Wangui Kimari

'We do not Want any more masters': ruins, planning and the " messy labours " of the urban poor

Wangui Kimari

Introduction

Amidst the animated conversations about recent national and county development plans for the city, one wonders when Nairobi's real residents will actually appear. In one of the most recent iterations of this master planning, the Integrated Urban Development Masterplan for the city of Nairobi (NIUPLAN), the now normalized images of futuristic highway networks and buildings (that appear to all derive from a singular neoliberal toolbox) persist unabated. Did the people fall from the many anticipated skyscrapers and super thoroughfares of this aspirational 'worldclass city'? Or are they buried under the literal and literary rubble of these master plans? While there are gestures towards participatory consultation that one notes in the NIUPLAN, in the litany of pages devoted to this document we still do not seem to see or hear the 60-70% of this city who live in the poor zones that account for only 6% of Nairobi's geography. Instead we encounter the globally sanctioned trademarks of what a city should look like; the fervent ambition to be 'world-class' 1 that endures as a "regulating fiction" for much of our urban life [START_REF] Robinson | Ordinary Cities: Between Modernity and Development[END_REF]. I argue here that despite their invisibility, it is the residents from these Nairobi ruins, its most marginalized spaces, who will determine the viability of the NIUPLAN in their neighbourhoods and in Nairobi broadly. For if we are to track the "long-running drama" (Manji 2015, 7) of master planning in Nairobi, we see that within and despite the imprints of colonial zoning, it is less the overdetermined templates of formal spatial The continuities between colonial and 1.

postcolonial urban spatial management, and in particular the persistence of "selective nonplanning" [START_REF] Yiftachel | Theoretical notes on 'gray cities: the coming of urban apartheid?[END_REF] in certain parts of the city that creates intentional zones of non-inclusion without even the most basic of infrastructural services; 2012) and also [START_REF] Roy | The 21st-Century Metropolis: New Geographies of Theory[END_REF].

The increasing use of militaristic violence to 2.

constitute urban governance in the neoliberal present, and; How the alternative urban histories and 3.

subjectivities that emerge from these ruins are what will direct us towards a truly more just city, and one that is altogether more inclusive than any master plan can ever conjure. This is principally the work of young people.

Before I contextualize and connect these three threads, I need to elaborate on the category of the ruin that is a key framing in my work. Informed by [START_REF] Stoler | Imperial Debris: Reflections on Ruins and Ruination[END_REF], thinking with ruins allows us to:

Thinking with ruins and Matigari connects the embattled reflections by 80 year old Monica Njeri about her 60+ years tenure in Village 1 where she has never "seen the government", with the plight of the youth group owned entrepreneurial car wash in Bondeni whose members are constantly besieged by the police, and often fatally, for "dragging water from the main pipes"; water that should have been in place decades ago. These stories spell out all the follies of Nairobi and belie the grand narrative that they are fixable through apolitical and technocratic sleights of hand contained in master plans, and whose genesis (and dependable saviour) is always more "business."

Inevitably the expansion of ruins and precarious lives will make us question the costs of what it means to become a "world-class city" (urbans that really should be called world-waste cities); clones of EuroAmerican provenance and that have been normalized to mean hyper-modern and hyper-inaccessible. At the same time, and as I argue here, these margins must be the locations from where we chart out city futures since the ruin is also always a site for "vital refiguration" (Stoler 2008, 194). That despite constituting long histories of grave neglect are at once "always broken, but always resounding" (Simone 2015, 2).

To demonstrate these arguments in a clearer fashion, below I give a brief history of Mathare space, and connect this to broader historical urban spatial management practices in Nairobi. Subsequent to this I also highlight some of the ground work that, consciously and unconsciously, contests the violence of urban planning from within ruins and in ways that challenges both the history and futures of this city. This exercise is provoked by the question that haunts and

Trace the fragile and durable substance and signs, the visible and visceral senses in which the effects of empire are reactivated and remain. But ruination is more than a process. It is also a political project that lays waste to certain peoples and places, relations, and things. To think with ruins of empire is to emphasize less the artifacts of empire as dead matter or remnants of a defunct regime than to attend to their reappropriations and strategic and active positioning within the politics of the present (Stoler 2008, 196).

My work focuses on these "durable substance(s) and signs" of colonial/postcolonial formal spatial management projects, how these are said to shape Mathare residents' subjectivities, and the manner in which these circulated positionings are engaged with by residents to develop different self and urban possibilities for present and future. In tracking the histories of Mathare through both the archive and collective memory of residents, consistent connections are made between urban governance in the present and past: the remnants and reactivations of empire that compel many residents to think of themselves as Matigari-"the ones who survived the bullets" and keep surviving them (Wa [START_REF] Thiongo | Matigari[END_REF]. Matigari in this sense can be a metaphor for shared consciousness in this place where the weight of the coeval histories of the city feels heavier on the backs of the poor, as it is them who have suffered through all of its urban charades.

Picture from apartment building in Kosovo, Mathare, from the authors own files undergirds this research endeavour: how is the value of life in Nairobi determined by spatial management grids that decide not just who lives and who dies, but also how should people who are left to die live?

"I live in Mathare not Nairobi" While Mathare constituency is seen by some of its residents as the land of temporary 10 x 10 shack dwellings and a place that signals no spatial planning or "modern" architecture, it is important to recognize that it has emerged from a long history of being part of the exclusions of both colonial and postcolonial urban spatial management practices [START_REF] Medard | City Planning in Nairobi: The Stakes, the People, the Sidetracking[END_REF]Torres-Rodrigues 2010;[START_REF] Myers | A World-Class City-Region? Envisioning the Nairobi of 2030[END_REF]. Although it covers an area that is only 3 square kilometres in size and is likely the smallest constituency in Nairobi, it is seen as a relentless impediment to the "modern" city and a constant thorn in the side to county authorities, the police and its elected political representatives.

Both oral narratives and archival data document that Mathare has existed for longer than the myopic "rural-urban" migration narratives about "squatter settlements" and "slums" in Nairobi would allow us to believe. While villages were documented in Mathare Valley, what is now part of Mathare constituency,2 from as early as 1921 [START_REF] Chiuri | County of Nairobi and the Japanese International Cooperation Agency (JICA). NIUPLAN/The project on Integrated Urban Development Master Plan for the City of Nairobi in the Republic of Kenya (Final Report). Nairobi: County of Nairobi and the Japanese International Cooperation Agency (JICA), 2014 Government of Kenya. Nairobi Metro 2030: A world class African metropolis[END_REF][START_REF] White | The Comforts of Home: Prostitution in Colonial Nairobi[END_REF]), it appears that human settlement began to increase in the 1940s and 50s. As the contours of Mathare confer, this area was a quarry and became a central site to excavate rocks that would contribute to building many of the architectures that constitute the 'functioning core' of the city centre. Many of those who were working here later settled in its vicinity. During these preindependence years, while a medley of employment laws controlled the presence of Africans in the city (White/Nairobi Master Plan 1948, 6;Chiuri 1978, 4), they were allowed to settle in this area as it is in the less favourable low lying eastern part of town where other African settlements such as Pumwani were and still are located [START_REF] White | The Comforts of Home: Prostitution in Colonial Nairobi[END_REF]). Furthermore, the East was also an area that was deemed less desirable for European settlement as it had poor drainage, and would later host sewage treatment plants and other noxious industry [START_REF] Otiso | Colonial Urbanisation and Urban management in Kenya[END_REF]Hake 1977, 88). In sum, despite the many measures taken to keep Africans away from Nairobi, they were needed for the machinations of the city and so were confined to the 'inferior' geography of these eastern coordinates. This zoning, intended to quell white fears of the social and biological contagion of African bodies, allowed that this community could persist, albeit precariously, within city limits.

In the postcolonial period Mathare continues to be a space that is not included even in the most basic urban planning infrastructure of the city. Despite this, not too long after independence, it was still regularly raided for tax collection (Hake 1977, 161). In all of these periods it is always depicted as a site of overcrowding, and conjured as the bedrock of crime, prostitution, illegal alcohol brewing and "parasites"; violent imaginaries that are said to be embodied, above all, by its young population. The "slum" is therefore taken as the immoral part of the urban form while it's more middle and upper class constituencies are seen to be its moral body. Furthermore, due to the perpetual preoccupations to make Nairobi a 'global' city this space becomes the "anti-city"; in straddling a super highway and two main transport arteries into the downtown core is seen to cast a long shadow over the established and aspired prosperities of this African urban.

Therefore, while surrounded by legal and tenured city infrastructures Mathare was and is still included in planning by its very exclusion. It is important to note that this is an disregard firmly anchored within the complicities of the state; a situation that evidences informality not as the antithesis of formality, but a state of affairs that includes both as co-constitutive processes that 'fold' into each other [START_REF] Mcfarlane | Rethinking Informality: Politics, Crisis and the City[END_REF]Roy 2009, 826). In this regard, while it maintains the area of space delimited by the quarry and extends slightly in an eastern direction, like many poor urban settlements in the city its presence is accepted with ambivalence while it continues to be omitted from the "development impulse" that [START_REF] Manji | Bulldozers, homes and highways: Nairobi and the right to the city[END_REF] discusses. Thus, over the years, it has taken on the contradiction of being at once within and without urban development; it is regarded as a necessary part of the city and yet planning continues around it without receiving any of the service benefits implied from being within the municipality's jurisdiction.

This historical neglect is evident in the dearth of basic water and sanitation services for residents, the absence of substantive educational and health facilities as well as a lack of tenure security (Muungano Support Trust et al 2012). What's more, this sinister exceptionalism further legitimized by the imagined positionings of its residents is also seen to provoke and justify a plethora of very violent acts committed, overwhelmingly, by the police and the city council. While these 'statesociety imaginaries' are never as inflexible as often understood, (see, for example, Garmany (2013) for an examination of this in Fortaleza, Brazil), the combined impacts of both structural neglect and extra-legal state violence indicate a spatial particularity that has engendered a situation often considered one of "the most difficult urban environments in East Africa" (Muungano Support Trust et al 2012, 4).

Not Joining the Masterplan Party.

A cursory history of Mathare was needed to contextualize the ruination that it is entangled within: a political, social, economic and ecological disregard that both creates and criminalizes this Throughout the history of the three previous plans for Nairobi (1927, 1948 and 1973), very little is seen to have changed in this part of the city-a feeling that is reiterated frequently by its residents as well as scholars [START_REF] Myers | A World-Class City-Region? Envisioning the Nairobi of 2030[END_REF][START_REF] Medard | City Planning in Nairobi: The Stakes, the People, the Sidetracking[END_REF]. Nonetheless, in recognizing this long history of indifference, residents, and particularly the young, work to fill the vacuums created by this abandonment. Mine is not a bid to romanticize often imperfect systems to deliver water, security and sanitation services etc., nonetheless, the persistent youth-work to equalize geography, in ways that have not been made possible by previous formal urban interventions, needs to be affirmed. And in embodying a reciprocated mistrust for these master plans, this work dampens the bourgeois fervour that accompanies the urban visions and proposals that do not make them visible. This is why many residents I spoke to conveyed multiple versions of the following statement: "I don't live in Nairobi, I live in Mathare".

In attending to this disregard in the areas of "blackness" that he conceptualizes, [START_REF] Simone | It's just the city after all![END_REF] speaks of these residents of ruins who:

Mambo! Vol XIV (4)
Learned to live in the implosion of old orders grinding to a halt, of being the example that teaches a populace how to watch what happens when a portion of its citizenry is unmoored from the basic supports of life. It has learned to live with incessant transience, quickly deciding how to recoup opportunity from sudden detours and foreclosures (Simone 2015, 7).

In contrast to improvement, it is often master plans that provide exactly these "detours" and "foreclosures." Evidencing this are the statements in the NIUPLAN about the need to tackle the housing crisis, but that are followed in quick succession by remarks about removal and resettlement (NIUPLAN 2015, 6-28). Similarly, contained within this new blueprint is information about enhancing, for example, transport facilities, but nothing about the displacements that will likely be occasioned by these processes, nor whether there will be adequate compensation given to those who will inevitably be forced to relocate. Furthermore, an unspoken though normalized practice that haunts these projects is the use of police force to make sure the ostensibly "business unusual" of Vision 2030 will go on (see [START_REF] Medard | City Planning in Nairobi: The Stakes, the People, the Sidetracking[END_REF][START_REF] Manji | Bulldozers, homes and highways: Nairobi and the right to the city[END_REF] and Amnesty 2013 on the violence of both the administrative police and city council during forced evictions). While the sheer murkiness of the numerous headings and subheadings of the NIUPLAN make it difficult to discern whether there is indeed a comprehensive pro-poor project at hand, the snapshots conferred above inspire what Myers refers to as a "jaundiced optimism" [START_REF] Myers | A World-Class City-Region? Envisioning the Nairobi of 2030[END_REF].

Conclusion

Notwithstanding the spectacle of hyper futuristic assemblages of city, and beyond the celebration of these global blueprints, the residents of this city appear where they have always been-on the ground and working for home. Examples of this are the marches and petitions to stop land-grabbing that most recently saw the removal of a corrupt village elder from Kosovo (Hospital Ward), or the establishment of ward based El Nino committees in a variety of villages. This resistance is also attested to by the large number, and continuous proliferation, of community groups that tackle garbage collection, incessant .5.

fires, bridge building, toilet maintenance, electricity and water provision. Unquestionably these actions manifest many situated contradictions and violences, but these are paradoxes that also reflect larger and more unequal local and global politico-economic practices.

Whether these imperfect actions confirm the "outlaw" beginnings and natures of a marginalized area is beside the point. What we should look at is how these 'barefoot activisms' are more about creating a city where people can live and not just "do business." Referring to the hegemony of these plans and their persistent ineffectiveness, one of my interlocutors stated the following: "Masterplan? We do not want any more masters'. They are going to build a sewage line but they don't even build toilets. We need houses."

This excoriation of technocratic planning is anchored within the painful realization that a 42 kilometre superhighway will be built faster than a house for a single mother; that the Nairobi Metro 2030 Vision Plan can make a decision as definitive as having a "Nairobi Philharmonic Orchestra" (Nairobi Metro 2030 2008, 79) but will not make any pronouncements about the exact number of toilets, clean water taps, clinics and access bridges that will be built in its margins. Therefore, a "strained relationship" with the ongoing proposals for urban planning will definitely persist in Mathare -if they are even noted. And in recognizing the "long-running drama", a telenovela actually, of master plan declarations that have never materialized in this part of the city, almost none of my interviewees speak with enthusiasm or even reference the spatial projects to come. These ruins will continue in their "own rules of operation" (Myers 2015, 332), uninspired by yet another proposal for Nairobi that does little to include them. Speaking to the inherent disparities within formal planning documents in Nairobi [START_REF] Myers | A World-Class City-Region? Envisioning the Nairobi of 2030[END_REF] states that: But until the yawning gap is narrowed between the elites' visions and the experiences of ordinary city residents, all the master plans in the world will not lead to the growth of inclusive, relational cities with an improving quality of life and declining rates of inequalities (343). It appears that it is not just Mathare residents who are unenthusiastic about planning proposals. Experientially, city dwellers know that it is only from their determinations that poor neighbourhoods will change, and so we can continue to expect more grounded inattention to the proliferating urban blueprints from within the ever expanding city margins. The more the 'world-class city' is pursued on a national level , the clearer it will be that it is only through the 'messy labours' of ruins that this city will survive, regardless of what it says in this or the next masterplan.

de Recherche sur l'Afrique de l'Est Volume XI n° 7-2013

MAMBO! IFRA Laikipia Road, P.O. Box 58480-00200 Nairobi, Kenya Tel : 254 20 43 43 446 -Fax : 254 20 43 43 052 E-mail: info@ifra-nairobi.net Web site: www.ifra-nairobi.net Travaux Rwanda, année zéro. C'est ainsi qu'apparaît l'année 1994 au pays des mille collines, ravagé par plusieurs années d'une guerre civile dont l'élément central et à sa tête Paul Kagamé, prend le contrôle d'un État fantôme, où tout est à reconstruire. Au-delà du travail le Rwanda nouvelle version va basculer d'un pays prend ses distances avec l'Afrique centrale en 2007, la Communauté d'Afrique de l'Est fut acceptée.

Le Rwanda se tourne à l'est

	Nairobi
	VOL XIV (5), 2016
	Jérémy Révillon
	management and more the "messy-labours" (Simone
	2015) of urban ruin residents that shape our city
	geographies for present and future.
	Not convinced by the topical promises of urban reform,
	and that are captured in neoliberal development plans
	such as Kenya Vision 2030 and its proliferating
	spatial progeny (of note are Nairobi Metro 2030 and
	the NIUPLAN 2014-2030) my PhD research dwells
	in the stories of residents from poor urban settlements
	in Nairobi, what I refer to here as ruins, to explore
	how both the material and discursive practices of
	these plans are experienced. I sit in these stories of
	urban entanglements-of innovation, violence, loss,
	resilience and redemption-in order to historicize
	urban spatial management in Nairobi from these
	marginalized spaces, and particularly those in the East
	of the city. Through tracking both the implementation
	and the absence of proposed urban interventions in
	Mathare, Nairobi (and it is usually the latter), I attend
	permet de mieux comprendre la réorientation to how urban spatial management has shaped residents
	on both a public and intimate level. To these ends I
	pays sont d'anciennes colonies britanniques, comme call attention to three urban events:
	2
	3
	de candidature en 1996 et en 2003. L'anglais, quant

1 1 En anglais, East African Community, d'où l'utilisation de l'acronyme EAC dans la suite du texte la Communauté Économique des États de l'Afrique d'intérêts commerciaux et un instrument de commerciaux avec l'Union Européenne. Le Rwanda rejoint ainsi son principal partenaire commercial actuel, le Kenya. Cette situation s'explique par le fait que Mombasa, le principal port kényan, est, de fait, aussi le principal port rwandais. Pays enclavé dans l'Afrique des Grands Lacs, le Rwanda importe 2 « Rwanda president accuses UN of betrayal and denies backing Tutsi rebels in Congo », The Guardian, 15 novembre 2008. 3 « Le Rwanda admis au sein du Commonwealth », Le Soir, 29 novembre 2009.

IFRA

. Mathare constituency has six county assembly wards; these are Huruma, Ngei, Mabatini, Hospital, Mathare and Kia Maiko wards.


Louis Lambrechts
email: louis.lambrechts@pasteur.fr

Sebastian Lequime

Borderia Av

G Diaz

Henningsson R Gonzalez

Evolutionary dynamics of dengue virus populations within the mosquito vector 1 2 3

To date, dengue virus evolution has mainly been addressed by studies conducted at the 17 between-host level. Like other pathogens with high mutation rate and rapid replication, 18 dengue viruses also evolve during the course of an infection. Over the last few years, the 19 advent of deep-sequencing technologies has facilitated studies of dengue virus populations at 20 the within-host level. Here, we review recent advances on the evolutionary dynamics of 21 dengue virus populations within their mosquito vector. We discuss how identifying the 22 evolutionary forces acting on dengue virus populations within the mosquito can shed light on 23 the processes underlying vector-virus interactions and the evolution of epidemiologically 24 relevant traits.

Introduction 26 27 Dengue viruses (DENV) are mosquito-borne RNA viruses in the genus Flavivirus (family 28 Flaviviridae) that cause more human disease than any other arthropod-borne (arbo) virus [START_REF] Guzman | Dengue[END_REF]. [START_REF] Smith | Venezuelan equine 356 encephalitis virus in the mosquito vector Aedes taeniorhynchus: infection 357 initiated by a small number of susceptible epithelial cells and a population 358 bottleneck[END_REF] They consist of four serotypes (DENV-1 to -4) that are phylogenetically related [START_REF] Holmes | The origin, emergence and evolutionary genetics of dengue 261 virus[END_REF] and 30 loosely antigenically distinct [START_REF] Katzelnick | Dengue viruses cluster antigenically 264 but not as discrete serotypes[END_REF]. Human DENV originate from sylvatic transmission cycles 31 that occur between non-human primates and arboreal mosquitoes [START_REF] Vasilakis | The history and evolution of human dengue emergence[END_REF]. They emerged and 32 became endemic in the human population in the last few hundred years [START_REF] Holmes | The origin, emergence and evolutionary genetics of dengue 261 virus[END_REF]. Today, DENV are 33 transmitted among human hosts by anthropophilic mosquito vectors, primarily Aedes aegypti 34 [START_REF] Lambrechts | Consequences of the expanding global distribution 268 of Aedes albopictus for dengue virus transmission[END_REF]. 35 36 Over the last three decades, phylogenetic and molecular evolution analyses based on viral 37 genomic sequences have provided valuable insights into DENV evolutionary history and 38 epidemiology [START_REF] Weaver | Molecular evolution of dengue viruses: contributions of 271 phylogenetics to understanding the history and epidemiology of the preeminent 272 arboviral disease[END_REF]. It is noteworthy that this large body of work on various aspects of DENV 39 biology such as the origin, spatio-temporal transmission dynamics, and pathogenesis has 40 mainly been conducted at the between-host level. In other words, viral genomic sequences 41 have generally been analyzed by considering a single genome per human or mosquito host 42 sampled, which makes the implicit assumption that a single DENV genome adequately 43 represents a given infection. 44 45 Due to their exceptionally high mutation rateabout a million times greater than that of 46 vertebrates [START_REF] Duffy | Rates of evolutionary change in viruses: patterns 274 and determinants[END_REF] -RNA viruses typically exist within their hosts as a heterogeneous population 47 of related genetic variants, often referred to as a viral quasispecies [START_REF] Lauring | Quasispecies theory and the behavior of RNA viruses[END_REF]. The within-host 48 genetic diversity of DENV was initially demonstrated by reverse transcription of viral RNA 49 purified from individual human plasma samples, followed by cloning and sequencing of 50 multiple PCR amplicons [START_REF] Wang | Dengue type 3 virus in plasma is a 278 population of closely related genomes: quasispecies[END_REF][START_REF] Wang | Sequence diversity of the capsid gene 280 and the nonstructural gene NS2B of dengue-3 virus in vivo[END_REF]. The majority of subsequent studies focused on the human 51 host [START_REF] Chao | Strategically examining 283 the full-genome of dengue virus type 3 in clinical isolates reveals its mutation 284 spectra[END_REF][START_REF] Descloux | Dengue 1 diversity and 286 microevolution, French Polynesia 2001-2006: connection with epidemiology and 287 clinics[END_REF][START_REF] Parameswaran | This comprehensive analysis of within-host dengue virus genetic diversity in human serum 293 samples pointed out correlations with clinical, epidemiological and virological 294 features[END_REF][START_REF] Thai | High-resolution analysis of intrahost genetic 298 diversity in dengue virus serotype 1 infection identifies mixed infections[END_REF], although the within-host genetic diversity of DENV populations was also 52 confirmed in mosquitoes [START_REF] Aaskov | This landmark study combined assessment of within-host dengue virus genetic diversity in 304 humans and mosquitoes and phylogenetic analyses to demonstrate sustained 305 transmission of a viral lineage associated with a truncated genome[END_REF][START_REF] Lin | Study 308 of sequence variation of dengue type 3 virus in naturally infected mosquitoes and 309 human hosts: implications for transmission and evolution[END_REF]. Within-host DENV genetic diversity was initially reported 53 to be lower in mosquitoes than in human patients [START_REF] Lin | Study 308 of sequence variation of dengue type 3 virus in naturally infected mosquitoes and 309 human hosts: implications for transmission and evolution[END_REF], but this observation did not hold in 54 later studies [START_REF] Aaskov | This landmark study combined assessment of within-host dengue virus genetic diversity in 304 humans and mosquitoes and phylogenetic analyses to demonstrate sustained 305 transmission of a viral lineage associated with a truncated genome[END_REF][START_REF] Sim | during a Dengue Virus Type 3 Cuban Epidemic: Deep Sequencing of Evolving 320 Viral Populations[END_REF]. 55 56 Early studies of within-host DENV genetic diversity based on clonal sequencing typically 57 examined partial genome sequences, such as the viral envelope gene, or a limited number of 58 complete genome sequences. With the advent of high-throughput sequencing technologies, genome-wide surveys progressively became the standard approach [START_REF] Parameswaran | This comprehensive analysis of within-host dengue virus genetic diversity in human serum 293 samples pointed out correlations with clinical, epidemiological and virological 294 features[END_REF][START_REF] Thai | High-resolution analysis of intrahost genetic 298 diversity in dengue virus serotype 1 infection identifies mixed infections[END_REF]18,[START_REF] Romano | Inter-and intra-host viral diversity in a large 323 seasonal DENV2 outbreak[END_REF], with some 60 exceptions [START_REF] Choudhury | Nature and 325 Extent of Genetic Diversity of Dengue Viruses Determined by 454 326 Pyrosequencing[END_REF]. The primary aim of these studies was to estimate the extent of within-host 61 DENV genetic diversity, and possibly correlate it with relevant clinical, epidemiological or 62 phylogenetic features. For example, a comprehensive study of within-host DENV-2 genetic 63 diversity in Nicaragua revealed non-uniform patterns across viral genes, variation across 64 phylogenetic lineages, and some degree of relationship with between-host genetic diversity 65 [START_REF] Parameswaran | This comprehensive analysis of within-host dengue virus genetic diversity in human serum 293 samples pointed out correlations with clinical, epidemiological and virological 294 features[END_REF]. One unresolved question is whether dengue disease severity in humans is associated 66 with the level of within-host DENV genetic diversity, as was reported in some instances [START_REF] Descloux | Dengue 1 diversity and 286 microevolution, French Polynesia 2001-2006: connection with epidemiology and 287 clinics[END_REF] 67 but not others [START_REF] Parameswaran | This comprehensive analysis of within-host dengue virus genetic diversity in human serum 293 samples pointed out correlations with clinical, epidemiological and virological 294 features[END_REF][START_REF] Thai | High-resolution analysis of intrahost genetic 298 diversity in dengue virus serotype 1 infection identifies mixed infections[END_REF]. 68 69 Only during the last two years did studies begin to monitor the evolutionary dynamics of 70 DENV populations at the within-host level [START_REF] Sim | during a Dengue Virus Type 3 Cuban Epidemic: Deep Sequencing of Evolving 320 Viral Populations[END_REF]18,[START_REF] Lequime | This study identified the main evolutionary forces at play during mosquito infection by 331 dengue viruses, and uncovered the influence of host genotype on the amount of 332 within-host viral genetic diversity[END_REF][START_REF] Plummer | Dengue Virus Evolution under a Host-336 Targeted Antiviral[END_REF][START_REF] Sessions | Analysis of Dengue Virus Genetic Diversity during Human 339 and Mosquito Infection Reveals Genetic Constraints[END_REF]. The abundant genetic diversity of 71 RNA virus populations, coupled with short generation times, fuels their rapid evolution over 72 the course of a single infection. Combining deep sequencing of within-host viral populations 73 (Figure 1) with computational and statistical methods allows empirical analyses of their 74 evolutionary dynamics [START_REF] Nelson | Within-host nucleotide diversity of virus populations: 342 insights from next-generation sequencing[END_REF]. Viral evolutionary analyses rely on the observed patterns of 75 genetic diversity to infer the underlying evolutionary processes [START_REF] Pybus | Evolutionary analysis of the dynamics of viral infectious 344 disease[END_REF]. Evolutionary forces 76 acting on within-host viral populations are not fundamentally different from those acting on 77 any population. They include genetic mechanisms (i.e., mutation and recombination), 78 demographic processes (i.e., genetic drift) and natural selection. 79 80 Here, we review our current knowledge on the within-host evolutionary dynamics of DENV, 81 focusing on the infection of mosquito vectors because they represent the bulk of published 82 studies to date. We illustrate how the analysis of within-host evolutionary dynamics can 83 provide important insights into DENV-vector interactions and DENV epidemiology. and Aedes albopictus mosquitoes ten days after intrathoracic inoculation with the same human 103 serum samples [START_REF] Sessions | Analysis of Dengue Virus Genetic Diversity during Human 339 and Mosquito Infection Reveals Genetic Constraints[END_REF]. They detected mutation hotspots and coldspots in the viral genome, some 104 of which were independent of the host type. In addition, there was evidence for frequent 105 reversion of low-frequency variants during human-to-mosquito transfer, and some degree of 106 mosquito species-specific evolution. Sim et al. also tracked changes in within-host DENV-1 107 genetic diversity during human-to-mosquito transmission, but using a more natural setting 108 [START_REF] Sim | during a Dengue Virus Type 3 Cuban Epidemic: Deep Sequencing of Evolving 320 Viral Populations[END_REF]. Twelve viremic dengue patients from Vietnam were experimentally exposed to naïve Ae. 109 aegypti mosquitoes, and three blood-fed mosquitoes per patient were analyzed after two 110 weeks of extrinsic incubation. Comparison of within-host DENV genetic diversity in matched 111 human and mosquito samples revealed substantial changes in the variant repertoire between 112 venous blood and the organs of mosquitoes that became infected by feeding on the 113 corresponding person. The majority of genetic variants were lost upon transition from venous 114 blood to mosquito abdomen, as well as from mosquito abdomen to salivary glands. 115 Nevertheless, several measures of genetic diversity per sample were similar between human 116 plasma, mosquito abdomens and mosquito salivary glands, indicating that the overall level of 117 within-host genetic diversity was maintained. 118 119 A study that our research group recently conducted investigated the evolutionary forces 120 underlying within-host DENV evolution in different genotypes of their mosquito vector Ae. 121 aegypti [START_REF] Lequime | This study identified the main evolutionary forces at play during mosquito infection by 331 dengue viruses, and uncovered the influence of host genotype on the amount of 332 within-host viral genetic diversity[END_REF]. We used deep sequencing to monitor the genetic diversity of full-length viral 122 genomes during mosquito infection by a wild-type DENV-1 isolate (Figure 2). Based on 123 neutral or quasi-neutral genetic markers to rule out a confounding effect of natural selection, 124 we detected a stochastic reduction in population size at initial midgut infection. We estimated 125 that infection was randomly initiated by tens of viral genomes, two orders of magnitude less 126 than the infectious dose of virus ingested by an individual mosquito in our experiment. The observed level of genetic diversity increased substantially during population expansion in the 128 midgut without evidence for positive selection. Importantly, the observed amount of viral 129 genetic diversity differed among mosquito genetic backgrounds, despite a similar census 130 population size. Although additional investigations are necessary to determine their exact 131 nature, this study thus pointed to a role for host genetic factors in the genetic breadth of 132 within-host DENV genetic diversity. This result extends to the host-genotype level the recent 133 finding that within-host West Nile virus genetic diversity is host-species dependent [START_REF] Grubaugh | Genetic Drift during Systemic Arbovirus Infection of 347 Mosquito Vectors Leads to Decreased Relative Fitness during Host Switching[END_REF]. 134 135 136

Inferring DENV-vector interactions from within-host viral evolutionary dynamics 137 138

The above studies illustrate how the evolutionary dynamics of within-host viral populations 139 can provide insights into DENV-vector interactions. Fluctuations in viral population size 140 caused by anatomical barriers, for instance, can be inferred by comparing the genetic variance 141 before and after traversal of the barrier [START_REF] Thebaud | Large Bottleneck Size in Cauliflower Mosaic 350 Virus Populations during Host Plant Colonization[END_REF][START_REF] Monsion | Large bottleneck size in Cauliflower 353 Mosaic Virus populations during host plant colonization[END_REF]. Such methods use neutral or quasi-neutral 142 genetic variants to estimate the effective population size (N e ), which is the size of an idealized 143 population that would drift at the same rate as the observed population. The relatively tight 144 population bottleneck observed at initial infection of the mosquito midgut [START_REF] Lequime | This study identified the main evolutionary forces at play during mosquito infection by 331 dengue viruses, and uncovered the influence of host genotype on the amount of 332 within-host viral genetic diversity[END_REF] indicates that 145 DENV likely establish infection in a small number of cells, as was observed for other 146 arboviruses [START_REF] Smith | Venezuelan equine 356 encephalitis virus in the mosquito vector Aedes taeniorhynchus: infection 357 initiated by a small number of susceptible epithelial cells and a population 358 bottleneck[END_REF]. One caveat of methods based on N e estimation is that the neutrality 147 assumption, even for synonymous substitutions, is difficult to verify for RNA viruses [START_REF] Cuevas | The fitness effects of synonymous mutations 360 in DNA and RNA viruses[END_REF]. 148

Nevertheless, these methods allow demographic analyses of wild-type viral populations that 149 would otherwise require artificial variants mixtures [START_REF] Ciota | 362 Quantification of intrahost bottlenecks of West Nile virus in Culex pipiens 363 mosquitoes using an artificial mutant swarm[END_REF], marked viral clones [START_REF] Forrester | Vector-borne 365 transmission imposes a severe bottleneck on an RNA virus population[END_REF], or viral 150 replicons expressing a reporter protein [START_REF] Gutierrez | Demographics of natural 368 oral infection of mosquitos by Venezuelan equine encephalitis virus[END_REF]. coding regions, it cannot detect certain forms of natural selection [START_REF] Yang | Statistical methods for detecting molecular adaptation[END_REF] and has several 160 limitations at the within-population level [START_REF] Kryazhimskiy | The population genetics of dN/dS[END_REF]. However, calculating the d N /d S ratio can be useful to compare the selection regime in different host compartments or different host types. 162

The within-host environment is typically heterogeneous (i.e., different cell types) and 163 spatially structured (i.e., organs), which can lead to the emergence of fitness trade-offs [START_REF] Alizon | Epidemiological and clinical consequences of within-375 host evolution[END_REF]. 164 Such within-host trade-offs remain to be been explored in the case of DENV. 165 166 Host immunity is expected to represent a major selective pressure on viral populations, and 167 mosquito hosts are no exception. Antiviral immunity in insects consists of several arms that 168 include RNA interference (RNAi), apoptosis, the ubiquitin proteasome pathway, autophagy 169 and the heat-shock response [START_REF] Marques | The diversity of insect antiviral immunity: insights from 377 viruses[END_REF]. RNAi is often described as the major antiviral immune 170 response of mosquitoes against arboviral infections [START_REF] Blair | Mosquito immune responses to arbovirus infections[END_REF]. Because RNAi results in viral RNA 171 degradation in a sequence-specific manner, there is potential for within-host co-evolutionary 172 dynamics between viral populations and the RNAi response. Under this hypothetical scenario, 173 rare viral variants are selectively advantaged if their sequence is sufficiently divergent to 174 escape the RNAi response targeted at the dominant viral sequence. Consistent with this 175 hypothesis, a study of West Nile virus infection in Culex mosquitoes showed that intense 176 RNAi targeting of the viral genome was associated with positive selection [START_REF] Brackney | RNAi targeting of West Nile virus in mosquito 381 midguts promotes virus diversification[END_REF]. As the escape 177 variants rise in frequency, they are increasingly targeted by the RNAi response, allowing 178 other rare variants to be selected. This mechanism of negative frequency-dependent selection, 179 whereby allele fitness decreases as a function of the relative allele frequency in the 180 population, is an example of balancing selection (that cannot be detected with the d N /d S ratio). 181 182

The shape of the viral phylogeny at the within-host level may also provide indications on 183 within-host processes. It has been suggested that different within-host processes may result in 184 a different phylogeny shape (i.e., different tree topology and distribution of branch lengths), 185 so that these mechanisms could be inferred from time series of within-host viral genetic 186 diversity [START_REF] Alizon | Epidemiological and clinical consequences of within-375 host evolution[END_REF]. For example, resource-mediated and immune-mediated competition are 187 expected to result in different trajectories of within-host evolution [START_REF] Alizon | Epidemiological and clinical consequences of within-375 host evolution[END_REF]. Thus, analysis of the 188 shape of viral phylogenies at the within-host level could help to elucidate the mechanisms of 189 DENV-vector interactions. One technical difficulty to overcome is haplotype phasing (i.e., 190 resolving the combination of nucleotide variants carried by one viral genome) using short-191 read sequence data, for which existing programs perform poorly when sequence divergence is 192 low [START_REF] Schirmer | Benchmarking of viral haplotype reconstruction 383 programmes: an overview of the capacities and limitations of currently available[END_REF]. 193 194

Connecting within-host and between-host DENV evolutionary dynamics 196 197

Dengue epidemiology (i.e., at the between-host level) can also benefit from a better 198 understanding of within-host evolutionary dynamics because within-host and between-host 199 levels are connected. Perhaps the most eloquent example of this connection is the long-term 200 transmission of defective DENV genomes in nature. A DENV-1 lineage that acquired a stop-201 codon mutation in the envelope gene was detected in both humans and mosquitoes over a 202 period of 18 months in Myanmar [START_REF] Aaskov | This landmark study combined assessment of within-host dengue virus genetic diversity in 304 humans and mosquitoes and phylogenetic analyses to demonstrate sustained 305 transmission of a viral lineage associated with a truncated genome[END_REF]. Such defective genomes, in which large sections have 203 been deleted, have a replication advantage over the full-length parental genomes but they are 204 only replicated by complementation in the presence of co-infecting functional viruses [START_REF] Li | Defective interfering viral 386 particles in acute dengue infections[END_REF]. 205

Defective interfering particles compete with functional viruses [START_REF] Huang | Defective viral particles and viral disease processes[END_REF] but may also contribute to 206 infection persistence [START_REF] Holland | Persistent noncytocidal vesicular stomatitis virus infections 390 mediated by defective T particles that suppress virion transcriptase[END_REF]. Maintenance of a defective DENV lineage over multiple 207 transmission cycles between humans and mosquitoes implies that population bottlenecks are 208 not severe enough to prevent complementation by a functional DENV lineage, consistent with 209 the estimated bottleneck size of tens of DENV genomes at initial mosquito infection [START_REF] Lequime | This study identified the main evolutionary forces at play during mosquito infection by 331 dengue viruses, and uncovered the influence of host genotype on the amount of 332 within-host viral genetic diversity[END_REF]. A 210 modeling study accounting for both within-host and between-host dynamics showed that co-211 transmission of defective and functional DENV can potentiate epidemic spread [START_REF] Ke | This study combined dynamical modeling and phylogenetic analyses to highlight the link 396 between within-host dynamics and epidemiological patterns in the case[END_REF]. Thus, 212 within-host interactions between viral variants (i.e., complementation) may provide a pathway 213 to viral emergence. 214 215 On a longer time scale, accounting for within-host evolutionary dynamics may help to predict 216 the between-host viral evolutionary rate [START_REF] Scholle | Viral substitution rate variation can arise 400 from the interplay between within-host and epidemiological dynamics[END_REF][START_REF] Peck | Connecting within-host dynamics to the rate of viral 403 molecular evolution[END_REF]. In the case of arboviruses, it was observed 217 that their evolutionary rate is on average slower than that of directly-transmitted RNA viruses 218 [START_REF] Jenkins | Rates of molecular evolution in RNA 405 viruses: a quantitative phylogenetic analysis[END_REF]. The fitness trade-off hypothesis postulates that arbovirus alternation between disparate 219 host types (i.e., vertebrate and invertebrate) constraints adaptation and slows down evolution, 220 but this hypothesis has only received partial support from empirical studies [START_REF] Coffey | Factors shaping the 407 adaptive landscape for arboviruses: implications for the emergence of disease[END_REF]. Another 221 important feature of the arbovirus life cycle are the recurrent population bottlenecks that occur 222 during transmission [START_REF] Forrester | Arboviral bottlenecks and challenges to 410 maintaining diversity and fitness during mosquito transmission[END_REF].

  knowledge, only five studies published to date monitored DENV genetic diversity at 89 the within-host level based on sequential observations of the same viral population. 90 Rodriguez-Roche et al. examined patterns of within-host genetic diversity in human serum 91 samples collected during a DENV-3 epidemic in Cuba [18]. Although this observation was 92 based on a single dengue patient, they detected significant changes in the genetic composition of the within-host viral population sampled two and four days after fever onset. Plummer et al. 94 monitored the evolution of DENV-2 genetic diversity during several passages in a mouse 95 model to evaluate the effect of an antiviral molecule [22]. The antiviral treatment resulted in a 96 higher number of viral genetic variants and signals of positive selection. In addition, viral 97 populations passaged in both the antiviral-treated mice and the control mice differed from the 98 input virus, indicating that evolution also took place independently of the antiviral treatment. 99 100 Sessions et al. examined DENV-1 genetic diversity in twelve paired human sera collected 101 early and late (i.e., four days apart) during the course of the infection, and in both Ae. aegypti 102

  151 152 Measures of natural selection can also shed light on the processes at play during DENV-153 vector interactions. The strength and direction of natural selection are commonly assessed by 154 computing the ratio of non-synonymous to synonymous changes (d N /d S ). A d N /d S ratio of one 155 means that non-synonymous substitutions occur at the same rate as synonymous substitutions 156 and is interpreted as evidence for neutral evolution. A d N /d S ratio greater than one implies 157 positive selection (i.e., driving change), whereas a d N /d S ratio smaller than one indicates 158 purifying selection (i.e., opposing change). Analysis of the d N /d S ratio can only be applied to 159

  These bottlenecks may act to slow down arbovirus evolution 223 independently of host alternation. Presumably, rare adaptive mutations are lost by genetic 224 drift, whereas more frequent deleterious mutations are efficiently removed by purifying 225 selection during phases of within-host population expansion. The combined action of 226 recurrent population bottlenecks and within-host purifying selection may thus promote 227 evolutionary conservation of arboviral genomes. 228 Conclusions 231 232Only a handful of studies investigated the dynamics of within-host DENV genetic diversity to 233 date. Most of these studies used a descriptive approach to correlate the extent of within-host 234 viral genetic diversity with epidemiologically relevant features. This is a time of great 235 opportunities to carry out longitudinal studies of within-host DENV evolutionary dynamics 236 that will go beyond the mere description of genetic diversity. By combining high-throughput 237 sequencing data with computational and statistical methods, within-host evolutionary analysis 238 of DENV has the potential to illuminate within-host processes and provide important insights 239 into epidemiological dynamics. Another critical aspect for future research will be to move 240 from the analysis of patterns of DENV genetic diversity to the analysis of phenotypic traits 241 underlying DENV fitness (e.g., drug resistance, replication rate, extrinsic incubation period). 242This will require the alliance of robust within-host viral evolutionary analysis and carefully 243 designed experiments of viral reverse genetics.

Figure legends 425 426 Figure 1 :

 4261 Figure legends 425 426 Figure 1: Deep-sequencing analysis of within-host viral populations. (1) RNA is extracted 427 from the sample, purified and reverse transcribed into complementary DNA (cDNA) using 428 random hexamers or a specific reverse primer located at 3' end of the viral genome. Reverse 429

Santana E, Alvarez M, Castro O, Fontes M, et al.: Increasing Clinical Severity

PLoS Pathog 2016, 12:e1005512.

programmes. Brief Bioinform 2014, 15:431-442.

Acknowledgements


Abdoul Aziz Mbacke
email: aziz.mbacke@inria.fr

Nathalie Mitton
email: nathalie.mitton@inria.fr

Herve Rivano
email: herve.rivano@inria.fr

RFID anticollision in dense mobile environments

Keywords: RFID, reader anticollision problem, MAC layer, resource allocation, distributed systems, mobile systems

The popularization of RFID systems has conducted to large deployments of RFID solutions in various areas under different criteria. However, such deployments, specially in dense environments, can be subject to RFID collisions which in turn affect the quality of readings. In this paper we propose two distributed and efficient solutions for dense mobile deployments of RFID systems. mDEFAR is an adaptation of a previous work highly performing in terms of collisions reduction, efficiency and fairness in dense static deployments. CORA is more of a locally mutual solution where each reader relies on its neighborhood to enable itself or not. Using a beaconing mechanism, each reader is able to identify potential (non-)colliding neighbors in a running frame and as such chooses to read or not. Performance evaluation shows high performance in terms of coverage delay for both proposals quickly achieving 100% coverage depending on the considered use case while always maintaining consistent efficiency levels above 70%. Compared to GDRA, our solutions proved to be better suited for highly dense and mobile environments, offering both higher throughput and efficiency. The results reveal that depending on the application considered, choosing either mDEFAR or CORA helps improve efficiency and coverage delay.

I. INTRODUCTION

The identification of goods has, for the past decades, been done thanks to the use of bar codes. In such systems, laser reader would have to be in direct line of sight with a given bar code to be identified. Having large warehouses where several products can be stored has made such a solution quite obsolete and even not appropriate. As such, RFID systems come to the rescue allowing for contactless and without direct line of sight identification of goods, using a technique known as back-scattering where the reader emits a radio signal towards a tag which in turn is activated/enabled and answers sending its ID. The use of radio signals and the size of tags, made it possible to have them embedded in different goods and still be identified even when disposed in lots. Such possibilities allowed new cases to come up where readers could be deployed and mobile in the system for a quicker coverage of all tags. In addition, mobile tags deployed with their successive positions being tracked by deployed readers. For instance, the case of factories could be explored. Using RFID technology in factories could allow substantial improvements regarding productivity, traceability, security. Using RFID tags deployed on each product could permit to follow all the movements of the products inside the covered area, being able to register incoming and outgoing products and somehow improve the quality of the delivery system for all goods. Having a solution that would allow the instantaneous identification of the entire content in a minimum delay would improve the traceability and reduce the burden of identifying them one by one enhancing the agility, security and productivity of the system. However, in order to establish such solutions, a very dense deployment of readers and tags need to be made. This alone creates a very collision vulnerable environment [START_REF]EPC TM radio-frequency identity protocols class-1 generation-2 UHF RFID protocol for communications at 860 MHz -960 MHz version 1.2.0[END_REF], but in order to further improve the system and to follow the needs of present setups, mobile readers and/or tags also need to be deployed. The addition of these latter deteriorates even more the system and thus delay.

We chose to study the design of anticollision algorithms for RFID readers taking dense deployments and mobility into account. Our proposal aims at building efficient RFID systems to reduce collisions and coverage delay while improving the throughput (in terms of tag identification).

The literature review shows different proposals [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for RFID dense reader environments[END_REF]- [START_REF] Joshi | A reader anti-collision MAC protocol for dense reader RFID system[END_REF] all highly performing in their considered environments but not acknowledging high density and/or mobility deployments. The few ones considering these characteristics are centralized approaches depending on the use of a central server for scheduling the readers [START_REF] Xia | Relative density based anti-collision algorithm in RFID networks with dense readers[END_REF]. This further increases the cost of the system and with the mobility taken into account, adds another needed layer of communication.

In this paper we introduce two new solutions for RFID anticollision. Both approaches are : -distributed and local: loose TDMA approach based on readers internal clocks handling clock drifts and relying on the information in their vicinity ; -efficient: in terms of throughput in spite of collisions ; -mobile-ready: algorithms are reliable to mobile scenarios thanks to their local and memory-less approach. The first proposal, mobile-Distributed Efficient and Fair Anticollision for RFID (mDEFAR) is an adaptation of a previous work to better comply with dense mobile environments by reducing the interference range considered by rendering it monochannel. The second proposal, Coverage Oriented RFID Anticollision (CORA) is a simple approach where readers accord each other locally. Some collisions are accepted in order to improve the throughput and coverage delay of the system. The concession made in terms of collisions makes CORA more suitable for less energy-constrained applications aiming at a quick coverage of deployed tags.

The rest of this paper is organized as follows, Section II reviews the problem statement and highlights our motiva-tion. Section III explores some state-of-the-art anticollision protocols for RFID and features their drawbacks regarding the tackled issues. Section IV details mDEFAR and CORA algorithms. Their performances relative to different metrics as well as the use cases are considered are evaluated and confronted in Section V. Finally, Section VI concludes by discussing future research directions.

II. PROBLEM STATEMENT

A. Dense environments

In order to increase the coverage area or improve the total reading delay in RFID systems, one solution was to increase the number of deployed readers. This reader densification within a given area was unfortunately not done without compromise. Indeed, the crowding of RFID readers results in the growth of the number of collisions. These latter ones can happen at different levels. For instance, when a reader tries to access multiple tags at the same time, all the interrogated tags answer at the same time resulting in a collision and the tags not being read. This type of collisions is referred to as a tag collision in the literature. They have been thoroughly studied and solved thanks to ALOHA [START_REF] Khandelwal | ASAP: a MAC protocol for dense and time-constrained RFID systems[END_REF], tree [START_REF] Myung | Adaptive binary splitting for efficient RFID tag anti-collision[END_REF], and frame-andtree [START_REF] Eom | An efficient framedslotted ALOHA algorithm with pilot frame and binary selection for anti-collision of RFID tags[END_REF] based protocols.

Another instance of collisions is when multiple readers try to read a given tag at the same time. The tag, unable to dissociate signals received from multiple sources, will just consider the multiple queries as radio noise and fails to answer to any of the readers. In Fig. 2, only tags T1 and T3 will be identified by readers R1 and R2 while T2 is in the colliding area and fails to be read. In order to avoid such an issue, R1 and R2 should either operate at different times or with a distance of at least d = 2 × d CRT with d CRT being the reading range. Another solution would be to have readers operate at different frequencies [START_REF]Electromagnetic compatibility and Radio spectrum Matters ERM Radio Frequency Identification Equipment operating in the band 865MHz[END_REF]. However in a very dense network, the distribution of available frequencies can be quite laborious and their number not sufficient. Thus having an efficient dynamic RFID reader anticollision algorithm becomes crucial to improve tag identification. Still in the aim to increase the coverage area to fulfill the needs of new applications, readers can be mounted on ambulatory devices for mobility. Mobile readers allow a better tracking of products inside a warehouse. Deploying a mix of static and mobile readers to follow the movement of tags within a large coverage area can also be considered. However, as for the drawbacks brought by dense environments, the mobility of readers also induces new collision issues.

B. Dedicated control channel

For the sake of contention resolution, a dedicated control channel between readers is needed. Indeed, in order for readers to exchange information regarding their reading behaviors a communication channel must be set between them. The hypothesis of a dedicated control channel between readers can be seen throughout the literature [3]- [START_REF] Yu | GENTLE: Reducing reader collision in mobile RFID networks[END_REF]. Some authors also considered this communication medium to be set between the readers and a central server [START_REF] Xia | Relative density based anti-collision algorithm in RFID networks with dense readers[END_REF] others even contemplated the idea of a link between readers and a central server as well as a link between readers themselves [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for RFID dense reader environments[END_REF], [START_REF] Ferrero | A fair and high throughput reader-to-reader anticollision protocol in dense RFID networks[END_REF].

In both our proposals, the hypothesis is made to have a single dedicated wireless communication channel between readers to allow exchange of beacons for contention resolution. We chose to set the range of the communication channel to d COM = d CRT . This value grants readers a knowledge of their vicinity potential colliding neighbors.

III. RELATED WORK

Many proposals of the literature have addressed reader collisions. They can mainly be classified in two categories: TDMA and CSMA-based. While the former relies on a prescheduled access of the readers to tags, the latter commits to listening the medium before attempting to read tags.

In TDMA-based DCS (Distributed Color Selection) [START_REF] Waldrop | Colorwave: An anticollision algorithm for the reader collision problem[END_REF], the readers randomly chose a slot which they use to access tags. In the event of a collision (supposing collisions can be detected), they randomly chose new slots within the maximum number of available slots. In a dense environment, this solution quickly proves to be unfitted since readers may get stuck in a new slot reservation procedure loop since the slots number might be insufficient. Willing to overcome this issue, authors then proposed VDCS (Variable DCS) [START_REF] Waldrop | Colorwave: an anticollision algorithm for the reader collision problem[END_REF] where the number of slots is not fix for the whole system but variable among readers. Collisions are then computed and matched to given upper and lower bounds. In case one of the bounds is reached, readers accordingly increase or decrease their maximum number of slots, chose a new one in the recently defined maximum slots number and broadcast it to their neighbors. This solution proved to resolve partially density issue but still fails to comply the constraints of mobility.

Another considered TDMA-based proposal is GDRA (Getric Distribution Reader Anticollision protocol) [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for RFID dense reader environments[END_REF] where authors assume the presence of a central server (CS) able to communicate with all the readers at the same time. The CS broadcasts an Arrangement Command (AC) to all readers letting them know the number of slots available. Readers then randomly chose a slot and a channel according to a given geometric distribution [START_REF] Jamieson | Sift: A MAC protocol for event-driven wireless sensor networks[END_REF]. The readers are expected to dispose of bistatic antennas allowing them to send a beacon announcing their chosen slot and channel to neighbors while receiving theirs. In case of readers sharing the same slot, they get disabled for the current round otherwise, if a reader finds no other peer with the same slot and channel as its own, it sends an Overriding Frame (OF). The OF disables all neighboring readers with no regards to chosen slot or channel for the current frame. Disabling neighbors as the OF does, induces throughput drops and uncovered tags.

Regarding CSMA proposals, we can consider Pulse. In [START_REF] Birari | Pulse : A mac protocol for rfid networks[END_REF], readers willing to access tags listen to the medium for a given period of time and if found idle, continuously broadcast a signal on a dedicated control channel to let the neighboring devices know that the medium is occupied. This proposal however does not fit much dense environments since several readers may get disabled for an undefined period of time to the benefit of a single one, thus reducing the throughput and fairness of the whole system. Also it is unqualified for mobile environments since enabled moving readers may collide.

In light of this brief review, it becomes clear that density and mobility should be taken into account in the design of an efficiently performing solution.

IV. PROPOSALS

In regards of the considerations exposed in Section III, we designed two proposals fitting a dense and mobile RFID system. Each proposal performs differently depending on the application constraints.

A. mobile-Distributed Efficient and Fair Anticollision for RFID (mDEFAR)

mDEFAR relies on our previous work DEFAR [START_REF] Mbacke | Distributed efficient & fair anticollision for RFID protocol[END_REF] algorithm. In order to well identify the contribution of mDEFAR compared to DEFAR, we give below a brief overview.

1) DEFAR: DEFAR is a multi-channel algorithm, unlike mono-channel CORA and mDEFAR. In DEFAR, readers randomly chose a communication token defined by a reading channel and slot. After observing a backoff, readers broadcast a beacon with their tokens on a dedicated control channel (different from the reading channel). This dedicated control channel operates on a separate antenna and frequency band, as such, it cannot interfere with reading operations. According to the received tokens, each reader then decides to read or not and switches its priority for medium access accordingly for the next round. Three (3) different priorities are identified: NEUTRAL, as the priority of all readers at start, LAZY for readers that successfully accessed to the medium and PUMPED UP for readers that failed to access the medium and raise their priority for the following round. Due to the multichannel [START_REF]Electromagnetic compatibility and Radio spectrum Matters ERM Radio Frequency Identification Equipment operating in the band 865MHz[END_REF] feature, the communication range is set to [START_REF] Amadou | HAMAC: High adaptive MAC protocol for dense RFID reader-to-reader networks[END_REF] with d COM the communication range between readers, d AC the adjacent channel interference range and d CRT the reader-tag interrogation range. While multiple channels allow less collisions since it is possible to have up to N = max slots × max channels tokens, it also disables more readers since the communication range has to account for the adjacent channel interference.

d COM = 2 × d AC = 2 × 3, 3 × d CRT
2) mobile-Distributed Efficient and Fair Anticollision for RFID (mDEFAR): mDEFAR is a distributed TDMA-based RFID anticollision protocol. The larger the communication range, the more disabled readers. As such, we chose to set d COM = d CRT by making the proposal monochannel in order to enable more readers at the same time for dense environments. This approach allows a better throughput and coverage delay albeit the single channel parameter induces more collisions since readers shift N = max slots × max channels available tokens to just N = max slots tokens.

Each slot has a duration of T slot . This duration is split into two parts, T beacon [START_REF]Electromagnetic compatibility and Radio spectrum Matters ERM Radio Frequency Identification Equipment operating in the band 865MHz[END_REF] used by readers to send beacons at the start of their slot and T CRT [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for RFID dense reader environments[END_REF], [START_REF] Amadou | HAMAC: High adaptive MAC protocol for dense RFID reader-to-reader networks[END_REF] used to access and read tags. A frame can then be defined as : T f rame = max slots * T slot with : T slot = T beacon + T CRT . To better understand the concept of mDEFAR, let's consider the graph depicted on Fig. 1a where the readers are the vertices and the contention between them are the edges. "N" states for NEUTRAL priority and the values in brackets represent the chosen slot. At bootstrap, readers R2, R4, R3 and R8 will collide as well as R6 and R7 and resolve their contention (Algo. 1-l6). All other potential collisions are resolved by the different slots selection (Algo. 1-l4). R2, R3 and R6 will win the contention since they hold the smallest IDs and will read tags (Algo. 1-l7) while R4, R7 and R8 will switch to a PUMPED UP priority "P" (Algo. 1-l9) and all other readers being LAZY "L" for the next frame (Algo. 1-l5-8). In the second frame (Fig. 1b), we see that R2 and R4 have chosen the same slot, as such R2 and R4 will collide (Algo. 2-l5). Since R4 did not access the medium in the previous frame, it has a PUMPED UP priority and will win the contention round (Algo. 2-l13-15). While R1 and R8 collide as well on slot 1, R1 loses the contention due to its lower priority (Algo. 2,l68). Regarding R5, R6 and R7 choosing the same slot, on one side R5 and R6 being on the same LAZY level (Algo. 2-l9) will compare their iD and R5 accesses tags (Algo. 2-l910). R6 being LAZY facing R7 in PUMPED UP loses the round. Yet, by running mDEFAR, all readers will eventually access tags being ensured to be the only one during its slot, guarantying no collision and all tags successfully read.

R1-N[2] R8-N[3] R2-N[3] R3-N[3] R4-N[3] R5-N[2] R6-N[3] R7-N[3] (a) 1 st frame R1-L[1] R8-P[1] R2-L[3] R3-L[4] R4-P[3] R5-L[2] R6-L[2]
R7-P [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for RFID dense reader environments[END_REF] (b) 2 nd frame Fig. 1: mDEFAR topology example

B. Coverage Oriented RFID Anticollision (CORA)

CORA is a monochannel TDMA-based proposal. For a while, RFID collisions have been considered as impactful as WSN collisions [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for RFID dense reader environments[END_REF]- [START_REF] Amadou | HAMAC: High adaptive MAC protocol for dense RFID reader-to-reader networks[END_REF]. As such, it was considered that colliding readers would not access any tag and collisions were just avoided at any cost, forgetting that tags that were not in the collision area would actually be successfully read. In Fig. 2a, R1 and R2 can resp. read tags T1 and T2 while they share a collision area over T2. In the event of R1 and R2 accessing the medium at the same time, both tags T1 and T2 will successfully be read while T2 being in the collision area will not, T2 will still be read afterwards by R3 (see Fig. 2b). Thus, instead of having the three readers at three different times we can have a total coverage within just 2 timeslots improving the coverage delay. The process of CORA is willingly kept simple as a monochannel and TDMA-based algorithm. The communication with tags is organized in frames themselves subdivided in slots. Every reader randomly chooses a slot within a value max slot. The frame is designed such as the frame as a beacon phase first during which every reader broadcasts its randomly chosen slot to its vicinity and receives other's beacons, second is a tag interrogation phase. The beaconing phase is organized with a backoff scheme to prevent beacon collisions at this level. Upon reception of all beacons in its vicinity (Algo. 3-l4,5), a reader then makes a decision according to the number of contenders that chose the same slot as its own (Algo. 3-l6-7) and the different ones (Algo. 3-l8-9). Each reader computes a number M = slot same -slot dif f erent .

-if M > 0 (Algo. 3-l13-14), reader considers there are too many neighbors on the same slot as its own and gets disabled. The potential size of the colliding area between all the contending readers in its vicinity involved makes it inefficient to read. In the example of Fig. 3, after beacon exchange, reader R5 will not access tags since it collides with both R4 and R6. Tags covered by R5 will be read on the following round; -if M ≤ 0 (Algo. 3-l15-16), reader accesses the medium even if it might collide with some of its neighbors considering that the uncovered tags due to collisions will be read by the neighboring readers on different slots within the same frame. As such, in Fig. 3, all readers except R5 access tags. Regarding tags laying between R2, R3 and R8, they will not be read in the current round but in the following round. end if 18: end if Contrarily to mDEFAR, readers here need to identify all their contending neighbors and their slots before the contention. The beaconing process is thus made prior to any reading phase. A frame has a length of:

T f rame = T beacon + (max slot × T CRT ). R1-[2] R8-[3] R2-[3] R3-[3] R4-[1] R5-[1] R6-[1]
R7- [START_REF] Birari | Pulse : A mac protocol for rfid networks[END_REF] Fig. 3: CORA topology example

V. PERFORMANCE EVALUATION

In order to evaluate our proposals, two use cases have been considered, both include density and mobility factors to comply with the issues of Section II. They are compared to GDRA which remains the best performing algorithm for RFID anticollision in the literature. Also, as a centralized and TDMA approach, it seemed to be the best suited to validate our distributed approaches.

A. Scenarios 1) Warehouse scenario: This scenario features a warehouse with 6000 tags deployed on shelves over 200 × 205m 2 . The shelves are considered to be aligned in rows spaced by 7meter-wide aisles. 10 readers are deployed on each aisle, moving at a speed of 0,7m/s linearly within the aisles and uturn once they reach an edge. Such a scenario challenges our proposals in a dense deployment with dynamic parameters.

2) Urban scenario: This scenario features an urban representation with 800 tags used to identify moving vehicles at an average speed of 10m/s. Static readers are placed on each building block corner. Mobile readers are mounted on bicycles riding at an average speed of 4m/s. This model deals with the coexistence of both mobile and static readers with sporadically dense environments when mobile readers reach an intersection and are in contention with at least 4 of the corner readers.

B. Performance results

We assessed the performance of our proposals using WS-Net [START_REF]WSNet -an event-driven simulator for large scale wireless sensor networks[END_REF], an event-driven simulator for large scale wireless networks. For each scenario, a random topology in respect to the models presented in Section V-A was generated. Each of our proposals was simulated 100 times on every model for 400s. According to the maximum transmission power defined in [START_REF]Electromagnetic compatibility and Radio spectrum Matters ERM Radio Frequency Identification Equipment operating in the band 865MHz[END_REF], we set the reader interrogation range d CRT = 10m. The value of max slot is set to 4 for a good compromise between high throughput and latency. T beacon and T CRT are resp. set to 5ms and 460ms according to [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for RFID dense reader environments[END_REF], [START_REF]Electromagnetic compatibility and Radio spectrum Matters ERM Radio Frequency Identification Equipment operating in the band 865MHz[END_REF], [START_REF] Amadou | How to improve csma-based MAC protocol for dense RFID reader-to-reader networks?[END_REF]. The performance metrics used are defined in [START_REF] Gandino | Evaluation criteria for reader-to-reader anti-collision protocols[END_REF].

1) Throughput: The throughput is computed as the average number of successful query sections over the simulation length. A better throughput means readers access tags more frequently. This can be useful to qualify tracking or security applications. The figs. 4a and 4b compare the throughput of mDEFAR, CORA and GDRA in both scenarios. In both applications, our proposals achieve better results than GDRA. Regarding the warehouse scenario, CORA has the best results since it allows collisions to happen in order to increase throughput. GDRA performs poorly because of the high density of readers deployed, the contention resolution techniques disable several readers. The same observations can be made regarding the urban scenario. The difference of throughput between CORA and mDEFAR is more noticeable since CORA offers a better contention resolution for static readers placed on corners where only one may be enabled in the case of mDEFAR against up to all 4 for CORA.

2) Collisions: Collisions are registered when multiple readers chose the same beaconing slot and when different readers in the same vicinity failed to choose different slots resulting in the case of mDEFAR, in disabled readers and in the case of CORA, in disabled or willingly colliding readers. The number of collisions in the warehouse model between mDEFAR, CORA & GDRA is presented in Fig. 4c while the collisions in the urban model are shown in Fig. 4d. GDRA records the highest number of collisions because of the proximity between readers. mDEFAR offers better results than CORA in the warehouse scenario thanks to the different priority levels, while CORA records collisions to increase throughput. In the urban scenario, performances of CORA and mDEFAR are similar with 20 times less collisions than GDRA.

3) Efficiency: Efficiency defines the ratio of the number of successful query sections (SQS) over the total amount of attempted query sections (AQS). Efficiency assesses how well an algorithm avoids collisions regarding the throughput it can achieve. Fig. 4e and Fig. 4f depict the computed efficiency of mDEFAR, CORA & GDRA resp. in the warehouse model and in the urban model. While GDRA struggles to reach 1% efficiency in the warehouse scenario, both CORA and mDEFAR have efficency levels of 72% and 75% respectively. mDefar is slightly better since it registers less collisions in that scenario. In the urban scenario, GDRA performs better with the static readers and manages to reach 42%, but is still far behind mDEFAR (73%) and CORA (76%). This implies that despite the collisions, throughput obtained by CORA makes it better fitted for this environment than mDEFAR.

4) Coverage Delay: This metric measures the speed of teh different protocols. The value is here expressed as the proportion of tags read over the number of rounds. Fig. 5 shows the coverage of our proposals in different scenarios. We notice that in the case of the warehouse, readers take more time (50 rounds) to detect all tags than in the urban scenario (20 rounds). This is due to the fact that readers only move at the speed of 0,7m/s thus taking long time to reach all the edges of the warehouse and designated tags. Both mDEFAR and CORA have overlapping plots, meaning similar results, and obtain full coverage after 50 rounds. GDRA, struggles to reach 10% coverage because of the low throughput and high number of collisions registered. In the urban scenario, results are slightly different, the compromise made by CORA regarding collisions allows it to reach full coverage after just 12 rounds while mDEFAR reaches 100% after 16 rounds. GDRA, although it does not reach full coverage (94%), has significant results, this is explained by the fact that it deals better with the static readers placed in the street corners but has a hard time scheduling the moving readers. The results observed corroborate the direction chosen with CORA regarding the willingly endured collisions in order to improve delay which here is significantly lower than one's of mDEFAR.

C. Discussions

Note that the gap in performance is explained by the inability for GDRA to deal with highly dense or mobile environments. Indeed, our proposals were designed from scratch for these constraints. It thus allows a certain level of interference for a better coverage, in the case of CORA for example, or different priorities level to induce greater fairness between devices, in the case of mDEFAR. The contention resolution process introduced by GDRA is also responsible for this high level of disabled readers. In case of a collision, all involved readers skip the current round and wait for the following contention. For the tested applications, this results in a high number of collisions and low throughput. The observed results confirm the belief that an all-around solution for RFID anticollision cannot be considered. As such, for a dynamic warehouse with mobile readers and tags, mDEFAR should be chosen for its efficiency, but if the goal is to offer the fastest coverage and higher throughput, CORA is the best choice. While in the case of a hybrid deployment of both mobile and static readers, CORA turns out to be the best in terms of efficiency, so is therefore better suited for long-term stability and quick coverage, while mDEFAR should be chosen if the system is vulnerable to collisions.

VI. CONCLUSION

We introduced two new distributed TDMA-based RFID anticollision algorithms. mDEFAR is a previous work improvement to fit mobility and high density deployments of RFID readers. CORA leverages on the particularity of RFID collisions to improve throughput and coverage delay. Both are compliant with [START_REF]Electromagnetic compatibility and Radio spectrum Matters ERM Radio Frequency Identification Equipment operating in the band 865MHz[END_REF] standard allowing their potential use all around different environments. Where CORA seems to be a better solution for applications requiring a quicker coverage of tags, it also shows lower efficiency than mDEFAR and thus might not be more suitable for energy-constrained systems.

Next steps will investigate the energy consumption of both approaches as well as the energy cost of collisions in order to leverage on the compromise made by CORA.

Fig. 2 :

 2 Fig. 2: Collision evaluation

  Fig. 4: Results

Fig. 5 :

 5 Fig. 5: Coverage delay

  Algorithm 1 First frame for a reader R i 1: slot i ← (int)random[1; max colors] 2: priority i ← N EU T RAL 3: if current slot == slot i then Send beacon else At least one other reader has chosen the same token 7: if Ri has the lowest ID then Ri wins the contention Next frames for a reader R i 1: slot i ← (int)random[0; max colors] 2: if current slot == slot i thenSend beacon

	4:	if no beacon received then	No collision
	5:	Read tags; priorityi ← LAZY
	6:
	8:	Read tags; priorityi ← LAZY
	9:	else priorityi ← P U M P EDU P
	10:	end if
	11:	end if
	12: end if
	Algorithm 2 3: if no beacon received then	No Collision
	4:	Read tags; priority i ← LAZY
	5:	else	At least another reader Rj has chosen the same
		token
	6:	if priorityi == LAZY then
	7:	if priorityj == P U M P EDU P then
	8:		priorityi ← P U M P EDU P
	9:	else if Ri has the lowest ID	Ri wins.
	12:	end if
	13:	else	priorityi == P U M P EDU P
	19:	end if
	20:	end if
	21:	end if
	22: end if

10: then Read tags; priorityi ← LAZY 11: else priorityi ← P U M P EDU P end if 14: if priorityj == LAZY then 15: Read tags; priorityi ← LAZY 16: else if Ri has the lowest ID Ri wins. then 17: Read tags; priorityi ← LAZY 18:

else priorityi ← P U M P EDU P end if

  Algorithm 3 CORA algorithm 1: slot i ← (int)random[1; max colors] , Send beacon 2: if no beacon received then No collision

	7:	slot same ++
	8:	else	Neighbor R j slot is different from R i
	9:	slot dif f erent ++
	10:	end if
	11:	end while
	12:	if slot same > slot dif f erent then
	13:		Too many colliding neighbors
	14:	Waits for next round
	15:	else	Enough neighbors on different slots
	16:

3: if current slot == slot i then Read tags end if 4: else R i receives beacons from neighbors 5: while R i receives beacons do 6: if slot j == slot i then R j chose the same slot if current slot == slot i then Read tags end if 17: