Prescillia Dupont

Manar Nesser

Olivier Maloberti

Julien Dupuy

Marc Lamblin

Maxime Ployard

Daniel Laloy

Jérôme Fortin

Préscillia Dupont

vector magnetic losses and general properties of Grain-Oriented Electrical Steels

Keywords: D Scalar and Vector Magnetic Properties, Grain Oriented Electrical Steels, Iron losses, Laser treatment

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. INTRODUCTION

RON losses can be reduced using surface laser treatments through the magnetic domain refinement technic [START_REF] Huang | Parameter optimization of Nd:Yag laser scribing process on core loss of grain-oriented magnetic silicon steels[END_REF]- [START_REF] Petryshynets | Magnetic losses reduction in grain oriented silicon steel by pulse and continuous fiber laser processing[END_REF]. These improvements are obtained for unidirectional excitation fields parallel to the Rolling Direction (RD) of Grain-Oriented Electrical Steels (GOES). However, in rotating electrical machine applications, the flux is bi-directional and/or rotational in some parts of the magnetic circuit (mainly within the yoke). In addition, alternating losses (occurring in the teeth) are due to flux variations in directions which are not always parallel to the preferred RD. Measurements under rotational fields are still not part of classical characterizations. Still, authors investigate the rotational behavior [START_REF] Zurek | Qualitative Analysis of Px and Py Components of Rotational Power Loss[END_REF]- [START_REF] Gao | Simple Numerical Calculation Method of Rotational Iron Loss in Silicon Steel Sheets[END_REF], its connection to unidirectional components and the dependence on the angle between applied field and the RD [START_REF] Appino | Static and dynamic energy losses along different directions in GO steel sheets[END_REF]. Meanwhile, few started to investigate the impact of an irradiation laser process on the rotational magnetic behavior of GOES [START_REF] Kajiwara | Effect of Laser Stress on Vector Magnetic Properties of Electrical Steel Sheets[END_REF] but recent advancement in laser technologies revealed new possibilities for beneficial laser treatments [START_REF] Nesser | Correlation between laser energetic parameters and magnetic properties of GO laminations under surface treatments with long, short or ultra-short pulsed lasers[END_REF] that have not been investigated under rotational fields yet. The present paper includes an experimental study of three laser process configurations that are investigated and compared in terms of impacts on the power losses of GOES for both unidirectional (in RD, but also in the Transverse Direction (TD)) and rotational excitation fields. Focus is made on new process called ablation (engravement obtained with Ultra-Short-Pulse (USP) laser technology) for which up to 4 laser parameters are compared.

The goal is to optimize the behavior of GOES in both unidirectional and rotational fields for electrical machines. Finally, the comparison of the different laser impacts emphasizes the difference between unidirectional and rotational magnetization processes and the necessity to use an adapted 2-D vector model to represent power losses and fully understand and optimize the impact of the laser.

II. EXPERIMENTS

A. Equipment and accuracy

Magnetic properties measurements have been carried out with two different apparatuses: a Single Sheet Tester (SST150) and a Rotational power loss Single Sheet Tester (RSST60). Magnetic observations were performed (for all samples, at each step, in the same location) using a Magneto-Optical Indicator Film technic (MOIF). Concerning the SST, an accuracy study was carried out by the authors (measurement of the same sample several times with (repeatability) or without (accuracy) complete removal). Measurement error varies with polarization and frequency but is always under 5%: about 5% error for low frequency (5 Hz) and low polarization (0.1 T), about 1% error for high polarizations (> 1 T) and frequencies (> 150Hz). Same error orders are considered for the RSST. Error bars are not plotted on upcoming figures for better legibility.

B. Material

The considered material is a laboratory Fe-3%Si GO grade from APERAM© company. It is 0.27 mm thick, isolated thanks I to a CARLITE® coating of 3-4 μm thickness and the mass density equals 7650 kg.m -3 . The samples size depends on the equipment: 150mm×150mm with the SST and 60mm×60mm with the RSST. As to avoid any sample dispersion, measurements are systematically carried out at each step (before/after laser treatment) on the same sample, with no significant mass variation (less than 0.1%) even for the ablation process. Remaining errors are caused by the apparatus precision and the repeatability of the measurement (see section II.A.).

C. Laser treatments

The samples have been laser treated with a pattern composed of distant parallel lines perpendicular to RD. The laser treatments were classified into three categories fully described in [START_REF] Nesser | Correlation between laser energetic parameters and magnetic properties of GO laminations under surface treatments with long, short or ultra-short pulsed lasers[END_REF]: Irradiation (Irr., thermal effect), Scribing (Scrib., thermal effect and weak engravement due to melting), and Ablation (Abl., predominant engravement due to sublimation). All laser treatments were performed with the same fixed spot size. Two different lasers have been used: an IPG pulsed Ytterbium fiber laser with 1.064 µm wavelength (for Irr. and Scrib.) and a USP laser of 1.030 µm wavelength (for Abl.). Laser configurations are given in Table I. The four ablation configurations differ in terms of Cumulative Energy Density (CED) (Abl.1 & Abl.3), peak power (Abl.1 & Abl.2) or pattern (Abl.1 & Abl.4). The CED can be deduced from the laser power at focal point and is related to the process parameters (frequency, speed, number of pass, etc.). The peak power of laser pulse is very high in the case of ablation due to the use of USP femtosecond laser.

D. Data processing and nomenclature

Measurements have been carried out twice per sample at each step: with the excitation field parallel to RD and parallel to TD when using the SST, in ClockWise (CW) and Counter-ClockWise directions (CCW) when using the RSST. RSST measurements were performed at fixed circular polarization and axis ratio has been kept as close as 1 as possible. Power losses are classically estimated by the measurements (see ( 1) and ( 2)). They will be noted PN,M (W.kg -1 ) with N corresponding to the field configuration (i.e. RD, TD, CW, CCW) and M corresponding to the considered step (i.e. B for Before or A for After laser treatment). PRD,B, PRD,A, PTD,B or PTD,A, represented by PU, can be retrieved using [START_REF] Huang | Parameter optimization of Nd:Yag laser scribing process on core loss of grain-oriented magnetic silicon steels[END_REF], with H the applied magnetic field (A.m -1 ) and B the average magnetic induction (T). PCW,B, PCW,A, PCCW,B or PCCW,A, represented by PR, can be retrieved using [START_REF] Petryshynets | Magnetic losses reduction in grain oriented silicon steel by pulse and continuous fiber laser processing[END_REF], with Hx, Hy, Bx, and By the orthogonal components of applied excitation field (Hx, Hy) (A.m -1 ) and of the average magnetic induction (Bx, By) (T).

. . W. kg (1)
. . . W. kg (2)
In ( 1) and ( 2), f represents the considered frequency (Hz), T equals 1/f (s) and dm is the mass density (kg.m -3 ). N can also take the following values: ROT (ROTational losses obtained by averaging CW and CCW power losses), ALT_AVG (AVeraGe ALTernating losses obtained by averaging RD and TD power losses) and ALT_SUM (SUM of ALTernating RD and TD power losses). Measurements have been performed for several polarization and frequency conditions.

III. RESULTS AND DISCUSSION

Fig. 1, Fig. 2, and Fig. 4 to Fig. 8 show the power loss variations after laser treatment ΔPN calculated using (3). The y-axis scale is fixed to ease comparisons between those figures. If ΔPN is negative, laser process allowed power loss reduction.

∆ ! " # $,& # $,' # $,' ( . 100 % (3)
A. Unidirectional fields (parallel to RD or TD) Fig. 1 and Fig. 2 show the evolution of power loss variation after laser treatment with polarization for several laser configurations with the excitation field parallel to RD (Fig. 1) and to TD (Fig. 2) at 50 Hz frequency. It is shown that it is possible to reduce the power losses in both RD and TD, and sometimes even more in TD than in RD, especially for irradiation and scribing processes and for low polarizations. For example, for Irr., the reduction rate in TD is nearly twice the one obtained in RD at 0.5 T / 50 Hz. The ratio between power loss variations ΔPTD/ΔPRD decreases as the polarization level increases. It can be shown in the MOIF observations of Fig. 3 that long 180° magnetic domains along RD are separated into shorter ones by the laser lines parallel to TD. The physical impacts of the different laser processes on the magnetic structure have already been discussed in [START_REF] Nesser | Correlation between laser energetic parameters and magnetic properties of GO laminations under surface treatments with long, short or ultra-short pulsed lasers[END_REF] considering fields parallel to RD. Depending on the process, the "new" domains configuration might involve more mobile walls, additional misoriented and/or closure domains, higher demagnetizing energy, etc. At low polarization and medium frequency (from 5 Hz to 500 Hz): the increase of walls density and mobility can explain the power loss reduction in RD. In TD, the spontaneous generation of misoriented and/or closure domains at the vicinity of the laser spots, that induce some stress and/or magnetic poles even at rest, can contribute to reduce the power loss. At low polarization, potentially stress-induced misoriented and 90° closure domains might ease the transverse magnetization by delaying and slightly lessening the domain rotation phase which is more energy consuming than the walls displacement mechanism. At high polarization, the material saturates. Thus, previously exposed domain structure specificities in comparison with the directions are no longer predominant as the wall density is decreased. As a result, effects of the laser tend to be the same for both RD and TD at higher polarizations.

Returning to Fig. 1 and Fig. 2, the tendency is globally the same for RD and TD configurations: the lower the polarization, the most beneficial the impact of the laser (at fundamental frequencies from 5 Hz to 500 Hz and especially at 50 Hz). Then, time harmonic losses with low magnitude may also be reduced with an appropriated laser treatment. Fig. 4 and Fig. 5 show the evolution of power loss variations after laser treatment with frequency for several laser configurations with the excitation field parallel to RD (Fig. 4) and to TD (Fig. 5) at 0.1 T. The reduction rates are in the same range from 5 Hz to 500 Hz in RD configuration (always better than -11.5% (Scrib.)). Similar behavior is observed in [START_REF] Nesser | Correlation between laser energetic parameters and magnetic properties of GO laminations under surface treatments with long, short or ultra-short pulsed lasers[END_REF] under close experimental conditions for several laser processes especially at low induction. On the contrary, in TD, better reduction rates are obtained for low (5 Hz) and medium (50 Hz) frequencies, but laser treatment can worsen the power loss when the frequency increases whereas polarization is kept constant at 0.1 T (for example: -10% power loss variation in RD against +21% in TD for Abl.2 laser configuration at 5 kHz). A laser treatment has a different effect depending on the frequency according to the field direction. From Fig. 4 and Fig. 5, it is clear that laser processes have a beneficial impact on both hysteresis and excess iron losses at low and medium frequencies (from 5 Hz to 500 Hz) in both RD and TD directions. Classical eddy current losses shall not be significantly changed by the laser (constant thickness) [START_REF] Bertotti | General Properties of Power Losses in Soft Ferromagnetic Materials[END_REF]. However, at higher frequencies (up to more than 5 kHz), the domains refinement in RD shown in Fig. 3 will still be responsible for an excess loss reduction in RD but can increase the TD losses at the same time. While the 180° walls are multiplying, 90° walls density and/or mobility may be reduced. This detrimental aspect must be taken into account when using laser processes in magnetic cores.

The behavior of laser treated GOES under unidirectional fields (RD and TD) was exposed through previous graphs. To ease comparisons with rotational behavior, RD and TD contributions have been lumped in average contributions ,-_,/0, and ,-_,/0,, that have been compared resulting in average loss variation (ΔPALT_AVG, see Fig. 8).

B. Rotational fields

Whereas all laser treatments were beneficial in both RD and TD at 50 Hz, some are detrimental when the field rotates (see Fig. 6 and Fig. 8). Indeed, there is a clear difference of behavior for ablation laser treated samples notably at 50 Hz whereas for Irr., which allows comparable reduction rates to the ones obtained in [START_REF] Kajiwara | Effect of Laser Stress on Vector Magnetic Properties of Electrical Steel Sheets[END_REF] under similar experimental conditions, alternating and rotating behaviors are very close. In [START_REF] Nesser | Correlation between laser energetic parameters and magnetic properties of GO laminations under surface treatments with long, short or ultra-short pulsed lasers[END_REF], it is mentioned that, in the case of irradiation, Lancet (zone n°3 in Fig. 3b), spike-like (zone n°2 in Fig. 3b), and 90° closure domains (zone n°1 in Fig. 3b) may be created in numbers and at a very small scale compared to other processes. Those closure domains could be beneficial for the material submitted to a rotating field, especially not in the rolling direction. Those misoriented domains are scarcer for Abl.2 process near the laser lines (see Fig. 3d). Ablation is usually used to mainly engrave the material without thermal stress effect, but it might have a pronounced purely mechanical impact inside the thickness at a certain depth (resulting in misoriented domains under the laser spots).

Then, concerning the study at fixed polarization (see Fig. 7), as for TD configuration, the higher the frequency, the less beneficial the impact of the laser (losses can even be worsened for some conditions). Laser treatments seem to reduce both hysteresis and excess iron losses but only at low and medium frequencies (from 5 Hz to 500 Hz). As aforementioned, at higher frequencies, the usual excess losses can still be reduced but only in RD. A detrimental effect occurs in TD and under rotational excitation which is probably due to an unknown and unusual excess loss term induced by the misoriented walls displacement. Rotational magnetization is very specific and involves not only the walls motion mainly but also the domains magnetization rotation. As exposed in section IV., mechanisms at stake should be better understood through the development of an adapted vector model.

C. Optimization of laser parameters

It results from both macroscopic and microscopic observations that the impact on power losses depends on the laser process. For Irr. and Scrib., both alternating and rotating losses behaves similarly. Concerning Abl., several laser configurations were tested as a first attempt of optimization (see Fig. 8). In the case of unidirectional fields, Abl.3 was more beneficial than Abl.2, itself more beneficial than Abl.1. Then, by comparing both alternating and rotational behaviors, and in relation with laser parameters exposed in Table I, one seems to tend towards an optimum being Abl.2: it has similar or better impact than Irr. in RD and TD, and it happens to be better under rotational fields. For the considered grade, for a fixed pattern, using an intermediate and optimized CED (between low (Abl.1) and high CED (Abl.3)) in parallel with a sufficiently high peak power (USP) seems beneficial.

IV. PERSPECTIVES

Finally, alternating and rotating power losses are compared in Fig. 8. In most cases here: P 234_256 7 P 894 7 P 234_:;< . This representation could also be expressed as in (4) depending on the contributions at stake with PCPGi (W.kg -1 ), a CouPlinG term between rotational and alternating contributions.

= > ,?

,-_,/0,? @#0 ,? > ,?

,-_A ?,? B @#0C,?

PCPGi might be related to the non-diagonal terms in 2-D vector magnetic properties such as in [START_REF] Enokizono | Two-dimensional (vector) magnetic property and improved magnetic field analysis for electrical machines[END_REF]. To highlight this, the evolution of the relative difference ΔPCPG1 (defined in (5)) versus polarization at 50 Hz is shown in Fig. 9.

∆ @#0 " ∆# DEFG,& ∆# DEFG,' ∆# DEFG,'
( . 100 % Fig. 9. shows that a high absolute value of ΔPCPG1 implies strong differences of impacts on power losses whether the field is rotational or unidirectional. The tendency observed in Fig. 8 (right part) also emerges in Fig. 9. The variations of PCPG1 loss part contributes to the total losses but can be analyzed separately in order to understand excess losses due to field rotations. For example, Abl.1 and Abl.3 were beneficial in RD and TD but detrimental under rotational field for medium polarizations, Abl.2 allowed reductions under both alternating (-5% in average) and rotating field (-12%) at 1 T / 50 Hz but in different ranges (see Fig. 8). Having ΔPCPG1 ≈ 0, means having the same impact on rotational and unidirectional losses. The next goal for the authors is to understand how to link this excess coupling term due to field rotations with the laser impact.

V. CONCLUSION

To conclude, outcomes of this paper may be summarized as follows. Several types of laser treatments including irradiation, scribing and ablation processes can be beneficial regarding power loss reduction in RD, TD but also under rotating field. However, laser treatment may be beneficial in RD and TD but not under rotational field unless it is optimized. After optimization, ablation process may be as interesting as classical irradiation process for reducing power losses, especially regarding rotational behavior. Finally, it is necessary to study vector 2-D magnetic properties to correctly characterize soft magnetic material in general, and more specifically when investigating the impact of laser treatments. In further work, the authors should investigate deeper the impacts of laser on 2-D vector magnetic properties, and especially on the coupling terms existing between RD and TD contributions.

Fig. 1 .

 1 Fig. 1. Power loss variation after laser treatment (ΔPRD) for several laser configurations under excitation field parallel to RD at 50 Hz.

Fig. 2 .

 2 Fig. 2. Power loss variation after laser treatment (ΔPTD) for several laser configurations under excitation field parallel to TD at 50 Hz.

Fig. 3 .

 3 Fig. 3. MOIF observations: before (a) and after (b) Irr. (1: misoriented and/or 90° closure domain, 2 spike-like domain, 3: Lancet domain); before (c) and after (d) Abl.2 (12.42mm×18.56mm images, center of 60mm×60mm samples).

Fig. 4 .Fig. 5 .

 45 Fig. 4. Power loss variation after laser treatment (ΔPRD) for several laser configurations under excitation field parallel to RD at 0.1 T.

Fig. 6 .

 6 Fig. 6. Power loss variation after laser treatment (ΔPROT) for several laser configurations under rotational excitation field at 50 Hz.

Fig. 7 .

 7 Fig. 7. Power loss variation after laser treatment (ΔPROT) for several laser configurations under rotational excitation field at 0.1 T.

Fig. 8 .

 8 Fig. 8. Average alternating (ΔPALT_AVG, left) and rotational (ΔPROT, other view, right) power loss variations after laser treatment at 50 Hz.

Fig. 9 .

 9 Fig. 9. Relative difference ΔPCPG1 between coupling terms (between rotational and average alternating power losses) at 50 Hz.

TABLE I LASER

 I CONFIGURATIONS (SAME PATTERN FOR ALL EXCEPT ABL.4)

	Laser treatment / Designation	CED [J/cm 2 ]	Peak power [kW]
	Irradiation	Irr.	509	0.25
	Scribing	Scrib.	611	3
	Ablation 1	Abl.1	1171	460.10 3
	Ablation 2	Abl.2	1426	560.10 3
	Ablation 3	Abl.3	2343	460.10 3
	Ablation 4	Abl.4	1171	460.10 3

ACKNOWLEDGMENT

This work was supported by JEUMONT Electric and was carried out with the help of financing from the Association Nationale de la Recherche et de la Technologie (ANRT) in France. It was carried out in parallel with the ESSIAL project. The ESSIAL project is funding by the European Union's Horizon 2020 research and innovation program under grant agreement No 766437. Authors would also like to thank the APERAM-Brazil company for having supplied the samples.


Eva Drevet Mulard

Sylvie Guibert

C Lefevre

M A Chauvin

Claudie Pinteur

M A Monet

M Godet

A M Madec

Béatrice Morio

J Rieusset

NMR Metabolomics Profiling Reveals Distinctive Signatures of an Obesogenic Diet in Mice

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Uncovering Metabolite Signatures

in Six Organs During a High-Fat, High-Sucrose Diet:

A Cross-Sectional NMR Metabolomics Study

Drevet Mulard E.

1 ; Guibert S. ; Rautureau GJP. This research of diet-related metabolic impairments in mice focuses on the effects of aging and a high-fat, high-sucrose diet.

Intro

Results

Discussion

• Aging significantly impacts all organs with different metabolite signatures for each organ • The brain is not impacted by the diet, while others organs and the serum are significantly impacted (up to 80% in concentration variation) • Some effects appear as soon as 6 weeks, while others were delayed to 10 or 14 weeks • Diet decreases metabolite concentrations corresponding to the one-carbon metabolism: betaine, dimethyl-glycine and sarcosine • Diet-time interaction evidences the need to evaluate carefully the influence of confounding factors over metabolic signatures

• 12 mice for each data point (72 total) • Insulin resistance was observed after 13 weeks of feeding • Organ weight was not impacted by the diet exept for liver weight • 43 metabolites were quantified • The heart and the liver are the most impacted by the diet while the brain is not impacted by the diet • Betaine, dimethyl-glycine, and myo-inositol were impacted by the diet in 5 organs and the serum • ANOVA reveals diet-time interactions in the liver and the lungs Obesity is a significant risk factor for diseases like type 2 diabetes and cardiovascular disorders. Identifying biomarkers for early obesity-related disorders is crucial for precision medicine. We used NMR to analyze metabolomic profiles of key organs and serum in male C57Bl/6J mice fed a high-fat, high-sucrose diet (HFHSD, 36 % w/w fat, 16.6 % w/w sucrose, Envigo, France) or standard diet (STD, Rhod16A, Genobio) for 6, 10, and 14 weeks. Our study reveals organ-specific metabolite concentrations, highlighting age and diet effects and their interaction.

Figure 1 :

 1 Figure 1:Betaine concentration in nmol/100mg of organ, depending on the diet (STD and HFHSD) and over time (6, 10, and 14 weeks respectively in red, blue, and green), for each organ except the brain. The 3 horizontal bars represent the median, the first, and the third quartile of the boxplots.

Figure 2 :

 2 Figure 2: Boxplots of 3 metabolites evolution in liver, each one representing the 3 effect detected by the Analysis of Variance (ANOVA): diet, time and diet-time interaction effect.

1

  University of Lyon, ICBMS UMR CNRS 5246, Bâtiment Raulin, 43 Bd du 11 novembre 1918, 69622 Villeurbanne Cedex, France 2 Centre de Résonance Magnétique Nucléaire à Très Hauts Champs, UMR 5082 CNRS / Ecole Normale Supérieure de Lyon / Université Claude Bernard Lyon 1, 69100, Villeurbanne, France 3 CarMeN Laboratory, INSERM, INRA, INSA Lyon, Université Claude Bernard Lyon 1, 69921, Oullins Cedex, France 4 Hospices Civils de Lyon, Faculté de Médecine, Hôpital Lyon Sud, 69921, Oullins Cedex, France

	How does ANOVA work?
	When more than 2 groups:
			Diet
	1 rst variable impact?	Standard	?	Obesogenic
	Interaction between the two variables?		?
			Time
	2 nd variable impact?		?
		6	10	14	weeks

NMR Metabolomics Profiling Reveals Distinctive Signatures of an Obesogenic Diet in Mice Spectra attribution and metabolite quantification

				Serum
						+
				Organs
	STD	6	Diet 10 HFHSD 14	Brain Heart Kidney Liver Lung Muscle				Sample preparation Organ Methanol PBS 100% Washing Extraction
	5-week-old mice	weeks
			•
		Statistics								Sample acquisition
	Multivariate								1 H NMR 800 MHz
	Univariate								1D: NOESY	NMR spectrometer
											2D
		Absolutes
	Concentrations table
	Spectra on Metabolight
	database
				9	8	7	6	5	4	3	2	1	ppm

72 mice • 2 different diets • 3 points in time • 6 organs • 504 NMR spectra


Mohammed Aissaoui

Chiraz Houaidia

Adrien Van Den Bossche

Thierry Val

Azouz Leïla

Saidane

Distributed Physical-layer Network Coding MAC Protocol

Keywords: Multi-hop Wireless Networks, MAC protocol, Carrier-Sensing Multiple Access, Physical-Layer Network Coding, Two-Way Relay Channel, Software Defined Radio

Physical-layer Network Coding (PNC) was first proposed for a Two-Way Relay Channel (TWRC) to improve the spectrum efficiency since it allows nodes to transmit simultaneously via a relay node. This technique requires multiple nodes to transmit their packets with accurate synchronization. Therefore, in many works of literature, centralized scheduling with perfect synchronization has been assumed to be employed on top of PNC.

Such assumptions are not applicable in general random access multi-hop wireless networks. Therefore, this paper proposes a distributed MAC protocol that supports PNC in static multi-hop wireless networks. The proposed MAC protocol is based on the Carrier Sense Multiple Access (CSMA) strategy, where RTS/CTS frames are used to detect PNC opportunities and to offer the appropriate scheduling of the involved transmissions that should occur simultaneously. This packet exchange process is coordinated by the relay node and was designed to guarantee compatibility with other conventional relaying schemes with specific concerns for the hidden node issues. Our solution was practically tested on a real testbed with different static wireless topologies and several physical settings.

With numerical results, we investigate the effectiveness of PNC in distributed wireless multi-hop networks. Compared to the conventional CSMA/CA and the PNC opportunistic (PNCOPP) MAC protocols, the proposed protocol's performances are advantageous in various scenarios, especially in large networks.

Introduction

Ad-hoc networks have been considered a sub-class of multi-hop wireless networks consisting of self-organized, mobile, and eventually large-scale networks. Distinctive features such as low establishment cost and simple expansion have popularized those networks as a promising technique for various Internet of Things (IoT) applications. However, since the capacity of these networks becomes increasingly constrained as the number of users grows, one of the main challenges is to meet the requirement of high throughput when the network is severely limited by interference.

Traditionally, Medium Access Control (MAC) protocols are designed to avoid interference scenarios and limit their appearance as much as possible. Recent advances in network coding have brought a new promising approach that exploits colliding signals to boost network performance.

In fact, the Physical-layer Network Coding (PNC) technique was first put forward in [START_REF] Zhang | Hot topic: Physical-layer network coding[END_REF] to benefit from the interference in wireless networks. The basic idea is to allow nodes to transmit simultaneously and then extract the colliding packets using appropriate mathematical or logical operations rather than being ignored and deleted [START_REF] Worlanyo | Network coding for wireless applications: A review[END_REF]. By this means, the data amount contained in each transmission will be increased, the bandwidth allocated to each node can be utilized more efficiently, and consequently, the overall network throughput will be improved.

The idea of the PNC technique could be illustrated using the Two-Way Relay Channel (TWRC) scenario or network, as shown in Figure 1. In this work, we adopt the TWRC for the PNC technique since it is the most representative, straightforward, and used scenario in the literature [START_REF] Zhang | Hot topic: Physical-layer network coding[END_REF] [3] [START_REF] Liew | A primer on physical-layer network coding[END_REF].

In this scenario, two nodes, A and B, exchange their data via the relay R. When considering a Half-duplex system, where each node has one antenna and can not transmit and receive simultaneously, in the traditional network (without NC), as shown in Figure 1 (a), the throughput is two packets by four-time slots.

Using the conventional network coding (NC) technique, as shown in Figure 1 (b), the relay encodes packets after receiving them in separate communication time slots. The encoding function is mainly an XOR operation for this TWRC topology [START_REF] Fragouli | Network coding: an instant primer[END_REF]. Each edge node can then extract the in-tended packet by XORing the coded packet with the initially sent packet. Therefore, the throughput is two packets by three-time slots using the NC technique, and a 33.33% of throughput improvement is achieved.

As shown in Figure 1 (c), the Physicallayer Network Coding (PNC) takes advantage of the naturally superposed electromagnetic waves and encodes packets through simultaneous transmissions in the Multiple Access Phase (MAP (Slot 1)). The encoding function of PNC can be either a multiplicative factor that amplifies the signal or an operator that maps the superposed signal to a series of bits representing the encoded packet [START_REF] Wang | Distributed mac protocol supporting physicallayer network coding[END_REF]. Hence, PNC further reduces the number of required slots and guarantees an improvement of the throughput of 100% and 50% compared to the traditional and NC systems, respectively. In [START_REF] Liew | A primer on physical-layer network coding[END_REF], network-analytical results were presented to prove the performance of the PNC technique in large networks.

In order to use and benefit from the PNC technique, the TWRC scenarios must be detected, and the PNC transmissions must be scheduled in a synchronized manner that allows such combinations. To do so and to coordinate channel access, a well-designed Medium Access Control (MAC) protocol is required.

In the literature, the centralized MAC protocols based on a Time/Frequency Division Multiple Access (TDMA/FDMA) are static and require centralized coordination, which is theoretically possible but is typically challenging to implement in distributed large-scale networks [START_REF] Wang | Distributed mac protocol supporting physicallayer network coding[END_REF]. Therefore, random access-based MAC protocols were widely adopted, mainly the Carrier Sense Multiple Access (CSMA) based MAC protocols. However, the design of such MAC protocols, when supporting the PNC tech- nique, leads to high complexity since synchronization on the symbol level is needed [START_REF] Lu | Asynchronous physical-layer network coding[END_REF] [8] [3] [START_REF] You | Reliable physicallayer network coding supporting real applications[END_REF].

To the best of our knowledge, there is no complete solution in the literature that makes the use of the PNC technique in multi-hop wireless networks possible, taking into consideration all the possible challenges, mentioning: the full realtime knowledge about the topology and the neighbor node's states required by nodes A, B, and R to detect the PNC opportunities, the synchronization of the edge nodes A and B in order to send their data to the relay node R simultaneously, and finally, the effect of the hidden stations (two nodes (stations) are hidden from each other if they are not in each other's transmission and interference range.) in large-scale networks.

Hence, this paper's focus is to design a distributed MAC protocol supporting the PNC and considering all the related issues previously introduced. Our solution, referred to as Distributed physical-layer Network Coding MAC Protocol (DNCP), is intended to be practically used in distributed large-scale static networks as tested and evaluated in a real testbed. DNCP is based on the well-known Distributed Coordination Function (DCF) of the IEEE 802.11 standard [START_REF] Committee | Wireless lan medium access control and physical layer specification[END_REF] where nodes randomly access the channel, and the MAC protocol detects the opportunity to perform PNC and offers the appropriate scheduling of the in-volved transmissions that guarantees the best throughput.

In the following, Section 2 presents the related issues in the state of the art. Followed by the proposed PNC MAC protocol (DNCP) presented in detail in the third section. The fourth section presents the implementation and the performance results of the proposed DNCP in three different topologies, using the Software Defined Radios (SDRs) as hardware and the GNU Radio platform as software. Finally, the fifth section concludes our work and outlines possible future improvements.

Related works

The distributed control of communication networks remains a challenging area [START_REF] Yomo | Distributed mac protocol for physical layer network coding[END_REF], [START_REF] Argyriou | Mac protocol for wireless cooperative physical-layer network coding[END_REF], [START_REF] Wang | Distributed mac protocol supporting physicallayer network coding[END_REF], [START_REF] Hoang | Cross-layer design of bidirectional-traffic supported cooperative mac protocol[END_REF], [START_REF] Liew | A primer on physical-layer network coding[END_REF], [START_REF] Mao | Anc-era: Random access for analog network coding in wireless networks[END_REF], [START_REF] Hoang | Phy-mac cross-layer cooperative protocol supporting physical-layer network coding[END_REF]. Recently, efforts on designing distributed MAC protocols that cover the PNC requirements have been proposed to make PNC applicable in practical systems.

The authors of [START_REF] Yomo | Distributed mac protocol for physical layer network coding[END_REF] propose an opportunistic and distributed scheduling scheme based on the RTS/CTS (Request to Send and Clear to Send) mechanism between edge nodes via a relay node. However, this prior exchange is used only to coordinate the channel access, and the relay cannot distinguish between the ordinary and PNC data. It always applies the same amplify and forward steps and cannot detect the PNC opportunities. Consequently, in the case of ordinary data, there might be additional delays for independent RTS/CTS between the relay node and the edge node, resulting in an 8-slot traffic pattern. Also, the channel access control of two source nodes uses only CTS messages, which are still unsubstantial until the two source nodes have already been synchronized with the relay node. On the other hand, the state information of the source node A must be known by the two-hop destination node B of the TWRC network.

In [START_REF] Mao | Anc-era: Random access for analog network coding in wireless networks[END_REF], authors propose that the node B of the TWRC scenario must send a specific Answer-To-Cooperate (ATC) message to the relay so that this latter can figure out that the subsequent transmission is a PNC transmission. However, hidden stations (the neighbors of one and two-hops of node A) can not figure out, whereas the transmission is an ordinary or PNC transmission. Therefore, they cannot wait until the end of the PNC transmission, which might cause collisions.

In [START_REF] Argyriou | Mac protocol for wireless cooperative physical-layer network coding[END_REF], a PNC MAC protocol for wireless ad hoc local area networks has been proposed. Based on the Channel State Information (CSI) of the concurrent transmissions of different communication pairs, the proposed MAC protocol chooses the relay nodes dynamically, but it is a one-hop MAC protocol as they consider a fully connected network.

The distributed PNC MAC protocol proposed in [START_REF] Wang | Distributed mac protocol supporting physicallayer network coding[END_REF] adopts a new policy by allowing the relay node to initiate the transmissions by sending an RTS message to the two edge nodes of the TWRC scenario, inviting them to coordinate their transmissions within a PNC opportunity. This process differs from the works in [START_REF] Yomo | Distributed mac protocol for physical layer network coding[END_REF] and [START_REF] Argyriou | Mac protocol for wireless cooperative physical-layer network coding[END_REF], where the source nodes initiate the transmissions.

To do so, the authors assume that the relay is already identified and has full realtime knowledge of the neighboring state and their MAC queues. Moreover, the authors assume that the nodes have real-time awareness of the network topology within at least a two-hop range, making the proposed protocol impractical for real implementation. Additionally, PNC communications will always have priority over ordinary transmissions. As a result, ordinary transmissions could never occur in some situations where PNC chances exist.

Authors in [START_REF] He | Building blocks of physicallayer network coding[END_REF] propose a method to detect the PNC atoms (different scenarios of PNC on star network) in the star topology, based on the Point Coordination Function (PCF) mode of the IEEE 802.11 standard in order to skirt the problem of the real-time knowledge about the neighboring node's assumption. The two phases of the proposed MAC protocol are as follows: The coordination point (the relay) takes data from the edge nodes in the first phase and uses the PCF mode's fundamental polling mechanism to find PNC atoms. This relay affects each edge node to its respective data time slot in the second phase.

To accommodate the bidirectional traffic for one-hop random-access networks, authors of [START_REF] Hoang | Cross-layer design of bidirectional-traffic supported cooperative mac protocol[END_REF] propose an elective scheme for an optimal choice of a helper node to serve as a relay node in a TWRC scenario. Similar to [START_REF] Argyriou | Mac protocol for wireless cooperative physical-layer network coding[END_REF], the selected helper node detects the PNC transmissions and plays the role of the relay node. The transmission process is as follows: first, the source node sends an RTS message to the destination node, and the destination node replays by sending a CTS message. After receiving the CTS message, the source node waits to receive a forwarder-to-send (FTS) frame from the helper node that indicates whether or not there is a PNC opportunity. When a PNC opportunity arises, the source and destination nodes simultaneously send their data to the helper node, which forwards the received PNC data to the source and destination nodes. Besides the synchronization and hidden stations issues, this work suffers from collisions during the optimal helper node (relay) selection process.

An improved version of the work proposed in [START_REF] Hoang | Cross-layer design of bidirectional-traffic supported cooperative mac protocol[END_REF] is presented in [START_REF] Hoang | Phy-mac cross-layer cooperative protocol supporting physical-layer network coding[END_REF], where many relaying nodes are chosen instead of just one relay node to enhance the system's performance and deal with the collision problem during the optimal helper node selection phase.

To summarize the state of the art, table 1 outlines the literature's limitations. We detail these primary shortcomings of the previous works in the following points:

• The synchronization of the edge nodes to the relay node: During the Multiple Access Phase (MAP) of the PNC transmission, the two edge nodes A and B of the TWRC scenario must be synchronized. This step is crucial for successful PNC transmission, ensuring that edge nodes A and B transmit simultaneously. Therefore, the relay node can effectively receive and map the data from both nodes A and B into PNC data. The synchronization process of the two edge nodes A and B to the relay node R requires multiple exchanges of messages (at least two) [START_REF] Huang | Phase-level synchronization for physical-layer network coding[END_REF][START_REF] Chen | Frequency offset compensation in physical-layer network coding systems[END_REF][START_REF] Liew | A primer on physical-layer network coding[END_REF][START_REF] Kramarev | Practical physical layer network coding[END_REF][START_REF] You | Reliable physicallayer network coding supporting real applications[END_REF][START_REF] Kramarev | Implementation of a two-way relay network with compute-and-forward in gnu radio[END_REF][START_REF] Hotescu | Synchronizing tiny sensors with sisp: a convergence study[END_REF].

• The full real-time knowledge about the topology and the neighboring state: In order to organize and schedule the node's transmissions, they must know the type of the outgoing transmission, whether it is an ordinary or PNC transmission. Therefore, the network's nodes must have real-time knowledge about each other's state (information about the data to be sent), at least, the two-hop range. Thus, the works in the literature are divided into three categories: works that deal with real-time knowledge with the messages exchange [START_REF] Liew | A primer on physical-layer network coding[END_REF] [14] [START_REF] He | Building blocks of physicallayer network coding[END_REF], works that use a specific typology [START_REF] Argyriou | Mac protocol for wireless cooperative physical-layer network coding[END_REF] [13] [START_REF] Hoang | Phy-mac cross-layer cooperative protocol supporting physical-layer network coding[END_REF], and works that put assumptions [START_REF] Yomo | Distributed mac protocol for physical layer network coding[END_REF] [6].

• The necessity of the transmission range to reach all the network's nodes in one-hop networks (fully connected networks): Some works like [START_REF] Argyriou | Mac protocol for wireless cooperative physical-layer network coding[END_REF][START_REF] Hoang | Cross-layer design of bidirectional-traffic supported cooperative mac protocol[END_REF][START_REF] Hoang | Phy-mac cross-layer cooperative protocol supporting physical-layer network coding[END_REF] have set this requirement to provide real-time knowledge about the topology and the neighboring state.

• The effect of the hidden stations:

The previous works did not consider the effect of the hidden stations of all three nodes of the TWRC network. At most, they treat the hidden stations of the edge node B and the relay R only [START_REF] Yomo | Distributed mac protocol for physical layer network coding[END_REF][START_REF] Mao | Anc-era: Random access for analog network coding in wireless networks[END_REF][START_REF] Wang | Distributed mac protocol supporting physicallayer network coding[END_REF], whereas the hidden stations on the side of node A are not resolved. Therefore, those MAC protocols can not be deployed in large-scale networks.

Hereafter, we present our distributed PNC MAC protocol DNCP, which supports the PNC technique and addresses the abovementioned literature's limitations, outlined in table 1. To the best of our knowledge, we are the first to practically implement a distributed solution on a real testbed using SDRs for TWRC extended topologies (more than three nodes of the TWRC network).

[11] X X X X [17] X X [12] X X [18] X X [6] X X [13] X X [4] X X X [16] X [19] X X [9] X X [20] X X [14] X X [21] X X [15]
X X

Distributed Physical-layer Network Coding MAC Protocol

Applying PNC approaches in large random wireless multi-hop networks may result in some design complexity, making performance analysis challenging. Therefore, most existing works use the three TWRC nodes scenario, make many restrictive assumptions, and evaluate the proposed solutions in the most straightforward way. To our knowledge, there is no existing complete work ready to be deployed in large and dense random wireless multihop networks. This paper presents the first full PNC MAC protocol, called Distributed Physical-layer Network Coding MAC Protocol (DNCP), based on the Distributed Coordination Function (DCF) of the IEEE 802.11 standard [START_REF] Committee | Wireless lan medium access control and physical layer specification[END_REF].

We address the major design considera-tions outlined hereafter:

• Detecting PNC opportunities: Before transmitting a packet, each edge node should check whether its data can be coded with another data flow so that there is coordination between the transmitting nodes and that their transmissions can occur simultaneously.

• Coordinating simultaneous transmissions: When performing PNC, the source nodes carrying the packets to be encoded must be aware of the appropriate transmission time slot.

• Considering the effect of hidden stations in large-scale networks: Because performing PNC may not always be possible and advantageous, the proposed protocol should guarantee compatibility with other conventional relaying schemes so that ordinary data transmissions (not part of a PNC opportunity) still occur correctly. When performing PNC, the neighboring nodes of the involved nodes in the PNC opportunity should also be informed in order to avoid destructive collisions.

• Real-world evaluation: In order to have a practical solution, we address effective assumptions and try to abide by the hardware requirements.

System Modelling

In this section, we discuss the basic principles of the proposed DNCP protocol. To do so, we model our network topology as presented in Figure 2, where two nodes, A and B, will exchange data via a relay node R. The remaining three nodes, C, D, and E, are hidden stations.

We assume that all nodes are static and the edge nodes are far enough apart to require a multi-hop relaying scheme to communicate. We also suppose that nodes are located at the same distance apart, and all nodes have the same transmission/communication range and the same interference/sensing range as depicted for the relay node in Figure 2.

All transmissions are conducted through single-channel mode. The half-duplex restriction mandates that a node cannot send and receive packets simultaneously. Note also that links are bidirectional, which allows PNC opportunities. When performing PNC, the transmitting nodes simultaneously transmit their original source packets, and we restrict the number of packets encoded to two so that each node in-volved in the PNC process XOR-encodes two packets belonging to different flows into a single coded packet. In this paper, since implementing a routing protocol is out of the scope of this work, we also assume a static routing, allowing nodes to be aware of routes toward all the possible destinations and the corresponding next-hop nodes in a complete system, where all layers are implemented (Physical, MAC, Network (routing), etc.). These informations could be extracted or received from the network layer (routing protocol) in a cross-layer system.

Considering this network modeling, we can distinguish two possible types of transmissions: one-hop or multi-hop transmissions, according to the number of hops to reach the final destination node. For the one-hop transmissions, the next node is the final destination node. Therefore, there is only one scenario, which is a one-hop ordinary transmission. Whereas, there are two possible scenarios for the multi-hop transmissions: a PNC or an ordinary multi-hop transmission. Indeed, the PNC transmission takes place if there is a PNC opportunity. Otherwise, the multi-hop ordinary transmission occurs (if there is no PNC opportunity). The details of each transmission's scenario will be presented in what follows.

For the one-hop transmissions, the same RTS/CTS process of the DCF (IEEE 802.11 standard) will be adopted.

As the present work aims to perform PNC, we are focusing only on multi-hop transmissions where at least one relay node is required to have a successful data exchange between any pair of nodes. Hence, the PNC technique will be applied if there is a PNC opportunity (i.e., two source nodes sending two bidirectional data flows via a relay node); otherwise, an ordinary trans- mission without the PNC technique takes place. Hereafter, we distinguish the two cases.

PNC Packet Exchange Process

To better understand PNC transmission, look at the timing diagram shown by Figure 3 as an illustrative example. In this scenario, node A wants to send data packet P 1 to the final destination node B via the node R. Simultaneously, node B has data packet P 2 for node A according to its queue status. As a result, a TWRC scenario exists between the nodes A, B, and R, which will act as a relay node. The DNCP was proposed in order to benefit from the PNC opportunities. To address the shortcomings of state of the art, as previously noted, DNCP introduces newly adopted policies based on the Distributed Coordination Function (DCF) of the IEEE 802.11 (RTS/CTS) standard [START_REF] Committee | Wireless lan medium access control and physical layer specification[END_REF]. The node A first senses the channel for a DIF S (DIFS: DCF Interframe Space, SIFS: Short InterFrame Space) then sends a Request To Send RT S AB frame to node R, which forwards a PNC Request P N CR AB to node B (in large-scale networks, the final destination is replaced by the two hop destination node of the source node). Since the node B has data to A, it answers by a PNC Confirm (P N CC AB ) frame to the node R to announce that it is ready to perform a PNC packet exchange with node A.

The node R, then, sends back a Clear To Send frame (CT S AB ) to the edge nodes A and B of the detected TWRC scenario. By receiving this CT S AB frame, node A broadcasts a final confirm frame P N C conf irm to inform the hidden station D of the relay node R that a PNC transmission is coming. In parallel, the two nodes A and B start a timeout; this timeout is used to synchronize simultaneous transmissions between the two nodes A and B. It will allow node B to wait until node A sends its control frame P N C conf irm . Node A also waits for the same timeout as node B after receiving the CT S AB frame to synchronize their simultaneous data transmissions.

Then, the two nodes already coordinated, transmit their respective data packets P 1 and P 2 simultaneously to the relay node R, which maps the two received packets into a PNC data packet P N CDAT A P 1 P 2 . Each edge node receiving the coded packet correctly decodes it and replies with an acknowledgment frame ACK to the relay node, indicating the end of this PNC transmission.

If these ACKs are sent at the same time, the relay will receive superposed (overlapped) ACKs. In this work, we adopt the relaying technique Decode and Forward (DAF) combined with the OFDM technology as described in section 4 to help the relay node to extract and decode the superposed ACK frames received from the two edge nodes A and B. Many other techniques proposed in [START_REF] Lu | Real-time implementation of physical-layer network coding[END_REF][START_REF] Hynek | Hardware implementation of distributed learning algorithm for mapping selection for wireless physical layer network coding[END_REF][START_REF] Tan | Mobile lattice-coded physical-layer network coding with practical channel alignment[END_REF][START_REF] Naves | When analog meets digital: Source-encoded physical-layer network coding[END_REF][START_REF] Wang | Adaptive transmission combining physical-layer network coding and successive interference cancellation for two-way relay system[END_REF][START_REF] Li | Design of physical layer network coded ldpc code for a multiple-access relaying system[END_REF][START_REF] Salim | Exchange of correlated binary sources in two-way relay networks using ldpc codes[END_REF] could be used for that purpose for which the main differences are about channel state information (CSI), carrier offset, modulation, and synchronization.

Therefore, the reception of the ACKs indicates that both A and B, respectively, are confirming the reception of the data packet P N CDAT A P 1 P 2 . Otherwise, the relay node will receive only one ACK packet or not receive any ACK frame at all, which will be addressed as failed transmission, and the relay node will perform the exception handling accordingly as in subsection 3.5.

Multi-hop Ordinary Transmission

In this case, as shown in Figure 4, when the relay forwards to B the request of a PNC exchange with A and since node B does not have any data to transmit to A at the same time, it sends back a Negative PNC frame (N EG P N C ) to indicate that it is ready to receive the data packet from A but cannot collaborate in a TWRC scenario. Therefore, the relay node sends a simple CT S A control frame to inform the source and the destination nodes A and B, respectively, that a multi-hop ordinary transmission is coming. After which, a unicast relaying scheme is employed to deliver A's packet to B with respect to the conventional IEEE 802.11 MAC protocol and the traditional NAV setting for hidden stations.

NAV Setting

This subsection presents the Network Allocation Vector (NAV) lengths (time length) for the different aforementioned scenarios. This technique is called Virtual Carrier Sensing (VCS) [START_REF] Gast | 11 wireless networks: the definitive guide[END_REF], which is used by the hidden stations (nodes D, E, and C) other than the TWRC nodes (A,B, and R) in order to sleep (do not transmit) during the PNC or Ordinary transmissions to prevent collisions.

In our examples shown by Figure 3 and Figure 4, for PNC and multi-hop ordinary transmissions, respectively, there are three hidden stations, nodes D, E, and C. The NAV of the two nodes D and C have two phases. The first phase is used before the reception of the message, which indicates whether there is a PNC transmission or not. The second stage is used after the encoded data message, which represents an update of the NAV based on the type of the upcoming transmission. For the node E, its NAV only has one stage, which begins after receiving the message P N CC AB or N EG P N C for PNC or Multi-hop ordinary transmission, respectively.

The NAV of the first node D (neighbor of node A) will be updated after receiving the P N C CON F IRM or ORD CON F IRM (Ordinary confirm) message, which indicates whether the transmission is a PNC or ordinary transmission, whereas the second node C updates the NAV after the reception of the CT SAB or CT S A message. The node E updates its NAV after receiving the P N CC AB or N EG P N C message, which already indicates the type of the upcoming transmission.

T N AV D _P N C = 6T SIF S + T P N CR AB + T P N CC AB +T CT S AB + T P N C conf irm + T P 1 +T P N CDAT A P 1 P 2 + T ACK (1)
T N AV C _P N C = 5T SIF S + T P N CC AB + T CT S AB +T P N C conf irm + T P 1 + T P N CDAT A P 1 P 2 + T ACK (2)
T N AV E _P N C = 4T SIF S + T CT S AB + T P N C conf irm +T P 2 + T P N CDAT A P 1 P 2 + T ACK (3)
T N AV D _ORD = 7T SIF S + T P N CR AB +T N EG_P N C AB + T CT S A + T ORD conf irm + + 2T P 1 + 2T ACK (4) T N AV C _ORD = 6T SIF S + T N EG_P N C AB + T CT S A +T ORD conf irm + 2T P 1 + 2T ACK (5) T N AV E _ORD = 5T SIF S + T CT S A + T ORD conf irm +2T P 1 + 2T ACK (6)

Handling Exceptions

We presented in Section 3.2 and 3.3 the basic process of PNC and Multi-hop Ordinary packet transmission, respectively. This section presents the exceptional cases caused by frame loss (or corruption) and introduces how DNCP handles these cases.

We discuss the most critical cases, depending on the lost frame, as follows:

• P N CC AB or N EG_P N C AB :
This means that the node B did not receive the P N CR AB message of the relay node R, or it does not exist. Therefore, the node R sends a CT S message (the same as the RTS/CTS process of the DCF) to authorize a one-hop ordinary transmission with node A.

• CT S or CT S AB or CT S A : In this case, there are two situations. The first one is that the node A wants to send a one-hop packet, and it did not receive the CT S message, the same as the RTS/CTS process of the DCF. So, it cancels the transmission and tries again in another round. In the second situation, the node A wants to send a multi-hop packet, and it did not receive a CT S AB or CT S A message. In this situation, the nodes A, B, and C all are concerned. The nodes A and B cancel the transmission and try again in another round. However, the node C will not update its N AV and considers that the transmission has failed.

• P N C conf irm or ORD conf irm : Here, the node D will not update its N AV , which means the transmission failed.

• ORD_DAT A : Here, there are three situations: the first is during one-hop packet transmission. In this case, the node R or any receiving node cancels the transmission and considers that the transmission has failed. The second case is during multi-hop ordinary packet transmission. Here, either node R or B, consider that the transmission has failed. Finally, the third situation is a PNC packet transmission. If the relay node receives only one ordinary data packet from one edge node, it forwards this data to the other destination edge node and considers it a multi-hop ordinary transmission. However, if it did not receive any ordinary data, it cancels the transmission and considers that it has failed.

• P N C_DAT A: If one of the edge nodes A and B, or the two, did not receive the P N C_DAT A, they consider that the transmission has failed and try again in another round.

• P N C_ACK : If the relay node R receives only one P N C_ACK for one edge node, it sends the received P N C_DAT A in another round to the other edge node. Whereas, if it did not receive any P N C_ACK, it broadcast the received P N C_DAT A in another round to both edge nodes A and B.

• ACK : If a node did not receive an ordinary ACK, it considers that the transmission has failed and tries to send this ordinary DATA again in another round.

• Duplicate packets : If a node receives a duplicate PNC or ordinary data packet, it sends a P N C_ACK or ACK, respectively, and deletes the received data packet.

The flowcharts shown in Figure 5 and Figure 6 summarize all the possible cases (sent, received, and lost frames) of the DNCP for the source node and any receiving node (relay, destination, and hidden station), respectively.

To sum up, the DNCP protocol stands for the following outlined policies that make it reliable, practicable, and suitable for multihop large-scale networks:

• Reliable coordination: Involved nodes in the TWRC scenario could carry their clock information in the control frames (RTS, CTS, ...) to coordinate the channel access between them and guarantee a simultaneous data transmission when performing PNC. The initial synchronization of the clocks of the nodes can be inspired from [START_REF] Van Den Bossche | Sisp: a lightweight synchronization protocol for wireless sensor networks[END_REF], [START_REF] Huang | Phase-level synchronization for physical-layer network coding[END_REF], [START_REF] Kramarev | Practical physical layer network coding[END_REF], [START_REF] Chen | Frequency offset compensation in physical-layer network coding systems[END_REF], [START_REF] Liew | A primer on physical-layer network coding[END_REF], [START_REF] Kramarev | Implementation of a two-way relay network with compute-and-forward in gnu radio[END_REF], [START_REF] You | Reliable physicallayer network coding supporting real applications[END_REF], and [START_REF] Hotescu | Synchronizing tiny sensors with sisp: a convergence study[END_REF].

• No full real-time knowledge: Using the DNCP, all information required to check and maintain a PNC opportunity is either local or extracted from the control frames. Therefore, there is no need for full real-time knowledge about the topology or the queuing status of neighboring nodes.

• No additional delays due to empty time slots: DNCP is a distributed MAC protocol, which implies that it does not suffer from the delay linked to the static TDMA-like MAC protocols, where there can be empty time slots and not used [3] [4].

• No necessity of dedicated network topologies: DNCP does not require specified or dedicated topologies to be deployed (for example, the transmission range to reach all the network's nodes as in [START_REF] Argyriou | Mac protocol for wireless cooperative physical-layer network coding[END_REF][START_REF] Hoang | Cross-layer design of bidirectional-traffic supported cooperative mac protocol[END_REF][START_REF] Hoang | Phy-mac cross-layer cooperative protocol supporting physical-layer network coding[END_REF]). Therefore, it can be deployed on any multi-hop wireless network.

• No hidden station's problem: Thanks to an efficient NAV configuration mechanism coupled with a systematic exchange of control messages, the hidden stations can properly delay their access to the channel and thus avoid what was initially called "destructive interference".

Framing

In this subsection, we discuss the framing of the proposed DNCP protocol. The frames are designed to contain the necessary information, as discussed in previous sections. Figures 7 (a The MAC header is illustrated in Figure 7(c), which includes the type of the message (Frame control), the duration, the destination MAC address, the source MAC address, the final destination MAC address, the Access point's Basic Service Set (BSS) MAC address, the sequence number (Sequence number 1), and the sequence number (Sequence number 2) of the second data frame if the data frame is a PNC data.

Implementation and Results

In this section, we present a real implementation of the DNCP using a Software Defined Radio (SDR) based testbed. More specifically, we have used GNU Radio [START_REF] Fsf | Gnu radio -gnu fsf project[END_REF] as software and Universal Software Radio Peripheral (USRP) [START_REF] Inc | Universal software radio peripheral[END_REF] as hardware. The version of the GNU Radio used is 3.7.14, which is well-documented and supported. While for the hardware, we used the FIT/CorteXlab testbed [START_REF] Massouri | Cortexlab: An open fpga-based facility for testing sdr & cognitive radio networks in a reproducible environment[END_REF], particularly the U SRP N 2932.

Experimentation Setup

To begin with, we used the TWRC network shown in Figure 8 to infer how the DNCP would behave with the TWRC scenarios.

Next, in order to analyze the impact of the hidden station's problem on our solution, we adopted the topology shown in Figure 9, which we refer to as the F ullT opology.

We consider, as illustrated in Figure 9, that nodes A and B are, on the one hand, involved in a TWRC scenario via the relay node R and, on the other hand, are exchanging data flows with their respective neighbor nodes D and E. This data configuration was motivated by the need to have ordinary transmissions as well as PNC ones.

Finally, the Line topology, depicted by Figure 10, is used to deduce the effects of successive relays in a large-scale multi-hop network. In this topology, the two edge nodes, A and F , are exchanging data simultaneously via the intermediate nodes B, C, D, and E. Thus, each intermediate node in this scenario could be a relay node.

We built a three-layered prototype using the GNU Radio Platform where the application layer generates data packets, the MAC layer implements our protocol DNCP, and the Physical layer performs on WIFI proposed in [START_REF] Bloessl | An ieee 802.11 a/g/p ofdm receiver for gnu radio[END_REF] [START_REF] Bloessl | [END_REF].

For the application layer, the block Message Strobe1 of the Debug Tools GNU radio module is used to create data packets from text. However, we built a new GNU Radio module for the MAC layer for our proposed MAC protocol called DNCP. We based our implementation on the wellknown CSMA/CA MAC protocol proposed in [START_REF] Gomes | gr-macprotocols: Gnu radio csma/ca mac[END_REF].

Node synchronization was the first challenge we had to handle in our implementation. All the existing distributed clock synchronization protocols that provide the level of synchronization required by the PNC technique are either implemented on an Application Specific Integrated Circuit (ASIC) [START_REF] Djukic | Soft-tdmac: A software tdma-based mac over commodity 802.11 hardware[END_REF] or a Field Programmable Gate Array (FPGA) [38] [9]. These solutions are not compatible with the General Purpose Processors (GPP) based SDRs, where most signal processing operations are performed in the personal computer on top of the SDR, and the synchronization does not take place according to deterministic time sequences as in ASIC and FPGA platforms [START_REF] You | Reliable physicallayer network coding supporting real applications[END_REF]. To the best of our knowledge, all the existing implementations that ensure the level of synchronization required by the PNC technique are centralized and based, either on Global Positioning System disciplined Oscillators (GPSO/GPS) [START_REF] Tan | Mobile lattice-coded physical-layer network coding with practical channel alignment[END_REF] [25], or based on the SDR's Gigabit Ethernet [START_REF] You | Reliable physicallayer network coding supporting real applications[END_REF] [39], for example using the Ettus Octoclock [START_REF]Ettus octoclock[END_REF]. When dealing with distributed implementations, we used OFDM technology [START_REF] Li | Orthogonal frequency division multiplexing for wireless communications[END_REF] to avoid such a level of synchronization [START_REF] Lu | Implementation of physical-layer network coding[END_REF][START_REF] Lu | Real-time implementation of physical-layer network coding[END_REF][START_REF] Ngo | Implementation of network coding scheme in universal software radio peripheral[END_REF][START_REF] Kramarev | Implementation of a two-way relay network with compute-and-forward in gnu radio[END_REF][START_REF] Quynh | Network coding with multimedia transmission: A software-defined-radio based implementation[END_REF][START_REF] Quynh | Network coding with multimedia transmission and cognitive networking: An implementation based on software-defined radio[END_REF] where all the schedules are done using the control bea-con frames as in the 802.11 standard [10] [22] and no more additional overhead is incurred.

When focusing on the physical layer, the key challenges were timing and frequency asynchrony in wireless transmissions due to the mismatch between the transmitter and receiver hardware clocks and channel effect [43][46]. Carrier Frequency Offset (CFO) between the transmitter and the receiver would cause inter-subcarrier interference if left uncorrected. Then, to tackle these challenges, the timing offset and the frequency offset have been estimated with OFDM Synchronizer block performing the Schmidl-Cox algorithm as described in detail in our previous work [START_REF] Aissaoui | Physical-layer network coding: A heterogeneous software-definedradio based implementation[END_REF]. The main idea of this solution is to take advantage of the OFDM frame structure to find the starting point of the frame as well as estimate and suppress the frequency offset of received symbols.

At the MAC layer, the result of our new module is a GNU Radio Companion (GRC) block called DNCP2 as shown in Figure 11. This block comprises multiple ports and parameters as summarized in Table 2 and Table 4, respectively.

It is worth noting that the parameter ALP HA is used to multiply the time slot, the SIFS, and the DIFS in order to increase the waiting time of the MAC layer since all the processes will be done by the GNU Radio software, which is not fast enough compared to integrated FPGA's software [START_REF] Bloessl | Timings matter: Standard compliant ieee 802.11 channel access for a fully software-based sdr architecture[END_REF]. The parameter T hreshold refers to the received power limit for which the channel is considered busy.

The MAC and PHY layer's parameters are shown, respectively, in Table 3 and Table 4, which are the default parameters recommended in literature [START_REF] Gomes | gr-macprotocols: Gnu radio csma/ca mac[END_REF].

In a prior experiment, we first check the channel state when idle and during transmission. This helps us to estimate the noise made by neighboring nodes properly and to correctly configure the signal power for which the channel is considered busy in our experiments. Figure 12 shows the channel's state during transmission and idle mode. During the idle mode (green dashed line), the received signal power is always around -100dBm since the CorteXlab environment is isolated (indoor environment), whereas, during the transmission mode (red continued line), there are four levels of the received signal power. The first level is around -100dBm, which indicates that no transmission is taking place at this time (or idle). The other levels (around -80dBm, -60dBm, and -40dBm, for weak, medium, and high signal power, respectively) represent that a transmission is taking place. In our experiments, we consider the medium signal power (-60dBm) as a threshold to indicate that the channel is busy by data transmissions, and any other weak signal (lower or equal to -80dBm) is considered noise.

The DNCP's performances are evaluated for the three aforementioned TWRC, Line, and Full topologies and are compared with the conventional CSMA/CA MAC protocol and the opportunistic protocol (PNCOPP) proposed in [START_REF] Yomo | Distributed mac protocol for physical layer network coding[END_REF] using four metrics: the throughput, the delay, the packet loss rate, and the packet drop rate since these metrics are the most impacted by the PNC technique. Each experiment lasted 50 minutes and was repeated five times. The considered confidence interval for the results is 95%.

Results Analysis

The average throughput of all the nodes is expressed in Kbits/s and measures the total number of bits of received packets during the experimentation period.

In order to estimate the overall delay induced by our MAC protocol, we estimate the average local delay of each data frame for each experiment. This delay identifies the average time between entering the local MAC layer buffer and transmitting the corresponding signal by the physical layer during a successful data packet transmission between any pair of nodes. We assume that Receives a data frame from the physical layer to the MAC layer

CS in

Receives the value returned by the Carrier Sensing operation there are no buffering delays in the physical layer so that the received data frame from the MAC layer is immediately transmitted. Also, we counted only successful transmissions for which the sending node received a positive acknowledgment. This period includes all potential delays for queuing, channel access, control frames transmission, inter-frame spaces, Etc.

The third metric is the packet's loss rate, which is computed based on the ratio of the number of lost packets from all the sent packets. This loss may be caused by error, data corruption, destructive interference, or breakable links, and we do not consider loss caused by saturated buffers.

Finally, the fourth metric is the packet's drop rate, which represents the ratio of dropped packets from all sent ones. We consider that a packet is dropped only if the MAC layer buffer is full or the maximum number of sending retries is reached. This metric estimates the impact of PNC transmissions on ordinary transmissions.

The performance results of the proposed DNCP, the PNCOPP, and the well-known CSMA/CA MAC protocols on the mentioned topologies are presented in the following sections.

Results of the TWRC topology

This section presents the results of the DNCP, PNCOPP, and CSMA/CA MAC protocols on the TWRC network. For this simple topology, we first study the impact of the modulation technique on the network's performance. This choice was motivated by the analysis made in a previous work [START_REF] Aissaoui | A new taxonomy of physical-layer network coding techniques in two way relay channel model with single antenna[END_REF], which revealed an important correlation between the modulation technique used in the physical layer and the performances drawn from the PNC. Indeed, it was proven that OFDM-based PNC is vulnerable to carrier frequency offset (CFO), which can be induced by node mobility and/or local oscillator (LO) mismatch between transmitters and receivers. This CFO cannot be estimated and eliminated in PNC at the relay node, which generally faces two different CFO values from the edge nodes A and B. CFO in PNC results in not only the inter-carrier interference (ICI) between the subcarriers of nodes A and B, but also a relative phase offset between the received constellations of nodes A and B, and both will cause performance loss in PNC, especially for high-order modulations including QPSK [START_REF] Surti | Impact of high speed modulation technique on wireless network[END_REF] [START_REF] Yeom | Performance analysis of physical-layer network coding with qpsk modulation in wireless iot networks[END_REF]. The rationale behind this first set of experiments is to evaluate our MAC protocol under different modulations in order to figure out the more appropriate PHY configuration to obtain the best performances.

The results of five experiments of each configuration are considered. Each composition experiment has been running for two minutes: for example, the sample rate 10MHz with the modulation BPSK1/2, and so on.

To take a broader view, Figures 13(a), 13(b), 13(c), 13(d) show the results average For the throughput, the DNCP outperforms the CSMA/CA MAC protocol regardless of the modulation used, except for the 64QAM modulation.

For all the performance metrics, we note that the modulation QPSK gives the best performance. Results also show that the higher the M-PSK modulation order, the higher the performance is, whereas, for the M-QAM modulation, the higher the modulation order is, the lower the performance is. The reason is that the M-QAM modulation is highly affected by the received Signal Noise Ratio (SNR) or Signal to Interference plus Noise Ratio (SINR), which requires higher levels of transmitter power and Receiver gain, especially with the PNC transmissions, where two signal received at the same time, in contrast with the M PSK modulation, which is highly resistant to the SNR or SINR [START_REF] Wang | Constellation mapping for physical-layer network coding with m-qam modulation[END_REF][START_REF] Wang | Rate and power adaptation for physical-layer network coding with m-qam modulation[END_REF][START_REF] Tan | Mobile lattice-coded physical-layer network coding with practical channel alignment[END_REF][START_REF] Haboobi | Impact study and evaluation of higher modulation schemes on physical layer of upcoming wireless mobile networks[END_REF][START_REF] Otsuka | Transmission performance of an ofdm-based higher-order modulation scheme in multipath fading channels[END_REF][START_REF] Singya | A survey on design and performance of higher-order qam constellations[END_REF].

We easily determine that for all the performance metrics, DNCP outperforms the conventional CSMA/CA protocol except for M-QAM modulation. This is caused by the Decode-and-forward (DAF) method used for PNC transmissions. Indeed, when using DAF with binary phaseshift keying (BPSK) or quadrature phaseshift keying (QPSK) modulations, the superposed signal can be mapped to the coded signal with the XOR operation. However, the XOR mapping method may not be suitable for higher-level modulations [START_REF] Wang | Constellation mapping for physical-layer network coding with m-qam modulation[END_REF].

Thus, in some cases, using this modulation, the encoding/decoding process will lead to erroneous packets, therefore retransmitted or ignored, which explains the drop in performance of our protocol in these scenarios.

Figures 13(a all the metrics. The reason is that the PNCOPP is particularly designed for the TWRC network (3 nodes only) and produces, therefore, the minimum possible control frames, the lowest delay, and the best throughput.

Results of the Full topology

In this section, we present the results of the DNCP, PNCOPP, and the CSMA/CA MAC protocols over the Full topology to see the hidden stations' effect on the TWRC network.

As we presented before, the most suitable modulation for the presented system is the QPSK3/4 modulation. Therefore, besides the parameters shown in Table 4, we used the modulation QPSK3/4 and the sample rate 10M Hz.

For figures 14 (a), 14 (b), 14 (c), and 14 (d), the results represent the minimum (Min), average (Avg), and maximum (Max) metrics values of five experiments (with confidence interval 95%). Each experiment took 50 minutes, and every 10 minutes, the results values were taken to see the stability of the MAC protocols over time.

For all performance metrics, we can note that DNCP always outperforms the PN-COPP and conventional CSMA/CA MAC protocols, except for the packet's loss rate It can be observed from this figure that the throughput gain of DNCP over CSMA/CA is about 1.82, which approaches the theoretical result of 2 as discussed in Section 1, and about 1.39 over PNCOPP.

Compared to the throughput obtained with the simple TWRC topology, in full topology, the throughput decreases slightly because of collisions or what we refer to as destructive interf erence caused by the channel contention, which becomes intense with the increasing node number. This ob-servation is particularly significant for the PNCOPP, which suffers from the hidden station issue since it is not designed to perform in large networks and does not use any control frame to reserve the channel, which makes it vulnerable to hidden terminal problems.

The error bars also show that the results of the DNCP MAC protocol are not very distracting, indicating our solution's stability.

In terms of delay, when it is an ordinary transmission, the DNCP will have the same behavior as the conventional CSMA/CA and PNCOPP and, therefore, achieves the same delay. Though, when dealing with PNC opportunities, DNCP requires an additional exchange of control messages in order to coordinate the two transmissions simultaneously, then more delays. Also, the increased throughput achieved by DNCP allows more packets to enter the network, and consequently, packets may have to wait longer in the queue. However, this delay is compensated by using single access control for two transmissions simultaneously and using a new policy to avoid the destructive interf erences caused by the hidden station, which explains the gain in delay provided by our DNCP protocol compared to CSMA/CA and to the PNCOPP which suffer from destructive interf erences caused by the hidden stations.

The packet loss is minor and almost the same for the DNCP and CSMA/CA protocols. This is due to a robust physical layer using the QP SK modulation technique coupled with reliable hidden nodes management at the MAC layer, which considerably reduces packet losses caused by transmission errors or interference. Nevertheless, for the PNCOPP MAC protocol, which gives the worst performance in terms of packet loss rate due to its vulnerability to hidden terminal problems and destructive interf erences as it does not include any control frame to reserve the channel prior to the data transmission.

Results of the Line topology

The Line topology is used to study the DNCP behavior when successive relaying nodes are added to the system.

It can be observed from Figure 15 that DNCP outperforms both the conventional CSMA/CA and the PNCOPP MAC protocol, with, respectively, an average throughput gain of 1.72 and 1.40. However, com-pared to TWRC and full topologies, the overall performance slightly decreases because the additional relay processing may result in more end-to-end delays, which consequently reduces the number of transmitted packets. Nevertheless, considering a single PNC opportunity between nodes A and F considerably reduces the risk of collisions, loss, and drop of packets, which justifies the stability of these results compared to previous topologies. Moreover, as we consider static networks (because routing issues need to be considered in a network with mobility, which is beyond the scope of this paper), it is not likely to have link failure for DNCP, PNCOPP, and CSMA/CA protocols, which explains the fairly low values of the loss rates.

In order to assess the goodput of our proposal, we evaluated, for the three considered topologies, the overhead involved by the PNC orchestration. Results are depicted in Figure 16. For all scenarios, PNCOPP gives the best performance for the overhead compared to DNCP and CSMA/CA. Indeed, to exchange two packets in a PNC transmission, PNCOPP needs only four control frames (RT S, CT S, 2 ACK), while DNCP uses seven control frames (RT S AB , P N CR AB , P N CC AB , CST AB , P N C_CON F IRM , 2 ACK) and CSMA/CA uses 12 control frames (4 RT S, 4 CT S, 4 ACK), which explains these findings. However, for Full and Line topologies, the results of DNCP are close to those of PNCOPP as DNCP compensates the large number of control frames used by the increasing number of PNC opportunities it detects. It can also be explained by the new policy introduced by DNCP to avoid the hidden station problems and the consequent losses.

Conclusion

In this paper, we have proposed the Distributed physical-layer Network Coding MAC Protocol (DNCP), which extends the IEEE 802.11 MAC protocol to support PNC.

The basic idea of our proposal was to detect the opportunities of PNC by using conventional RTS/CTS frames and to coordinate the channel access between the concerned source nodes via a common relay node to carry simultaneous transmissions.

The focus of the proposed protocol is on compatibility with other conventional relaying schemes so that ordinary data transmis-sions (not part of a PNC opportunity) still occur correctly. Several mechanisms were proposed to address such issues caused by the hidden node problem, such as NAV enhancement and ACK diversity, which has led to a reliable packet exchange that avoids destructive interference.

In order to have a practical solution, we addressed effective assumptions related to the hardware requirements and the network topology, and we carried out a real-world evaluation using an SDR-based testbed. The DNCP's performances are evaluated for the TWRC, Line, and Full topologies and are compared with the conventional CSMA/CA MAC and PNCOPP MAC protocols [START_REF] Yomo | Distributed mac protocol for physical layer network coding[END_REF] using four metrics: the throughput, the delay, the packet loss rate, and the packet drop rate since these metrics are the most impacted by the PNC technique.

The experimentation results show that the proposed DNCP brings considerable throughput improvement in various scenarios while maintaining a similar trend to the packet loss results. It follows that DNCP is advantageous, particularly for throughputsensitive applications of wireless networks.

A deeper analysis of the multi-relay line topology is worth investigating in the future. Moreover, as we considered, in this work, a static routing, we intend, in the future, to couple and adapt the PNC technique and the MAC level scheduling to have an opportunistic routing capable of directing the data packets on the paths that support the PNC opportunities.

Figure 1 :

 1 Figure 1: The three relaying techniques of the TWRC network.

Figure 2 :

 2 Figure 2: The considered network topology.

Figure 3 :

 3 Figure 3: Example of a multi-hop PNC transmission.

Figure 4 :

 4 Figure 4: Example of a multi-hop ordinary transmission.

  ) and 7 (b) summarize the formats of control and data frames, which are modified according to the IEEE 802.11 standard.

Figure 5 :

 5 Figure 5: Flowchart of the source node.

Figure 6 :

 6 Figure 6: Flowchart of the relay, destination, and hidden station.

Figure 7 :

 7 Figure 7: DNCP's MAC Frame.

Figure 8 :

 8 Figure 8: The TWRC Topology.

Figure 9 :

 9 Figure 9: The full topology.

Figure 10 :

 10 Figure 10: The Line topology.

Figure 11 :

 11 Figure 11: The DNCP GRC block.

Figure 12 :

 12 Figure 12: Channel State Check.

  ), 13(b), 13(c), and 13(d) also show the good performance of the PN-COPP over the DNCP and CSMA/CA for

Figure 13 :

 13 Figure 13: Results for the TWRC topology.

Figure 14 :

 14 Figure 14: Results of the Full topology.

Figure 15 :

 15 Figure 15: Results of the Line topology.

Figure 16 :

 16 Figure 16: Overhead results.

Table 1 :

 1 The literature's limitations

	Ref.	Nodes	Topology	Neighbor-	Custom	Hidden
		synchro-	awareness	ing	state	topology	station
		nization		awareness		issue

Table 3 :

 3 MAC Layer Configuration

Parameters Values Size of the application layer buffer 250 data frames Size of the local MAC layer buffer 3 data frames Average delay of the GNU Radio block 1000 µs (1ms) Maximum retries to send a data packet 10 Physical layer delay to receive a data frame

		1 (µs) for a maximum distance of
		300m between two nodes
	CWmin	16
	CWmax	1024

Table 4 :

 4 PHY Layer Configuration

	Parameters	Values
	Central frequency f	f = 2.45GHz
	Shift frequency β	β = 250KHz
	Frequency 1	f + β
	Frequency 2	f -β
	Sample Rate	10MHzs
	Modulation	BPSK1/2 (1/2 is the code rate), BPSK3/4
		, QPSK1/2, QPSK3/4, 16QAM1/2,
		16QAM3/4, 64QAM1/2, and 64QAM3/4
	Transmission Power	0.75 (Normalized gain ∈ [0.0, 1.0])
	Received Gain	0.03 (Normalized gain ∈ [0.0, 1.0])
	Threshold CS	-60
	Alpha	10
	DATA packet's length	500 bytes
	Data packet's production time interval 500 ms
	of all the modulations used for the through-
	put, delay, packet loss rate and packet drop
	rate, respectively.

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