Pierre Bourdieu was probably the most influential French sociologist of the last third of the twentieth century. Synthesising diverse philosophical and sociological traditions, his research practice and theory aimed at unveiling correspondences between social and mental structures, and at giving an historical account for their emergence -a research orientation that Bourdieu himself once called 'constructivist' or 'genetic structuralism'. He became internationally renowned for introducing or re-actualising the concepts of 'practice', 'habitus', 'capital', and 'field' in social science. Having graduated in philosophy, Bourdieu turned to anthropology and sociology during a stay in Algeria in the late 1950s. He taught at the

Relatively unawares and without following any conscious rule, people of similar social origin share homologous attitudes, categories, and perceptions, and engage in similar practices.

The sense of honour of Kabyle men, and the humble attitude of Kabyle women, in Algeria, are examples of such dispositions. The habitus is history materialised and embodied; it is inscribed in the Kabyle house (which is organised along an opposition between a dark, damp, lower, 'female' part and a light-filled, noble, upper part) and in the (upright, male, or stooped, female) bodies of the Kabyle. Pierre Bourdieu used the Kabyle material as an illustration in Outline of a Theory of Practice [1972] and The Logic of Practice [1980], two highly influential books that summed up his social thought in mid-career, about two decades before the comprehensive presentation of his approach in Pascalian Meditations [1997].

Bourdieu drew on concepts derived from economics to conceptualise social action. Agents have different kinds and amounts of 'capital'. These include material resources in the original sense of the word, but also 'social capital' (social relations, networks), 'symbolic capital' (prestige, honour), and 'cultural capital'. Cultural capital takes three forms: it is materialised as books and art works in one's possession, incorporated as a certain cultivated 'habitus', and institutionalised in the form of diplomas and degrees.

With the help of these conceptual instruments, Bourdieu analysed the workings of the French educational system. The Inheritors: French Students and Their Relation to Culture [1964] and Reproduction in Education, Society and Culture [1970], two books co-authored with Jean-Claude Passeron, showed that students from lower social classes, being less familiar with the dominant culture, are more likely to fail in school than those with a certain social and cultural capital. Notwithstanding an appearance of neutrality, the school system reproduces and legitimises preexisting social differences.

Generally, and with the complicity of the educational system, the dominant classes accumulate all kinds of capital. In strategies that are designed to bring about the reproduction of their social standing, they sometimes transform one kind of capital into another, for example when they invest in the education of their children. Having graduated from prestigious schools, the latter obtain leading positions in industry and the State, so converting their cultural and symbolic capital back into economic capital.

In accordance with the amount and structure of their capital, agents occupy different 'positions' in the 'social space'. In Distinction: A Social Critique of the Judgement of Taste [1979], Bourdieu made a fine-tuned analysis of the class visions and divisions of French society in the 1960s and 1970s. He correlated social positions with preferences for certain types of music, art and theatre, but also seemingly trivial cultural forms such as foodstuffs, table manners, sports, and interior design. Such seemingly natural tastes and 'choices', Bourdieu argued, are in reality strategies that serve to differentiate oneself from (or sometimes to emulate) members of other social groups.

The social space can be seen as a juxtaposition of various 'fields', for instance the economic and the political field, or the fields of science, fine arts, and literature. A field is a relatively autonomous microcosm with its own rules and logic. Agents and institutions compete with each other over a highly priced value or good that all agree is at stake, e.g. truth in the field of science, or salvation in the field of religion. Everyone employs strategies (either consciously or, more often, out of practical sense, the 'feel for the game') in order to maintain and increase their capital. The existence of a field implies struggle and a power relationship between a dominant and a dominated section within the field.

These fields have evolved historically. For instance, Bourdieu investigated the emergence of an autonomous field of the arts in the late 19 th century. He argued that only such a historical analysis could unearth the conditions for 'understanding' art in our time, e.g. for judging artworks according to purely formal aesthetic criteria rather than appreciating their (religious, economic or decorative) function.

Bourdieu advocated a similar historization for every scientific endeavour. In an exercise of self-reflexivity or 'participant objectivation', social scientists should question their research interests and strategies in three ways: how much are these research orientations influenced firstly by the scholars' social origin, and secondly, by their current position in the scientific field; and do they have an 'intellectualist' bias, i.e. is sufficient attention paid to the fact that most social action is practice rather than the conscious creation of meaning or application of rules? In Homo Academicus [1984], Bourdieu proposed an analysis of the field of higher education in France prior to and during the events of May 1968, investigating correspondences between scholars' social and symbolic capital, their career paths, research interests and outlooks; and he included his own career and research activities in this review.

In the 1990s, Bourdieu had amassed enough scientific authority (or symbolic capital) to make his voice heard outside his own field. Obviously, there was a political agenda in Bourdieu's research virtually from the beginning. By exposing the mechanisms of domination, social science should provide weapons of defence against symbolic violence, as he argued repeatedly. From the mid-1990s however, having published The Weight of the World [1993], an account of the daily suffering of the disadvantaged in France, Bourdieu intervened more and more often directly in public debate, speaking out against neo-liberalism and in defence of the welfare state. If these statements secured him a considerable following outside the field of social science and stirred increasing media attention, they also provoked sometimes harsh criticism from political adversaries and some peers.

Social scientists have also questioned certain of Bourdieu's central concepts. The actual impact of childhood experiences on the habitus has come under scrutiny. What is the influence of the plural, perhaps contradictory settings in which many humans grow up? What role do media and other discourses play in the formation of habitus? The notion of the field might be extremely pertinent for the analysis of socio-professional domains, but possibly much less so for other social realities, such as the family. Is there necessarily conflict and domination as soon as relations between human beings are established? Is it an appropriate to suspect (conscious or unconscious) interests behind expressions of friendship, compassion, and solidarity? As for 'capital', how can one account for the social action of those who do not possess any at all? Many of Pierre Bourdieu's analyses are highly pertinent criticisms of French society, where the republican ideology of egalitarianism and meritocracy often clashes with a reality of racism, class contempt, and relative social immobility. His writings have had a decisive impact on the sociological study of culture and education, as well as on the anthropology of Algeria. Bourdieu's social thought offers suggestions about how to overcome theoretical impasses associated with dichotomies such as the social and the individual, free will and determinism, body and mind, structure and agency. One of the most important legacies of his work is his own modus operandi, i.e., the ethos of a social science that crosses boundaries between disciplines such as sociology, anthropology, philosophy, and history; between different national intellectual traditions; and between the purely academic exchange of ideas and public debate.


Husain Mubashir

Stéphane Rehmani
email: rehmani@univ-mlv.fr

Abderrezak Lohier
email: lohier@univ-mlv.fr

Rachedi
email: rachedi@univ-mlv.fr

Channel Bonding in Cognitive Radio Wireless Sensor Networks

Keywords: Cognitive radio wireless sensor networks, wireless multimedia sensor networks, adjacent-channel interference, channel bonding

Recently, wireless sensor networks are gaining a lot of attention due to the availability of various low cost sensor devices. For example, Wireless Multimedia Sensor Networks (WMSNs) have lot of new potential applications in different domains including, environmental monitoring, home automation, tracking, health care, just to name a few. Another example is the Body Area Networks (BANs), where different sensors are attached with the patient's body. However, these sensor networks also bring several challenges, such as they are resource constraint, requires application specific QoS requirements, high bandwidth demands, and resistance to interference. In this paper, we argue that wireless sensor nodes with cognitive radio capability, together with channel bonding, can help to address these challenges. In this perspective, we discuss cognitive radio wireless sensor network for hospital environment as a case study. We then discuss the way of spectrum characterization and criteria for channel bonding in such networks. Finally, we highlight issues and challenges, which provides the basis to develop algorithms and protocols for the future cognitive radio wireless sensor network deployments in different application scenarios.

I. INTRODUCTION

Wireless Multimedia Sensor Network (WMSN) [START_REF] Akyildiz | A survey on wireless multimedia sensor networks[END_REF] is an emerging technology and gaining a lot of attention by the researchers and industry. This is due to the availability of low cost sensor devices. Moreover, WMSN have a lot of new potential applications, such as, environmental monitoring, home automation, tracking, surveillance, and hospital environment applications [START_REF]Wireless multimedia sensor networks: Applications and testbeds[END_REF]. However, there are several challenges in the practicability of WMSN and their performance is suffered by resource constrained wireless sensor nodes.. For instance, wireless multimedia sensor nodes require application specific QoS requirements, and high bandwidth demands. Another example is of Wireless Body Area Networks (BANs), which are prone to interference (cf. Section II).

In this paper, we argue that wireless sensor nodes with cognitive radio capability, i.e., Cognitive Radio Wireless Sensor Networks (CRWSNs) [START_REF] Akan | Cognitive radio sensor networks[END_REF] can help to address these challenges. Additionally, channel bonding can further satisfy the bandwidth requirements of sensor nodes. In fact, in channel bonding, a set of contiguous non-overlapping channels are bonded This work is part of the project entitled "Wireless Sensor Networks for vehicle and pedestrian traffic assessment applications", funded by the university of Paris Est under the PPS (Projet Pluriannuel Structurant) program.

together to create a single bonded broadband channel. This results in large aggregated bandwidth, increase in the packet transmission rate, and the better satisfaction of the nodes bandwidth requirements [START_REF] Chandra | A case for adapting channel width in wireless networks[END_REF]. Note that channel bonding [START_REF]Draft Standard for Wireless Regional Area Networks : Channel Bonding versus Channel Aggregation[END_REF] and channel aggregation [START_REF]Draft Standard for Wireless Regional Area Networks : Channel Aggregation Summary[END_REF] are two different concepts. In channel aggregation, aggregated channels need not to be contiguous as in the case of channel bonding. Channel aggregation need higher complexity and cost because of sophisticated channel management, scheduling, and load balancing.

In traditional wireless networks, channel bonding has been used for load balancing [START_REF] Chandra | A case for adapting channel width in wireless networks[END_REF] and QoS provisioning [START_REF] Raman | A practical approach for providing qos in multichannel ad-hoc networks using spectrum width adaptation[END_REF]. The current draft version of IEEE 802.11n also discusses channel bonding in both the 2.4GHz and 5GHz spectrum, where two 20MHz channels are bonded into one 40MHz channel to improve transmission rates [8]. However, aforementioned techniques can not be directly applied to CRWSNs. This is due to the constraints imposed by Primary Radio (PR) nodes such as the time varying PR occupancy [START_REF] Akyildiz | Next generation/dynamic spectrum access/cognitive radio wireless networks: a survey[END_REF]. Moreover, for successful reception of data packets, the CRWS sender and receiver nodes are required to bond the same frequency channels. Thus, intelligent channel bonding techniques are required in CRWSNs that keeps into account the PR occupancy, cause less interference to PR nodes, and tune the CRWS sender/receiver nodes to the same bonded channels.

In this paper, our goal is to provide a clear picture of the potentials of channel bonding in cognitive radio wireless sensor networks. Furthermore, we also want to discuss how to minimize interference to PR nodes and satisfy the wireless sensor nodes' bandwidth requirement by equipping the wireless sensor nodes with cognitive radio capability. In this perspective, we first consider a cognitive radio wireless sensor network for hospital environment as a case study. Second, we describe the architecture and functionality of cognitive radio wireless sensor network in hospital environment. Third, we discuss the way of spectrum characterization and criteria for channel bonding in CRWSNs. And finally, we highlight issues and challenges, which provides the basis to develop algorithms and protocols for the future cognitive radio wireless sensor network deployments in different application scenarios.

The remainder of this paper is organized as follows: Section II discusses the CRWSN for hospital environment as a case study. In section III, we discuss channel bonding characterization. Issues and challenges are discussed in section IV, and finally, section V concludes the paper.

II. COGNITIVE RADIO WIRELESS SENSOR NETWORK FOR HOSPITAL ENVIRONMENT: A CASE STUDY

In hospital environment, different e-health applications have been proposed to deliver patient's data, such as blood pressure, ECG, temperature, heart rate etc., to the clinical staff. This data is captured by the wireless sensor devices attached with the body of the patient, also called Body Area Networks (BANs) [START_REF] Latré | A survey on wireless body area networks[END_REF]. With the help of this data, doctors will be able to remotely monitor the patient and it will also helpful in telemedicine services [START_REF] Phunchongharn | A cognitive radio system for e-health applications in a hospital environment[END_REF], [START_REF] Feng | Providing telemedicine services in an infrastructure-based cognitive radio network[END_REF]. Normally, the data transfer from the patients' body to the monitoring room is done through wireless system such as ZigBee.

However, there are two main challenges. First, several overlapping wireless systems exists in the hospital environment besides ZigBee, for e.g., 802.11 b/g/n, telemetry (which operate on WMTS bands or ISM bands), and Bluetooth [START_REF] Liang | Surviving wi-fi interference in low power zigbee networks[END_REF]. All these wireless systems operates on the same band as ZigBee i.e., 2.4 GHz ISM band and overlap with each other. This leads to harmful interference between the existing wireless systems.

More precisely, in cognitive radio wireless sensor networks, when CR wireless sensor nodes transmit on channels that are adjacent to the primary radio bands, it cause harmful interference to PR nodes [START_REF] Tang | Some physical layer issues of wide-band cognitive radio systems[END_REF]. For this reason, during channel bonding, the information regarding the adjacent channels' occupancy is crucial in mitigating interference to its adjacent primary radio nodes [START_REF] Ghosh | Spectrum occupancy modeling and validation using real-time measurements[END_REF]. This problem is referred as Adjacent Channel Interference (ACI) problem [START_REF] Raman | Adjacent channel interference reduction in multichannel wireless networks using intelligent channel allocation[END_REF], [START_REF] Mishra | Cooperative sensing among cognitive radios[END_REF] in traditional wireless networks. Keeping this in mind, the Federal Communication Commission (FCC) in IEEE 802.22 standard has also restricted fixed devices from transmitting on adjacent channels of the i th active channel. Furthermore, in cognitive radio wireless sensor networks, the first priority is to protect the PR nodes, therefore, non-overlapping channels will be used for communication. In summary, it is essential, but extremely challenging to consider adjacent channel interference during channel bonding because of time varying PR occupancy.

Second, wireless communication by wireless sensor devices to the monitoring room may cause harmful interference to medical devices, which are sensitive to electromagnetic interference (EMI) caused by the wireless transmissions. Consequently, the medical devices may start malfunctioning, which cannot be tolerated due to the sensitivity of medical environment.

Recently, cognitive radio based architecture has been proposed to deal with the EMI problem in hospital environment [START_REF] Phunchongharn | A cognitive radio system for e-health applications in a hospital environment[END_REF], [START_REF] Feng | Providing telemedicine services in an infrastructure-based cognitive radio network[END_REF]. However, in this architecture, additional cognitive radio devices are required besides wireless sensor nodes. Moreover, different sensors may have different bandwidth requirements according to the application. For example, ECG data provided by the sensors may have high priority over temperature sensors. In addition, previous approaches [START_REF] Rahul | Learning to share: Narrowband-friendly wideband wireless networks[END_REF], [19] [20] did not consider the adjacent channel interference problem while performing channel bonding.

Thus, to deal with these issues, we extend the idea of [START_REF] Phunchongharn | A cognitive radio system for e-health applications in a hospital environment[END_REF], [START_REF] Feng | Providing telemedicine services in an infrastructure-based cognitive radio network[END_REF] and propose a Cognitive Radio Wireless Sensor Networks (CRWSNs) [START_REF] Akan | Cognitive radio sensor networks[END_REF] architecture (cf. Fig. 1) for hospital environment. In addition, we propose that the channel assignment should be done in such a manner that it should consider the PR occupancy on the adjacent channels.

We now describe the system model and assumptions.

A. System Model and Assumptions

We assume single-hop cognitive radio wireless sensor network architecture. In this architecture, cognitive radio access point (AP) is responsible for continuous spectrum monitoring of the PR nodes. Moreover, we assume that the AP is not resource constrained and is equipped with multiple transceivers. The AP performs channel characterization, channel assignment, and communicate the channel switching decision to CRWS nodes periodically.

We consider total N channels in the network. We assume that the multiple sub-channels can be combined to create a single bonded channel (cf. Section III). We assume that cognitive radio wireless sensor nodes are equipped with a single transceiver, where a single channel can be assigned by the access point. This single channel is used for transmission or reception by the cognitive radio wireless sensor nodes. Both, the cognitive radio access point and cognitive radio wireless sensor nodes can freely switch to channels. We further assume that CRWS nodes may have different bandwidth requirements depending upon the application types.

B. Architecture

Cognitive Radio Wireless Sensor Network for Hospital Environment is a four-tier architecture tailored to implement and deploy real cognitive radio wireless sensor network applications in hospital environments. A general overview of our framework is depicted in Fig. 1 In this architecture, we consider medical devices in the hospital as primary network and their nodes as primary nodes. Indeed, our objective is to detect on-going communications of the medical devices in order to protect them and then satisfy the bandwidth requirements of cognitive radio wireless sensor nodes. In our proposed architecture, the medical devices are considered as the Primary Radio (PR) nodes. These PR nodes have the higher priority to use the channels. In addition, when any PR transmission is going-on, all the other transmissions will be halted. We consider that each wireless sensor node is equipped with cognitive radio capability, which we refer hereafter as Cognitive Radio Wireless Sensor (CRWS) nodes [START_REF] Akan | Cognitive radio sensor networks[END_REF]. In order to deal with bandwidth-hungriness problem of CRWS nodes, we propose to use channel boding.

We now describe individually the functionality of components:

C. Functionality

• Cognitive Radio Wireless Sensor (CRWS) Nodes: They are based on software defined radio. These nodes can access the AP for channel assignment. They can also create multi-hop network to reach nearest access point.

• Cluster Heads (CHs): They are cognitive radio sensor nodes and their responsibility is to coordinate between cognitive radio access point and cognitive radio wireless sensor nodes. Cluster heads will help to create multi-hop network of CRWS nodes. Moreover, the election of cluster head can be done locally by using any appropriate cluster head technique [START_REF] Abbasi | A survey on clustering algorithms for wireless sensor networks[END_REF].

• Cognitive Radio Access Point (AP): These devices are responsible for monitoring the spectrum usage by the primary radio devices. Moreover, they are also responsible for channel assignment to cognitive radio wireless sensor nodes based upon their bandwidth requirements.

• Medical Devices: These devices can be Oscilloscope, ECG etc. They operate on the ISM band and have higher priority to use the channels. We now describe channel bonding characterization.

III. CHANNEL BONDING CHARACTERIZATION

The main responsibility of AP is spectrum characterization. During spectrum characterization, the AP classifies the N channels into occupied and unoccupied ones. The AP then characterizes the unoccupied channels into 2 major subcategories: (1) interior free channels and (2) border free channels. These two sub-categories are on the basis of the location of the i th free channel [START_REF] Ghosh | Spectrum occupancy modeling and validation using real-time measurements[END_REF]:

• Interior Free Channel Case: In this case, the i th free channel is any interior free channel, i.e. i = 1 and i = N . This results in four different types of channels based upon free adjacent channels, i.e. Type I-1, I-2, I-3, and I-4 (cf. Fig. 2).

• Border Free Channel Case: In this case, the i th free channel is not any interior free channel and is located at the border of the spectrum, i.e. i = 1 or i = N . This results in two different types of channels based upon free adjacent channels, i.e. Type B-1, and B-2 (cf. Fig. 3).

P R o c c u p i e d c h a n n e l U n -o c c u p i e d c h a n n e l R e m a i n i n g c h a

We divide these six channel types into two sub-categories:

(1) bonded channels category and (2) non-bonded channels category. In the bonded channels category, we propose to use channel Type I-1, I-2, I-3 and B-2. While, Type I-4 and B-1 will be treated as non-bonded channels.

After spectrum characterization, the AP will preform the spectrum assignment. The AP assigns bonded channels i.e. Type I-1, I-2, I-3, and B-2 channels to CRWS nodes that has higher bandwidth requirements, while Type I-4 and Type B-1 channels will be assigned to CRWS nodes that has lower bandwidth requirements.

We now define the criteria for channel bonding, which is based on the availability of the number of free adjacent channels. This criterion will ensure the AP to mitigate the adjacent channel interference during channel bonding. Type I-1 channel has no adjacent neighbor channels occupied and results in three available contiguous non-overlapping channels. In Type I-1 channel case, the three channels will be bonded to create a single broadband channel. In Type I-2, I-3 and B-2 channel case, two contiguous non-overlapping channels are available. Therefore, these two channels will be bonded to create a single broadband channel. Finally, Type I-4 and Type B-1 channels have their adjacent channels occupied. Thus, in this case, no channel will be bonded and no transmission will be taken place.

We consider the channels of homogeneous bandwidth. A channel i with center frequency F c(i) is shown in Fig. 4(a).

The bandwidth BW i of channel i can be written as:

BW i = F e(i) -F s(i) (1)
where F s(i) , F e(i) are the starting and ending frequencies of channel i. The bandwidth of the channel i from the starting or ending frequency to the center frequency F c(i) can be calculated as:

BW c(i) = F e(i) -F c(i) = F c(i) -F s(i) (2)
where F s(i) , F e(i) , and F c(i) are the starting, ending and center frequencies of channel i. We now take the case when three channels are combined to create a single bonded channel i.e. Type I channel. The bandwidth of bonded channel i BW 3 BC(i) (cf. Fig. 4(b)) that bonds the two adjacent channels (i-1) and (i+1) can be calculated as:

BW 3 BC(i) = F e(i-1) -F c(i-1) + GB (i-1)→i + F e(i) (3)
-F s(i) + GB (i)→(i+1) + F c(i+1) -F s(i+1)
where (i-1) is the prior channel and (i+1) is the posterior channel of channel i. The symbols F s(i) , F e(i) , and F c(i) are the starting, ending and center frequencies. GB (i-1)→i is the guard band between channel (i-1) and i and GB (i)→(i+1) is the guard band between channel i and channel (i+1).

In order to avoid adjacent channel interference, we adopt an straightforward approach, in which the bandwidth of the bonded channel is increased till the center frequencies of adjacent channel (i -1) and (i + 1) (cf. Fig. 4(b)) and the bandwidth is expressed in Eq. 3. We adopted this approach primarily because a large guard band is required at the band edges when performing channel bonding [START_REF]Draft Standard for Wireless Regional Area Networks : Channel Bonding versus Channel Aggregation[END_REF]. It is worth mentioning here that the guard band in between channels can be reused through channel bonding.

IV. ISSUES AND CHALLENGES

In this section, we discuss the issues and challenges related with channel bonding in CRWSNs:

• How to conserve the energy of CRWS nodes? CRWS nodes are energy constrained devices. One challenge is to investigate the impact of channel bonding on the energy of CRWS nodes. More precisely, the higher the number of channels to be bonded, the higher the energy consumption, thus, there is a need to quantify this relationship of energy consumption vs. number of channels to be bonded.

• How to use channel for bursty traffic? Different types of CRWS nodes may generate bursty traffic. Thus, it is important to assign channel opportunistically to CRWS nodes to increase reliability and to decrease packet losses.

• How often the channel assignment is triggered? In traditional wireless networks, the channel assignment is often triggered when a new node join/leave the AP or a new data flow start by a node. In the context of CRWSNs, the appearance of PR nodes on the assigned channel may lead to trigger the channel assignment again. The channel assignment will become more frequent in the case of channel bonding, as the probability of PR appearing nodes increase with the number of used channels during channel bonding. The triggering of channel assignment can be minimized by predicting the PR activity.

• How to select stable free channels during channel bonding? PR nodes can re-appear on the bonded channel. Consequently, the bonded channel is broken and given back to PR nodes for transmission. The question is, how the transmission should be handed over seamlessly to another bonded channel with the same bandwidth size. One approach to deal with this problem is to maintain a pool of bonded channels at the AP. This pool contains the list of most probable channels for channel bonding w.r.t. their sizes. E.g., when a bonded channel with 2 sub-channels is broken then the AP select and assigns the next bonded channel of the same size to the CRWS node. Note, if the pool of the bonded channels is already being used by the neighboring CRWS nodes and no free bonded channel is available, then CRWS node may use the nonbonded available channel for communication. However, this will reduce the data rate of the CRWS node.

• How to achieve maximum capacity gain while causing less interference to PR nodes? We suggested to increase the bandwidth till the center frequency of the adjacent channels. Although, this ensures significant reduction in PR interference but there would be some capacity losses.

In order to achieve maximum capacity gain while causing less interference to PR nodes, we have to consider the tradeoff between the channel separation distance and PR interference. If the channel separation distance is small, PR may get interference, while if it is larger, capacity loss will be there. Thus, we need to quantify this tradeoff.

• How many channels can be used for Bonding? One may think to bond as many channels for transmission but there exists a tradeoff; the higher the number of bonded channels, the higher is the probability of being interrupted by PR transmissions. Thus, one approach is to consider channels with longest remaining idle time for channel bonding.

• Which type of channels to be used for channel bonding?

In CRWSNs, the available channels can be categorized into three types: (1) Overlapping, (2) Non-overlapping, and (3) Partially overlapping. The first priority in CR-WSNs is to protect the PR nodes, therefore, overlapping and partially overlapping channels cannot be used for channel bonding. The only candidate for channel bonding in CRWSNs is non-overlapping channels (case of ZigBee Technology). However, the challenge resides in finding the consecutive non-overlapping idle channels.

• How interference can be handled? In CRWSNs, interference can be caused between: (1) CRWS-CRWS nodes, (2) CRWS-PR nodes, and (3) AP-AP. The interference between CRWS nodes can be caused when the AP assigned the channel to the CRWS node whose i th active channel is already being used by another CRWS node. The CRWS-PR interference can be caused when a channel is assigned to CR node whose i th active channel is already being used by PR node. While, inter-AP interference can be caused if the same channel is assigned to the neighboring AP. All these three types of interference can be handled by employing an intelligent channel assignment on the AP. For instance, one way to reduce the CRWS-PR interference by the AP is to select those channels that have longest remaining idle time.

• Impact of channel bonding on Transmission Range and Throughput? When two or more channels are bonded together, the transmission range will vary and it will also impact the throughput of the wireless nodes. Thus, there is a need to investigate this issue also.

• How to deal with CRWS nodes occupancy? During channel assignment, besides PR occupancy, the AP also requires to consider the CRWS nodes occupancy on the adjacent channels. Otherwise, the CRWS nodes cause harmful interference among each other, which may degrade the performance of CRWS nodes. In order to deal with CRWS occupancy, the AP can assign those channels to CRWS nodes which are non-consecutive.

V. CONCLUSION AND FUTURE WORK

In this paper, we have discussed how wireless sensor nodes with cognitive radio capability, in conjunction with channel bonding, can satisfy the cognitive radio wireless sensor nodes' bandwidth requirements and interference problem. In this context, we took cognitive radio wireless sensor network for hospital environment as a case study. We have identified the working principle and the challenges related with the deployment of this architecture. Then, we discussed how the PR occupancy and adjacent-channel interference can be considered. We have also highlighted issues and challenges in the deployment of this architecture. We believe that this work can serve as a basis to build new algorithms and protocols for cognitive radio wireless sensor networks that can work in the presence of heterogeneous networks and communications technologies in different application environments.

Fig. 1 .

 1 Fig. 1. Cognitive Radio Wireless Sensor Network in Hospital Environment

  . The building blocks of this architecture are: (1) Cognitive Radio Wireless Sensor (CRWS) nodes, (2) Cluster Heads (CHs), Cognitive Radio Access Point (AP), and (4) Medical Devices.

P R o c c u p i e d c h a n n e l U n -o c c u p i e d c h a n n e l R e m a i n i n g c h a n n e lFig. 2 .

 2 Fig. 2. Types of channels (Interior Free Channel Case): (a) Type I-1, higher bandwidth category, (b) Type I-2, higher bandwidth category, (c) Type I-3, higher bandwidth category, and (d) Type I-4, lower bandwidth category.

Fig. 3 .

 3 Fig. 3. Types of channels (Border Free Channel Case): (a) Type B-1, lower bandwidth category, and (b) Type B-2, higher bandwidth category.

F r e q u e n cFig. 4 .

 4 Fig. 4. An example showing channel bonding (a) when the channels are not bonded and (b) when three channels are bonded to create a single broadband bonded channel.

978-1-4673-0937-0/12/$31.00 ©2012 IEEE

ACKNOWLEDGEMENTS

The author, Mubashir Husain Rehmani, would like to thank Aline Carneiro Viana from INRIA, France for the helpful discussions and suggestions.


Beyond reified categories. Multidimensional identifications among 'Black' and 'Indian' groups in Columbia and Mexico

The integrationist approach that formerly characterized the social and cultural policies of many Latin American governments has been displaced in recent years.

The new principle, variously interpreted by different states, is that the cultures and institutions of the diverse 'indigenous peoples' who constitute the nation should be respected, taken into account, and 'activated' to participate in national debates and decisions (Sieder 2002; Hoffmann & Rodríguez 2007). This view of 'indigenous peoples'-widely debated concerning its essentialist or pragmatic aspect (Béteille

remedy-have since the Conquest affected also other groups in colonial history identified by traits perceived as 'different,' such as the descendants of black African slaves. Nevertheless, this recognition of difference involves a normative usage of the relevant social categories, notably those describing 'Indian' and 'black' groups, based on supposedly 'cultural' criteria, but which range from language use to skin color and include specific social and cultural institutions (kinship, oral tradition, law), while other sectors of the population remain 'ethnically neutral' (Alexander 2006: 156).

These categorizations are perceived in different ways depending on locus (Canessa 2008), thus creating, between the institutional center and the community out in the field, 'indigenous spaces' (Hathaway 2010) where specific social universes emerge out of a vast range of interpretations and implementations of these identity-related and identity-creating concepts and categories. Yet the sociocultural identities lived daily by the groups in question do not necessarily match those attributed to them by institutions (Chakrabarty 2002: 88, cited by Alexander 2006: 160). As [START_REF] Omura | Comments to Adam Kuper's 'The return of the native[END_REF] points out, only the daily practices of the groups classified as 'indigenous' can show us which social categorizations are relevant for the individuals they describe. How, then, can we capture their daily reality without naturalizing the categories created 'from the outside'? What sort of categorization emerges from praxis, and what does it teach us?

The first part of this paper explains how the historical and political contexts interfere in the analytical approaches. Next we examine four groups in Colombia and in Mexico, identified as 'Indians' or 'blacks,' by their neighbors and/or institutions; we analyze local narrations that they use to situate themselves in the national societies to which they belong. The third part of the article explores possible, non-exclusive ways in which the different kinds of 'otherness' arising out of these narrations might be categorized. This comparative approach will shed some light on what drives the similarities and differences between groups. In this way, the internal social ideologies underlying the categorizations applied in each case become more evident.

Analyzing the Institutional Categorizations of Subaltern Populations

Despite the extensive ethnic intermixing that characterizes most Latin American countries, contrasting positions persist and are transmitted from generation to generation through the national histories of social and cultural diversity and the commemorative systems instituted by the state-systems that include official and school-taught history, specialized institutions, specific legislation, memorial ceremonies, etc. The 'Indian,' who as a descendant of pre-Hispanic civilizations occupies a historic and generic place in the nation, has greater prestige than the 'black,' whose place, as the descendant of slaves, is denied and nearly non-existent.

Regardless, these two population categories are socially considered of lower status than, and basically subordinated to a third one, namely that of the 'mestizo' (Mexico) or the 'white' or the 'criollo' (Colombia), which are all associated with the image of Western culture and the idea of progress.

In order to grasp the contemporary logic of these categories, their possible inadequacies, their effects, and their issues, many researchers have undertaken to analyze them not only in terms of their historical context, but also in their discursive relations with the state, with capital (the market, the economic elites), and with the whole constellation of actors in the local, national, and international spheres, nongovernmental organizations (NGOs), etc. [START_REF] Alexander | Cowboys and Indians and Comuneros: Policypositioned ascriptions of ethnicity, identity and history in Chile[END_REF]Canessa 2008;[START_REF] Dupuy | Wayana et Aluku: les jeux de l'altérité dans le haut Maroni[END_REF]Gros & Strigler 2006;[START_REF] Obarrio | Remains to be seen. Third encounter between state and 'customary' in Northern Mozambique[END_REF]). Yet these analyses often return to the binary opposition of state versus Indians-or some other minority-just like the simplistic classifications that reduce indigenous peoples to representatives of the impoverished class.

In fact, however, the issue is much more complex. As an example, the dialectical borrowings between self-designation and legal-administrative terms do not derive solely from the Machiavellian machinations of the state or from purely self-serving motives on the part of the groups in question. Our concern, then, is with the debate over the categories of indigeneity in the broad sense of the term-that is, we would like to move beyond the official categorizations of subaltern sectors that have passed from invisibility to statutory visibility, and include those categories that are created by these sectors themselves. Our study seeks to analyze the social representations used out in the field, outside the context of a public assertion of difference-in other words, within the groups themselves, where categories preconceived elsewhere are not mentioned and where no one boasts of association with any specific 'culture' [START_REF] Briones | Meta)cultura del Estado-nación y estado de la (meta)cultura, Popayán[END_REF]. These are groups that have not emerged on the political stage.

Legitimizing Local Narratives

We are interested in rural groups that are 'integrated' in the national collective but relatively marginalized and geographically peripheral, and that do not make political claims based on their 'ethnic' (or racial) 1 identities. That does not mean that their discursive productions do not include reinterpretations and reappropriations of the national discourse-a discourse which is itself far from being homogeneous and disembodied. However, these groups, unlike others (in some cases in the same region), do not rely on a politicized rhetoric of difference.

We studied four groups. On one hand we had two modern-day neighboring Indian groups, Teeneks and Nahuas, in the Huastec region of northeastern Mexico, who do not emphasize their Amerindian culture, even while taking it for granted and living it fully through social and magico-religious practices. We paired them with two groups of Afro-descendants, one in Colombia in the 1980s and the other in current-day Mexico, who, similarly, make no show of their racial appearance, although they fight the discrimination and racism from which they suffer on account of their skin color and their 'distinction' as 'blacks.' Obviously none of these groups is isolated from any identity movements that exist in the region, and in which, in fact, they may often participate (the black mobilization of the 1980s and 1990s in Colombia is one striking example, or the programs of cultural renewal in Mexico). 2 However, at the times and in the places described in this study (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010), political and ideological struggle for recognition of ethnic or racial difference is or was not a significant aspect of the social context, nor expressed by the people themselves.

Our study does not compare the 'situations' of these groups, which are extremely diverse and complex, but rather examines a common process and its local variants: the recourse to commemorative narrative resources based on a cultural language or, to paraphrase Comaroff and Comaroff (1992), the poetics narrative of history, as a basis for the collective identity. To this end, we looked at the founding stories of each community, which may be mythical or historical-myth being understood here as a historical narrative, objective or not, in which a supernatural element is involved to explain the group's present-day situation.

The comparative method involved a specific examination of the role played in these narratives by two criteria of identity mentioned earlier which official discourse brackets together in defining the term 'autochthony' applied to Indian and black populations-that is, the length of time in the country and 'ethnic or racial difference.'

The Black Populations of Mexico and Colombia: Contrasting Settlement Histories MEXICO:

Although their importance was recognized back in the mid-twentieth century by an important Mexican historian and anthropologist [START_REF] Aguirre | La población negra de México: estudio ethnohistórico, México: Fondo de Cultura Económica --1989[END_REF][START_REF] Aguirre | La población negra de México: estudio ethnohistórico, México: Fondo de Cultura Económica --1989[END_REF], the black populations of Mexico have attracted little attention from contemporary researchers. Their near invisibility as a group is obviously largely responsible; for one thing, they have no clearly recognizable phenotype, owing to a long history of frequent ethnic intermixing, so that few individuals match the 'black' stereotype recognized in other countries: Africans, Cuban 'blacks,' and North-American 'blacks.' In addition, they are few in number, since not many individuals identify themselves with a category that has been officially 'abolished' (in censuses, official documents, public policies) since independence in 1821. The final deathblow was the dominant discourse concerning the melting pot as the foundation of the nation, and hence the favored means of social mobility and integration, to the detriment of other-thereby devalued-identities. Today in Mexico, populations who collectively self-identify as black are a tiny minority concentrated in a few regions-where, however, they do maintain a real presence and a certain social and cultural dynamism (the Pacific coast, certain regions of the state of Veracruz, and smaller enclaves in a number of other regions). Counted individually, they could easily total anywhere from a few thousands to several millions, depending on the criterion used to identify them (phenotype, self-identification, ancestry, cultural practices, etc.).

For the purposes of this study, we will focus on the black populations residing in the small area known as the 'Costa Chica' (the 'small coast'), along the Pacific coast between the states of Guerrero and Oaxaca.

The very origin of these populations has been disputed. Certainly one established reference point is the beginning of slavery in the handful of enormous haciendas that extended along the coast, where in most cases slaves were used to aid in cattle raising, but sometimes to work cotton or sugar-cane plantations as well. A few researchers, however, also mention the existence-hitherto undocumented in any archives-of palenques or villages of runaway slaves in that remote area, which was nearly inaccessible until the mid-1900s. Others report nineteenth-century migrations of free 'blacks' supposedly from Veracruz or other parts of Mexico, attracted by very lightly populated regions where land was still available, with or without legal title. Family legends offer almost no additional information. Ancestral memories do not go back further than three or four generations, but they do highlight an interesting trait common in many Latin-American black societies: the very high residential and matrimonial mobility of not only individuals, but also households and sometimes entire villages.

A third source of collective memory, after scholarly accounts and family chronicles, is a 'mythical' legend pervasive in the villages, according to which a slave ship was wrecked on the coast, permitting the escape of men and women who consequently set foot on the American continent as free people, founding villages that had nothing to do with slavery. This particular legend, sometimes interpreted as a source of identity, dignity, and possible integration without the stigma of slavery (Lewis 2001), is found in nearly all the coastal regions inhabited by the descendants of slaves in America (Colombia, Ecuador-where in fact the event is well documented-Venezuela, Honduras, etc.), as well as in the Caribbean Islands. These commemorative accounts appear to have a great narrative heterogeneity. The events-historical or mythical-are not clearly situated in space and time; the accounts all tend to be standardized in the format of a simple legend, and there appears to be little interest in determining which version might be the true or legitimate one. Depending on the village or the day, the same individual may not tell the same version twice, as though, ultimately, historical veracity is not the issue. The story of their origins, their foundation, is of little value if it evokes social and racial discrimination rather than serving as a foundational element worthy of group 'recycling' and patrimonialization. The cultural record (dance, music, festivals), certainly very rich (Moedano 1997), also demonstrates the intensity of the ethnic intermixing that has historically characterized the region.

The attitude to the land is also very ambiguous. As long as the population was still very sparse and the legal owners of large landholdings were absentee landlords (before the liberal reforms of the end of the nineteenth century, and especially the Revolution at the beginning of the twentieth century), access to land did not pose a problem: 'everyone took what he needed' to farm, without any property title but with the consent of the landowner, who thereby controlled labor. The elders accept their 'foreignness': 'we are 'newcomers,' we grabbed [the land] wherever there was any' (Somos arribeños-from'arribar,' 'arrive,' not 'arriba,' 'above,' as in other regions-agarramos donde había). After the Revolution (1910Revolution ( -1920)), the agrarian reform redistributed the land, legally transferring it to agrarian 'communities' which then began to consolidate around these freshly acquired land rights.

COLOMBIA

The situation is very different in the areas of black population in Colombia, such as the Pacific coast. This population, which has undergone very little intermixing, has had unlimited access to tropical lands not much prized by the Spaniards and their descendants, and also very tenuously linked to the interior of the country. Some of the inhabitants have been building their villages and cultures on these lands for more than two centuries. The villagers, descended from slaves liberated in the midnineteenth century but also, primarily, from blacks free since the eighteenth century, have made use of the forest (lumber), the subsoil (gold mines), natural resources (ivory nut), and the rivers (fishing) for the greater benefit of white businessmen settled in the few towns, but without any daily restrictions on their work habits and their mobility. In these villages, the elders can give a precise account of the migrations that brought them to their current homes, name the 'founders' of family lines and villages, and describe past trials and tribulations and relocations, voluntary or enforced. Their oral tradition is extremely rich, combining ancient Hispanic poetry (sixteenth and seventeenth centuries) long forgotten elsewhere, a record of national popular culture (Velázquez 1959), and expressions clearly African in origin.

Their funeral rites (De Granda 1973), kinship system [START_REF] Friedemann | Forjando patria, México: Editorial Porrúa GREENE, SHANE 2007 'On race, roots/routes, and sovereignty in Latin America's Afro-indigenous multiculturalisms[END_REF], and musical and linguistic habits (De Granda 1977) all connect back to specific systems, as do the myths of origin and healing rites, which are often inspired by those of the neighboring Indians but are largely reappropriated [START_REF] Losonzcy | Los afromestizos y su contribución a la identidad cultural en el pacífico sur: El caso de la tradición oral en la Costa Chica, in Pacifico Sur: ¿Una región cultural?[END_REF]. Without falling into exoticism or blind essentialism, we must nevertheless note a complex, coherent originality unique to the Pacific region and the black populations who live there.

Here history pops up everywhere, the tools of memory-whether explicit or notare diversified and consistent with each other, and the group acts fully as an everyday culturally mobilizing force. There is no mention of 'autochthony,' but the ancestors are definitely present and the rites of healing define a group distinct from the neighbors, with shifting borders of identity (Hoffmann 2004;[START_REF] Losonzcy | Los afromestizos y su contribución a la identidad cultural en el pacífico sur: El caso de la tradición oral en la Costa Chica, in Pacifico Sur: ¿Una región cultural?[END_REF] that are nevertheless recognizable by anyone, whether outsider or insider. That does not imply, however, a unanimous identification as 'blacks' or 'black communities,' which would reduce and to some degree mutilate the complexity of the social and cultural field, but rather a collective consciousness shaped around memory and its tools.

Two Neighboring Indian Populations in Northeastern Mexico

In the township of Tantoyuca, in northeastern Mexico (Huasteca region), two Indian groups, the Teeneks and the Nahuas, live side by side. They belong to two of the numerous ethnic groups enumerated in Mexico by government agencies. However, the two groups put down roots in fundamentally different processes. The Teeneks, who have lived in the region since the second century A.D., have a social and territorial organization that dates back to the end of the nineteenth century, and they bought the lands where they lived and worked during the colonial period through their own efforts. The Nahuas, in contrast, came from the other side of the mountain, fleeing the violence of the Revolution, and settled in the area in the early twentieth century; consequently, their social and territorial organization is more recent, and they gained official access to their lands as a result of a post-Revolutionary (1910)(1911)(1912)(1913)(1914)(1915)(1916)(1917)(1918)(1919)(1920) grant after negotiation with the mestizo landholders and the state.

The two groups express these different temporalities of territorial settlement symbolically in distinctive narratives that support their collective history and lay out their relationship to the land. In fact, the Teeneks of Loma Larga, a very strongly marginalized group, tell a creation myth in which the arrival of the sun led their prehuman ancestors to seek refuge underground, making them authochtonous figures and providing justification for Teenek ownership of the land. This myth engendered worship of these prehuman ancestors, the Baatsik'-expressed through certain rituals that safeguard a balanced cohabitation between humans and supernatural beings necessary to ward off illnesses. The myth thus underlies a narrative structure that explains different misfortunes. In fact, the same narrative framework that explains both the presolar cataclysm that drove the ancestors underground and the diseases to which their descendants fall prey is also applied by the Teeneks to other narratives that explain the Spanish Conquest and the accompanying Christian evangelization, as well as the Mexican Revolution that stripped them of their landin short, social misfortune in general, including their extreme poverty. It is not a fixation on the past, but rather a narrative resource that organizes the contemporary Teenek world in the form of binary opposites-on one hand, these supernatural forefathers who cause illness, but with whom the Teeneks identify by continuing to pay them homage; and, on the other, the intruders of each era-attributing a unicultural space to each (Ariel de Vidas [2002] 2004). Thus, the Teeneks' myths and practices are a form of interpretation allowing them to understand and legitimize the present by transferring the represented contemporary world to the past. The Teenek myth of origin reflects the depth of this group's territorial roots in the region and a historical consciousness of the metamorphoses undergone since the Spanish Conquest and the Christian evangelization. Thus, for the Teeneks it is a structural transmission, repeatedly updated to reflect new circumstances, of the memory of that encounter in the past with the cultural Other, each new reiteration allowing them to cement their belonging to the place and to justify the group's marginal position.

The Nahuas of La Esperanza, a group that is somewhat better integrated socially and economically in its region, tell a much more recent foundation story than their Teenek neighbors. This story takes place in the mid-twentieth century, when a terrible drought struck the Huastec region and was followed first by an outbreak of foot-and-mouth disease that ravaged the cattle ranches, and subsequently by a famine that killed many. According to local history, a man from a neighboring community decreed that to bring rain, the ritual of Chicomexochitl should be performed on the top of the mountain near the village. In the Nahuatl language, Chicomexochitl means 'seven flowers'-it is the name of the spirit of the corn that provides sustenance and nourishes the human soul, invoked in the course of a common ritual of the Nahua culture in the Mexican high plains (Sandstrom 1991: 133). The villagers accordingly made offerings to the mountain, consisting in food, dances, and music; and, all at once, ears of corn began to rain down. From then on, each year on their patron saint's day, the villagers would leave offerings at the top of the protecting, nourishing mountain, 'the seat of the rulers of the place,' according to one of the ritual specialists of the village. This experience, recounted by the elders of La Esperanza with great emotion, was an initiation into the desired state of coexistence with the masters of the earth, a coexistence that had previously been ignored by the villagers because they were 'new.' 'We will not forget this custom again. Before, it was abandoned because the elders who knew of it had died.' Thus, the Nahuas of La Esperanza, a group that, although newly constituted in the area, derives its cultural patrimony from the common core of the Mesoamerican culture, resorted to this foundation myth dating from the mid-twentieth century for an explanation of the modalities of convivial relations with the rulers of the recently appropriated land. Although the La Esperanza villagers are culturally and linguistically part of the majority ethnic group in Mexico, which claims descent from the Aztec civilization, they rarely evoke those remote pre-Hispanic times. The identity discourse in the village is local and non-ethnic, and the miracle on the mountain is its point of departure. The establishment of La Esperanza's mountain ritual, which came during the same period as the official confirmation of the community's real property rights (1955), seems thus to have served as a symbolic construction of local identity during a moment of crisis.

Through their mythical-historical narratives, the Teeneks and the Nahuas studied here draw upon different time periods that express not only the relative historical depth of their territorial settlement in the region, but also their concept of belonging, which is ultimately not dependant on that historic depth but rather on the establishment of a relationship with the land on which and from which they live.

Beyond Ethnic Displays: The Contribution of Cross-Comparisons

Keeping these ethnographic data in mind, we now want to explore the different perspectives on the position occupied by these 'indigenous peoples' in the larger society, beyond the one in which they live. This exploration will allow us to grasp the complexity, the diversity, and perhaps even the contradictions inherent in the relations maintained between the state, social groups, and individuals at different levels.

The first configuration refers to the state's relationship with 'its' minorities, as explained earlier in this article, with a clear division between groups defined, respectively, as black and Indian. This 'traditional' binary view invites several comments. It reveals a 'normalized' image of the country's composition in terms of 'ethnic groups,' cultural diversity, even multiculturalism (which is not the same thing), but always based on the idea of a mosaic, of groups that are separate and adjacent. It treats the designation criteria differently: 'racial' and 'regional' for one group, 'ethnic' and 'cultural' for the other, according to two different logics that in fact share a broader ethno-racial outlook (Greene 2007: 332). It is accepted and used by actors who are in some cases antagonists but who all find in it a kind of strategic convergence: States, ethnic activists (all with their own specific agendas), international agencies with programs targeted at the populations involved. In short, this rather simplistic interpretation 'works,' masking historical truths such as, perhaps, the difference in many basic ways between a history of slavery on one hand and a history of colonial domination on the other.

However, this black/Indian cleavage can be completely turned around if, for example, we no longer consider the state's relations with 'its' subaltern groups, primarily in terms of public policies, but rather look at the images conveyed in the popular collective imaginaries-in literature, for instance, or the visual arts (painting, drawing, caricatures, etc.). Indigenous people ('Indians') are present there in the form of either the historical, imperial Indian (in Mexico, Nahuas are the archetype of this, as the presumed direct descendants of the Aztecs), or the poor, marginalized, or contemptible outcast or charity case, depending on the circumstance. Similarly, 'blacks,' where they exist at all, appear in national imaginaries as either a bothersome, 'atypical' enclave, poor, foreign folks whom people would prefer not to see, or-when they can no longer be overlooked, as in Colombia-as a group 'apart,' heirs to a special history worthy of being disseminated in textbooks and taught in schools and universities (Maya 2000).

If we adopt the perspective of the popular imaginaries, we see similarities first between the Costa Chica inhabitants and the Teeneks, in Mexico, who are both groups that have long been unknown or misunderstood, enclaves 'that stick out like a sore thumb,' and 'do not manage to blend in.' Thus, up until the 2000s, the 'blacks' of Costa Chica have not presented an image of otherness that could be integrated into the national story. They are characterized primarily as a minority, a 'special group' apt to be exoticized and naturalized, with no cultural roots other than the mythical slave ship. The Teeneks, for their part, conduct a discourse of autochthony through their myths about their underground ancestors, and, through these myths, maintain a clear distinction between 'us' and 'others' (whites, mestizos). Using self-denigration as a defense against the world around them (Ariel de Vidas [2002] 2004), the Teeneks have erected a cultural 'barrier' of inferiority that has enclosed them in a symbolic enclave-as well as real economic and geographic marginalization.

Another similarity exists between the 'black communities' of the Pacific coast in Colombia and the Nahuas in Mexico, who represent, each in their own way, an image of the 'other,' historicized and recognized as a 'partner' of the national society, at least in discourse. The Colombian 'blacks,' who have long roots in their own lands, have constructed their own collective discourse, nourished by the relative autonomy deriving from nearly two centuries of isolation and demographic strength.

They formed a black 'regional society' of their own (Hoffmann 2004), rich in specific cultural practices and narratives. They have not really needed a myth to justify their presence; that presence is undeniable and their 'difference' cannot be disputed-so much so that in the 1990s, when the new constitution was drafted, it provided the basis for 'official' black otherness in Colombia. In short, they represent an emblematic difference in the nation. In that respect they constitute a special case, as noted by [START_REF] Restrepo | El 'giro al multiculturalismo' desde un encuadre afro-indígena[END_REF] and [START_REF] Ng'weno | Can ethnicity replace race? Afro-Colombians, indigeneity and the Colombian multicultural state[END_REF], since they can be most easily assimilated to the case of 'Indian' groups, unlike so many other Afro-descendants who do not fit into any cultural-ethnic pigeonhole. 3 Similarly, the Nahuas do not have the same sense of inferiority vis-à-vis the world around them that their Teenek neighbors in the Huastec region do. They know they are numerous, visible, historically 'present,' with or without a founding myth.

Their history, which they can find in textbooks, is sufficient to prove their legitimacy. Even unspoken and lacking any cultural elaboration, their relation to the ancient Aztecs is known and accepted. In a way, they are the Mexican Indian, the figure that is honored in the twentieth century by intellectuals and artists as the source of the national identity (Gamio 1982(Gamio [1916];];León Portilla 1997[1956]).

In short, viewed from the perspective of a group's position relative to or within the national history, a comparative configuration takes shape, in which the parallels have nothing to do with the ethnic or racial 'nature' of the groups in question, nor with their degree of autochthony as measured by the objective, archivally documented age of their settlements: the black groups of Mexico and Colombia were both established in the eighteenth century or even earlier, yet have very different memory 'systems' or tools. Similarly, the Teeneks and the Nahuas, although both 'Indians' in the same region and both subject for centuries first to Spanish, then mestizo rule, do not carry the same 'baggage' where their memories are concerned.

What seems to matter here is not a static situation, but a dynamic relationship, a relation to the nation: a relationship of recognized, even emblematic, otherness (the black communities of the Pacific in Colombia, the Nahuas of Mexico) in one case, and a denied, or in any case 'forgotten' relationship in the other (Costa Chica, the Teeneks).

The two divisions noted so far (public policy, national history) do not exhaust the possibilities for different positionings, notably because they still involve a state/group opposition, thereby eliminating any chance of understanding from within the emic positions on these subjects. Let us consider a third option, the view that the groups themselves have of their history, their own stories, and those of their neighbors.

To begin with, we should recall that in none of the cases mentioned is the criterion of 'autochthony' locally relevant in the sense given it by the governmental categories ('Indians' versus 'blacks'). On the other hand, it does seem, in the sense elaborated by the local populations themselves, to establish a collective identity on the basis of a subjective attitude towards the group's origin: Are not the Teeneks' Baatsik' witnesses to their autochthony? And the black communities of Colombia are definitely 'at home' in the Pacific, whereas, in the recent history retransmitted by oral tradition, the Nahua groups of La Esperanza and the 'black' communities of Costa Chica in Mexico are 'newcomers' on the lands 'of others.' These narratives, through which 'memory is incorporated into the constitution of identity' (Ricoeur 2000: 103), give rise to a new configuration which analyzes commemorative tools by introducing something we will call 'insularity' versus 'extroversion'.

Insularity is typical of groups that have long been settled in the region (the Teeneks of Loma Larga, the black populations of the Colombian Pacific). With or without official title to their lands but very strongly rooted in space or time (millions of inhabitants in the Colombian Pacific, a settlement a few hundred years old in the case of the Teeneks) and 'protected' from the dominant majority by their extreme geographic marginality, these groups have managed to develop diversified cultural creations that are very rich in meaning and that are endowed with both stability and great historic depth. Their view of history intertwines with mythical times to explain the present-a phenomenon that evokes Detienne's 'myth-ideology ' (2005: 20).

Extroversion, for our purposes, applies to groups whose territorial roots are recent, or challenged by a different, dominant socio-ethnic environment (the Nahuas of La Esperanza, who arrived in the early twentieth century in flight from the violence of the Revolution; the black villages of Costa Chica, which had no land rights before the agrarian grants). Survivors of previous relocations, these groups face a hostile environment while in conditions of social break-down. They are made up of small population nuclei, either unstable or recently stabilized, formed during the agrarian reform of the twentieth century, which gave them territorial and political legitimacy after they had already arrived in the area. Cultural creation comes after social creation. The community establishes itself and only subsequently achieves cultural legitimacy. The institutions of commemoration are devised a posteriori, both from materials gleaned from individual memories and from those that are made available by the historical context of the moment, the government, the institutions, and the national narrative. This belongs to the realm that [START_REF] Bhabha | The location of culture[END_REF] calls 'the third space,' that in-between space that accommodates the negotiation of values and the articulation of opposites.

Extroversion implies an openness to the other. In Costa Chica, the 'blacks' see themselves as newcomers on their land in their own narratives, organize themselves in relation to other neighbors, and espouse an ancient logic of mixed race that is cultural as well as biological. The Nahuas, for their part, draw on a recent founding narrative, and although possessed of an accepted ethnic identity, they easily handle their relations with the non-Indian world, which are consequently not as traumatic as they are for the Teeneks.

These case studies reveal two locally differentiated modalities of collective affiliation: affirming 'insularity' on one hand, and accepting 'extroversion,' on the other. The division here is not between 'Indians ' and 'blacks,' nor between 'emblems' and 'enclaves,' but rather between 'newcomers' and 'already there.' This cleavage is part of a social phenomenon that is probably quite widespread, analyzed in the classic works of [START_REF] Elias | The established and the outsiders: A sociological enquiry into community problems[END_REF][START_REF] Elias | The established and the outsiders: A sociological enquiry into community problems[END_REF]) and noted by [START_REF] Dupuy | Wayana et Aluku: les jeux de l'altérité dans le haut Maroni[END_REF] in Guyana, for example.

From comparison to its prospects

If, instead of categorizing populations by ethnic or racial criteria or by their priority in time, we use our system of multiple comparisons based on types of group representation, we can see the cross-similarities between the groups, with each configuration underlining similarities or differences that are not based on a hypothetical supra-local ethnic affiliation, but rather on different uses of collective discourses that, depending on the case, infuse a coherent mythology/cosmology and/or cobble together or invent group traditions in the midst of contradictions and impositions of all kinds.

The flexibility of these concepts is, furthermore, easily confirmed by the recent changes made in the definitions of, for example, the 'black' populations in Latin America. For proof of the feasibility and efficacy of identity reinvention, we have only to look at how, in Colombia from the 1990s onward, an intensive popular mobilizing effort, arising in part from legislative changes that finally opened the door to the recognition of 'blacks' as an 'ethnic group' forming part of the nation, permitted a complete, almost essentialist, redefinition-political, cultural, social-of that group. Moreover, it can be expected that sooner or later Mexico will have to join the other Latin American countries in recognizing a 'black mobilization' of which we are now seeing the first nascent signs. Lately Mexico's Indian populations, too-currently undergoing major structural changes due largely to the processes of emigration-have been reformulating their concept of 'Indian identity.' That concept is no longer directly related to the land and to the system of magic and ritual that went along with it in the days of agrarian culture, when the social reproduction of Indian groups was closely associated, both practically and ideologically, with their immediate natural environment. Contemporary identification with the community of origin takes different forms-and those forms, too, are occasionally tinged with essentialism and shaped by the external discourses produced by the state apparatus or ethno-political movements.

Conclusion

In this article we have explored three different ways of configuring related identity categories: the official break-down, in which the two Indian groups are contrasted with the two black groups; the national imaginary in which one of the Indian groups studied is classed with one of the black groups in the category of national emblem, albeit a subaltern one, while the other two groups-black and Indian-come under the rubric of marginalized enclave; and finally, the third configuration, which derives from another cross-positioning between the black and Indian groups studied, this time based on the criteria of insularity or extroversion. Each of these configurations could be set out as is or modified according to other parameters. Above all, these three options can combine, coexist, reinforce each other, or conflict with each other. Accordingly, these data do not allow here to propose 'a model' of relations. These configurations rather illustrate how social multi-dimensionality can be conceptualized for the purposes of better understanding the ways of relating in specific contexts.

Let us recall that the four groups concerned here do not make ethno-political claims and do not make use of the administrative ethnic categories usually brought into play in the interactions with other political actors. This situation makes it thus possible to draw attention to original and local forms of identifications. As Escobar stated about Colombian Pacific coast, those forms do not borrow Indians or black identity politics rhetoric, but leave room to specific narrative productions, anchored in the local cultures and histories (Escobar 2008:10). For sure, these modes of local identifications are always related to specific historical structures of domination.

Finally, the stories of domination as constructed and transmitted through a group's representation of itself and the other in memorial narratives reflect an autonomous sense of history. Indigeneity is thus always a negotiated position, a structural relationship with history and power (Canessa 2008), and never an essentialized, static situation.

The analysis presented in this paper, concerning the positions of four social groups categorized by their respective governments as 'Indians' or 'blacks,' shows that their ways of identifying themselves socially cannot be systematically considered as 'Indian culture' or 'black culture,' but rather reflect a relationship to place and history, a relationship that cuts across these classifications and suggests others. We do not reject the relevance of the governmental categorizations, which are historically and culturally constructed (de la Cadena & Starn 2007), nor do we deny the processes of ethnification that may result from them. We seek rather to understand how social identity is categorized, by examining the subjective perceptions of groups that are targeted by contemporary Latin American multicultural policies.

Far from being a stand on principle, our approach arose out of an intellectual exchange over two prolonged and quite different (both anthropologically and geographically) field experiences carried out in two different countries with distinctly different national histories. A comparison of our case studies revealed some congruencies: first, the impact of distinct national traditions on the relationship between the state and minority groups-traditions that affect ethnic configurations; and second, the existence of unexpected intersections among the positions of individuals and groups both between the two countries and within each of them.

Accordingly, we have made a systematic, concentrated comparison of the positionings of four social groups officially classified as 'blacks' or 'Indians' by the state, in order to grasp the way that the group's place in relation to other social groups is understood from within. The result is a weakening in the 'logic' of ethnic or racial borders, and a blurring of externally created identifications and classifications-which are based on cleavages assumed to be 'traditional' or even 'natural' between groups.

By paying attention to the locally expressed collective memory and the vision it proposes of the historical formation of the group, we were able to contrast national ideological constructions concerning different sectors of the population, on one hand, with the ideological constructions of four groups, blacks and Indians in Colombia and Mexico, concerning their own positions in relation to other social groups, on the other hand. The focus of our comparison was the tools of memory construction-that is, the myths, rituals, and narratives that feature the group versus 'others.' Our demonstration confirmed an idea that is widespread but that has never been adequately documented ethnographically: Identities are always negotiable and flexible, resisting the classifications devised and imposed by the hegemonic powers.

The three showed cases, or 'systems,' ultimately raise the question of the social attitude towards time and space, inflected according to both the regime of historicity (Hartog 2003) and that of temporality [START_REF] Dubar | Les temporalités dans les sciences sociales: introduction[END_REF] activated by the societies studied. Depending on how history is written or retold, the collective insertion in the national scene varies according to the different configurations of the group and the 'others' built by this history. It is not, in fact, 'ethnicized (or racialized) nature' that determines a nation's relation to the other, but rather one's own self-perception that allows-or does not allow-one to approach others. And this connection is accomplished by 'putting it into words,' or 'organizing' and sharing history in the form of a narrative or more or less mythic story.

Our comparison between cases also underlines the role of appropriated space as a foundation for myths and identity narratives-although the appropriation is neither 'cultural' nor immanent, contrary to that currently represented by the image of the sacred ancestral territory. It is thus not official, declarative 'autochthony' that determines a people's relation to land, but a historically constructed 'insularity' that makes possible, or impossible, the elaboration of mythical or historical stories that explain and justify the group's presence in a given spot.

CANESSA, ANDREW 2008 'The past is not another country: Exploring indigenous histories in Bolivia ', History and Anthropology, vol. 19, no. 4, pp. 353-69 CAREY, DAVID 2007 'Elusive identities: indigeneity and nation-states in central America ',Ethnohistory,vol. 54,no. 3,pp. ', Anthropology Today, vol. 20, no. 2, pp. 4-9 KUPER, ADAM 2003'The return of the native', Current Anthropology, vol. 44, no. 3, pp. 389-402 LEÓN-PORTILLA, MIGUEL 1997[1956] La filosofía náhuatl, estudiada en sus fuentes, México: Instituto Indigenista Interamericano LEWIS, LAURA 2001 'Of ships and saints: History, memory, and place in the making of Moreno Mexican identity ',Cultural Anthropology,vol. 16,no. 1,

  547-54 CHAKRABARTY, DIPESH 2002 Habitations of modernity: Essays in the wake of subaltern studies, Chicago: University of Chicago Press CHILDS, JOHN B., & DELGADO-P., GUILLERMO 1999 'On the idea of the indigenous', Current Anthropology, vol. 40, no. 2, pp. 211-12 CHIVALLON, CHRISTINE 2007 'Retour sur la 'communauté imaginée' d'Anderson. Essai de clarification théorique d'une notion restée floue', Raisons Politiques, vol. 27, no. 3, pp. 131-72 COMAROFF, JOHN, & COMAROFF, JEAN 1992 Ethnography and the historical imagination, Boulder: Westview Press CUNIN, ÉLISABETH, coord. 2010 Mestizaje, diferencia y nación. Lo "negro" en América Central y el Caribe, Mexico: Instituto nacional de Antropología e Historia / Centro de estudios Mexicanos y Centroamericanos / Universidad Nacional Autónoma de México / Institut de Recherche pour le Développement DE GRANDA, GERMÁN 1973 Chigualo en el litoral Pacífico del departamento de Nariño (Colombia), Madrid: Talleres gráficos Vda de Bermejo --1977 Estudios sobre un área dialectal hispanoamericana de la población negra: las tierras bajas accidentales de Colombia, Bogotá: Instituto Caro y Cuervo DE LA CADENA, MARISOL, & STARN, ORIN, eds. 2007 Indigenous experience today, Wenner-Gren International Symposium Series, Oxford: Berg GROS CHRISTIAN & STRIGLER, MARIE-CLAUDE eds. 2006 Être indien dans les Amériques. Spoliations et résistances, mobilisations ethniques et politiques du multiculturalisme, Paris: Éditions de l'Institut des Amériques and Éditions de l'Institut des Hautes Études de l'Amérique Latine HARTOG, FRANÇOIS 2003 Régimes d'historicité. Présentisme et expériences du temps, Paris: Seuil HATHAWAY, MICHAEL 2010 'The emergence of indigeneity: Public intellectual and an indigenous space in southwest China', Cultural Anthropology, vol. 25, no. 2, pp. 301-33 HOFFMANN, ODILE 2004 Communautés noires dans le Pacifique colombien. Innovations et dynamiques ethniques, Paris: IRD-Karthala HOFFMANN, ODILE, & RODRIGUEZ, MARIA-TERESA eds. 2007 Los retos de la diferencia. Los actores de la multicuturalidad entre México y Colombia, México: CEMCA-CIESAS-ICANH-IRD KENRICK, JUSTIN, & LEWIS, JEROME 2004 'Indigenous peoples rights and the politics of the term 'indigenous

Notes

1 The word 'racial' refers here to the social and political construction that attributes to individuals and groups a 'difference' based on a combination of criteria (origin, culture, etc.), the main one being phenotypical appearance (skin color).

2 See Agudelo (2005) for Colombia and Ariel de Vidas (1994) for Mexico.

3 [START_REF] Restrepo | El 'giro al multiculturalismo' desde un encuadre afro-indígena[END_REF] contends that in Latin America there are three different models of state recognition of black and Indian populations: equivalence, circumstantial equivalence, and unthinkable equivalence. The standard is always the Indian and the trinomial 'culture + language + territory = un pueblo' (Green 2007: 345).


Davide Cuda

Raluca-Maria Indre

Esther Le Rouzic

James Roberts

Dynamic bandwidth allocation for all-optical wide-area networks †

Keywords: all-optical network, wide area network, dynamic bandwidth allocation, flow-aware networking

Nous proposons une architecture pour un réseau WAN tout-optique basée sur la notion de connexions optiques multipoint à multipoint, que nous appelons multipaths. L'ensemble des noeuds d'accès est partitionné en groupements pour l'émission et la réception. Une (ou plusieurs) longueur d'onde est allouée à chaque paire groupement-source groupement-destination. Les noeuds d'un même groupement-source se partagent la capacité de cette longuer d'onde selon un protocole MAC adapté. Les données transmises sur une longueur d'onde sont diffusées à tous les noeuds du groupement-destination, et chaque récepteur extrait alors les données qui lui sont destinées à partir du flux rec ¸u. Le réseau que nous proposons ne nécessite que des composants existants et se compare favorablement en termes de complexité et d'efficacité énergétique à des solutions alternatives comme la commutation optique par paquet (Optical Packet Switching -OPS) ou la commutation optique par rafale (Optical Burst Switching -OBS). Nous présentons d'abord l'architecture multipath et comparons sa consommation d'énergie celle d'un réseau classique à base de routeurs. Nous proposons ensuite un algorithme d'allocation dynamique de la bande passante. Nous évaluons la performance de l'algorithme proposé à l'aide de simulations et nous montrons que notre solution permet d'atteindre d'excellentes performances en terme de délai et temps de réponse.

Introduction

In today's networks, optical technologies such as Wavelength Division Multiplexing (WDM) are used to provide high-capacity point-to-point lightpaths consisting of a wavelength channel carried over a succession of fibres. As each wavelength has a capacity of 10 Gb/s or more, a lightpath is only used efficiently when its end points concentrate a large amount of traffic. In current networks, traffic is usually concentrated through a hierarchy of electronic transit routers. In this paper we propose a solution for performing lightpath sharing in the optical domain. Previous proposals for lightpath sharing include both point-to-multipoint and multipoint-to-point sharing. Point-to-multipoint lightpath sharing can be realized using splitters and optical time divison multiplexing as proposed in [START_REF] Petracca | Neri Design of WDM networks exploiting OTDM and lightsplitters[END_REF]. Multipoint-to-point lightpaths are obtained by merging signals from two or more incoming fibres on to a single outgoing fibre. This principle is used notably for upstream transmission in passive optical access networks (PONs). The capacity of the upstream channel is shared by means of a dynamic bandwidth allocation (DBA) algorithm.

We propose to use DBA to control sharing of multipoint-to-multipoint lightpaths, called multipaths for short, interconnecting edge nodes in a wide-area network. The proposed network uses presently feasible optical technology, namely tunable transmitters, burst mode receivers and wavelength selective cross-connects, to replace electronic routers in a country-wide ISP network.

We present the envisaged multipath infrastructure before proposing an EPON-inspired MAC protocol to control bandwidth sharing. We describe the scope for flexibly defining DBA algorithms within this framework and present some performance evaluations illustrating the attractiveness of a preferred realization.

From PON to WAN : extending the reach of dynamic bandwidth allocation

Proposed DBA algorithms are realized using either EPON or GPON standards. Our proposal derives mainly from EPON solutions where the OLT attributes to its ONUs non-conflicting time slots defined flexibly in terms of a start time and a duration. We believe this is sufficient to meet requirements and is preferred to frame-based GPON solutions for reasons of simplicity.

The distributed control implementation of TWIN proposed by Sanièe and Widjaja [START_REF] Saniee | Design and performance of randomized schedules for time-domain wavelength interleaved networks[END_REF] employs a form of dynamic bandwidth allocation. Each destination has its own wavelength and receives light signals from source nodes via a tree network created over an optical infrastructure. The destination grants source nodes time slots on its particular wavelength tree in much the same way an OLT grants time slots to its ONUs. The algorithm ensures bursts from remote sources do not collide within the network or at the destination. An EPON-like MAC protocol for TWIN and an associated DBA algorithm are described in [START_REF] Robert | A flow-aware MAC protocol for a passive optical metropolitan area network[END_REF].

TWIN is not suitable for a wide-area network. It has limited scalability since a distinct wavelength must be assigned to each destination. Its geographical span must be relatively small to limit delays due to report/grant signaling. While time slot collisions are avoided within each destination tree, destinations act independently and blocking can occur (i.e., grants cannot be completely fulfilled) when time slots overlap at a source, significantly reducing traffic capacity. On the other hand, a single destination may not have sufficient traffic to justify a dedicated multipoint-to-point lightpath.

Our proposed network of multipoint-to-multipoint lightpaths, or multipaths, avoids these problems [START_REF] Cuda | Getting routers out of the core : Building an all-optical network with 'multipaths[END_REF]. A cluster of sources shares a wavelength tree as in TWIN except that the root of the tree is not the destination but a controller that attributes slots to each source. The tree is extended beyond the controller towards a cluster of destinations. Light in this part of the multipath is split and amplified as necessary to ensure each destination receives a copy of the combined signal from the source cluster. Each destination converts this signal to electronic form and retains its own packets (as downstream transmission is performed in a PON). Only the control signals are converted to electrical form at the controller. Figure 1 illustrates a sample of the multipaths needed to interconnect a toy network of 100 edge nodes. Each node is assumed to send an average of 9 Gb/s distributed uniformly over all the other nodes. We form clusters of 10 nodes and create multipaths to interconnect them. Intra-cluster multipaths are also created, as in the lower righthand corner of the figure. Multipaths are assumed here to have a capacity of 10 Gb/s, sufficient to handle inter-and intra-cluster demand. The controller in each source cluster coordinates sharing on all its multipaths and can thus avoid blocking. It is close to the source nodes ensuring negligible signalling delays. Each edge node is equipped with one receiver for each of 10 incoming multipaths and one tunable transmitter.

Figure 2 represents a traditional network interconnecting the same 100 edge nodes using 10 electronic transit routers to concentrate traffic onto high capacity point-to-point optical links. The multipath network avoids this transit layer leading to a significant economy in equipment and energy. A more realistic example Dynamic bandwidth allocation for all-optical wide-area networks is worked out in [START_REF] Cuda | Getting routers out of the core : Building an all-optical network with 'multipaths[END_REF], demonstrating that the multipath approach readily extends to a country-wide ISP network of 420 edge nodes. It realizes a power saving of 1.7 MW by eliminating a total of 92 metro, regional and core transit routers.

3 Slot allocation : a MAC protocol ensuring maximal utilization All multipaths accessed by a source cluster share the same fibres up to the controller. A further wavelength on these fibres is used to constitute upstream and downstream control channels for report and grant signalling, respectively. The control channels are also used for synchronization and ranging. The EPON time stamp exchange protocol is used, allowing the controller to measure the round trip time RTT i between itself and each source i of its cluster and synchronizing each source clock to the controller clock time plus an unknown one-way propagation delay (see [START_REF] Robert | A flow-aware MAC protocol for a passive optical metropolitan area network[END_REF]). These data are used in the following grant timing algorithm.

Grant timing must account for a guard time ∆ g between burst emissions and for grant signalling delay that we suppose is bounded by some value τ. In evaluations reported below we assume ∆ g = 100 ns and τ = 1 ms. The process of grants emitted by the controller to the source nodes of a given multipath j is specified by the functions g j (.), s j (.) and d j (.) defined as follows. The n th grant sent for multipath j to some source is formulated by the controller at time g j (n) and instructs the source to transmit for duration d j (n) starting at source local time s j (n). Assume the (n + 1) th grant is issued to source i. Epochs g j and s j are calculated recursively, as follows.

g j (n + 1) = g j (n) + d j (n) + ∆ g , (1) s
j (n + 1) = g j (n + 1) + ∆ O -RTT i , (2)
where

∆ O is an offset satisfying ∆ O ≥ max i (RTT i ) + τ.
It can be proved that this defines a feasible schedule ensuring maximal lightpath utilization (see [START_REF] Robert | A flow-aware MAC protocol for a passive optical metropolitan area network[END_REF]). The choice of source i for the (n + 1) th grant is not specified in the above recursion. If the choice were made independently for each multipath, grants might overlap leading to capacity loss due to transmitter blocking. This can be avoided here since, as proved in [START_REF] Cuda | Getting routers out of the core : Building an all-optical network with 'multipaths[END_REF], the controller can always select a source i for the (n + 1) th grant that is known to have an available transmitter at the calculated start time s j (n + 1).

Reporting and granting : a flexible choice of DBA algorithm

As for a PON, it is possible to design a wide variety of DBA algorithms using the above MAC protocol. These are distinguished by the content of reports and the manner in which grants are attributed. We briefly outline the scope for choice before presenting the algorithm we recommend for a wide-area ISP network.

Reports for all multipaths of a source cluster are delivered using a common signalling channel. Management of this channel is flexible beyond the requirement that the arrival of new traffic must be signalled in a timely manner to the controller. Each report indicates the current status of its packet queues, distinguishing flows of diverse granularity and/or classes of service, as required by the implemented service model.

Grants are calculated at the instants g j defined above using the latest reports received for the considered source and multipath. The source may be selected in simple round-robin order or by using a more complex state dependent algorithm, accounting for a given priority policy, say (while avoiding blocking between multipaths, as previously discussed). It is also possible that grants be attributed without prior reports, as envisaged in the GPON "non-status reporting" option.

Our preferred approach for the wide-area network is a flow-aware DBA. We assume application flows can be reliably identified and that sources report the current number of active flows, i.e., flows having at least one packet in the multipath buffers. The controller calculates grant time slot durations d j to accommodate one "quantum" for each such flow. Sources are given grants in round-robin order. They use the grants to serve active flows also in round-robin order. To give priority to low rate flows, any flow that newly becomes active is placed at the head of the round-robin schedule. This scheme realizes network-wide, per-flow fair bandwidth sharing with well-known traffic control advantages (e.g., see [START_REF] Oueslati | Roberts A new direction for quality of service : Flow aware networking[END_REF]). It minimizes the delay of low rate flows ensuring streaming audio and video flows, that naturally fall into this category, see negligible packet latency. We have simulated the toy network of Figure 1 under the following traffic model. Flows arrive according to a Poisson process and are one of two types : high speed data flows able to transmit at up to the 10 Gb/s multipath rate or video streams emitted at the constant rate of 2 Mb/s. The size of data flows is 10 MB on average and has an exponential distribution. The duration of video flows is also exponential with a mean of 30 s. The flows share multipath bandwidth according to the flow-aware DBA scheme outlined above. Each source has a single tunable transmitter. Network parameters are τ = 1 ms, ∆ g = .1µs, RTT i = i × 100µs for i = 1, . . . , 10.

Figure 3 shows the mean response time of data flows for a traffic mix with 20%, 40% or 80% of data. We see that this parameter has limited impact on performance which is excellent until demand attains multipath capacity (load→1). Figure 4 plots the delay experienced by packets of the video flows. These packets suffer an average delay of 2.5 ms at very low load due to the signalling exchange (1-2) and the assumed control channel implementation. Delay then decreases due to the priority mechanism before exploding when demand attains capacity.

Conclusion

The present proposal demonstrates that is possible to do without costly and power hungry electronic transit routers in a wide-area network using presently available optical technology. Specifically, we propose to use available technology such as rapidly tunable transmitters, burst mode receivers and cross-connects to create multipoint-to-multipoint lightpaths called multipaths. Multipath bandwidth is shared between sourcedestination traffic flows under the control of a MAC protocol designed to avoid collisions at merge points and destinations. The use of DBA in this context is an original approach to all-optical networking that we believe has considerable potential. We are currently working on applying this approach to interconnect servers in a datacenter network.

FIGURE 1 :

 1 FIGURE 1: Network of multipaths -showing just 4 multipaths.

FIGURE 2 :

 2 FIGURE 2: Traditional network using transit routers.

FIGURE 3 :

 3 FIGURE 3: Mean response time of data flows .

FIGURE 4 :

 4 FIGURE 4: Mean packet delay of video flows. 5 Performance of the flow-aware DBA

† The research leading to these results has been partially funded by the FP 7 TREND Network of Excellence.