Fabrice Theoleyre
email: theoleyre@unistra.fr

Georgios Z Papadopoulos
email: georgios.papadopoulos@telecom-bretagne.eu

Experimental Validation of a Distributed Self-Configured 6TiSCH with Traffic Isolation in Low Power Lossy Networks

Keywords: IoT, 6TiSCH, IEEE802.15.4-2015-TSCH, distributed reservation, self-configuration, traffic adaptive, experiments

Time Slotted Channel Hopping (TSCH) is among the proposed Medium Access Control (MAC) layer protocols of the IEEE 802.15.4-2015 standard for low-power wireless communications in Internet of Things (IoT). TSCH aims to guarantee high network reliability by exploiting channel hopping and keeping the nodes time-synchronized at the MAC layer. In this paper, we focus on the traffic isolation issue, where several clients and applications may cohabit under the same wireless infrastructure without impacting each other.

To this end, we present an autonomous version of 6TiSCH where each device uses only local information to select their timeslots. Moreover, we exploit 6TiSCH tracks to guarantee flow isolation, defining the concept of shared (best-effort) and dedicated (isolated) tracks. Our thorough experimental performance evaluation campaign, conducted over the open and large scale FIT IoT-LAB testbed (by employing the OpenWSN), highlight the interest of this solution to provide reliability and low delay while not relying on any centralized component.

INTRODUCTION

During the last years we have experienced the emergence of a new paradigm called Internet of Things (IoT) in which smart and connected objects cooperatively construct a (wireless) network of things [START_REF] Atzori | The internet of things: A survey[END_REF].

In an IoT deployment, energy-efficiency is one of the most important parameters, since smart objects have to save energy to meet the lifetime requirements of typical applications. Since MAC layer is responsible for controlling the main source of energy consumption [START_REF] Zhi | A Survey and Projection on Medium Access Control Protocols for Wireless Sensor Networks[END_REF], therefore, a strong focus has been put on the medium access: the devices should turn OF F their radio for most of the time.

Besides, many modern applications cannot accommodate the best effort approach. For instance, losing one packet for a smart meter application means the measures are incorrect for this subscriber. More importantly, several applications or even clients should cohabit under the same wireless infrastructure to reduce the cost of deployment [START_REF] Gaillard | Service level agreements for wireless sensor networks: A wsn operator's point of view[END_REF]. These requirements lead researchers to design deterministic algorithms for medium access. Thus, the wireless infrastructure is able to provide stable and predictable performance. IEEE 802.15.4-2015 standard was published in 2016 [START_REF]IEEE Standard for Low-Rate Wireless Personal Area Networks (LR-WPANs)[END_REF], to offer a certain quality of service for deterministic industrialtype applications. Among the MAC schemes defined in this standard, Time-Slotted Channel Hopping (TSCH) is for lower-power and reliable networking solutions. Indeed, by adopting a channel hopping approach, the standard makes the transmissions more reliable [START_REF] Watteyne | Reliability through frequency diversity: Why channel hopping makes sense[END_REF]. Besides, TSCH relies on scheduling where the contention is solved deterministically, and each radio link receives a given amount of bandwidth. Thus, TSCH is particularly efficient to save energy: a node has to stay awake only when it transmits or receives a frame.

6TiSCH IETF Working Group (WG) is currently defining a set of protocols to fill the gap between IPv6 and TSCH [6]. It aims at executing IPv6 over a reservation based MAC by reserving TSCH slots. Moreover, 6TiSCH introduces the concept of tracks, where certain timeslots are mapped to a given flow, guaranteeing thus traffic isolation. Different flows from different applications are mapped to different tracks. We consider this feature as a key enabler for reliability in IoT. Recently, Orchestra proposed an innovative way to exploit the 6TiSCH architecture, highlighting the relevance of a distributed version [START_REF] Duquennoy | Orchestra: Robust mesh networks through autonomously scheduled tsch[END_REF]. However, it does not guarantee traffic isolation since it does not implement any track management solution.

Our 6TiSCH version is resilient: we propose to adapt the schedule locally, based on the radio link quality and the actual amount of traffic to forward. We do not rely on a centralized component which would increase the delay to react to changes. In this paper, we present a distributed version of 6TiSCH to guarantee traffic isolation and deterministic performance.

The contributions presented in this paper are threefold:

1. We first propose a distributed implementation of 6TiSCH with traffic isolation using tracks, where each pair of nodes negotiates the required bandwidth.

2. We then describe mechanisms so that each node decides locally which timeslot to use, and for which track;

3. We perform a thorough experimental evaluation over the FIT IoT-LAB testbed, to highlight the feasibility of an high reliability and low delay solution.

BACKGROUND AND RELATED WORK

Hereafter, we first provide the necessary background about TSCH and 6TiSCH. We then review the existing contributions that provide high reliability with deterministic performance for Low-Power Lossy Networks (LLNs).

TSCH Overview

As previously mentioned, TSCH is among the MAC protocols defined in the IEEE 802.15.4-2015 standard [START_REF]IEEE Standard for Low-Rate Wireless Personal Area Networks (LR-WPANs)[END_REF] that provide high-level reliability and low-power operation in LLNs.

In TSCH networks, a schedule is computed and distributed among the nodes, therefore nodes must remain time synchronized throughout the network deployment. Moreover, under TSCH operation, time is divided into timeslots of equal length, large enough to transmit a frame and to receive an acknowledgement. A set of timeslots constructs a slotframe. At each timeslot, a node may transmit or receive a frame, or it may turn its radio OF F for saving energy. Finally, each timeslot is labelled with Absolute Sequence Number (ASN), a variable which counts the number of timeslots since the network was established. Thus, based on ASN and its scheduler, each node decides when to transmit or receive a frame.

Furthermore, to defeat noise and interference, and consequently to enable high reliability, TSCH proposes to implement a channel hopping scheme. To each transmission opportunity is attached a channel offset. In TSCH, a cell is a transmission opportunity described by a pair of timeslot and channel offset. Thus, at the beginning of each timeslot, the selection of the actual channel is derived from the channel offset and the ASN.

Finally, in TSCH, a cell may be either shared or dedicated, see Fig. 1. In the former mode, several interfering nodes are authorized to transmit: they must execute a slotted CSMA-CA mechanism to avoid collisions. In the latter case, a collection of non interfering transmitters are the owners of the cell: they transmit in contention-free mode.

6TiSCH Overview

6TiSCH IETF WG aims at defining protocols to bind IPv6 (i.e., 6LoWPAN) and reservation based MAC layer (i.e., TSCH). In 6TiSCH minimal [START_REF] Vilajosana | Minimal 6TiSCH Configuration[END_REF], one shared cell is reserved at the beginning of the slotframe to exchange control packets (cf. Fig. 1). For instance, Enhanced Beacons (EBs) are transmitted during the shared timeslot so that the neighbors may associate with the existing network, while the rest of the slotframe comprises dedicated cells.

Furthermore, 6TiSCH makes a distinction between the protocol which defines how to negotiate the cells (i.e. 6P [START_REF] Wang | 6top protocol (6p)[END_REF]) and the algorithm deciding how much cells to allocate in the schedule (the Scheduling Function such as SF0 [START_REF] Dujovne | 6TiSCH 6top Scheduling Function Zero (SF0)[END_REF]). The solution is very flexible since any scheduling algorithm may be practically implemented: a new Scheduling Function has just to be defined and interfaced with 6P. Thus, 6P may work either in a centralized manner (e.g. a node asks a Path Computation Element for new cells to use) or in a distributed manner (e.g. SF0 decides how many cells to allocate based on the local measures).

6TiSCH introduces the concept of track [START_REF] Thubert | An Architecture for IPv6 over the TSCH mode of IEEE[END_REF]. A track corresponds to dedicated radio resource, along with a multihop path. More precisely, a set of cells (a bundle) is reserved for each hop. By selecting different cells for different data flow, 6TiSCH may provide traffic isolation. A track forwarding scheme is in this case applied: when 6P receives a frame to forward, it automatically finds the outgoing bundle associated with the incoming cells. Furthermore, 6TiSCH introduces the concept of chunk : each node is able to separate the scheduling matrix in non overlapping chunks [START_REF] Dujovne | 6tisch: deterministic ip-enabled industrial internet (of things)[END_REF]. Then, the churns are allocated (in a centralized or distributed manner) to different nodes, so that each of them can pick in its churn when it has to allocate new cells.

In Fig. 1, the flow from A to the border router R, via node B, will be assigned to track 1. Besides, the same node (e.g., B) may forward an additional flow, for instance from C, using a different track (i.e., 2).

Scheduling

6TiSCH may rely either on an centralized scheduler (i.e., the Path Computation Element (PCE)) or on a distributed algorithm.

Centralized Scheduling

Several scheduling algorithms can be used by 6P. In [START_REF] Tsitsiklis | On the power of (even a little) centralization in distributed processing[END_REF], the tradeoff between a centralized and a distributed scheduling is studied. By adopting a queue theory approach, the authors showed that a centralized one is more efficient.

Ghosh et al. [START_REF] Ghosh | Multi-channel scheduling algorithms for fast aggregated convergecast in sensor networks[END_REF] proposed to minimize the schedule length in a multichannel TDMA environment. Similarly, TASA proposed to construct a centralized scheduling for a multihop TSCH network achieving the same objective [START_REF] Palattella | On Optimal Scheduling in Duty-Cycled Industrial IoT Applications Using IEEE802.15.4e[END_REF].

Yigit et al. [START_REF] Yigit | On the interdependency between multi-channel scheduling and tree-based routing for WSNs in smart grid environments[END_REF] studied the impact of routing on scheduling. The nodes have to use a larger number of timeslots when the radio links are unreliable. Schedex [START_REF] Dobslaw | End-to-End Reliability-aware Scheduling for Wireless Sensor Networks[END_REF] proposed to schedule additional timeslots until an end-to-end delay constraint is not anymore respected.

All these centralized approaches assume the interference can be estimated accurately. They are particularly well suited for industrial networks in controlled environments.

Distributed Solutions

To not rely on a centralized controller, certain distributed solutions have been proposed. DeTAS presented a decen-tralized version of TASA [START_REF] Accettura | Decentralized traffic aware scheduling for multi-hop low power lossy networks in the internet of things[END_REF], assembling micro-schedules. Phung et al. [START_REF] Phung | Schedule-based multi-channel communication in wireless sensor networks: A complete design and performance evaluation[END_REF] proposed to use a Reinforcement Learning based scheduling algorithm to cope with a variable traffic. However, the authors do not propose dedicated cells: the nodes have always to execute a CSMA-CA phase before transmitting their packets. In the same way, Z-MAC [START_REF] Injong Rhee | Z-MAC: A Hybrid MAC for Wireless Sensor Networks[END_REF] proposes a mix between CSMA and TDMA, where timeslots are assigned distributively, solving iteratively the collisions.

Orchestra was proposed recently [START_REF] Duquennoy | Orchestra: Robust mesh networks through autonomously scheduled tsch[END_REF]. As exposed previously, Orchestra uses dedicated cells selected according to a pseudo-random sequence. However, it does not implement tracks (i.e., traffic isolation) and, thus, may suffer from the funneling effect [START_REF] Wan | Siphon: Overload traffic management using multi-radio virtual sinks in sensor networks[END_REF], since each nodes reserves the same number of timeslots with its parent. SF0 [START_REF] Dujovne | 6TiSCH 6top Scheduling Function Zero (SF0)[END_REF] proposed a Scheduling Function to compute the number of cells to allocate/remove. Palattella proposed also to use an hysteresis function to decide when new cells have to be added or removed [START_REF] Rita | On-the-Fly Bandwidth Reservation for 6TiSCH Wireless Industrial Networks[END_REF]. While it presents a promising approach, they did not present experimental evaluation, and moreover, they didn't describe the integration of tracks to provide traffic isolation (the bandwidth estimation algorithm is let to a future work).

A FULLY DISTRIBUTED VERSION OF 6TISCH

Hereafter, we will present our distributed 6TiSCH architecture where all decisions are taken locally, and guaranteeing traffic isolation: different applications may be assigned to different tracks, and use consequently different cells.

• the shared cells are only used to transmit 6P packets (unicast) and Enhanced Beacons and DIO (broadcast);

• the other unicast packets such as DAO or data packets use dedicated cells, reserved for a given track.

In other words, data transmissions use the deterministic part of TSCH, while random access is only reserved for 6P negotiations and beacons.

Track Management

6TiSCH track consists in one track owner (64 bits) and one track ID (16 bits). We propose here a way to exploit these tracks to enable the following features: Isolated track: a source node has to reserve one end-toend track for its flow. To guarantee traffic isolation, the track must be only used by one data flow. Thus, the track owner is its own MAC address (64 bits) and the track ID (16 bits) corresponds to the application in execution on the node. The track ID may be assigned uniquely before the deployment, following the same approach as the well known ports. Thus, a network may support at most 65,535 different applications.

Convergent track (shared track, e.g., alarms): because data transmissions are in this case unfrequent, reserving one cell during each slotframe may waste bandwidth. However, we must guarantee small end-to-end delays for alarms, and the maximum time between two cells should be minimized. In particular, if an event is detected by several nodes, the delay of the first exemplary received by the border router is the most important one. We propose for these applications to reserve a common convergecast track, shared among all the sources of the same application. Thus, the track owner is the address of the destination, and the track ID is specific to the application. All the other nodes which generate data packets to this destination will, by definition, use the same track. It is important to note that the packets from different sources do not collide: they use dedicated cells. However, the packets after the first one have to be buffered, increasing the delay, and the probability of a buffer overflow if a large amount of traffic is generated at the same time.

When packets are routed via the border router, the track owner is extracted from the RPL DAG Information Object (DIO).

Furthermore, we may mix different applications in the same network. In Fig. 2, the smart metering application uses isolated tracks (blue and green for two applications). Bandwidth reserved for the the blue track cannot be used by the green flow. On the contrary, the application on A and C use a convergent track (in red). Let's imagine that both A and C generate one packet during the same slotframe. B has only one outgoing cell to forward the two packets: the second one has to be forwarded in the next slotframe.

The convergent track strategy represents well the behavior of classical wireless sensor networks: each source may generate an indefinite number of packets, and the relay nodes expects to forward them in a best effort manner. However, different applications use different tracks, and therefore orthogonal radio resources. In other words, we multiplex only the transmissions from the same application (and different source nodes).

Note that we do not implement a GMPLS-like approach to manage the tracks, as advocated by [START_REF] Thubert | An Architecture for IPv6 over the TSCH mode of IEEE[END_REF]. We insert rather information in the 6P transaction to specify which track is associated with the cells to insert in the schedule. For this purpose, our Scheduling Function may use the metadata field of the 6P Add Request message. Later, when a node receives a data frame, it has to extract the track associated with the incoming cell, and tag the frame accordingly in the queue. We also implemented a FIFO discipline: the node picks up the first packet from the queue with the trackID corresponding to the outgoing dedicated cell.

Bandwidth Requirement Estimation with SF

We propose a simple strategy to allocate the bandwidth on-demand. The Scheduling Function SF0 [START_REF] Dujovne | 6TiSCH 6top Scheduling Function Zero (SF0)[END_REF] collects the bandwidth requirements from neighbor nodes. Note that it does not detail how per track requirements are computed. We here propose a Scheduling Function SF loc which takes decisions considering only local information. Besides, SF loc allocates different cells for different tracks. In this way, we are able to compute a schedule in a distributed manner while maintaining traffic isolation: two applications using different tracks will be managed independently by SF loc . Besides, we do not require each application defines a priori its requirements: SF loc is self-adaptive and learns from the forwarded packets.

In our explanations, we focus on the convergecast traffic pattern. A node has consequently to negotiate with its RPL preferred parent a list of cells to use for each of its tracks. A node, which has to decide how many cells to reserve, considers only its own buffer and its schedule utilization.

Cell insertion

We have to decide when a new cell has to be negotiated with a neighbor. We follow a similar approach to SF0, while only using the amount of forwarded traffic to compute the bandwidth requirements:

1. when a packet is inserted in the queue, SF loc is triggered;

2. if the node already has an on-going 6P transaction, SF loc stops: only one track may be modified at the same time;

3. for each track tr, the node computes:

(a) the list of outgoing cells Cout toward the parent and the average number of transmissions required before receiving an acknowledgment (∀k ∈ Cout, ET X(k));

(b) the number of packets present in the queue for this track N pk (tr) (c) if the following condition does not hold:

k∈C out 1 ET X(k) ≥ N pk (tr) (1)
SF loc asks 6P to reserve the following number of cells:

N pk (t) - k∈C out 1 ET X(k) (2)
(upper bounded by 3 since an 6P Information Element may contain at most 3 timeslots).

Our approach is robust to interference: a colliding cell will present a larger Expected Transmission Count (ETX), and will be detected quickly, using the same mechanism as [START_REF] Muraoka | Simple distributed scheduling with collision detection in tsch networks[END_REF] (tx-housekeeping feature). A bootstrapping node will reserve quickly new cells for the given track: this over-provisioning efficiently deals with lossy links. The colliding cells will be detected and removed later.

We plan in the future to implement a more sophisticated mechanism. Recently, Domingo-Prieto et al. [START_REF] Domingo-Prieto | Distributed pid-based scheduling for 6tisch networks[END_REF] proposed to avoid oscillations by considering the dynamics of the bandwidth requirement. The solution was evaluated by simulations and seems promising.

We will detail in the next subsection how 6P selects the actual cells (timeslot and channel offset) to use.

Cell removal

The routing topology might changes since RPL is dynamic. In the same way, a node may be too aggressive and may have reserved too much bandwidth, when e.g., a new parent is selected and it has to empty its buffer. A node has to later remove the unused cells.

To select the cells to remove, we consider a given cell becomes useless if no ack is received during a long time. This cell is considered useless even if unicast packets are transmitted but not acknowledged. In this way, we remove both the colliding and the unused cells. Indeed, no contention is implemented for dedicated cells: the data packets will be deterministically lost if a pair of interfering radio links uses the same cell.

To this end, we propose a simple timeout based approach: when a cell is not used during at least T imeoutSF loc -tx seconds, the owner of the cell (the transmitter) asks the receiver to release this cell. More precisely, 6P sends a link-removal request, which specifies the timeslot and the channel offset to remove from the schedule.

Possibly, the radio link may not be anymore usable. Thus, the transmitter removes silently a cell if the 6P link-removal failed after all the possible retransmissions.

Inversely, a receiver removes silently a cell if it is unused during T imeoutSF loc -rx. To avoid inconsistent decisions, the following condition must hold:

T imeoutSF loc -tx << T imeoutSF loc -rx (3)
In particular, it must take into account the time to transmit the link-removal request and to receive the associated ACK.

Distributed Scheduling With 6P

To make 6TiSCH entirely distributed, a pair of nodes must be able to decide which cell should be used to exchange packets, without the help of a centralized PCE.

We present here two different approaches to either reduce the number of collisions (random) or to reduce the end-toend delay (contiguous).

Random Scheduling

When the nodes select autonomously the cells to use with their parent, they do not have a vision of the cells already reserved by an interfering transmitter. To reduce the probability of collision, we propose to select randomly the cells to use: we minimize the probability to have colliding cells in the global schedule. As it is shown in performance evaluation Section, this simple and greedy approach is efficient and robust.

When a node has to reserve k new cells, 6P must ask its preferred parent to reserve some additional bandwidth for a given track, using a 2-step 6P transaction:

1. the transmitter Nt selects j timeslots (j ≥ k), selected randomly among the available timeslots in its schedule. It selects one random channel offset for each of these timeslots;

2. Nt constructs a link-request including these cells and transmits the packet during a shared cell;

3. the receiver Nr selects the first k cells present in the link-request and available in its schedule. It enqueues a link-reply, which will be transmitted during one shared slot. If there is not enough available cells (< k), Nr refuses the transaction and sends an empty negative link-reply. Else, the reply includes the list of the selected cells.

4. Nt receives the link-reply and updates its schedule with the cells allocated by its parent if the reply is positive.

Selecting randomly the cells to use minimizes the probability of collision, but tends to increase the end-to-end delay. Intuitively, the outgoing and incoming cells being chosen randomly, the end-to-end delay equals on average:

delay = hops * SF length * T slot 2 (4)
With hops being the number of hops in the route, SF length the slotframe length, and T slot the timeslot duration (by default 15ms). Besides, a node, which forwards many flows, will have to buffer many packets since the outgoing and incoming cells may be far away. Thus, some packets may be dropped because of buffer overflows. However, as we highlighted in our experiments this effect is limited.

Contiguous Scheduling

In this second approach, we focus on minimizing the endto-end delay. To this aim, we propose to minimize the buffering delay:

1. With SF loc , the transmitter Nt computes the number of cells (= k) to reserve toward its parent for the track tr. It identifies in its schedule the last slot number T rx last (tr) which is used to receive a packet for the track tr.

• If no incoming cell exists (i.e., Nt generated the packet), T rx last (tr) is chosen randomly;

2. Nt selects the first available cells after T rx last (tr) and chooses randomly one channel offset for each of them. It constructs a link-request to piggyback this list;

3. The receiver Nr selects the first k cells from the linkrequest also available in its schedule. The following cases may occur:

(a) at least k cells are available: Nr selects the k first of them, and constructs the associated linkreply as usually;

(b) not enough cells are available:

i. Nr constructs a link-reply and piggybacks an Information Element which lists the busy cells. ii. Nt receives this link-reply and will insert in its own schedule the blacklisted cells, linked with the parent. When Nt will send another link-request to Nr, theses busy cells cannot be selected.

PERFORMANCE EVALUATION 4.1 Experimental Setup

Our experimental campaign was conducted over the FIT IoT-LAB platform in Grenoble [START_REF] Georgios | Adding value to WSN simulation using the IoT-LAB experimental platform[END_REF]. This federated platform provides access to a large collection of motes. We employed the m3 nodes, based on a STM32 (ARM Cortex M3) microcontroller (ST2M32F103REY). It embeds an AT86RF231 radio chipset, providing thus, an IEEE 802.15.4 compliant PHY layer.

Topology

FIT IoT-LAB allows for several experiments to be executed concurrently. Thus, we evaluate the behavior of 6TiSCH under real-world conditions, with potential external interference. The nodes are placed in corridors, in false ceilings and floors, to mimic smart buildings scenarios. To construct multihop topologies, we select nodes sufficiently far from each other: we alternate between ceils and floors, and two consecutive nodes are separated by approximatively 1.5meters. We selected the nodes in the corridor located in the middle (cf. Fig. 3).

Default parameters

Furthermore, we employed the OpenWSN * , that provides an open-source implementation of the 6TiSCH stack (i.e., IEEE802.15.4-2015 TSCH, 6P, SF, 6LoWPAN and RPL). We implemented in OpenWSN our proposal to handle tracks, schedule appropriately the cells, etc. Our modifications are freely available on GitHub † .

We use the default parameters value as depicted in Table 1. All nodes take local decisions, based on the information exchanged with their neighbors (EBs, 6P requests and replies). Each mote sends its packets to the border router according to a CBR traffic, see Table 1. To dissociate the impact of traffic and of the network size, we maintain the number of packets generated constant in the network, whatever the number of nodes. Thus, each node generates by default one data packet every 10 * nbN odes seconds.

Queue management

Because of tracks, the buffer does not follow a FIFO approach: data packets assigned to a track are buffered until the node enters in a dedicated cell for this particular track. Due to memory constraints, the data buffer is limited, and thus, it may be filled quickly. To guarantee sufficient space for the control packets in the buffer, we reserve by defaults 4 slots in a buffer of 10 frames (cf. Table 1). To avoid privileging old packets in the queues, we implemented a timeout based mechanism, where each data packet is tagged at reception and is dropped after 8 seconds (≈6 slotframes) if it is still present in the queue. We first study the network behavior during the convergence, on a simple 2 hops topology, by selecting an appropriate set of nodes in the same corridor of the Grenoble's IoT Lab topology (cf. Fig. 3). The DODAG is constructed by RPL in a distributed manner.

Convergence in a simple topology

We first measure the delay performance (see Fig. 4a). As it can be observed, most of the packets are delivered very quickly to the border router, even for nodes several hops far from the sink. A few packets need more time to be delivered: they correspond to retransmissions, since radio links may be unreliable. Finally, only a few packets are delivered in more than 101 slots (i.e., the slotframe length, 1, 010 ms).

Considering the packet losses distribution (see Fig. 4b), we can remark that most of the packets are lost at the beginning, during the convergence time. Indeed, each node has to request bandwidth to its parent, negotiating which timeslots and channel offsets should be used for the transmission. Meanwhile, the packets are buffered, and are dropped when they are stored in the buffer for a long duration (i.e., 8 seconds). However, after the convergence period, the reliability significantly increases, only a few packets are dropped, mainly due to the external interference (i.e., other experiments are executed simultaneously on the same testbed).

It is worth mentioning that medium access is deterministic after the convergence: the schedule does not change anymore, and the same cells are used for a given track (i.e., application). 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 17000 18000 19000 20000 21000 22000 23000 24000 25000 26000 27000 28000 29000 30000 31000 32000 33000 34000 35000 36000 37000 38000 39000 40000 41000 42000 43000 44000 45000 46000 47000 48000 49000 50000 51000 52000 53000 54000 55000 56000 57000 58000 59000 60000 61000 62000 63000 64000 65000 66000 67000 68000 69000 70000 71000 72000 73000 74000 75000 76000 77000 78000 79000 80000 81000 82000 83000 84000 85000 86000 87000 88000 89000 90000 91000 92000 93000 94000 95000 96000 97000 98000

Contiguous versus Uniformly Distributed Shared Cells

In 6TiSCH, a node may use a shared cell to transmit a frame under contention. Typically, these shared cells are used for EBs, broadcast packets (e.g., DIO), as well as to reserve bandwidth (Link-Request, and Link-Replies). By default, the N shared first cells are shared in the slotframe (i.e., the contiguous scenario).

We here propose rather to distribute uniformly the shared cells. Indeed, IEEE802.15.4-2015-TSCH does not use a backoff at the beginning of a shared cell. It rather implements a CSMA-CA slotted approach among the shared cells: the backoff counts the number of shared cells to wait before transmitting a packet. Thus, collisions are very frequent during the shared cells.

We compare the contiguous and the distributed strategies during the convergence (Fig. 5). We measure the packet losses in a topology with 10 nodes and one sink. With contiguous cells, many collisions occur in the shared cells: links requests tend to be lost, retransmitted, and a node has to wait for a long time before receiving a dedicated cell to use. On the contrary, having uniformly distributed cells in the slotframe tends to reduce the collisions and to accelerate the convergence. Thus, we should either use very short slotframes with one single shared cell, or a long slotframe with uniformly distributed shared cells.

RPL metric

RPL may use any objective function to compute the DAG rank of the nodes. However, minimizing the number of hops tends to perform poorly [START_REF] Dhananjay | Measurement and characterization of link quality metrics in energy constrained wireless sensor networks[END_REF]. Thus, the ETX metric is often privileged: it consists in estimating the number of transmissions before receiving an acknowledgement.

To highlight the impact of the routing metric, we evaluate the behavior of the 6TiSCH stack with the following routing metrics:

Minhops: we minimize here the number of hops: the link metric is set to minHopRankIncrease for all the links; ETX: we use ETX, reported by the MAC layer: ratio of the number of acknowledged packets and the number of transmitted packets for a given neighbor;

RSSI: all links with a RSSI superior than a threshold value have the best link quality (=1). Then, the link metric is incremented for each dBm less.

To compute the DAG rank, a node sums the rank of its parent, and the link cost (i.e. the OF0 objective function in RPL). Besides, we maintain the default blacklisting method of OpenWSN: a radio link is marked as stable when its RSSI exceeds -80 dBm for 3 consecutive different packets. Inversely, links become unstable when its RSSI is inferior than -80 dBm for 3 consecutive packets.

Figure 6 presents the impact of the routing metric on the reliability. ETX converges slowly: a significant part of the packets is lost during the beginning (first hour). On the contrary, the packet losses decrease more quickly with minhop. Finally, using the RSSI metric constitutes a tradeoff between minhop and ETX.

After 20 minutes, the packet losses increase drastically with minhop. A link performs badly and the RPL DAG structure is reconfigured: minHop tends to select unreliable long links. Thus, a large ratio of the nodes have to change their preferred parent, and a storm of requests use the shared cells. The network re-converges very slowly to a legal state. On the contrary, RSSI and ETX perform better: the performance is more stable after 1.5 hours.

In conclusion, ETX needs more time to converge because it represents an instantaneous metric and is subject to large variations, caused by the radio instability and the depen-dency on the traffic (i.e., more packets on a route impacts negatively the link quality) [START_REF] Iova | Stability and efficiency of RPL under realistic conditions in Wireless Sensor Networks[END_REF]. However, 6TiSCH achieves to exhibit stable performance after the network convergence.

Scheduling algorithm

We finally evaluated the performance of the scheduling algorithms (Fig. 7), comparing our random strategy and the random contiguous strategy.

The random contiguous strategies selects the outgoing cells to minimize the buffering delay. Mechanically, the random contiguous strategy achieves a smaller end to end delay than the random strategy.

Besides, we can remark a reconfiguration with the random strategy: after 30 minutes, the DODAG is reconfigured, leading to better performance. However, the end-to-end delay remains superior to that of the contiguous scheduling strategy.

Due to lack of space, we did not represent here the performance for larger topologies. While the random contiguous strategy keeps on reducing the buffering delay, the convergence time is longer: more link-requests have to be transmitted before finding an available cell. Besides, shared cells suffer from many collisions when we have more than 20 nodes. Thus, the packet delivery ratio is lower with the random contiguous strategy during the convergence time (30% versus 50% after 30 minutes). In conclusion, 6P has to be improved to enable more reliable transactions.

LESSONS

Guidelines

Based on our experimental results, we conclude the following guidelines should be applied in 6TiSCH:

Non contiguous shared cells: when shared cells are all placed at the beginning of the slotframe, all the requests are buffered during a whole slotframe before being transmitted in the first shared cells. The collisions are consequently very frequent.

On the contrary, we propose to distribute uniformly the shared cells in the slotframe, decreasing thus the pressure, and making 6TiSCH much more efficient.

Routing metric: a blacklisting approach is surprisingly efficient, while the minhop metric converges quickly to a legal schedule. However, it selects the worst radio links, which may dysfunction: these local faults impact globally the network, penalizing the convergence.

On the contrary, both the ETX and RSSI metrics perform better. However, some heuristics should be proposed to accelerate the convergence.

Open Challenges

While our distributed version of 6TiSCH works efficiently in many situations, we highlighted several challenges that still have to be addressed: Radio link quality estimation: a node has currently to select a parent without exchanging packets (i.e., no dedicated cells has been reserved so far with this possible parent).

However, RSSI or the packet losses of EBs may be misleading to estimate the link quality [START_REF] Baccour | Radio Link Quality Estimation in Wireless Sensor Networks: A Survey[END_REF]. Worse, the We should propose a mechanism to estimate the link quality of a possible parent, while not being already attached to this node;

Dense topologies: the radio link quality estimation is even exacerbated with dense topologies: which neighbor should be selected as parent? While the path metric (its DAG rank) is important, the link quality is also of prime importance. We have to propose schemes, that discover an acceptable parent, with a tradeoff between discovery cost and optimality.

Centralized versus Distributed: we expect in the future work to compare quantitatively the performance of a centralized and a distributed scheduling solution. However, we must be able to report to the PCE the link quality of all the neighbors and to estimate the level of interference: this constitutes a challenging objective. Besides, the shared cells are subject to many collisions: how could we increase the end-to-end reliability of the communication to the PCE?

CONCLUSION AND PERSPECTIVES

In this paper, we presented a distributed solution of 6TiSCH: a node decides autonomously when and which cells to use with its parent. Moreover, we proposed to exploit isolated tracks to provide flow isolation, and to make the transmissions reliable and independent: each application has dedicated (i.e., reserved) bandwidth for its packets transmissions. Inversely, we are also able to mutualize the bandwidth reservation, using a convergent track : all the DAO in our experiments use the same convergent track toward the border router. This flexibility of the 6TiSCH architecture and the stability of the performance prove the relevance of a deterministic network for the low power lossy networks.

In a future work, we plan to explore the impact of very large topologies, and to study in depth the maximum distance and traffic TSCH may support efficiently. We also plan to study how we could accelerate the convergence time: we should first reduce the number of collisions among the 6P requests and replies, which increase the delay to negotiate a set of cells with its parents. Furthermore, we should explore which routing metric should be used to avoid re-changing the parent when the radio link quality is initially misestimated.

Figure 1 :

 1 Figure 1: Schedule in a 6TiSCH network, using two different tracks for traffic isolation.

2 Figure 2 :

 22 Figure 2: Mixing heterogeneous applications in the same 6TiSCH instance.

Figure 3 :

 3 Figure 3: FIT IoT-LAB: topology in the Grenoble testbed.

  -end delay (in ASN) (a) End-to-end delays for each received packets

Figure 4 :

 4 Figure 4: 6TiSCH behavior during the convergence -topology with 12 nodes and one sink (1.5 hours).

Figure 5 :

 5 Figure 5: Packet losses / received during an experiment of 30 minutes with 10 nodes and one sink.

Figure 6 :

 6 Figure 6: Packet losses / received during an experiment of 1.5 hours with 20 nodes and one sink.

Figure 7 :

 7 Figure 7: Packet delays during an experiment of 1.5hours with 5 nodes and one sink

  openwsn -open-source implementations of protocol stacks based on Internet of Things standards, https:// openwsn.atlassian.net/

		22 motes		69 motes
		27 motes
					80 motes
	79 motes	50 motes	50 motes	82 motes	Nodes are selected in this corridor
					80 motes
					69 motes

* † https://github.com/ftheoleyre/openwsn-fw/ and https: //github.com/ftheoleyre/openwsn-sw/ † In this way, we have a constant number of packets generated in the whole network, even when increasing the number of nodes.

Table 1 :

 1 Default parameter values(the inter-packet time depends on the number of nodes to maintain a constant load with a variable number of nodes)

	Experiments Duration	1.5 hours
		Topology	line (floors and ceils)
		Separation	≈ 1.5 meters
		Slotframe length	101
	TSCH	Nb.	shared cells	5
		(N shared )
		Timeslot duration	15 ms
	RPL	DAO period	50 s
		DIO period	8.5 s
	CoAP	CBR		10 s * nbNodes
	6TiSCH	T imeoutSF loc -rx	25 s
		T imeoutSF loc -tx	20 s
	Queues	Timeouts	8 s
		Queue size	10 packets
		incl. data packets	at most 6 packets


D Barat

V Bardinal
email: bardinal@laas.fr

I Dika

O Soppera

A Rumyantseva

B Reig

M Renault

A Bruyant

J B Doucet

T Camps

J P Malval

E Daran

Microlens self-writing on vertical laser diodes by Near Infra-Red photo-polymerization

Keywords: micro-optics, laser writing, photo-polymerization, optical probing, maskless fabrication

We report on the improvement of a fabrication method for micro-optics integration on vertical semiconductor laser diodes (VCSELs). Our approach is based on microlens self-writing by the laser beam itself thanks to self-guided Near-Infra-Red photopolymerization. Fabrication conditions have been optimized to control and take benefit of process sensitivity to oxygen quenching and to make possible the study of photo-chemical parameters influence on final tip geometry. The interest of this simple method for fabricating focused-beam VCSELs is finally discussed.

Introduction

Due to a high parallel operation and a low power consumption, VCSELs (Vertical-Cavity Surface-Emitting Lasers) have become laser sources of great importance for optical interconnects and for compact optical sensors [START_REF] Iga | Vertical-Cavity Surface-emitting Laser : its conception and evolution[END_REF][START_REF] Larsson | Advances in VCSELs for Communication and Sensing" Selected Topics in Quantum Electronics[END_REF]. To be exploited in such systems, VCSELs are associated to external micro-optical elements to adapt their natural far-field beam divergence (from 10° to 22° half angle at 1/e², depending on fabrication technology) to the aimed application. Consequently, VCSEL beam shaping using collective and simple methods is strongly in demand [START_REF] Bardinal | Collective micro-optics technologies for VCSEL photonic integration[END_REF]. In particular, beam focusing up to the diffraction limit stays a key issue for exploiting singlemode VCSELs for sensing, measurement or printing [START_REF] Heinis | Miniaturized scanning near-field microscope sensor based on optical feedback inside a single-mode oxide-confined Vertical-Cavity Surface-Emitting Laser[END_REF][START_REF] Mukoyama | VCSEL array based light exposure system for laser printing[END_REF]. With this aim, we have developed a new method for polymer microtips self-writing on GaAs-based VCSELs using self-guided Near Infra-Red (NIR) photo-polymerization. This approach implements the use of photopolymers that are sensitive at the VCSEL lasing wavelength. In these conditions, the fabrication process of polymer microlens at the laser output is triggered by the laser source itself and can be applied to devices arrays during their collective test under probes, without the need for a lithographic step. The method feasability was first demonstrated using a local dispending technique [START_REF] Bardinal | A microtip self-written on a Vertical-Cavity Surface-Emitting Laser by photopolymerization[END_REF]. More recently, we investigated the properties of 760nm tipped-VCSELs with different geometries, by playing on photochemical parameters [START_REF] Barat | Photo-chemical study and optical properties of microtips self-written on vertical laser diodes using NIR photo-polymerization[END_REF]. This study was possible thanks to the development and to the optimization of a wafer level fabrication process that we detail in this paper.

Experimental methods

NIR self-writing on VCSELs

Our method is derivated from a simple and low cost technique earlier proposed in the visible spectral range for implementing a polymer micro-component at the extremity of an optical fiber. A drop of photopolymer is deposited at the end of a fiber in which a green laser light is coupled [8]. Light exposure initiates a photopolymerization process and a self-guiding effect occurs thanks to a change in the refractive index of the photopolymer upon irradiation.

After a standard developing step, a self-aligned microtip remains at the surface of the fiber and allows for a strong focusing of the output beam. This effect can be exploited for nearfield optical microscopy or scanning probe microscopy applications.

To apply this method to VCSELs devices, several conditions must be met: first, the dye has to absorb light in the Near-Infra-Red spectrum and initiate polymerization, since this range corresponds to the typical emission range of most advanced VCSELs, i.e. in GaAs technology (700nm-1µm). Secondly, the resist has to be totally bleached upon irradiation so that it becomes transparent after fabrication. Indeed, for VCSEL case, the operating wavelength will be indeed inside the actinic polymerization wavelength span. Furthermore, the resist has to be easily dispended on the VCSELs wafer. Finally, laser operation should be minimally disturbed by microtip fabrication so that main VCSEL features (modal behaviour, low threshold, emitted power) are kept. To fulfill these requirements, we have developed new chemical systems based on free radical photopolymerization. Our formulation is a mixture of three components: a NIR sensitizer, a cyanine (HITC), associated with a co-initiator (MDEA) to trigger the polymerization of a triacrylic monomer (PETIA). Such a monomer presents good performances in terms of spatial resolution and leads to a crosslinked polymer with suitable mechanical and optical properties for integrated micro-optics on laser devices. The molecular structure of the different components is shown in Fig. 1, as well as the formulation transmission coefficient spectrum before and after irradiation. As required, it presents a maximum value in the NIR region, and is totally bleached after fabrication. The thickness of the inhibition layer can be controlled by NIR light intensity. Indeed, the origin of the inhibition layer is due to the continual replenishment of oxygen by diffusion from the surrounding atmosphere that does not allow reaching an oxygen concentration low enough to efficiently start the polymerization reaction. Increasing the light power creates more reactive species that consume more Oxygen and the result is a thinner inhibition layer. A more detailed description of involved physical and chemical phenomena can be found in [START_REF] Soppera | Photopolymerization with microscale resolution : influence of the physico-chemical and photonic parameters[END_REF]. More details on photochemistry properties can be found elsewhere [START_REF] Soppera | Fabrication of micro-optical devices by self-guiding photopolymerization in the near IR[END_REF][10].

Effect of initial thickness of the film

Figure 2 illustrates the prominent role of initial film thickness on the geometry of fabricated microtips on VCSELs devices. Indeed, the initial film thickness not only governs the final tip height (because, obviously, the size of the tip cannot exceed the film thickness), but also the shape of the tip. In fact, the overall process is ruled by light-driven processes (photo-polymerization) but also by oxygen inhibition. The thickness of the inhibition layer can be controlled by NIR light intensity. Indeed, the origin of the inhibition layer is due to the continual replenishment of oxygen by diffusion from the surrounding atmosphere that does not allow reaching an oxygen concentration low enough to efficiently start the polymerization reaction. Increasing the light power creates more reactive species that consume more Oxygen and the result is a thinner inhibition layer. A more detailed description of involved physical and chemical phenomena can be found in [START_REF] Soppera | Photopolymerization with microscale resolution : influence of the physico-chemical and photonic parameters[END_REF]. The consequence of oxygen inhibition and of diffusion from the surface is the presence of a thin layer at the surface of the film in which polymerization is not efficient. Figure 2 illustrates what happened for 3 different thicknesses ranging from 5 to 60 microns. In all cases, we observed that the thickness of the tip was smaller than the initial film thickness. The difference was, for the light intensity that was chosen, around 5 microns. For the thinnest film (5 microns), the tip could hardly be observed. Indeed, in this case, the thickness of the film is of the same order of magnitude of the inhibition thickness. For the thicker layer (60µm), polymerization proceeds as well up to the limit of inhibition but in this case, the aspect ratio of the tip exceeds the maximum acceptable value for the mechanical properties of this material. The consequence is a tip with a twisted end, unsuitable for applications. In consequence, the thickness range suitable for the tip fabrication is comprised between 20 and 50 microns. Only in this range, it is possible to obtain straight tips and to tune their end shape in a wide range of radius of curvature, by changing the photonic parameters (optical power and exposure time).

Film deposition using spin-coating

First experiments on VCSELs were performed using a local dispensing method for NIR resist deposition [START_REF] Bardinal | A microtip self-written on a Vertical-Cavity Surface-Emitting Laser by photopolymerization[END_REF]. Tip creation was demonstrated with VCSELs emitting just above their laser threshold. However, the precise control of resist thickness in the above-mentioned required range (20-50 µm) was not possible and tip shape was thus not reproducible. To solve this issue, a spin-coating process was developed. As seen in Fig. 3, this method present other advantages, as it does not require an alignment of the droplet relatively to the emitting device and it is more suited for a collective fabrication at a wafer-scale. Viscosity of the photopolymer has been adjusted by a judicious choice of monomers to reach the targeted thickness value and to present a behavior similar to the one of a commercial negative tone resist (SU-3025). In parallel, substrate functionalization has been proceeded in order to guarantee an optimal anchoring of the polymer on the VCSEL surface and a good surface quality. Thickness profile after spinning and polymerization on a 2" diameter wafer was characterized by means of a mechanical profilometer for two different spinning speeds (1000 and 2000 rpm, during 30s, acceleration: 5000 pms -1 ) (Fig. 4). In both cases, the targeted thickness was reached (48µm and 24µm) and uniformity was found to be better than 2µm on the whole surface, except in a ring-shape area located at the wafer edges (2.5mm). This is due to a typical material built-up occurring during spin coating. This edge-bead could be removed but this is not necessary since in our case no mask will be used for photo-fabrication.

Photofabrication under optical control

After NIR formulation deposition, photofabrication was performed using a simple electrical probe station commonly used for L-I-V (Light-Intensity-Voltage) measurements of vertical laser diodes emission properties. Thanks to an optical microscope, a metallic microprobe was placed on the VCSEL top contact pad (anode) to allow for an electrical injection. Precise probe positioning was possible thanks to formulation transparency in the visible range (see Fig. 1). As illustrated in Fig. 5, a silicon photodiode was also placed above the emitting device to measure the light transmitted through the polymer during fabrication. This real-time control allowed for the control of the end of the reaction by checking formulation transparency. Bleaching time was thus found to vary from 4s to 16s depending on the initial VCSEL optical power. The signal was also acquired as a function of the optical dose for different VCSEL biasing conditions, i.e. different initial emitted powers (Fig. 5). The normalized curves saturate for the same value of optical dose (40s.kW/cm²). This means that the photo-fabrication mechanism is not dependant of optical power level and that in this range of incident power tip geometry can be tuned by playing only on the exposure dose.

Parametric study and discussion

The influence of chemical and opto-electrical parameters was investigated for two different types of singlemode VCSELs devices presenting two different emission sizes (5µm and 3.µm with beam divergences of 17° and 22°, respectively). Figure 6 shows a parametric study carried out for VCSELs fabricated with a 5µm-diameter metal aperture and for an initial resist thickness comprised between 45 and 50 microns. For this series of experiments, the dye content was chosen to be 0,09% of HITC. The VCSEL emission powers were chosen between 40 and 350 μW and resist irradiation times between 2 and 600 s. Note that missing data in the table correspond to samples that were damaged before SEM characterizations. For the lowest power (40 μW) the extent of polymerization remains very weak. This is due to the inhibition and diffusion of oxygen that stop the polymerization in the film. Indeed, the thickness of inhibition described in the previous figure is directly dependant on the used power. Low power favors the influence of the diffusion of oxygen as it allows a continuous replenishment of oxygen from the surface. Under such conditions, the inhibition can never be overcome by polymerization, resulting in a saturation of the tip height to very low value. Interestingly, this gives an evaluation of the inhibition thickness for such power, which can be estimated to 40 microns. When the power is increased to 170 and 280 μW, the extent of polymerization is increased and resulting tips are higher, as expected. Interestingly, these values of power correspond to a wide dynamic range in tips geometry, which is obviously the regime of interest for future applications since the tips properties can be adjusted in a wide range by playing on photonic parameters. Finally, the use of higher powers (350 μW) showed low interest since in this case, the polymerized volume can be hardly controlled and resulting tips exhibit irregular shapes.

The previous results were confirmed with a second series of experiments carried out on a second family of VCSEL (namely "oxide-confined" VCSELs that exhibit a smaller emission zone of 3µm, as well as a more divergent beam) (Fig. 7). In this case, the dye concentration was 0.18% and the resist thickness was kept between 45 and 50 microns.

Though these results cannot be directly compared since the emission surface is different from the 2 series of VCSELs, the same trends were observed. Indeed, for the lowest power (40 μW), we observed a limited size of the microtip (less than 5 microns in height), which again can be explained by the inhibition layer at the surface of the photopolymer. When the power was increased, the tip height was found to increase and to saturate around 30 microns for powers greater than 100 μW. Interestingly, it was observed that tuning the tip height was more complicated as for the previous VCSEL devices. For the lowest irradiation times (2s), the tips had the same length than for highest irradiation but resulted in poorly cross-linked polymer with low mechanical properties. The optimal fabrication parameters appear thus around 60 μW, for which straight tips with tunable length and radius of curvature at the extremity can be reached. These results shows that self-guided polymerization remains possible even in the case of large numerical aperture VCSEL such as oxide-confined VCSELs. As for the discrepancies observed on tip shapes and tip heights between the two types of devices, they can be mainly accounted for the differences of initial beam profiles [START_REF] Barat | Photo-chemical study and optical properties of microtips self-written on vertical laser diodes using NIR photo-polymerization[END_REF].

Once optimal fabrication conditions are defined, typical dimension ranges of microtips fabricated at the surface of NIR single-mode VCSELs were extracted (Table 1). Tip ROC (Radius of Curvature), tip height and tip basis diameter were found to increase with the exposure dose. Highest values were indeed obtained in the saturation regime, when the actinic light reaches the inhibition zone, leading to longer, larger and flatter tips. Note that the ROC range could be further extended by depositing an intermediate transparent layer between the VCSEL and the NIR formulation (to increase the beam size before reaction) and/or by modifying the chemical composition of the formulation. Taking into account these experimental data, focusing properties of tipped-VCSELs were modeled using a FDTD (Finite-Difference Time-Domain) method, as illustrated in Fig. 8. Our simulations indicated that tip insertion on VCSEL leads to a strong beam focusing near the tip surface (working distance <1µm). These trends were successfully compared to experimental measurements performed on mounted devices using a high numerical aperture objective for spot imaging [START_REF] Barat | Photo-chemical study and optical properties of microtips self-written on vertical laser diodes using NIR photo-polymerization[END_REF].

Spot diameters as low as 500nm could be measured near the tip surface for the sharpest tips. It is worth noting that other VCSELs features (lasing wavelength, modal behavior) were not significantly disturbed by tip presence, except regarding the laser threshold that was slightly increased due to a modification of VCSEL top reflectivity. This optical study is still undergoing. Nevertheless, the ability of our technique to fabricate collectively focused-beam VCSELs sources has been demonstrated.

Conclusions

We have presented a NIR self-writing process suited for the integration of self-aligned microtips on vertical cavity laser devices (VCSELs). This single-step and room temperature technique is based on the use of NIR photopolymers that were optimized for spin coating to make possible a precise control of initial thickness prior to photofabrication. As a result, limitation of oxygen quenching effects was possible as well as a collective fabrication at a wafer-scale. Thanks to this improvement, a parametric study was also led on two different kinds of VCSEL devices. Suitable exposure doses and corresponding dimensions ranges were determined. The focusing abilities of such tipped-devices were also evaluated, showing that our method could be exploited for the fabrication of compact optical micro-probes for scanning probe microscopy or for near-field optical microscopy with an optimization of the polymer probe [START_REF] Sedaghat | Near-field optical imaging with a nanotip grown on fibered polymer microlens[END_REF]. Fig. 2 Role of initial film thickness on the final shape of fabricated microtips: the inhibition zone due to oxygen quenching prevents optimal fabrication if the layer is too thin (5µm). On the other hand, for layer that are too thick (60µm) the tip aspect ratio is too high to lead to a symetric shape.

Figure captions

Fig. 1 .

 1 Fig.1. Molecular structure of the formulation chosen for self-aligned photo-fabrication on

Fig. 3

 3 Fig.3 Comparison of local dispending (left) and spin-coating (right) methods for NIR-

Fig. 4 :

 4 Fig.4 : Thickness profiles measured after spin coating and photopolymerisation on a 2" wafer

Fig. 5 :

 5 Fig.5: (left) Principle of microtip NIR self-writing on VCSELs under real-time optical

Fig. 6 .

 6 Fig. 6. SEM (Scanning Electronic Microscopy) images of microtips self-written at the surface

Fig. 7 .

 7 Fig. 7. SEM (Scanning Electronic Microscopy) images of microtips self-written at the surface

Table 1 :

 1 Possible dimensions ranges for tip ROC, tip height and tip basis diameter of microtips fabricated on singlemode oxide confined VCSELs (emission zone 3µm).

Fig. 8 .

 8 Fig.8. (top) Description of main parameters used for tipped-VCSELs optical simulation and

Fig. 1 16Fig. 2

 12 Fig.1

Fig. 3 18Fig. 4

 34 Fig.3

  Fig.5

Acknowledgements

The authors gratefully acknowledge French National Research Agency (ANR) for financial support (ANR-09-BLAN-0168-01 and RENATECH (French Network of Major Technology Centres) within LAAS-CNRS for technological support. The authors would also like to thank Oclaro Inc. (Zürich, Switzerland) and Philips Technology GmbH U-L-M Photonics (Ulm, Germany) for providing 760nm-VCSEL devices.


New and on-going challenges for structural engineers regarding existing structures

Editorial for SEI Journal

New and on-going challenges for structural engineers regarding existing structures Bruno Godart, Deputy-Director of the Materials and Structures Department at IFSTTAR Marne-la-Vallée (France)

Considering the level of infrastructure development achieved in many developed countries, works conducted in the field of civil engineering will increasingly focus on the maintenance and repair of structures. The maintenance of existing structures will therefore become an significant challenge, whether for infrastructure managers, who will have to improve their management methods, or for companies that will have to carry out works under specific, and sometimes difficult conditions, or for the structural engineers who will have to design and plan innovative solutions. Concerning this last point, we could mention, as an example, the development of the so-called, somewhat caricatured term "stealth work sites". This term refers to construction on sites that will increasingly have to be carried out with the least possible interference to the operation of the structure, and which will require imaginative approaches in order to find solutions adapted to the severe operation constraints of certain road and railway infrastructures, particularly in urban areas.

In addition, accidental actions such as earthquakes, floods and cyclones will pose an increasing number of questions to the structural engineer, both in the design of new structures and in the dimensioning of cost-effective strengthening of existing structures. The considerable destruction that occurred on the islands of St-Martin and St Barthelemy during the very recent passage of the IRMA hurricane (95% of the existing building stock was destroyed or damaged) remind us that reinforcement is necessary so that existing structures can withstand exceptional climatic events, the frequency of which is unfortunately likely to increase in the future (a displeasing fact for some climatic skeptics ...). The collapse and destruction of structures should also favor the development of forensic engineering based on all the methods and techniques applied to the investigation in order to analyze and understand the origin of the disasters, and draw lessons from them. It seems that lessons from failures have not sufficiently been drawn on in the past and one of the solutions consists in significantly reinforcing the teaching of the pathology of structures and forensic engineering at universities and schools, teaching which appears to still be very weak in most countries.

Another challenge awaits the structural engineer in the management of existing structures. This involves the assessment and detailed evaluation of structures with specific material pathologies such as internal swelling of concrete, freeze-thaw damage, corrosion of rebar in reinforced concrete, fatigue of metal or deterioration of wood. Collaboration with material scientists will be essential to move forward on advanced models and numerical calculations that allow for safety, serviceability and durability limit state checks, and facilitate the design of reinforcements adapted to increase the lifespan of structures.

The challenge ahead is therefore to evaluate and maintain our heritage of existing buildings and civil engineering structures, while enhancing and modernizing it to make it safer and more sustainable.