Christophe Lepoittevin
email: christophe.lepoittevin@neel.cnrs.fr

Structure resolution by Electron Diffraction Tomography of the complex layered Iron-Rich Fe-2234-type Sr 5 Fe 6 O 15.4

Keywords:

    Structure resolution by electron diffraction tomography of the complex layered iron-rich Fe-2234-type Sr 5 Fe 6 O 15.4

Introduction

In the search for new high Tc superconducting materials (HTSC), large structural families have been found out. They are characterized by n rock-salt (RS) type layers (n = 1, n = 2 or n = 3) and m perovskite-type layers. In the Sr-A-O system (A = 3d transition element), the RS block consists of (n -1) intermediate [AO] layer(s) sandwiched between two [SrO] layers, and the perovskite block is made of (m -1) [SrO] layers alternating along the stacking axis with m [AO 2 ] layers. Then a simple notation, based on the cation indices (n -1)2(m -1)m, has been adopted by the community to denote these materials. For A = Fe, in the iron rich part of the Sr -Fe -O, the incommensurate modulated phase Sr 4 Fe 6 O 13-δ , was structurally analyzed in detail by single crystal X-ray diffraction and the results led to the formulation (Fe 2 )(Sr 2 )FeO 6.5-δ/2 (n = 3, m = 1), which highlights the particularity of the double intermediate layer in the RS block to contain only transition metals (instead of 5d 10 cations in the HTSCs) [START_REF] Pérez | A new light on the iron coordination in Sr 4 Fe 6 O 13±δ : Super space formalism and structural mechanism[END_REF] . The second member of the iron-rich strontium oxides family (n = 3, m = 2) was obtained by substituting Sr 2+ with Bi 3+ and the formalism used to describe its structure is (Fe 2 )(Bi 0.69 Sr 1.31 )SrFe 2 O 9.5±1/2δ [START_REF] Grebille | Disordered commensurate structure of the 2212-related phase Fe 2 (Bi 0.69 Sr 2.31 )Fe 2 O 9.5±1/2δ and structural mechanism[END_REF] . Thus these first two members were denoted Fe-2201 and Fe Bi -2212 respectively, in order to outline the structural difference with the other Bi-2201 or Bi 2212 structures in the Bi-rich Bi -Sr -Fe -O system. In 2008 a third member of this iron-rich family was isolated, and its preliminary structural characterization highlighted a first schematic representation of the structural model in accordance with electron diffraction (ED) studies and High Resolution Transmission Electron Microscopy (HRTEM) and Z-contrast imaging observations. This schematic model led to the formulation (Fe 2 )(Sr 2 )(Sr 3 )Fe 4 O 14.65 (n = 3, m = 4) [START_REF] Lepoittevin | A Layered Iron-Rich 2234-Type with a Mixed Valence of Iron: The Ferrimagnetic Tl-doped Fe 2 (Sr 2ε Tl ε )Sr 3 Fe 4 O 14.65[END_REF] , and then can be named Fe-2234. Attempts to synthesize single crystal of this phase for solving the structure by single crystal X-ray diffraction have been unsuccessful, and solving the structure ab-initio by powder X-ray diffraction is almost impossible due to the numerous diffraction peaks. In the present article we propose to solve the structure of this phase using tools provided by electron crystallography, in order to highlight structural details like oxygen positions. Different methods have been developed these past few years to collect three dimensional electron diffraction data : zone axis Precession Electron Diffraction (PED) [START_REF] Gjønnes | Structure Model for the Phase Al m Fe Derived from Three-Dimensional Electron Diffraction Intensity Data Collected by a Precession Technique. Comparison with Convergent-Beam Diffraction[END_REF][START_REF] Boulahya | Ab initio determination of heavy oxide perovskite related structures from precession electron diffraction data[END_REF][START_REF] Novitskaya | crystal structure and physico-chemical properties of the new quaternary oxide Sr 5 BiNi 2 O 9.6[END_REF][START_REF] Dorset | Direct electron crystallographic determination of zeolite zonal structures[END_REF][START_REF] Gemmi | Structure solution with three-dimensional sets of precessed electron diffraction intensities[END_REF][START_REF] Klein | Precession electron diffraction of Mn 2 O 3 and PbMnO 2.75 : solving structures where X-rays fail[END_REF][START_REF] Boullay | Structure determination of a brownmillerite Ca 2 Co 2 O 5 thin film by precession electron diffraction[END_REF][START_REF] Gemmi | Structure solution of the new titanate Li 4 Ti 8 Ni 3 O 21 using precession electron diffraction[END_REF][START_REF] Hadermann | Solving the Structure of Li Battery Materials with Precession Electron Diffraction: Application to Li 2 CoPO 4[END_REF] , Electron diffraction tomography (EDT) 13 -22,27 and Rotation Electron Diffraction (RED) [START_REF] Zhang | Precession electron diffraction using a digital sampling method[END_REF][START_REF] Yun | Phase identification and structure determination from multiphase crystalline powder samples by rotation electron diffraction[END_REF][START_REF] Su | Structure analysis of zeolites by rotation electron diffraction (RED)[END_REF][START_REF] Wan | Three-dimensional rotation electron diffraction : software RED for automated data collection and data processing[END_REF] . The advantage of all these techniques is to solve structures analogously to conventional single crystal X-ray diffraction, on micro or nano-sized particles present in a powder, which is particularly interesting in the case of multiphased samples. Solving structures from electron diffraction data is a quite recent science, but numerous structures have already been solved, including complex structures presenting incommensurate modulation [START_REF] Boullay | Precession Electron Diffraction Tomography for Solving Complex Modulated Structures : the case of Bi 5 Nb 3 O 15[END_REF] or containing light cations like Li +11,12 .

In the present work, manual electron diffraction tomography was the technique used for solving the structure of Sr 5 Fe 6 O 15.4 (Fe-2234). The resulting structural model was refined from powder X-ray diffraction data.

Experimental section

For the TEM observations the black powder sample was synthesized by conventional solid state reaction [START_REF] Lepoittevin | A Layered Iron-Rich 2234-Type with a Mixed Valence of Iron: The Ferrimagnetic Tl-doped Fe 2 (Sr 2ε Tl ε )Sr 3 Fe 4 O 14.65[END_REF] . The specimen for Transmission Electron Microscopy (TEM) analysis was prepared by crushing a small part of the pellet in an agate mortar containing ethanol, and a drop of the suspension was deposited on a copper grid covered with a holey carbon film. Electron diffraction patterns were recorded using a 4k × 4k CMOS-TVIPS TEM camera with a 16 bits dynamic range, and the Energy Dispersive Spectroscopy (EDS) analysis was performed using a Brüker Silicon Drift Detector. Zone Axis Selected Area Diffraction (SAED) was performed with a LaB 6 Philips CM300ST transmission electron microscope operating at 300kV, equipped with a ±30° double tilt sample holder. Manual electron diffraction tomography was performed using a single tilt tomography sample holder with a tilt range from -60° to +49°. The dataset was collected by recording an ED pattern each time the sample holder was manually 1° tilted. A precession angle of 1.2° was applied to integrate the whole intensity of the reciprocal space rods. Then the dataset were processed using PETS program [START_REF] Palatinus | PETS-program for analysis electron diffraction data[END_REF] . The program performs peak extraction on each ED pattern, and then refines the azimuthal angle between the horizontal axis and the projection of the tilt axis. After having analyzed the peak positions, PETS performs a difference vector space analysis to produce a complete representation of the reciprocal space. The program JANA2006 [START_REF] Petřiček | Structure Determination Software Programs[END_REF] was used as a graphical interface to find the unit cell and to refine the orientation matrix that allows integrating the reflection intensities of each ED pattern by PETS. Intensities belonging to the same reflection on adjacent patterns are integrated together, and the final list of intensities contains one value per each hkl indices with the corresponding estimated standard deviation [START_REF] Palatinus | Structure refinement using precession electron diffraction tomography and dynamical diffraction: tests on experimental data[END_REF] . Thus the resulting intensity file was used as an input for ab initio structure determination by direct methods with the software SIR2011, which contains special algorithms adapted to Electron Diffraction [START_REF] Gemmi | Structure of the new mineral sarrabusite, Pb 5 CuCl 4 (SeO 3 ) 4 , solved by manual electron-diffraction tomography[END_REF][START_REF] Cascarano | Crystal structure solution via precession electron diffraction data: The BEA algorithm[END_REF] .

Powder X-ray diffraction (XRD) pattern was recorded on a D8 ENDEAVOR diffractometer in reflection mode using Cu K α radiation (λ = 1.54181 Å), in the range 10° ≤ 2θ ≤ 100° with 0.02° stepsize. The structure was refined by the Rietveld method using FULLPROF SUITE program 32 integrated in WINPLOTR software [START_REF] Roisnel | WinPLOTR: A windows tool for powder diffraction pattern analysis[END_REF] .

Results

3-1. Crystal structure determination

The EDS analysis, performed on over 40 particles of the phase, exhibits a ratio Sr/Fe ≈ 0.80±0.04. By tilting around crystallographic axes, SAED patterns were recorded for reciprocal space reconstruction, and the three main zone axes [100],

[010] and [001] are presented as example in Fig. 1(a The structure resolution was performed by SIR2011 software using the direct methods. The input file contains the following informations :

-The cell parameters : cell parameters determined from electron diffraction being much less accurate than those from powder X-ray diffraction, the cell parameters refined by Lebail refinement on powder X-ray diffraction data were used :

-The possible space groups to test : Fmmm, Fmm2, Fm2m, F2mm or F222.

-The nominal composition : 100Sr 120Fe 280O obtained by a simple proportional calculation between the number of atoms in Fe 2 (Bi 0.69 Sr 1.31 )SrFe 2 O 9.33 (Fe Bi -2212, V = 2498 Å 3 ) and the number of atoms in our unit cell (Fe-2234, V = 6426 Å 3 ).

-The resolution d resolution : radius of a sphere containing the number of reflections taken into account for structure determination. Calculations were made for d resolution = 0.8 Å.

-The two-beam approximation of dynamical theory I ∝ |F | : amplitudes of diffracted beams are comparable to the amplitude of the incident beam. Two-beam approach assumes that only the incident and the diffracted beams interact in the crystal.

First trials for solving the structure were carried out using the highest symmetry space group Fmmm. Among the 14755 measured reflections, 1746 reflections were retained with I > 3σ; leading to R int of 34 and a redundancy of 3.6. After SIR2011 calculations, 1020 independent reflections were taken into account for a d resolution of 0.8 Å, giving rise to a completeness of 58 %. The output of the direct method calculations is a density map that is interpreted as atomic positions by SIR2011. The structure solution model is presented in Fig. 2 (a), and can be described as a regular alternation of Sr rows with Fe rows along c axis, with however two successive Fe rows at 0.2 < z < 0. Other trials in ab-initio structure resolution were carried out using a lower symmetry, by replacing one of the m mirror by a 2-fold axis. The position of the 2-fold axis was chosen identical to the one of the parent compound Fe Bi -2212, which is along b axis [START_REF] Grebille | Disordered commensurate structure of the 2212-related phase Fe 2 (Bi 0.69 Sr 2.31 )Fe 2 O 9.5±1/2δ and structural mechanism[END_REF] . In Fm2m space group, 1737 reflections were retained for I > 3σ, leading to a R int of 32.5 and a redundancy of 2.1. The [010] projection of the 3D electron density map generated by VESTA after SIR2011 calculations is presented in Fig. 3(a), and its interpretation by SIR2011 in term of atomic positions is presented in Fig. 3 In Fig. 4, four sections are presented in the area of the unit cell where Sr and Fe rows alternate regularly along c axis, which corresponds to 0.30 ≤ z ≤ 0.70. In Fig. 4(b), the strong electron densities with the electrostatic potential maxima around 36 e.Å -1 are attributed to Sr atoms, and those with the maxima around 24 e.Å -1 are attributed to Fe atoms (electrostatic potential values were calculated from the mathematical expression established by Cinthia Corrêa and Lukas Palatinus : U (e.Å -1 ) = 3.32367(Å)× |e|ρ JANA (r)(Å -2 ), with e = electron charge and ρ JANA (r) = electrostatic potential given by JANA2006). The three other sections in Fig. 4 According to the raw structural model in Fig. 3(b), some oxygen atoms were missing in this area. Sections at y = 0.05 (Fig. 5(a)) and y = 0.40 (Fig. 5(c)) allowed to determine these oxygen positions by identifying electron densities whose electrostatic potential maxima were above 10 e.Å -1 on the one hand, and on the second hand with reasonable distances (around 2 Å) between them and Fe atoms present at the section y = 0.02 (Fig. 5(b)). The resulting determined oxygen atoms were O18 (Fig. 5(a), electrostatic potential maxima = 13 e.Å -1 , d O18-Fe9 = 1.8 Å) and O20 (Fig. 5(c), electrostatic potential maxima = 11.6 e.Å -1 , d O20-Fe9 = 1.9 Å, d O20-Fe9 = 2.2 Å and d O20- Fe7 = 1.9 Å). A slight shift of O19 and O23 positions was applied in order to fit better with the maxima of the electron densities. The atomic arrangement between oxygen and iron atoms is presented in Fig. 5 (d) and Fig. 5(e) in the range of y = 0.05 to y = 0.4 and y = 0.05 to y = 0.9 respectively. The arrangement in this slab is very analogous to the double rock-salt type layer observed in intergrowth type structures called Bi-22(m-1)m, with Fe atoms instead of Bi in the double layer, reducing thus the distance between the two layers and conferring larger displacements of oxygen atoms, in such a way that an extra oxygen O18 is periodically inserted every 5 Fe atoms along a axis between the two Fe layers. These structural features were already highlighted in the phases called Fe-2201 and Fe Bi -2212, confirming the belonging of our phase to this family of layered iron oxides.

3-2. Structure refinement

The Fm2m space group and the atomic positions calculated by the direct methods on SIR2011 were used as starting sets in the refinement procedure. 24 points were picked up on the experimental diagram to create the background, while the diffraction peaks were fitted using a pseudo-voigt function. The half width parameter of the diffraction peaks was evaluated by the Caglioti equation [START_REF] Caglioti | Choice of collimators for a crystal spectrometer for neutron diffraction[END_REF] . The cell parameters and atomic positions of the secondary phase SrFe 12 O 19 have been considered and refined in order to index correctly the whole set of diffraction peaks on the diagram. After this refinement, all parameters of this phase were fixed and the cell parameters of the phase Fe-2234 as well as the profile function parameters were refined. Then the scale factor and the cationic positions were refined. Considering the high number of independent atomic positions in the structure, the fact that the structure to refined is one of two phases present in the powder and the fact that oxygen is poorly visible by X-rays, it is clear that refinement of oxygen positions is rather irrelevant. So to maintain stable the oxygen positions during the Rietveld refinement, restrain in cation-oxygen distances was applied and cation positions were fixed, with application of a low damping factor. The refinement led to satisfactory reliability factors as well as a good agreement between the experimental and calculated XRD diagrams. Details of the refinement conditions, cell parameters, phase contents and reliability factors are given in Table 1. Atomic positions are given in Tables 2 and in supporting information with the interatomic bond distances. The observed and calculated XRD diagrams with their difference curve are presented in Fig. 6 displaying a good agreement between observed and calculated diffraction profiles. The final structure, presented in Fig. 7(a) (with atoms) and Fig. 7(b) (with polyhedra), is described as a stacking of a quadruple perovskite-type layer with a double rock-salt type layer, where the sequence of polyhedra along a axis consists of three corner-sharing tetrahedra (in yellow) linked by the corner with one distorted tetragonal pyramids (in green). The chemical formula per unit cell is Sr 100 Fe 120 O 308 , which is reduced to Sr 5 Fe 6 O 15.4 .

Results obtained from Rietveld's profile analysis method

Step scan increment (°2θ) 0.02 2θ range (°) 10 -100 phase 1 : Sr

Conclusion-Discussion

The crystal structure of Sr 5 Fe 6 O 15.4 has been successfully solved by electron diffraction tomography combined with powder X-ray diffraction refinement by Rietveld method. The direct methods applied on the electron tomography dataset were perfectly suitable to determine the structure model in Fm2m space group with correct detection of all the cation and oxygen positions. This structure is described as a stacking along c axis of a quadruple perovskite-type layer composed of Fe distorted octahedra, with a Fe double row similar to a double rock-salt type layer where iron is found in tetrahedral and tetragonal environments. This structure is comparable to the parent structures Fe 2 Sr 2 FeO 6.5-δ/2 (Fe-2201) and Fe 2 (Bi 0.69 Sr 1.31 )SrFe 2 O 9.33 (Fe Bi -2212), both solved on single crystal X-ray diffraction some years ago and consisting of the alternation of single or double perovskite layer with a similar Fe-double row.. The substitution of Sr 2+ by Bi 3+ in Fe-2201 led to a double perovskite-type layer in the phase Fe Bi -2212 by adding more oxygen. Its structural analysis by single crystal X-ray diffraction demonstrated that Bi atoms were situated at the periphery of the double perovskite-type layer. In our phase Fe-2234, the quadruple perovskite-type layer is not a consequence of strontium substitution by a trivalent cation, but is due to a Fe 3+ /Fe 4+ mixed valence state of iron, which was confirmed by Mössbauer study in a previous work [START_REF] Lepoittevin | A Layered Iron-Rich 2234-Type with a Mixed Valence of Iron: The Ferrimagnetic Tl-doped Fe 2 (Sr 2ε Tl ε )Sr 3 Fe 4 O 14.65[END_REF] . If we consider Fe 4+ cations present in the first and fourth layer of the quadruple perovskite-type layer, in a same way as bismuth is present at the periphery of the double perovskite-type layer in Fe Bi -2212, these irons refer to Fe1 (position 16e), Fe2 (position 8c) and Fe3 (position 16e), which leads to 40 iron cations in 4+ valence state and then the 80 remaining iron cations are 3+. By a consequence the number of oxygen atoms in the structure is reduced to 300 per unit cell, involving 8 oxygen vacancies in the structure, and so the deletion of one oxygen position with multiplicity 8 is needed. Among all the oxygen positions with this multiplicity, the most relevant to remove is O22, in 8d Wyckoff site since the oxygen vacancies are situated in the middle of the quadruple perovskite-type layer (Fig. 8). This hypothesis of structure with oxygen vacancies is important to consider since their presence would bring more relaxation in the structure and therefore reduce the polyhedra distortions. Obviously this hypothesis is impossible to verify by a structure refinement from powder X-ray diffraction, only refinement from neutron diffraction data could confirmed the presence or not of these oxygen vacancies.

Fig. 1 .

 1 Fig. 1. (a) [100], (b) [010], (c) [001], [3 50] and [05 1] SAED patterns. All patterns are indexed in the F-lattice type orthorhombic unit cell.

Fig. 2 .

 2 Fig. 2. (a) [010] projection of the structure model obtained in Fmmm at d resolution = 0.8 Å, (b) [010] projection of the electron density map generated by SIR2011 in Fmmm after direct method calculations. For better viewing, only 1/8 of the unit cell is represented for 0 ≤ y ≤ 0.5. Sr (in green), Fe (in blue) and O (in red) atoms are superimposed on it.

Fig. 3 .

 3 Fig. 3. (a) [010] projection of the 3D electron density map generated by VESTA after SIR2011 calculations in Fm2m space group, (b) Raw structural model as an interpretation by SIR2011 of the electron density map.

Fig. 4 .

 4 Fig. 4. [010] observed Fourier map sections at (a) y = 0.05, (b) y = 0.277, (c) y = 0.356 and (d) y = 0.554. They all exhibit electron densities corresponding to Sr, Fe and O in the part of the unit cell included between z = 0.30 and z = 0.70.

Fig. 5 .

 5 [010] observed Fourier map sections at (a) y = 0.05, (b) y = 0.20, (c) y = 0.40, (d) 0.05 ≤ y ≤ 0.40 and (e) 0.05 ≤ y ≤ 0.90. They all exhibit electron densities for Fe and O in the part of the unit cell included between z = 0.21 and z = 0.29.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Final plot of the Rietveld refinement, showing the experimental (red circles), calculated (black line) and difference (blue line) curves in the range 2θ = 10° -100°. Green vertical markers refer to the calculated positions of the Bragg reflections for Sr 5 Fe 6 O 15.4 and SrFe 12 O 19 respectively.

Fig. 8 .

 8 Fig. 8. [010] schematical representation of the Fe-2234 structure with the hypothesis of oxygen vacancies in the middle of the quadruple perovskite-type layer. The chemical formula becomes Sr 5 Fe 6 O 15 .

Table 1 .

 1 Parameters and reliability factors obtained by the Rietveld refinement of the powder Xray diffraction diagram containing both phases Sr 5 Fe 6 O 15.4 (Fe-2234) and SrFe 12 O 19 .

	5 Fe 6 O 15.4 (space group F m2m/Z = 20)
	scale factor	0.0000188(1)
	U	-0.0105(18)
	V	0.0239(17)
	W	-0.00144(34)
	shape	0.17(1)
	a(Å)	27.4047(3)
	b(Å)	5.48590(7)
	c(Å)	42.7442(4)
	V(Å 3 )	6426.14(13)
	phase content(%)	56.25(22)
	Rbragg (%)	18.5
	phase 2 : SrFe 12 O 19 (space group P 6 3 /mmc/Z = 2)
	scale factor	0.0000360(3)
	U	0.0217(5)
	V	-0.003(5)
	W	0.005(1)
	shape	0.09(2)
	a(Å)	5.88618(4)
	c(Å)	23.0540(3)
	V(Å 3 )	691.742(12)
	phase content(%)	43.75(26)
	Rbragg (%)	15.8
	global χ 2 (%)	2.57

Table 2 .

 2 Atomic positions in Sr 5 Fe 6 O 15.4 (Fe-2234) after powder X-ray diffraction refinement.

ACKNOWLEDGMENTS

The author thanks Dr. Olivier Leynaud for his help in Rietveld refinements on powder XRD. The author thanks also Pr. Sylvie Malo and Pr. Maryvonne Hervieu (CRISMAT, Caen) for providing a piece of sample.


Michelle Wetterwald

Damien Saucez

Xuan-Nam Nguyen

Thierry Turletti

SDN for Public Safety Networks

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.

Introduction

Mobile communications and the Internet have revolutionized our ways of life. They have even merged together with the introduction of the smartphone, which delivers performances at an order of magnitude beyond comparison with the first personal computers sold on the market. Computing and digital technologies now progress at an meteoric rate towards higher data speed, larger bandwidth, seamless mobility or the Internet of Things (IoT). All these new features mandate improved characteristics of mobile networks, including enhanced flexibility, security or scalability. A recent and growing trend in network management and control aims at meeting these new requirements by the introduction of Software Defined Network (SDN) and Network Function Virtualization (NFV) technologies. Their objective is to bring to the networks the software flexibility and scalability that will allow them to better fulfill the upcoming needs of network operators and their users.

However, some of the mobile communications users that would most need these new technologies still stay behind, namely the Public Safety Networks (PSNs) users. When the LTE and 4G deployment base grows at a fast pace, PSN users still depend on 2G data communications, when not restricted to voice. Until recently, the digitalization of their Professional Mobile Radio (PMR) was led by a few major projects such as Terrestrial Trunked Radio (TETRA) or Project 25 [START_REF] Doumi | LTE for public safety networks[END_REF]. TETRA has been deployed in Europe, Middle East, Asia and Latin America since 1995. It provides mostly voice and dispatch services to mobile terminals operating in direct mode (DMO), trunk mode (TMO) through a dedicated infrastructure, or by using neighbour terminals as relays. Data services are also available, but at very low speed, from 80 kbps up to potentially 500 kbps [START_REF] Forge | Is commercial cellular suitable for mission critical broadband? A study prepared for the European Commission DG Communications Networks[END_REF], using TETRA Release 2 also known as TETRA Enhanced Data Service (TEDS). The wide geographical coverage enabled by the usage of low frequency bands permits one to deploy an infrastructure with a reduced number of base stations. On its side, the collaborative Project 25, or P25, standardizes an equivalent system for Land Mobile Radio (LMR), mainly deployed in Northern America, Australia and New Zealand. The latest version defines data services at a low level of data rates similar to TEDS.

PSNs carry a mission critical objective, with human lives depending on the successful exchange of information such as devices location, dispatch services or alert messages. Fire brigades or police departments are organized in highly mobile groups of individuals respecting a strong hierarchical configuration, which necessitates different levels of priorities and end-to-end security. They may work in very harsh environments such as earthquake destruction, fires or flooding and are expected to provide relief under any circumstances and in any location. Beyond the voice and specific services that they receive from their rugged mobile devices, PSN users tend to start taking advantage of more evolved and powerful applications, such as real-time video, temperature or heart-rate sensing from their private commercial smartphones.

Under the consideration that even teenagers own a device much more powerful than first responders, several countries and specialized organizations have launched the definition or funding of the evolution of the PSN. Their objective is to integrate the latest advances of mobile communications and offer technologies at a level equivalent to public commercial networks to their users. In the UK, the Emergency Service Network (ESN) will start its deployment in 2016, targeting police, firefighters and ambulances [START_REF] Nokia | LTE networks for public safety services[END_REF]. In the US, the FirstNet authority [START_REF]First Responder Network Authority[END_REF] plans the establishment of one single inter-operable, highly reliable and resilient wireless broadband network to serve police, firefighters, paramedics, and command centers. The TETRA and Critical Communications Association (TCCA) has recently selected LTE (Long Term Evolution) as the technology to complement TETRA for broadband communications. Enhancements to LTE and LTE-Advanced (LTE-A) have been designed and standardized under Release 12 of the 3GPP, as described in [START_REF] Doumi | LTE for public safety networks[END_REF] to cope with the additional features necessary to support the PSN operations, such as those described in Section 2.1. At the same time, various new technologies aiming to enhance the performance and efficiency of digital communication networks have sprung up in the past years. Software Defined Networking (SDN) is presented in Section 2.2. It introduces a clear decomposition of control and data plane, which permits one to perform the forwarding at flow level according to fine-grained rules. This section also introduces Network Function Virtualisation (NFV) and Service Function Chaining that complement SDN by adding further flexibility and increasing scalability. Hence it is a natural tendency to propose the benefits of these new technologies to the design of future PSNs, as shown in Section 3. In Section 3.1, we analyze what are the features required for the PSN scenarios. Section 3.2 presents the C-RAN technology, which introduces flexibility on the fronthaul network. A set of solutions based on virtualization is sketched in Section 3.3. They address the resiliency of the backhaul network and enable the operation of isolated cells. In Section 3.4, SDN-based frameworks are presented, where the dynamic management of flows enables improved flexibility for the PSN services. Finally, Section 4 concludes this study.

Background

Public Safety Networks

An emergency scenario: the earthquake

Public Safety (PS) agencies must accommodate a very large range of events, which can be classified into two categories. A first category of events requires mainly prevention. The PS users must ensure the safety of the community during big events like sports or festivals. A main characteristic of this category of events is that it is usually planned well in advance and the technical staffs have sufficient time to anticipate and extend, or re-organize the communication network as required by the event. The second category of events requires healing to respond and restore a viable situation after a natural disaster or a criminal event has occurred. Earthquakes, major storms, bombing and mass transport accidents fall into this category. Contrary to the first category, the latter events are unplanned. They may even witness the destruction of the existing infrastructure, drastically reducing the means of communications for first responders and for the public citizens in general, at a time when they have to deal with emergency requirements.

A typical event of the second category is the earthquake, which scenario has been described in [START_REF] Etsi | Reference scenario for the deployment of emergency communications[END_REF] and is pictured in Figure 1. In order to remain as Figure 1: Illustration of the earthquake scenario. comprehensive as possible, this scenario pictures the event in two sub-areas, a residential neighbourhood and an industrial estate. The residential neighbourhood is composed of apartment buildings as well as individual houses, schools and stores including a petrol station. The earthquake brings down the school and a few other buildings, starts a fire in one of the shops in the commercial center, threatening to spread to other stores and, even worse, to the petrol station. A lot of people get trapped under the buildings, in varying conditions, while others who could escape run away from the danger into the streets. Globally, a large part of the buildings show a high risk of collapsing. The telecommunications network is partly destroyed, creating large coverage holes in the area. The industrial estate contains mainly factory buildings hosting a large number of employees, a bus garage and a gas tank. The area can be accessed only through two roads, one large and a second narrow one spanning a river on a small old bridge. Here as well, the shock brings down the factory buildings, trapping the employees inside. Moreover, it sets a fire in the bus garage, triggers a landslide that closes the main access road and breaks the utility infrastructure for water, electricity and telecommunications. Some cracks can be observed on the small old bridge, which restricts the access through the narrow road only to light vehicles.

Very rapidly, the emergency call centers receive a large number of messages requesting help and inform the first-aid teams and local authorities. The event brings into the affected area an increasing number of actors. In the first minutes after the quake strikes, firefighters, Search and Rescue (SAR) teams, medical services and police units arrive on scene to secure the hazard area, i.e., the area that was most struck and where individuals may be endangered, as well as a larger and safer zone around it, where most of the first responder teams can be installed, the incident area. They prevent unauthorized access and start their relief operation. An emergency Casualty Collection Point (CCP) is established to receive and triage the victims according to their condition. Rapidly, the whole site gets organized under the governance of a hierarchical management structure of Emergency Control Centers (ECC) receiving instructions and reporting to an off-site Local Emergency Management Authority (LEMA). Other off-site services, e.g., ambulance stations, fire stations, hospitals, shelters such as sports hall belonging to local authorities call up their staff for on-site or remote assistance. The different teams are dispatched in the incident and the hazard areas by their relevant ECC. Relief locations, with temporary shelters and medical equipment are settled in the incident area to receive the victims, care for them according to their conditions and bring them suitably out of the incident area, where they will be further treated or released. Until the last patient has been helped, the SAR teams keep moving between the hazard areas and the relief locations. Additional firefighter teams try to reach the scene, but their big trucks are blocked at the obstructed accesses, delaying the resolution of the fires. Technical experts start assessing the level of the damages to the infrastructure. Road department experts evaluate the stability of the old bridge, exchanging pictures with remote engineers. Construction companies begin evacuation of the mud on the main access road to restore the vehicle traffic flow, assisted by defense forces, under the remote and local direction of geologists. Power, water and food supply are gradually restored by local utility companies and authorities. Repair teams start installing an emergency communication network to compensate for the destroyed infrastructure, after which they tackle the restoration of the original telecommunications networks.

All along, lightly affected and off-site citizens try to contact their relatives to learn whether they have been affected or not, have been moved to hospitals, to shelters or to other places. The media try to exchange information with their headquarters, sending reports for the news magazines and TV shows. Both types of users can only rely on the possible usage of the remnants of the telecommunications infrastructure that are still available, and as soon as they are operational, on the authorized parts of the restored networks.

Requirements

The scenario presented in the previous paragraph, which shows a worst case event, allows deriving the main requirements for future PSNs.

In a first step, it shows the large diversity of actors and other citizens that have to be accommodated by the networks, as well as their needs. Today, the existing PS communications rely mainly on voice conversations, but real-time pictures, video, access to databases, geolocation, purpose-made PS applications could highly improve the impact and efficiency of the rescue missions. Secondly, the scenario also demonstrates that the different groups require different types of communications. The rescue personnel is linked through group communica-tions to their teammates and dispatching commandments, because they keep sharing real-time information. In existing networks, they use a push-to-talk broadcast mechanism, providing mainly group voice communications. Only one device transmits at one time and all the others listen, based on priority, e.g., the dispatcher's device has a higher priority. The devices are hardened against external conditions, such as high temperature, and are very easy to use, even with heavy protective gloves [START_REF] Forge | Is commercial cellular suitable for mission critical broadband? A study prepared for the European Commission DG Communications Networks[END_REF], which are not part of the common features of commercial smartphones. The PS communications must be restricted to a specific geographical area, covering the hazard area where the teams operate. From the ECC point of view, reporting upwards in the hierarchy or requesting information from the local control center rather requires one-to-one communications. These communications must be granted a high priority level in the network, either by owning a guaranteed and private spectrum, or by enforcing QoS mechanisms that increase their priority compared to commercial users in the case where the network is shared. Even more, emergency PS users ought to have higher priority than regular PS users. [START_REF] Nokia | LTE networks for public safety services[END_REF] gives the different levels of deployment options for future PSNs, especially when taking into account the sharing of the mobile broadband (MBB) network. It ranges from a separate private PSN, to RAN sharing, to considering the PSN as a prioritized virtual operator network or as an overlay over the commercial MBB network. It also considers the possibility for the PS devices to roam to any available MBB network, while operating at a higher priority level when this is made relevant due to external conditions. Roaming also allows to benefit from heterogeneous services such as Wi-Fi or satellite access when they are available, either because they were not destroyed or because they could be established as a temporary emergency solution. The cost for such solutions varies in a wide range. Using a dedicated allocated frequency spectrum with specific devices does not allow taking advantage of scalability savings, as could be the case with devices operating in commercial frequency bands. The network deployment must be flexible enough to easily accommodate any or a mix of the previous options.

These deployment options introduce three major requirements on future PSNs: (1) full geographical coverage, potentially through heterogeneous accesses; (2) robustness and reliability of the network to avoid losing the connection at a critical time; and (3) resilience provided by pooling, back-up connections, traffic load balancing or automated management tools. The full coverage and the resilience may also be achieved using technologies that allow deploying rapidly extensions to the existing and still usable infrastructure, e.g. by pre-installed cells in emergency vehicles. When the full coverage is not feasible because of the failing infrastructure, the mobile devices must be able to interact directly with one another or to serve as relays or gateways to neighbour devices. This is known as Device-to-Device (D2D) communications. With the objective to guarantee communications between the incident area and the off-site locations, the PSN must take into account the large geographical distribution of devices and external server locations. Accordingly, the network planning must be flexible and scalable to accommodate a rapid increase of the number of PS actors brought on-site in the incident area and able to cope with the high mobility of teams such as SAR, which permanently move back and forth between the hazard area and the ECC, or of the ambulances that transport casualties to the off-site hospitals.

The security of the network, from a physical or cyber point of view, is of extreme importance. Network peripherals and data centers must not be brought down easily after their installation. Capability to access the network must be restricted both at device and at user level. All the traffic must be protected, whether it relates to the management, control or user plane of communications. The confidentiality of each transaction and conversation must be preserved, either inside each team or between a local ECC and the off-site LEMA, with the possibility to separate the different PS teams and agencies inside the PSN.

Network Softwarization

Networks today became so complex that it was necessary to rethink the way they were conceived to provide higher level of abstraction to increase their efficiency (e.g., cost, energy) and simplify their management. To that aim, Software-Defined Networking (SDN) [START_REF] Nunes | A Survey of Software-Defined Networking: Past, Present, and Future of Programmable Networks[END_REF][START_REF] Xia | A survey on softwaredefined networking[END_REF][START_REF] Kreutz | Software-defined networking: A comprehensive survey[END_REF] has been proposed. SDN advocates a clear separation between the data-plane, composed of the various forwarding devices, and the control-plane implementing the network control logic. This principle of separation is not so novel but was hardly reachable so-far because of technical limitations. However, nowadays with the generalization of virtualization and the potentially high performance that can be reached by commodity hardware, it is possible to implement efficiently the separation between the control and the data planes.

The general idea of SDN is illustrated in Fig. 2. The separation between the control and the data planes is realized by a Southbound API while the network logic is implemented by a logically centralized Controller Platform to which network applications interact by the means of a Northbound API. The controller platform manages the state of forwarding elements (e.g., switches and routers) directly with the southbound interface. This approach reduces the complexity of forwarding elements as their only role is to forward data packets in the network.

OpenFlow [START_REF] Mckeown | Openflow: enabling innovation in campus networks[END_REF] is by far the most mature implementation of the southbound interface [START_REF] Nunes | A Survey of Software-Defined Networking: Past, Present, and Future of Programmable Networks[END_REF][START_REF] Xia | A survey on softwaredefined networking[END_REF][START_REF] Kreutz | Software-defined networking: A comprehensive survey[END_REF][START_REF] Jain | B4: Experience with a globallydeployed software defined WAN[END_REF][START_REF] Kobayashi | Maturing of Open-Flow and Software-defined Networking through deployments[END_REF]. Interestingly, OpenFlow started in the academia but rapidly received much attention from the industry such that nowadays, all major vendors support OpenFlow in their commercial products [START_REF] Nunes | A Survey of Software-Defined Networking: Past, Present, and Future of Programmable Networks[END_REF]. The project became so important that the Open Network Foundation (ONF) [START_REF] Foundation | Optical transport working group otwg[END_REF] driven by the most influential actors of the Internet (e.g., Google, Facebook, Microsoft, Cisco) has been created to design, improve, and maintain the OpenFlow protocol and its satellite components.

While usually forwarding decisions are taken on a per destination basis, in OpenFlow all forwarding decisions are performed on a flow basis. This approach permits a fine-grain granularity in the way traffic is managed within the network. An OpenFlow switch consists of flow tables, each containing a prioritized The key to simplify network management is raising the level of network abstraction. Higher abstraction hides the complexity of underlying networks, and helps operators to manage the network in an e cient, less error prone way. To that goal, Software-Defined Networking (SDN) [NMN + 14, XWF + 14, KRV + 14], an emerging network architecture that advocates the separation between forwarding devices (the data plane) and the network control logic (the control plane), is proposed. SDN is not an entire new thing, but it appears at the intersection of ideas, technology maturity and future needs. Fig. 1.1 shows the SDN architecture. The separation of the control plane and the data plane is realized by a well-defined southbound application programming interface (API). In SDN, network control logic is implemented in a logically centralized entity, called the controller. The controller is logical centralized but can be physically distributed for scalability reason. Also, a switch may have multiple controllers. Via the southbound API, the controller directly manages the state of forwarding devices to respond to a wide range of network events, for example, a link is congested. Furthermore, the controller also exposes high level northbound APIs for operators to write network applications to automate management tasks, such as firewall, load balancing. With this architecture, SDN promises to dramatically reduce costs and to simplify network management thanks to commodity, vendor-independent forwarding hardwares and high level interfaces for management. list of rules that determine how packets have to be processed by the switch. Conceptually, a rule is composed of three components: a matching pattern, an actions field, and counters. The matching pattern forms a predicate whose value is computed on-the-fly by the switch based on packet meta-information (e.g., source IP address, destination port, VLAN tag). All packets making true the matching pattern predicate are said to belong to the same flow. The actions specified in the actions field of the rule are applied to every packet of the corresponding flow. The most common actions are: forwarding, dropping, or rewriting the packets. Finally, the counters are used to keep statistics on the flows. As a packet may match multiple matching patterns, each rule is associated with a priority and only the rule with the highest priority that matches the packet is considered to take actions on it. The prioritization of rules permits constructing multiple levels of granularity. Fig. 3 summarizes the architecture of OpenFlow and the interactions between the various network components.

In operation, when an OpenFlow switch receives a packet, it checks its flow tables. If the packet matches no rule, the switch may send a request to the controller to determine what actions to perform on the packet. The switch then stores the information so it can process packets accordingly for any further packet belonging to the same flow as the initial packet.

SDN allows to separate data-plane and control-plane components by the means of abstractions such as OpenFlow. However, with the generalization of software solutions in networking, it is possible to complement this abstraction with the virtualization of the network functions themselves. This approach known as Network Function Virtualization or NFV relies on the fact that network functions can be implemented in software and thus be virtualized and operated on commercial off-the-shelf (COTS) machines. A direct benefit of this approach is in the costs reduction. However, as functions are virtualized and run on virtual machines, it also results in more flexibility and scalability as virtual Basically, forwarding devices are called OpenFlow switches and all forwarding decisions are flow-based instead of destination-based like in traditional routers. An OpenFlow switch consists of flow tables, each containing a prioritized list of rules that determine how packets have to be processed by the switch. 1 A rule consists of three main components: a matching pattern, an actions field and a counter. The matching pattern forms a predicate whose value is computed on-the-fly by the switch based on packet meta-information (e.g., source IP address). All packets making true the matching pattern predicate are said to belong to the same flow. The actions specified in the actions field of the rule are applied to every packet of the corresponding flow. The most common actions are: forwarding, dropping, or rewriting the packets. Finally, the counter is used to count the number of packets that has been processed (i.e., that made the predicate hold true) along with the lifetime of this rule. As a packet may match multiple matching patterns, each rule is associated with a priority and only the rule with the highest priority that matches the packet is considered to take actions on it. The prioritization of rules permits constructing default actions that can be applied on packets only if no other rule can be used. Examples of default actions are dropping packets, forwarding packets to a default interface, or even to the controller. For e ciency and flexibility reasons, the latest versions of OpenFlow [of1] support pipeline processing where a packet might be processed by several rules from di erent flow tables.

The behavior of OpenFlow switches strongly depends on the set of rules it holds. Many network applications can implemented from OpenFlow, for example, monitoring, accounting, tra c shaping, routing, access control and load balancing [MAB + 08a]. To implement these applications and operators policies, it is important to select a set of corresponding rules to be installed each OpenFlow switches, called rules placement solutions. An ine cient rules placement may lead to frequently flow table miss, high signaling overhead, high setup delay, that degrades machines can be migrated at anytime or hardware can be upgraded without interrupting operations. When network functions are virtualized and SDN is used, then Service Function Chaining (SFC) is straightforward. SFC consists in linking network functions to produce services. As every component is software and virtualized, service chains can be modified at anytime to provide new services or to adapt to the load.

How can SDN and NFV improve PSN communications

Required features for PSN scenarios

By their very nature, Public Safety Networks are challenging as they are supposed to work properly even in chaotic situations such that priority management, fault tolerance, and rapidity of deployment are the key features to implement in order to build PSN solutions. The generalization of the software approach in networking with SDN, NFV, and SFC permits to design the new generation of PSNs in an elegant way and sections below show examples of solutions leveraging it. OpenFlow is a key technology to enable efficient priority management. Indeed, in OpenFlow network decisions are decided on a flow basis, for arbitrary flow definition, and implemented in the form of rules with priority levels. It thus means that flows can be treated differently based on policies implemented by the controller and as the controller has a full view of the network, it ensures the strict respect of the policies. The centralization of decisions may appear as a weakness of SDN. However, the centralization of decisions avoids transient situations where all network components try to converge to a new stable state thanks to distributed algorithms, it thus avoids inconsistencies and potential transient loops. The centralization also simplifies fault diagnosis and recovery. In addition, with the virtualization of network functions, it is straightforward to migrate network functions on different pieces of hardware in case of fault of the underlay. Nevertheless to benefit from all that, it is necessary to build logically centralized control planes that keep the good centralization properties without impairing the robustness of the entire system, as shown in the sections that follow. Finally, the usage of virtual functions and SDN allows rapid deployment of PSNs as functions can be configured and updated on the fly without any particular intervention on the device itself.

Cloud Radio Access Networks (C-RAN)

In current mobile communication networks, Baseband units (BBUs) are usually co-located with the cell towers on-site. They run on proprietary hardware and are designed for worst-case peak loads. However, through NFV, BBUs can be virtualized and run on general purpose computers. Such centralized virtual BBUs clusters can be provisioned on demand and represent a good replacement for on-site installations of BBUs in distributed geographical locations. This new mobile network architecture, called Cloud 1 Radio Access Network (C-RAN), can bring many advantages in a PSN context. The first, an antenna-integrated Remote Radio Head (RRH) is easy to deploy close to an affected area. Second, a higher system capacity and lower power consumption can achieved when installing RRHs close to the rescue teams since the wireless signal will propagate on a short distance to reach them. Third, smart cooperative multiple-radio resource management can be used (and higher bandwidth) as the baseband processing is centralized in a data center (BBU pool), as shown in Figure 4. However, C-RAN involves a huge overhead on the fronthaul link because very high bandwidth is required on the common packet radio interface (CPRI) between the RRH and the BBU pool to transmit In-phase and Quadrature (IQ) data. For instance, in order to carry a 2x2 MIMO, 20 MHz LTE signal, 2.5 Gbps of bandwidth is needed on the CPRI. At the same time, strict constraints on latency and jitter have to be fulfilled, in particular, the RTT delay of user data should be less than 5µs [START_REF] Wang | C-ran: the road towards green ran[END_REF], and jitter should be less than 65ns [START_REF] Gpp | Base Station BS Radio Transmission and Reception[END_REF]. Several transport options are possible for CPRI transport such as Dedicated fiber, Optical transmission network (OTN), Passive optical network (PON) and Microwave Radio (MWR). For PSN scenarios, as fiber is likely not to be available, microwave transport is usually the best option for CPRI, provided that the distance between RRH and BBU is less than 1 kilometer. Note that MWR can only support a subset of the CPRI line bit rate options. But with 3:1 compression of CPRI, it is possible to fronthaul a 2x2 MIMO LTE 3-sector RRH site under full load over two times 1 Gbps MWR links [START_REF] Lorca | Lossless compression technique for the fronthaul of lte/lte-advanced cloud-ran architectures[END_REF].

Resilient Backhaul solutions

In case of major emergency situations, Evolved Universal Terrestrial Radio Access Network (E-UTRAN) may become isolated from the core network and the backhaul access (i.e., the X2 air interface in LTE) has to be rebuilt on the fly. The 3GPP 22.897 specification [START_REF] Gpp | Study on isolated Evolved Universal Terrestrial Radio Access Network (E-UTRAN) operation for public safety[END_REF] describes different public safety scenarios and the corresponding requirements to maintain basic services to users. Note that the problem of isolated eNBs is also frequent in moving cells scenarios, where the connectivity remains limited to only a subset of eNBs that can still communicate to each other through the remaining part of the backhaul. Recently a few solutions have been proposed to re-establish the communication among different network partitions.

In [START_REF] Apostolaras | Evolved user equipment for collaborative wireless backhauling in next generation cellular networks[END_REF], the concept of evolved User Equipments (eUEs) is proposed where eUEs are used as relays between eNBs and are able to be associated with and convey trafic from multiple eNBs, making possible backhaul access to moving cells or core-isolated cells as illustrated in Figure 5. In other words, eUEs play the role of a virtual MIMO antenna with collaborative transmission, which could be implemented as a new coordinated multipoint (CoMP) mechanism. In this way, a virtual overlay wireless mesh can be formed on top of the LTE cellular topology.

Another solution is proposed in [START_REF] Favraud | Wireless mesh backhauling for LTE/LTE-A networks[END_REF] for the scenario where eNBs are in range of each other. This proposal introduces the concept of enhanced evolved Node B (e2NB) that leverages the existing LTE air interface to allow communication between eNBs through a Tx/Rx transmitter, and this, without requiring modifications of the LTE components and protocols.

As shown in Figure 6, the architecture of e2NB adds to the eNB different LTE components that makes it still operational in the case it is isolated from the core network. Examples of components include the Mobile Management Entity (MME) and the Home Subscriber Server (HSS) components, but also two new functions: (1) Virtual UEs (vUEs) that are deployed as a service by the e2NB to Figure 5: eUE. Source: [START_REF] Apostolaras | Evolved user equipment for collaborative wireless backhauling in next generation cellular networks[END_REF] establish a connection with some of neighboring e2NBs; and (2) a Coordination and Orchestration Entity (COE) that handles the inter-e2NB communication and coordinates with other COEs to establish the network topology and optimize the network. Furthermore, the COE manages the IP addressing space and performs mesh routing algorithms.

Note that the COE functionality could be implemented in a software defined networking (SDN)-based network control plane as well as the CoMP functionality [START_REF] Cvijetic | Sdn-controlled topology-reconfigurable optical mobile fronthaul architecture for bidirectional comp and low latency inter-cell d2d in the 5g mobile era[END_REF].

[13] proposes a similar software architecture design to enhance the resilience of LTE networks and enable the operation of isolated base stations (eNB) to serve in PSNs. The authors introduce the notion of Hybrid-eNB (HYeNB) that includes the usual entities of an eNB, collocated with a flexible management entity (FME) handling the main Evolved Packet Core (EPC) functions required to serve the attaching user equipments (UEs). The HYeNB uses its backhaul link only to synchronize the UE context with the physical EPC. The virtual EPC inside the FME is pictured in Figure 7. It is made of a gateway-agent (GW-A) that manages the user plane functionalities, a Mobility Management Entity-Agent (MME-A) that manages the control plane functionalities, including virtual X2 interfaces and handover procedures, and a D2D-Agent ( D2D-A) that manages the mechanisms to enable D2D communications, as explained below. A link management unit (LMU) manages the link with the physical EPC through a single or multi-hop connection. A routing management unit (RMU) routes the packets in the network and maintains the active path with the physical EPC. A topology management unit (TMU) optimizes the topology of a network when it contains several HYeNBs. Moreover, when UEs are not in the coverage of an HYeNB, the authors propose a device-to-device (D2D) Figure 6: e2NB. Source: [START_REF] Favraud | Wireless mesh backhauling for LTE/LTE-A networks[END_REF] communication mechanism where the head of a group of PS users broadcasts a D-Beacon to create and advertise a D2D network with its neighbouring devices. As soon as one of the UEs recovers the connectivity to an HYeNB, all resources allocated to the D2D communications are disabled.

SDN-based frameworks for PSNs

Network Embedded On-line Disaster (NEOD)

Network Embedded On-line Disaster (NEOD) is a globally deployable SDNbased network disaster management system that focuses on agility, accuracy, reliability, and scalability issues. It provides abstraction layers to deploy vendoragnostic event detectors and to facilitate dynamic configuration of network policies. As illustrated in Figure 8, NEOD uses a two-tier architecture: (1) a disaster event detection and filtering segment embedded in an OpenFlow Switch as an OpenFlow firmware extension; and (2) a disaster correlation and detector management segment residing in one or more controllers. The first segment includes Event Detectors, a Dynamic Event Detector Manager, an Event Publisher, and the System Event Adaptation Layer. Event Detectors use configured policies to monitor the disaster events, which are sent by the Event Publisher. In the second segment, the NEOD manager performs correlation of disaster events to provide root cause classification and prediction of events that have been considered as Figure 7: FME software elements and interfaces. Source: [START_REF] Gomez | Enabling disasterresilient 4g mobile communication networks[END_REF] Figure 8: NEOD Architecture. Source: [START_REF] Song | Neod: network embedded on-line disaster management framework for software defined networking[END_REF] not scalable or impossible to conduct. Three types of NEOD Disaster event detectors are provided: (1) new flow attack consisting in injecting randomly a large number of new flows to an OpenFlow switch; (2) interface flapping where a switch interface has a hardware failure that generates alternately "up" and "down" events; and (3) Event Storm that can occur in case of multiple status changes on a network object that is containment relationship with a large number of other objects.

Decentralize-SDN (D-SDN)

Decentralize-SDN (D-SDN) [START_REF] Santos | Decentralizing sdn's control plane[END_REF] is a framework that aims to distribute the SDN control in presence of multiple administrative authorities. It provides both physical and logical control distribution through a hierarchy of controllers composed of main controllers and secondary controllers, as illustrated in Figure 9.

To operate, a secondary controller needs first to be activated by a main con- The scenario exhibits different public safety authorities that organize themselves for exchanging valuable information regarding an emergency situation. Secondary controllers are running on agency vehicles and serve as gateways to a network of the different agency actors including firefighters and police officers. An agency typically has many decentralized secondary controllers that exchange messages with other agencies' secondary controllers. The D-SDN framework is Figure 11: Joint SFC control architecture. Adapted from: [START_REF] Sonkoly | Multi-domain service orchestration over networks and clouds: a unified approach[END_REF] used to provide fault tolerance communication mechanisms. In particular, in such extreme environments, connectivity disruptions are frequent and should trigger a new controller to take over and manage remaining reachable SDNenabled devices. An algorithm is proposed to elect in a secure way another secondary controller in case the previous one is no more reachable. Such a control delegation is challenging to perform as the system should maintain flow consistency and share flow state between controllers in presence of extreme conditions. Finally, D-SDN provides security mechanisms services which are particularly important in this context, including secure communication with authentication between controllers and secure association between switches and controllers.

UNIFY Service Function Chaining (SFC)

As opposed to ISP or data-center networks that are deployed, operated, and maintained by network engineers, in some situations PSNs will be deployed and operated by emergency services personal that are not engineers. It is therefore essential to simplify the usability of the system and the UNIFY framework offers a first step towards this simplification. The UNIFY common orchestration framework [START_REF] Sonkoly | Multi-domain service orchestration over networks and clouds: a unified approach[END_REF] proposes a joint cloud and resource virtualization Application Programming Interface (API) to optimize resource management in multi-technology domains, including legacy technologies. The proof-of-concept (PoC) pictured in figure 11

Conclusion

This report presented the evolution of PSNs from voice communications as defined in TETRA or P25 networks towards the more recent networking technologies. Starting from the detailed example of an earthquake emergency scenario, the main requirements of future PSNs could be derived. Fast network deployment, flexibility in the control of the communication flows for robustness, reliability, secure resources management, or resilience are the main features that naturally bring forward the recent softwarization and virtualization of the networks, as defined in SDN, NFV or SFC novel technologies. A set of applications or PSN frameworks that take advantage of these new technologies, either at node or at system level has been presented, involving the RAN, the backhaul, and the service layer. They picture how software networks help complying with the critical requirements of PSNs, benefiting from the clear separation between data and control plane, the faster convergence of newly installed networks, the centralization of policy decisions, and the flexibility of reconfiguring network functions in case of overload or hardware damage. PSNs are currently moving towards existing technologies to provide their users with capabilities equivalent to commercial data networks. Softwarization and virtualization will be their next step.

  Figure 1.1: SDN architecture

  OpenFlow [MAB + 08a], depicted in Fig.1.2, is the most popular implementation of the southbound interface of SDN [NMN + 14, XWF + 14, KRV + 14, JKM + 13, KSP+ 14]. Although OpenFlow started as academic experiments, it is receiving much attention from both academic and industry. Many vendors have supported Open-Flow in their commercial products [NMN+ 14]. Open Network Foundation [OTW13], an industrial-driven organization including the biggest operators such as Google, Facebook, Yahoo, Microsoft, is promoting OpenFlow as the standard southbound API. The latest version of OpenFlow is 1.5 [of1].

Figure 2 :

 2 Figure 2: Generic architecture of a SDN network.

Figure 3 :

 3 Figure 3: OpenFlow architecture.

Figure 4 :

 4 Figure 4: CRAN Architecture. Source:[4] 1 C-RAN also refers to Centralized-, Cooperative-or Clean-RAN.

Figure 9 :

 9 Figure 9: SDN control delegation

Figure 10 :

 10 Figure10: Broadband scenario for inter-agency communication. Source:[START_REF]CHORIST[END_REF]

  supports a recursive orchestration mechanism and automated dynamic service creation. A network function (NF) in a client virtualization can also be replaced with the inter-connection of several NF subcomponents, chained to perform the initial function objective. The core of the framework is the ESCAPE global Orchestrator, designed to meet the requirements of the joint SFC control plane. It supports recursive orchestration, the UNIFY resource abstraction model and can address different technological domains via Controller Adapters. A GUI and Service Layer sits on top of the northbound interface to the Orchestrator. It revolves around a Service Orchestrator, which uses a Service Graph mapper to map the service requests (bandwidth, delay) defined by the consumers of the UNIFY resource service into a Virtualizer. The global Orchestrator manages the virtualized resources and network functions. It passes the service requests to an Abstract Resource Virtualizer over different NF execution environments through specific domain adapters via its southbound interface. The lower layer of the PoC demonstrating the SFC is mainly implemented in the Mininet domain, even though Open-Flow or a standard Universal Node can also be accommodated. In Mininet, the YANG component contains the data model of the Virtualizer. A dedicated local orchestrator identical to the global Orchestrator manages the NFs and deploys the service chains over the domain resources. It takes advantage of recursive algorithms over the Virtualizer-Manager interface to benefit from the NF decomposition.

We follow the OpenFlow model terminology where a packet consists of any piece of information traveling through the network and a switch stands for any device processing such a packet.