Emmanuel Lochin
email: emmanuel.lochin@isae.fr

Guillaume Jourjon
email: guillaume.jourjon@nicta.com.au

Sébastien Ardon
email: sebastien.ardon@nicta.com.au

Design and Validation of a Reliable Rate Based Transport Protocol: The Chameleon Protocol

TFRC protocol has not been designed to enable reliability. Indeed, the birth of TFRC results from the need of a congestion controlled and realtime transport protocol in order to carry multimedia traffic. Historically, and following the anarchical deployment of congestion control mechanisms implemented on top of UDP protocol, the IETF decided to standardize such protocol in order to provide to multimedia applications developers a framework for their applications. In this paper, we propose to design a reliable rate-based transport protocol based on TFRC. This design is motivated by finding an alternative to TCP where its oscillating behaviour is known to be counterproductive over certain networks such as VANET. However, we found interesting results partly inherited from the smooth behaviour of TFRC in the context of wired networks. In particular, we show that TFRC can realize shorter data transfer compare to TCP over a complex and realistic topology. We firstly detail and fully benchmark our protocol in order to verify that our resulting prototype inherits from the good properties of TFRC in terms of TCP-friendliness. As a second contribution, we also propose a ns-2 implementation for testing purpose to the networking community. Following these preliminary tests, we drive a set of non-exhaustive experiments to illustrate some interesting behaviour of this protocol in the context of wired networks.

I. INTRODUCTION

Transport protocols suffer of an evolutionary pressure due to the large diversity of recently identified networks properties. For instance, High Speed networks raise several performance problems while Delay Tolerant Networks are at the source of multiple new routing and end-to-end communications issues. As a result, the networking community shows a deep interest in rethinking the way to carry data over Internet. In particular, the IETF Transport Area Working Group (TSWG) is looking at specifying new congestion control and the recent Transport Architecture Evolution mailing list, discuss of ideas and issues surrounding the medium to long-term architectural evolution of the transport layer. Even the OSI model is now under question. Indeed, in a recent paper [START_REF] Ford | Breaking up the transport logjam[END_REF], the authors argue that the transport layer should be now sliced in three sub-layers to cope with new network characteristics.

Although Internet transport protocols must be redesigned due to congestion control lack of fairness and the difficulty of TCP to achieve high throughput, some issues must also be tackled in the context of satellite links, DTN and Intermittently connected Mobile Ad-hoc Networks (ICMAN). Those networks are notably characterised by long delay, lossy links and asymmetric path for which the classical transports protocol are known to poorly perform. New proposals such as Saratoga [START_REF] Wood | Saratoga: a delay-tolerant networking convergence layer with efficient link utilization[END_REF] and LTP-T [START_REF] Farrell | Evaluating LTP-T: A DTN-Friendly transport protocol[END_REF] with pure ARQ scheme (indeed SACK or SNACK based), allowing a faster and efficient use of the available bandwidth in the case of one-hop transfer, have been proposed for interplanetary and satellites communications. However, we must remark that these protocols are deeply linked to the intrinsic store and forward nature of the DTN architecture and the characteristics of the link layer.

Furthermore, we can note that at least for the ICMAN scenario, a huge attention from the community have been given to the routing problem while giving a little interest to end to end transfer and transport. In dense PSN the radio channel might be lossy with variable bandwidth due concurrent communications and the fluctuating distance between nodes. A transfer protocol deployed in such an environment might take in account these parameters that lead to a problem sensibly different than the one solved by deep-space transfer protocol like Saratoga and LTP.

Recent work on transport protocols [START_REF] Handley | TCP Friendly Rate Control (TFRC): Protocol Specification[END_REF], [START_REF] Kohler | Datagram Congestion Control Protocol (DCCP)[END_REF] have proposed alternatives to the generally used window based congestion control. These protocols compute a sending rate which reproduces the TCP behaviour and have been defined as an alternative to UDP to carry multimedia traffic while respecting the fair-share principle introduced in [START_REF] Jacobson | Congestion avoidance and control[END_REF]. In a wireless or lossy channels context, previous studies [START_REF] Sharafkandi | Simple and Effective End-to-End Approach to Increase TCP Throughput over Ad-hoc Networks[END_REF], [START_REF] Kawadia | Experimental Investigations into TCP Performance over Wireless Multihop Networks[END_REF] have demonstrated the poor TCP performances over wireless and multi-hop networks while others emphasise the good behaviour of rate controlled congestion control over these networks [START_REF] Chen | The utility of explicit ratebased flow control in mobile ad hoc networks[END_REF], [START_REF] Anastasia | Towards a novel transport protocol for ad hoc[END_REF]. Therefore, the design of a reliable rate-based transport protocol is perceived as a alternative data-delivery reliable service, that might be suitable for wireless multi-hop network such as vehicular networks (VANET) [START_REF] Leiggener | Analysis of Path Characteristics and Transport Protocol Design in Vehicular Ad Hoc Networks[END_REF].

All these facts motivate the present study which detail a complete rate-based reliable transport protocol implementation as an alternative to the current domination of TCP in terms of reliable service.

There exist a range of reliability mechanisms from basic stop and wait to the more advanced Selective Acknowledgment mechanism [START_REF] Mathis | TCP Selective Acknowledgment Options[END_REF], [START_REF] Floyd | An Extension to the Selective Acknowledgement (SACK) Option for TCP[END_REF]. In a previous contribution [START_REF] Jourjon | Design, implementation and evaluation of a qos-aware transport protocol[END_REF], we outlined the design of a SACK-like mechanism, suited for rate-based congestion control such as TFRC [START_REF] Handley | TCP Friendly Rate Control (TFRC): Protocol Specification[END_REF]. While TFRC offers a smooth traffic dynamic property to the network, on the application side, this congestion control allows a direct exchange between the transport and application layers. Indeed, the metric used by both layer is identical (i.e. a sending rate in bit per second) and is not related anymore to the discrete and unusable value of window of packets per RTT. This previous study, mainly focused on the benefit of using such rate-based protocol to efficiently reach a negotiated throughput over a QoS-enable DiffServ network, did not specify the whole mechanism that could be used over a best-effort network. Thus, in this paper, we also consider the problem of flow control implementation, i.e. how to prevent packet loss at the receiver due to the receiving application not reading packet fast enough from the socket buffer. Such flow control mechanism is obviously mandatory to implement an efficient reliable transport protocol and require specific adaptations compare to the well-known TCP flow control version in order to be used conjointly with a rate-based congestion control mechanism. At last, the design of this implementation is not static and allows enabling or disabling the reliable mechanism plugged into TFRC. On the contrary, except in a recent study proposing a unreliable TCP mechanism (TCP-UREL [START_REF] Ma | Tcp urel, a tcp option for unreliable streaming[END_REF]) to offer an alternative to DCCP/CCID#2 mechanism [START_REF] Floyd | Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion Control[END_REF], none TCP versions allow to switch between a reliable and unreliable congestion control service.

The capability to directly interacts with the transport layer in order to either adapt the QoS required by applications and to enable or disable cross-layer mechanisms [START_REF] Lochin | gTFRC, a TCP Friendly QoSaware Rate Control for Diffserv Assured Service[END_REF] and reliability have motivated the name of this protocol: Chameleon. Following the presentation of the whole structure of this new protocol in the following Section II we demonstrate that the present composition of SACK and TFRC does not impact on the TFRC TCP-friendliness property and validate the addon of SACK and flow control mechanisms to TFRC by showing that there is no packet loss at the receiver, in case of a slow receiving application (Section IV). Before this, we discuss other possible designs in Section III. We investigate the behaviour of Chameleon protocol over a realistic topology which aims to represent an Internet provider backbone inspired from Free operator 1 in Section V. Finally, Section VI gives the conclusion and future work.

II. SPECIFICATIONS OF THE CHAMELEON PROTOCOL

We present in this section the protocol used for the integration of a flow control and, in particular, the composition of the TFRC congestion control and SACK reliable mechanisms.

The concept of Selective ACKnowledgments (SACK) was originally introduced in [START_REF] Mathis | TCP Selective Acknowledgment Options[END_REF] as a TCP option that aims to optimize its reliable service by allowing a faster recovery in 1 http://www.free.fr/ the case of a burst of lost packets [START_REF] Floyd | An Extension to the Selective Acknowledgement (SACK) Option for TCP[END_REF]. By sending selective acknowledgements, the receiver of data can inform the sender which segments or packets have been successfully received and which ones have to be selectively retransmitted.

In [START_REF] Jourjon | Design, implementation and evaluation of a qos-aware transport protocol[END_REF], we have defined and validated the composition of SACK and TFRC mechanisms in order to provide a transport protocol compliant with the QoS negotiated with the network layer. This composition implied modification of the SACK mechanism to make it compatible to a datagram-oriented mechanism such as TFRC.

The modifications of the TFRC messages headers to integrate SACK are shown in Figure 1. The two first protocol data units represent respectively the TFRC header and the new header including SACK. The two last Protocol Data Units (PDUs) represent respectively the feedback for the classical TFRC protocol and for the TFRC-SACK composition. As TFRC mandates a new sequence number for each packet sent, we have to introduce a new identifier, linked to Application Data Units (ADUs), to perform the reliability. In the following and in the remaining of this paper, we refer to this identifier as sequence number. In all these headers, each field is either encoded over 4 or 8 bytes except for the proto ID (two bits), the type (two bits), processing time (one byte), and the SACK payload (variable length). We defined the datagram oriented SACK mechanism similarly to the stream oriented mechanism: the SACK payload, constituted by a sequence pair numbers. These pairs represent the edge of a continuous sequence of corrected received packets. The length represents the number of pairs in the SACK payload. Finally the Offset represents the sequence number of the first packet of the first pair.

In the following of the paper, we refer to TFRC with the SACK mechanism and the complete protocol with flow control as TFRC-FC-SACK, and TFRC with only the SACK mechanism as TFRC-SACK.

Based on this composition, the design of a flow control mechanism compatible with TFRC is presented and validated in the following.

Since the SACK mechanism requires receivers to maintain a buffer for the in-order delivery of packet to the application, we base our design on the introduction of a new window variable, avail_win, representing the space available in this buffer. This window should not be confused with the congestion-control window of TCP. The only purpose of this variable is to maintain, at the sender, the amount of buffer space available at the receiver and prevent the sender from transmitting more packets than there is available buffer space. Other candidate solutions for the design, including modification of the TFRC equation, are discussed in section III.

Figure 2 shows the sender and the receiver window. In this figure, the dark boxes represent data packets already sent or received.

At the sender, the flow control mechanism should stop transmitting data packets if the receiver's buffer is full. To achieve this, we use the avail_win variable, which, at the receiver, represents the available space in the receiver buffer, in number of packets. This variable is integrated in the TFRC-SACK feedback messages as a one-byte field after the Receiving Rate field of the last header in Figure 1.

The avail_win variable therefore indicates, at the sender, the supposed number of packets which can be sent. Avail_win is never negative and upper bounded by the total window size. When this variable is non nil, the sender sends data packets at the rate computed by TFRC algorithm. Each time a packet is sent, avail_win is decreased by one at the sender. When avail_win is nil, the sender has already sent the maximum number of data packets which could have been accepted by the receiver. Note that the TFRC rate still condition the speed at which packets are sent, the avail_win variable condition the maximum number of packets which can be sent between receiving two feedback messages. Indeed, as mentioned previously, each feedback message sent by the receiver contains the available buffer space. At the sender, upon reception of a feedback message, the avail_win variable is computed by withdrawing the number of packets sent since the header's Offset from the header's avail_win. A feedback message can therefore unfreeze the sender if the newly computed avail_win is non-nil or the SACK vector indicates that some packets need to be retransmitted.

At the receiver side, when a data packet is received, if its sequence number (S new ) is higher than the highest previously received sequence number (S old ), avail_win is reduced by S new -S old . Otherwise, this packet is out-of-order and is therefore placed in the reception buffer. When the application reads packets from the buffer, the avail_win is increased by the corresponding number of read packets.

III. DISCUSSIONS

In this section, we discuss the design of our flow control for TFRC and explore alternative solutions.

The main feature of a rate-based congestion control mechanism is the use of an equation to determine the sending rate. This equation typically uses network measurements (or estimations) to calculate the theoretical rate at which TCP would send in similar conditions. Following this observation, we first investigated two other possible solutions to the flow control problem.

The first solution is to obtain the reading rate of the receiving application and send it back to the sender. This can be done either by estimating the reading behaviour of the application or by assuming that the application can communicate this reading rate to the transport protocol. The sender would then adjust its sending rate to the minimum between its computed congestion control sending rate, twice the receiver's receiving rate, and the application's reading rate. However, this solution has two major drawbacks. Firstly, the reading rate depends on different parameters such as application type, CPU usage, etc. and may therefore follow complex patterns, which can be difficult to estimate. This may result in erroneous values leading in buffer overflow. Secondly, in order to provide packet ordering, the receiver temporarily buffers out of order packets. This can lead to a situation where the application's reading rate is nil, therefore the sender would stop even if there were space in the buffer.

The second possible solution would have been to modify the equation used to compute the sending rate. TFRC computes its rate (X) with an equation, defined in [START_REF] Handley | TCP Friendly Rate Control (TFRC): Protocol Specification[END_REF]. This equation takes into account the RT T and loss event rate (p). We note this equation:

X = F (p, RT T ) (1)
We propose to model the flow control impact on the transmitting rate as follows:

X = F (p, RT T ) + G(avail_win, p, RT T ) (2)
where G(avail_win, p, RT T ) is a model of the flow control impact on the transmitting rate which takes into account the avail_win variable and F (p, RT T ) is the TFRC equation as defined in [START_REF] Handley | TCP Friendly Rate Control (TFRC): Protocol Specification[END_REF]. We chose not to pursue this solution, as it seems to introduce too much complexity to the rate computation.

Moreover, if we want to avoid losing even one packet at the receiver due to a slow reading application, we would need to underestimate the sending rate, which would negatively impact on the protocol performance. From this, it follows that there is no improvement or benefit in including the flow control in the TFRC's sending rate algorithm.

IV. MECHANISM VALIDATION In this section, we validate our Flow Control mechanism using simulation in ns-2.30. We first implemented the SACKlike mechanism within ns-2.30's TFRC. We also extended the ns-2 simulator to include the application layer to simulate an application reading from the socket buffer at different rate. Using this implementation, we conduct a set of simulations to demonstrate the effectiveness of our flow control mechanism, and quantify the potential impact of the SACK and flow control modifications over the TFRC flow dynamics.

A. TCP-friendliness conservation and Reliability

The first experiment aims at verifying the TCP-friendliness of TFRC-FC-SACK when sharing a bottleneck with other TCP flows. These days, the definition of the TCP-friendliness is still being debated [START_REF] Briscoe | Flow rate fairness: Dismantling a religion[END_REF]. In this study, we will first follow the definition in RFC3448: "[...] a flow is "reasonably fair" if its sending rate is generally within a factor of two of the sending rate of a TCP flow under the same conditions.". This definition concerns instantaneous values. Another common view is that, on average, a flow is TCP friendly if the non-TCP source obtains a long-run term average sending rate not larger than the one TCP would have obtained under the same circumstances [START_REF] Floyd | Promoting the use of end-to-end congestion control in the Internet[END_REF].

To quantify the TCP-friendliness we therefore use an expression of the means ratio as shown on equation (3):

T (X) = 1 n n i=1 x i 1 m m i=1 y i (3)
where X is the protocol being studied, x i the average throughput of the i th X flow, n the number of X flows, y i the average throughput of the i th TCP flow and m the number of TCP flows. In this formula if T is inferior to 1 then the non-TCP flow is TCP-friendly, if T is equal to 1 then we have an ideal friendliness and finally if T if greater than 1 then the non-TCP flow overruns TCP.

In this simulation scenario, we use the butterfly topology shown on Figure 3. There are two sources transmitting to two destinations over a shared link between two intermediate nodes. These two flows are competing for the bottleneck link bandwidth. We perform two experiments where TFRC-FC-SACK is first competing with TCP-SACK, then with TCP New Reno. All three protocols are set to the same packet size of 1KByte and a maximum window size 64KBytes. In both experiments, the application reading rate is infinite. Results are presented in Figure 4. Each graph shows the flow instantaneous throughput at the receiver computed with an average sliding window throughput estimation with a 1ms window.

From Figure 4, we can see that the TFRC-FC-SACK flow instantaneous throughput is slightly inferior to both TCP SACK and TCP New Reno flows. In addition, the TFRC-FC-SACK instantaneous throughput is well within the 2x factor imposed by our TCP friendly definition previously mentioned. We can therefore conclude that this TFRC-FC-SACK implementation remains friendly with both TCP SACK and TCP New Reno2 . In Figure 4 (a), TFRC-FC-SACK equally shares the bottleneck with TCP during almost 130s. At t = 130s, TFRC suffers from consecutive losses and therefore sharply decreases its throughput. TFRC-FC-SACK then attempts to re-adjust its throughput to the equilibrium with TCP, but this process converges slowly, which is a well-known shortcoming of TFRC [START_REF] Widmer | Equation-Based Congestion Control[END_REF]. When competing against TCP New Reno, as shown in Figure 4 its behaviour with TCP SACK except that it stays longer at the first equilibrium (200s instead of 130s). Furthermore after the consecutive losses TFRC-FC-SACK reaches the equilibrium with TCP New Reno faster than with TCP SACK. These differences can be explained as the TFRC equation models TCP Reno. Table I presents the TCP-friendliness index of TFRC-FC-SACK calculated using equation [START_REF] Farrell | Evaluating LTP-T: A DTN-Friendly transport protocol[END_REF]. As all figures are below one. This confirms that TFRC-SACK is friendly with both version of TCP.

TCP version

T(TFRC-SACK) TCP/Newreno 0.82 TCP/SACK 0.72

For these experiments, we also validate the SACK mechanism, i.e. verify that all lost packets are retransmitted. In Table II, we summarize the number of sent and lost packets for each flow in the previous experiments. We can see from this table that TFRC-FC-SACK flows send less packets than both TCP versions. This is explained as the TCP flows overall throughputs are higher than the TFRC-SACK and the packet statistics are collected during a fixed time period of 400s. Furthermore, we can see that the TFRC-FC-SACK flows experience less packets loss than both TCP flows (in absolute value and in percentage). This is explained by the fact that the rate-based congestion control mechanism produces a smoother sending rate compared to a window-based mechanism which is more aggressive. Finally, by using packet marking (not shown in the table), we verify that TFRC-FC-SACK retransmit all dropped packets until correctly received.

B. Impact of the Application Read Rate

The objective of this experiment is to validate the flow control mechanism, by measuring the sender throughput when varying the application read rate, i.e. simulating a slower application. We also want to confirm that there is no packet lost due to a slow receiver unable to accept incoming packets. In addition, in this section, we quantify the impact of our SACK and Flow Control mechanisms over TFRC smoothness, by measuring the throughput stability during the data transfer.

In order to quantify this stability, we consider the average throughput for each time unit interval. For each time interval we compute each flow's throughput standard deviation [START_REF] Jin | FAST TCP: motivation, architecture, algorithms, performance[END_REF] and obtain the following metric equation (4):

S = 1 n n i=1 1 x i 1 m -1 m j=1 (x i (k) -x i ) 2 (4)
where x i is the average throughput of the i th TFRC-FC-SACK (resp. TFRC) flow, n is the number of flows, x i (k) is the throughput of the i th TFRC-FC-SACK (resp. TFRC) flow for the k th time interval and m is the number of time intervals.

For these experiments, we use a simple topology where two nodes communicate through a third one. Packets are crossing two consecutive links of respectively 10 Mbps and 1Mbps bandwidth, for an overall 20 ms RTT (5ms delay on each link).

Figure 5 (a) shows the throughput of a TFRC-FC-SACK flow as the application read rate is set to 600kbit/s at the receiver.

Each packet loss event is illustrated on Figure 5 (a) by a cross on the x-axis. At the beginning of the transmission, the sender sends packets according to the slow start algorithm. This phase stops when the first packet loss event occurs. TFRC then enters the congestion avoidance phase. As soon as the receiver's buffer is full due to the application limited read Hence, the sender is only allowed to send new packets when the receiver has delivered some packets to the application. Consequently, Figure 5 (a) confirms that the flow control mechanism operates correctly as the throughput is adapted to the receiver application read rate. Furthermore, Figure 5 (a) shows that the receiver does not drop any packets.

In Figure 5 (b), we mix one TFRC-FC-SACK and one TFRC flow in the same network conditions as previously. However, contrary to the previous experiment, the application read rate varies in time and follows a specific pattern as shown in Figure 5 (b). We have chosen this specific pattern as it represents a mix of above, below and equal to the fair share throughput.

From Figure 5 (b), we can first see that a read rate above to the theoretical fair share value (500kbit/s) does not impact on the behaviour of TFRC-FC-SACK: TFRC and TFRC-FC-SACK equally share the link bandwidth. Furthermore, the transition from this read rate to another one inferior to 500kbit/s does not induce any packet loss at the receiver buffer. Between t = 100s and t = 150s, the application read rate is set to 100kbit/s, i.e. under the theoretical fair share value. During this phase, we can see from Figure 5 (b) that TFRC-FC-SACK sending rate is following the application reading rate while TFRC flow can uses the rest of the bottleneck. At t = 150s, the application reading rate is set again to values above to the fair share for 100s. We can see from the graph that during this period TFRC-FC-SACK and TFRC equally share the bottleneck bandwidth as expected. Finally, for the remaining variations of application reading rate, TFRC-FC-SACK continues to behave in a fair manner.

To quantify the impact flow control when the receiver application drives the transmission over the throughput smoothness, we use the stability metric, as defined in equation 4. We applied this criterion on a set of experiments that aims at checking that the flow control does not introduce any degradation in the smoothness characteristic of TFRC.

In Table III, we present the results of experiment when two identical flows share a bottleneck of 1M bit/s during 400s. We show in this table that TFRC-FC-Sack remains as smooth as TFRC when it is not limited by the application read rate. Furthermore when we introduce for both flows a read rate of 300Kbit/s, the resulting stability of the system is increased. This result can be explained by the fact that the oscillations in the throughput are usually due to the congestion control mechanisms that tries to increase until the detection of a loss. In the case of a system limited by the application read rate the two flows do not try to increase nor decrease and therefore are more stable.

V. EXPERIMENTATIONS OVER A REALISTIC TOPOLOGY

In order to evaluate our proposal over a more realistic case, a more complex topology is used in the next simulations. This topology is similar from this presented in [START_REF] Dedu | A study on the benefit of tcp packet prioritisation[END_REF]. According to a small study of the xDSL backbone of a Internet provider 3 , most of these networks are built around a central core where several loops are connected. These loops are composed of a small number of routers. The aim of the closed loop is to have a fault tolerance.

For the simulation, a flower with five loops is considered as backbone, each loop has 8 routers, shown in figure 6. Each router (except the 5 core routers) has 2 DSLAMs connected to it, and each DSLAM has 3 hosts connected to it. Each link has the following characteristics: 10Mb/s bandwidth, 10ms delay, DropTail.

We emit 250 FTP over TCP/Newreno/Sack connections (flows) with random and non-identical hosts as source and destination. Then, we realise exactly the same experiments with TFRC/FC/Sack proposal. Each connection starts at a random time between 0 and 20 seconds and sends a random number between 10 and 600 packets.

Table IV provides the global result obtained for the whole simulation. We observe on average the transfer time of TFRC/FC/Sack flows is globally lower than TCP/Newreno/Sack.

In Figure 7, we classify each flow in ascending order of their size (i.e. the number of packets of each flow is ranging from 10 to 600 and is reported on the x-axis) and we report their respective time values on the y-axis. If several flows gets the same number of packets (as this number is randomly choosen), we report the highest value obtained by one of these flows. As we used the same seed for each simulation, the distribution of the flows' size was the same for both TCP/Newreno/Sack and TFRC/FC/Sack experiments. This figure tends to show that the transfer times of long TFRC/FC/Sack flows are smaller than TCP/Newreno/Sack flows. In order to better see this trend, in Figure 8, we classify in ascending order each transfer time value of each flow. Following these two figures, we clearly see that long TFRC/FC/Sack flows obtain a lower transfer time than TCP/Newreno/Sack. However, the shorter the TFRC/FC/Sack flows are, the higher their transit time. This means that TFRC/FC/Sack is not a good trade-off in the context of short data transfers such as HTTP requests but outperforms TFRC/FC/Sack in the context of long data transfers.

We explain this effect by the slow dynamic convergence of TFRC [START_REF] Widmer | Equation-Based Congestion Control[END_REF]. Indeed, TFRC is known to converge slowly to the nominal capacity of the link over high delay and congested links just after the SlowStart phase. However, when TFRC enters the steady state, its long run behaviour is more stable than TCP which operates in a more oscillatory behaviour and then, might reduce several time its congestion window due to the highly dynamic character of the network.

In complement to the arguments presented in the introduction (Section I), these results show another interesting property of this protocol that might be helpful in high dynamic environment where RTT is constantly oscillating. As an example, due to the tremendous progress in physical and data layers reliability, in clear weather condition 4 , a satellite link is considered as mostly error free (following DVB-S standard, BER ≈ 10 -7 ) while RTT might constantly moving due to mobility. Thus, we claim it would make sense, in certain condition, to prefer the use of rate-based mechanism rather than window-based mechanism as presented in Section I.

We reserve for a future work measurements dealing with vehicular networks. Indeed, current ns-2 simulator is not well designed either to implement correct MAC and physical layers models or to produce realistic VANET scenario. We are currently analysing various possibilities to realize this task and in particular investigating ns-2 extension software such as [START_REF] Feliz Kristianto | Rapid generation of realistic mobility models for vanet[END_REF].

VI. CONCLUSION AND FUTURE WORK

In this paper we have investigated and proposed a complete reliable rate-based protocol based on TFRC and SACK mechanisms. Our design also introduces a flow control variable, which regulates the sender to avoid packet loss at the receiver due to a slow receiver. We show that the modifications resulting from this composition does not affect the TCP-friendliness property of TFRC. We validate our proposal through ns-2.30 simulation and verify TCP-friendliness metrics. We further show that there is no packet loss due to flow control, at the receiver, and apply a stability criterion to demonstrate that the introduction of the flow control inside TFRC does not alter the smoothness property of this mechanism. We finally show the benefit of using this protocol in the context of long data transfer over a complex and realistic topology.

There is room for potential improvements. Following a recent study in [START_REF] Sarwar | Improvements in dccp congestion control for satellite links[END_REF], in a future work, we will investigate the benefit of tuning the number of feedback messages in order to adapt the frequency of feedback messages depending of the RTT, to guarantee optimal operation under various network conditions. Others experiments must be propose in order to verify hypothesis concerning the adequacy of such proposal over VANET and multihop scenario. We are currently specifying such scenarios based on real VANET traces in ns-2 and hope to obtain interesting results to discuss.

1 Figure 3 .

 13 Figure 3. Topology of the scenario

  (b), TFRC-FC-SACK behaves similarly to Instantaneous throughput of TFRC-FC-SACK and TCP New Reno

Figure 4 .

 4 Figure 4. Validation of TFRC-FC-SACK composition in ns-2.30

Figure 5 .

 5 Figure 5. Validation of flow control mechanism

Figure 6 .

 6 Figure 6. Backbone of the flower network.

Figure 7 .

 7 Figure 7. Transmission time of all the flows; as a function of the packet flow size (from 10 to 600 packets).

Figure 8 .

 8 Figure 8. Transmission time obtained by all flows.

Several experiments with different RTT and bottleneck capacity have confirmed these results. Due to space reason, we give in this section a general sample of these experiments.

In[START_REF] Vazquez | Voip cross-layer control for hybrid satellite wimax networks[END_REF] the authors show that, for some rain events, the DVBS2 ACM modes can not cope with the deepest attenuations where the E b /N 0 of the tropical rain is below the intervention threshold of the minimum ACM mode, which is -1.5 dB for a 10 -7 QEF FEC (Quasi Error Free Packet Error Rate).

ACKNOWLEDGEMENTS

The authors would like to thank Yong-Han Lee for the implementation of this proposal during his internship done at NICTA and Eugen Dedu for the flower network settings. This work has been supported by funding from National ICT Australia.


Dino M Lopez-Pacheco

Emmanuel Lochin

Golam Sarwar
email: golam.sarwar@nicta.com.au

Roksana Boreli
email: roksana.boreli@nicta.com.au

Understanding the Impact of TFRC Feedbacks Frequency over Long Delay Links

TFRC is a transport protocol specifically designed to carry multimedia streams. TFRC does not enable a reliable and in order data delivery services. However TFRC implements a congestion control algorithm which is friendly with TCP. This congestion control relies in a feedback mechanism allowing receivers to communicate to the senders an experienced drop rate. Although the current TFRC RFC states that there is little gain from sending a large number of feedback messages per RTT, recent studies have shown that in long-delay contexts, such as satellite-based networks, the performance of TFRC can be improved by increasing the feedback frequency. Nevertheless, currently it is not clear how and why this increase may improve the performance of TFRC. Therefore, in this paper, we aim at understanding the impact that multiple feedback per RTT may have (i) on the key parameters of TFRC (RTT and error rate) and (ii) on the network parameters (reactiveness, fairness and link utilization). We also provide a detailed description of the micromechanisms at the origin of the improvements of the TFRC behavior when multiple feedback per RTT are delivered, and determine the context where such feedback frequencies should be applied.

I. INTRODUCTION

TCP-Friendly Rate Control (TFRC) [START_REF] Floyd | TCP Friendly Rate Control (TFRC): Protocol Specification[END_REF] is a protocol which provides a congestion control compatible with TCP without reordering and reliability services. Currently, TFRC is one of the most promising approach for the transfer of multimedia flow in shared networks such as Internet.

In order to adapt the rate of TFRC to the network conditions, receivers estimate a drop rate p, which is sent to the senders (the TFRC feedbacks). In the sender side, at every received feedback, TFRC updates its rate according to (1) :

X = s R 2bp 3 + t RT O 3 3bp 8 p(1 + 32p 2 ) (1)
Where :

• X is the TCP's average transmit rate in bytes per second;

• s is the segment size in bytes (excluding IP and transport protocol headers);

• R is the round-trip time in seconds;

• t RT O is the TCP retransmission timeout value in seconds;

• b is the maximum number of packets acknowledged by a single TCP acknowledgment (set to one following the current RFC). Several studies have shown that TFRC offers good responsiveness and higher TCP-friendliness level compared to others TCP-friendly schemes in a wide range of wired-network topologies [START_REF] Tsao | Taxonomy and evaluation of tcp-friendly congestion-control schemes on fairness, aggressiveness, and responsiveness[END_REF]. As a consequence, many efforts have been made to optimize [START_REF] Jourjon | Optimization of tfrc loss history initialization[END_REF], [START_REF] Xu | Media streaming via tfrc: An analytical study of the impact of tfrc on user-perceived media quality[END_REF] and to adapt TFRC to network dynamics introduced by wireless networks (such as in the context of vertical handovers [START_REF] Gurtov | Effect of vertical handovers on performance of tcp-friendly rate control[END_REF], [START_REF] Panagiotis Stathopoulos | Performance of tfrc during vertical handover events in heterogeneous networks[END_REF]).

In parallel, the use of satellite in IP networks, as a mean to transfer multimedia streams to fixed or mobile terrestrial nodes, motivates the analyze of TFRC in a satellite context. Thus, in a previous contribution [START_REF] Sarwar | Improvements in dccp congestion control for satellite links[END_REF] we have reported in a preliminary study that in long delay link, the goodput of TFRC can be improved by increasing the numbers of feedbacks per RTT. Thus, following empirical measurements, we advise to send at least one feedback every 100ms when RTT reaches one second delay.

Therefore, in this paper, we aim at providing a detailed study of the impact of the feedback frequency on the TFRC key parameters, like the experienced RTT and the estimated drop rate. Later, we focus on the impact of multiple feedback per RTT on the reactiveness and the fairness. The main goal of this study is to determine the context where the increase of the feedback frequency may improve the behavior of TFRC.

Well understand the whole behavior of TFRC in presence of multiple feedbacks per RTT would allow us to better tune TFRC stack and avoid unnecessary overhead. Indeed, increasing the amount of feedback messages might impact on the return link capacity of satellite link (quite low in general). Thus, a particular attention must be taken to minimize this value.

II. TOPOLOGY

In order to investigate the impact of the feedback frequency on the TFRC behavior, we drive a set of simulations with the ns-2.33 network simulator and using the default TFRC parameters, only varying the feedback frequency. The topology used is shown in Figure 1. In our simulations, we send two, three and four TFRC flows sharing a bottleneck with a maximum capacity of 1Mbps. The propagation delay of the bottleneck link is fixed to 250ms to simulate LEO satellite links and 500ms to simulate GEO satellite links. Those propagation delay values are identical for both forward and reverse paths. Finally, TFRC is configured to send one, two, three, four, five and ten feedback messages per RTT.

III. IMPACT OF FEEDBACK FREQUENCY ON TFRC

PARAMETERS

In order to observe the benefits or drawbacks of increasing the feedback frequency on TFRC, next subsections present a detailed view of two parameters that drive the overall TFRC behavior (see Eq. 1): the computed loss rate p, and the experienced RTT. Note also that in this study, simulation results with one feedback per RTT are used as reference as this frequency corresponds to the value set by default in the standard TFRC implementation.

A. Impact on the computed loss rate

After every simulation done, we compute the cumulative average of the reported loss rate p by the receiver of each connection. In Table I

TABLE I AVERAGE LOSS RATE

From Table I it can be observed that in most cases, feedback frequencies higher than one per RTT lead to a lower computed drop rate. However, the computed drop rate does not follow a uniform behavior. Indeed, the feedback frequency cannot be related to the value of p, since those values seem to follow random distribution. Therefore, the feedback frequency has not much impact on the computed loss rate from a macroscopic point of view.

B. Impact on the computed RTT

In order to explain the impact of the feedback frequency on the experienced RTT, we present in this section the evolution of the computed RTT during the simulation with a 500ms link delay, two competing flows and one feedback per RTT (Figure 2(a)) and four feedbacks per RTT (Figure 2(b)). We have chosen those feedback frequencies since they represent the general behavior of the computed RTT in all our simulations.

Since after the Slow-Start, TFRC suffers from link underutilization and slow convergence [START_REF] Widmer | Equation-based congestion control[END_REF], during the first hundred seconds, TFRC experiences an RTT similar to the link propagation delay. However, after second 100, when TFRC enters in a steady state, it can be observed that the increase of the feedbacks frequency leads to a higher experienced RTT.

When TFRC sends only one feedback per RTT (Figure 2(a)), at second 165, the experienced RTT decreases to 1 second. In addition, around seconds 250 and 310, the senders estimate an RTT equal to 1.2 second.

Increasing the feedback frequency increases the experienced RTT. Indeed, when TFRC sends up to four feedbacks per RTT (Figure 2(b)), the experienced RTT is most of the time equal or higher than 1.3 second. Only at second 180 and during for few seconds, the experienced RTT falls down to 1.2 second. Sending more than 4 feedbacks per RTT does not introduce significantly variations to the experienced RTT showed in Figure 2(b).

Since in our simulations we did not introduce reverse traffic, occupancy link introduced by 4 feedback per RTT in the reverse path remains negligible. Thus, the higher experienced RTT means higher link utilization in the forward path.

IV. UNDERSTANDING THE IMPACT FREQUENCY ON TFRC

We have seen that when the feedback frequency increases, the rate of the senders tends to decrease while the experienced RTT seems to increase. At first glance, this phenomenon seem to violate the next basic rule : the decrease of the rate of the senders should decrease the congestion in networks. Therefore the experienced RTT should also decrease.

After analyzing our simulation results, we find out that higher feedback frequency only improve the accuracy of the estimated RTT. Consequently, the accuracy of the estimated p increases and TFRC flows are able to grab the network resources with a minimum needed sending rate. In next paragraphs we will clarify this phenomenon.

Suppose that due to a congestion, a TFRC flow loses two packets (as described in Figure 3). Let us consider for this case two scenarios : (i) a feedback frequency #1 which computes the RTT #1 (in dashed lines) and (ii) another feedback frequency #2 (in plain lines) which results in the computation of a slightly lower (but more accurate) RTT #2. In this context, the way the number of loss events is computed is different. Indeed, in the first case both losses belong to the same loss event while in the second case, TFRC will see two losses events. As a consequence, TFRC will react differently. We could think that in this example, finally, the accuracy of the RTT value is counterproductive for the source as with a wrong RTT the receiver would estimate one Congestion Event only. Obviously, we will get the reverse case if the RTT given by samples from the frequency #2 is larger, but still accurate, than the RTT estimated by the frequency #1. As a conclusion, whenever the RTT increases or decreases conjointly with the distribution of the loss events, higher feedback frequency per RTT can result in a better adapted behavior of TFRC to the network conditions.

A more accurate view of the network state is suitable in high dynamic networks with long delay. Indeed, a more accurate perception of the congestion parameters leads to a faster adaptation of TFRC to network conditions. Since in real conditions, network are mostly dynamic environments, the benefits of higher feedback frequencies will remain more visible that in simulations environments [START_REF] Sarwar | Improvements in dccp congestion control for satellite links[END_REF]. In non-dynamic networks (e.g. networks where the assigned bandwidth to a flow remains almost constant), the benefits of increasing the feedback frequency remain limited even in presence of long propagation delay. This phenomenon is produced by the Congestion Avoidance mechanisms implemented in TFRC which introduce very slow oscillation of the rate after reacting to losses events, and when no more losses are detected (the steady state of TFRC). Finally, we note that an increase of the TFRC feedback has limited benefices. Indeed, decreasing the inter-arrival time of feedback packets by only a few milliseconds does not introduce significantly variations in the behavior of TFRC.

V. IMPACT OF THE FEEDBACK FREQUENCY ON THE

NETWORK

In this new section we aim at knowing if such benefits can have an impact on macroscopic networks parameters, like the responsiveness of TFRC and the fairness.

Thus, we have simulated 3 flows sharing a bottleneck, coming in to the network with an inter-arrival time of 200ms. In this case, we used a 250ms propagation link delay (RTT ≈ 500ms). Figures 4(a), 4(b) and 4(c) show graphically the evolution of the rate seen by the receivers for simulations with 1, 4 and 5 feedback/RTT respectively. Functions were plotted by calculating the goodput in a one second interval.

A. Reactiveness and Convergence

In the first case, with a frequency of one feedback per RTT, it can be observed from Figure 4(a) that every time a new flow enters the network, it needs almost 200 seconds to converge and enter in a steady state phase.

Increasing the feedback frequency to four per RTT (second case) improves significantly the responsiveness of TFRC. Indeed, at second 200, flow labeled "TFRC-0" quickly yield 0.2 Mbps to flow labeled "TFRC-1". Thus, new incoming flows need around 100ms to converge, which represent only 50% of the total time needed by the first case. In this second case, flows converged to a point very close to the fairness point.

When TFRC is configured to deliver up to 5 feedbacks per RTT (third case), it can be observed that at second 340, flows far from the fairness point after one of them detect losses events. When a third flow comes into the network, a faster convergence between last two flows can be observed. However, the "TFRC-1" flow, which has been less penalized by losses events remains far of the fairness line.

Results from the third case are useful to illustrate that higher feedback frequencies can improve the convergence speed of flows. Moreover, the resulting fairness level will depend on how lost packets were distributed between flows.

Another parameter which is important to explorer is the link utilization. After calculating the average link utilization of the bottleneck, we have found that in general, the available bandwidth is well filled when multiple feedback per RTT are delivered. Indeed, in our simulation, the bottleneck utilization ranges from 97% for the worst case to 98% for the best case.

VI. EXPERIMENTS WITH REAL IMPLEMENTATION

For the sake of completeness, additionally, we present some results of TFRC on an emulated satellite network. We have used the same implementation as in [START_REF] Sarwar | Improvements in dccp congestion control for satellite links[END_REF] where the number of feedbacks sent during an RTT, N F b , is periodically calculated as :

N F b = max(int( RT T RT T ref ), 1) (2)
where RT T is the current estimated RTT and RT T ref , the targeted interval between two feedback messages. Note that to obtain the standard TFRC behavior, in Equation 2, RT T ref = RT T . To analyze the behavior of TFRC in presence of multiple feedbacks in a single RTT we used RT T ref = 100ms.

We emulate a satellite network by using the Linux Netem emulator in a PC configured as a router. This replaces the satellite link shown in Figure 1. The results presented here are for the GEO satellite configuration, with a 1000ms RTT and a 1Mbps capacity for uplink and downlink. No packet loss rate was introduced.

Figure 5 show the resulting throughput obtained by one TFRC flow, with one feedback per RTT shown in 5(a) and the dynamic feedback computation scheme shown in 5(b). We can clearly see in these figures the benefit of using the multiple feedbacks scheme on the instantaneous throughput obtained by TFRC.

VII. CONCLUSION

In this article we show that increasing the numbers of feedback per RTT may improve the perception of the congestion level parameters at the sender. The key at the origin of such improvement is an accurate value of the experienced RTT by the senders. Since a more accurate RTT value implicitly improves the drop rate seen by the receivers, this result in faster adaptation (high responsiveness) of TFRC to the network congestion levels.

Our analysis have also shown that the improvements carried in by the increase of the feedback frequency may have important effects on long-delay dynamic networks. Indeed, in short-delay networks, one feedback per RTT is enough to get an accurate RTT value. Also, in non-dynamic networks, the congestion avoidance mechanisms of TFRC which seek to avoid oscillations and losses events, limit the benefits of having a more accurate RTT value.

However, in long-delay dynamic networks, like satellitebased networks, increasing the feedback frequency leads to improvement of the fairness and the reactivity.

Following our simulation and emulation results, we find that in a context where the base RTT values are between 500ms and 1000ms, increasing feedback from one to 4 or 5 feedbacks per RTT (i.e. around one feedback every 200ms) improves the performance of TFRC. Also that higher frequency of feedback does not provide further performance improvements.

We hope this study will help TFRC users to better understand the impact of the feedback frequency and to the deployment of TFRC over long-delay links.

Fig. 1 .

 1 Fig. 1. Topology

  Fig. 2. Experienced RTT

Fig. 3 .

 3 Fig. 3. Example of the impact of frequency feedback on losses

Fig. 4 .

 4 Fig. 4. Active flows arriving with a 200s

Fig. 5 .

 5 Fig. 5. Throughput obtained by one TFRC flow

  , we present the average value of p as a function of the reference value (i.e. one feedback per RTT). Thus, values bigger than one reveal a higher computed drop rate and respectively, values smaller than one reveal a lower computed drop rate.

	No. flows	RTT (ms)	2	Feedback per RTT (p N /p 1 ) 3 4 5	10
	2	500	0.88	0.98 0.89	0.94	0.90
	2	1000	1.06	1.14 0.99	0.89	0.92
	3	500	0.84	0.95 0.86	0.91	0.89
	3	1000	0.92	0.94 0.90	0.95	1.00
	4	500	1.04	0.98 1.01	0.92	0.93
	4	1000	1.00	1.06 0.97	1.11	1.00


S Calvez

P Dubreuil

M Diallo

E Daran

A Monmayrant

O Gauthier-Lafaye

Si Z O X N Y BY INDUCTIVE-COUPLED-PLASMA PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITION FOR MULTI-LAYER OPTICAL THIN FILMS

Introduction

Material characterisation Optical thin-film coatings

Conclusions Materials & processes

Low-temperature-grown thin films are interesting for numerous applications in micro-electronics / photonics. The ability to grow materials whose refractive index can span a wide range of values facilitates the fabrication of optical multilayer stacks and, by proxy, of opto-eletronic devices.

Silica (SiO 2 ), silicon nitride (Si 3 N 4 ) and amorphous silicon (a-Si) are conventionally used materials for micro-electronics and photonics.

Here, we study two types of alloys

• SiO 2 /Si 3 N 4 • Si 3 N 4 / a-Si
The SiO 2 /Si 3 N 4 compounds are created by variation of the N 2 O gas flux in the SiH 4 /N 2 O/N 2 gas mixture.

The Si 3 N 4 / aSi materials are obtained by modification of the plasma RF power for fixed SiH 4 /N 2 gas injection.

 Found the conditions to deposit mixed alloys between SiO 2 /Si 3 N 4 and Si 3 N 4 / a-Si  Use real-time in-situ monitoring tools to gain insight into/monitor the depositions  SiON layers with compositionally-adjustable refractive index between 1.48 to 2.78+i0.007 @ 775nm with roughness < 1nm  Demonstrated multilayer coatings Here, we establish the deposition conditions of SiON thin films by Inductively-Coupled-Plasma Plasma Enhanced Chemical Vapor Deposition (ICP-PECVD) at low temperature (100°C) et demonstrate their use to crea te anti-reflective and mirror optica lcoatings.

Instrumented equipment

In-situ real-time characterisation tools:

• Optical Emission Spectroscopy • Reflectometry OES SiON #1 Fused SiO 2 SiON #2 Design criteria 1. R=0% 2. Wide Bandwidth (1) => n 2 ~(n subs /n air ) 0.5 *n 1 (2) => low dispersion => SiON #1 = SiO 2 (& n 2 ~1.67) Anti-Reflective coating SiON #1 Fused SiO 2 SiON #2 … Design criteria 1. R>99% 2. Wide Bandwidth (1) & (2) => high n 1 /n 2 => SiON #2 = SiO 2 & SiON #1 = high-Si Si-SiN Mirror @ 1550nm
Mirror coating Ellipsometry Atomic Force Microscope


S Sa

Novel (super)hard SiCN from crystal chemistry and first principles

Keywords: Silicon carbonitride, DFT, phonon dispersions, superabrasive, electronic band structure

The purpose of this work is to predict the existence of novel equiatomic SiCN based on tetragonal C 6 structure (glitter), the elementary building unit being the 1,4 cyclohexadiene molecule comprising both tetrahedral (sp 3 ) and trigonal (sp 2 ) carbons. From crystal chemistry rationale the structural transformations of C 6 to SiC 2 and then to SiCN ternary phase were fully relaxed to the ground states using first principles DFT-based calculation.

Like early proposed C 6 and SiC 2 , SiCN was found bonding and structurally stable from the elastic properties on one hand and dynamically stable from the phonons, on the other hand. The Vickers hardness of SiCN is higher than that of cubic silicon carbide, a conventional superabrasive, whereas hardness of tetragonal SiC 2 is slightly lower. Besides the abrasive properties, the electronic band structure indicates metal-like behavior of SiCN thus suggesting the potential for heat dissipation in operating conditions, oppositely to insulating SiC.

Introduction

Whereas SiC is a rather well-studied binary compound that is widely used for various applications [START_REF] Kimoto | Fundamentals of Silicon Carbide Technology: Growth, Characterization, Devices, and Applications[END_REF], ternary compounds of the Si-C-N system are ill-studied. In 1965, equiatomic SiCN was reported as made by SiC crystals grown in nitrogen atmosphere [START_REF] Kawamura | Silicon carbide crystals grown in nitrogen atmosphere[END_REF]. The stated structure shown in Fig. 1a is a cubic fluorite-like one with face centered Si substructure, and C and N atoms occupying the fluorite F8 cube corners. Later, Chen et al. [START_REF] Chen | Crystalline SiCN: a hard material rivals to cubic BN[END_REF] identified crystalline and amorphous SiCN in the process of thin films growth by microwave CVD; but no crystal structure was reported. The conducted nano-indentation study revealed a hardness value of 30 GPa for the crystalline phase and 22 GPa for the amorphous phase, therefore, the authors' claim that SiCN is a hard material that rivals cubic BN is unjustified. In 1997, Riedel et al. [START_REF] Riedel | The first crystalline solids in the ternary Si -C -N system[END_REF] claimed the first ternary crystalline solid with Si 8 C 4 N 16 stoichiometry built of SiN 4 tetrahedra interconnected via carbon atoms, thus leading to N 3 SiN--C--NSiN 3 -like chains of tetrahedra (blue, Fig. 1c) along vertical b-axis, and corner sharing along horizontal planes The latter feature is also characteristic of the novel SiCN proposed herein, with a mixed N-C coordination polyhedron for Si.

In this work, a novel equiatomic SiCN phase is proposed based on crystal chemistry rationale then quantified by ab initio calculations within the well-established density functional theory (DFT) [START_REF] Scholl | Density Functional Theory. A Practical Introduction[END_REF].

Crystal Chemistry rationale

The chemistry of the silicon with carbon and nitrogen is ruled by several factors involving:

(i) the difference of -Pauling-electronegativities with (N) = 3.04 > (C) = 2.55 > (Si) = 1.80 leading to polar Si-C and Si-N polar bonds.

(ii) the larger size of covalent radius: r Si = 1.14 Å versus r C = 0.76 Å and r N = 0.70 Å resulting from the atomic configuration exceeding carbon and nitrogen with one more shell, i.e., Si (1s 2 2s 2 2p 6 3s 2 3p 2 ) versus C (1s 2 2s 2 2p 2 ) and N (1s 2 2s 2 2p 3 ) leading to more compressible Si based compounds.

(iii) the tetrahedral coordination (sp 3 -like), common to Si, C, and N which also exhibit linear sp and trigonal sp 2 types. However, in rare occurrences, Si=Si (double bonds) and SiSi (triple bonds) are known mainly in organic chemistry as in tetramesityldisilene [START_REF] West | Tetramesityldisilene, a Stable Compound Containing a Silicon-Silicon Double Bond[END_REF].

As a case study on the interconnectedness between solid state and molecular chemistry regarding carbon, we have shown recently that the rarely occurring tricarbon C 3 molecule can be a building block of 3D covalent ultra-hard carbon networks, with carbon showing two hybridization types: linear-and tetrahedral-like [START_REF] Matar | First-principles investigations of tricarbon: From the isolated tricarbon C 3 molecule to a novel ultra-hard anisotropic solid[END_REF]. Earlier, Bucknum and Hoffman in 1994 [START_REF] Matar | First-principles investigations of tricarbon: From the isolated tricarbon C 3 molecule to a novel ultra-hard anisotropic solid[END_REF] conceived a dense hexacarbon tetragonal structure with linear C=C (sp 2 ) and tetrahedral C(sp 3 ) based on 1,4-cyclohexadiene. Such molecule depicted in Fig. 2a is used as an effective hydrogen donor for catalytic hydrogenation reactions. Whereas in butadiene there are alternating double and single bonds between the six carbon atoms (Kekulé formulation) in two resonant formulas, 1,4-cyclohexadiene shows a localization of two C=C double bonds.

The corresponding 3D (D = dimension) structure called 'glitter' [START_REF] Bucknum | A Hypothetical Dense 3,4-Connected Carbon Net and Related B 2 C and CN 2 Nets Built from 1,4-Cyclohexadienoid Units[END_REF] i.e., shining due to the conductive character of the electronic structure as shown in the bad structure section development of this work (cf. Figure 5). The crystal structure featuring a C6 hexahedron is shown in Fig. 2b. It consists of two types of carbons labeled C1 (tetrahedral) and trigonal C2, letting express the chemical formula as C1 2 C2 4 .

Later, a silicon dicarbide Si 2 C 4 (Fig. 2c) was theoretically proposed based on tetragonal C 6 , by replacing tetrahedral C1 by Si leading to SiC4 tetrahedra connected through the two C2 characterizing the structure [START_REF] Andrew | A theoretical investigation of the stability of crystalline silicon dicarbide[END_REF]. With the objective of identifying SiCN ternary compound in view of the inefficiency of the cubic setup proposed earlier [START_REF] Kawamura | Silicon carbide crystals grown in nitrogen atmosphere[END_REF], or of cyanide-like SiCN, we come up with the equiatomic stoichiometry through replacing one of the two trigonal carbons (C2) by N leading to SiC2N2 tetrahedra. The ad hoc structure submitted to unconstrained geometry relaxation led to a lowering of the tetragonal space group symmetry to P4 2 mc N°105. The ground state equiatomic structure is depicted in Fig. 2d. To the best of our knowledge, no propositions have been made of equiatomic SiCN structure or existence other than the 1965 paper [START_REF] Kawamura | Silicon carbide crystals grown in nitrogen atmosphere[END_REF].

Computational framework

The search for the ground state structures and their energies such as those devised from crystal chemistry rationale, is a prerequisite to discriminate stable from unstable chemical systems prior to further investigations of the physical properties. Such a task is best carried within quantum mechanical computations based on the DFT [START_REF] Scholl | Density Functional Theory. A Practical Introduction[END_REF]. In present work, our investigations, were carried out using the Vienna Ab initio Simulation Package (VASP) code [START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF][START_REF] Kresse | From ultrasoft pseudopotentials to the projector augmented wave[END_REF] and the built-in projector augmented wave (PAW) method [START_REF] Kresse | From ultrasoft pseudopotentials to the projector augmented wave[END_REF][START_REF] Blöchl | Projector augmented wave method[END_REF] considering atomic potentials. The DFT exchange-correlation XC effects were considered within the generalized gradient approximation (GGA) [START_REF] Perdew | The Generalized Gradient Approximation made simple[END_REF] found herein to perform like more sophisticated hybrid functional as the GGA-HSE06 [START_REF] Heyd | Hybrid functionals based on a screened Coulomb potential[END_REF]. From heavy preliminary calculations, cohesive energies with hybrid functional were found close but slightly larger than the plain GGA results. Such overestimation could lead to validate unstable structures/compositions as the SiCN found unstable also from elastic and dynamic properties in the elaboration of this work.

From the ground state structures the sets of elastic constants C ij were calculated, and the mechanical properties such as hardness and fracture toughness were estimated using contemporary theoretical models. To verify the dynamic stability of the different phases, the phonon dispersion bands were calculated. The approach consists of computing the phonon modes through finite displacements of the atoms off their equilibrium positions to subsequently obtain the forces from the summation over the different configurations. The phonon dispersion curves along the main lines of the tetragonal Brillouin zone (BZ) were obtained using first-principles phonon calculation code "Phonopy" [START_REF] Togo | First principles phonon calculations in materials science[END_REF]. Finally, the electronic band structures showing small gap insulating to metallic behaviors were assessed.

Results and Discussions

Energies and crystal structures

The structures presented in the introduction were submitted to unconstrained geometry relaxation to obtain the ground state configuration. Energy is the prevailing criterion for a first assessment of the crystal structures.

From the total electronic energies (E tot ) the cohesive energies were obtained by deducting the atomic contributions. In view of the different stoichiometries, only the atom averaged cohesive energies can be compared.

Cubic SiCN structure templates

The structure of cubic SiCN (space group F-43m, N°216) is shown in Fig. 1a resembling fluorite-type is built of face centered Si substructure with C and N occupying cube corners [START_REF] Kawamura | Silicon carbide crystals grown in nitrogen atmosphere[END_REF]. Replacing N by C to model cubic SiC 2 for the sake of comparison in present work leads to higher symmetry fluorite-type (Fm-3m N°225) shown in Fig. 1b. Both cubic SiCN and SiC 2 were submitted to unconstrained geometry optimization calculations of the energy. The numerical results of the cohesive energies obtained by subtracting the atomic contributions from the total electronic energy are shown in the first lines of Table 1. They reveal small positive magnitudes leading to doubt the validity of cubic SiCN structure and to search for alternatives.

The ordered structure of orthorhombic equiatomic sodium cyanide NaCN [START_REF] Fontaine | Structure du cyanure de sodium en phase ordonnée[END_REF] was considered by replacing Na by Si and operating full geometry optimizations. The resulting geometry consisted of a layered structure with square planar SiC2N2 motifs (Fig. 1d). Such coordination is unusual for Si. The cohesive energy, while being negative, i.e., favored with respect to cubic SiCN, is of a lower magnitude versus the other configurations based of C 6 Glitter as shown in next paragraph to feature tetrahedral coordinated silicon. Such layered structure hypothesis with square planar SiC2N2 motifs was subsequently abandoned.

Lastly, the other alkaline cyanides ACN (A = Li, K, Cs) characterized solely by A-N connections were also considered in the preliminary calculations but all led to lower cohesive energies versus square-planar NaCN-type SiCN (Fig. 1c). Such results can be chemically expected since ACN cyanides are A+ CN-, i.e.,"ionic", whereas SiCN cannot be a cyanide but rather a carbonitride, whence the interest in proposing a stable phase with tetrahedral SiC2N2 motifs.

Glitter-derived SiCN

The second part of Table 1 shows cohesive chemical systems all larger than the preliminary results at upper part of the table. C 6 which contains both tetrahedral and trigonal carbon types is characterized with E coh. /at.= -2.037 eV/at., i.e. less cohesive than diamond made of purely C(sp 3 ) -cf. [START_REF] Matar | Novel ultra-hard hexacarbon allotropes from first principles[END_REF]. Si 2 C 2 N 2 is cohesive with E coh. /at. magnitude smaller but close to that of Si 2 C 4 .

Crystallographically, the replacement of C1 by Si in C1 2 C2 4 ("glitter" C 6 , Fig. 2b) leads to hypothetic silicon dicarbide Si 2 C 4 (Fig. 2c) [START_REF] Andrew | A theoretical investigation of the stability of crystalline silicon dicarbide[END_REF] which shows the same tetrahedral stacking as in C 6 . Both structures are defined in tetragonal space group P4 2 /mmc, N° 131 (cf. Tables 1 and2). Since the two C2 belong to the same Wyckoff site (4i), the replacement of one carbon by nitrogen leads to Si 2 C 2 N 2 where Si is tetrahedrally coordinated with trigonal N and C. The initial tetragonal space group P4 2 /mmc N° 131 symmetry is lowered and subsequent crystal structure determinations after geometry relaxation led to define the ternary phase in a new space group P4 2 mc N°105 (Fig. 2c), i.e., with the loss of a mirror (m) symmetry due to the occupation of the fourfold (4i) ½, 0, z position in Si 2 C 4 (Table 2b) by two different atoms, N and C, which are now are in twofold (2c) ½, 0, z positions (Table 2c). The trigonal carbon interatomic distances increase from C 6 with d(C2-C2) = 1.34 Å to ~ 1.37 Å in Si Note that the tetrahedral angle C-Si-N = 109.76° presents an interesting feature of sp 3 -like environment. This is a relevant result in view of the symmetry lowering to tetragonal space group P4 2 mc N°105. Consequently, SiCN is likely to exist in a structure with mixed N, C coordination expressed in regular SiC2N2 tetrahedra connected via C-N along the tetragonal c-axis direction. Figure 2e shows a 221 supercell of SiCN highlighting the SiC2N2 tetrahedra shown in the same blue color as Si.

Charge density projections

Further illustration of electronic  crystal-structure relationship is provided by the charge density projections. Figure 3 shows the charge density yellow volumes in the different panels.

In "glitter" C 6 (Fig. 3a) the charge density is expectedly different between tetrahedral C1 with sp 3 -like 4-lobes yellow shapes versus trigonal C2 (sp 2 -like with linear-like yellow volumes) thus confirming the difference of chemical behavior between carbons belonging to the two crystal sites.

Upon replacing C1 by Si the charge density changes significantly with a transfer from Si to C2. C2-C2 groups show a larger charge envelop versus C2-C2 in C 6 (Fig. 3b). Nevertheless, Si keeps a tetrahedral coordination with C, forming SiC4 tetrahedra.

The replacement of one C2 by N, leading to Si 2 C 2 N 2 equiatomic compound results in a differentiated charge envelop along the C-N pairs with the largest charge volume around N (Fig. 3c). These results are chemically sound is so far that they go along with the electronegativity trends: (N) = 3.04 > (C) = 2.55 > (Si) = 1.80 which coincide with the trends of charge transfers from Si to both C and N, i.e., (N) = -2.12, (C) = -1.72, and (Si) = +3.84.

Mechanical properties

Elastic constants

The elastic constants C ij are needed to derive the mechanical properties. They were determined by performing finite distortions of the lattice and deriving C ij from the strainstress relationship. Then the bulk B and shear G moduli are obtained by averaging the singlecrystal elastic constants using, here, Voigt's method [START_REF] Voigt | Über die Beziehung zwischen den beiden Elasticitätsconstanten isotroper Körper[END_REF] based on a uniform strain. The calculated sets of elastic constants are given in Table 3. All values are positive, which is the first indication of mechanical stability. Further proofs were obtained from C ij combinations obeying rules related to mechanical stability: C ii (i =1, 3, 4, 6) > 0; C 11 > C 12 , C 11 +C 33 -2C 13 > 0; and 2C 11 + C 33 + 2C 12 + 4C 13 > 0.

The equations providing the bulk B V and shear G V moduli as function of the elastic constants C ij are as follows for the tetragonal system:

G Hardness

Vickers hardness (H V ) was predicted using two contemporary theoretical models. The thermodynamic (T) model [START_REF] Mukhanov | The interrelation between hardness and compressibility of substances and their structure and thermodynamic properties[END_REF] is based on thermodynamic properties and crystal structure, while Lyakhov-Oganov (LO) approach [START_REF] Lyakhov | Evolutionary search for superhard materials: Methodology and applications to forms of carbon and TiO 2[END_REF] considers topology of the crystal structure, strength of covalent bonding, degree of ionicity and directionality. The fracture toughness (K Ic ) was evaluated using Mazhnik-Oganov model [START_REF] Mazhnik | A model of hardness and fracture toughness of solids[END_REF] which is the only model that allows such predictions. The results on mechanical properties of the considered phases of the Si-C-N system are presented in Table 4. It is worth noting that hardness and bulk moduli (B 0 ) calculated in the framework of the thermodynamic model are in perfect agreement with available experimental data [START_REF] Chen | Crystalline SiCN: a hard material rivals to cubic BN[END_REF][START_REF] Vandeperre | The hardness of silicon and germanium[END_REF][START_REF] Vaidya | Compressibility of 22 elemental solids to 45 KB[END_REF][START_REF] Baloga | Nano-versus macro-hardness of liquid phase sintered SiC[END_REF][START_REF] Strössner | High pressure X-ray investigations on 3C-SiC[END_REF]. As follows from our findings, Vickers hardness of Si 2 C 2 N 2 is higher than that of cubic silicon carbide c-SiC (Table 4), a conventional superabrasive, whereas the hardness of tetragonal Si 2 C 4 is slightly lower.

Dynamical stabilities from the phonons

Further stability criteria can be obtained from the phonon dispersions. Following the method presented in the 'Computational framework' section, the obtained phonon dispersion curves for C 6 , Si 2 C 4 and Si 2 C 2 N 2 are shown in Fig. 4. In each panel the bands run along the main lines of the tetragonal Brillouin zone.

Along the vertical direction the frequency ω is given in units of Terahertz (THz). Since no negative energy magnitudes are observed, the three structures can be considered as dynamically stable. There are 3N-3 optical modes at high energy and 3 acoustic modes. The acoustic modes start from zero energy (ω = 0) at the  point, center of the Brillouin Zone, up to a few Terahertz. They correspond to the lattice rigid translation modes in the crystal (two transverse and one longitudinal). The remaining bands correspond to optic modes.

Remarkably, in C 6 (Fig. 4a) the highest bands are at 50 THz, a magnitude higher than in diamond where ω max. ~40 THz [START_REF] Krishnan | Raman spectrum of diamond[END_REF] observed with Raman vibration spectroscopy. Such high frequency is assigned to the C2-C2 stretching vibrations. The phonons of Si 2 C 4 (Fig. 4b) have similar shape but the highest magnitude is now at ω max ~45 THz. This is concomitant with the overall smaller elastic constants especially C 33 and the resulting B and G values as well as the hardness. The phonons dispersions of the equiatomic Si 2 C 2 N 2 show different features versus the two binaries with many flat bands resulting from the unique structure of SiC 2 N 2 tetrahedra and the strong C-N interaction.

Electronic band structures

The electronic band structures were calculated within DFT based on the Augmented Spherical Wave ASW method [START_REF] Eyert | Basic notions and applications of the augmented spherical wave method[END_REF]. The crystal input data of Table 1 were used. The band structures are depicted in Figure 5. The energy along the vertical direction is with respect to the Fermi level E F since all systems are metallic-like with bands crossing E F ; note that Si 2 C 4 shows vanishingly small band crossing at E F . Additional calculations using hybrid XC functional didn't change the metallic character identified for C 6 and SiCN, while bringing little changes to SiC 2 that could be considered as semi-conducting with a very small band gap. However, all three phases are far from the behavior of electronically insulating diamond characterized with a large band gap of 5 eV. The present results on electronic band structures can be approached with those of tricarbon allotropes [START_REF] Matar | First-principles investigations of tricarbon: From the isolated tricarbon C 3 molecule to a novel ultra-hard anisotropic solid[END_REF] where two kinds of hybridizations were identified for the carbon sites.

Conclusions

The present work focused on the identification of a viable structure of SiCN equiatomic following a protocol involving crystal chemistry rationalization of structures submitted in a second step to unconstrained geometry optimization with DFT-based quantum mechanical calculations. Among different hypotheses, the most stable structure consists of SiC 2 N 2 tetrahedra with a stacking resembling tetragonal "glitter" C 6 and the "glitter"-derived silicon dicarbide. Confirmation of the stabilities for "glitter" C 6 , silicon dicarbide and SiCN were inferred from the cohesive energies. The mechanical (elastic properties) and dynamic (phonon dispersions) stabilities have been confirmed. From the quantitative hardness assessments SiCN was found to be (super)hard which makes it a prospective superabrasive. Furthermore, metallic properties of SiCN revealed from the electronic band structure provide the potentiality of evacuating heat during operation (machining, etc.), oppositely to SiC which is an insulator.

  2 C 4 due to large Si atom resulting in the increase of a and c lattice constants. Si 2 C 2 N 2 structure has larger d(C-N) = 1.38 Å while d(Si-C) = 1.82 Å and d(Si-N) = 1.81 Å become close in magnitudes.

9 (

 9 2C 11 + C 33 + 2C 12 + 4C 13 ).

  15 (2C 11 + C 12 + 2 C 33 -2C 13 + 6C 44 + 3C 66 ).The calculated C ij are given in Table3. All values are positive, and their combinations obey the rules pertaining to the mechanical stability. C 6 Glitter characterized by shortest interatomic distances has the highest elastic constants. In Si 2 C 4 obtained by the replacement of C1 by larger Si, the resulting bulk and shear moduli lead to drastically lower C ij , as well as and . value agrees with literature [9]. The replacement of carbon by silicon possessing a larger radius and the overall resulting larger volume can explain the larger compressibility of Si 2 C 4 . Turning to the ternary phase Si 2 C 2 N 2 , the calculations of the two moduli and lead to significant increase, explained by the smaller d(Si-C) and d(Si-N) versus d(Si-C) in the dicarbide (

6 ,

 6 space group P4 2 /mmc (N° 131). (Fig. 2b). a = 2.589 Å; c = 5.979 Å. -C2) = 1.34 Å; d(C1-C2) = 1.54 Å. C2-C1-C2 = 106.795°, C1-C2-C2 = 122.517° b) Si 2 C 4 (SiC 2 ) space group P4 2 /mmc (N° 131). (Fig. 2c). a = 3.170 Å; c = 6.878 Å. -C) = 1.37 Å; d(Si-C) = 1.89 Å. C-Si-C =113.32° C-C-Si =123.19° c) Si 2 C 2 N 2 (SiCN) space group P4 2 mc (N° 105). (Fig. 2d). a = 2.9492 Å; c = 6.978 Å. -N) = 1.38 Å; d(Si-C) = 1.82 Å; d(Si-N)= 1.81 Å. C-Si-N =109.76°

Figure 1 .

 1 Figure 1. Si-C-N ternary structures within preliminary assessments. a) Cubic fluorite-like equiatomic c-Si 4 C 4 N 4 [2], and b) Si 4 C 8 derived from cubic Si 4 C 4 N 4 ; c) Si 8 C 4 N 16[START_REF] Riedel | The first crystalline solids in the ternary Si -C -N system[END_REF] showing SiN4 tetrahedra in blue color, d) optimized SiCN derived from orthorhombic NaCN[START_REF] Togo | First principles phonon calculations in materials science[END_REF]. Blue, brown, and grey spheres correspond to Si, C and N atoms, respectively.

Figure 2 .Figure 3 .

 23 Figure 2. Sketches of the different structures: (a) 1,4-cyclohexadiene isolated molecule highlighting sp 3 and sp 2 carbons; (b) tetragonal C 6 ; (c) tetragonal Si 2 C 4 (SiC 2 ); (d) tetragonal equiatomic Si 2 C 2 N 2 (SiCN); (e) 221 supercell of SiCN highlighting the SiC2N2 tetrahedra. Blue, brown, and grey spheres correspond to Si, C and N atoms, respectively.

Figure 4 .

 4 Figure 4. Phonon dispersions along directions of the tetragonal Brillouin zone. (a) «glitter» C 6 ; (b) «glitter»-based Si 2 C 4 (SiC 2 ); (c) Si 2 C 2 N 2 (SiCN).

Figure 5

 5 Figure 5 Electronic band structures along main directions of the tetragonal Brillouin zone: (a) "glitter" C 6 ; (b) "glitter"-based Si 2 C 4 (SiC 2 ); (c) Si 2 C 2 N 2 (SiCN).

Table 2 )

 2 . Such results should be further rationalized with the calculation of the hardness in following section.

	. Voigt tetr B	. Voigt tetr G	. Voigt tetr B
		. Voigt tetr B	. Voigt tetr

Table 1

 1 Total energies (E tot ) and atom averaged cohesive energies (E coh /atom.) of different phases considered in the present work.

	Phase	Space Group Number E Tot. (eV)	E Coh. /at. (eV)
	c-Si 4 C 4 N 4 [2] F-43m Fm-3m c-Si 4 C 8 * o-Si 2 C 2 N 2 [15] Pmmn	N°216 N°225 N°59	-73.38 -72.32 -44.12	+0.054 +0.340 -1.210
	C 6 Si 2 C 4 (SiC 2 ) P4 2 /mmc P4 2 /mmc Si 2 C 2 N 2 (SiCN) P4 2 mc	N°131 N°131 N°105	-51.82 -45.21 -45.37	-2.037 -1.458 -1.418

* Structure based on Ref. 2

Atomic energies in eV: Si (-5.03); C (-6.6); N (-6.8).

Table 2

 2 Calculated crystal structures parameters.

Table 3

 3 Calculated elastic constants and resulting bulk (B V ) and shear (G V ) moduli (in GPa).

		C 11	C 12	C 13	C 33	C 44	C 66	B V	G V
	C 6	715	45	95	1172 69	71	340	296
	Si 2 C 4 (SiC 2 )	342	5	103	523	36		181 * 122
	Si 2 C 2 N 2 (SiCN) 429	5	135	718	4	115	236	155

* In agreement with Ref. 9 results.

Table 4

 4 Mechanical properties of some phases of the Si-C-N system: Vickers hardness (H V ), bulk modulus (B), shear modulus (G), Young's modulus (E), Poisson's ratio () and fracture toughness (K Ic ).

	Phase (space group)	T *	H V LO †	exp.	B 0	*	B B V	exp.	G V	E ‡	 ‡	K Ic	§
									GPa					MPa•m ½
	c-Si (227)	12	14	13 [22]	102	90	101 [23]	48	122	0.27	0.7
	C 6	(131)	83	75	--	376	340	--	296	688	0.163	5.7
	c-SiC (216)	34	30	33 [24]	236	216	248 [25]	126	316	0.26	2.5
	Si 2 C 4	(131)	31	32	--	183	181	--	122	299	0.225	2.1
	c-SiCN (216)	30	41	30 [3]	335	--	--	--	--	--	--
	Si 2 C 2 N 2	(105)	38	35	--	228	236	--	155	381	0.231	3.0

*

Thermodynamic model

[START_REF] Mukhanov | The interrelation between hardness and compressibility of substances and their structure and thermodynamic properties[END_REF]

† Lyakhov-Oganov model

[START_REF] Lyakhov | Evolutionary search for superhard materials: Methodology and applications to forms of carbon and TiO 2[END_REF]

‡ E and  values calculated using isotropic approximation § Mazhnik-Oganov model

[START_REF] Mazhnik | A model of hardness and fracture toughness of solids[END_REF]

Acknowledgements

Computational facilities of LGU are acknowledged.


M Hoyos

Gilbert Teyssedre

P Tiemblo

J M Gomez-Elvira

Chemiluminescence

" Chemiluminescence

J M Gómez-Elvira

Chemiluminescence spectral evolution in metallocene polyolefin films under a cold helium plasma at low temperature

teaching and research institutions in France or abroad, or from public or private research centers.

Cold plasma surface treatments are increasingly being used as a non-polluting method for surface transformations. It is well known that the interaction of a cold plasma discharge with a polymer induces a number of processes such as the formation of new chemical groups, crosslinking, formation of short chains and electrons and ion implantation which modify the properties on the surface in a way that under conventional conditions would require high temperatures. These modifications can be considered as degradation or, when done in a controlled fashion, as a means to achieve additional surface properties, being classical examples adhesion, optical reflection, permeability and biocompatibility 1 .

Previous investigations on Plasma Induced Luminescence (PIL) on polyolefins led to identify three spectral components in the PIL spectra recorded in time intervals from 1s to several min after the application of the plasma, each having different excitation mechanism and thus different decay kinetics 2,3,4 . In the typical results shown in the figure below, the fastest component is due to the decay of photoluminescence excited by the UV of the plasma (with spectrum -1-peak close to 430 nm), the second is dominated by the production of carbonyl groups (conspicuous in spectrum -2-and responsible of the shift to near 400 nm), and the third, with the slowest kinetics, is due to charge recombination at the most conjugated chromophores which are preferentially charged by the plasma (long wavelength shift up to about 470 nm in spectrum -3-). These previous works were done on commercial polymers, remaining hence the doubt as to whether traces of catalysts, antioxidants or other additives could contribute to PIL.

In this work, PIL is investigated on a sequence of laboratory-synthesised metallocene polypropylenes, polyethylenes and ethylene-propylene copolymers films at -130°C. All polymers have been thoroughly studied in order to characterise all defective structures in the chain and the semicrystalline morphology. The PIL spectral features of these (co)polymers conform to those obtained previously on commercial polyolefins, thereby discarding potential contributions from additives and residues in the later since synthesis techniques are completely different. While the origin of photoluminescence is relatively clear, the existence of a chemiluminescence -CLemission contained in PIL implies the formation of carbonylcontaining species in the fresh material. The understanding of the features of such emission requires a greater knowledge of the polymers structure. The first results of this study are presented and analysed with the purpose of understanding the existing link between chain ends, chromophores, configurational and compositional defects and the semicrystalline morphology of polymers, on the one hand, and the shape of the CL spectrum issued from cold plasma interaction on the other hand.

Typical PIL spectra obtained consecutively after plasma discharge switch-off.


E Aubert

G Teyssedre
email: gilbert.teyssedre@laplace.univ-tlse.fr

C Laurent

S Rowe

Monitoring electrically-active defects in silicafilled epoxy using light detection

Epoxy resins have been used for long as the insulation of electrical systems. It is generally formulated with a dispersion of micro-fillers to improve thermal and mechanical properties. However, there are concerns on the possible influence of these fillers on the electric behavior, especially on the long term ageing under functional stresses. Defects (decohesion between matrix and fillers, macro and micro-voids in the resin, etc.) can be detected using tomography but there is no indication of the harmfulness of these defects under electrical stress. The possibility of monitoring defects by electroluminescence is put forward in this work.

I. INTRODUCTION

The use of silica to reinforce polymeric insulations like epoxy resin is used in industries essentially for its dielectric along with thermal properties since the adjunction of silica improves the thermal dissipation. Such materials may undergo ageing under thermo-electrical stresses, and interfaces between the filler and the matrix may constitute weak points with the formation of voids and delamination, particularly as the temperature is cycled. The objective in this work is to determine if interfaces can be damaged under electrical stress and if electroluminescence (EL) can be used as optical diagnosis.

Usually, EL is defined as light emitted by molecules excited by an electric field. It refers to the light produced by the material itself through interaction of electron of high kinetic energy with the molecules, producing excited states or through recombination of electron-hole pairs. If voids or delaminated areas are present in the material, light is expected to be emitted also by the gas occupying such defects even though microdischarges cannot be detected by conventional means. The search for a sensitive tool is of particular relevance here as experiments are carried out at low field with r.m.s. values typically up to 9kV/mm. This is a challenging aspect as, even though optical techniques are well known for their sensitivity, to our knowledge electroluminescence spectra at such low field values have never been reported in the literature. The devices used for detecting such low signal are presented in the next section along with materials under study.

II. EXPERIMENTAL

A. Material -Samples

The material under study is a silica-filled epoxy resin; with 60% by weight of silica. The matrix is made of equal parts of resin (Diglycidyl ether of bisphenol A) and of a phtalic anhydride as hardener, and of a small amount of tertiary amine as catalyzer [1]. The silica particules size is widely dispersed, their diameter ranging from 1 to 100µm.

The samples have been molded into disks of 1 mm (± 20µm) thickness and a diameter of 110mm and cured at 100°C for 16h. For the needs of EL experiment, semitransparent gold electrodes, 50mm in diameter, have been deposited by sputtering. The thickness of the upper electrode, put to the HV and through which light is detected, is of 30nm. For the lower electrode, a larger thickness was used, 60nm, to improve the electrical contact with the ground electrode. A silicone rubber ring was laid at the periphery of the electrodes in order to avoid emission in regions of uncontrolled field. Fig. 1, shows a schema of the sample once conditioned.

B. Experiment environment

A light-tight chamber is used to carry out experiments. It is equipped with pumping units allowing to make experiments under secondary vacuum. Most often, EL experiments are carried out in vacuum to avoid any gaseous discharges in the environment of the sample. If some signal is related to microdischarges in the sample, then its features should be sensitive to the ambient atmosphere. For these reasons, in the present experiments, we have carried out measurements under vacuum (typically 2 to 4x10 -6 mb), in air, and under nitrogen atmosphere at 1200 mb (since the oxygen present in air could quench part of the luminescence being detected).

Two brass electrodes are used to make measurements: the lower one is a massive electrode connected to the ground. The upper one is put to the HV; it has a ring shape to allow light detection, cf. Fig. 2. Finally, a mask is put on the ensemble to Three kinds of measurements are carried, that are integral light detection by photon counting, spectra analyses of the emitted light and imaging of the emission in the sample plane.

The integral light detection is made using a photomultiplier (model R943-02 from Hamamatsu) working at -30°C. The spectral detection range is 180 to 900 nm. A dwelling time of two seconds is used for monitoring the emitted light. The average noise level is 1.49 counts per 2 seconds with a standard deviation of 1.02 counts per 2 seconds.

The image of the emission in the sample plane has been carried out using a cooled (-130°C) charge coupled device -CCD-camera (model spectrum one D-91 from Jobin-Yvon). The CCD is made of 512 x 2048 pixels of size 13.5x13.5 µm². The principal advantage of this device is the possibility of using long exposure times (several hours) without perturbation due to the noise level. The CCD is sensitive in the wavelength range 200-1000nm.

Finally, spectral analyses of the light are made with the same CCD, introducing in front of it a grating monochromator. With the grating used (groove density of 133 per mm), the region 285-1000nm is resolved into about 1800 points. As shown in Fig. 2, a mirror installed on a translational guide allows deviating light towards the CCD when such experiments are carried out.

All experiments have been realized under 50Hz AC stress; all field values reported herein refer to r.m.s. values. For recording EL-voltage characteristics, the voltage was increased from 0 to 9kV in steps of 1 kV lasting for 5 min. The voltage is raised at a rate of 300V/s between steps. For acquiring spectra or images, as the integration time can be very long, the voltage is set manually to a given stress. Through a systematic analysis of the emission for a population of samples, being filled or not, we have evaluated the impact of the atmosphere on the shape of the EL-field characteristic and on the EL level. Fig. 4, shows an example of results obtained for the same sample, in air, nitrogen and vacuum (2.5x10 -6 mb). Some differences in the level of light being detected could be observed, depending on the atmosphere; however, there was no systematic trend on the effect of such or such atmosphere when characteristics were smooth. In the example the signal is lower for air and higher for nitrogen. In fact, differences could be observed only when instabilities appear on the characteristics. A second aspect which was treated is the statistic variation in the form of the characteristic with samples of different roughness (due to different mould being used) and with either silica-filled samples or pure epoxy samples. intervals: roughly a sample-to-sample variation of x40 can be obtained at the extreme. It can also be observed that the variation factor is larger for high field than for lower ones.

Looking in details at these characteristics, it has been observed that some instabilities appear during the increase of the voltage, in the form of a step in the signal above a voltage ranging from 4 to 8kV, depending on samples. Such steps are responsible for the strong signal observed in some samples (>1000c/2s at 8kV/mm).

The criterion on the apparition of instabilities has been used to classify samples into two groups having different typologies of EL-field characteristics. The typology 1 corresponds to light emission which begins to appear at 1-2 kV and increases regularly with the field, being stable at each voltage level. This concerns most of silica-free samples: over 80% of them are of type 1 (under 1000c/2s). For typology 2, a steep increase at a given voltage level occurs and the emission becomes unstable under constant voltage. This concerns a majority (60%) of silica-filled samples.

B. EL spectra

Several wavelength-resolved spectra have been recorded at different voltages and with different samples. Due to the low signal being measured, the time necessary to get an exploitable signal could be long (of the order of 1 hour). Also, for the same reasons, spectra could be recorded only in the high field range of the characteristics. We have observed that different spectra are obtained depending on the typology of the characteristic of the sample under test. To characteristics of typology 1 correspond broad non-structured spectra in the red domain, Fig. 6.a (the spectrum peaks at about 780nm). In case of typology 2, the emission appears in the same wavelength range; however clear structures appear in the spectrum, cf. Fig. 6.b.

Emission in the red domain has already been reported when investigating EL of insulating polymers. As it is difficult to envisage chemical groups having emissions in that range, this red emission has been interpreted as surface plasmons -SPrelaxation [2]. A plasmon is a collective oscillation of electrons in a metal. If these oscillations can couple to surface roughness, they can radiate light. SP spectra reported in the literature for MIM junctions are usually broad. Structured spectra have been reported in the case of SP excited by electrons tunneling between a scanning tunneling microscope -STMtip and a sample [START_REF] Picardi | Raman spectroscopy and light emission at metal surfaces enhanced by the optical near-field of a scanning tunneling tip[END_REF]; however, the structure has been explained by geometrical factors in the arrangement.

It is not clear yet as to why samples exhibiting instabilities in their EL-field characteristic do lead to these specific structures in the EL spectrum. Are the conditions of excitation of SP in STM gathered here? Are there other mechanisms of emission like gaseous discharges? The point is that there are no tabulated emission bands for gases in this wavelength range.

Finally, we have not identified specific signal linked to the epoxy itself, presumably because the red (SP) emission overlaps it.

C. Imaging of the EL in the sample plane

Images of the emission in the sample plane have been recorded with the CCD as described before and an example of such image is shown on Fig. 7.a. Homogeneous emission has been observed irrespective of the typology of the corresponding EL-field characteristic. The homogeneous aspect of the image is apparently not related to the long integration time used here since in case of porous materials in which discharges are known to appear, bright spots could be observed even for long integration time, see Fig. 7.b. The result of these images have shown homogenous signal in the two types of spectra. The promising aspect here is that a region of enhanced emission is clearly visible in Fig. 7.a), a feature which was not observed in un-filled materials and that could be related to the defects advocated above. Cite as: E. Aubert, G.

D. Routes for discarding SP emission

One way to discard light related to SP emission consists in inserting an optical filter that cuts emission at wavelengths above 600nm. Fig. 8 compares the characteristics obtained with and without filter. The fact that the field dependence of the 2 characteristics are not proportional implies that the spectrum is field-dependent and hence that possibly several emission processes contribute to the emission. As regards potential emissions in pure epoxy, it has been shown that the longer wavelength emission detected is related to oxidation of the hardener of the epoxy [START_REF] Gallot-Lavallée | [END_REF], which peaks at 580nm. Clearly, such emission would be in the short wavelength tail of the SP emission and the two processes are difficult to separate using a red-censoring filter. Nevertheless, the shape of the characteristic indicates that the relative amplitude of the emission under 600nm increases with the field.

The best solution to avoid SP emission consists in using an electrode material that radiates out of the wavelength range of interest, which is the case of indium tin oxide (ITO) since its SP emission is in the far-IR. Another advantage of ITO is its transparency in the visible range, which is not the case of gold; the drawback being that the deposition technique is somewhat more complex. Fig. 10 shows an example of characteristic obtained with 220 nm -thick ITO electrodes. The shape of the characteristic is approximately linear and conforms more to that obtained with red-sensoring the signal of gold-metallized samples. Most importantly, the spectrum obtained with ITO electrodes is clearly different from that obtained with gold, cf. Fig. 9. The peak is shifted to about 570nm, consistently with that obtained during thermal oxidation of the resin [START_REF] Gallot-Lavallée | [END_REF] and also with cathodoluminescence measurements (results not shown here), being strong supports that this emission is really characteristic of the material. Work is in progress to develop measurements with ITO electrodes.

IV. CONCLUSION

Two typologies have been identified in the EL spectrum of gold-metallized pure and silica-filled epoxies. The spectrum is always in the red domain, with either no structures or clearly identified structures. In the first instance, the EL-field characteristic are smooth, while in the second one, they exhibit a sharp increase in the signal at a given field level. As the later concerns in majority filled samples, the behavior could reflect the existence of voids at the epoxy-silica interface. The use of ITO instead of gold electrodes appears as a promising solution for avoiding SP-related EL, allowing so to isolate materialrelated emission.

Fig. 1 .

 1 Fig. 1. Sample for EL measurement

Fig. 2 .

 2 Fig. 2. Arrangement of the experimental set-up

Fig. 3 .Fig. 4 .

 34 Photomultiplier

  Fig. 6. Examples of EL spectra obtained on samples having characteristics of different typology.

Fig. 5 .

 5 Fig. 5. Superposition of EL-field characteristics for various samples and with either air, vacuum or nitrogen as atmosphere. Solid symbols: silica-filled materials; open symbols: un-filled samples.

Fig. 7 .Fig. 8 .Fig. 9 .Fig. 10 .

 78910 Fig. 7. Images of the EL emission in the sample plane. The size of the images is 27.6 x 6.7 mm².