Ahmed Junaid
email: junaid.khan@inria.fr

Khan

Cedric Westphal
email: cedric@soe.ucsc.edu

Yacine Ghamri-Doudane

Offloading Content with Self-organizing Mobile Fogs

Keywords: Content Centric Networking, Distributed Content Caching, Fog Networking, Content Offload, Coalition Game

Mobile users in an urban environment access content on the internet from different locations. It is challenging for the current service providers to cope with the increasing content demand from a large number of collocated mobile users. In-network caching to offload content at nodes closer to users alleviate the issue, though efficient cache management is required to find out who should cache what, when and where in an urban environment, given nodes limited computing, communication and caching resources. To address this, we first define a novel relation between content popularity and availability in the network and investigate a node's eligibility to cache content based on its urban reachability. We then allow nodes to self-organize into mobile fogs to increase the distributed cache and maximize content availability in a cost-effective manner. However, to cater rational nodes, we propose a coalition game for the nodes to offer a maximum "virtual cache" assuming a monetary reward is paid to them by the service/content provider. Nodes are allowed to merge into different spatio-temporal coalitions in order to increase the distributed cache size at the network edge. Results obtained through simulations using realistic urban mobility trace validate the performance of our caching system showing a ratio of 60 -85% of cache hits compared to the 30 -40% obtained by the existing schemes and 10% in case of no coalition.

I. INTRODUCTION

The increase in the number of smart mobile devices resulted in the growth of content demand by lots of consumers in closer urban proximity, some even using multiple portable devices. For example, a large number of mobile users in an urban environment can be interested to watch the video of a latest episode of a popular TV show/drama or sports highlights. Provisioning of such popular content to each user requires lots of redundant connections between users and the service provider, given that the content is requested by lots of spatiotemporally co-located users with similar social interests.

Nowadays, it becomes more and more challenging for the current "connection-centric" network infrastructure to facilitate content availability for such large number of mobile users in close proximity while offering attractive tariff plans with no bandwidth limitations. One of the solutions advocated by the research community is the use of Content-Centric Networking (CCN) [START_REF] Ahlgren | A survey of information-centric networking[END_REF] paradigm which addresses the issue by decoupling the content provider and consumer along in-network caching at intermediate nodes. Content caching at individual nodes exploit different content replacement strategies such as First-In First-Out (FIFO), Least Recently Used (LRU) and Least Frequently Used (LFU). However, there is a need to go beyond individual caching policies towards a collaborative content caching approach near mobile users.

In this paper, we target the content caching at the edge problem where nodes in an urban environment can subscribe to offer distributed caches to a service provider [START_REF] Rebecchi | Data offloading techniques in cellular networks: a survey[END_REF]. We define Mobile fogs as set of co-located mobiles nodes that can self-organize to offer distributed resources (computing, communications, caching) as an ad-hoc network at the network edge. The aim is to allow nodes to collaborate in order to increase the "virtual cache" closer to mobile users, maximize content availability and minimize cost for the service provider as well as the users. This however invokes the following questions; first, given the massive content constantly generated and consumed by mobile devices [START_REF]Cisco visual networking index: Global mobile data traffic forecast update[END_REF], which content is important to cache with respect to the social interests of geographically co-located users at different locations and times of the day? Second, which nodes can be considered suitable candidates for caching, given limited bandwidth and monetary resources, both, for the service provider and the mobile nodes? Third, given the existence of lots of such subscribed nodes including rational nodes, how to incentivize mobile users and compensate for their incurred cost of caching content at different urban neighborhoods, specially, in populated areas (spatial) or during peak traffic hours (temporal) to maximize content availability? Thus, it is challenging to address questions regarding "Who should cache what, when and where?", requiring an understanding of the content as well as the node's spatio-temporal profiles. To address this, inspiring from social networks metrics (centrality), we first define a novel relation considering simultaneously the content spatio-temporal popularity and availability to characterize its importance. Based on the content profile, a node autonomously finds its caching capability by computing its local [START_REF] Khan | Car rank: An information-centric identification of important smart vehicles for urban sensing[END_REF] centrality as a measure of its connectivity and reachability in the network. The existence of rational caching nodes cannot be ignored, where such users opt to reduce the consumption of their resources such as battery and storage dedicated for caching. Thus, we present a coalition game to incentivize co-located nodes already interested in the content to merge into coalitions and collaboratively offload a maximum amount of content locally to other nearby users. Nodes in coalition receive a monetary reward proportional to the amount of content offloaded in a neighborhood where coalitions or mobile fogs formed by all subscribed nodes in each neighborhood can be considered as the grand coalition providing a larger city-wide distributed virtual cache. Since the distributed caching problem is NP-Hard [START_REF] Golrezaei | Femtocaching: Wireless video content delivery through distributed caching helpers[END_REF], an optimization problem is thus formulated with a heuristic to maximize the offered distributed cache in the network. The contribution of this paper can be summarized as follows:

• A novel content spatio-temporal availability-popularity relation is proposed to decide a content importance in order to be cached by nodes at the network edge. • A new node eligibility metric is presented for socialaware content caching. It classifies a node's caching capability proportional to its relation to content, connectivity, cost and reachability in the network. • A coalition game is proposed to incentivize caching nodes, allowing a service provider to optimally select nodes as caches in an urban environment, while compensating for their costs and maximizing content availability in different locations and times. • A distributed algorithm is proposed for nodes with caching capability to self-organize and collaboratively provide a maximum virtual cache near end users at different locations and times. In order to validate our approach, we compare the cache hits and offload benefit of mobile fogs formed with state of the art node centrality schemes as well as a case when no coalition is formed. Results show that our approach outperforms such approaches as well as the case without coalitions by substantially offloading the content at the distributed mobile fogs while achieving around 60% of cache hits compared to the 30 -40% obtained by the existing schemes and 10% in case of no coalition.

The remaining of the paper is organized as follows. Section II discusses the related work. In Section III we define the metrics for the content and node eligibility. The coalition game for the formation of mobile fogs along the optimal coalitions selection is described in Section IV. In Section V, we discuss the performance evaluation and results. Section VI concludes the paper along insights into future directions.

II. RELATED WORK

Content Caching is studied for some time by the research community mainly in Small Cell Networks (SCNs) [START_REF] Bas ¸tug | Cache-enabled small cell networks: Modeling and tradeoffs[END_REF], [START_REF] Bharath | A learning-based approach to caching in heterogenous small cell networks[END_REF], Content Distribution Networks (CDNs) [START_REF] Hamidouche | Many-to-many matching games for proactive social-caching in wireless small cell networks[END_REF] and Information/Content Centric Networking (ICNs/CCNs). For example, in [START_REF] Sourlas | Distributed cache management in information-centric networks[END_REF], distributed cache management decisions are made in order to efficiently place replicas of information in dedicated storage devices attached to nodes of the network using ICN. Similarly [START_REF] Wang | Design and evaluation of the optimal cache allocation for content-centric networking[END_REF] address the distribution of the cache capacity across routers under a constrained total storage budget for the network. A sub-optimal heuristic method is proposed based on node centrality for the case of frequent content publishing. The authors found that network topology and content popularity are two important factors that affect where exactly should cache capacity be placed. In a recent work [START_REF] Li | Pricing and resource allocation via game theory for a small-cell video caching system[END_REF], game theory is exploited for caching popular videos at small cell base stations (SBSs). A Stackelberg game model is presented for the service providers to lease its SBSs to video retailers in order to gain profits as well as reduce the costs for back-haul channel transmissions. Another work, [START_REF] Mangili | Bandwidth and cache leasing in wireless information centric networks: a game theoretic study[END_REF] proposed a game theoretic approach in ICN to stimulate wireless access point owners to jointly lease their unused bandwidth and storage space to a content provider under partial coverage constraints.

An example of social-aware caching approach in Delay Tolerant Networks (DTNs) is presented in [START_REF] Le | Social caching and content retrieval in disruption tolerant networks (dtns)[END_REF] where the authors proposed a cooperative caching scheme based on the social relationship among nodes in DTNs. Content is dynamically cached at selective locations (i.e. nodes along the common request forwarding paths) in the network with the highest social levels identified as cluster head nodes. A cache replacement policy based on the content popularity is presented as a function of both the frequency and recency of data access. Similarly, Socially-Aware Caching Strategy (SACS) [START_REF] Bernardini | Socially-aware caching strategy for content centric networking[END_REF] for Content Centric Networks uses social information in order to privilege influential users in the network by pro-actively caching the content they produce. The authors detect the influence of users within a social network by using the Eigenvector and PageRank centrality measures. The above mentioned caching schemes rely on typical centrality measures which are unsuitable and unstable in dynamic connectivity scenarios in an urban environment. Moreover, existing schemes considers only content popularity along policies such as FIFO, LRU an LFU, which do not completely capture the spatiotemporal content characteristics to address the requirements of urban mobile users. Thus, none of existing approach is able to model the content profile with respect to the economic and social interest of large (scalable) number of users.

We address the caching problem to find out who should keep what and how, jointly with the existence of rational caching nodes. Unlike the above mentioned approaches, (i) we derive a relation between spatio-temporal content availability and popularity allowing a node to decide its caching decisions. It considers the extent it is related to the content based on the frequency of interest it satisfied for the content. (ii) We propose novel content caching metrics which go beyond typical FIFO, LRU, LFU schemes towards a collaborative strategy, while simultaneously considering recency, frequency as well as accounting for large number of users with socially similar interests. (iii) Our popularity metrics are adapted to urban mobile environments considering node local connectivity (topology) as well as its reachability and (iv) the mobile fog formation ensures maximum content availability with the best candidate nodes in coalition at different locations and times, while, at the same time incentivizing nodes. To the best of our knowledge, there exist no scheme to provide such a scalable content caching model to offload traffic at user devices including mobile nodes in urban environment.

III. WHO SHOULD CACHE WHAT?

The first step towards efficient content caching is to understand the eligibility of the content to be cached based on a relevant amount of user demands relative to the content as well as its availability in the network. Before discussing the content importance, we first present the system model to consider for the rest of the paper.

A. System Model 1) Business Model: We consider a service provider and a set of subscribed users requesting content at different locations and times. There are three players involved with their respective business gains and costs, (i) the service provider whose benefit is proportional to the amount of content downloaded by the subscribed users. (ii) User nodes with sufficient resources (communication, caching, computing, energy etc.) interested in the content downloads it and are capable to cache it at their fixed access points, small-cells or even mobile devices in order to provide it to other nearby users. However, caching and delivering content incur cost for such users proportional to their provided resources such as battery and storage. Therefore, they are paid a monetary reward by the service provider proportional to the amount of content delivered to other users where such reward is more than their consumed resources and less than the cost of using the infrastructure network. (iii) The user node subscribed to consume the content with no caching capability, thus paying an amount to the service provider proportional to the downloaded content. The cost for a user to download content from a nearby user in an ad-hoc network is less than using the infrastructure network.

Caching content at the edge and allowing users to use the low cost ad-hoc network to retrieve content locally from nearby nodes is economically beneficial for all the three players; (a) The reduction in service provider cost and bandwidth for not using the infrastructure network. (b) The reward paid to the nodes caching content, greater than their incurred cost, and (c) the reduction of cost for the user downloading content from nearby nodes in an ad-hoc network.

2) Network Model: We consider an ad-hoc network modeled as an undirected graph G(V(t), E v (t)), where V = {v} is the set of nodes and where the time instant t integrates the temporal network characteristics.

E v (t) = {e jk (t) | v j , v k ∈ V, j = k}
is the set of edges e jk (t) modeling the existence of a communication link between nodes j and k at time instant t. To consider the dynamic nature of the network topology, we assume the time T = (t 1 , t 2 , ...) as a sequence of regular time-slots, where the k th time-slot is represented as t k = [t k , t k+1 ). Similarly, the urban environment is represented by the undirected graph G(L, E l ), the set of vertices L = {l} represents different locations l and the set of edges E l = {e pq | l p , l q ∈ L, p = q} are the respective boundaries that connects different neighborhoods. It is to note that there is a precision versus overhead trade-off, i.e. The more locations and time-slots, the more computing overhead. The granularity of such spatio-temporal division depends on the application requirements. To provide a generic solution, we consider a We define the set of content as X = {x} where x is an indivisible content chunk in the network. We need to take into account a spatio-temporal relation of its availability versus popularity at a location l ∈ L and a time slot t. Therefore, in the remaining of this section we model and map the content availability and its popularity with respect to location and time.

f v I Content importance function f v L,T Spatio-temporal availability function M I v (l, t k ) Mutual information between location & time P v (l, t k ) Joint probability between location & time for node P v (l), P v (t k ) Joint probability of location & time for node f v Γ Neighborhood connectivity B v /B t v Available/total buffer at node v f C v /f R v Node cost/

B. Spatio-temporal Content Profile

It is important to consider both the spatial and temporal characteristics of the content available as well as demanded in the network. Therefore, we let the node extrapolate from the past information for the cached content availability and popularity to decide regarding its future profile. We define below the proposed content availability and popularity respectively: Definition 1: (Content Availability) We formally define the spatio-temporal availability for a content piece x ∈ X cached at a node v ∈ V as A x v (l, t) = 1, x cached at v 0, otherwise , where t is the time slot at which the content is available at the node v at a location l ∈ L. Content availability differs with respect to time and location, similarly, the content availability is zero for all the content x ∈ X not cached at the node at a particular location and time slot.

To model the storage requirements for a particular content x, we denote Q x as the number of chunks/pieces cached at a node, assuming the possibility of having multiple content chunks. The file size for the content can be defined as F x = Q x • |x|. Thus, the file size of all the content cached at a node v is given as

F v (l, t) = x A x v (l, t) • F x , i.
e. the sum of file sizes for all the possible content at location l and time slot t.

The content popularity can be shared with the nodes using three approaches, (i) offline method by the content operator as a control message; (ii) part of content header shared by the service provider; and (iii) local monitoring by the nodes taking into account the number of user interests for the content. (i) and (ii) requires global knowledge only available to the service provider. We define below (iii) for a node, the cached content spatio-temporal popularity with respect to a location l and time-slot t by local monitoring.

Definition 2: (Content Popularity) The spatio-temporal content popularity at a node is represented by the probability p x (l, t) = λ(l,t) Λ as a measure of the user interests for content x at location l and time t.

Here, λ(l, t) represents the number of interest for the content x at location l and time t and Λ = Xv λ(l, t) is the total interests for all contents cached X v at the node v at the required location l and time slot t. The above definition considers varying content popularity at different times and locations, thus modeling the existence of users with different social interests in different neighborhoods.

It is important to find the node relation to the cached content with respect to the received user interests in order to efficiently cache content of interest for the intended users in the network. Therefore, we define below a user interest satisfaction based metric which complies with such criteria: Definition 3: (Interest Satisfaction Frequency) We define

I x v (l, t) = rv(l,t)
Rv as the frequency of user interests satisfied by the node for content x at location l and time-slot t, where r v (l, t) are the number of successful responses in the previous time slot and R v = Xv r v (l, t) are the overall successful responses for all the contents X v cached at the node v ∈ V at the respective location and time slot.

We assume each user interest specifies a hint (such as Time To Live -TTL) as the content validity for the intended user. Since such TTLs vary for interests received from different users at the node v. We let the node compute the average interest validity hint from the received user interests. It is defined as the average time slot specified by the user interest Let t

x f v be the time instant when the interest for the content x was previously satisfied by node vand the average interest validity time-slot is represented as t xav v for the content. Definition 4: (Content Validity) Leveraging the user interest validity hint, we define τ x v (l, t) as the measure of the content validity scope for the next time instant t k+1 . Different functions can be used, however we use here the commonly used exponential decay function:

τ x v (l, t) = 1 e -δ(t k -t xav v ) t k ≤ t x f v + t xav v t k > t x f v + t xav v
where, δ is the tuning parameter to adjust the exponential decay depending on the application requirements as in Figure 1. For the current time instant t k less/greater than the sum of the time instant the content was last responded t x f v and the time slot t xav v specifying the average user interest validity. In case the node receives no active interest in the previous time slot and the average interest validity is expired, the content validity follows an exponential decay since the information is of less importance in the network. On the other hand, τ is set to unity for the content required to be constantly cached at the node.

Definition 5: (Content Availability vs Popularity Relation) We present a metric on the content importance for a node to model together a cached content's "availability" while considering its "popularity" with respect to the received user interests. The content availability at a location l and time t can be mapped together with its popularity in the network by the relation:

ψ x v = p x • τ x v • I x v , A x v = 0 (1)
where, A x v is a binary variable modeling the content x availability at a node v. For a cached content i.e. A x v = 0, Equation 1 allows nodes to estimate the cost and reward for caching and delivering content as well as allowing nodes to autonomously decide content replacement decisions. For a given content set X v , it can classify content by looking into the user interests (p x ) and find out how long it needs to keep the content (τ x v ). Similarly, the node can find the degree of relevance with respect to cached content from the amount of interest it already satisfied for the content (I x v ). At the same time, ψ ensures diversity in caching content as there might be content not popular overall, though more related to the individual node (high I x v ), and therefore, should be kept at the node. It is to note that ψ x v is a single metric that incorporates a hybrid of content recency, popularity and availability as a linear content eligibility metric at a node depending only on its local knowledge. At the same time, each term in Equation 1 consider content as a stable metric, independent of unstable metrics such as nodes interactions and mobility.

C. Node Eligibility

We believe content caching should be made at socialaware mobile nodes identified in the network using centrality measures. The idea is, more precisely, to classify a node for content caching based on the social needs of users in an urban environment. However, it is not possible to compute typical centrality measures (Degree, Closenesss, Betweenness, Eigenvector) in a dynamic network topology for mobile nodes. Furthermore, existing schemes follow a network-centric approach requiring complete network connectivity in order to analyze the network, thus, ignoring the content importance with respect to the user relevance. Similarly, using the nodes contact frequency and duration to decide its capability to cache in the network is a challenging task because of the rapid changes in the time evolving network topology due to mobility. To overcome this, we propose the use of a novel social aware metric, which simultaneously considers three essential parameters, the content user relevant importance, the node spatio-temporal availability and its network connectivity [START_REF] Khan | Car rank: An information-centric identification of important smart vehicles for urban sensing[END_REF] allowing it to find its local caching capability without affected by its mobility.

• Content importance: It measures the cached content eligibility ψ (Equation 1). • Spatio-temporal availability: reflects the node's recursive presence in a particular location l at a time-slot t, reflecting its correlation to the location. It is the mutual information the location l and time-slot t share about the node expressed as:

M I v (l; t) = ∀l∈L ∀t∈T P v (l, t) log P v (l, t) P v (l)P v (t) , (2)
where, P v (l, t) is the joint and P v (l) and P v (t) are the marginal probabilities of the node presence at the location l and time-slot t respectively, usually obtained prior to the eligibility computation time using different localization measures. • Neighborhood importance: Neighborhood importance considers the node topological connectivity. Different metrics can be used to to find a node's local connectivity, we use an easier to compute average degree (number of neighbors) k v for a node v observed in a prior time-slot.

The nodes first classify the content using the relation ψ x v in Equation 1 and then use the content eligibility to find its relative importance in the network as a local information hub responsible for the efficient caching. The local centrality is represented as C v , computed at a time instant t k :

C v (t k ) = f v I (t k ) • f v L,T (t k ) • f v Γ (t k ) (3)
where, f v I , f v L,T and f v Γ are the content importance, node's spatio-temporal availability and its neighborhood, respectively. Thus, a node more related to a content (high f v I ), more frequently present at the location l and time-slot t (high f v L,T ) and better connected (high f v Γ ) is the best candidate to cache the content in that location and time-slot.

Node Utility: The node utility is composed of its reward and associated cost proportional to the amount of content delivered to users.The node cost function is defined as:

f C v (l, t) = 1 |X| x d x v (l, t)×C d (l, t), A x v = 0, ∀x
where, d x v is the amount of content chunks delivered and C d is the cost per content chunk delivery incurred to the node. The monetary reward paid to the node for caching besides the natural incentive for the node to be itself interested in the content is given by the function

f R v (l, t) = d x v (l, t) × R d .
It is proportional to the content delivered, where R d is the reward paid per delivered content chunk dedicated by the service provider. We derive the node's utility as:

U v (l, t) = f R v (l, t) -f C v (l, t), where, f R v (l, t) and f C v (l, t
) are the node's reward and cost functions, respectively. Thus, the node computes its utility based on the locally known information regarding the amount of delivered content, its cost and the reward per unit delivery.

IV. DISTRIBUTED FOGS FORMATION AS COALITION GAME

The nodes collaborate to offer their buffer to the service provider by forming mobile fogs for distributed caching at set of co-located nodes aiming to maximize their profit/payoff. To manage this, we use game-theoretic concept of coalition formation where the service provider optimally selects the best coalitions formed by nodes in all areas at all time slots. The coalition game G(V, U ) for the set of nodes V targets to form the set of coalitions S = {s}, s ⊆ V as the respective mobile fogs. The real-valued function U : 2 |V| → R associates to each coalition s a value U s (l, t) as the total payoff available to players in coalition s at the location l and time slot t in an urban environment.

We define below the preference relation for the nodes preferences to merge into a particular coalition yielding a larger amount of utility. The service provider uses the same preference relation to select the best coalition of nodes with the maximum utility among the set of coalitions formed in the network at different locations and time slots.

Definition 6: (Coalitions Preference Relation) For any node v, a preference relation is defined as a complete, reflexive and transitive binary relation over the set of all coalitions that node v can possibly form {s ⊆ V : v ∈ s}.

Each node prefers to merge to a coalition with larger utility to increase its reward. For example, given two coalitions s 1 and s 2 for the node v to join, s 1 , s 2 ⊆ V, such that v ∈ s 1 and v ∈ s 2 , s 1 s 2 indicates that node v prefers to merge with s 1 over s 2 . The node adds its utility to the coalition it joins, thus increasing the coalition utility.

The optimization problem can thus be formulated for the service provider as follows to choose the coalition (form mobile fogs) offering the maximum utility over preferences at different locations and times. edymaximize

S s∈S U s (l, t) ∀v, ∀l, ∀ t edysubject to v∈s A x v (l, t) ≥ A xth v (l, t), ∀s, (C 1 ) v∈V f C v (l, t) ≤ f C th v (l, t), (C 2 ) f R v (l, t) ≥ f Rth v (l, t), (C 3 ) max v∈V B v (l, t) ≤ B t v (l, t), ( C 4 )
The objective function maximizes the utility of coalitions formed by spatio-temporally co-located nodes offloading content for the service provider. The constraint C 1 states that the content availability in each coalition at each location and time should be more than a desired threshold specified by the service provider. It allow fairness by specifying a minimum content availability requirements at all nodes, locations and times. The constraint C 2 addresses the cost limitations where the cost paid to nodes as incentives is limited by a threshold cost a service provider can afford. Constraints C 3 and C 4 deal with the node individual resources, where, for all nodes in different coalitions, (i) the reward offered to the node for providing its storage should be greater than the associated cost and (ii) the buffer offered by the node should not exceed the total buffer size available at the node.

A. Solution: The Core

The target is to motivate each node with cache to participate towards forming the grand coalition of all nodes also known as the core of the game i.e. the node receives

U v (l, t) = U v (l, t), if joins coalition in l & t 0, otherwise
To do so, we first define for each location and time slot an imputation y = (y 1 , y 2 ..., y M ), M = |V| as the distribution of the total payoff/reward among nodes such that (i) y v ≥ U ({v}) , ∀v, i.e. a node receives at least as much as it could obtain on its own, without cooperating with anyone else (individual rationality). (ii) v y v =U (M ) (efficiency). The core is the set of imputations y such that y s = v∈s y v ≥ U s , ∀s ⊆ V and equality if s = V.

Non-emptiness and Uniqueness of the Core: In order to derive the non-emptiness and uniqueness of the core, we define Algorithm 1 Self-organizing Coalition Formation

1: Initialize s = φ, 2: for each location l ∈ L, time-slot t ∈ T do 3: v = arg max v∈V C v (t k ) 4: s = s ∪ v 5: U s ← U s + U v 6:
for each content x in v do 7:

A x s ← A x s + A xth v 8:
end for 9:

f C s ← f C s + f C v 10: while (f C s ≤ f C th s ) and (A x v ≥ A xth v ), ∀x do 11:
for each node u , u = v do 12:

if U s > U u and B u ≤ B t u then

13:

s = s ∪ u 14:

U s = U s + U u 15:
for each content x in u do 16:

A x s ← A x s + A xth u 17:
end for 18: w s • U s ≤ U V . We consider the weights for each coalition as the percentage of total nodes in the coalition. Then, we check whether the game is balanced by finding the condition in Definition 7. Thus, in a balanced game, any arrangement feasible for the different coalitions is feasible for the grand coalition as well. We can now state the Bondareva-Shapley Theorem as:

f C s ← f C s + f C
Theorem 1: (Bondareva-Shapley Theorem) The coalitional game (V, U ) has a non-empty core if and only if it is balanced.

Proof: There can be at least one caching node in a coalition, thus i.e. min(|s|) = 1, ∀l, ∀t. For each location and time, V C s combinations of nodes are possible in a coalition. The combinations of coalitions for all locations at all times can be given as l t

V C s (l, t). A single coalition in each location and time slot i.e. V C s = 1, ∀l, ∀t, results in

l t V C s (l, t) = 1.
Thus, we are able to reduce 2 M possible combinations for coalitions to a single coalition, i.e. 1 << 2 M , as the "grand" coalition.

B. Algorithm: Optimal Selection among Coalitions

Algorithm 1 summarizes the coalition formation for distributed caching at spatio-temporally co-located nodes. For a location l and time-slot t, the service provider selects the node The following performance metrics are used for evaluations:

• Cache Hits: Ratio of content responds from node cache for the received consumer interests. • Offload Benefit: The amount of content files downloaded/retrieved from local nodes to relieve infrastructure, while nodes utility proportional to the content delivery is taken into account. • Spatio-temporal Coalitions: Graphical depiction of the mobile fogs formed in the network in order to validate the coalition formation.

C. Simulation Results

1) Cache Hits: We evaluate the overall cache performance by finding the total number of cache hits by the caching nodes in the network. Figure 2 depicts the cache hit rate in percentage by comparing different schemes obtained at 10 each minutes. This analysis clearly shows that the coalitions formed at the nodes using our scheme outperforms existing schemes with a minimum of 60% and up to 85% cache hits. Moreover, the case of no coalition between nodes resulted in the poorest performance in terms of cache hits reaching less than 10% during the entire simulation. Existing social metrics such as typical centrality schemes yielded a cache hit rate of around 20-30%, thus, validating the efficiency of using our proposed scheme.

We also observe an increase in the cache hits over time for our approach. It is because the coalition formed at the nodes with high utility caches more content over time and subsequently satisfies more interests for the incoming requests. Such increase is not observed by the other schemes since each time slot at different location yields different set of nodes, thus cannot be relied for content caching. Most importantly, this also validates the fact that coalitions improves caching performance as all the cases where coalitions were formed (i.e. degree, betweenness, closeness, eigenvector based coalition) resulted in higher cache hits than the case where no coalition is formed.

2) Offload Benefit: The benefit of offloading at mobile fogs is validated by finding the total number of files cached in the mobile fogs. Figure 3 shows the offload benefit at different times in the simulation with shown after each 10 minutes. It Figure 3: Total number of files cached using each scheme over an average of ten different simulations is clear that the total number of content files made available for the user by caching at nodes by our approach is higher than with all existing schemes as well as in the case of no coalition. The case of no coalition failed to cache files substantially in the network with five times lesser number of files, where other schemes cached half the amount of files compared to our approach. This proves that caching content at mobile fogs, formed around high centrality nodes identified by our approach maximizes the overall content availability in the network. It is because such nodes received, cached and thus satisfied more user relevant interests in the network. The formed coalitions not only reduced the network overhead with respect to the number of files but also considered the content popularity to relieve the infrastructure network.

3) Spatio-temporal coalitions: It is important to evaluate the existence and uniqueness of the core in the proposed coalition game. Therefore, Figure 4 shows the mobile fogs formed by nodes, vehicles in this case, around 10 minutes, 30 minutes and 60 minutes into the simulation for the Cologne City center (6km 2 ). The central node of each coalition is marked as '•' and the remaining nodes in the coalition are marked as ' * ' . First, we observe that all nodes form coalition resulting in the existence of grand coalition as the core of the game. Most of the nodes are isolated nodes and thus form fog in the form of self-coalition as shown by a high number of single superimposed '•' and ' * '. Secondly, we noticed that most coalition central nodes are located at the edge of the network. This validates the existence of mobile fogs in closer proximity to consumers. We also observe that a single central node represents the coalition of the most denser part (relatively center) of the network. This proves the efficiency of our game motivating nodes to join coalition in order to form a grand coalition. The network analysis at three different snapshots highlighted the benefit of considering spatio-temporal factor in our approach as we can see different set of coalitions are formed at different times. Thus, we can clearly observe the union of smaller coalition in different neighborhoods forming the grand coalition in the network.

D. Summary of Findings

Results shown that offloading content can be efficiently managed by forming mobile fogs allowing nodes to perform As indicated in Figure 4, it is important for individual nodes to consider the spatio temporal content profile while considering both its availability and popularity towards caching decisions. We observe that in case of absence of other nodes in the vicinity, the node can also form self-coalition or self-fog as the only node caching content in the location. We also found that most coalition central nodes reside at the edge of the network which simultaneously reduce back-haul network traffic and improve user quality of experience. Thus, our analysis based on results from scalable simulation scenarios clearly validates our proposition that offloading content at mobile fogs in the form of coalitions at the network edge substantially benefits the network.

VI. CONCLUSIONS AND FUTURE DIRECTIONS

This paper proposed a collaborative content caching approach at high centrality nodes in an urban environment. To do so, we first modeled the content profile by defining its spatiotemporal popularity and availability relation allowing each node to find the content eligibility, then it finds its capability to cache content taking into consideration the offered resources and cost with respect to social interest of users. We proposed a coalition game for the best co-located candidates to selforganize and share storage close to mobile users. We performed scalable simulations for an interesting case of vehicles as mobile caches on a realistic trace. Results shown that the our proposed coalitions offload twice more content and achieved 60% cache hits compared to other approaches which yielded around 25% cache hits in the network.

Caching at mobile nodes need further investigations. Once we identified mobile fogs, the study of how to route user interests to nearby high centrality nodes for its efficient retrieval in the fog would be of interest. Therefore, our future work includes the study of content and node profile towards efficient content distribution in delay sensitive dynamic networks with multiple content providers while avoiding duplicate content downloads in the network.

Figure 1 :

 1 Figure 1: Content validity hint indicated by user interests

Figure 2 :

 2 Figure 2: Average cumulative cache hit rate by all nodes with utilities computed using each scheme in ten simulations

Figure 4 :

 4 Figure 4: Spatio-temporal Mobile Fogs for 6X6 km area, central nodes for each coalition identified in green

Table I :

 I List of Notations

	Notation	Description
	V	Set of nodes
	L	Set of locations
	X	Set of content
	E l /E v (t)	Edges set between locations/nodes at time t
	E	Edges set between nodes and locations
	T /t/t t xf v	Total time / time instant / Time slot Time instant: last interest satisfied
	t xav v	Time-slot: average content validity
	t k	k th time-slot under consideration
	λ	Number of received interests for content x
	Λ	Number of received interests at v for all contents
	M	Total number of nodes
	N	Total number of content chunks
	A x v	Content x availability at node v
	Q x	Number of chunks for content x
	F x	File size for content x
	F v	File size for all content at v
	p x	User interest probability for content x
	τ x v	Content validity metric
	δ	Content replacement tuning parameter
	I x v	Interest satisfaction frequency
	r v /R v	Successful/Overall interests satisfied
	ψ x v	Content popularity/availability relation
	C v	Node centrality

  set of weights w s for coalitions, where 0 ≤ w s ≤ 1, ∀s ⊆ V, is a balancing set of weights if ∀v ∈ V,

		u
	19:	end if
	20:	end for
	21:	end while
	22: end for
	23: return s ∈ S, A x s , f C s , U s
		w s = 1.
		s,v∈s
	Definition 7: (Balanced Coalition Game) The coalition
	game (V, U ) is balanced if and only if, for every balancing
	set of weights w, we have
		Ø =s⊆V

a

v ∈ V with the highest centrality as the delegate to locally cache content. Such high centrality node initializes the fog formation by creating the coalition subset s ⊂ S as in Line 4. It adds its associated utility and cost (Lines 5 and 9) to the corresponding coalition utility and cost, respectively. For each content x cached at the node v , it updates the content availability in the corresponding coalition s as stated in Lines 6 -8. The first condition in Line 10 ensures the cost of nodes f C s merged into coalition s do not exceeds a threshold cost f C th s dedicated by the service provider.

Moreover, the content availability A x v for all content should also meet the minimum requirements by the service provider for contents availability at different locations and time-slots. The high centrality node requests the other subscribed nodes in the location l and time-slot t to join the coalition. Any other node u ∈ V, joins the the coalition s if it gets a higher utility than it can get by itself, at the same time, not surpassing buffer limits. It subsequently merges into the coalition and adds its utility to the coalition as in Lines 12 -14. For each content x cached at the node u , the content availability (Lines 15 -17) and its associated cost (Line 18) is updated at the set s. Finally, the set of nodes in the coalition s, their respective cached content availability A x s , their utility U s , and cost function f C is returned (Line 23) to the service provider by the high centrality node in the location l and time-slot t.

The above distributed algorithm allows the nodes to forms a distributed fog in each location and time-slot, where, colocated nodes both spatial (in a location l) and temporal (within time window t) join coalition as a stable set s, thereby not affected by mobility while in the same location and time window. Theorem 1 states that the existence such fogs can be combined to form a city-wide grand coalition as the set of coalitions S ∈ V. The advantage of the grand coalition is that both the user and the service provider can benefit from the content cached at the large size virtual cache as a stable set of all the subscribed node with storage, thus maximizing the content availability near end users.

Complexity and Convergence: Algorithm 1 requires the nodes to only compute their respective utility which is a linear relation of its reward and cost function. It then shares this local information with the service provider and neighboring nodes where to avoid network overhead, a service provider can request a high utility node in a neighborhood to initiate the fog (coalitions) formation. The nodes are not required to know the global network knowledge, thus, resulting in substantially a reduced complexity algorithm.

The algorithm converges when the utilities of all the coalitions at each location and time-slot are maximized. This is possible when (i) the content availability requirements in the mobile fogs for each location and a time-slot are fulfilled, and, (ii) all the nodes subscribed for caching content form coalitions in the respective location and time-slot also referred as the "grand coalition" in the network.

V. PERFORMANCE EVALUATION A. Use Case: Vehicles as Mobile Fogs

We evaluate the proposed coalition game model using ns-3 where the named-data networking model of the CCN architecture is implemented with a discrete time granularity of 1 second. The node mobility is extracted from a realistic model for large scale vehicular mobility in Cologne, Germany [START_REF] Uppoor | Generation and analysis of a large-scale urban vehicular mobility dataset[END_REF]. We choose vehicles as potential candidates for to form urban fogs as the relatively high computing, caching and communication capabilities of modern vehicles makes best candidates to form city-wide mobile fogs [START_REF] Khan | Saving: Socially aware vehicular information-centric networking[END_REF]. For the impact of fading and shadowing due to obstacles such as buildings, we use the combination of Nakagami and Log distance path loss model. A total of 2, 986 nodes are used to validate the scalability of our caching approach at mobile fogs. The analysis is performed by dividing an area of 6X6km 2 into 36 uniform neighborhoods (fogs) each of 1X1km 2 , where, l = 36 and t = 10 minutes.

B. Simulation Scenario

The simulation scenario implements two types of nodes, the content caching or source nodes and the consumer nodes interested in the content. Consumer nodes generate interests for a pre-known content sequence of 100 chunks of size 1kb each following a Zipf distribution to model the content popularity profile allowing frequent interests for more popular content. Any caching node already containing the content responds to the consumer interest. We allow intermediate nodes to perform in-network caching where uniform buffer sizes are assigned to each node. 30% consumers, 30% caching nodes and the remaining nodes are intermediate nodes with no caching to better evaluate caching nodes. The content availability is modeled by randomly assigning it to each node as its cached content. The content validity metric τ = [0, 1] is extracted from its popularity as well as its availability profile by tracing the generated interests to compute Equation 1.

For each node, the centrality is computed using Equation 3 and the corresponding node utility is derived from its centrality, reward and cost extracted from its buffer space. Utilities for spatio-temporally co-located nodes are computed towards the respective coalition formation. Algorithm 1 is implemented to find the best coalitions as mobile fogs from the set of all coalitions formed in the network. We use STRIVE [START_REF]Strive: Socially-aware three-tier routing in information-centric vehicular environment[END_REF] as a social aware protocol to forward the user interests to retrieve content from caching nodes.

We perform each simulation up to ten times by analyzing different set of nodes as content caches in order to compute up to 95% confidence intervals. The Fogs formed using the proposed approach is compared with the following schemes:

• Centrality-based Fog: Benchmark social-aware approaches use Degree, Closeness, Betweenness, Eigenvector centrality, thus we implement each centrality scheme to form fogs using Algorithm 1.

• No Fog: approach where no coalition is formed and each node greedily implement individual caching policy LRU in a non-collaborative fashion.


Dr Florin Constantin Mihai

Dr Adrian Grozavu

Spatial assessment of construction waste generated by residential buildings in rural areas

Keywords: construction waste, spatial assessment, Neamț County, Romania

The management of construction waste it is at an early stage in urban areas and lacking in rural areas where this fraction is frequently uncontrolled disposed on public lands. Despite the fact some items of construction waste are considered inert for the environment (soil, concrete) it also contains hazardous items (paint additives, cans, and containers) or recyclables (plastics, metals, wood). Potential recovery of this waste stream is high if it is properly managed. The paper estimates the potential amounts of waste resulted from residential constructions across rural municipalities of Neamț County between 2002 and 2010. These statistical values are calculated at commune level (rural territorial administrative unit) in order to outline the disparities between various geographical areas using thematic cartography. This approach is correlated with demographic features in order to reflect such spatial patterns. The map of population density within built-up areas reveals where the construction sector is emerging at the county scale. Field observations highlight the existence of illegal disposal practices of construction waste in the proximity of settlements or water bodies. This waste stream should receive proper attention in following years in order to achieve recycling and recovery targets imposed by EU regulations.

INTRODUCTION

Construction and demolition waste (CDW) is a specific waste stream according to the EU regulations. This fraction needs to be separately collected, recycled and treated through proper facilities in order to achieve a minimum recovery rate of 70% (by weight) until 2020. EU challenges the member states to improve their current waste management systems according to "waste hierarchy" concept which promotes the 3R policy (reduce-reuse-recycle). Urban expansion and population growth led to an increased amount of CDW across the world and scientists start to focus on this field [START_REF] Yuan | Trend of the Research on Construction and Demolition Waste Management[END_REF][START_REF] Melo | Construction and Demolition Waste Generation and Management in Lisbon (Portugal)[END_REF][START_REF] Llatas | Model for Quantifying Construction Waste in Projects according to the European Waste List[END_REF]. These wastes contain cement, bricks, asphalt, wood and other building materials which are usually inert and they can be used as fillings, a foundation for roads or even as a coating material for urban landfills.

The main measures that can be applied to these waste streams according to the environmental agency [START_REF] Bacău | Report on the state of the environment in the North-East Region 1, Regional Environmental Protection Agency of Bacau (REPA)[END_REF] are: source separate collection by type of material (hazardous and non-hazardous); to promote recycling and the reuse of construction and demolition waste (CDW); to provide capacity for treatment and sorting; proper disposal site for wastes which cannot be recovered. On the other side, the municipal waste management systems from rural areas are poorly developed and illegal dumping is still an environmental threat because of the low coverage rates of waste collection services [START_REF] Mihai | Accessibility of waste collection services in Romania: a multi-scale analysis in EU context using thematic cartography[END_REF].

Rural dumpsites often contain mixed fractions such as recyclables (PET/plastics, glass, aluminum cans) textiles, construction, and demolition waste, agricultural waste (straw, branches, sawdust, weeds) polluting the local environment. The paper examines the construction waste generated from residential buildings which are susceptible to be uncontrolled disposed in the proximity of built-up areas.

MATERIALS AND METHODS

Residential constructions increase the population density in built-up areas as an indicator of human pressure on local environment and on the other hand, it leads to increased quantities of construction waste. Firstly, the population density (2010) within built-up areas is calculated and mapped using the color range across rural municipalities (communes) of Neamț County. The data are correlated with a total number of authorizations released for the construction of private residential buildings in the period 2002-2010. These data layers are overlapped (color range and proportional circles) resulting in the first map. Subsequently, it is estimated the potential amount of construction waste (Q cw ) resulted during the study period using a waste generation rate (WGR) of 21.38 kg/m 2 per net usable floor areas (A ufa ).

Such calculations are performed at commune level based on the total usable floor areas (UFA) derived from authorizations according to following formula:

Q cw (t) = A ufa (m 2 ) * W GR (kg/m 2 )/1000
Raw data are provided by National Institute of Statistics, Department of Neamț County. The guide for construction and demolition waste of EPA Sibiu (2011) presented as a case study the management of construction waste in a high-income country (HIC) such as Norway [START_REF]EPA Sibiu, Ghid privind gestionarea deșeurilor din construcții și demolări (Guidelines on the management of construction and demolition waste), Environment Protection Agency of Sibiu County (EPA), Romania, in partnership with Regional & Local Authorities Associations from Norway)[END_REF]. In that case, the waste generation rate per square meter (m 2 ) per usable floor area is much higher such as 41.6 kg/m 2 for larger buildings than 500 m 2 and 56.1kg/m 2 for smaller ones. These rates are too high considering the socio-economic conditions in Romania. The waste generation rate specific for residential areas 21.28 kg/m 2 used as case study in Thailand [START_REF] Kofoworola | Estimation of Construction Waste Generation and Management in Thailand[END_REF] is more suitable taking into consideration that Romania and Thailand are classified as upper middle-income countries (UMIC) by World Bank.

The second map reveals, on the one hand, the spatial distribution of net usable floor area (UFA) or net internal area (NIA) per building using Jenks Natural Breaks of statistical data, and on the other hand, the estimated amounts of construction waste during 2002-2010 associated with building permits.

Local disparities are outlined at county scale by comparing these maps. The third map indicates the potential recovery of construction waste streams based on waste composition data specific for rural buildings.

The data are extracted from a case study performed in Buzău County (South-East Region) as shown in figure 1. The data reveal the construction waste composition specific for rural residential areas and these are not compatible with large infrastructure projects or major demolition/construction activities. Potential recovery streams include recyclables (paper/cardboard; metals, wood, plastics, glass) almost 15.6 % of total fraction, concrete (7%), bricks and tiles (30%) and other local materials (inert-38%). The recyclables can be processed by the waste operators. The concrete, brick, tiles and other material can be sent to a crush plant or to be reused (if hazardous items are extracted) by local councils.

RESULTS AND DISCUSSION

A proper system for recovery and recycling of C&D waste is lacking in Romanian rural regions and only the reuse or recovery of this fraction at household level is performed. There are no relevant data concerning the streams of this waste fraction across the Neamț County. Furthermore, no such data were available for urban areas until 2004. These wastes were mixed collected and disposed in the non-compliant urban landfills. Piatra Neamț, the capital city of Neamț County, has a crushing station for CDW waste for following specific fractions: broken bricks, asphalt waste, concrete waste, mixed construction waste. This station is operational since 2007 when a new integrated urban waste management system were implemented through ISPA funds. Also, there are two collection points for this fraction located in Dărmăneşti and Mărăței neighborhoods. Despite this fact, the uncontrolled waste disposal of CDW fraction on improper sites (riverbanks, local roadsides, public places) was overlooked by local authorities in the study region including urban or rural localities.

Most of the rural buildings permit overlap on localities that have the highest density in built-up areas (> 800 inhab/km 2 ) in 2010 which are located in the proximity of major rivers such as: terraces of Moldova river or in the proximity of its floodplain (Botești, Gherăeşti, Săbăoani, Cordun, Cotu Vameş communes); terraces of Siret river (Tămăşeni, Adjudeni and Sagna communes); terraces of Bistrita river from Subcarpathian sector (Dumbrava Roșie, Săvineşti, Zăneşti and Podoleni communes).

Figure 2. Spatial distribution of built-up density and residential buildings permits

These communes mentioned above are also located on the one hand, in the proximity of two major cities (Piatra Neamț and Roman) which have a regional polarizing role and on the other hand, the localities are passed by major traffic roads or located in their proximity (European and national roads). These geographical regions are most suitable for residential constructions which lead to higher waste generation rates of the main streams (household waste/construction waste). The peri-urban communes (of county capital city) located in the mountain region (Gârcina, Alexandru cel Bun and Pângărați), are well populated areas and attractive areas for construction sector which may generate large amounts of construction waste in as revealed by figure 2.

Neamț Depression is another densely geographical area due to a significant tourism potential in the proximity of Târgu Neamț city (Vânători Neamț, Agapia, Petricani and Grumăzeşti communes). A lower number of building permits (2002-2010) is observed in the subcarpathian hills from central county (Dragomireşti, Bârgăoani communes etc.) and Moldavian Plateau (peripheral municipalities from South-East of the county such as Boghicea, Stănița, Poienari and Pâncești communes) as shown in figure 2.

Such areas are remote from the main urban areas (Piatra Neamț and Roman cities) and the difficult socio-economic conditions reflect the lower amounts of potential waste generated (< 150 t). Furthermore, the usable floor areas per building is lowest in hill regions (southeast of the county) where deprivation index calculated by [START_REF] Burlea | Geographic and socio-economic health inequalities in Neamt County, Romania[END_REF] is higher compared to western half of county (mountain region) with average values above 116 m 2 per building where deprivation index is lower.

Figure 3. Construction waste generated by the new residential buildings in rural areas

The "Alexandru cel Bun" commune has the largest usable floor area per building (209.2 m 2 ). This value is explained by a large number of touristic pensions in the proximity of Piatra Neamț city. Peri-urban municipalities are the main construction areas for rural residential areas which may generate over 1000 t of construction waste. The largest amounts of construction waste (> 2000 t) are generated in northern area of the Roman city by Săbăoani and Tămășeni communes followed by Doljești, Gherăești, Girov and Alexandru cel Bun municipalities (1000-1500 t). The Subcarpathian sector of Bistrița river located in downstream of Piatra Neamț city is another peak area for construction waste generation (400 -1000 t). The same part is revealed in mountain region along the route between Piatra-Neamț and Bicaz cities (Pângărați, Tarcău) where construction sector is supported also by the touristic pensions.

There are significant disparities between such emerging areas and poorest regions of Neamț County where the development construction sector is limited. The construction waste is under 100 t in the center (Bârgăuani commune) and south-eastern part of the county.

In peripheral areas of Neamț County, the construction waste associated with new residential buildings is around 50 t (Poienari commune) or under this threshold such as Stănița commune (36 t). The total amount of construction waste generated by rural buildings during 2002-2010 at the county level is 31393 t of which 4897.3 t are recyclables, 9417.9 t brick, and tiles, 2197 t concrete. Potential recovery of construction waste represents the waste fractions which could be diverted from landfilling or waste dumping on public lands via recycling and recovery operations. This potential varies from one commune to another as shown in figure 4.

Figure 4. Potential recovery fractions of construction waste in rural areas

The values are ranging between 1963 t to under 50 t in peripheral rural areas. The large amounts of recyclable fractions are generated in the proximity of urban areas. These fractions could be collected by the urban waste operators. However, during 2002-2010 the waste collection services are poorly developed across rural municipalities. Frequently, these fractions were mixed and disposed on improper sites in the proximity of the built-up areas (riverbanks, floodplains, pastures, roadsides) as revealed by field observations during 2009-2011. The largest dumpsites were encountered in the case of localities located on fluvial terraces at a distance more than 1.5 km from river main course [START_REF] Mihai | Quantitative assessment of household waste disposed in floodplains of rivers from extra-Carpathian region of Neamț county, Romania[END_REF]. Such areas are also prone for construction sector reflected by a dense population and numerous building permits (Săbăoani, Gherăești, Botești).

The Siret corridor valley is another area where construction sector is expanding with large amounts of waste (Tămășeni commune -2166 t; Doljești -1356 t) disposed in wild dumps, frequently located on floodplains due to the lack of appropriate waste management services. On opposite side, the rivers and tributaries in the proximity of households are most vulnerable to illegal dumping, particularly in mountainous western half of the county. These dumpsites, which may also include household and agricultural fractions, are potential threats during the floods for downstream localities (particularly the households in the proximity of rivers and their tributaries) and local bridges.

Household annexes which are frequently built on floodplains of rivers are the most vulnerable to these natural hazards. Rural municipalities were forced to close the rural dumpsites until July 2009, several sites were compacted and covered with construction and demolition waste as shown in figure 5. As regards the waste management sector, the current priority should be the extension of waste collection services and the improvement of the existing ones in densely populated communes of Neamț County. New waste disposal centers for CDW waste is needed (which does not require major investments like the municipal fraction) where these wastes can be collected by sanitation operators or reused by authorities for local infrastructure projects (road foundations, embankments, dikes etc.) in order to avoid the uncontrolled waste disposal practices. The industrial symbiosis between private and public sector could avoid the landfill of CDW fraction. Almost 30,000 t of CDW waste was reused as cover material (instead of virgin soil) in order to rehabilitate a fly ash landfill site of Termica power plant in Suceava County [START_REF] Danubianu | A new fate for the demolition wastea case study[END_REF].

Figure 1 .

 1 Figure1. C&D composition (%) in rural residential areas. Source of data[START_REF] Musuroaea | Valorificarea deşeurilor din construcţii şi demolări din judeţul Buzău VAL-C&D[END_REF]

Figure 5 .

 5 Figure 5. Uncontrolled waste disposal of CDW waste in rural areas and reuse of this fraction as coating material for local dumpsites

CONCLUSIONS

The paper reveals spatial implications across Neamț County of rural residential buildings between 2002 and 2010. The quantitative and geographical approaches are necessary to reveal the most attractive rural areas for new residential constructions which also correspond to the hot spots of construction waste generation. The local environment is vulnerable to illegal dumping in the proximity of these built-up areas especially if the population is not fully covered by waste collection services.

Construction waste is inert to environment but some fractions are hazardous and improper disposal of these streams may facilitate the pollution of sensitive environmental factors (rivers/groundwater/local biodiversity). Also, these fractions disposed on improper sites favor the waste dumping of other streams (household and agricultural waste, sawdust) including toxic waste such as WEEE, batteries etc. The new regional approach of the integrated waste management system is under implementation supported by EU funds. This system will change the current waste management infrastructure from urban and rural areas in following years. Rural waste management is a complex issue which Romania is facing in order to provide sustainable waste management facilities.


Method

• Dataset: High-quality resting-state fMRI (rfMRI) and diffusion MRI data in 85 participants (age range: 31-35 years old; 46 Females) from the HCP.

• Functional ROIs: All ROIs were defined as a 6mm-radius geodesic disc centering on the MNI coordinate. The VWFA (-45, -57, -12); Language-related regions: Left IFG (-53, 27, 16), SMG (-49, -57, 28), AG (-56, -43, 31), and ITG (-61, -33, -15); Parietal attentional regions: IPS (-25, -62, 51).

• Resting-state functional connectivity: Surface-based resting-state functional connectivity (rsFC) for each grayordinate vertex was computed from the FIX-denoised grayordinate-space rfMRI data. For each cortical vertex, the correlation between its time course and the average VWFA time course was computed, and the Fisher r-to-z transformation was applied. The groupaverage rsFC map was thresholded at p < .05 (FDR corrected).

• Structural connectivity with probabilistic tractorgraphy: For each surface vertex in the grey-white matter interface, we computed the corticocortical connectivity through probabilistic tractography seeding 5,000 streamlines per vertex. We averaged the connectivity profiles for all VWFA vertices, computed the group-wise [START_REF] Gong | Mapping anatomical connectivity patterns of human cerebral cortex using in vivo diffusion tensor imaging tractography[END_REF] , and thresholded at p < 0.0001 (FDR corrected).

• Concordance between functional and structural connectivity:

We fit, at the individual level, a regularized linear model [START_REF] Grosenick | Interpretable whole-brain prediction analysis with GraphNet[END_REF] , to predict the functional connectivity matrix from the structural connectivity matrix. Then, we conducted a t-test for significance at the group level. Finally, we analyzed which vertices' structural connectivity predicted resting-state connectivity by rejecting vertices with a 0 weight on the linear model parameter at p < 0.0001 (FDR corrected).

• Behavioral measures: Language-related measures: word reading and vocabulary; Attention-related measures: Flanker task from NIHToolbox.

Concordance between white-matter pathways and functional circuits linking the VWFA and IPS

Lang Chen 1 , Demian Wassermann 2 , John Kochalka 1 , & Vinod Menon 1

Department of Psychiatry and Behavioral Sciences, Stanford University, Stanford, CA 2. Athena Project Team, INRIA Sophia Antipolis-Méditerranée, Sophia Antipolis CEDEX, France

Specific aims of the current study

• Delineate the structural and functional connectivity of VWFA in a large HCP sample, focusing on the contrast between fronto-temporal language regions vs. fronto-parietal attention networks, particularly, IPS. • Examine the concordance of structural and functional connectivity of VWFA.

• Investigate the brain-behavioral relationships of the functional and structural circuits of VWFA with respects to language and attentional skills.

Background

• The visual word form area (VWFA) is thought to be functionally specialized for words based on its preferential activation to written words 1 . • However, recent studies have suggested that the VWFA shows stronger intrinsic functional connectivity with fronto-parietal attention systems along the dorsal-evetral axis than fronto-temporal language systems 2 .

• VWFA white matter pathways with fronto-parietal systems and their correspondence with functional connectivity remain poorly understood [START_REF] Chen | A unified model of human semantic knowledge and its disorders[END_REF] , limiting our understanding of its core cognitive functions. • According to the "connectivity constraints" account [START_REF] Chen | A unified model of human semantic knowledge and its disorders[END_REF][START_REF] Plaut | Complementary neural representations for faces and words: A computational exploration[END_REF] , the origin and function of VWFA depends on its connectivity fingerprints. For example, if it is specialized in processing word stimuli, it should show stronger structure-function correspondence with the language systems, more than with the attention systems.

VWFA Resting-state Functional connectivity associated with VWFA and IPS

The VWFA showed significant functional connectivity locally with VTOC, as well as distal regions such as IPS, dlPFC and pMTG/STG. VWFA connectivity was stronger with IPS than with language-related regions.

Structural connectivity associated with VWFA and IPS

The VWFA showed significant structural connectivity locally within VTOC, as well as distal regions such as IPS, ATL, STS/STG, and cuneus. VWFA connectivity was weaker with langauge regions compared to that with IPS.

Language regions IPS ITG AG SMG IFG

Functional connectivity of VWFA with IPS associates with attentional/control skills

We found that VWFA-IPS rs-functional connectivity was associated with the performance on the Flanker task, but no association of any other behavioral measures with any other VWFA circuits.

  functional connectivity of VWFA with IPSWhite matter tract probability significantly predicted rsFC of the VWFA (r 2 = 0.186, SD = 0.081), t = 21.18, p < .001). They converged most strongly in local VTOC circuits, but also extended to the posterior bank of IPS, and in dlPFC.

•

  Structural connectivity of VWFA was stronger with the IPS than language-related regions • VWFA Functional connectivity was stronger with the IPS than language-related regions • VWFA-IPS functional connectivity predicted attention but not word reading skills.• Our findings support the view that the VWFA plays a broader role in cognition than word reading. Interaction of the VWFA with the IPS may support visuospatial attention required for processing letter and number strings distributed in space.

Acknowledgement: This work is supported by NIH funding and Lucille Packard Foundation to V.M., and Stanford CHRI funding to C.L . Contact info: lchen32@Stanford.edu


P Morand-Fehr

S Giger-Reverdin

The value chain in Mediterranean sheep and goats. Industry organisation, marketing strategies, feeding and production systems 497 BADOCAP (Database on goats): advantages and limits

Keywords:

BADOCAP is a reference database of AgroParisTech enclosing 21,500 references (in December 2014) from 174 countries and dealing with all the subjects concerning the goat sector: husbandry, production, products (dairy, meat or hair) and commercial channels. Conditions to include a paper in BADOCAP are its goat specificity (more than 50% of its content must concern a goat subject) and the rigor and originality of its scientific, technical and socio-economical information. BADOCAP covers a broad diversity of subjects, allows searching references by four different methods (one is specific to BADOCAP and gives results more accurate by avoiding interferences with references out of the subject), the easiness of search and the presence of abstracts, which allows answers to a specific question to be obtained more rapidly. 73% of papers are written in English, but the rest has frequently an English abstract. Initially, a high proportion of papers (40-50%) came from "grey documentation" (papers unpublished in acknowledged international reviews) and provided original and interesting information. This proportion has declined steadily for 50 years. The quality, the rigor and the originality of the available information in BADOCAP make an essential tool for goat specialists, decision makers for agricultural policy and goat lovers.

II -What is BADOCAP?

BADOCAP is a reference database which includes all the references dealing with different subjects concerning goats: husbandry, production, products (dairy, meat or hair), commercial chan-nels…. with various approaches: scientific, technical, sociological, economical or cultural.

The database is in a computerized form that uses the software EndnoteX7 (or later versions). A reference (one for each paper) is composed of several specific fields. Beside the usual information necessary to refer a paper in an article (list of authors, year of publication, title of the paper, where it has been published), BADOCAP may contain other characteristics as abstract, results, conclusions or geographic origin.

III -How BADOCAP was implemented?

BADOCAP was started in 1966. It was the bibliographic tool of the INRA Research Station of the Department of Animal Science of the National Institute of Agronomy (Paris) which has developed researches on goat nutrition in relationship with lactation, growth, digestion, metabolism, milk yield and milk composition, carcass and meat composition and quality, dietary requirements and feeding programs.

In 70's, requests for information from ITOVIC (French Technical Institute on Small Ruminants) and the closed relationships between the two institutes induced to expand the subjects of articles accepted by BADOCAP. In 80's, the information needs of the FAO -CIHEAM Research Network on Sheep and Goats and International Goat Association induced BADOCAP to expand its field of investigation and above all, to become an international database interested in all topics involving goats and their productions throughout the world.

Initially, BADOCAP consisted of sheets filled by hand or typed. Around 1980, the database has been computerized using several softwares. Finally in the late 90's, the software Endnote has been adopted and it is still used in 2015.

IV -How are papers accepted in BADOCAP?

Whatever their geographic origin, all the subjects on goats, their products or their commercial channels may enter BADOCAP since 1975-1980. Lots of papers concern ruminants, but only those where 50% of contents deal with goat subjects or where information on goats is interesting and original can enter BADOCAP; if not, it would lose its specificity and the reference number would increase threefold.

The main criterion for selection of papers to enter BADOCAP is their quality. For example, a paper with a poor experimental design, a poor statistical interpretation or conclusions not consistent with experimental results is automatically discarded and cannot be included in BADOCAP. On the opposite, the rigor of the reasoning and the quality of information upon which the conclusions are based are elements that promote the acceptance of articles. Finally, originality and topicality of the subjects and the interest of the results are important for the article to be included. In fact over the years, research teams and authors are known mostly by the specialists in charge of BADOCAP.

Options Méditerranéennes, A, no. 115, 2016

V -What is the originality of BADOCAP?

BADOCAP therefore contains references selected on the quality of the items evaluated by recognized experts about goats, which secures the quality of papers included in BADOCAP. It also has the advantage of containing a large number of references published before the computerization of the documentation and many items belonging to the gray literature which includes articles not published in recognized international journals. During the first years of BADOCAP, the papers dealing with goats were in some cases not accepted for publication in these journals. Towards the years 70-80 years, the articles of "grey literature" were often more interesting and sometimes more original than the ones published in international journals. Nowadays, they are fewer and their importance has diminished somewhat.

BADOCAP concerns a wide range of stakeholders in the goat sector: scientists, lecturers, students, farmers, technicians, veterinarians, decision-makers of agricultural policy and planning, actors in goat products (milk, meat and hair etc…).

BADOCAP allows them to get accurate, well-targeted and often original information and to respond to their concerns for the following reasons:

• The diversity of the information enclosed in BADOCAP.

• The possibility to question all the information included in each reference as methodology used, nationality of authors or the place where work reported was made.

• The ability to query by four different methods.

• The fast, flexible and easy methods of query.

• The accuracy of the interrogation by avoiding some parasites or inadequate responses.

• Information on each reference to know where to get full texts.

VI -What are the characteristics of the reference list?

In December 2014, BADOCAP contains 21,500 references from 174 countries. In 1970, 60% of selected references were from developed countries, mainly from Europe and United States while their numbers of goat heads accounted for only 7% of global population. In 2014, it is the opposite: 75% of references originate in emerging or developing countries, particularly China, India, Brazil, Nigeria, Turkey, Bangladesh and Pakistan and 25% industrialized countries which less 4% of global livestock goats.

73% of the articles are written in English, 21% in French and 6% in another language. Approximately, 59% of the articles have a summary in English. The others have no summary in English because the authors have not written one or a summary in another language or finally it was not possible to recover the computationally executive summary.

60% of papers contain key-words chosen by authors or librarians as some papers have been published without key-words. However, all papers of BADOCAP were characterized by the specialists in charge of BADOCAP with" descriptors" included in the "Label" field. They are coded with numbers from 1 to 6 figures except for country descriptors which have been coded with their FAO code in 3 letters.

VII -Advantages and limits of each of the four interrogation methods

The four methods of possible queries in BADOCAP are:

• Method 1: on the title words of the paper,

• Method 2: on the words of the summary,

• Method 3: on the key-words of the "Keywords" field,

• Method 4: using the descriptors of the "Label" field.

The advantages and limits of these four methods are:

Method 1: It can be applied on all items of BADOCAP. If the query is done in English, only 76% of papers can be taken into account. If it is done in English and French, 98% of papers are taken into account, which is satisfactory but complicates the query. It is a little longer to query in 2 languages and we can meet some difficulties concerning deceptive cognates of words: you are exposed to the difficulties of false friends between the two languages and the words which do not have the same meaning in English and French. Generally the title words are not sufficient to mention all the interesting aspects of a paper.

Method 2: It can be performed only on items with summaries. Same as for Method 1, it can be applied in 2 languages for querying more papers. This method is a satisfactory one when the summaries are well written, which is not always the case.

Method 3: It can only be applied on items with key-words (65%). Generally, even if the full text of the paper is not written in English, the authors write an English summary with English key-words. They are generally well chosen.

Method 4: It can be applied to all the articles of the data base. The descriptors are coded with numbers or letters (see chap. V). Consequently, there is no language difficulty. The descriptors cover areas as diverse as science, techniques or current queries. They are chosen by goat specialists and not by librarians.

VIII -What are the different steps of a query? Advices to carry out an inquiry correctly

A query on BADOCAP consists of the following steps:

-Define the question and write it (Step 1)

The question must be as accurate as possible without ambiguous words. For example, an inaccurate question would be: "Effects of diet composition on the performances of young goats". The more accurate question on the same topic to find adequate key-words should be: "Influence of the level of concentrate in diets on the growth and nutritional efficiency of weaned young goats".

-Select key-words or descriptors (Step 2)

The second step is to choose the key-words or descriptors corresponding to all the aspects of the question. This requires a sufficient knowledge of the list of key-words or descriptors used in BADOCAP.

There is often an interest in limiting the subject of the question to a given period or geographic area. If, for example, a methodology appeared after 1990, the query should be made only on items published after 1990. On the other hand, it can may be appropriate on a subject concerning Angora goats to select items from countries that have experience in the production of haired Angora: South Africa, Turkey and United States.

Options Méditerranéennes, A, no. 115, 2016

The value chain in Mediterranean sheep and goats. Industry organisation, marketing strategies, feeding and production systems

-Design of the query (Step 3) Specifically, it fills the interrogation device with a maximum capacity to question key-words or descriptors depending of the EndNote Version. In the last ones, it is possible to query on a subset of the database. Like in other databases, Boolean operators are used: "and", "not" and "or".

The result of the query is presented as a reference list of articles that answer the question. They can be included easily in a word file with a specified output form.

-Control and possible improvement of the result of the query (Step 4) Analysis of selected references may be evaluated to assess whether the result is satisfactory. It often happens that the first approach gives references either inadequately selected or precise or contaminated with parasites or irrelevant. In this case, we must return to step 2 by modifying descriptors.

-Analysis of result references of the query response (Step 5) At this stage, it is a classical literature review articles that should be done. The method to use depends on research objectives. If for example, the purpose is simply to find the average value of vitamin E in goat milk reading of several papers summaries may be sufficient. However, knowing the factors influencing the content of this vitamin in goat milk requires a thorough analysis of full papers dealing with the subject.

To get the full text of papers, the easiest way is to ask either the corresponding author (especially when his e-mail address is contained in the database) or visit the site of the journal in which they were published. Some of them are open-access, others accept to open the access 12 months after publication. In fact, the access to full texts depends on the policy of each journal.

Last but not least, the EndNote program is compatible with Word. References used to write a paper can be directly integrated and the author can cite them as requested by the publisher.

IX -Conclusions

BADOCAP database is operational at Paris (UMR INRA-AgroParisTech MoSAR), but not available to outsiders. However, due to its interest for goat specialists, BADOCAP should be available in the Documentation Centres of Institutions interested by small ruminants as the one of the Mediterranean Agronomic Institute of Zaragoza, especially for members of the FAO-CIHEAM Network of Cooperative Research for Sheep and Goats and for the participants of the seminars organized by the network. This is justified since some of the information in BADOCAP originated from the work and review articles achieved by the members of the network for over 30 years. The authors of this short paper are ready to help them so that their use of BADOCAP will be efficient by taking into account their specific objectives.


S Giger-Reverdin

D Sauvant

goats. Industry organisation, marketing strategies, feeding and production systems

Keywords:

Feed efficiency in ruminants is largely linked to rumen digestive efficiency. Moreover, some recent research pointed out that feeding behaviour influences rumen pH. This work showed that twelve dairy goats (six Alpine and six Saanen) characterized by very different intake rates with a standard diet increased their intake rate when the diet was changed to a high concentrate one. The between-goat ranking for intake rates was not modified (P = 0.70) by the shift in diet composition. Organic matter digestibility of the high concentrate diet decreased as level of intake increased, with a 2.2 points difference between the Alpine and Saanen breeds. This difference is mainly due to a difference in cell wall digestibility and can be explained neither by the rate of intake, nor by the sorting behaviour or by the fat corrected milk yield. Feed efficiency was, in this trial, linked to metabolic efficiency. Both items were explained by the fat-corrected milk yield. Feed efficiency was not correlated with digestibility and there was no difference (P = 0.72) between breeds. Differences between animals in feed efficiency were explained by differences in metabolic efficiency.

Keywords. Dairy goat -Feeding behaviour

II -Material and methods

One month after their second kidding, thirty five dairy goats were given for three weeks a total mixed diet (control): 30% dehydrated alfalfa, 20 meadow hay, 30% pressed sugar beet pulp and 20% concentrate on a dry matter (DM) basis. They were fed ad libitum, after milking, with 2/3 of the diet offered in the afternoon and 1/3 in the morning after milking in agreement with the intervals between milking.

Feed intake was recorded every 2 minutes in individual crates by weighing devices fitted under the feed trough. For each afternoon feeding, the quantity of diet eaten 180 min after feeding was divided by the total feed intake for this feeding (fractional intake rate, P180), because the greatest variability between animals was observed at that moment [START_REF] Giger-Reverdin | Variations individuelles du comportement d'ingestion d'un ruminant. Application au phénotypage de chèvres laitières[END_REF]. It was measured during a four day period on the control diet at the end of the 3 rd week (P180c).

Two groups were formed with six Alpine goats and six Saanen. The twelve goats were chosen out of the 35 to present a wide range of variation for P180c within a group, but a similar P180c and similar DIM (days in milk) in the two groups. They were then shifted to an acidogenic diet: 15% dehydrated alfalfa, 15% meadow hay, 20% pressed sugar beet pulp and 50% concentrate on a DM basis after a transition week between control and acidogenic diets. After an adaptation period of three weeks, their fractional intake rate was measured during a four day period (P180a). The goats were then moved to digestibility crates, and a five day digestibility measurement was performed after an adaptation period of one week to digestibility crates.

Dry matter, ash, cell wall content (or NDF) and crude protein were measured on feed offered, refusals and feces according, respectively, to ISO (1983), ISO (1978), [START_REF] Giger | Etude de l'influence préalable de différents traitements amylolytiques sur la teneur en résidu NDF d'aliments du bétail[END_REF] and ISO (1997) to calculate the digestibility of the corresponding constituents. Body weight (BW) was measured once a week. Milk yield was measured at each milking and fat and protein contents were measured each week on two consecutive milkings. Fat corrected milk (FCM) corresponds to a standard milk with 35 g/kg fat and 31 g/kg protein.

III -Results and discussion

Intake

When the diet was shifted from control to the acidogenic one, one goat exhibited an off-feed period two weeks after the beginning of the allowance of this diet. Its dry matter intake was as low as 0.337 g/kgBW, but the goat recovered its former intake in three days.

The mean value on the whole data set for P180c was 0.759 (standard deviation or SD = 0.0771) and ranged from 0.628 to 0.912 with a normal distribution. For the twelve goats chosen to be fed the acidogenic diet, the P180c had a mean value of 0.736 (SD = 0.0717) and varied from 0.628 to 0.860.

The rate of intake with the acidogenic diet (P180a) was directly proportional to the rate of intake with the control diet (P180c): P180a = 1.15 x P180c (n = 12, r = 0.69, RSD = 0.066) There was no interaction between breed and P180c and no breed effect (P = 0.70). This means that the animals exhibited the same ranking for control and acidogenic diet, and that they ate at a 15% higher rate the high concentrate diet, even for the goat which exhibited the off-feed episode with the acidogenic diet. This result is in agreement with the repeatability within goats of intake behaviour previously observed on the whole data set with a control diet fed at four different times [START_REF] Giger-Reverdin | Variations individuelles du comportement d'ingestion d'un ruminant. Application au phénotypage de chèvres laitières[END_REF]. This is also in agreement with previous data showing that when goats shifted from a medium concentrate diet to a high concentrate diet, their rate of intake was not modified during the first weeks of allowance [START_REF] Serment | Effect of the percentage of concentrate on intake pattern in midlactation goats[END_REF]. The goat who exhibited an off-feed period had surprisingly the lowest intake rate (P180c). The large between-animals variation for P180a should also be noticed.

Organic matter digestibility

The organic matter digestibility (OMD) was 61.8% (SD = 1.32) and ranged from 59.9 to 63.5. OMD decreased as the dry matter intake (DMI) expressed on a body-weight basis increased, there was no interaction between breed and the covariate DMI, but a significant difference (P <0.001) of 2.2 points for OMD with a digestibility higher for Alpine than for Saanen (Fig. 1). OMD = 80.1 (4.64) -0.372 (0.0941) DMI/BW (n = 12, n breed = 2, r = -0.88, RSD = 0.718)

The between breed difference can be explained neither by the fractional rate of intake (P180c or P180a), nor by the sorting behaviour (expressed as the ratio between the NDF composition of intake and offered) or by the milk yield. It is mainly due to a better digestibility of NDF by the Alpine goats compared to the Saanen ones, as the mean difference between the digestible NDF content was 18 g/kg DM. The remaining difference was explained by the difference in the digestible crude protein content: 5 g/kg DM.

The decrease in OMD with the level of intake was higher than the slope of 0.253 that we observed in various ruminants [START_REF] Sauvant | Modélisation des interactions digestives et de la production de méthane chez les ruminants[END_REF]. This decrease is probably due to the transit time of the feeds which is negatively correlated to digestibility [START_REF] Najar | Influence de la quantité de matière sèche ingérée et du temps de séjour des aliments sur la digestibilité de la ration par la chèvre laitière[END_REF].

Feed efficiency

Feed efficiency (FE) was calculated as the ratio of FCM yield/DMI which is the product of digestibility (DOMI/DMI) by metabolic efficiency (ME = FCMY/DOMI). FE was firstly linked with ME (n = 12, r = 0.97). These two items were mainly explained by FCMY (n = 12, r = 0.66 and r = 0.72 for FE and ME respectively). FE was not correlated with digestibility (n = 12, r = -0.38, P = 0.22). This could be explained by the low range of digestibility in this trial.

Individual variation of FE were mainly explained by the level of performance. Individual variation of digestibility which were mainly linked to the levels of FCMY and DMI cannot explain individual FE. There was no breed effect (P = 0.72).

IV -Conclusion

This work pointed out that intake rates of dairy goats were modified by the type of diets, but that between goat ranking remained unchanged. Between animal variability for organic matter digestibility depends on the level of intake, but there was also a between-breed difference due mainly to a different ability in cell wall digestion. Differences between animals in feed efficiency were linked to the level of performance and were explained by difference in metabolic efficiency. Digestibility had no influence on feed efficiency in this trial, as it did not vary a lot.

Fig. 1 .

 1 Fig. 1. Effects of dry matter intake and breed on organic matter digestibility.

Options Méditerranéennes, A, no. 115, 2016

The value chain in Mediterranean sheep and goats. Industry organisation, marketing strategies, feeding and production systems

Acknowledgments

The authors thank J. Tessier, J.P. Bonte, O. Dhumez, G. Durandet and A. Eymard for taking care of the goats and for their technical assistance during the experiment at the INRA experimental farm of Grignon and for the analysis of the samples.