Duncan Fisher

Minesh Khashu

Esther A Adama

Nancy Feeley

Craig F Garfield

Jillian Ireland

Flora Koliouli

Birgitta Lindberg

Betty Nørgaard

Livio Provenzi

Frances Thomson- Salo

Edwin Van Teijlingen

Edith Cowan

Fathers in neonatal units: Improving infant health by supporting the baby-father bond and mother-father coparenting

Keywords: neonatal care, fathers, coparenting, bonding, skin-to-skin 1. Introduction

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

A randomised controlled trial (RCT) compared Family Integrated Care in a neonatal unit (where parents are supported to become the primary carers of the baby) with standard neonatal care. The study involved about 1,000 fathers and 800 mothers in Australia, Canada and New Zealand. The study found greater weight gain in babies and more exclusive breastfeeding at 21 days in Family Integrated Care [START_REF] O'brien | Effectiveness of Family Integrated Care in neonatal intensive care units on infant and parent outcomes: a multicentre, multinational, cluster-randomised controlled trial[END_REF]. The father-baby bond, like the mother-baby bond, benefits the baby. Researchers at Hallym University Sacred Heart Hospital in South Korea ran an experiment to show the benefit to infant health of fathers giving "tactile stimulation" to their premature babies. They trained 20 fathers to stroke their babies for 10 minutes a day over five days. They found that babies who had received such additional tactile care had higher oxygen saturation levels [START_REF] Kim | Effects of tactile stimulation by fathers on physiological responses and paternal attachment in infants in the NICU: A pilot study[END_REF]. A review in 2016 of 12 studies recorded the following benefits of father skin-to-skin care for infant health [START_REF] Shorey | Skin-to-skin contact by fathers and the impact on infant and paternal outcomes: an integrative review[END_REF]:

 Increase in baby's temperature, similarly to mother skin-to-skin care (Sweden, Germany)  Better blood glucose level (Sweden)  Lower salivary cortisol level, indicating less stress  More settled baby (Sweden, Canada) The early presence of fathers can help to reduce the time of parent-baby separation after the birth [START_REF] Koliouli | Fathers' life experiences in the NICU: a qualitative study[END_REF]. In research from outside of neonatal unit care, increased father-child attachment has been shown to improve mother-baby interaction as measured by increased breastfeeding rates. Recent research from Taiwan shows that supporting father-baby skin-to-skin care improved rates of exclusive breastfeeding in the first three months, even when not accompanied by any additional education about breastfeeding itself [START_REF] Wang | The effective of early skin-to-skin contact with fathers on their supporting breastfeeding[END_REF]. This complements research from Vietnam, showing an association between increased breastfeeding rates and support for the father-baby bond [START_REF] Bich | Fathers' involvement and its effect on early breastfeeding practices in Viet Nam[END_REF]. Benefit: service savings In the United States (US) babies of mothers and fathers who attended a four-session Creating Opportunities for Parent Empowerment programme stayed in the hospital on average four days less [START_REF] Melnyk | Reducing premature infants' length of stay and improving parents' mental health outcomes with Creating Opportunities for Parent Empowerment (COPE) Neonatal Intensive Care Unit Program: a randomized, controlled trial[END_REF]. This was, however, not reproduced in the later Family Integrated Care RCT and needs further exploration [START_REF] O'brien | Effectiveness of Family Integrated Care in neonatal intensive care units on infant and parent outcomes: a multicentre, multinational, cluster-randomised controlled trial[END_REF].

Engagement with fathers in neonatal care remains suboptimal

There is a well-established global interest in an equal "partnership of care" or "therapeutic alliance" between health professionals and parents in neonatal care. The aim is to enable parents to be active carers of their baby, supporting them to bond with the baby and engaging them in decision-making. UNICEF's Baby Friendly Initiative (www.unicef.org.uk/babyfriendly/baby-friendlyresources/guidance-for-health-professionals/implementing-the-baby-friendlystandards/guide-to-the-baby-friendly-initiative-standards/) defines a set of standards for this kind of partnership. Family Integrated Care (familyintegratedcare.com) is an advanced form of partnership with parents, which evolved originally in Estonia and has been pioneered in Canada. The process is designed in collaboration with parents and staff are trained to be coaches, mentors and counsellors for parents. In UK the Bliss Baby Charter defines a set of standards for family-centred care (www.bliss.org.uk/the-bliss-baby-charterguide). Engagement with fathers, however, needs to be improved. A study of family-centred care in 11 European neonatal intensive care units found variable performance in the support offered to fathers. In six units they were less present than mothers and perceived less support than mothers. In the others they reported receiving the same level or even more support than mothers did. The survey found that fathers' contribution to the total parental presence varied between the units from 12% to 46% and accounted for 14-43% of the time the babies were held and 10-37% of the provision of skin-to-skin contact. The study was carried out by the Separation and Closeness Experiences in Neonatal Environment (SCENE) research group in Finland. It involved 262 families in six European countries -Finland, Sweden, Norway, Estonia, Spain and Italy [START_REF] Scene | Parent and nurse perceptions on the quality of family-centred care in 11 European NICUs[END_REF]. A French study found an association between fathers' rating of neonatal care and how well they felt treated by the staff [START_REF] Koliouli | Healthcare professionals in a neonatal intensive care unit: source of social support for fathers[END_REF], whereas in contrast, in Ghana, there is total exclusion of fathers in neonatal care. Fathers are sent away, left out of discussion and planning, uninformed and subject to unfriendly exchanges with staff [START_REF] Adama | Ghanaian fathers' experiences of caring for preterm infants: a journey of exclusion[END_REF].

Fathers in neonatal units can and should become very close to their babies

The experience of mothers and fathers trying to maintain closeness with their babies in a neonatal unit has been described as "clinging to closeness" during a rollercoaster ride of closeness and separation. Closeness helps fathers feel like 'real parents' [START_REF] Mäkelä | Clinging to closeness: the parental view on developing a close bond with their infants[END_REF]. Neonatal care can offer additional opportunities for fathers to be close to their babies, compared to birth at term, because of the additional needs of the baby that draw both parents in. A 2008 study of fathers in neonatal units found that they reported being closer to their babies than fathers of babies born at term [13]. For example, they were quoted as saying:

"I'm the one who accompanied my daughter to neonatology. I was able to be at her side. It made me very happy." [START_REF] Martel | Establishment of the relationship between fathers and premature infants in neonatal units[END_REF] "It's important that I can see him, talk to him, simply touch him so that he knows I'm there." [START_REF] Martel | Establishment of the relationship between fathers and premature infants in neonatal units[END_REF] If the mother is immobilised in another location, the father often shuttles back and forth, supporting the mother-baby connection as well as he can [START_REF] Martel | Establishment of the relationship between fathers and premature infants in neonatal units[END_REF]. The feeling of being caught in between mother and baby is common immediately after birth, because fathers want to be with both the mother and the baby at the same time [START_REF] Lindberg | The birth of premature infants: experiences from the fathers' perspective[END_REF]. Skin-to-skin contact between fathers and babies generates particularly strong feelings in fathers -of gratitude, happiness and love. To describe the experience, fathers use words like "powerful", "strong", "fantastic", "amazing" and "incredible" [START_REF] Olsson | Skin-to-skin contact facilitates more equal parenthood -a qualitative study from fathers' perspective[END_REF], for example:

"I did a bit of kangaroo with him and when I looked at him….wow! I'm going to be paternal, I know." [START_REF] Martel | Establishment of the relationship between fathers and premature infants in neonatal units[END_REF] "When I first saw M., it was magic, a miracle! I was all alone in the bloc, when they were preparing her for the NICU… I felt like I had to touch my daughter. A very emotional moment, firstly the eye contact and then the physical, a magic contact…" [START_REF] Koliouli | Soutien social et expérience paternelle des pères de nouveau-nés prématurés[END_REF] "Kangaroo care together…these are the unforgettable moments. When we cried together." (mother of twins) (youtu.be/RGdaKY2HXqc) "The first time I held him skin-to-skin, it was really, like, wow! It was like a communion, like a really privileged contact." [START_REF] Feeley | The father at the bedside: patterns of involvement in the NICU[END_REF] "I looked at my son and then my daughter and then my wife and I just felt, damn I'm so happy." (father of twins) [START_REF] Olsson | Skin-to-skin contact facilitates more equal parenthood -a qualitative study from fathers' perspective[END_REF] After skin-to-skin care, some fathers report relief from anxiety, pride (e.g. wanting to share photos with friends), increase in confidence in care and feeling more equal with the mother as a carer [START_REF] Olsson | Skin-to-skin contact facilitates more equal parenthood -a qualitative study from fathers' perspective[END_REF]. The aforementioned study from Taiwan about teaching fathers tactile stimulation of babies was associated with fathers reporting more attachment to their baby according to a 35point attachment questionnaire [START_REF] Kim | Effects of tactile stimulation by fathers on physiological responses and paternal attachment in infants in the NICU: A pilot study[END_REF]. The feeling of closeness between father and baby is not associated with a lack of support for mother-baby closeness. For many fathers, the separation of mother and baby is a source of anxiety.

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Father-baby attachment has a biological basis, like mother-baby attachment, but has not received adequate attention

When fathers are more engaged in caring for their babies, they experience stronger hormonal and neurobiological changes than when not engaged. These changes are, in turn, associated with short and longer-term benefits for the baby.

 Oxytocin is associated with human sociality, including empathy, social collaboration, care of babies and romantic love. It increases in fathers as in mothers through physical contact such as skin-to-skin, and decreases less quickly than in mothers post skin-to-skin [START_REF] Cong | Parental oxytocin responses during skin-to-skin contact in pre-term infants[END_REF]. When fathers are administered oxytocin (via a nasal spray), they become more attuned to their babies. The babies' own oxytocin levels also become higher and their social orientation and socialising behaviour increase [START_REF] Abraham | Network integrity of the parental brain in infancy supports the development of children's social competencies[END_REF].  Testosterone decreases in men when a baby is born. If the father is more involved in caring, the testosterone decreases further [START_REF] Gettler | Longitudinal perspectives on fathers' residence status, time allocation, and testosterone in the Philippines[END_REF]. Fathers with lower testosterone are more affectionate and sensitive towards their babies [START_REF] Weisman | Oxytocin administration, salivary testosterone, and father-infant social behaviour[END_REF]. Testosterone also drops more if the father is more committed to the relationship with the mother after the birth of the baby. In these couples, there are stronger correlations in testosterone levels between the mother and father [START_REF] Saxbe | Fathers' decline in testosterone and synchrony with partner testosterone during pregnancy predicts greater postpartum relationship investment[END_REF].  Cortisol is commonly associated with stress, but not necessarily negatively. Fathers with higher cortisol levels are more alert and responsive to a baby crying [START_REF] Fleming | Testosterone and prolactin are associated with emotional responses to infant cries in new fathers[END_REF]. This increase in cortisol might help new parents pay attention to their baby's signals. In contrast, higher cortisol levels in both mothers and fathers are associated with a poorer relationship between them [25]. Skin-to-skin care is associated with a drop in cortisol in fathers, as in mothers, with the effect being longer lasting in fathers post skin-to-skin [START_REF] Cong | Parental oxytocin responses during skin-to-skin contact in pre-term infants[END_REF].  Prolactin is associated with breastfeeding, but it increases in new fathers too.

Experienced fathers exhibit increases in prolactin when they hear a baby crying [START_REF] Fleming | Testosterone and prolactin are associated with emotional responses to infant cries in new fathers[END_REF].

Prolactin levels are highest when babies are most needy and vulnerable [START_REF] Gettler | Applying socioendocrinology to evolutionary models: fatherhood and physiology[END_REF]. Neuroscience has identified two areas of the brain associated with caregiving and both can be activated in fathers like mothers, the more so when the father cares for his baby [START_REF] Abraham | Father's brain is sensitive to childcare experiences[END_REF].

 The "emotional empathy" brain network. This network enables an automatic understanding of the baby's mental state, allowing the parent to "feel" and experience in herself/himself the physical pain or emotional distress of the baby.  The "socio-cognitive" brain network. This is a later developing circuit, including cortical and frontal brain areas. It is associated with mentalizing, cognitive empathy, and social understanding. In relation to caring for a baby, it enables a parent to infer baby's mental state from behaviour, to predict the baby's needs and plan future caregiving activities. Actively caring fathers show greater activation in the "emotional empathy" brain network. When fathers engage in regular active day-to-day care of children -particularly in the extreme case when fathers raise their babies with no maternal involvement -they become as attuned and as sensitive to the baby's cues as mothers do [START_REF] Abraham | Father's brain is sensitive to childcare experiences[END_REF]. Longitudinal research has found that emotional empathy brain activity in a father in year one is associated with better emotional regulation in the child four years later. Similarly, higher activity in the socio-cognitive brain network is associated with a child's higher social skills four years later [START_REF] Abraham | Network integrity of the parental brain in infancy supports the development of children's social competencies[END_REF].

The active engagement of fathers in neonatal units triggers these biological and neurobiological processes. This new science helps to explain the feelings of love expressed by fathers holding their babies, as described in the previous section.

Social expectations of mothers and fathers influence their experiences and behaviour as well as the behaviours of healthcare professionals towards them

A focus of the most recent research on fathers in neonatal units has been the different social expectations of mothers and fathers and how these influence -but by no means define -their responses to having a baby in a neonatal unit. The father of a premature baby starts in a different place from the mother [START_REF] Olsson | Skin-to-skin contact facilitates more equal parenthood -a qualitative study from fathers' perspective[END_REF]. In a Canadian study, one third of fathers were highly involved, equally with mothers. Another third visited regularly but believed the role of the mother was to care for the baby and the role of the father was to support the mother. The final third did little or no caring, were fearful about handling the baby and had to be strongly encouraged [START_REF] Feeley | The father at the bedside: patterns of involvement in the NICU[END_REF]. Such variation is not see among mothers, whose roles are more clearly defined. Three particular gender attitudes -some would say myths -emerge as significant:

(1) employment and work are mainly a man's responsibility [START_REF] O'brien | Effectiveness of Family Integrated Care in neonatal intensive care units on infant and parent outcomes: a multicentre, multinational, cluster-randomised controlled trial[END_REF] women are better than men at caring (3) men should be strong [START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF][START_REF] Thomson-Salo | A fathers' group in NICU: recognising and responding to paternal stress, utilising peer support[END_REF][START_REF] Provenzi | A comparison of maternal and paternal experiences of becoming parents of a very preterm infant[END_REF][START_REF] Hugill | Fatherhood in midwifery and neonatal practice[END_REF]. Research and practice in neonatal parenting support should not ignore gender. It is not possible to design the best service for mothers or fathers without understanding different gender influences of each of them [START_REF] Provenzi | The lived experience of fathers of preterm infants in the Neonatal Intensive Care Unit: a systematic review of qualitative studies[END_REF]. Healthcare professionals should understand their own attitudes to and assumptions about fathers. All that follows describes tendencies among fathers and mothers. In reality, neonatal unit care needs to deal with diversity of responses and needs of both parents.

Men are expected to be the lead earner

In Denmark, like many other countries, fathers only get two weeks of paternity leave in the first 14 weeks. This creates a dilemma for fathers of babies admitted to neonatal units. They want to take leave in the first two weeks when their babies are in the most critical condition; but they also need it later on, including when the baby comes home. Furthermore, fathers are typically left in sole care of any older siblings of the baby. This lack of time off for the baby places fathers in a particularly difficult situation [START_REF] Provenzi | A comparison of maternal and paternal experiences of becoming parents of a very preterm infant[END_REF].

"I have never been this stressed before….I take care of the other children at home and of my job, but I also need to be here -I want to be here as well." [START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF] "I hate the fact that I get to see my baby 1 hour a day now as I work 8 am-8 pm. Just understand that even though you have a little one in the NICU, it does not mean that the mortgage company or credit company would care." [START_REF] Kim | Use of social media by fathers of premature infants[END_REF] Work commitments also mean that sometimes fathers get no information directly from staff, only via the mother. When this happens, it leads to feelings of being out of control and not a full parent.

Women are considered "natural" carers; men are not

There is a widespread belief in society as a whole that mothers are "primary" carers and so fathers are secondary. Hence, fathers in neonatal units can see themselves as secondary parents and staff can treat them as helpers, rather than full parents [START_REF] Ireland | Experiences of fathers with babies admitted to neonatal care units: a review of the literature[END_REF]. "As a father, you feel left out." [START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF] "I think that the mother is more important than the father, even though both would like to hold her." [START_REF] Martel | Establishment of the relationship between fathers and premature infants in neonatal units[END_REF] "I didn't always feel comfortable holding him or giving him a bottle. I didn't want to do anything wrong." [START_REF] Martel | Establishment of the relationship between fathers and premature infants in neonatal units[END_REF] Some studies observe that fathers are less intimate with the baby when the mother is present -they defer to the mother when she is present [START_REF] Ireland | Experiences of fathers with babies admitted to neonatal care units: a review of the literature[END_REF]. Such beliefs, contrasting with the heightened degree of father-baby connection that can occur in neonatal care, generate a diversity of reactions. Sometimes the beliefs impact on the father's own relationship to the child -they hold back from touching or holding the baby [START_REF] Provenzi | A comparison of maternal and paternal experiences of becoming parents of a very preterm infant[END_REF]. Meanwhile, at other times, such beliefs make little difference to the father's own investment in relating to the baby [START_REF] Martel | Establishment of the relationship between fathers and premature infants in neonatal units[END_REF]. The primary/secondary construction has long been challenged by development psychologists who observe the importance of the system of care around the baby, involving multiple parent-child attachments and support by one parent for the other parent's relationship with the child; in this perspective there is no primary or secondary, rather a team [START_REF] Cabrera | Fathers are parents, too! Widening the lens on parenting for children's development[END_REF]. The idea that mothers are natural carers and men are not can lead to mothers and fathers reacting differently to their own uncertainty and lack of confidence. Fathers may need and want more detailed and specific instruction, fearful of their perceived innate inability to care [START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF].

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Men are expected to be strong and protective

Research shows that fathers of pre-term babies are considerably more at risk of depression and anxiety than fathers of babies born at term, though they are less affected than mothers of pre-term babies [START_REF] Candelori | The experience of premature birth for fathers: the application of the Clinical Interview for Parents of High-Risk Infants (CLIP) to an Italian sample[END_REF]. Multiple earlier studies from France, Canada, Sweden, Australia and UK have shown that many fathers in neonatal units do not feel at liberty to 'let go' emotionally [START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF]. Some of the fathers in a Danish study expressed constant worry and an inability to relax. "There is chaos in my mind," one father said. But they also described how they strive to appear calm and show none of this, in line with masculine ideals of independence and selfsufficiency [START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF][START_REF] Hugill | Fatherhood in midwifery and neonatal practice[END_REF].

"I don't want to be weak in front of my wife. I don't think she knows how bad I am hurting right now." "We guys like to fix things and this is something that cannot be fixed and there's got to be some frustration about that." [START_REF] Kim | Use of social media by fathers of premature infants[END_REF] A recent US study found that, on returning home, cortisol levels in fathers showed they were more stressed than mothers, and yet they did not report this in a Perceived Stress Scale questionnaire [START_REF] Garfield | home: paternal and maternal cortisol rhythms in parents of premature infants[END_REF].The fathers stated that their role was to fulfil their partner's needs and expectations, and that they did not expect reciprocal support from their partners [START_REF] Olsson | Skin-to-skin contact facilitates more equal parenthood -a qualitative study from fathers' perspective[END_REF][START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF].

"I have to cheer her up, but no one helps me. It is difficult to bear. I do not show that I am burnt out; instead, I suppress my feelings." [START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF] "My wife does not know that I am about to lose patience." [START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF] "I wouldn't want my wife asking me how I feel." [START_REF] Kim | Use of social media by fathers of premature infants[END_REF] Fathers said they needed time alone with staff, to be able to talk freely about their difficulties without showing their vulnerability to their partners. [START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF]

Evidence based recommendations for engaging fathers in neonatal units and optimising practice

On the basis of this review of recent evidence and practice this working group makes 12 recommendations for practice.

Assessing fathers' and mothers' needs individually

Fathers' needs can be different from mothers' needs, and so assessment should consider parents from the same family as separate individuals. The National Perinatal Association in the US has established guidelines for the psychosocial care of parents (www.nationalperinatal.org/Support4NICUParents). They recommend that mental health professionals meet with all parents/primary caregivers within one to three days of admission to establish a working relationship, normalize emotional distress and evaluate risk for psychological distress (e.g., depression, post-traumatic stress). Both mothers and fathers should be screened for distress within the first week after admission to the NICU and re-screened prior to the baby's discharge. Measures such as the Clinical Interview for Parents of High-Risk Infants (CLIP) have been found to be less good at predicting depression and anxiety in fathers of pre-term babies than in mothers [START_REF] Candelori | The experience of premature birth for fathers: the application of the Clinical Interview for Parents of High-Risk Infants (CLIP) to an Italian sample[END_REF]. This supports the need to adapt tools designed for mothers before applying them to fathers. Fathers are, of course, diverse, just as mothers are. Gender norms influence but do not define the behaviours of fathers. Fathers come with widely different experiences -socioeconomic position, culture, race, age [START_REF] Provenzi | A comparison of maternal and paternal experiences of becoming parents of a very preterm infant[END_REF][START_REF] Provenzi | The lived experience of fathers of preterm infants in the Neonatal Intensive Care Unit: a systematic review of qualitative studies[END_REF]. Individual assessment will enable recognition of such diversity.

Care plans

Care plans drawn up with parents should specify what each parent needs individually, as well as jointly, to allow for differences between parents. Of particular significance will be the timing of support around each parent's availability [START_REF] Walmsley | Are fathers supported by neonatal teams? An exploration of the literature[END_REF].

Accessibility of neonatal unit

Family-centred neonatal care involves making neonatal units family friendly and that, of course, includes fathers. Complete flexibility of access for fathers is important, to enable visiting at any time of day around their work and their care of other children. The physical space is important -one survey showed that units providing opportunities for overnight stays for at least one parent had a significantly higher parental presence [START_REF] Raiskila | Parents' presence and parentinfant closeness in 11 neonatal intensive care units in six European countries vary between and within the countries[END_REF]. The imagery and language in printed materials and the attitudes and body language of professionals are both important [START_REF] Hugill | Fatherhood in midwifery and neonatal practice[END_REF]. A hospital in Poland has task lists by the cots, divided into "mother", "father" and "nurse" (youtu.be/RGdaKY2HXqc).

Coparenting education

Parenting education should be directed equally at both parents within the neonatal unit. Education in the hospital should also prepare parents for the return home, where they have to work as a team, caring for the baby and for each other. One study found that parents were worried about each other on returning home -mothers tended to worry about fathers' work-life balance and fathers about mothers' mental health [START_REF] Garfield | Paternal and maternal concerns for their very low-birth-weight infants transitioning from the NICU to home[END_REF]. As a future innovation we propose the development of parenting education based on a coparenting approach. This approach rejects the "primary"/"secondary" idea and replaces it with the idea of teamwork. Parenting skills are considered, but also how parents can work together and support each other, fathers for mothers and mothers for fathers. Coparenting is particularly important in stressful situations where parents need to look after each other more. 5. Father-baby closeness Neonatal units can support father-baby closeness in a variety of ways.

 Provide privacy and time to bond with their baby in a peaceful and calm environment [START_REF] Mäkelä | Clinging to closeness: the parental view on developing a close bond with their infants[END_REF][START_REF] Olsson | Skin-to-skin contact facilitates more equal parenthood -a qualitative study from fathers' perspective[END_REF].  Help fathers to recognise a baby's responsiveness to them, for example, how the baby calms down when the father cares [START_REF] Mäkelä | Clinging to closeness: the parental view on developing a close bond with their infants[END_REF].  Help fathers be autonomous carers for the baby -involved in decision-making and understanding the technology [START_REF] Feeley | Fathers' perceptions of the barriers and facilitators to their involvement with their newborn hospitalised in the neonatal intensive care unit[END_REF][START_REF] Treherne | Parents' perspectives of closeness and separation with their preterm infants in the NICU[END_REF].  Allow fathers to see other fathers on the unit being close to their babies [START_REF] Olsson | Skin-to-skin contact facilitates more equal parenthood -a qualitative study from fathers' perspective[END_REF][START_REF] Feeley | Fathers' perceptions of the barriers and facilitators to their involvement with their newborn hospitalised in the neonatal intensive care unit[END_REF]. This communicates a different social norm. The new understanding of biology and neurobiology of fatherhood marks out father-baby skin-to-skin as a particularly important factor, triggering instincts that tune the father into caring and introducing them into an active paternal role [START_REF] Koliouli | The role of the father in the caregiving process: parental sense of competence, paternal stress and skin-to-skin contact[END_REF]. Skin-to-skin care has a unique role in helping fathers to build confidence as equal carers, not just "helpers", and to discover the joy of loving their baby [START_REF] Ireland | Experiences of fathers with babies admitted to neonatal care units: a review of the literature[END_REF][START_REF] Helth | Fathers' experiences with the skin-to-skin method in NICU: competent parenthood and redefined gender roles[END_REF]. Skin-to-skin requires adequate time and time alone with the baby, and the father should feel able to practise skin-to-skin even when the mother is present. Mothers too, of course, need protected time and space to develop skinto-skin care for the baby.

Be attentive to fathers hiding stress

Given that many fathers feel the need to be "the strong one" and to provide support to, but not receive support from their partners, the neonatal staff needs to take particular care to afford fathers the space to express their feelings safely, and possibly confidentially from their families [START_REF] Hugill | Fatherhood in midwifery and neonatal practice[END_REF][START_REF] Ireland | Experiences of fathers with babies admitted to neonatal care units: a review of the literature[END_REF]. The staff needs to be mindful that fathers can mask fear through withdrawal, or the opposite, such as anger and bravado [START_REF] Hugill | Fatherhood in midwifery and neonatal practice[END_REF]. Simply communicating to fathers that their reactions are normal is important [START_REF] Lindberg | The birth of premature infants: experiences from the fathers' perspective[END_REF].

''I wanted to hear this is a normal reaction and you are going to handle it, but nobody spoke to me." [START_REF] Lindberg | The birth of premature infants: experiences from the fathers' perspective[END_REF] 7. Inform fathers directly Fathers typically require information as a way of feeling less out of control [START_REF] Lindberg | The birth of premature infants: experiences from the fathers' perspective[END_REF][START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF]. It is important to inform the fathers directly, not solely via the mother, and that means being able to communicate at times when he is available around work and care for older siblings [START_REF] Ireland | Experiences of fathers with babies admitted to neonatal care units: a review of the literature[END_REF]. It is preferable to provide important updates to parents when they are together: two people will understand the information in more diverse ways, can discuss and support each other after, and one can ask questions the other might not feel comfortable to ask (youtu.be/RGdaKY2HXqc). A study in Iran found that running an education class specifically for fathers a few days after the baby's admission to a neonatal unit improved their knowledge. The researchers, working with 23 fathers, used a US programme, HUG Your Baby (www.hugyourbaby.org). This consisted of one session, a DVD and a handout. The programme focused on baby sleep, crying and attachment [START_REF] Kadivar | Supporting fathers in a NICU: effects of the HUG Your Baby program on fathers' understanding of preterm infant behaviour[END_REF]. Information for fathers should include the biology and neurobiology of fatherhood, as outlined above. Understanding of the science helps fathers understand the difference between what is "natural" and what is social convention. Professor Khashu in UK has developed a new resource for fathers of babies in neonatal units, with Inspire Cornwall CIC, called DadPad Neonatal©. This is comprehensive and information rich and it addresses all the concerns that are identified in the research, as well as facilitating a father's direct communication with the neonatal unit. It has been developed in such a way that it can be kept as a memento of the neonatal journey. Meanwhile in Australia, a text messaging service for fathers, SMS4Dads, is about to be tested with fathers of premature babies -SMS4Dads Early Arrivals (www.sms4dads.com). In the US, a NICU-2-Home smartphone app has been trialled, and found to improve parenting self-efficacy, discharge preparedness and length of stay by those who use it regularly [START_REF] Garfield | Supporting parents of premature infants transitioning from the NICU to home: a pilot randomized control trial of a smartphone application[END_REF]. A recent US study finds that fathers would like both mothers and fathers to have the same frequent direct updates on their mobile phones. These fathers observed the tendency for health staff to inform mothers preferentially. They also wanted to be able to access more emotional support from professionals [START_REF] Kim | Social support provision: perspective of fathers with preterm infants[END_REF].

Facilitate peer-to-peer communication for fathers

A US study looked at what fathers of babies in neonatal care talked about with each other on social media, in their own space [START_REF] Kim | Use of social media by fathers of premature infants[END_REF]. The researchers found that fathers ask questions, offer suggestions to each other, share information they have found elsewhere and share personal experiences. Topics of conversation include the health of the baby, healthcare costs, the pressure of work and caring for older siblings, the lack of social support and the difficulty of having to be the strong one.

"I think that one thing I could have used was communication with other dads who had been through the NICU experience. The moms were able to bond in the pumping room but I had no place to meet or talk with other dads that had gone through what I was experiencing." "The whole preemie thing is terrifying. Everybody around the situation is focused on mom/baby. Dads are left to worry about everything and everyone. As a dad, you may feel lonely. Hopefully, you will get some support by talking to people in here." The same researcher later published evidence about what fathers want, with peer-to-peer communication one of the wished for aspects of support [START_REF] Garfield | Supporting parents of premature infants transitioning from the NICU to home: a pilot randomized control trial of a smartphone application[END_REF].

"It would have been helpful to have maybe more contact with NICU fathers such as men with children who were either currently or had been in the NICU at some point." "My wife and I went to one of the support group meetings that were run by a social worker. I was surprised there was only one other father. Overall, I would say fathers wouldn't have the time to actually go to those support groups, so maybe something like Facebook would be better for fathers." One mother said:

"I had a very hard time at first. I felt very guilty. There were some other NICU moms who sat me down and talked to me….It was very encouraging! But there was not really any of that for fathers." In Australia a weekly two-hour fathers group is organised. Group discussions are animated, typically featuring strong expressions of sadness, anxiety and anger. Fathers attending the groups say they particularly appreciate the connection with others in a similar position. In addition to the meetings, a closed Facebook page has been set up, moderated by a member of neonatal staff. To organise groups like this, the neonatal unit must provide staff, a room and active promotion to fathers on the unit [START_REF] Thomson-Salo | A fathers' group in NICU: recognising and responding to paternal stress, utilising peer support[END_REF]. Ideally there should be a variety of provisions, since not all fathers want the same thing. Nevertheless, if fathers' groups, on-line and off-line, are well run, they are popular [START_REF] Hugill | Fatherhood in midwifery and neonatal practice[END_REF].

Differentiate by gender in evaluation outcome measurement

Service evaluation should always collect data individually from both parents and analyse results broken down by gender. This is relevant to the evaluation of staff quality, parental involvement in caring, communication and information, unit environment and discharge planning [START_REF] Hugill | Fatherhood in midwifery and neonatal practice[END_REF]. Engaging fathers in research requires specific recruitment techniques, for example:

 Don't require fathers to disclose their answers to their partners -access the parents separately.  Access fathers around their working hours.  Offer the option of a male researcher to interview fathers in case any would find that easier.  Persist in engaging fathers who are more difficult to reach.  Offer different venues for data collection, including on-line. The home may not be a suitable environment for some [START_REF] Hugill | Fatherhood in midwifery and neonatal practice[END_REF].

Staff training

Studies have shown that nurses have an important role as facilitators of father engagement. [START_REF] Garfield | Supporting parents of premature infants transitioning from the NICU to home: a pilot randomized control trial of a smartphone application[END_REF] Nurses, however, tend to have more difficulty relating to fathers than to mothers and may benefit from coaching to overcome this [START_REF] Johnson | Engaging fathers in the NICU: taking down the barriers to the baby[END_REF][START_REF] Ammentorp | Coach training can improve the self-efficacy of neonatal nurses. A pilot study[END_REF].

Staff training should include engaging fathers across all aspects of care. A special section on "engaging fathers" in a training course all about supporting mothers is not enough. We propose an innovation in staff training, namely teaching a coparenting approach that focuses on enabling parents to work together effectively.

Father-friendly service audit tool

An audit tool for neonatal services, how father-friendly are we? could built to analyse any service in relation to all the recommendations above. An existing tool of this kind is the Fatherhood Institute's 10-point father-friendly commissioning checklist, covering such things as organisation policy, staff training and assessment of fathers' needs [START_REF]Commissioning father-inclusive parenting programmes: a guide[END_REF]. 12. Review international standards for neonatal care in the light of recent evidence The importance of supporting father-baby bonding and mother-father coparenting are not adequately reflected in international standards, such as UNICEF's Baby Friendly Standards. A review is now needed.

Conclusion

This paper sets a new agenda for international action, with 12 recommendations focusing on how neonatal units can support father-baby bonding and mother-father coparenting, both of which improve infant health. The Family Initiative's International Neonatal Fathers Working Group will take forward these recommendations, seeking to disseminate knowledge, host discussion among professionals and develop new tools for best practice. The authors invite all those interested to get involved by visiting www.familyinitiative.org.uk/neonatal-fathers.

  Tell me what it means when my son raises arms. It means, "I want peace." It is helpful to receive specific instructions. Try to interpret it your way [the nurses say].What am I supposed to interpret? I have no idea."[START_REF] Noergaard | Fathers' needs and masculinity dilemmas in a neonatal intensive care unit in Denmark[END_REF]


Michel Bakni
email: m.bakni@estia.fr

Guillaume Terrasson
email: g.terrasson@estia.fr

Octavian Curea
email: o.curea@estia.fr

Alvaro Llaria
email: a.llaria@estia.fr

Dos Jessye

Santos

Jessye Dos Santos
email: j.dossantos@estia.fr

An Approach for Modelling Wireless Sensor Networks: Focusing on the Design Concept and Energy Awareness

Keywords: Energy-aware design, Cross-level, Energy modelling, Wireless Sensor Networks. I

published or not. The documents may come

INTRODUCTION

A Wireless Sensor Network (WSN) is a set of batterypowered nodes that include sensors coupled with processing units and wireless transceivers. Since energy stored in the batteries is limited, both the autonomy of the node and the system's lifetime are impacted. Thus, energy-aware design is an important research topic and different solutions have been proposed in recent years to address the design concept from an energy point of view [START_REF] Bakni | Energy-aware Cross-level Model for Wireless Sensor Networks[END_REF] [START_REF] Chéour | Evaluation of simulator tools and power-aware scheduling model for wireless sensor networks[END_REF]. In this context, and to achieve the high-energy efficiency required by WSN to be implemented in domains like agriculture, industry, and healthcare [START_REF] Yick | Wireless sensor network survey[END_REF], developers of WSN applications and researchers working in this field need to make the right decisions in the early stages of the design process. Consequently, and in order to deal with this challenge, simulators and emulators are widely used.

The scientific literature presents a large variety of singlelevel simulators, especially node-level or system-level [START_REF] Bakni | WSN Simulators Evaluation: An Approach Focusing on Energy awareness[END_REF]. This distribution can be traced back to the models on which the simulators were built. From what we studied, there is no WSN model designed to reveal the impacts of a given parameter on energy consumption from a cross-level perspective, and not only from a specific-level point of view [START_REF] Haase | Power-aware system design of wireless sensor networks: Power estimation and power profiling strategies[END_REF]. Therefore, in [START_REF] Bakni | A Cross-level model for power-aware Wireless Sensor Networks design[END_REF], we briefly addressed the need of the cross-level design in WSN modelling. Later, we suggested a cross-level energy-aware model for WSN where interactions among model parameters were also highlighted, but only for the radiofrequency (RF) unit onboard the node [START_REF] Bakni | Energy-aware Cross-level Model for Wireless Sensor Networks[END_REF]. In this paper, the application of the model is expanded to include other node components, precisely the processing unit and a temperature sensor, together with more detailed examples of cross-level interaction.

The structure of this paper is as follows. First, the evolution of WSN modelling is reviewed focusing on design concept and energy-awareness. Then, we propose a crosslevel energy-aware model for WSN. After that, the result obtained from the implemented model is compared with results from a well-known WSN simulator: NS2. Additionally, a set of scenarios will be presented to show cross-level interactions among model parameters. This includes distance between the nodes and fragmentation threshold. Finally, we conclude with the perspective and future works.

II. WSN MODELLING AND SIMULATION

In order to better understand the main challenges related to WSN modelling and simulation in a power-aware design context, a short study on the evolution of WSN models is first provided. Throughout, the development of modelling techniques is traced. Then, WSN simulators, which were built on the previous model, are to be classified based on two characteristics: design concept and energy-awareness.

A. WSNs Models: from Layered-stack to Cross-level Design

Classical data network models, such as the Open Systems Interconnection (OSI) model or the Internet model (TCP/IP), suggest a layer-based design approach where a set of layers are stacked together. In this kind of model, the layers are defined based on their functionality, such as physical or logical connections. Each layer can only interact with the two adjacent layers in the stack using software interfaces [START_REF] Zimmermann | OSI Reference Model -The ISO Model of Architecture for Open Systems Interconnection[END_REF]. However, in WSNs, parameters residing at different layers need to interact with each other [START_REF] Yick | Wireless sensor network survey[END_REF]. Thus, services are to be provided across stack layers rather than a specific layer. Thereby, the traditional stack-layered model cannot answer the modelling challenges of this kind of networks [START_REF] Goldsmith | Design challenges energy-constrained ad hoc wireless networks[END_REF].

For many years, several works have handled this problem by developing different WSN-oriented solutions. In [START_REF] Zimmermann | OSI Reference Model -The ISO Model of Architecture for Open Systems Interconnection[END_REF], the authors suggest a modelling approach based on cross-layer design, where adaptivity and optimization across multiple layers of the stack is supported. A comparable cross-layered model is introduced in [START_REF] Akyildiz | Wireless sensor networks: a survey[END_REF]. Furthermore, in [START_REF] Charfi | A layered model for wireless sensor networks[END_REF], the cross-layer point of view is applied to network security, and additional considerations are included in this regard. In [START_REF] Korkmaz | A 2-hop coloring-based collision free infrastructure design for Wireless Sensor Networks[END_REF] a cross-layer concept is suggested through an infrastructure that supports a cross-layer design approach. According to the authors, the cross-layer interaction means that some data from a layer may be used as a parameter by a protocol that resides in another layer or it may affect another layer's operating process.

The suggestion of the "tier" or "level" concept is another approach that is used to answer the modelling challenges in WSN [START_REF] Egea-López | Simulation tools for wireless sensor networks[END_REF]. In the previous proposal, a level stands for a group of parameters that belong to different functions and features of the entire system, and not only limited to the network model. Alongside the previous methods, the solution outlined in [START_REF] Kuorilehto | High abstraction level design and implementation framework for wireless sensor networks[END_REF] divides the model's layers into two levels. The first level is mainly focused on non-physical parameters related to both software and application. On the other hand, the second level is concerned with hardware where the protocols are implemented, especially linking and routing, along with RF unit and sensor parameters.

This resulted in the development of a new approach for modelling WSNs: the multi-level approach. It expands the standard layered-stack model to cover not only node hardware and software, but also wireless media and parameters from the surrounding environment, such as temperature. Examples of this kind of multi-level models can be found in [START_REF] Korkalainen | Survey of wireless sensor networks simulation tools for demanding applications[END_REF] and [START_REF] Du | Towards a taxonomy of simulation tools for wireless sensor networks[END_REF].

After that, several evolutions were suggested to develop a cross-level model starting from the multi-level approach. As an example, in [START_REF] Imran | A survey of simulators, emulators and testbeds for wireless sensor networks[END_REF], a description is provided for using the multiple-level model in the design and development of WSN from a cross-level perspective. However, as illustrated in [START_REF] Issariyakul | Introduction to Network Simulator 2 (NS2)[END_REF], many proposed cross-level approaches are not used in an effective manner. Consequently, although energy modelling is a crucial issue in WSN design, this aspect is also not well implemented in the modelling stage.

B. WSN Simulator classification: design concept and energy awareness

In general, WSN models focus on one level of abstraction. Therefore, WSNs simulators built on these models are typically specific level [START_REF] Chéour | Evaluation of simulator tools and power-aware scheduling model for wireless sensor networks[END_REF]. Consequently, they trace parameters related to that particular level [START_REF] Yick | Wireless sensor network survey[END_REF]. In this context, a parameter is a numeric value that characterizes one property of a given level of abstraction, such as power consumption or payload length.

For example, Network Simulator version 2, well-known as NS2 [START_REF] Issariyakul | Introduction to Network Simulator 2 (NS2)[END_REF], is aimed at simulating network protocols. Thus, it has poor support for hardware simulation. Quite the opposite, TOSSIM [START_REF] Levis | TOSSIM: Accurate and scalable simulation of entire TinyOS applications[END_REF] can precisely emulate hardware, but it provides a very abstract perspective for high-level network or routing protocols. Moreover, there are several multi-level simulators. In this category, the simulator can take into account parameters belonging to different levels simultaneously. By way of an example, Jsim [START_REF] Sobeih | J-Sim: A simulation and emulation environment for wireless sensor networks[END_REF] is a multilevel simulator dedicated to WSNs. It is multi-level because it can simulate both environment and network parameters at the same time. Finally, few simulators are classified as crosslevel because they have the ability to show cross-level interaction among parameters belonging to different levels of abstraction. COOJA [START_REF] Osterlind | Cross-Level sensor network simulation with COOJA[END_REF] is an example of this kind of simulator.

None of the simulators mentioned above is energy oriented. These simulators could trace energy parameters, but they were not really built purpose for that. Based on that, a level-based energy classification for WSN simulators is proposed in [START_REF] Bakni | WSN Simulators Evaluation: An Approach Focusing on Energy awareness[END_REF] and illustrated in Fig. 1. In this figure, simulators are first classified according to their energyawareness: energy-oriented and non-energy-oriented. This classification is extended to include the simulator design concepts that are listed above.

Looking at energy-oriented simulators, PowerTOSSIM [START_REF] Perla | PowerTOSSIM z: Realistic energy modelling for wireless sensor network environments[END_REF] is an extension of TOSSIM that is dedicated to the emulation of energy in hardware. There is also IDEA1 [START_REF] Du | IDEA1: A validated system C-based simulator for wireless sensor networks[END_REF]. It is a multi-level energy-oriented simulator that handles three abstract levels: the wireless medium, the node and the environment.

The previous review shows that several non-energyoriented simulators, based on multi or cross-level models have been suggested. However, as demonstrated, regarding the energy aspect, the cross-level approach is not implemented. Therefore, with existing simulators, it is difficult to analyze the influence of parameters belonging to different levels of abstraction on both WSN lifetime and total energy consumption in a given node.

III. CROSS-LEVEL APPROACH FOR ENERGY AWARE MODELLING

Considering the previously mentioned limitations of existing simulators, we propose a cross-level approach for modelling WSN in the energy-aware context. First, the main definitions on which the model is built are introduced. After that, a global overview of the suggested concept is provided. Based on that, the relationship between parameters within each node component is explained to demonstrate the crosslevel interactions.

A. General definitions

As previously mentioned, we start by defining the principles on which the proposed concept is based.

Definitions relating to the design concept are first introduced:

• Parameter: a configurable value that represents a specific property of the level it belongs to. • Level: an abstract design concept. It stands for a set of parameters that describes the same part of the Figure 1. Energy-aware level-based classification for WSN simulators system that is being modeled. • Interaction: a relationship between two parameters that mutually affect each other. If the parameters belong to the same level, it is to be called an intralevel interaction. Otherwise, if the two parameters reside at different levels, the interaction is to be called cross-level. Secondly, definitions related to the energy-aware design process are provided:

• Phase: a time period corresponds to one circuit activity, each phase is combined with a current consumption level. • Pattern: a set of subsequent phases for several circuits. Each pattern corresponds to a particular node and it describes its behavior in terms of current consumption and time. • Pattern frequency (Fp): the number of pattern occurrences in one second.

B. Global Concept

Regarding the definitions above, the global overview of our cross-level is illustrated in Fig. 2. Based on that, a WSN is divided into four separate levels of abstraction. Within each level, a set of parameters describes the level's properties. Interactions between the parameters can be crosslevel or intra-level. A short description of each level is provided below.

• The Use case Level (UL) is the highest level in the concept hierarchy. It is concerned with the WSN application requirements. Parameters such as pattern frequency (Fp) or node activity sequences reside at this level. • The System Level (SL) reflects the topological aspect of the WSN. It focuses on how algorithms and high-level protocols influence the performance of WSNs. Parameters related to the network topology, like the distance between nodes, or protocols specifications, such as fragmentation threshold belong to this level. • The Node Level (NL) is concerned with the interactions among node software, such as the operating system, and onboard hardware, for example, the RF unit. In other words, this level focuses on the interactions between the node components. The parameters relating to the structure of node patterns, such as the order of the phases reside at this level. • The Circuit Level (CL) is the lowest level in the model's hierarchy. It is used particularly to characterize node hardware. The modelling of the electronic circuits takes place at this level, and this is where hardware-specific parameters, such as the power levels or the supply voltage, reside. The circuit level includes an RF unit, a processing unit, a set of sensors, and a battery. Therefore, as the four levels are stacked together, the higher the level is, the more general the parameters are. For example, parameters belonging to the UL are related to the scenario description, and they theoretically fit into any WSN application. On the other hand, CL parameters are very specific, they describe particular electronic circuits.

C. Cross-level Interactions for Node Circuits

In this section, the interactions, both intra-level and cross-level, between parameters that concern each circuit are explained.

As the proposed concept is energy-aware, in the following section, we focus on the energy aspects of WSN. Energy consumption in WSN is circuit-based, i.e., each circuit consumes energy independently. However, consumption is governed by the activities of the circuits which, in turn, depend on the interactions between parameters.

In the description below, next to each parameter's name, the abbreviated name of the level to which the parameter belongs will be added.

1) RF unit

The proposed concept is first applied to describe RF activities in a WSN. Fig. 3 provides an overview of all the interactions that takes in the different stages of RF activities.

In the first stage, the total number of bits to send is calculated. This includes the payload (UL) created by sensors or other applications that generates data on the node, as well as high-level protocol headers (SL) and the link layer protocol header (SL). Note that interactions at this stage are both intra-level and cross-level.

After that, the total amount of data is confronted against the fragmentation threshold (SL) identified by the wireless link protocol. If fragmentation is needed, the process will take place in this stage, and will result in two or more data frames. Next, the preamble (SL) is added to each data frame. Then, the length of each frame is calculated in term of seconds, thanks to the bitrate(s) (SL) supported by both the link protocol and the selected RF chipset.

The next step is to build the energy consumption pattern for the node's RF activities. Activity sequences (UL) and phase order (NL) are to be considered. The activity sequence helps to specify the actions taking place, such as sending or receiving frames. The link protocol defines the phases within each activity, as well as their order. For example, in the sending activity, the order of phases as follows: accessing the channel (phacc), exchanging data frames (phexch) and then waiting for acknowledgment (phack).

After that, the sequence of activities is to be matched with the power levels (CL) provided by the RF unit datasheet. This includes considering the distance (SL) that has a direct impact on the power level of the sending activity. Then, considering the supply voltage of the RF circuit (CL), the energy consumed by the RF activities is calculated.

In the final stage, the total simulation time (UL), the frequency of the pattern Fp (UL), as well as the initial amount of the energy stored in the battery (NL) are taken into consideration in order to estimate the system's lifetime.

2) Processing Unit All the activities relating to the processing unit are nodebased. Interactions only occur with other onboard components. However, the processing unit controls the RF unit's activities in terms of wake-up and sleep times. Additionally, it manages the activities of the sensors in the same way. Thus, synchronizing the node's activities is the main task of the processing unit.

The time sequence for the processing unit pattern starts in the sleep phase, in which, all node components are in sleep state. Then, the unit wakes up and enters the active phase where all other component activities take place. Finally, the processing unit return to the sleep state. Fig. 4 shows an abstract pattern of the processing unit, where the three phases: sleep, wake-up, and active take place, respectively. As illustrated, the amount of time spent in each phase depends on cross-level and intra-level interactions between the parameters.

After that, the processing unit's pattern will be created. To achieve that, the corresponding power level (CL) for each phase is derived from the circuit datasheet.

As a result, and considering the supply voltage (CL), the energy consumption of the processing unit in one pattern is calculated. Finally, the simulation time (UL), frequency pattern (UL) and battery energy (NL) are added as well, and the effects of the processing unit's activities on the node's lifetime can be estimated. Fig. 5 displayed in the sequential stages is used to calculate energy consumption in the processing unit.

3) Sensing Unit Sensors interact with the physical environment and with other nodes' components only, which means there will be no inter-node activities. Fig. 6 provides an overview of all the interactions that take place in one sensing unit. Note that multiple sensing units can exist onboard the WSN node.

When sensing a physical phenomenon (SL), the sensor measures a physical quantity and converts the measured value into an electric signal. The conversion time (CL) is specified in the sensor datasheet provided by the manufacturer. This is the time when most energy is consumed by the sensor. We consider that the result of this stage is a set of bits captured from the sensor's environment.

The sensor's activity sequence (UL) is then to be included. This includes the number occurrences in which the measurements take place in one pattern, as well as intervals between measurements. When the sensor is not sensing, it is in sleep mode. The result of this stage is the time sequence of the sensor's phases.

Next, the power level (CL) derived from the sensor datasheet is included. For each phase, there will be a corresponding consumption level. After that, the battery's voltage will be added, and energy consumption for one pattern of sensor activities is calculated. Finally, simulation time, pattern frequency and the energy level are to be included, thus, an estimation for the effects of the sensor's activities on the node's lifetime can be generated.

D. Model design

As illustrated previously, the proposed model is divided into four levels of abstraction. Going one step further in the modelling process, we introduce the block. It is an abstract modelling object that exists in the level's boundary. Each block includes a set of parameters. Blocks connect with each other using one of the following types of relationships:

1) Deriver-source connection: this connection links two blocks: the derivative and the source. The derivative block inherits all source attributes and behavior. Moreover, it has additional extensions that create variants of the source to serve the derivation purpose. 2) Child-parent connection: it couples two blocks: the child and the parent. The child block is part of the parent, which can have more than one child block of the same type.

The blocks can be classified based on the level in which they reside. Fig. 7 shows a block diagram for the proposed model, the levels are separated using dashed lines, and the highest level of abstraction is at the far right of the diagram. Blocks are represented using a rectangle. Additionally, block connections are shown, as well as the number of possible children or parents for each child-parent connection. Note that interactions link the model's parameters, while connections represent relationships between the blocks.

Hereafter, starting from the highest level of abstraction, there is a short description of the blocks used in the modelling process.

• UL blocks: UL includes one or more scenario blocks, each of which represents a full scenario. A given scenario includes scenario-level parameters such as the scenario time as well as blocks from the topology level like the network topology block. In the modelling hierarchy, the scenario block is the highest. As mentioned above, the model supports network heterogeneity. This is a property describing the ability to simulate heterogeneous systems where different types of nodes can exist in the same scenario [START_REF] Bakni | Methodology to Evaluate WSN Simulators: Focusing on Energy Consumption Awareness[END_REF]. The connection between the type and the node blocks at the node level answers heterogeneity. Each node includes one type block, but they can vary in its components and protocols. Note that the pattern block is not a part of the type, which means that two identical nodes can have different energy consumption patterns, where each consumes energy based on its parameters. The distance to the destination parameter is an example of that. Different distances will create various patterns, thus, non-equal energy consumption among the nodes, although they might be identical in terms of components.

As we have seen in the WSN modelling, this is the first model to include circuit regulators. Although the benefits of this addition were not directly explained in this paper, the transferring from one unit to another when representing consumption, precisely from current to energy, was done thanks to the regulators. The regulator modelling allows to distinguish the voltage levels for each point on the node, thus, facilitating the consumption calculation and make it more accurate.

IV. APPLICATION OF OUR MODEL

A. General settings

Our model is implemented using Matlab. Then tested scenarios will be proposed. These are designed for two purposes. First, presenting interactions between parameters, both cross-level and intra-level. Second, comparing the results obtained from the proposed model with those of a well-known simulator, namely NS2.

All the scenarios take place in an open area where there are two wireless nodes named Node A and Node B. These settings for the scenarios where selected to show the interactions between the parameters. Thus, simple two-node scenarios were developed. Periodically, Node A sends a fixed-length payload to Node B through the wireless medium using its RF unit.

Each node applies a TCP/IP network model. The implementation of the protocols starts at the network layer where Internet Protocol version 4 (IPv4) [START_REF] Postel | RFC 791, Internet Protocol[END_REF] has been chosen. The Internet Control Message Protocol (ICMP) [START_REF] Postel | RFC 792, Internet Control Message Protocol[END_REF], which is an integral part of IPv4, is used to create echo messages when necessary. Consequently, when the IPv4 module receives a data packet, it is going to send the same data back to the original source. In all scenarios, the length of IPv4 and ICMP headers are 20 and 4 bytes, respectively.

Next, the link-layer parameters are to be set. Two different protocols are to be used, these are: IEEE 802.15.4 as [START_REF]IEEE Standard for Information technology --Local and metropolitan area networks --Specific requirements--Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low Rate Wireless Personal Area Networks (WPANs)[END_REF] and IEEE 802.11a is used [START_REF]IEEE Standard for Telecommunications and Information Exchange Between Systems -LAN/MAN Specific Requirements -Part 11: Wireless Medium Access Control (MAC) and physical layer (PHY) specifications: High Speed Physical Layer in the 5 GHz band[END_REF]. The energy specification for the IEEE 802.15.4 module is derived from the CC2420 transceiver (Texas Instruments) [START_REF][END_REF]. For IEEE802.11a the specifications will be derived from an implemented chipset named HDG204 (H&D wireless) [START_REF]HDG204-WiFi 802.11b+g System in Package[END_REF]. Table I shows the settings for the two wireless link protocols. For each scenario, the considered time used to calculate energy consumption is 100 seconds, and it begins after initializing the nodes.

The previous settings are general. They are to be implemented in all the scenarios. However, in the following sections, additional settings are to be added will be explicitly mentioned.

B. Payload and pattern frequency

Pattern frequency and payload size are UL parameters. In this set of scenarios, we trace the cross-level effects of the two on the consumed energy, which is a CL parameter. First, we suggest a scenario using only the RF unit and we compare the obtained results with those from NS2. The reason behind this choice is the need to have comparable results, i.e., NS2 does not include models for other hardware components. Second, the scenario will be extended to include other hardware components, namely a processing unit and a sensor. This will highlight the contribution of the proposed modelling approach.

In order to illustrate the cross-level interaction, we go back to Fig. 7. When the payload size (UL) is modified, the length of the data frame to be sent (CL) by the RF unit will be changed. This, in turn, will impact the pattern structure (NL) and will have an influence on the total energy consumed by the node (CL), as well as the system's estimated lifetime (UL). The cross-level interaction for the frequency pattern can be traced in the same way. However, in this case, the change of FP (UL) will directly affect the pattern structure (NL), without passing by the circuit level. Then, the chain interaction will follow the same way as the payload size interactions.

1) Scenarios with only RF unit

To effectively trace payload size and FP, the following settings are to be added to the general settings. First, the two nodes will reside 10m apart. Second, both IEEE 802.11a IEEE 802.15.4 are to be used. For each protocol, three different values of Fp: 0.1, 1 and 2 Hz, and ten values for the payload length ranging between 10 to 100 bytes are used. Each scenario requires a combination of the three parameters previously mentioned. As a result, there are 30 scenarios to be run for each wireless link protocol. Table II summarizes the setting for the scenarios with only RF unit included.

These scenarios are configured both in NS2 and in the proposed model implemented in Matlab. The obtained results are the energy consumed by different activities of the RF unit. These activities are categorized into 4 phases:

• Access phase (phacc): RF unit tries to access the wireless channel. • Exchange phase (phexch): RF unit sends or receives data frames. • Acknowledge phase (phack): RF unit sends or receives acknowledgment frames. • Sleep phase (phslp): The RF unit is in sleep state. In these phases, the cross-level interaction between parameters is taking place, i.e., the energy consumed in each phase is a result of interplay between parameters related to different levels. For example, in the exchange phase, the consumed energy depends on of the headers' lengths, payload length, and bit rate, which belongs to the following levels: UL, SL, and NL, respectively.

The results obtained from the implementation of the previously described scenarios in NS2 and in Matlab using our model can be found in Table III. The upper part shows the obtained results from IEEE 802.11a and the lower part for IEEE 802.15.4. Each part is further divided into two subparts, corresponding to scenarios with 10 and 100 bytes of the payload length, respectively.

For the two protocols, when comparing obtained results from NS2 and our proposed model, differences and similarities can be found as follows. The energy consumed in phacc or in phack are identical and there is a slight difference in the energy consumed in phslp. In phexch, the difference is notable, but stable and this is due to different interpretations of the link protocol specifications. For example, in our model, the ICMP header is considered to be part of the data packet, contrary to NS2 where this header is added to the data packet later.

Fig. 8 shows the pattern obtained from the proposed model. The left side of the figure is dedicated to IEEE 802.11a and the right side to IEEE 802.15.4. Different values of Fp are displayed, namely 0.1, 1, and 2 Hz. For each of these values, a set of corresponding errors is provided. Each of these is also related to a simulation where the payload length is 10, 50, or 100 bytes.

Next, the relative error between the results obtained from NS2 and our model is to be calculated. For each scenario, the relative error is calculated as follows. The value obtained from out model is subtracted from the value obtained from NS2, and the result is to be divided by the latter. In all the simulation results, the relative errors obtained from IEEE 802.11a are greater than those of the corresponding scenarios of IEEE 802.15.4. This difference can be explained by unplanned and non-periodic radio activities that appear periodically in NS2 IEEE 802.11a simulations. These activities have a fixed duration regardless of Fp and the payload length. Each of these activities appears as a single pulse of transmission or reception causing an additional energy consumption of around 5 μJ and 2 μJ, respectively.

Finally, as illustrated in Fig. 9, the value of the relative error between NS2 and the proposed model did not exceed 3.5%. Based on that, we can consider the proposed model is validated with NS2. Although not shown in this scenario, our proposed WSNs model also allows adding energy consumption phases for other hardware on the node, such as the processing unit or sensors. As a result, an accurate pattern can be constructed representing precisely the real consumption of the node contrary to NS2 that has a poor support for hardware, as mentioned before.

2) Complete node Scenario

A typical WSN node includes other components in addition to the RF unit, such as processing unit and sensors. Together, they serve the node functionality. In order for the components to serve the node, they consume a considerable amount of energy that cannot be neglected. For example, energy profiling in [START_REF] Terrasson | Energy Model for the Design of Ultra-Low Power Nodes for Wireless Sensor Networks[END_REF] shows that the RF unit is responsible for 62% of the node consumption, where the sensors and the microcontroller share the remaining 38% of the energy consumed. Another study shows different energy profiles based on the scenario setting, the consumption of the RF unit varying between 7 to 65% of the total consumed energy, while the other components are responsible for the remaining consumption [START_REF] Navarro | Energy profile for environmental monitoring wireless sensor networks[END_REF].

NS2 is not capable of tracking the energy consumed in sensors and the microcontroller. However, the proposed model is able to do that. This is simply achieved by extending the consumption pattern to include new phases for the other components.

In this scenario, the setting of the RF only scenario will be implemented considering the following extensions. We assume that the node includes a microcontroller and a sensor. The microcontroller is PIC18F4620 [START_REF]-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology[END_REF] and the sensor is TMP102 [START_REF]TMP102 Low-Power Digital Temperature Sensor With SMBus and Two-Wire Serial Interface in SOT563[END_REF], a temperature sensor. The scenario is still the same, except for the following: The node wakes up after 0.5 seconds from the pattern start. Then, the sensor captures a value directly after the node is up it sends the data through the RF unit. Finally, it goes back to sleep again without waiting for the response. In addition to that, the size of the payload is only 2 bytes since the sensor creates a 12-bit sample.

All the circuits are supplied with 3.3 V. The electrical characteristic for the RF unit will not be changed. Regarding the processing unit and sensor, Table IV provides supply current in the active and sleep phases. Note that the wake-up duration of the processing unit lasts for 1 μs, during which we consider the supply current to be identical to the active phase.

In Fig. 10, a pie chart is shown for a per-component energy consumption percentage for the node A. The energy consumed by the processing unit is 60% of the total consumption and sensor consumes only 4% of the total energy. However, energy consumed by the RF unit is 36%. However, the selecting the hardware components, such as those using different processing units or the scenario settings, like activity sequences can significantly impact the energy profile.

In addition to that, the model is also capable of tracing the energy consumption down to the circuit level, i.e., the consumption of the phases for each circuit. Table V provides results obtained from the sending node, the energy consumed by the processing unit is divided into three phases: Sleep, wake up and active, and consumption in the active phase has the higher value. For the sensor, the energy consumed is categorized into two phases: sleep and active. Following the same consumption trend, most consumption takes place during the active phase. The proposed model answers the needs of WSN designers and researchers. Regarding the first group, the model provides a way to combine different circuits and protocols. This is useful in the application design stage where decisions relating to the construction of the node are to be made. On the other hand, the model can be used in the field of research as well, because it can show the cross-level interactions, and this is interesting for energy-related studies.

The suggested model is extensible, i.e., new blocks can be added to support new functionalities. For example, a wave propagation model can be added at the system level. Additionally, an energy harvester unit can extend the model at the circuit level. However, the model activities and the corresponding phases need to be added to the pattern at the node level as well.

C. Distance and power levels

Distance between nodes has an impact on energy consumption in the RF module. Manufacturers of the RF chipsets provide a range of transmission power levels. The further the destination is, the higher the transmission power level selected. The proposed model provides a way to trace this cross-level interaction.

The cross-level interactions are illustrated using arrows in Fig. 11. First, a distance between two nodes changes, it is a system-level parameter. This, in turn, interacts with the power level selection in the RF unit at the circuit level. Through intra-level interaction, the activity sequence in the RF unit is affected. This is illustrated using a blue arrow on the same figure. After that, the cross-level chain interaction reaches the node pattern at the node level. Finally, energy consumption in the battery at the circuit level, and thus, the system's lifetime at the scenario level are affected.

In order to show the previous cross-level interactions, the general settings will be extended as follows. First, the payload length will be 20 bytes in all scenarios. Second, the distance between the two nodes will change to 25, 50, 100, 200, and 400 meters, respectively. Those values were intentionally selected to cover the whole power level range proposed in the RF unit's datasheet.

Fig. 12 shows the obtained values for energy consumption of the RF unit exchange phases from the distance and power level scenario. As the distance between nodes increases, the energy consumption in the exchange phase increases linearly. This can be explained from the chipset datasheet specification. As the distance rises above a particular threshold, the model automatically changes the transmission threshold to use the next value. This change has an impact on the energy consumed by the node. For example, when the distance between the two nodes changed from 25 to 400m, the corresponding consumed energy in the transmission phase changed from 60 to 120 μJ.

D. Fragmentation threshold

Fragmentation is a mechanism to divide Protocol data units; i.e., frames or packets, into two or more units that are shorter in length. Fragmentation is used when WSN applications contain a constraint on the size of data units. For example, IEEE 802.15.4 has a maximum fragmentation threshold set to 127 bytes, but any lower value down to one, can be set. Fragmentation threshold is a system-level parameter which impacts the energy consumption of the RF unit at the circuit level. The proposed model can be used to show the previous cross-level interaction.

In this case, the cross-level interactions take place as follows: first, the fragmentation threshold will be changed. It is a system-level parameter. This change will affect the protocol object, which is attached to the RF unit object, and both are circuit-level parameters. These changes will affect the pattern structure at the node level, which, in turn, affects the energy consumption in the node, which is a circuit-level parameter. In the end, the system's lifetime will be affected by this cross-level chain interaction.

In practice, the settings of the complete node scenario will be implemented. However, the following changes will take place. The scenario will include sending a payload of 100 bytes, and the fragmentation threshold is set to {40, 60, 80, 100} bytes, respectively. TMP102 is not used. It will always remain in the sleep state, thus, its consumption is constant in all the scenarios and it will be ignored. On the other hand, the consumption of the RF unit and the processing unit is changed as a response to the fragmentation setting, and these changes are to be traced. The node wakes up after 0.5 seconds from the pattern start and begins sending the data packets.

Fig. 13 presents the results obtained from the fragmentation scenario. The energy consumption by the RF unit phases is illustrated according to the fragmentation threshold. The consumption of access, exchange, and acknowledgment phases drop significantly after the first test. Energy consumption in the sleep phase changes slightly.

These results can be explained with the help of the number of frames sent in each scenario: they are 15, 4, 3, and 2, respectively. This sharp drop in the number of the sent frames is due to the effect of another system-level parameter, namely protocol headers. The used protocols need to add headers to the payload. It is 33 bytes in total, and that leaves only 7 bytes for the payload when the fragmentation threshold is set to 40 bytes. However, the payload size is up to 27 bytes when the threshold is 60 bytes. Thus, 100 bytes of payload need only 4 frames to be completely sent. In general, these results show that energy consumption in the node decreases as the fragmentation threshold increases.

V. CONCLUSION AND FUTURE WORK

In this paper, a cross-level energy-aware modelling approach for WSN is proposed. This approach was applied by mean of several scenarios to reflect how parameters from different levels can interact and affect energy consumption. The suggested model is built on the assumption that WSN node activities can be described in a pattern, which is periodically repeated. A pattern consists of sequential phases belonging to all the circuits composing the node.

Moreover, the obtained results show the model's ability to provide energy consumption at different levels of abstraction. This includes the total energy consumed by each node, the consumption of one specific circuit, as well as that of a particular phase. Furthermore, this capability will be extended in future work to include the estimation of the system's lifetime.

The characterization of energy consumption requires handling time on a very large scale. On the one hand, it is necessary to calculate energy consumption in the node pattern accurately using microsecond scale. On the other hand, the system's lifetime tends to be expressed in years.

Our approach can answer those issues because the simulation time is a linear gain between the pattern energy and the total amount of consumed energy.

However, for the moment, our approach cannot integrate non-repetitive or unpredictable activities. For example, whenever the network topology changes during the scenario, routing protocols should be activated to find new routes and these activities will consume energy in an irregular way.

Currently, we are developing a WSN simulator based on the proposed approach. The simulator will be designed for both researchers and WSN application developers. In parallel, we are developing a test bench based on physical nodes. The objective is to compare the simulation results with measurement from real applications, when the same scenarios are implemented in both environments.

Figure 2 .

 2 Figure 2. The Proposed Cross-level concept for Wireless Sensor Network

Figure 3 .Figure 4 .

 34 Figure 3. Cross-level design for parameters interaction in the RF unit

Figure 6 .

 6 Figure 6. Cross-level design for parameters interaction in the consumption in the sensing unit

Figure 7 .

 7 Figure 7. Block diagram for the proposed model

Figure 8 .Figure 9 .

 89 Figure 8. Energy consumption patterns for different wireless link protocols (Node B, Payload length = 100 Bytes, Fp = 1 Hz)

Figure 13 .Figure 12 .

 1312 Figure 13. Per-phase energy consumption from the fragmentation scenario for the RF unit

  • SL blocks: this is the level where information relating to the whole system is parameterized. Examples of parameters are the number of nodes and the distance separating them. This level includes two topology-related blocks: network topology and wireless medium blocks. The topology block includes a set of node blocks, each of which represents one node in the network, and all those nodes are node-level blocks. The wireless medium characterizes the communication channel.• NL blocks: these reside at the node-level and cover the parameters at that level, such as node position.There are three blocks at this level: node, type, and pattern blocks. The node block stands for the physical node in the modeled system. Each node is associated with a pattern and type blocks.

	The type
	serves a design requirement: heterogeneity. A type
	includes circuit blocks. Additionally, a type block
	can be shared among a set of nodes. The pattern
	block represents the node's periodic behavior. If
	two nodes share the same pattern, they will behave
	identically, in terms of activities (sending, sensing,
	etc.), but the consumption associated with each
	activity might be different based on other
	parameters, such as node positions. The type block
	also includes a battery, processing unit, RF unit, and
	sensors block. These are all circuit-level blocks.
	• CL blocks: these describe the hardware components
	of the node. The battery block is a circuit-level
	block. It represents the physical battery and
	includes energy parameters, such as the battery's
	nominal values. The circuit is a circuit-level
	abstract block, i.e., other blocks can be derived
	from it, they are the processing unit, RF unit and the
	sensors, each of which describes one specific
	component. These blocks include parameters
	related to the physical circuits. The parameters can
	usually be obtained from the circuit's datasheets,

like the power level. Finally, the RF unit block is associated with the protocol block. The protocol block describes the wireless link protocol used by the RF unit. It covers parameters related to the link protocol such as bit rates.

TABLE I

 I

	.	WIRELESS LINK PROTOCOL SETTINGS
	Parameter		IEEE 802.11a (HDG204)	IEEE 802.15.4 (CC2420)
	Bitrate [bps]		12 M	250 K
	Carrier Sense Mechanism	Pure CSMA/CA	CCA-ED
	Transmitter power [mW]	725	52
	Receiver Power [mW]	220	59
	Sleep Power [mW]		0.2	0.06
	TABLE II.	GENERAL SETTINGS OF THE RF UNIT ONLY
			SCENARIOS
	Parameter	Value
	Number of the nodes	2
	Node positions		(10,10), (10,20) [m]
	Scenario duration TSce	100 [s]
	Pattern Frequency Fp	0.1, 1, 2 [Hz]
	Payload length		10, 20, …, 100 [Byte]
	Link protocol		IEEE 802.11a, IEEE 802.15.4

TABLE IV .

 IV ELECTRICAL CHARACTERISTICS FOR THE PROCESSING UNIT AND THE SENSOR IN THE COMPLETE NODE SCENARIO

	Phase	TMP102	PIC16F4620
	Sleep [μW]	1.65	0.33
	Active [μW]	280.5	4290
	Figure 10. Per-component energy consumption profile for the
		complete node scenario

TABLE III

 III

		.	ENERGY CONSUMPTION OF THE PHASES IN
		DIFFERENT SCENARIOS (FP = 1 HZ)
			Consumed energy per phase [μJ]
	Simulation	Sleep	Exchange	Access	Ack.	Total
			IEEE 802.11a
			10 Bytes
	Matlab	51.03	50.08	23.63	199.959 324.69
	NS2	51.03	55.76	23.63	199.950 330.37
			100 Bytes
	Matlab	51.03	106.79	23.63	199.935 381.38
	NS2	51.03	112.46	23.63	199.927 387.04

IEEE 802.15.4 10 Bytes Matlab

		18.89	145.73	39.07	59.78	263.47
	NS2	18.88	152.84	39.07	59.78	270.47
			100 Bytes
	Matlab	18.89	465.37	39.07	59.44	582.77
	NS2	18.88	472.42	39.07	59.45	589.82

TABLE V .

 V ENERGY CONSUMPTION FOR THE NODE COMPONENTS IN ONE PATTERN FOR THE COMPLETE NODE SCENARIO. (PAYLOAD = 2 BYTES)

		Processing unit: PIC16F4620
	Phase	Sleep	Wake-up	Active
	Energy [μJ]	0.359	4.81 x 10 -3	143.03
		RF Module: CC2240
	Phase	Sleep	Access	Exchange	Ack
	Energy [μJ]	1.31	17.8	40.75	24.48
		Sensor: TMP102
	Phase	Sleep		Active
	Energy [μJ]	1.8		7.69

Figure 11. Block diagram for the proposed model

ACKNOWLEDGMENT

The work presented in this paper has been financially supported by the Regional Council of New Aquitaine (Call for Projects 2016 funds), as part of the OUDINI research project.