing tools. This is especially difficult when the tools to link have different data structures. This becomes increasingly apparent with innovative tools like VR. Indeed, they are often developed with game engines that represent 3D models in a discrete manner using triangles, called tessellation. This is highly efficient for the real-time rendering that is necessary for video games and VR. However, this model is incompatible with Computer-Aided Design (CAD) data that mostly uses continuous representations like NURBS. The conversion from CAD to VR, from a geometrical standpoint, is possible thanks to many algorithms that aim to discretize and optimize 3D models for real-time rendering [START_REF] Guo | Automatic and high-quality surface mesh generation for CAD models[END_REF]7]. However, when it comes to metadata such as part hierarchy or textures, the transformation often requires some amount of work that cannot be streamlined and depends on both the CAD and VR file formats [START_REF] Mchenry | An Overview of 3D Data Content, File Formats and Viewers[END_REF]. There is a push for CAD data structures to represent tessellated geometry, for example with the new versions of STEP AP242 (ISO 10303-242) files but most design software cannot yet interpret this new norm. The CAx-Interoperability Forum regroups the companies actively working on this topic with the aim of accelerating the development of STEP AP 242 translators [START_REF]CAx Interoperability Forum[END_REF]. Those companies evolve in different industries such as automotive, defence, aerospace and energy. CAD software developers such as Autodesk, Dassault Systèmes, Datakit and others, accompany them in this endeavour. This shows there is a growing demand from every industry for better interoperability of CAD file formats especially when it comes to representing tessellated geometry.

Unity, one of the most commonly used game engine to develop VR applications, acquired Pixyz in 2021, which is a plugin that aims to let users import CAD files directly into the game engine. The import is automatically optimized for 3D rendering and supports files coming from most CAD software. There also exist file conversion software such as CAD Exchanger, that lets users transform CAD file formats into others, including ones compatible with game engines. Also, there are open libraries that aim to ease the development process of conversion software such as Assimp [START_REF]Open Asset Import Library (assimp)[END_REF]. This type of transformation, no matter the software used, often comes with a partial loss of geometrical data and metadata that prevents two-way data transfer.

Indeed, VR to CAD is even more complex as this conversion cannot be done strictly with only the deteriorated databecause it has been discretized and tessellatedthat emanates from the VR system. The process of converting back geometric data from a tessellated format into one that is readable by CAD software mainly relies on interpolation [START_REF] Pal | A reconstruction method using geometric subdivision and NURBS interpolation[END_REF][START_REF] Pal | Object shape reconstruction through NURBS surface interpolation[END_REF] or primitive shape extraction techniques [START_REF] Bénière | A comprehensive process of reverse engineering from 3D meshes to CAD models[END_REF]. In some cases, it might be possible to reconstruct fullyincluding metadata the CAD data but this would rely on accessing 3D model databases or creating a third-party software that has access to both domains. Some software use this kind of technology to create conversion-less import of CAD models in VR scenes and even allow some amount of bidirectional real-time modifications :

 Exxar CAD integrates with standard CAD applications such as Aveva Marine, CADMatic, Catia… to stream the 3D data directly in a VR review application. Users working on the same model both in a CAD software and in VR can annotate the models. Those annotations propagate in real-time.  Techviz interfaces with CAD applications like Catia and NX for real-time visualization of 3D models.

Modifications made in the CAD software can be visualized while a VR session is ongoing. Some interactions with the models are possible in VR such as measuring or moving parts, but it is not possible to modify the CAD model in VR.  Mindesk also integrates with many CAD applications like Solidworks, Revit or Rhino. It allows streaming CAD data directly into VR for visualization purposes. Leveraging Rhino's data format, Mindesk allows VR modifications to be reflected in real-time on the CAD model through Unreal Engine, which is another game engine often used for developing VR applications.

These observations highlight the need for industries, including shipbuilding, to tame their data. Data must become easily consumable which means having a unique model used throughout the lifecycle of the project. Having access to up-to-date models is necessary to widen the range of possible VR applications [START_REF] Martin | Accessible immersive visualisation for shipbuilding[END_REF][START_REF] Morais | The Evolution of Virtual Reality in Shipbuilding[END_REF], as they are currently limited to non-modifiable visualization when it comes to CAD. In the context of Industry 4.0, many powerful models are emerging:

 Building Information Modelling (BIM): a technology that aims to simulate construction projects in virtual environments. A unique model is constructed for "design, procurement, fabrication, construction […], operations and maintenance" [START_REF] Azhar | Building Information Modeling (BIM): Trends, Benefits, Risks, and Challenges for the AEC Industry[END_REF]. The model is accessible by all teams for the whole lifecycle of the project. This model serves as the basis for digital transformation in the architecture and construction industry, which are highly similar to the shipbuilding industry. A recent open-source Unity plugin aims to allow bidirectional communication between BIM authoring platforms and the game engine [START_REF] May | UNITYREV -Bridging the gap between BIM authoring platforms and game engines by creating a real-time bi-directional exchange of BIM data[END_REF].  Product Lifecycle Management (PLM): a technology that aims to manage the complete lifecycle of a project from ideation to disposal. This model contains all the necessary information from the specifications, technical solutions, to fabrication and maintenance details. Data is streamlined as to create a "coherent data flow, avoiding redundancies and gaps" [START_REF] Terzi | Product lifecycle management -From its history to its new role[END_REF]. In BIM, the digital mock-up is at the center of the model while PLM also formalizes the processes of modification and validation of the model.  Digital Twins: the virtual representation of real world assets with the goal of representing the real time state of said assets. This virtual representation should span the whole lifecycle of the asset. The virtual model can be used for monitoring, diagnostics, design iterations, simulation… to improve productivity. The link between digital twins and VR is a popular research topic [START_REF] Pirker | Immersive Virtual Reality for Virtual and Digital Twins: A Literature Review to Identify State Of The Art and Perspectives[END_REF].  Industrial Internet of Things (IIoT): application of Internet of Things (turning every object into a connected device) to industry. It enables ubiquitous monitoring and communication between machines for better automation [START_REF] Sissini | Industrial Internet of Things: Challenges, Opportunities, and Directions[END_REF]. This kind of technology requires data standardization and is often paired with augmented reality to monitor interactively the state of a machine [START_REF] Husinsky | Situated Visualization of IIoT Data on the Hololens 2[END_REF].

 Knowledge-Based Engineering (KBE): aims to increase productivity and reduce costs by automating design tasks through the formalization of expert knowledge [START_REF] Verhagen | A critical review of Knowledge-Based Engineering: An identification of research challenges[END_REF]. The technology often uses an inference engine to determine if a set of facts validates a set of rules [START_REF] Shahzad | KBE/AI for Ships Construction -A Feasibility Study[END_REF]. The rules should be representative of the expert knowledge.

The objective of such technologies and processes is to create a unique data model that is easily consumable during the whole lifecycle of the project: from early designs all the way to the supply chain. It can enable multidisciplinary teams to interact simultaneously with the same data. They can also reduce iteration time and costs if used as decision-helping tools. This makes these systems ideal candidates for application in the ship building industry.

EASE OF USE AND DEVELOPMENT

The communication between VR and CAD tools is important but is not the only condition preventing wider adoption of VR. Martin [START_REF] Martin | Virtual reality: tool or toy for shipbuilding?[END_REF] stated that VR tools must be easy-to-use even for non-CAD experts. Indeed, the expertise needed to run and be proficient with VR tools nowadays far outweighs the benefits. First, as stated before, the amount of work required to prepare data for VR is non-negligible and non-trivial. While there are solutions to convert the data for visualization as demonstrated in Section 2.1, they often require human intervention or expertise. As of today, teams that want to use VR regularly and efficiently for design reviews purposes would need a dedicated VR expert.

Secondly, the development of VR prototypes is difficult and often lacks collaboration with field experts. This means systems end up missing features or are clunky to use. It might not be a priority to create a perfect user experience because we are still experimenting and trying to prove the potential of VR, but it does not help with convincing experts of VR capabilities. VR development is not easy also because there are many systems that are not all compatible with each other. Indeed, different levels of VR can be experienced depending on the output (head-mounted displays (HMDs), CAVEs, 3D screens…) and input (controllers, navigation, body and gesture tracking) devices [START_REF] Anthes; R | State of the art of virtual reality technology[END_REF]. This means some amount of work is required to standardize the experience across platforms.

Finally, when it comes to actual usage, it may be difficult for the less tech-savvy to get the hang of VR. New users must first realize what it means to be immersed in a virtual environment, and they must then learn how to interact with said environment. New users may also fall into the trap of expecting too much from VR. Misconceptions about the capabilities of VR can lead to disappointment, which may harm one's ability to adapt to the technology. We can however expect VR acquaintance to grow as time passes. Nick Clegg, President for Global Affairs at Meta Platforms, recently published a manifesto about the vision of Metaex-Facebookregarding the future of VR [START_REF] Clegg | Making the metaverse: What it is, how it will be built, and why it matters[END_REF]. This vision revolves around standardisation and a new social way to experience the internet called metaverses.

With more people acquainted with VR, it will be easier for industry to adopt the technology. Moreover, Meta's short-term objectives are all about releasing new HMDs designed with productivity and industrial use in mind. It is currently pushing for more collaboration with lawmakers, academics and industrials. This is a major opportunity to define properly what is necessary for VR in industry. For the first time, workshops [START_REF]Metaverse as a promise of a bright future? -social interactions in a world of isolation[END_REF][START_REF]Metabuild -Building the Foundations for the Metaverse[END_REF] were dedicated to these topics during the IEEE Virtual Reality 2022 conference (the premier international conference on VR) and the push for VR in industry is one of the main factors that will influence the transition towards metaverse-like VR implementations. Such standardization of the VR ecosystem would help greatly the development of industrial applications.

VR applications are often developed using game engines and SDKs that are suited for video games. As such, visualization, interaction and manipulation techniques in virtual environments are vastly different from CAD software. VR apps are not traditionally created with productivity and industrial use in mind, and this shows even when industrials are involved in the making. As such, making VR easy-to-use is probably not sufficient, and new innovative ways to interact with data in VR, that differ radically from the ones used today, must be studied with industry as a priority.

THE NEED FOR INDUSTRY-TAILORED DATA INTERACTION TECHNIQUES

The main hindrance to adopting VR in industry remains data compatibility. As stated before, major VR and CAD actors globally are actively working on this issue. We can realistically believe it will be resolved in a not-so-far future. We must then anticipate the next problem in adopting VR, which is how to make it productive enough to rival current processes. Indeed, current data interaction and manipulation techniques in VR are not compatible with productivity and industrial use. For example, in a traditional CAD software, a keyboard and mouse setup lets the user select and modify data. This setup is efficient in both interacting with and manipulating data and does not really require any training, as everyone has some experience with the setup. In VR though, while there exist many performant and intuitive ways to select data [START_REF] Sanz | A Survey of 3D Object Selection Techniques for Virtual Environments[END_REF], manipulating and changing it is challenging. Indeed, it is difficult to embark a physical keyboard in a VR setup, even if doing so results in unmatched data entry speedsmeasured in words per minutes (WPM)varying from 40 to 80 WPM [START_REF] Pham | HawKEY: Efficient and Versatile Text Entry for Virtual Reality[END_REF]. This kind of system however requires specific setups that are difficult, from a logistical and technological standpoint, to integrate with standard HMDs. In general, "plug and play" VR apps have only tried to emulate and virtualize classickeyboard and mouse setupsinteraction methods. This makes sense in a first approximation because the familiarity helps when it comes to understanding VR. However, ultimately, since being immersed in data is wildly different from interacting with it through a flat-screen, the virtualization of classic techniques (such as virtual keyboards) proves not to be efficient enough. As such, it is understandable that we cannot expect VR to succeed in an industrial settingwhere productivity is paramountif even basic tasks such as typing text are daunting.

There are a lot of research and commercial works exploring new ways to handle locomotion, interaction and collaboration in virtual reality. However, entertainment industries are the ones driving this exploration. The needs in those fields is vastly different from industrial needs. Industry must be the driver behind the creation of efficient VR data interaction and manipulation techniques. Three factors are primordial when designing VR applications for industrial use: efficiency, ergonomics, and ease-of learning and mastering. Combining these factors and bidirectional communication between VR and CAD will allow, in the end, to use VR as a design tool beyond its traditional usage of visualization and design reviews.

VR AS A DESIGN TOOL

There are three main reasons VR would be great for design purposes. First, VR is immersive, which makes it an ideal tool for visualizing difficult situations at 1:1 scale. Indeed, in standard CAD tools, it may happen that some design tasks, and especially in the highly constrained environment of shipbuilding, are rendered difficult because the less than optimal visualization strategies [START_REF] Alonso | Advantages of using a virtual reality tool in shipbuilding[END_REF]. Secondly, VR is becoming more and more of a social platform that lets its user interact and share experiences with each other. Some amount of standardization in the domain of identity and avatars in VR will happen thanks to the rise of metaverses, which will make it easier to integrate collaborative components into industrial VR tools. Collaborative tools are great for better communication between designers and between teams, which will in turn improve design efficiency and reduce iteration costs. Finally, users in VR are able to interact with the environment in a more intuitive and close to the real world manner. As such, VR tools can help the decision making process. For example, one could verify if doors can be opened by trying them, could walk through maintenance spaces to check if they are accessible, or even simulate the process of evacuation after pushing in an incendiary alarm button. We are leveraging the advantages of VR through the development of an immersive, interactive, and collaborative VR design tool for ship outfitting. Outfitting is a highly multi-disciplinary process that involves many trades such as piping, electrical, plumbing, HVAC (heating, ventilation, and air-conditioning) and even furnishing. This process is often sequential and iterative. Its goal is to ensure that all the different constraints and norms from each trade are respected in contrived spaces. Combining the visualization power of VR with a decision-helping component will make the outfitting process more efficient. The goal of the tool is to let outfitters immerse themselves at 1:1 scale in the technical locals for better visualization, and be able to place new objects. Interacting with those objects will let them verify certain properties such as accessibility. Additionally, we are developing a decision-helping tool which goal is to let outfitters know when constraints are not respected.

By working hand-in-hand with outfitting experts, we aim to formalize knowledge [START_REF] Zhu | A KBE Application for Automatic Aircraft Wire Harness Routing[END_REF] and exploit it in real-time in the VR tool. An example is given in Figure 1: placing an electric box in the environment comes with an accessibility constraint. Indeed, it must remain possible to open the door at all times, even after subsequent changes in the environment. The tool under development must be able to verify in real-time that all constraints are respected, and lets the outfitter know if there are any problems. As such, by formalizing knowledge, the amount of work necessary to verify everything is in order should be reduced.

In the other hand, while developing this VR design tool, it became abundantly clear that text entry in VR, which is a common task in design, was not efficient enough for industrial use.

A NOVEL TEXT ENTRY TECHNIQUE

When it comes to conception and design, one of the main fundamental tasks is data entry. Indeed, creating and manipulating textual and numerical data is necessary in all design processes. In industry, these processes rely heavily on the keyboard as a mean to input data. Keyboards are overall highly efficient: they reach high words per minute (WPM), are not physically and mentally tiring, and most importantly they require little training since they are so common. In the context of using VR tools for design, creating and interacting with this type of data is not trivial. Three types of virtual keyboards are often used for tools that want to only rely on a standard HMD setup [START_REF] Dube | Text Entry in Virtual Reality: A Comprehensive Review of the Literature[END_REF]:

 Head/eye pointing: the user uses the movement of its eyes or head to move a pointer on a virtual keyboard.

Pressing a button on a controller will select the pointed-at key [START_REF] Yu | Tap, Dwell or Gesture? Exploring Head-Based Text Entry Techniques for HMDs[END_REF].  Controller pointing: the user uses a laser-like apparatus associated with a controller to point directly at a virtual keyboard. Pressing a button on a controller will select the pointed-at key [START_REF] Speicher | Selection-based Text Entry in Virtual Reality[END_REF].  Drum-like: the user uses virtual drumsticks to hit on a virtual keyboard that reacts to the collisions between the drumsticks and keys. No other input than the collision is required to type a character [START_REF] Boletsis | Text Input in Virtual Reality: A Preliminary Evaluation of the Drum-Like VR Keyboard[END_REF].

Compared to a classic keyboard that has a 75 WPM average typing speed for commonly skilled users [START_REF] Arif | Analysis of text entry performance metrics[END_REF], these virtual keyboards all have drawbacks that make them not practical for industrial purposes. Head/eye pointing keyboards have low typing speeds (16 WPM) and are physically demanding. Controller pointing keyboards suffer from the same problems in a lesser extent. Finally, drum-like keyboards report the best performance (21WPM) but it is still far from the performance of traditional keyboards and it remains physically tiring [START_REF] Boletsis | Controller-based Text-input Techniques for Virtual Reality: An Empirical Comparison[END_REF]. A lot of work has been done trying to create innovative data input techniques for VR using T9-like keyboards [START_REF] Yu | PizzaText : Text Entry for Virtual Reality Systems Using Dual Thumbsticks[END_REF] or gestures-based methods [START_REF] Chen | Exploring Word-gesture Text Entry Techniques in Virtual Reality[END_REF]. While they can be better when it comes to physical and mental load, they are slower than virtual keyboards for beginners and their mastery requires a lot of training. It is then important to create a VR text input system that is suitable for industry: efficient, ergonomic and easyto-learn.

HANDWRITING IN VR

In our work to create an interactive and collaborative VR tool for ship outfitting, we faced the problem described previously that it is hard to do design without performant data input, which does not really exist in the context of virtual reality. The scenarios where we need to input text or numbers are, non-exhaustively, the following:

 Filter databases to find 3D models to import in the virtual environment.  Find items in large lists.  Create or modify textual values like object names and annotations.  Create or modify numerical values like object positions, scales… All these scenarios are concise but reoccurring typing tasks. As such, we need an input technique that is efficient and reactive mainly for singleor fewword scenarios. In a real-life setting, when asked to provide short textual information without the use of a keyboard, one would grab a piece of paper and a pen. We believe mimicking this type of behaviour in VR could be more efficient than virtualizing keyboards. As such, we developed a system that enables handwriting in VR on a board with a pen. An off-the-shelf handwriting recognition software (HWR), that we integrated into the Unity game engine, transforms the handwritten text in real-time in an exploitable data format as shown in Figure 2. Such a system would indeed answer all the criteria for an industry-suitable input system in VR: handwriting speeds surpass the performance of virtual keyboards for short writing sessions as they average ~38 WPM [START_REF] Van Drempt; | A review of factors that influence adult handwriting performance[END_REF][START_REF] Summers | Assessment of handwriting speed and factors influencing written output of university students in examination[END_REF] and requires no training since virtually everyone knows handwriting. Moreover, writing in VR has been gamified many times (The Lab, Half-Life Alyx, DryErase…) and can become an entertaining task, which would reduce drastically the mental load associated with typing in VR.

Nevertheless, writing on a virtual surface that is not present physically comes with two challenges:

 It is necessary to create appropriate feedback for the user to know when the pen is indeed in contact with the virtual whiteboard.  It can be tiring as it is not possible to rest against the writing surface.

To make the experience more intuitive, we added visual and haptic feedbacks. Visual feedbacks consist of enabling collisions between the virtual pen and the virtual whiteboard, shadows that help estimate the distances to the board, and an adaptive line thickness that varies following the amount of virtual pressure applied on the board. Haptic feedback consists of a varying controller vibration based on the pressure applied to the board. We are also experimenting with visual haptics by representing the contact of the pen with the board by adding a rippling texture on the board, substituting a haptic cue by a visual one. These feedbacks allow for writing that is more natural and lifelike, even if the cues do not perfectly match reality [START_REF] Cooper | The effects of substitute multisensory feedback on task performance and the sense of presence in a virtual reality environment[END_REF]. Adding guides, such as horizontal lines to follow when writing, will maximize the effectiveness of the handwriting recognition software because its performance depends greatly on each character's size and verticality.

Figure 2 -Handwritten text on a virtual blackboard in a Unity VR scene is transformed into exploitable string data thanks to the HWR software. The number provided with the data represents the recognition confidence of the algorithm on a 0-100 scale.

We are also studying the influence of posture on the physical load of the handwriting task. Indeed, for long writing sessions in a classroom whiteboard setup, not being able to rest on the board makes the task tiring. As such, it is possible to imagine new writing positions where the arm can be at rest either by providing a physical support or by finding an ergonomic position for the board. However, this problem only arises for long writing sessions, which are not the use case we envision for this system. Preliminary testing has demonstrated the potential of the system when it comes to writing speed, ease-of-use and general appreciation compared to virtual keyboards in a MacKenzie type test [START_REF] Mackenzie; R | Phrase Sets for Evaluating Text Entry Techniques[END_REF]. We are planning a two-part user study that will first determine implementation details of the system (feedback types, posture…) and then prove the effectiveness of the system in a more industrial task that is related to design in shipbuilding.

DESIGNING A USER STUDY TO ASSESS INDUSTRIAL USABILITY

One common flaw found in many of the VR tools developed for shipbuilding [START_REF] Lödding | Virtual Reality Supported Assembly Planning in the Shipbuilding Industry[END_REF][START_REF] Cassar | Integrating virtual reality software into the early stages of ship design[END_REF][START_REF] Alonso | Advantages of using a virtual reality tool in shipbuilding[END_REF] is that there is no quantification of their usability. It is important to prove the potential of VR in both quantitative and qualitative manners. In the rare cases where user or case studies were conducted [START_REF] Garcia Agis; | The potential of virtual reality (VR) tools and its application in conceptual ship design[END_REF][START_REF] Nordby | Efficient Use of Virtual and Mixed Reality in Conceptual Design of Maritime Work Places[END_REF], either there were not enough participants to be relevant, the study was not structured in a way a quantitative measure could be performed or statistically analysed, or the population of the study was made of students who could not give an appropriate industrial evaluation of the system. If we do not make more effort to provide metrics for the usefulness of VR in industry, it will remain nearly impossible to justify investing massively into this kind of system, especially if no comparisons with current systems are available. The "lack of perceived benefits" described by Morais et al. [START_REF] Morais | The Evolution of Virtual Reality in Shipbuilding[END_REF] is in part due to this lack of metrics.

Quantitative measures will always depend on the task; it is not possible to provide a general framework to assess quantitatively the performance of an industrial VR system. However, measuring qualitatively the usability of a system is a well-known exercise [START_REF] Kun | Utilitarian or Experiential? An Analysis of Usability Questionnaires[END_REF]. The SUS (System Usability Scale) questionnaire is the most often used: it evaluates 10 items related to usability in a task-abstracted fashion. This evaluation is often conducted in pair with the NASA-TLX questionnaire that aims to quantify the mental and physical loads of a task [START_REF] Hart | Development of NASA-TLX (Task Load Index): Results of Empirical and Theoretical Research[END_REF]. Combining these questionnaires should provide an objective metric of a VR tool's potential as well as a way to compare them to the currently used tools. It is important for the questionnaires that the tested population in a user study is representative of the target population. In academics settings, it is often the case user studies are conducted on students. It is debatable whether such results can justify industrial use. In industrial settings however, while technicians and engineers are often involved in the development process of VR tools, proper case studies with quantifiable metrics are rarely performed. As such, more collaboration would benefit greatly the implementation of VR in industry. Indeed, such collaboration can lead to better-controlled environments for testing systems with relevant populations. We will follow these principles when carrying out the studies for the handwriting input system presented in Section 4.1 and the ship outfitting design tool presented in Section 3.

CONCLUSIONS

The difficult communication between current tools and processes and VR is the main limiting factor to large-scale VR adoption. Indeed, in industry, processes require smooth and streamlined transitions between the different cycles of a project. Additionally, this makes VR cumbersome to use because of the amount of work necessary to prepare data even for basic visualization. We showed these are popular research topics that industrials and academics are actively trying to resolve.

We argue there are additional limiting factors that have not been yet properly identified when it comes to expanding the scope of VR in industry to tasks like conception and design. there is a dire need for more performant, more ergonomic, and easier-to-use data interaction techniques in VR that are suited for industrial use. Moreover, it is also important to start evaluating industrial VR applications in a more rigorous manner in controlled studies that can provide metrics of VR usefulness and usability. Morais & al. stated VR has to "offer a solution that is ten times better than the current way of doing things" [START_REF] Morais | The Evolution of Virtual Reality in Shipbuilding[END_REF]. It is important to be able to measure this statement, and compare VR to current systems, to justify massive investments towards industrial adoption. We believe that close collaboration between industry and academia would better advance this topic.

6.

Figure 1 -

 1 Figure 1a) The green zone represents an accessibility constraint. It is associated with an electric box object that has an open door highlighted in yellow. The green zone must remain free to let the door open. b) Whenever another object enters the area, the constraint is detected as being violated and the volume turns red to indicate the problem to the user.


St€phane Nahrath

Philippe Warin

Rediscovering Public Law and Public Administration in Comparative Policy Analysis: Tribute to

. 1 However, this quest is unlikely to be successful if there is no empirical endeavour to explain the interactions between the three concepts. This is why Peter Knoepfel has constantly formulated the equation between actors, resources and rules, in operationalizable terms -in both his research work carried out in close proximity to public action (through various mandates) and in his teaching in Switzerland and abroad. 2 The policy analysis defended by him is constantly shaped by a pragmatic perspective. The aim is to gain insight into concrete (observable) practices, through which actors produce a shared normative signification, particularly in terms of an hypothesis on problem causation (who or what is 'guilty' or 'objectively responsible' for the collective problem to be resolved?) and an hypothesis on state intervention (how can the collective problem be alleviated or resolved?). This pragmatism is located in a constant effort at methodological operationalization, which is systematically aimed at exploring the explanation for this trilogy (actors, resources, rules) and its interactions as a pragmatic concept. Peter Knoepfel always expects the explanation of a policy to cite factors that are important. For him the idea of importance (as for the pragmatists, Hilary Putnam (1990) in particular) is always

dependent on the reason for asking the question why? In this instance, why such-and-such an actor, rule or resource?

The constructivist pragmatism which, in my opinion, characterizes Peter Knoepfel's approach to policy analysis can be explained. This is my objective here. To that end, I will: (I) indicate the systemic and non-positivistic nature of his approach; (II) highlight his adhesion to the methodological principle of the contingency of analysis; and (III) demonstrate the axiomatic neutrality of the approach. This will reveal the utility of his analytical framework in finding answers to a fundamental question: i.e. how are public policies made?

A systemic and non-positivistic approach to policy

The pragmatic constructivism characterizing the framework for policy analysis proposed by Knoepfel can be explained, first, in terms of its simultaneously systemic and non-positivistic conception of policy. He sees policies as a cascade of decisions and activities, the interactions of which produce observable results. This is what makes the system. The idea of system implies the consideration of both decisions and activities as perpetual combinations which aim to establish equilibriums between actors -i.e. from deciders to stakeholders -and through them between different social interests. These equilibriums are necessarily partial and provisional at each step of the policy cycle. In this regard, Peter Knoepfel postulates a renewal of them. In a sense it is summed up in the reproduction of one of his works (Peter Knoepfel is also an artist) found on the cover of Analyse et pilotage des politiques publiques, which brings to mind 'Tinguely's Machine'.

At the same time, Knoepfel's approach is also non-positivistic. It starts with observed facts which give policies their substance and avoids referring to them as an ideal type [START_REF] Coenen-Huther | † Le type id€al comme instrument de la recherche sociologique ‡[END_REF]. However unlike scientific positivism (as defined by Auguste Comte) it does not give up asking the question why? Consequently, for Knoepfel, policies are inevitably an act that is visible and can, therefore, be grasped by the analyst. For this reason, the proposed approach seeks to develop a scientific analysis of public policy, based on the study of variables to be explained (the 'products' of a policy) by means of explanatory factors (the actors, resources and institutional rules). The idea is to consider these six products3 from two points of view: i.e. substantial ('How to solve the public problem?') and institutional ('Which actors are going to contribute to the next step in solving the problem, based on which rules and using which resources?'). The kaleidoscope of explanatory variables changes for each product examined, however the questions remain the same: why such-and-such an actor, rule or resource? By reasoning in terms of dependent and independent variables, the production of any proposal pertaining to the explanation of a phenomenon ('Why this definition of this public problem? Why this politico-administrative programme?' etc.) is subject to the control of its statements.

The analyst is urged to follow a logical reasoning inspired by the Popperian principles of The Logic of Scientific Discovery (Popper, 1959 4 ) for this purpose. To be valid, the approach must respect the 'golden rules'. For example, Knoepfel always recommends the question 'Why is there not such-and-such an actor, resource, or rule?'. Thus an analysis of policies, whose statements can be falsified, is produced. According to Knoepfel, 'the proposed analytical model can be applied in various scientific perspectives, that is, to describe, interpret, explain or even anticipate the content of a policy' (Knoepfel, Larrue & Varone, 2006: 276). If it is to be more than an impressionistic account, under no circumstances may the analysis exclude the study of observed facts. Its relevance lies in the reconstruction of the overall logic of a system of decisions and activities taken as a whole. To that end, it is necessary to highlight observed facts and to note their compositional effects. The analysis thus requires a depersonalization and complexification of the logical reasoning ('Why these variables and not others?') by integrating multiple feedback loops. In this way, linear reasoning loses its plausibility and relevance. Two aspects are particularly important here: first, the fact of highlighting the validity of each observed fact, insofar as it is possible to find the 'right reasons' for their presence or absence by reconstructing explanatory variables; second, and above all, the fact of highlighting the simultaneity of explanatory variables, from which it is possible to specify the dynamics of policies on the basis of parts of their content.

To explain the logical reasoning at work, consider for a moment what Peter Knoepfel and his co-authors call the 'Product n€ 1' of a policy, 'the political definition of the public problem'.

The initial idea is that there are no policies without decisions or activities (legislative, regulatory, administrative, etc.) designed to implement them. The problems addressed by policies are highly specific however: they are problems that are politically defined as public.

If a social problem is the subject of a political controversy it becomes a political problem.

From this point of view there is nothing original about the proposed approach. It fits into the agenda-building perspective, which shows that the emergence of a problem is accompanied by a process of problem-framing, during which the actors propose their definition of the situation based on their own perception of the problem (Cobb & Elder, 1972). But it is also part of the constructivist approach to public problems, now dominant in North American research,5 which, under the impetus of John Kitsuse's and Malcolm Spector's group at the Society for the Study of Social Problems, strongly emphasizes the social construction of the meaning of things and events [START_REF] Gusfield | The Culture of Public Problems : Drinking-Driving and the Symbolic Order[END_REF][START_REF] Gusfield | On the Side: Practical Action and Social Constructivism in Social Problems Theory[END_REF][START_REF] Spector | Constructing Social Problems[END_REF]. It also embodies the cognitivist conception of 'agenda-setting as a 'continuous operation of construction and symbolic reconstruction'' (Padioleau, 1982: 26). The importance of ideas in the political definition of the public problem is not overlooked. The authors point out that 'the assessment of problems depends not only on the objective conditions of the situation deemed to be problematical, but also on its evaluation and subjective weighting (that is, the political definition) by the actors concerned' (Knoepfel, Larrue & Varone, 2006: 145). Knoepfel nevertheless approaches the issue of agenda-setting (like everything else) from a pragmatic perspective. Like the North Americans, he advises the analyst to strive, above all, to account for the stakeholders' representations of the nature of the problem. To that end, and somewhat akin to researchers in ethnomethodology, the interactionists (Blumer, 1971) or sociologists of science (Latour & Woolgar, 1979), he proposes the same dimensions of operational analysis ('intensity, perimeter, novelty, urgency of the problem') as those, around which the actors struggle concretely to reach a political definition of the public problem. It is in this struggle, and not only for pre-established reasons linked to statuses or institutional context, or for existing cognitive schemes (or framing), that the 'political entrepreneurs' so dear to Cobb and Edler effectively appear as actors. The analyst then has the possibility of recognizing them if he or she is prepared to undertake an empirical inquiry. For that purpose, it is necessary to examine these dimensions (and others, Knoepfel recommends) as precisely as possible, to show that a situation becomes a public problem not because of its intrinsic characteristics, but because of the way in which the actors construe these dimensions to ensure that the situation appears on the political agenda.

The contingency of policy analysis

This systemic and non-positivistic approach means ipso facto that it is necessary to try to inter-relate decisions and activities; it is naturally situated in the here-and-now of interactions between the two. The epistemological foundation of this approach to public policy analysis is the contingency of the analysis. As Erhard Friedberg, who defends the pragmatic value of the scientific analysis of organizations, says, it is based 'on the implementation of an intuitive and progressive choice of the outlines and central points or sensitive areas whose study allows for the best progress to be made in the understanding of the whole' (Friedberg, 1993: 244).

Hence, policy analysis and its results are historically situated. It is an empirical explication of a construct that the historically and spatially situated actors ('public actors', 'target groups' and 'beneficiary groups', in Knoepfel's terminology) have instituted to produce the processes of exchange and power that interweave the interactions between decisions and activities.

Clearly, an explanation such as this is also historically situated and its demonstration, as both Knoepfel and Friedberg say, cannot claim to exhaust reality or to propose a general model.

The validity of the analysis lasts no longer than the policy itself; it loses its relevance as soon as the policy changes. The course of public policies is by no means automatic. It is subject to the uncertainties of the interactions between decisions or activities. Moreover, their duration is subject to the possibility of challenging the 'hypotheses on the causes' of the problems to solve and the 'hypotheses on the state solutions' to provide, on which they are based at some point.

The contingency of the analysis does not mean that one should disregard the diachrony and study of change in the definition of the problem causation and state intervention hypotheses, as is so often believed (Friedberg, 1993: 244). Public policies are not eventless; they really happen. Moreover, they have a historicity in so far as -produced by historically and spatially situated actors -they are only meaningful for a limited period. Peter Knoepfel situates himself, therefore, in a synchronic perspective and warns us that the systemic and nonpositivistic approach to public policy, which he defends, must recognize the temporal contingency of the analysis thus produced. In his opinion, there is nothing stable or timeless about a policy. On the contrary, it is a dynamic process of interaction. Hence, the empirical knowledge of the structural mechanisms -especially the general and specific 'institutional rules' influencing the actors' games and stabilizing interactions between decisions and activities -is what enables the analyst to describe and understand policy transformation (or abandonment) processes. The highly instructive example of the public problem of "poisoned water" enables him to get this theoretical message across. From the witches of yesteryear burned at the stake to today's farmers forced to change their production techniques, Peter Knoepfel uses allegory to talk about the historicity of the definition and treatment of a public problem. Moreover, the historicity of public policies is a principal component in this approach in that this dimension accommodates enduring and international comparisons as precisely explained in this book.

The (Hegelian) conception of the historicity of the problem causation and state intervention hypotheses underlying the choice and way of acting explains Peter Knoepfel's interest in 'causal stories' or, in other words, in public policy discourses which define the problems and legitimize the meaning of the action to be undertaken. His approach to public policies draws explicitly on the assets of cognitive perspectives which, in public policy analysis, serve to indicate the structuring importance of the 'causal stories' [START_REF] Stone | Causal stories and the formation of policy agendas[END_REF] and, more recently, the 'normative frameworks' or 'scalar narratives' [START_REF] Mahon | NAFTA, the Redesign and Rescaling of Canada's Welfare State[END_REF]) that point out the 'right' key known for the definition of public problems and legitimate policies and the appropriate means of dealing with them. With Peter Knoepfel (unlike Erhard Friedberg and Michel Crozier) the contingency of the analysis does not lead to a refusal of the importance of ideas in the policy production process for fear of a conception postulated in political terms, like 'totalism', i.e. as a producer of a general meaning whose effective scope cannot be demonstrated empirically [START_REF] Leca | † Le d€p€rissement de l'Etat. A propos de 'l'Acteur et le syst‰me' de Michel Crozier et Ehrard Friedberg ‡[END_REF]: 1149-1153). Knoepfel believes that the analyst's work consists in identifying competing causal stories so as to understand how a dominant 'problem causation hypothesis' emerges in a conflictual debate. We are very close here to 'the approach by reference frameworks' developed in France on the basis of Bruno Jobert's and Pierre Muller's work [START_REF] Jobert | L'Etat en action[END_REF][START_REF] Faure | La construction du sens dans les politiques publiques. Dƒbats autour de la notion de rƒfƒrentiel[END_REF]. Yet with Knoepfel, as with Friedberg [START_REF] Warin | La contingence des acteurs : apports et limites pour une analyse de la n€gociation[END_REF], the existence of a community of rules, a collective project, founding agreements and higher principles -all explanatory variables taken into account in the cognitivist approaches to policy -structures the field of action less explicitly than it does the capacity of the actors -who are placed in necessarily unequal negotiating positions in relation to the portfolio of resources at their disposal6 -to impose a definition of the causal hypothesis and the intervention hypothesis.

The pragmatism of the approach is measured, therefore, in terms of the deliberately a minima treatment reserved for the actors' personal variables (interests, motivations, intentions, beliefs). These exist and influence the actors' behaviours but they are excluded from the analysis because they are not independent of the actors. Pragmatism here resembles a form of realism. The analyst retains only those statements which enable him or her to relate any consideration of the actors and their rationale empirically to the reciprocal conditioning between context (set of resources and rules) and action (interactions between decision and activities). In other words, apart from the observation of this mutual conditioning, nothing can be said about the actors' action from the scientific point of view. The proposed approach thus excludes the personal variables that reduce the predictive capacity of the analysis. From a theoretical point of view, we can see that it is carefully kept apart from propositions designed to surpass the rustic utilitarianism of the neoclassical theory of homo economicus [START_REF] Etzioni | The Moral Dimension[END_REF], and before him an entire series of heterodox economists: [START_REF] Leibenstein | Beyond Economic Man: A New foundation for Microeconomics[END_REF]Akerlof, 1984). It is however based on a logical positivism, which to some extent brings to mind the ethnomethodologists' position when they contend that, apart from analyzing the processes through which the actors construct the situations they are simultaneously living and producing, the analyst has nothing but 'stories for the telling' [START_REF] Garfinkel | † Sur le probl‰me des correctifs ‡, Pratiques de formation (analyses)[END_REF].

In this type of approach the actors' power is concentrated in this capacity to control the resources and to mobilize them in a context of institutional rules so that they can impose both their opinions and demands on others. From this point of view, Peter Knoepfel does not problematize the political aspects of relations between actors much. He confines himself to an a minima conception of the political, amounting to the exchange of resources between the types of actor that he proposes to study: 'public authorities', the 'target groups', whose behaviours have to be changed to solve the public problem, and the 'beneficiary groups' of the policy that is made and implemented. The power is incorporated into the political interaction that guides the definition of the problem causation and state intervention hypotheses, through the mobilization of resources within the framework of pre-established rules. The pragmatic constructivism characterizing this approach to policy is then locked in by the granting of (systemic and non-positivistic) methodological priority to the endogenous explication of explanatory variables. This complicates the connection with the substantive approaches to politics in the production of policies. Starting with this comment, we can now try to discuss this consequence of constructivist pragmatism in public policy analysis, to characterize its specificity with even more precision.

An axiomatic neutrality

From the perspective of pragmatic constructivism, policies mobilize configurations of actors, resources and rules with the aim of directly producing or coordinating the responses required to meet the collective needs recognized as falling within the province of the state or other public authorities. From this point of view they are 'instituted policies'. In this respect the policy approach concerns the actual realization of outputs that are supposed to influence the behaviour of the groups or individuals believed to be at the root of the public problem to be resolved. It studies the performance of these configurations, sometimes following explicit engineering goals, as in the case, in particular, of the evaluation of policies or action programmes. The complex configurations of actors, in particular at the level of the 'politicoadministrative arrangement' of a policy, representing the structured whole of public and parapublic actors responsible for its implementation, produces what we now call policy governance. Many research studies are devoted to it, both in France and abroad. They analyze how the logics of reconstruction of public action are structured by the forms of mobilization of the social actors, especially by the shifting of traditional boundaries between legitimate (or incumbent) actors and challengers. Analytical tools have been developed for examining these interdependencies -such as the notions of 'networks' for linking up actors [START_REF] Thatcher | † R€seau (policy network ) ‡[END_REF] and 'forums' for the channels of influence on public action (Boussaguet, 2004) -and identifying the importance of the qualification of the objects of action in the processes of actors' interference, especially around the notion of a 'public problem' [START_REF] Sheppard | † Probl‰me public ‡[END_REF]. This presentation of policies is not, however, the first one selected to explain the role of politics in policy-making. Another approach consists in considering policies as 'instituting policies', in so far as the process and political implications of their production lie in the very definition of the problems to be resolved [START_REF] Giraud | Politiques publiques et dƒmocratie[END_REF]. Within this approach, policies effectively mobilize institutional configurations. However these are studied less for the outputs of the policies that they decide on and implement (resource allocation, setting legal or administrative rules, decisions on equipment programmes, etc.), than for the relations of domination that they institute in order to structure the political field of action. In this instance, policies are processes involving the social definition of reality, and are analyzed as such.

The policies are 'instituting' in so far as they develop a representation of the action to be undertaken, which integrates a dual hypothesis on the causes of the problem to be dealt with and the solutions to be provided. The construction of what is then called a 'normative frame' is at the very heart of the policy production process [START_REF] Jobert | † Des Etats en interactions ‡[END_REF]. This is of particular interest to policy analysts in France. One of the pioneers in this respect, Robert Axelrod, demonstrated it in his own way in terms of the cognitive maps of political elites which are mobilized in public decision-making (Axelrod, 1976). This construction is an eminently political moment, which determines which actor wins or loses. Contradictory interests are expressed and sometimes open up crisis zones which politics has to manage. The return to stability then consists in drawing on a repertory -representative of the political culture -of symbolic and cognitive elements that serve to guarantee the maximum possible compliance on the part of the actors.

The approach to public policies as 'instituting policies' is presented as a structural approach to politics. It highlights the political regulation of the social order or, in other words, the need to contain the centrifugal forces stemming from contradictory interests, which would otherwise make it impossible to produce any policies at all. The public policy approach to politics raises specific questions with original theoretical foundations. In particular, the importance granted to contradictory interests explicitly takes into account the Habermassian analysis of the fragility of social cohesion and, more implicitly, the analysis of the state proposed by Nobert Elias. 7 Social cohesion is threatened by both the disparities between the different ensembles that structure society and the non-adhesion of subjects, capable of analysis and judgement, with values and symbols that justify the permanence of a social order. The politicians' specific task is to maintain the political community by safeguarding its structured components and the integration of individuals. To analyze it as such, this alternative approach to public policies asks two questions: How do the main components hold together as a whole? And, how do the different individuals, situated in relation to these structural components, adhere to the political community? This conception of the politicians' work refers to a set of actions and functions conceived of as a whole. Politics is, therefore, more a dynamic, a process that tends to include actors and institutions, than a passively occupied space.

The mastery of this fundamental function of politics spurs the ambitions of actors who understand that by dominating the representation of the social reality on which it is necessary to act, they occupy a key position de facto. The regulation of these ambitions is needed to ensure social order. It functions in the very production of policies, and determines their form and content. This regulation of ambitions is directly political; it is the business of whosoever accepts the pre-eminence and continuity of stabilized symbolic and cognitive elements. The fact of mastering discourse on the values, norms and rules that have to be complied with for problems to be addressed, puts him or her at the heart of the configuration of actors. We find an anthropological conception of politicians (and politics) here, insofar as this power is that of the pacifier whose means are limited to speech (Clastres, 1974: 175-176) and whose hegemony (or monopolization) constitutes the state as a function of arbitration [START_REF] Elias | La dynamique de l'Occident[END_REF], as per the conception of the state theorized by [START_REF] Hobbes | Le Citoyen ou les fondements de la politique[END_REF][START_REF] Hobbes | Le Citoyen ou les fondements de la politique[END_REF]. This political regulation is nevertheless a result of a mobilization of legitimate constraint [START_REF] Jobert | † La r€gulation politique : le point de vue d'un politiste ‡[END_REF]. Public action cannot do without arguments for ranking and making more or less compatible the use of political resources. The study of this regulation or the practice of political power is, therefore, crucial here. It concerns above all the clashes between representations of social reality, which necessarily highlight the divergence of interests. This point of view also corresponds to recent analyses. In the late 1990s Renate Mayntz concluded a dialogue with Niklas Luhmann, who pointed out the inanity of politics faced with the irrepressible reinforcement of interests contradictions and with auto-referentiality of ensembles that structure society, by redefining politics as the 'management of interdependencies' [START_REF] Mayntz | Zur Selektivit‹t der steuerungstheoretischen Perspektive[END_REF]. The function of politics, thus defined by the famous German political scientist, is compatible with Luhmann's diagnosis: i.e. to avoid the shocks and contradictions resulting from the pursuit of the process of differentiation. This conflict management, central in policy-making, reveals the underlying ambiguity of politics. In a democratic context, politics does not amount to avoiding the shocks of differentiation. It must also find ways of reconciling political equality with social inequalities, as the latter also depend on the ways in which social interests are structured. This debate, which first emerged between Niklas Luhmann and Fritz Scharpf [START_REF] Luhmann | Politische Steuerung: Ein Diskussionsbeitrag[END_REF][START_REF] Scharpf | Politische Steuerung und Politische Institutionen[END_REF]), has had an enduring influence on Peter Knoepfel, who participated directly in the founding research programme directed by Scharpf and Mayntz in the 1980s (Knoepfel, Weidmer, 1982). Indeed, the approach to policies as 'instituting policies' is clearly apparent in Knoepfel's approach around the question of the potentially conflictual relations between target groups and end beneficiaries and other actors, who are also affected by the effects of a policy. While the politics in the theoretical model may be rather subtle (and in Knoepfel's teaching), this main dimension is never entirely lacking. The demonstration of its existence is not, however, the ultimate goal in Knoepfel's policy approach.

The understanding of policy in the approach to instituted policies outlined under the pragmatic constructivism perspective implies an understanding of the processes through which this shared signification of problems and solutions is not only produced, but also produces effects of meaning throughout the course of a policy. That is why it seeks to consider each product of a policy from both a substantial and institutional point of view. Peter Knoepfel equips us with useful tools for identifying the variables to be explained and the explanatory variables, and for understanding the substance of the outcome of the interactions between decisions and activities. He is, however, more vague when it comes to the question of the semantic productivity of problem causation hypotheses and state intervention (ideas and social representations) -that is, when the subject is not totally avoided. Policy analysis does not stop when have defined the play of hypotheses characterizing the raison d''tre and orientation of a policy. It could even be said that it starts when one wonders which effects of meaning this shared normative signification is going to produce next, particularly during the shift from the 'product' of one public policy to another. This remark prompts, therefore, the question as to whether the 'Knoepfelian approach' to policy contains a theory of 'shared cooperative action', at least if the pragmatic hypothesis is applied [START_REF] Bratman | Shared Cooperation Activity[END_REF][START_REF] Coleman | The Practice of Principle: In Defence of a Pragmatist Approach to Legal Theory[END_REF], according to which the semantic productivity of the definition of problems and solutions could be judiciously analyzed in terms of cooperative action. The approach developed by Peter Knoepfel seems somewhat allusive on this point. It appears to have little concern for integrating reflection on the 'social system' when the actors interact. Yet it also seems to show that the definition and implementation of a policy cannot be summed up in the achievement of objectives that are intentional or attributed to one or more actors; they also stem from the renewal or transformation of shared beliefs. In other words, the pragmatic constructivism proposed by Peter Knoepfel takes policy analysis to the extreme, insofar as the explanation of a policy is based on a combination of independent observable variables (actors, resources and rules). The advantage of this epistemological position is to ensure that policy analysis does not go astray -at the other extreme -in the study of the meaning attributed to and produced by public action, but remains apprehendable from the outside. It is probably correct to point this out at a time when cognitivist approaches are conquering much of the field of policy studies (for France, see: RFSP, 2000). The risk that is run here would be to lose sight of the (supposed) realism needed by the scientific approach, and to focus exclusively on the knowledge, ideas, representations and beliefs of the actors involved. The reason is that they make it possible to describe what politics really resembles, at the observable level of the reciprocal conditioning between context (a set of resources and rules) and action (interactions between decisions and activities).

The proposed approach does not aim, therefore, to show that policies are the result of conflicts between groups of actors who mobilize to guarantee the satisfaction of 'causes' stemming from shared beliefs and norms (Sabatier & Jenkins-Smith, 1993). This possibility is not, however, denied. It is just that Knoepfel does not consider that the aim of policy analysis is to say that the actors share the same 'worldview', no more than it is to show that they act out of self-interest. Peter Knoepfel's analysis is noteworthy in so far as it does not impose a cardinal theory of the actors as explanatory factors in the birth and implementation of policies. His pragmatism is characterized -as I have said -by a deliberately a minima treatment of the actors' personal variables (interests, motivations, intentions, beliefs). He sees beliefs and interests as inputs that are potentially relevant within the explanatory variables as a whole. If they exist, the analyst will have to account for them by induction. This approach is resolutely oriented towards a methodology for policy analysis. It is based, therefore, on axiomatic neutrality so as to avoid theoretical debates and standpoints which are sterile for the analyst whose analysis helps to steer policy. For all of these reasons, the constructivist pragmatism underpinning Peter Knoepfel's approach is clearly distinguished from policy analysis as 'pragmatic of democracy', as it appears from the cognitivist perspective [START_REF] Giraud | Politiques publiques et dƒmocratie[END_REF]. By proposing to approach policies as essentially intellectual constructions, loci of debate, and places of transformation of social representations of the reality to be dealt with this other perspective chooses explanatory variables which are substantially different (actors' logics and interests; social representations; institutional rules that strongly frame -or not -political interaction; possibility -or not -of rigorous and transparent procedures; scenes of debate, controversy or communication; alliances and power struggles etc.). At the same time, and above all, the variables that are to be explained change. They are 'products' not of policies, as with Knoepfel, but of the permanent production of the social order through the political work of reconciliation of contradictory interests, which policies support. Policies are characterized here by the necessary adjustment of social representations which change sooner or later. In this case they are studied because, through the possibility of viewing problems and solutions differently, they can call for changes in institutional configurations and sometimes even in aspects of the political culture. As a field for political expression, public policies appear to be a democratic affair; they help to maintain the social order by making its renewal possible. In other words, these two perspectives do not have the same scientific goal. In the one, policy analysis serves reflection on politics and democracy. In the other, it aims to understand how policies are made -and that is the underlying intention of the approach developed by and around Peter Knoepfel.

* *

In the final analysis, the constructivist pragmatism which I see as characterizing the public policy analysis proposed by Peter Knoepfel can probably be explained by his idea of his profession as a university lecturer, researcher, expert and, at the heart of all that, teacher and adviser. The axiomatic neutrality characterizing his approach corresponds to his main objective of equipping analysts, scientists and practitioners with a framework for understanding situations. His scientific contribution lies in the fact of making possible an ethnography of public policies. By being wary of the analyst who has only 'stories to tell', Peter Knoepfel reminds us in a way of an international figure in anthropology, Claude Levi-Strauss, who started his famous popularized book Tristes tropiques ([1955] 1985) with the following incipit: 'I hate journeys and explorers'. Let us therefore dare to compare pragmatic constructivism at the service of a logical reasoning in policy analysis to Lƒvi-Strauss's break with the travel novel genre, with the aim of establishing an intellectual experience of another kind, that of structural logic.

The axiomatic neutrality that is the framework of the pragmatic constructivism of this approach nevertheless raises a fundamental question that I will leave open: the question of whether it reduces policy analysis to a technique. The history of the social sciences shows how axiomatic neutrality has served as a requisite for intellectual strategies for promoting reformist formulae that do not challenge the socio-economic order [START_REF] Guilhot | † Une vocation philanthropique. Georges Soros, les sciences sociales et la r€gulation du march€ mondial ‡[END_REF]. I certainly do not believe that Peter Knoepfel's scientific contributions serve this type of goal. Along with other European academic institutions (e.g. the International Management Institute, and the Environmental Research Institute of Berlin), the Institut des Hautes Etudes en Administration Publique, where Peter Knoepfel worked for a long time and of which he was also Director, is one of the main places in Europe where the interaction between research and application have contributed to the complete renewal of training in public management, and to the development of the 'policy sciences' on the basis of a critical view of 'public administration theory' (Thoenig, 1985: 2). In the context of this international perspective, his approach to policy analysis contributes deliberately to a questioning of the policy dimensions that defy the mode of analysis so dear to this theory. Like Dror and others, who strove to introduce organizational or institutional factors, conflicts, ideologies, and even the social value of time -outrageously neglected by institutions such as the Rand Corporation in their biased analysis of government action [START_REF] Dror | Public Policy Making Reexamined[END_REF][START_REF] Dror | Prolegomena to Policy Sciences[END_REF] -, Peter Knoepfel endeavours to explain the importance of a methodical approach to the explanatory variables of public policies, which denaturalizes categories and gives depth to the analysis of government decisions and activities. His aim is to provide the means of producing complete scientific statements on which the debate on public policy can be based. From this point of view, his approach helps to make policy analysis a real government science as it shows -mainly through the study of resources -the diversity of approaches (and sciences), from which government authorities can act.

  A. (1984). An Economic Theorist's Book of Tales, Cambridge (Mass): Cambridge University Press. Axelrod R. (ed.) (1976). Structure of Decision: the Cognitive Maps of Political Elites, Princeton: Princeton University Press. Blumer H. (1971). "Collected Behavior", in A.M. Lee (ed.), New Outline of the Principles of Sociology, New-York: Barnes and Noble: 167-222. Boussaguet L. (2004). † Forums ‡, in L. Boussaguet, S. Jacquot et P. Ravinet (dir.), Dictionnaire des politiques publiques, Paris : Presses de Science Po: 226-232.

This reference work is the main theme of this chapter.

As I was able to witness by accompanying him for several years at a seminar in the Institut d'Etudes Politiques de Grenoble.

The products are: 'the political definition of the public problem', 'the politico-administrative programme', 'the politico-administrative arrangement', 'the action plans', 'the implementation outputs', 'the evaluative statements on the effects of a public policy'.

First English translation.

For an overview of precursory debates, see[START_REF] Schneider | Social Problems Theory: The constructivist View[END_REF] Holstein & Miller, 1993.

The main resources studied are: 'force (violence resource); law (legal resource); staff (human resource); money (monetary resource); information (cognitive resource); organization (interactive resource); consensus (trust resource); time (temporal resource); infrastructure (patrimonial resource); political support (majority resource)'.

For Elias, both the problem and raison d''tre of social order is to render the interdependence of individuals (individuals need one another) and their conflicts (but their interests oppose them) compatible, to ensure the conditions of living together[START_REF] Elias | La dynamique de l'Occident[END_REF].