Ph Jeanneaux
email: philippe.jeanneaux@vetagro

; Blasquiet

Localised Agro-Food Systems in France and Dairy Farms performance

Keywords: Dairy farm, Economic performance, Mountain, Cheese

In France, the dairy sector within the mountain areas turned strongly to a strategy of cheese production under quality signal like of differentiation, two strong disparities concerns the Agricultural Income differential per dairy farm per year; the other -linked to the previous disparity are getting older and fewer y shed light on the impact of economic and social performance one (PDO Cantal), few firms have taken control on the supply chain and have chosen with the time a model based on costs leadership, imposing low milk price at farm gate. The second one (PDO Comté) is a specific model with a strong and structured collective a these two models have an impact on the dairy farms economic for the PDO Comté case, some business companies (farmers and their cooperatives which are agro-food processing firms), related within a achieved to control for the better the rules which govern the PDO cheese production chain. This collective action in Jura Mountain and its market power benefit to farmers thanks to the better milk price at farm gate than in others mountain areas. Thanks to data FADN (survey concerning European Network for farm accountancy) , the dairy sector within the mountain areas turned strongly to a strategy of cheese production under quality signal like Protected Designation of Origin (PDO). Despite this strategy strong disparities exist in the dairy farms in France over a long time Income differential per dairy farm (profit before tax) linked to the previous disparity-concerns the renewal in agricultu are getting older and fewer young people are entering farming. The objective of shed light on the impact of a collective action and a substantial Gross Output performances. We scrutinized two contrasted cases studies. For the first e (PDO Cantal), few firms have taken control on the supply chain and have chosen with the time a model based on costs leadership, imposing low milk price at farm gate. The second one model with a strong and structured collective a these two models have an impact on the dairy farms economic and social performance. Indeed, for the PDO Comté case, some business companies (farmers and their cooperatives which are essing firms), related within a localised agro-food system in Jura Mountain, have achieved to control for the better the rules which govern the PDO cheese production chain. This collective action in Jura Mountain and its market power benefit to farmers thanks to the better e than in others mountain areas. Thanks to data FADN (survey concerning European Network for farm accountancy) and data from the French Agricultural Census 2010 1 Food Systems in France and Dairy Farms performances making; Collective actions.

, the dairy sector within the mountain areas turned strongly to a strategy of cheese (PDO). Despite this strategy ance over a long time: One (profit before tax), close to 10 000 € concerns the renewal in agriculture. Farmers objective of this paper is to Gross Output to the farms' . We scrutinized two contrasted cases studies. For the first e (PDO Cantal), few firms have taken control on the supply chain and have chosen with the time a model based on costs leadership, imposing low milk price at farm gate. The second one model with a strong and structured collective action. We show that performance. Indeed, for the PDO Comté case, some business companies (farmers and their cooperatives which are food system in Jura Mountain, have achieved to control for the better the rules which govern the PDO cheese production chain. This collective action in Jura Mountain and its market power benefit to farmers thanks to the better e than in others mountain areas. Thanks to data FADN (survey concerning and data from the French Agricultural Census 2010, we bring to light the main factors of explanations of the highest economic and social performance of the dairy farms. The substantial Agricultural Income per farm is due to the substantial Gross Output. This extra Gross Output is partly saved:

to increase the farm size and to organise regrouping of lands; -to increase the herd and therefore the production; -to develop technologies and assets. These numerous actions have strongly carried farmers to develop new skills, to increase the labour productivity and to ensure greater the renewal in dairy farms. This evolution has begun for few decades.

Introduction

In France, the dairy farms of mountainous areas have on average an income lower (22'400€) than the farms located in plain areas (31'500€) [START_REF] Chatellier | La production laitière dans les montagnes françaises : une dynamique particulière pour les Alpes du Nord INRA Productions[END_REF]. However, this situation hides disparities within mountainous areas. Over a long time the economic performance of the dairy sector in mountainous areas is lower than in plain areas, the dairy production and the cheese-making processes are important activities for the economic development of this kind of areas in France. The dairy sector of the mountainous areas turned strongly to a strategy of cheese production under quality sign. Out of twenty-six cheeses made with cow's milk and produced in mountainous areas, fifteen cheeses benefit by a quality sign like the Protected Designation of Origin (PDO). Despite this strategy of differentiation, strong disparities of Agricultural Income per dairy farm, close to 10'000€, exist in the dairy farms in French mountainous areas over a long time. Moreover, there are income differential between the various mountain massifs. For example, the dairy farms of the Jura Mountain (Easter France) have income upper to those of the Massif Central (Middle of France). Yet, one of the important challenges in the economic development of the mountainous areas comes from the capacity of the dairy farms to have high incomes. We suggest that the ability to seek wealth is due to farmers' collective action which helps them to improve their skills and to ensure greater the renewal in dairy farms. Dairy farms renewal rate (94%) in the Jura Mountain is higher than in Massif Central.

So, this paper brings to light the determiners of the economic performance of the dairy farms observed on two French contrasted cases studies located in mountainous areas: the Jura Mountain and the Massif Central. The aim of this paper is to shed light the impact of the farmers' collective action on the Agricultural Income and renewal in dairy farms.

In a first part, we develop the theoretical framework and the methods (2.). In a second part, we describe the two case studies (3.). Then, we propose a typology of farms. Their characteristics are explained by descriptive statistics (4.). The discussion highlights the reasons why there are disparities of income and indirectly of renewal between the two PDO systems (5.), before the conclusion (6.).

The framework and the method

We develop this issue at the intersection of Agricultural Economics and Law and Economics.

On the one hand, the agricultural economics theory allows us to analyze the individual strategic behaviour concerning particularly the milk production from dairy farmers. This work is included in the general research on economic efficiency [START_REF] Battesse | Frontier production functions and technical efficiency: a survey of empirical applications in agricultural economics[END_REF] which has begun by Farrell in 1957. Afterwards, development concerning economic efficiency has combined both allocative and technical efficiency. This theoretical framework deals with some factors (assets characteristic, education, household, age, human capital) explaining the significant difference in the level of allocative generally between two groups of farmers. It is achieved when the farmer combines resources in the relevant combination to generate maximum output (technical) as well as ensuring least cost to obtain maximum income (allocative) [START_REF] Christopher | A Quantitative Determination of Allocative Efficiency[END_REF].

On the other hand, our analytical framework has to be considered in the vast domain of Laws and Economics based on the observation of "Law in Action" using direct legal sources. This discipline allows to analyse the farmers collective action and its economic consequences [START_REF] Kirat | L'analyse économique, le droit et les règles : les questions méthodologiques de l'interface droit-économie dans les travaux francophones[END_REF]. The legal elements form the relevant dimensions for the stakeholders for setting and implementing a collective economical strategy while seeking to influence their institutional environment and the relations between the economic stakeholders.

For estimating the characteristics and the economic performances of the dairy farms due to collective action, two different PDO cheese supply chain strategies were studied (PDO Cantal and PDO Comté in France) at different stages (dairy farmers, cheese-makers, and cheeseripeners) using three major sources:

(1) The first source characterizes the dynamic of each cheese supply chain over a long period. The characteristics and the evolution of the productive structures at the different stages were analyzed in order to recreate the agents' paths. To obtain the data, we conducted a survey among the stakeholders and also used documentary sources [START_REF] Jeanneaux | Durabilité d'un compromis territorial dans un contexte de pression compétitive accrue : le cas de la filière AOC Comté[END_REF][START_REF] Meyer | Filières fromagères sous AOC en zone de montagne et performances économique : Etude des filières AOC gruyère et cantal[END_REF]. For characterizing the collective strategy of the production systems, we based the analysis on legal documents for each PDO. These documents set the content of the PDO's codes of practices and throw light on the production's organization and the constraints put on competitors. We try to analyze the value creation and the mechanism for its distribution among the different stages of the supply chain. We have identified the specific resources used, the productive structures of the supply chains (technical and social division of labour between stakeholders), the mechanisms for managing supply, as well as the fundamental inter-professional agreements that set the payment modalities for cheese.

(2) The second source characterizes the performances of the dairy farms in the Massif Central and in the Jura Mountain thanks to the FADN database (Farm Accountancy Data Network; RICA in French) for the year 2007. FADN is an instrument for evaluating the income of agricultural holdings and the impacts of the Common Agricultural Policy. "It consists of an annual survey carried out by the Member States of the European Union. The services responsible in the Union for the operation of the FADN collect every year accountancy data from a sample of the agricultural holdings in the European Union. Derived from national surveys, the FADN is the only source of microeconomic data that is harmonized, i.e. the bookkeeping principles are the same in all countries. Holdings are selected to take part in the survey on the basis of sampling plans established at the level of each region in the Union. The survey does not cover all the agricultural holdings in the Union but only those, which due to their size, could be considered commercial. The methodology applied aims to provide representative data along three dimensions: region, economic size and type of farming" 1 . FADN database brings details of the structure of the farms (surface, herd), their economic results (agricultural production, loads, subsidies, income) and their financial situation (liability). We particularly resumed for the analysis the intermediary management balances to assess the economic efficiency of the dairy farms.

(3) The third source characterizes the social performance of the dairy farm in the Massif Central and in the Jura Mountain thanks to the last French Agricultural Census carried out in 2010. This census gives information about renewal in agriculture, farmers' age, and demographic trends from 2000 to 2010. This information is useful to understand relation between economic efficiency and farming transition. Thanks to theses investigations, we bring to light the existence of two opposite models of cheese making under quality signs in the French mountain's areas, which strongly influence the performance of the dairy farms.

Economics results of dairy farms in Jura Mountain and Massif Central

We show that these two models have an impact on the dairy farms economic performance. In a first part, we present the specificities of the two localized agro-food systems due to the nature of the collective action. Then, we present the farms' typology before to scrutinize the factors of their performance disparities.

PDO Cantal vs. PDO Comté, two relevant case studies to highlight milk price disparities

The Cantal supply chain: a French PDO cheese supply chain dominated by industrial cheesemakers Agriculture in the mountains of the centre of France is strongly dedicated to milk production that feeds into cheese supply chains benefiting from a legal recognition through official quality labels (several PDOs: Cantal, Saint-Nectaire, Bleu d'Auvergne, Fourme d'Ambert, Salers). In 2007, these products from the Auvergne Region, brought together around 8'000 dairy farmers, producing more than 1'200 million litres of milk. Among them, 2'800 farmers produced milk necessary for 17'800 tons of PDO Cantal. Since 1990, the PDO Cantal production is stabilized around 18'000 tons.

From a historical perspective, the PDO Cantal was the most important vector to the organizational development of the cheese supply chain in the Cantal department (located in the Auvergne region). It was based, at the beginning, on a specific technical and social division of labour before a modernization turn, during which some industrial cheese-making units have taken control over the supply chain. As a result, they have drastically gained in market power, having reached the quasi-exclusive access to market. Today, two industrial processors control around 70 % of the PDO Cantal production. Finally, industrial cheese-makers have imposed a production system mostly based on cost leadership strategy to increase their competitive advantage. As a result, the dairy farmers have lost their bargaining power toward their buyers. The regional price is disconnected from the PDO Cantal cheese market price, because the farmers in the supply chain do not know in which product is processed their milk. The milk price depends as well on the bargaining skills of the dairy farmers when discussing with the cheese-makers. Because milk producers are just suppliers, their bargaining abilities are substantially restricted. Moreover, the cheese-makers must not declare their sales' volumes and selling prices for cheese to the interprofessional organization. The organization does not publish reference prices of Cantal cheese, which might help market transparency. Consequently, over the last 15 years (excepted 2007), the milk price for PDO Cantal has been the same as standard milk and has reached a 25% lower price compared to some French PDO (i.e. PDO Comté) [START_REF] Jeanneaux | Diversité des modes de gouvernance dans quatre filières fromagères sous AOC en Europe : un cadre d'analyse pour comprendre l'origine des prix aux producteurs[END_REF].

PDO Comté: A traditional and dynamic collective cheese supply chain

Agriculture in the Jura mountain is mainly dedicated to milk production that feeds into cheese supply chains benefiting from a legal recognition through official quality labels (several PDOs : Comté, Morbier, Mont d'Or, Bleu de Gex). These products originate from the Doubs and Jura Departments, gathered in 2008 3'420 dairy farmers, producing more than 760 million litres of milk. Among them, 3'000 farmers produced 52'000 tons of Comté. Since 1990, the Comté production significantly has increased by more than 15'000 tons. From a historical perspective, the PDO Comté was the most important vector to the organisational development of the cheese production system in the Jura, based on specific technical and social division of labour (Perrier-Cornet, 1986). The dairy farmers, on the one side, are organised in collective cheese-making units (150 cheese-making cooperatives in 2005), control the processing from milk to fresh cheese (not ripened) and do not have market access. The cheese-ripeners (around 10 units in 2005), on the other side, have the quasi-exclusive access to market without being involved in the first processing stage. This labour division is still present and generates a long-lasting collective value that is safe-guarded by the successful setting of the Protected Denomination of Origin Comté.

The surplus resulting from the organization comes from the PDO code of practices [START_REF] Barjolle | Some factors of success for origin labelled products in agrifood supply chains in Europe: market, internal resources and institutions In The socio-economics of origin labelled products in agri-food supply chains: spatial, institutional and co-ordination aspects[END_REF]. The Comté supply chain is often considered as a model of collective action. It set a powerful organization and giving extended authority for bargaining and implementing Comté supply control measures, as well as promoting the differentiation strategy, which protects the interests of the Jura dairy farmers, small-scale cheese-makers and ripening facilities. The Comté supply chain shaped the agro-food sector in this mountainous region and supported the selling price of that cheese. In 2007, the average price was 10.20 €/kg compared to 7.10 €/kg for French Emmental,2 that is to say a positive price difference of 47% in favour of the Comté 3 .

The milk price depends as well on the bargaining skills of the dairy farmers (as they also control most of cheese-making cooperatives) when discussing within the collective organization about the calculation ratio for fresh cheeses they sell to the ripeners. Until today, the supply chain organized the surplus' distribution between « production » and « market ». The system's efficiency was based on the regulated distribution of the collective surplus that allowed dairy farmers, over the last 15 years (excepted 2007) to reach a 25% higher price for milk compared to national average [START_REF] Jeanneaux | Stratégie d'élévation des coûts des concurrents pour préserver un système productif agroalimentaire : le cas d'une filière fromagère d'appellation d'origine[END_REF].

The main factors of explanations of the highest economic performance of the dairy farms in Jura Mountain

After this first presentation of the two localized agro-food systems and their different collective strategies, the objective is to highlight the diversity of the farms' performance and their potential links with the specificities of each localized agro-food systems.

A first analysis: the factorial analysis, to present the disparities between farms

Thanks to a Multiple Correspondence Analysis and an Ascending Hierarchical Classification, we have identified five classes of dairy farms. This typology helps us to scrutinize the first results:

-The first class (36 farms) includes the small farms (AWU < 1.5; Total Utilized Agricultural Area < 56 ha) which produce few benefit (low milk price) and which are rather located in Auvergne (Massif Central).

-The second class (25 farms) is the class of the big farms located also in Auvergne (AWU > 1.5; Total Utilised Agricultural Area > 96ha) with a high production, partially modernized, and with a milk price intermediate.

-The third class concerns dairy farms small to average (AWU < 1.5 and Total Utilized Agricultural Area between 56 and 96 ha) with a strong benefit (FNVA/AWU > 21 062 €, high EBITDA/AWU). They have also a high production and a high milk price (33.2 €) and are preferentially located in Franche-Comté (Jura Mountain).

-The fourth class (25 farms) groups big farms (Total Utilized Agricultural Area > 96 ha) which are very efficient (FNVA/AWU > 21 062 €, high EBITDA/AWU). Theses farms have high milk price and are modernized (high fixed assets). They are located also in Franche-Comté.

-Finally, the fifth class groups the farms (28) that are rather average (in terms of Total Utilized Agricultural Area) with lots of AWU (> 1.5). Theses dairy farms make low benefits (low FNVA/AWU and low EBITBA/AWU) even if the production and the milk price are mostly important. Moreover, these farms are not much modernized (few fixed assets).

Thanks to this analyze, we are able to distinguish two different forms of dairy farms for each region. These forms tend to oppose Auvergne and Franche-Comté. In Auvergne, the small dairy farms have not been able to invest to modernize their production system unlike the small dairy farms in Franche-Comté. Concerning the biggest dairy farms in Auvergne, they have invested, but in an expensive extensive model in terms of inputs and which valuing badly the forage resource (so the specific resources of the region).

A second descriptive statistics analysis to present the origin of the disparities

The substantial Agricultural Income per farm is due to the substantial Gross Output. Jura Mountain farm Production for the accounting year balance is up to 26'000 € than Massif Central farm. This disparity is due to a better milk price at farm gate (close to 12'000€) because of the power market ability of the Jura Mountain farmers.

This extra Gross Output on its other part is due to technical efficiency. Indeed, extra Gross Output is partly saved (cf. table 1):

to increase the farm size and to organise regrouping of lands. Indeed, Jura Mountain farms owned 19 ha more than Massif Central farms.

to increase the herd and therefore the production. Thanks to the access to more areas, Jura Mountain farmers can breed more cows and thus produce more milk (33'000 L) while their farms employ the same number of workers.

to develop technologies and assets. The extra Gross Output is more invested in agricultural equipments in Jura Mountain farms than Massif Central farms (more than 25'000 €).

These numerous actions have strongly carried farmers to develop new skills and to increase the labour productivity. New technical skills are identified with the yield of the cows (750 L/cow up for Franche-Comté compared to Auvergne) and the production costs (fertilizers are less in Franche-Comté close to 10€/ha). The labour productivity is also higher for Jura Mountain farmers than for Massif Central farmers (5000 €/farmer). Consequently, the Gross operating income compared to the Gross Output is better, close to 4 points (42% compared to 38%). As a conclusion, Jura Mountain farms are more effective than Massif Central farms. We show there is a relation between economical performance and localized agro-food system. Main factors of explanations of the highest economic performance of the dairy farms could be scrutinized. The substantial Agricultural Income per farm is due to the substantial Gross Output. The origin of this extra Gross Output is partly explained to the better milk price at farm gate and partly explained by the individual strategies developed by dairy farmers. We suggest that the ability to seek wealth is due to farmers' collective action which helps them to improve their skills and to ensure greater the renewal in dairy farms. Indeed, dairy farms renewal rate (94%) in the Jura Mountain (Doubs Department) is higher than in Massif Central (79% for Cantal Department).

In this last department, farmers are getting older and fewer young people are entering farming. In the Doubs Department 32% of farmers are under 40 years old, 10 points more than Cantal Department farmers (22%), and moreover 33% are over 50 years old, 10 points less than Cantal Department farmers (47%).

Discussion

The results bring to light two opposite models of cheese-maker production, which drive towards two farm systems and their performances.

Dairy farms of Massif Central and Jura Mountain have similar conditions of production, sell both under PDO label, but there are an important difference of performance between them. This evolution began a few decades ago and continues. We discuss the idea that two aspects can explain the differences in the agricultural incomes between the dairy farms of the different massifs.

On the one hand, the difference of income can be explained by external determinants, that is the collective strategies and the forms of coordination between the stakeholders of the cheesemakers industry and the farmers of the dairy farms (Perrier-Cornet & Sylvander, 2000).

Concerning the external determinants, the farmers cannot control them directly but they can have an effect on this type of determinants in getting involved in collective strategies as dairy cooperatives. The specific rules in the codes of practices set restrictions so to give decisive advantages to small-scale enterprises thank the imposition to their competitors (large dairy industry companies and intensive dairy farmers) specific production techniques and its related costs. This constitutes a lever for imposing to the certain economic agents a certain level of production costs which corresponds to a certain type of producers/processors (Jeanneaux & al., 2010). Raising Rivals' Costs theory [START_REF] Scheffman | 20 Years of Raising Rivals' Costs : History, Assessment, and Future[END_REF]) helps to answer the question whether the PDO cheese production costs are actually imposed to competitors by the defenders of the local PDO production system. Indeed, associations of farmers can have an impact in the price formation of milk and in the distribution of the benefits between farmers. It's not the case for PDO cantal, but true particularly within PDO Comté local agro-food system which has an impact on the dairy farms economic performance thanks to a best milk price. Thus:

-For the first one (PDO Cantal), few agro-food firms have taken control on the supply chain and have chosen with the time a model based on costs leadership, imposing low milk price at farm gate. The PDO Cantal is a local agro-food system with little structure and with few constraints of production based on a historical tradition of individual behaviour.

-The second one (PDO Comté) is a local agro-food system with a strong and structured collective action. Indeed, for the PDO Comté case, some business companies (farmers and their cooperatives which are agro-food processing firms), related within an agro-food local productive system in Jura Mountain, have achieved to control for the better the rules which govern the PDO cheese production chain. The PDO Comté is a model with a strong and structured interprofessional structure based on a historical tradition of collective action. This collective action in Jura Mountain and its market power benefit to farmers through a better milk price at farm gate than in others mountain areas and dynamizes the Jura region.

On the other hand, the difference of income can be explained by internal determinants as the individual strategies of the farmers. However, we saw that these strategies result, at the beginning, to the capacity of the farmers to keep the Gross Output in the farm to modernize the equipments, to develop the size of the farm and to improve consequently the human skills. These numerous actions have strongly carried farmers to develop new skills and to increase the labour productivity. We know the nature of social capital (bonding and bridging) in a territory is an important factor to explain economical development disparities [START_REF] Callois | Towards indicators of social capital for regional development issues[END_REF]. To more understand the relation between social capital, collective action and economical and social performance of dairy farms in mountainous areas some works would be developed in this way.

We are though very much aware of the limits of our approach as the hypothesis was only tested on some PDOs and it cannot be generalized. Further research is presently conducted to test this analytical grid of the collective regulation modes of localized production systems and its impact on global performance of farm in order to discuss the validity of our preliminary conclusions.

Conclusion

This paper shows that the collective action mode developed for managing PDO leads to different performances in PDO cheese supply chains, in regards to milk price at farm gate. With these two case studies, we show that the disparity Agricultural Income per farm (profit before tax) reaches 10000 €, and farm renewal is better in this situation. We could highlight the role of collective action which develops an incentive atmosphere right to create human, technical skills and to improve economical and social performances. Annexe 1: characteristics of dairy farms in the two studied regions

  .jeanneaux@vetagro-sup.fr Dairy farm; Economic performance; Mountain; Cheese-making; Collective actions.

  Thus, the reasoning can be sum up with the diagram below:The study concerned only the dairy farms (Otex n°41 ) located in mountainous areas (up to 600m) of two French regions: Franche-Comté in Jura Mountain (PDO Comté) and Auvergne in Massif Central (PDO Cantal) (see topographic map below). There are 133 dairy farms in the database in 2007: 39 for Franche-Comté and 94 for Auvergne. Consequently, the characteristics and performances of the dairy farms for both studied regions can only give an idea of the reality about the economic performances and renewal of the dairy mountains farms concerned by PDO Comté and Cantal.Data mining were carried out on SPAD v.7.0 by descriptive statistics (minimum, maximum, mean, standard deviation) and by factorial analysis (Multiple Correspondence Analysis, and Ascending Hierarchical Classification).We have taken into account ten variables for the Multiple Correspondence Analysis and Ascending Hierarchical Classification: Total Utilized Agricultural Area (ha); Total Utilized Agricultural Area/AWU (ha/awu); Milk price (€/l); Production for the accounting year (€/year); Production/number of dairy cow (€/cow); Production/AWU (€/awu); Farm net value added/AWU awu); Total input (€); Fixed assets/AWU (€/awu). Region (Franche-Comté or Auvergne) and EBITDA/AWU (€/awu) have been added in the analyses as illustrative variables.

Source : European Union, http://ec.europa.eu/agriculture/rica/concept_en.cfm

Source : Syndicat français de pâtes pressées cuites 2007

Source CIGC : http://www.comte.com/le-marche-du-comte,4,0,8,1,1.html


Hayat Doukkali
email: hayat.doukkali@enst-bretagne.fr

Loutfi Nuaymi
email: loutfi.nuaymi@enst-bretagne.fr

Analysis of MAC protocols for Underwater Acoustic Data Networks

Keywords: Underwater Acoustic Network, MAC Layer, acoustic channel, TDMA, CDMA, Power control

This paper presents a state of art of existing Underwater Acoustic Networks. We study specifically the Media Access Control (MAC) Protocol design of research work done in this field. A comparison of different protocols is presented according to several criteria: propagation delay, node connectivity and energy consumption. This comparison allows us to present some ideas for a new design of a MAC Underwater Acoustic protocol.

I. INTRODUCTION

Underwater Acoustic Networks constitute an important topic of interest for the offshore oil industry. It allows wireless communication between different submarine control vehicles and elements above the surface of the sea. Thus, the use of fewer physical links allows a lower cost in installating an oil platform. An example of the different nodes of an underwater acoustic network is illustrated in figure 1. We assume that we work with a depth of up to 700 meters.

The paper is organized as follows: section II presents the specific problem of the underwater acoustic channel.

In section III, we present some existing proposals for underwater acoustic networks. The comparison between these protocols based on some metrics is presented in section IV. Our ideas for a new underwater acoustic MAC protocol are detailed in section V, followed by a conclusion in section VI.

II. THE SPECIFIC PROBLEM OF UNDERWATER

ACOUSTIC DATA NETWORKS Sound propagates well in water because attenuation due to absorption and diffusion is much smaller than for electromagnetic waves. Thus acoustic waves constitute the best and, currently, the only way of underwater data transmission.

The underwater acoustic channel present analogies with the radio mobile channel with regard to propagation phenomena. The underwater channel is a multi-path and frequency selective channel. But the propagation speed of the acoustic wave under water is very low compared to the propagation speed of radio electromagnetic waves (ratio of 1/200000): the propagation speed of acoustic signals is 1500 m s -1 under water, instead of the speed of light c=3*10 8 m s -1 .

In the literature, the approach used to model these disturbances (signal fading and interferences) is the discrete multipath model that we used in [START_REF] Doukkali | Cdma based approach for transmit diversity over frequency selective fading channel[END_REF]. The channel response is given by equation 1:

c(t,τ ) = N i=1 c i (t)δ(t -τ i (t)) ( 1
)
where N is the number of paths, c i and τ i are the attenuation and the delay of the channel respectively.

The main differences with the radio mobile channel which may be taken into account in the design of a data transmission protocol are :

-Propagation delay : the low propagation velocity in the aquatic environment increases the latency of transmission. For usual distances of the order of a kilometer (Km), the delay of propagation is about a second (s). This limits applications with strong temporal constraints and reduces considerably the load of the traffic on the network. For example, in [START_REF] Yeo | Modified mac(media access control) protocol design for the acoustic-based underwater digital data communication[END_REF] and [START_REF] Xie | A networking protocol for underwater acoustic networks[END_REF], the authors state that the Carrier Sense Multiple Access (CSMA) protocol does not work properly under these conditions. -Data rate: very low compared to radio mobile communication. In underwater acoustics, the data rate cannot exceed some tens of kbits per second. This is due to the material constraints of acoustic transducers. -Frequency: in underwater communication systems, the frequency band resources are severely limited.

Long distance communication systems (a few tens of kilometers) are limited to frequencies of the order of a few tens of KHz. Systems operating at distances of a few meters can use frequencies of the order of some hundreds of KHz. This limitation of bandwidth resource is explained by the fact that the higher the frequency, the more significant the absorption.

III. MAC LAYER FOR UNDERWATER ACOUSTIC NETWORKS

A. The Network topologies

The three basic topologies existing for underwater acoustic networks are :

centralized topology: communication between nodes pass through a central station. This topology is preferred for deep water acoustic networks [START_REF] Sozer | Underwater acoustic networks[END_REF][5] [START_REF] Sozer | Initialisation and routing for ad hoc underwater acoustic networks[END_REF]. One disadvantage of this architecture is that the network depends fully on the central station. Another problem is the difficulty of scalability. single hop distributed topology: this topology provides point-to-point links. The radio access protocol may be less optimal. multihop topology: each node has the possibility to communicate with any other neighboring node. This architecture covers a large area because the range of the network is determined by the number of nodes and not by the coverage of one node. The protocol of [START_REF] Sozer | Underwater acoustic networks[END_REF] [START_REF] Sozer | Design and simulation of an underwater acoustic local area network[END_REF] proposes several nodes per master node (base station) and proves that using multihop links, the system minimizes energy consumption. In [START_REF] Salva-Garau | Multi-cluster protocol for ad hoc underwater acoustic networks[END_REF], the authors propose a multi-cluster protocol over a multihop distributed topology. In [START_REF] Xie | A networking protocol for underwater acoustic networks[END_REF], the authors propose a centralized dynamic architecture (possibility of adding or moving a node). The configuration of the topology is performed by the master node using Topology Discovery Massage (TDM).

B. Multiple Access Techniques

Multiple access can been done using FDMA, TDMA or CDMA. The use of TDMA techniques needs precise synchronization between users. Moreover, it increases the latency of the system. Therefore, TDMA techniques are generally not suitable in the case of underwater communications. The authors in [START_REF] Sozer | Underwater acoustic networks[END_REF] propose using FDMA but CDMA is also suitable. In [START_REF] Salva-Garau | Multi-cluster protocol for ad hoc underwater acoustic networks[END_REF], the authors use a combination of the two techniques: CDMA to communicate between clusters and TDMA inside each cluster (see below).

C. Media Access Protocol

In this section, we present some currently proposed MAC protocols for underwater acoustic data networks.

1) Adapted MACA to underwater acoustic networks: In [START_REF] Sozer | Underwater acoustic networks[END_REF] [START_REF] Proakis | Shallow water acoustic networks[END_REF], the authors propose a media access control protocol based on the MACA (Multiple Access with Collision Avoidance) protocol, which uses CTS-RTS-DATA exchange. Error detection is done using STOP&WAIT ARQ (Automatic Repeat Request): the source waits for an Acknowledgment before transmitting the following packet (see figure 2-a). If after a specified time the source does not receive the ACK, it retransmits the packet until it receives an ACK. The source drops the communication after K retransmissions not followed by an ACK packet (see figure 2-b). Using this classical MACA scheme, energy consumption may increase in the case where the source repeats its request several times. To solve this problem, the authors of [START_REF] Sozer | Underwater acoustic networks[END_REF] propose adding a WAIT command (see figure 2-c), so the destination tells the source that it is busy and will send the CTS as soon as it can. To avoid deadlock problems (see figure 2-d), an assignment priority is added to packets directed toward the master node.

2) A MAC protocol for multi-channel underwater acoustic environment: The authors of [START_REF] Yeo | Modified mac(media access control) protocol design for the acoustic-based underwater digital data communication[END_REF], propose a multiple access protocol with different frequency channels. The purpose of this system is to transmit high data rate applications.

The frame transmission mechanism begins by detecting the candidate channel which can transmit the frame. Within these candidate channels, a selection of the longest idle channel is performed to define the transmitting channel.

The frame reception procedure manages the transmission of the ACK frame if there are no transmission errors, and the transmission of an NACK if there are errors after cancelling the received frame.

After the frame reception mode, the system returns to a carrier sensing mode, which calculates the idle time for each channel [START_REF] Yeo | Modified mac(media access control) protocol design for the acoustic-based underwater digital data communication[END_REF]. To our knowledge, this is the only paper that considers a multiple channel environment with dynamic channel allocation.

3) A scalable centralized protocol for underwater networks : in [START_REF] Xie | A networking protocol for underwater acoustic networks[END_REF], the authors propose a centralized topology. This scheme reduces the propagation delay compared to the 4-way handshake mechanism (see Table I): the central station provides a dedicated transmission channel to each node and pre-computes all routes. The proposed protocol allows nodes to be added or removed dynamically. The master node configures periodically the network topology by transmitting Topology Discovery Messages (TDM). Topology discovery message propagation allows the master node to have complete information regarding membership of the network and each node's immediate neighbors. A power control mechanism is integrated into the transmission of TDM.

4)

Clustering and CDMA/TDMA multiple access: Work presented in [START_REF] Salva-Garau | Multi-cluster protocol for ad hoc underwater acoustic networks[END_REF] proposes a multi-cluster protocol for distributed ad hoc (no master node) underwater acoustic networks. Communication in each cluster is done using using TDMA, which means that each node transmit during its assigned time slots. Allocation of time slots is carried out in the network initialization phase. In this phase, three main operations are carried out:

-division of the network into clusters; -division of the frame into slots; -assignment of CDMA codes to clusters.

A maintenance phase is needed due to node mobility. Communication between two clusters is performed using CDMA which allows the simultaneous use of slots across clusters (see figure 3).

CDMA is used by the assignment of orthogonal codes to clusters (adjacent clusters do not overlap). Thus, each cluster has a code for transmission and each node from that cluster transmits using this code. The number of codes is limited and this limits the number of clusters. To solve this problem, authors in [START_REF] Salva-Garau | Multi-cluster protocol for ad hoc underwater acoustic networks[END_REF] propose to reuse code as in cellular networks (see figure 3). This technique of reusing code is possible because the acoustic signal fades due to distance. The authors show that using this clustering concept, it is possible to achieve a more efficient use of network resources through the spatial reuse of resources (CDMA code reuse).

IV. COMPARISON

In this section, we compare the protocols mentioned in section III-C. These protocols are evaluated according to the following metrics:

-Delay: calculated as the difference between the time of a frame transmission and the time of its successful reception. An example of a simulation that provides some delay values in the case of 4-way handshake (with RTS/CTS) and 2-way handshake (without RTS/CTS) is presented at table I.

TAB. I -Acoustic delay of transmission in a network of 8 nodes

This example illustrates the fact that the transmission of the RTS/CTS packet before each packet increases the delay significantly. These values are obtained using Network Simulator 2 (NS2) after its Protocol Principle Delay (the average delay)

Energy consumption

Node connectivity Comments

Sozer,2000 [START_REF] Sozer | Underwater acoustic networks[END_REF] [8]

• Multi-hop distributed topology. Absence of collisions by using the WAIT command which also allows preservation of battery power, but this results in increased delay of propagation Yéo,2001 [START_REF] Yeo | Modified mac(media access control) protocol design for the acoustic-based underwater digital data communication[END_REF] • Multi-channel environment.

• Carrier sensing based on the use of idle-timer of each channel known by all nodes (no need for RTS/CTS) Delay equivalent to the case of a centralized network because carrier sense is done using the information of the state of the channel (flag and idletime values) which is known by all nodes of the network Absence of a power control protocol so there is no optimization of energy consumption.

The connectivity of each node depends on the number of its transmitting and receiving channels Specific case due to multiple channels. Local information about the state of channels known by nodes.

Xie,2000 [START_REF] Xie | A networking protocol for underwater acoustic networks[END_REF] • Centralized network around a master node.

• Possibility of adding and removing nodes.

• Topology control is done by periodically transmitting special messages (TDM).

Smaller than the delay of the scheme of [START_REF] Sozer | Underwater acoustic networks[END_REF] (thanks to the absence of RTS/CTS packets). • Grouping of nodes into clusters.

• Multi-hop distributed scheme. The clustering concept allows us to achieve a more efficient use of network resources through spatial reuse of resources (CDMA code).

TAB. II -Comparison between different MAC protocols for underwater acoustic networks

adaptation to the context of underwater acoustic channel (by modifying the value of the propagation speed, frequency and data rate). The simulated network is composed of 8 nodes. We consider TCP traffic between two nodes separated by a distance of 100 m and other TCP traffic between two other nodes separated by 80 m. Each node transmits at the power level p t = 0.313W and the range of the nodes for this power level is equal to 100 m. For the two scenarios (transmission with and without RTS/CTS), we transmit 50 TCP packets.

-Energy consumption: this equals the total energy used for frame transmission. -Node connectivity: defined as the percentage of the transmitter's neighbors that successfully receive a frame. Table II summarizes the comparisons between the MAC underwater acoustic networks protocols presented above.

Comments concerning each protocol are given.

V. COMMENTS AND PROPOSAL

In this section, we present our design of a new MAC underwater acoustic protocol based on a comparison between existing protocols.

Regarding the mobility of the nodes of the network (see figure 1), our work considers protocols similar to those presented in [START_REF] Sozer | Underwater acoustic networks[END_REF][7] [START_REF] Xie | A networking protocol for underwater acoustic networks[END_REF]. The MAC protocol of [START_REF] Yeo | Modified mac(media access control) protocol design for the acoustic-based underwater digital data communication[END_REF] being more suitable in the case of fixed nodes. The centralized scheme presented in [START_REF] Xie | A networking protocol for underwater acoustic networks[END_REF] has the disadvantage that it cannot work properly for a large number of nodes.

We recommend a CDMA/TDMA clustering scheme for the following reasons: first, the cluster algorithm is robust with regards to changes of topology (adding or removing nodes); second, the use of CDMA techniques allows one to combat interference and multi-path fading; finally, such a scheme provides spatial reuse of resource and this is important in the case of the limited bandwidth of the underwater acoustic channel.

Our proposal is the integration of a power control protocol. The advantage of a power control protocol is that it extends the battery life of stations and increases the traffic capacity of the network. The formation of clusters may then be based on the available power levels. Based on these power levels, we can build up clusters where minimal transmitted powers are used (see figure 4). As in [START_REF] Salva-Garau | Multi-cluster protocol for ad hoc underwater acoustic networks[END_REF], we propose to use CDMA for inter-cluster communication. More details of the proposed protocols and its performance are provided in [START_REF] Doukkali | Clustering and power control for underwater acoustic networks[END_REF].

FIG. 1 -

 1 FIG. 1 -. Control elements of control constituting an underwater oil platform

FIG. 2 -

 2 FIG. 2 -. Adaptation of MACA protocol to underwater acoustic communications (based on [4])

FIG. 3 -

 3 FIG. 3 -. Example of a clustering network using CDMA with the reuse of code 1 by cluster 4 in addition to cluster 1

VI. CONCLUSION

In this paper, we study the design of a MAC protocol for underwater acoustic networks. The acoustic channel used has some specific aspects with regards to the wireless channel : long propagation delay, limitation of bandwidth and frequency, etc. We recall these aspects and then briefly describe some existing proposals for underwater acoustic networks. These proposals are analyzed and compared. Interesting conclusions can be made based on these comparisons. We suggest the design of a new MAC protocol for underwater acoustic networks based on the idea that the integration of power control in a clusterized MAC protocol may solve the problem of limitated battery power while improving the performance of the network. Future work [9] will give the details of this new protocol and analyze its performance.

ACKNOWLEDGMENT

The authors would like to thank Ms Janet Ormrod for her helpful comments.

Code 2 50 mW cluster 10 mW cluster 1 mW cluster Code 1 Code 4 Code 6 Code 5 Code 7 Code 1 Code 2 Code 3 Code 1 FIG. 4 -. Assignment of CDMA codes using three power levels 1mW, 10mW and 50mW.