A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications

ARTICLE IN PRESS

JID: ADHOC

[m3Gdc;September 14, 2015;11:56]

Ad Hoc Networks xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Ad Hoc Networks journal homepage: www.elsevier.com/locate/adhoc

A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications✩ Ignacio Foche-Pérez a, Javier Simó-Reigadas a, Ignacio Prieto-Egido b,∗, Eduardo Morgado a, Andrés Martínez-Fernández a

Q1

a b

Department of Signal Theory and Communications, Rey Juan Carlos University, Camino del Molino s/n, 28943 Fuenlabrada, Madrid, Spain EHAS Foundation, Ciudad Universitaria s/n, 28040 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 6 February 2015 Revised 2 August 2015 Accepted 20 August 2015 Available online xxx Keywords: Multi-hop wireless networks IEEE 802.11 IEEE 802.15–4 Energy efficiency

a b s t r a c t Energy efficiency has become a regular hot topic in the research on wireless communications networks. However, existing proposals leave room for improvement when multi-hop wireless broadband networks are used to provide on-demand connectivity under low traffic load conditions. For this kind of networks, the capability of bringing the network up and down on demand wirelessly would help significantly to design energy-efficient, compact and low-cost network nodes. This paper presents a new approach for multi-hop energy-efficient networks based on a dual network architecture called dualWireless. Each node belongs to two parallel wireless multi-hop networks, an on-demand IEEE 802.11n network for data communications and a permanent IEEE 802.15–4 network for control purposes. The paper explains the proposal, the development of prototypes and the empirical validation with a testbed. In addition, a power consumption analytical model is used to measure the energy savings achieved by dualWireless and for comparison with other authors proposals. The dualWireless approach is shown to be more energy efficient than other alternatives as long as the occupation time of the network is under 40%, which is the case for many particular applications. © 2015 Published by Elsevier B.V.

1 Q2 2

3 4 5 6 7

1. Introduction Urban areas around the world tend to have full coverage of high speed networks through both wired and wireless access networks, and acceptable levels of connectivity are usually available in medium-size towns as well as along the roads. However, large rural areas that are not densely populated remain unconnected to terrestrial telecommunication

✩

This work has been supported by Research Project TUCAN3G IST-601102 STP, supported by the European Comission’s FP7 Programme. ∗ Corresponding author. Tel.: +34914888741. E-mail addresses: [email protected] (I. Foche-Pérez), javier.simo@ urjc.es (J. Simó-Reigadas), [email protected], [email protected] (I. Prieto-Egido), [email protected] (E. Morgado), [email protected] (A. Martínez-Fernández).

networks in most countries. Multi-hop wireless networks have been proven as a suitable solution to cover the increasing demand of ubiquitous broadband communications in these scenarios. These rural networks have a wide variety of applications that go from tele-educational or e-health services to environmental monitoring or remote video surveillance. Some of these applications only generate intermittent traffic with low-intensity, while others show long inactivity periods, for example at night. In these cases, the network wastes a significant amount of energy staying alive and idle just because it is not obvious how to bring it up on demand remotely. This fact is especially relevant when we take into account the figures from the International Communications Union, who indicates that 50% of the operating costs (OPEX) in rural networks are related to power consumption [1].

http://dx.doi.org/10.1016/j.adhoc.2015.08.028 1570-8705/© 2015 Published by Elsevier B.V.

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

JID: ADHOC 2

24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66

67 68 69 70 71 72 73 74 75 76 77 78 79 80 81

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

Many authors have already proposed different solutions to control the energy consumption of the nodes. Although their contributions are referenced in the next section, none of them propose a scalable and efficient solution for wirelessly controlling nodes in a multi-hop path. This paper proposes a solution called dualWireless that was inspired by the dual paradigm control network / data network, very common in network management architectures. Both the data network and the control network are wireless, using IEEE 802.11n and IEEE 802.15–4 respectively. Although the capacity of a network based on 802.15–4 is very low, it suffices for exchanging control messages. Hence, all of the nodes are dual, incorporating an embedded computer with 802.11n cards and a mote with a 802.15–4 controller. The control network is always available, with very low power consumption, while the rest of the system is powered on and off on demand by the control part. Simple commands and routing algorithms have been implemented in the 802.15–4 firmware to provide the functionality for propagating wake-up and shut down commands along defined paths. Nodes can automatically discover and establish links with their closest available neighbors, both at the data and control network level. In order to validate the proposal, several dualWireless nodes have been built, and a testbed consisting of an experimental chain network has been evaluated for more than one month. Measurements of power consumption and performance have been obtained, showing that our design may reduce the energy consumption significantly for many applications. A projection of these results would suggest that dualWireless could be advantageous even for more conventional applications such as WiFi networks in commercial or administrative buildings, where they are not used during the nights and weekends. The rest of the paper has the following structure. Section 2 summarizes the background and related work. Section 3 states clearly the problem and the requirements to be met by any possible solution. Section 4 gives a high-level description of the node and the network architecture. Section 5 describes the components of a node. Section 6 proposes a formal framework to analyze the power consumption and measure the energy savings systematically. Section 7 illustrates the testbeds deployed for proof of concept of the proposed solution. Finally, Section 8 presents the conclusions.

2. Related work Many works in scientific and technical literature have proposed mechanisms to save energy in WiFi systems. A few of them proposed solar-powered autonomous telecommunication systems. In these systems, the power consumption is critical because it determines cost and size of the solar panel and the batteries. The EHAS Foundation [2] developed a low-cost WiFi router with some self-configuring functions and QoS support. The node was powered with solar photovoltaic panels and used a distributed protocol called WiFiSleep, which put the network in standby mode, waking it up periodically. However, this solution was not proved and has many drawbacks such as high unavailability periods. Modern networks, as the one presented in [3], use the node but not the WiFiSleep protocol.

A similar project, called GreenWiFi [6], has developed an autonomous solar-powered WiFi router, in which the node can be programmed for being powered on and off. Also, the TIER group at Berkeley University [7,8] has used solarpowered WiFi nodes that enable the management of energy consumption through manual configuration. In these works, systems cannot be turned on or shut down on demand. Other research works focused on optimizing the use of 802.11 power saving mode (PSM) [9–11] combining it with traffic schedulers [12–14], saving a significant amount of power in the wireless interfaces, but requiring a nonstandard MAC and needing the embedded computer to be available all the time. Inside this line of work, some authors have specifically considered the impact on latency [15], that is one of the constrains considered in this study. Other authors have tried to save energy by adapting the transmission rate and other parameters to the traffic requirements [16–18]. These works don’t require MAC modifications, but they can only save energy in the WiFi cards. Other approaches like [19–21] have tried to save energy by changing higher level protocols or the network topology, but these systems need to be on all the time as well. Some researchers have applied a dual node architecture to save energy in mobile devices while keeping the connectivity (they turn-off the WiFi interface while there is no traffic, and during that period they use a control channel to maintain connectivity) [22–25]. The main difference between their work and our present proposal is that they do not turn-off any devices (only the interfaces), therefore the power saving is much lower (as will be shown in Section 6). The same approach was applied later in [26] to ad-hoc networks, but with the same limitations as the mobile version. Precedent works that are closer to our proposal, [27–30] propose high density WiFi infrastructures in which APs are woken up on demand; however, these nodes are wired, and wake-on-LAN methods are used in all cases (which is not possible in rural scenarios without wired infrastructure). A closer work [31] proposed to increase PDA-based phones (Personal Data Assistant) battery live (in Voice-Over-IP applications) by shutting down the device when it was not being used and waking it up through a wireless control channel when an incoming call is received. The use of the control channel is similar in the current work, but applied to a different scenario with different constrains (related to ad-hoc networks). A scenario very similar to ours, presented by Raman et al. [32], proposes the use of a Wake-on-WLAN schema [33], using dual nodes with a 802.15–4 mote and a WiFi system. However, both subsystems live alternatively, and systems along a path are woken up in sequence, not simultaneously, which makes the wake-up time of the network proportional to the number of hops, and therefore, not scalable to large networks. There were also reliability problems when deciding in the 802.15–4 mote that the main system had to be waken-up, though this was subsequently solved in [34] by introducing a signature for authentication. In [35], authors define a node design very similar in hardware to the one presented in this paper, although the network architecture and the software part are for delay tolerant networks. In [36] the authors analyze two approaches to reduce the energy consumption of networks. The first one is to adapt the rate of network operation to the required workload and is

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142

JID: ADHOC

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

160

not consistently supported in that work. The second is based on putting network components (not only wireless interfaces) to sleep during idle times, reducing energy consumed in these periods. The authors obtain energy savings over 80% with low traffic load subject to the existence of good predictions about when the systems need to be awake. Although these results are not immediately achievable, they will be helpful for comparison with this paper’s proposal. In the context of the related work presented in this section, this paper proposes and implements a solution that means a step forward to improve energy efficiency in multihop wireless networks. A set of prototypes have been built and tested for proof of concept. Measurements of the power consumption of each system have been taken in the different operational states, as well as the start-up time and other useful parameters. With those outputs, a power consumption analytical model is developed and used to compare this proposal with others.

161

3. Problem statement and proposal

162

Based on what has been previously exposed, this paper aims to propose a multi-hop broadband wireless solution that meets the following requirements:

143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159

163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199

1. The proposed solution must permit the exchange of highspeed data traffic over a multi-hop architecture. 2. In order to keep the spectrum of potential applications as open as possible, we assume that multi-hop paths may have a high number of hops. The network must be robust, and failure in one node must not interrupt the normal network operation if an alternative path exists. 3. The network must be very energy-efficient under low traffic load conditions. The power consumption must be reduced to a minimum when there is not any relevant traffic crossing the network. 4. The system must be able to recover from failure of one node in order to avoid a fast decrease of the end-to-end availability in multi-hop networks with many hops. Based on the first requirement, 5.6–5.8 GHz band is proposed for the communications network. The use of nonlicensed bands is a key for this decision, and the 2.4 GHz band is left for the control subsystem. Other technologies such as IEEE 802.16–2009 WirelessHUMAN might provide comparable speeds but the power consumption is usually higher. The second requirement itself suggests that each node must have at least two wireless interfaces for data switching/routing. When routing, if the same wireless interface is used for receiving/forwarding traffic, then the end-to-end capacity decreases very fast as the number of hops grow [5]. On the contrary, if incoming traffic and forwarded traffic involve different (non interfering) wireless interfaces, the capacity drops very little as the number of hops increases [3]. The third requirement must be studied more carefully. As it has been explained, WiFi can be considered as a relatively efficient technology in terms of power consumption. However, the applications suggested will produce intermittent network utilization, and consequently the network will be idle for long periods. As we have seen in the previous section, there are solutions for powering down the nodes while

3

there is not any traffic, but the problem arises when the network must be powered up on demand. The only previous solutions to have a start-up time proportional to the number of hops (Nh ), as it is formally calculated with (1), which enters into conflict with the second requirement:

Ta = (Ts + Tc )Nh ,

201 202 203 204

(1)

where Ta is the overall time to start up a whole path, Ts is the time to start up the wireless router, and Tc is the time to transmit the power-on command to the next hop. Our proposal will be oriented to obtain a multi-hop network in which nodes can be switched off by default, with a standby consumption close to zero. On demand, only routers belonging to the required path would be switched on. Secondly, the switching times should remain almost independent on the number of hops so that the time for a whole path to come to life is given by:

Ta = Tc Nh + Ts .

200

205 206 207 208 209 210 211 212 213 214

(2)

In order to do so, there must be a control network that acts almost simultaneously on all the nodes along a path. The control network needs to be available at any time in order to propagate power-on commands issued upon traffic demand and effect the switching-on. Fig. 1 illustrates the proposal. In the figure, a surveillance model is included as an example of application-dependant module that could be integrated in some nodes. This control network requires a very low-power wireless technology whose marginal power consumption in the whole system may be neglected. In addition, it must permit the establishment of control links parallel to those supporting data traffic. A very interesting technology that may meet these requirements is IEEE 802.15–4. Conceived for wireless sensor networks, 802.15–4 motes usually have ultra-low power consumption. Non-licensed channels are used as well (5 MHz wide channels in the 2.4 GHz ISM band), and its low capacity is enough for control purposes. In principle it is feasible, with a careful design, that a 802.15– 4 network be established in parallel to a 802.11n/e network, being the first for control and the second for data switching. If the 802.11 subsystems can be powered on physically by the 802.15–4 subsystems on demand, the expected result could be achieved.

215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237

4. Node architecture and network operation

238

A formal description of the dualWireless node architecture and the network operation is presented in this section. The objective is to describe clearly what are the elements and the connections among them, and propose a notation that can be used later in the power consumption analysis.

239

4.1. Node architecture

244

The dual network has N nodes ni , ∀ 1 ≤ i ≤ N. Each node ni is formed with a IEEE 802.11n/e subsystem tagged as di (for data subsystem) and a 802.15–4 subsystem named ci (for control subsystem). Subsystems di , ∀ 2 ≤ i ≤ N, have two wireless interfaces wi,1 and wi,2 whereas n1 only has w1,2 as explained in the next paragraph. All di have two ethernet interfaces ei,1 and ei,2 , the first one being normal and the second one with PoE (Power over Ethernet) capabilities,

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

240 241 242 243

245 246 247 248 249 250 251 252

JID: ADHOC 4

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

Fig. 1. Logical schema of the dual wireless router.

253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281

so that external devices can be plugged to ei,2 and get data connectivity and power from di . All ci have one 802.15–4 interface zi . Node n1 will be different from the rest because it acts as the gateway between the rest of the network and the outer world, and it is supposed to be plugged to an external power supply. The whole node n1 is in on state all the time and does not need the w1,1 interface because its upstream connection to the outer world is through e1, 1 . Subsystem di , ∀ 2 ≤ i ≤ N, may be powered on and off with the help of an actuator controlled by ci . di and ci are connected through a local serial data bus (cabled) while di may connect to any neighbor dj , j = i through a high-rate wireless connection and ci may connect to any neighbor cj , j = i through a low-power wireless connection. Hence, di are normally off (except for d1 ) and go on only on demand. ci activates di when a command is received signaling that ni must become available for data traffic. di is powered off when a command is received in ci indicating to do so. Those commands may be generated in several ways, e.g.: • Any other node may issue that command, and particularly n1 that may offer a management service to an operator for powering on and off any path in the network. • Subsystem di may monitor the traffic going through it and hence it can issue the power-off command when it is no longer needed. • A software agent in any ci processing events from a sensor might generate power-on commands in order to send an alarm or a report to the network operators.

282

4.2. Network operation

283

Let us consider a dual wireless mesh network in which every node belongs to two parallel networks: a control network with very low power consumption and low bitrate, and a data network with higher power consumption and high bitrate. A

284 285 286

basic condition for a consistent operation in the dual network is that radio subsystems are designed in such a way that the set of nodes seen as first neighbors by the data subsystem di is a superset of the set of nodes seen as first neighbors by the control subsystem ci . Hence, any route found from one node to another in the control network must also exist in the data network. A second condition is related to faulttolerance: which has been said for first neighbors must also be true for second neighbors, so that the failure of a single node in a path may be solved by connecting directly to a node that was previously a second neighbor. Although the most general topology for a typical ad-hoc network is a mesh topology, this paper assumes a tree topology at the control network, proposing a balanced solution between complexity and generality based on the following considerations: • Chain networks or tree structures are very typical when it comes to extend connectivity from well connected locations to remote isolated positions [4]. • The concept of network topology itself is diverse in this paper. There is a control network topology that determines how the control subsystems are connected among them. Then, there is a data network topology that determines how the WiFi subsystems will be connected among them when powered on. • The data network has a mesh topology due to the basic conditions exposed above. A dynamic ad-hoc routing protocol with an appropriate metric permits that the nodes that are alive establish the optimal path dynamically, leaving aside any node that is not really required, or even including other nodes that are permanently alive like n1 if they exist and improve the path. There is no reason to simplify this assumption, as there are well tested implementations of appropriate existing routing protocols like OLSR with ETX metric.

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302

303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321

JID: ADHOC

ARTICLE IN PRESS I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

[m3Gdc;September 14, 2015;11:56] 5

Fig. 2. Example of the network operation in three basic stages. Although all nodes have all the elements explained in the text, only the active interfaces are represented in each node for the sake of clarity. Permanent Ethernet links are represented with straight red lines, while 802.15–4 wireless links are represented with blue discontinuous lines. Red arrows represent active 802.11 connections. Subsystems that are powered off are marked with a grey background color. The icon that ends the figure on the right represents a webcam, which is just an example of device that needs to be accessed through the network.

322 323 324 325 326 327 328 329 330 331 332 333 334 335 336

• The control network just needs to be efficient at exchanging short control messages with the nodes belonging to the optimal route between any pair of nodes that need to exchange traffic. The most general and flexible approach to find the optimal route would be using a dynamic routing protocol for mesh topologies with an appropriate metric for wireless networks in the control network. However, this would make much more difficult to accomplish the basic conditions indicated above in the first paragraph of this Subsection. In addition, the first consideration explained above has motivated a most simpler and practical approach, so that the focus of this paper is kept on the node architecture, the empirical proof of concept and the power consumption analysis.

Hence, a tree topology is assumed for the control network, which only has consequences in the routing protocol designed for the control network, and a mesh topology is assumed for the data network. Future works may extend this paper’s proposals to a more general mesh case. Fig. 2 illustrates a typical sequence of operations in a network. In Fig. 2(a), the ci subsystems are alive and connected in a tree topology, while all di are off. In Fig. 2(b), a command has been issued for bringing up the path from n1 to n7 . The command propagates through the control network and all the nodes in that path switch on the di subsystems. Once all the nodes are on, the WiFi connections are established and Internet Protocol (IP) routes have been dynamically created, traffic is normally exchanged. Fig. 2(c) illustrates the recovery

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

337 338 339 340 341 342 343 344 345 346 347 348 349 350 351

JID: ADHOC 6

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

Fig. 3. Cabling among elements in an dualWireless node.

363

from a node failure. Node n5 fails but, as every node sees the second neighbors, n7 connects to n3 directly and end-to-end communication is recovered after a few seconds. Once each node ni detects that the traffic is finished, di issues a command to ci to be switched off. Only with the previous description, one can imagine many interesting applications for this architecture. However, the usefulness of the proposed solution depends very much on the energy savings and the end-to-end time-to-operation. Therefore, the operation of the proposed solution must be further analyzed in terms of availability periods and energy savings.

364

5. Hardware and software design of a dual node

365

Once the expected operation of dualWireless systems has been described, the details for building such systems are now presented. A complete set of dualWireless nodes have been built as a proof of concept in order to demonstrate that our proposal is feasible. In addition, building the systems for real will permit the measurement of times and power consumptions that will be later be needed for analyzing the power consumption. While Section 4 focuses on the details of the node design, the testbeds deployed for proof of concept of the dual network operation are described in Section 7.

352 353 354 355 356 357 358 359 360 361 362

366 367 368 369 370 371 372 373 374 375

5.1. Hardware architecture

376

A prototype of node has been designed according to the schema previously introduced in Fig. 1. There are two main hardware subsystems representing the broadband communications subsystem and the control subsystem, connected through a serial communications bus and, additionally, the powering line for the first being switched on and off by an actuator embedded in the second. Both subsystems are

377 378 379 380 381 382

integrated in an outdoor enclosure, and both are powered with a photovoltaic subsystem. The enclosure chosen is an Acconet AN5820-WB that integrates a 20dBi directional antenna, connected with a pigtail to interface w1 except for n1 , where this antenna is connected to w2 . The interface w2 is connected through an external connector to an ELBOXRF antenna with 19dBi gain. The zi interface is also connected to an external 5dBi dipole through another external connector. The antennas have been chosen to balance the link budget of both the data and the control links. Due to the big difference in the sensitivity between 802.11n interfaces (−87 dBm at 6 Mbps bitrate) and the 802.15–4 interface (−100 dBi), and accounting for the higher propagation loss in the 5 GHz band, the control channel can use small omnidirectional dipoles and still get the same range as the data channel with directional antennas with much higher gain. A simple calculation with the free space propagation model permits to obtain these results. Some nodes may also incorporate an additional application subsystem such as a video camera or a sensor node. This would be connected to an Ethernet port in the enclosure that corresponds to the e2 interface, which is prepared for PoE (Power over Ethernet). The other Ethernet connector corresponds to the e1 interface, which is just for communications purposes; although only the gateway node uses this interface regularly, ei,1 permits physical access to any node for maintenance when the wireless interfaces fail. Fig. 3 shows the interconnection among all these elements and Fig. 4 provides the detail of the electrical connections among them. The broadband communications subsystem is made up with a PCEngines ALIX3D2 embedded computer having a Linux operating system installed in the CompactFlash card. This node has two Mikrotik r52 WiFi interfaces installed in

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417

JID: ADHOC

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

7

Fig. 4. Electrical connections among elements in an dualWireless node. Due to the DC powering, positive and negative lines are differentiated.

Fig. 5. Communications enclosure with the communications subsystem and the control subsystem already mounted.

418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435

both mini-PCI slots. One of them is connected to the antenna embedded in the enclosure, and the other one goes to an external SubMiniature version A (SMA) connector. The control subsystem is make up with a Libelium Waspmote having a Digi XBee Pro 802.15–4 radio and an extra auxiliar card with the actuator that may switch the power on and off for the previous subsystem. An additional Traco Power TSR 1–2433 DC/DC converter is installed on this subsystem for adapting the 12 V DC coming from the photovoltaic subsystem to the 3.3 V required by the waspmote. Besides the necessary cables and connectors the system was completed with the powering subsystem, made up with an ATERSA A-40P solar panel, an ATERSA Mino V2 regulator and an ATERSA S12/41A battery. The last two elements were installed in a Himel PLM-43 enclosure. This subsystem can be significantly reduced for most real applications, but the prototypes were over dimensioned for the sake of flexibility.

Figs. 5 and 6 show the elements as they are placed in the two enclosures described above.

436

5.2. Software components in the communication subsystem and self-configuration strategies

438

The operating system used is Voyage GNU/Linux 0.6.5, well suited to be embedded in the Compact Flash card and flexible enough for installing additional packages. Several additional third-party packages were added:

440

• pmacctd: traffic monitor that may trace the different classes of traffic being routed by a node, and issue commands based on timeouts after the last packet of each class. It is mainly used to monitor the traffic in order for the node to know when a power-off order must be issued because a previously defined idle time has elapsed. This monitor is able to distinguish between traffic that has to

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

437

439

441 442 443 444 445 446 447 448 449 450

JID: ADHOC 8

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

Fig. 6. Powering enclosure.

451 452 453

•

454 455 456

•

457 458 459 460

• •

461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491

•

be considered and other packets that must be ignored for this purpose. olsrd: dynamic routing control daemon. It uses the Expected Transmission Count (ETX) metrics, well suited for wireless mesh networks. dnsmasq: permits to assign IP address to child nodes that connect as station (STA) to the second (Master) WiFi interface. ifplugd: permits to monitor the link state. hostapd: for WiFi Protected Access (WPA2) authentication and Access Point (AP) management. wpa_supplicant: client for WPA2 authentication.

In addition, a few configuration scripts have been customized and some additional modules have been developed. A daemon named eWifid has been programmed to issue control commands to the waspmote through the serial port. Several self-configuration strategies have been introduced for the sake of robustness. The aim is not as ambitious as could be in SON (self-organized networks), because the network is supposed to maintain a tree topology and nodes are prepared for certain relative positions in the network. However, the aim is to resist to node failures, giving the nodes the capability to regenerate a path through a second neighbor if a first neighbor fails. IP addresses are the first aspect considered for selfconfiguration. Each node has two IPv4 address. The first interface (disabled for the first node) is configured in STA mode and connects to the best AP with SSID=dualWireless. The IP address is obtained with Dynamic Host Configuration Protocol (DHCP) from the AP. The second interface is configured as AP and gets an automatic IP address that is built as 10.X.Y.1, being X.Y the last 16 bits of the MAC address. Of course, this mechanism only works properly if all the WiFi cards used are manufactured by the same vendor. The DHCP daemon is self-configured to offer 10.X.Y.2–254 addresses to any stations connecting to the AP. In addition, a “black list” with the MAC address of the first WiFi interface is given to the second in order to avoid loops. Only the first node need to be manually configured to give an appropriate IP address to the passive Ethernet interface. This first node also incorporates an embedded web server

and proxy that can be programmed for accessing servers of application devices through the network. A special web form is also included in the first node that, together with a cgi script, permit the passing of commands to the eWifid daemon just accessing this node remotely with a web navigator, which in turn communicates through the serial port with the control subsystem.

492 493 494 495 496 497 498

5.3. Software for the control subsystem

499

A special firmware has been developed for the control subsystem in this project based on the Waspmote Application Programming Interface (API) and the control libraries provided by the manufacturer for the different components. The objective of this software module is to transform the 802.15–4 node in an efficient control module that propagates commands to the right neighbors as appropriate. A specific protocol with short messages to transfer commands has been designed for this work, and the software required for issuing, parsing and executing these commands has been developed. Although this development has been complex itself and the details go beyond the scope of this paper, the basic operation is as follows:

500

1. When the node comes to live, it sends an “I’m alive” broadcast message, and it keeps doing so once per minute. 2. The control subsystem keeps listening for any incoming commands. If a command is received, it is parsed and, depending on the range of addresses specified in it, it can be executed and/or forwarded to a neighbor. 3. A table of neighbors is maintained. Neighbors are introduced when commands or “I’m alive” broadcast messages are received from them. Neighbors are eliminated when these messages are not received for a configurable period of time. 4. Commands destined to the present node other than the “On” command are forwarded through the serial bus to the communications subsystem, where it is processed by the eWifid daemon. 5. Commands received from the serial bus are also processed as the ones arrived from remote nodes.

513

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

501 502 503 504 505 506 507 508 509 510 511 512

514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530

JID: ADHOC

ARTICLE IN PRESS I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554

The following commands are implemented and programmed both in the eWifid daemon and in the control firmware: • IMALIVE → For sending broadcast “I’m alive” messages. • KEEPALIVE → With a range of nodes, it disables the algorithm that controls the traffic and issues an “off” command after a certain idle time. • AUTOOFF → Re-activates the algorithm disabled by the previous command. • ALIXON → Switches on the communications system if the present node is in the address range and forwards the command to neighbors whose addresses are in that range. • ALIXOFF → Switches off the communications system if the present node is in the address range and forwards the command to neighbors whose addresses are in that range. • SWITCHUART1 → The serial bus has two Universal Asynchronous Receiver-Transmitter (UART) ports. This commands switch to UART1 the serial port in which the control subsystem waits for commands. • SWITCHUART2 → The serial bus has two UART ports. This commands switch to UART2 the serial port in which the control subsystem waits for commands. Packets exchanged among waspmotes have the following syntax: NIDO=nodeID; BAT=bat; COMMAND=Command∗

581

Commands all have the syntax “command|interval1| interval2∗ ”. The waspmote is given an address of the form “uppertree-branch:leaf#”. For example, “1:3-2:5#” would be the leaf 5 in the 2nd branch, connected upstream to node 3 of the 1st branch. This addressing strategy and the routing protocol developed have advantages and drawbacks compared to Zigbee. On the one hand, the addresses indicate a fix position of the node in a tree topology, imposing a restriction that prevents self-configuration. On the other hand, there is no need for discovering neighbor nodes, and messages are forwarded to neighbors only when it is strictly necessary (when the message should be received by a neighbor different from the one the message comes from). This increases performance efficiency both in terms of throughput -higherand latency -lower-. In addition, this addressing strategy imposes a logical topology, that must be a subset of the physical topology in which some physical neighbors are ignored, thus permitting to control what nodes are seen as neighbors. This is an important tool that permits to assume the topology of the WiFi network, which is very much conditioned by the use of directive antennas, and impose that topology to the control network, in which 802.15–4 subsystems may see more neighbors through omnidirectional antennas. In summary, this addressing strategy is not good for self-configuration, but permits the very efficient operation of the the control channel. If self-configuration is really a priority, the use of Zigbee with broadcast messages should be explored instead.

582

5.4. Operation foreseen for a node

583

Any node in the network is initially in standby mode except for the first one. This means that the control subsystem is alive and the rest is powered off. When a user (human or software) wants to wake up a path in the network,

555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580

584 585 586

[m3Gdc;September 14, 2015;11:56] 9

an ALIXON command with the appropriate address range is issued. The command propagates through the path in a few milliseconds and the control subsystems of the nodes in that path switch them on almost simultaneously. Once the nodes are on, the wireless interfaces detect neighbors and establish connections. Then, IP addresses are assigned or obtained for the WiFi interfaces, and once this point is achieved the routing protocol can perform its initial tasks until routes converge and the user gets end-to-end connectivity. Traffic can then go through the network as desired. When the path remains idle during enough time, nodes issue an ALIXOFF command to themselves. Alternately, this command can be manually issued by the user. 5.5. The routing strategy As explained in Section 5.3, the routing protocol in the control network has a hierarchical nature. Each node’s address indicates in which branch of the tree it is situated, what is its position in the branch and how the branch connects to the rest of the network. Hence, the route between any pair of nodes is deterministic and, at the same time, fault tolerant. For example, a message that has to be forwarded from node 1 : 1# to node 1 : 2 − 2 : 5# has to follow the path: 1 : 1# → 1 : 2# → 1 : 2 − 2 : 1# → 1 : 2 − 2 : 2# → 1 : 2 − 2 : 3# → 1 : 2 − 2 : 4# → 1 : 2 − 2 : 5#. However, if one of those nodes fails, the message would keep being forwarded as long as all nodes are suposed to see their second neighbors. At the data layer the routing philosophy is very different, as an ad-hoc dynamic routing protocol is used. Whatever set of nodes are alive at any time, they will have the capability of establishing the best possible path for the pair of nodes that have to be connected. OLSR with metric ETX has been chosen because there are very solid implementations of OLSR, it is an appropriate proactive state routing protocol for static wireless networks and the ETX metric takes into account the quality of wireless links together with the number of hops, which makes the route decisions very close to optimality. In addition, if the best route in the data network contains a subset of the nodes that have been waken, the nodes that are not needed are not used and may power off themselves after a period of inactivity. Moreover, if one or more nodes are connected to permanent energy sources and do not need to incorporate the control subsystem because they are permanently active, they could be incorporated seemlessly in the data network, opening the doors to heterogeneous networks in which dual nodes coexist with simple nodes. Once dual nodes in a path are alive, both dual and simple nodes may collaborate to establish the best path. Then, nodes that are alive but left out of the path stay idle, and those among them that are dual nodes will eventually power off themselves.

587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636

5.6. The cost of a dual node

637

The cost of a dual node may vary significantly depending on many factors, and the specific decisions taken in this research for all the components of a prototype may not be generalized, as other hardware choices may drive to different cost structures. However, assuming these limits, it is useful to offer some simplified figures that allow to estimate how

638

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

639 640 641 642 643

JID: ADHOC 10

644 645 646 647 648 649 650 651 652 653 654 655 656 657

ARTICLE IN PRESS I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

much the cost of a node is increased because of the introduction of the control subsystem. The dual nodes built in this research had a long list of components that could be grouped in three sets: • Elements that would be in a similar outdoor wireless router that does not incorporate a control subsystem, excluding those belonging to the powering system. The cost of this set of elements in a dual node prototype was $564. • Elements that are included specifically to introduce the control subsystem, excluding those belonging to the powering subsystem. The cost of this set of elements in a dual node was $214. • Elements belonging to the powering subsystem. This set of elements costed $521 for each prototype.

6.1. Power consumption model

667 668 669 670 671 672

676 677 678 679

682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699

3.48 2.86 3.22 4.02 5.17 2.72

4.17 3.42 4.17 4.91 6.87 3.30

0.336 0.116 0.175 0.207 0.306 0.098

1. Total Time (Tt ). Duration in seconds of the total time of observation. It is assumed that this time is chosen between two instants in which the network is idle. 2. Start-up Time (Ts ). Time in seconds required by the dualWireless node ni to switch on di and get ready to exchange IP packets with neighbors. 3. Wait Time before sleep mode (Tm ). After a transmission period in a dualWireless node, this is the idle time that

681

666

2.48 2.78 3.13 3.62 4.59 2.66

S1 It corresponds to the state of the WiFi router when it is booting up. S2 WiFi router on, with only wi,1 installed but idle (no traffic is being routed). S3 WiFi on with both radios wi,1 and wi,2 installed but idle. S4 WiFi on with only wi,1 wireless interface installed. Traffic is being routed through ei,1 and wi,1 with the higher MCS (Modulation Coding Scheme).

680

The power consumption of the proposed solution is analyzed in this section with a simple analytical model. Then, the model is fed with the power consumption levels and times measured in the prototypes built. This permits to provide some results on the power savings of this proposal and the comparison of them with other related works.

665

Std.

We need to characterize the power consumption of a dualWireless node as well as that of a typical 802.11n system in order to compare them. In order to do so, we will firstly measure the power consumption of a 802.11n system in five defined states, which will be later combined to estimate the consumption in normal operation. We use as reference a wireless router with two WiFi radios (wi,1 and wi,2 ) and two ethernet ports (ei,1 and ei,2 ). The defined states are related with the main operational phases of the router, which will have different power requirements:

675

664

Max.

704

6. Power consumption analysis

663

(boot) (idle 1) (idle 2) (AP) (routing) (idle)

Mean

The extra sixth state (S6) does not exist in dualWireless systems, but it is defined in order to make possible the comparison with some related works in Section 6.3. DualWireless systems may be in states S1, S2 and S4 in the case of end systems with only one wireless interface (e.g. the gateway), or in states S1, S3 and S5 for the rest of nodes. The consumption of a node has been measured in all those states by using a high performance programmable multimeter that sends results to a computer through a serial port. States S4 and S5 have been reproduced by injecting User Datagram Protocol (UDP) traffic through the node beyond the saturation point of the wireless interfaces. The iperf traffic injector has been used for this. Resulting measurements for each state are shown in Table 1, including minimum, mean, maximum and standard deviation values for all the states. These measurements have been done using the maximum transmission power of the WiFi radios because the solution proposed is oriented to long distance links, where the maximum transmission power is usually used. The second step is to characterize the 802.15–4 subsystem consumption. This subsystem has two main states in this application: (Pidle ) transmitting with 802.15–4 protocol and keeping the WiFi system in off state; and (Prelay ) keeping the WiFi system in on state by means of the relay circuit. We measured the mean power consumption in both situations and we get that the consumption in Pidle is 0.312 W, and the consumption in Prelay is 0.792 W. The introduction of more states associated to the transmission activity in the 802.15–4 radios has been dismissed for the sake of simplicity, as measurements show that the transmission of control messages has not a significant impact on energy consumption in dualWireless systems. Considering this information, consumption models for both the WiFi and the dualWireless networks are going to be built using the following parameters:

674

661 662

PS1 PS2 PS3 PS4 PS5 PS6

Min.

700

673

659 660

Table 1 Power consumption measurements. This table shows the power consumption measured with a high performance programmable multimeter. Values are provided in Watts (W).

S5 System on, both wi,1 and wi,2 installed and traffic is being routed through wi,1 and wi,2 with the higher MCS. S6 System on, without radio interfaces and with no traffic being routed.

These figures seem to reveal that the control subsystem increases the price of a prototype in 20%, but this is not the case. The powering subsystem is dimensioned for 3 h of daily use in the prototypes built. A node that cannot be remotely activated because it lacks of a control subsystem must have a powering subsystem dimensioned for 24 h of operation, resulting in an overcost of almost $200 compared to a dual node. Accurate cost calculations reveal that a dual node dimensioned for less than 15.7 h of daily operation is cheaper than a simple node dimensioned for permanent operation. Of course, if nodes are to be powered with an external source and the powering subsystem is not included in the cost calculations, the dual node is then 38% more expensive than an equivalent simple node. This makes interesting the possibility of establishing heterogeneous networks formed by dual and simple nodes.

658

[m3Gdc;September 14, 2015;11:56]

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

701 702 703

705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746

JID: ADHOC

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

747 748 749

4.

750 751

5.

752 753 754 755 756

6.

757 758 759 760 761

7.

has to elapse since the last packet was routed until the system goes off. Switching-on rate (λ). Number of times per second that di goes from off state to on state. Activity rate (tx ). Fraction of the total time that the WiFi router will be on and actively involved in transmission activity. The maximum time that a WiFi node can spend transmitting is Tt − λ(Ts + Tm ) (the total time less the time spent in booting and waiting for sleep). Real throughput (S), where Smax is the maximum end-toend throughput that can be achieved. Number of nodes in the network (N), not including the gateway node.

(3)

762

s = λTs ,

(4)

763

δtx = S/Smax .

(5)

764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782

δ tx represents the intensity of use of the radio during tx , 1 being the highest possible value that corresponds to the saturation throughput. The value of δ tx depends on the type of traffic: video traffic will imply a heavy traffic load requiring a high δ tx , whereas a sensor monitoring session represents a light traffic case with low δ tx . In both cases the router will be required to be on for a fraction of time tx , and the radio will be really transmitting for a fraction of time δ tx tx . Using the previous parameters, the power consumption of a network is modeled below. Firstly, we will consider a network of common 802.11n/e nodes ni that are only formed by the di part and are always on. This nodes have two basic states (transmitting and idle). The gateway differs from the rest of the network in that it has one 802.11n radio (6), while the other nodes have two 802.11n radios (7). The WiFi radios will be on a fraction of time tx δ tx , while the WiFi router will be idle but available for transmitting a fraction of time (1 − δtx tx ). Thus, the power consumption of the gateway (Pgw ) and the rest of the nodes (Pn ) are obtained as Pgw = tx δtx PS4 + (1 − tx δtx )PS2

783

785 786 787 788

790 791 792 793 794 795

Tm Ts

δ tx

N

λ

Set 1

Set 2

Set 3

Set 4

Set 5

10 42 0.1 10 3,10,20,25

10 42 0.1 2,3,5,10,15 10

1,10,100 42 0.1 10 10

10 0.69,42,92 0.1 10 10

10 42 0.01,0.5,0.8 10 10

Table 3 Zero saving points. This table presents the points where the combination of the fraction of transmitting time (tx ) and the number of accesses in one hour (λ) make the power saving equal to 0%.

λ

tx

3 10 20 25

0.71 0.61 0.46 0.15

be on Pidle state. Thus, the power consumption for the waspmote subsystem (Pwasp ) can be estimated as

796 797

Pwasp = (tx + s + m )Prelay + (1 − tx − s − m )Pidle , (9) We can obtain the power consumption of the dualWireless  ) and the rest of the dualWireless nodes (P  ) as gateway (Pgw n

P

gw

= Pgw + Prelay

798 799

(10)

and

800

Pn = (tx δtx PS5 + (m + (1 − δtx )tx )PS3 + s PS1 ) + Pwasp ; (11) where, to obtain (11), note that, in the rest of dual nodes, the WiFi system will only be activated during s = λTs , m = λTm and the active rate tx . Therefore, the power consumption of the whole dualWireless network (PT ) is obtained as  . PT = NPn + Pgw

801 802 803 804 805

(12)

6.2. Analysis of the model results

806

(7)

where PSX represents the power consumption in state SX, with X ∈ {2, 3, 4, 5}. Then, the total power consumption in the 802.11n/e scenario (PT ) is the combination of the power consumption of all the WiFi nodes in the network (the gateway plus N WiFi routers):

PT = NPn + Pgw . 789

(6)

and

Pn = tx δtx PS5 + (1 − tx δtx )PS3 , 784

Table 2 Sets of parameters used to feed the model for the different results represented in Figs. 7,8,9,10, and 11.

We also define some intermediate variables to simplify the equations:

m = λTm ,

11

(8)

Now, let us consider a network formed with dualWireless nodes. Each node ni has di and ci subsystems, and the di part can be switched on and off except for the gateway, where d1 is always on. It is important to note that the control subsystem ci should be always activated in all the nodes. When the WiFi router is starting, transmitting or waiting to go to sleep, the control subsystem is in Prelay state. The rest of time it will

In this subsection we are going to analyze the results provided by the model developed in the previous section, and we will try to understand the influence of each parameter. Several parametric representations of the consumption model will be presented, and the parameters for each of them are provided in Table 2. First, the power saving between the dualWireless network (Pt ) and the usual WiFi network (Pt ) is calculated in function of the active rate tx for different values of λ (rate of starts and shut down). Results are shown in Fig. 7, where we can observe that saving for small values of tx and λ is among 60% and 80%. As these parameters are increased, the power saving ((1-Pt /Pt )∗ 100) becomes smaller and eventually reaches the 0% (Table 3 gives tx and λ values for which this happens). High values of tx or λ imply an intense use of

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

807 808 809 810 811 812 813 814 815 816 817 818 819 820 821

JID: ADHOC 12

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

Fig. 7. Power consumption savings evolution with respect to λ.

Fig. 9. Power saving dependence with Ts .

Fig. 10. Power saving dependence with Tm . Fig. 8. Power saving dependence with N.

822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840

the WiFi subsystem, making the savings negligible or even negative. Negative savings are due to the additional power consumed by the waspmotes. The dualWireless nodes spend time in booting and waiting before going to sleep, that’s why in some cases the curve doesn’t reach the end of the x-axes. In a second set of calculations, the influence of the number of nodes (N) is studied in Fig. 8, which shows the power saving versus tx for different values of N. It can be observed that when N increases, the power saving of the dualWireless system also grows up, because the cost of having the gateway on permanently is mitigated by the saving in the rest of nodes. However, this improvement is significant only up to N = 10. The impact of the start time Ts and the waiting time to sleep Tm is reflected in Figs. 9 and 10, respectively. If Tm values are not too big, it has low influence on the overall performance of the system from the energy point of view. However, TS can impact on the power saving if very slow boot devices are used. These parameters are also important from

the application perspective. As an example, applications with very strict time access requirements will need to use WiFi routers with low Ts . In the same line, if the number of connections to the system (λ) is very high, it would be also interesting to have routers with low Ts . Finally, the influence of δ tx proves to be negligible as shown in Fig. 11, because the difference in power consumption between transmission and idle states is small in the WiFi subsystem. 6.3. Comparison with previous work’s proposals In this section the solution proposed in this paper will be compared with others that have been introduced in Section 2. The most representative models of previous works have been compared in regards to the energy savings that could be achieved with them on a common WiFi network. The works compared here use similar models to analyze the power consumption of a network, with small modifications to adapt the model to each scenario. For example in our case,

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

841 842 843 844 845 846 847 848 849

850 851 852 853 854 855 856 857 858

JID: ADHOC

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

Fig. 11. Power consumption savings evolution with respect to δ tx .

859 860 861 862 863 864 865 866 867 868 869 870 871 872 873

an additional term had to be added to include the power consumed by the waspmote. The models selected are: • In [13] the WiFi Radio is turned on only when there are packets to be transmitted or received, keeping the router in idle mode and the WiFi radios off (PS6 ) the rest of time (1 − δtx tx ). The advantage of this mechanism is that the time required to start a transmission is smaller than the dualWireless one (the time to start the radio is much smaller than the time to start the whole device). However the WiFi router is never off, so the network always have a flat consumption equivalent to Pidle . This mechanism will be identified as “Radio Sleep”, and the translation of that mechanism to our model is represented by (13), where the time to turn on the radio is supposed to be neglectable compared to the start time of the whole WiFi router.

Pn = 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895

tx δtx PS5 + (1 − δtx tx )PS6

(13)

• In [36] the authors propose two mechanisms to reduce the energy consumption of a WiFi node. First, they assume that it is possible to put the network routers in sleep mode and wake them up automatically when transmissions or receptions are expected. This assumption is not of practical use because there aren’t any predictive mechanisms that can implement this currently for most applications, but the theoretical approximation will be used as reference for comparisons. The main difference with our proposal is that it tries to predict the traffic based on historical data, keeping systems in sleep mode when no transmission is expected. The capacity of traffic prediction makes unnecessary the addition of control subsystems, permitting to save more energy. This mechanism will be identified as “Device Sleep”, and its translation to our model is presented in (14). This work also assume that link rate adaptation may also contribute to save energy, especially under heavy traffic workloads (which is not the case that we tackle in this paper). This part of the proposal in [36] is not sufficiently supported and hence will not be considered for the comparison in this section. However, future works may analyze more accurately the impact of

13

Fig. 12. Comparison with the ‘device sleep’ mechanism in [36].

link rate adaptation in dualWireless systems.

Pn =

tx δtx PS5 + (1 − δtx )tx PS3 + PS1 s

896

(14)

• The power saving of the dualWireless network is calculated considering λ = 10 connections in one hour, Ts = 42 seconds and T m = 10 seconds.

897 898 899

Results of this comparison can be seen in Fig. 12, where it can be seen that the dualWireless network provides a higher energy saving than the Radio Sleep mechanism if the percentage of transmission time is lower than 55%. When comparing the dualWireless curve with the “Device Sleep” curve, it can be observed that this paper’s proposal has a similar behavior, although the consumption of the waspmote and the wait time before sleep make our solution less energy efficient. From this Fig. 12, it can be concluded that the proposed solution provides an efficient power saving mechanism for low traffic load conditions. Regarding the rate adaptation mechanism, an estimation has been done using a transmission rate of 12 Mbps (instead of 54 Mpbs), but no saving is observed in this analysis. The reason is that the WiFi routers used in this paper do not match the rate adaptation assumption: its consumption doesn’t decrease quadratically with the rate. Therefore, using rate adaptation with our devices actually increases the power consumption. However, the rate adaptation mechanism is in theory compatible with the mechanism prosed in this paper (if the WiFi router supports it), and it would be possible to benefit from both approaches at the same time.

900

6.4. Reference case studies

922

In this subsection two possible scenarios for the proposed solution will be presented. The first scenario is a sensor network used for monitoring purposes. Typically, a sensor sends periodic information with low transmission rates, so it would be feasible to shut down the WiFi routers between transmission periods. One of the scenarios used as reference is a group of five meteorological stations placed in Doñana National Park in Spain. The stations send information about sun radiation, humidity, wind speed and precipitation two times per

923

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921

924 925 926 927 928 929 930 931

JID: ADHOC 14

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

Fig. 13. Testbed for the first set of scenarios for validation.

932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965

hour, and the duration of each transmission is one minute. With this information, it is possible to apply the model previously explained, being the complete set of parameters: • • • • • • •

Tt = 3600 s Ts = 30 s Tm = 5 s tx = 2 ∗ 60/3600 δ tx = 0.02 N=5 λ=2

The result of the model for this case will be that Pt = 19.08W, Pt = 5.88W, and the percentage of power saving would be 69.13%. Another reference scenario is a monitoring camera placed in the Gredos mountain range. This camera was installed by the Transport Ministry to allow remote monitoring of the roads state during the snow season. General public can access the camera through a web page that receives a mean of four visits per hour during the busy hour. The parameters of the model are then: • • • • • • •

Tt = 3600 s Ts = 0.61 s Tm = 1 s tx = 4 ∗ 5 ∗ 60/3600 δ tx = 0.2 N=2 λ=4

The result of the model for this case will be that Pt = 10.66 W, Pt = 7.81 W, and the percentage of power saving would be 26.8%. Both reference networks would be powered through an isolated system composed by solar panels and batteries. Therefore, this paper’s proposal would permit a significant reduction of solar panels and batteries.

966

7. Proof of concept

967

After analyzing the power saving of the proposed solution, several scenarios are presented in order to demonstrate the dualWireless system. Although this proof of concept is presented after the analytical work, it was actually done before, and basic measurements of power consumption and start-up times were done on the testbeds.

968 969 970 971 972

The first set of scenarios is composed exclusively of 802.15–4 motes and is designed to test the control channel (see Fig. 13). Three different network topologies are established to progressively test the behavior of the routing protocol. 1. In the first scenario all nodes have the same branch identifier. The routing method is very simple and is based on increasing or decreasing the node identifier. This scenario is also used to test the “mote bypass” functionality, which consists in sending packages to nodes at length two or three when the neighbor node is down. 2. The second scenario is composed of several branches that have a common root (scenario of depth 1). It presents the challenge of deciding the right branch to send the package. In this scenario, the “mote bypass” must be able to work even if the first node in a branch is down. 3. The third scenario is composed of several branches without a common root (scenario of depth 2). In this case the routing decision is more complex because the destination node may belong to a branch that is not common to the source node. In addition, the “mote bypass” functionality must be able to switch to new branches if needed.

973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996

The second set of scenarios aimed to test the performance 997 of the WiFi links. No energy saving scheme was used and the 998 devices were powered by PoE (Power over Ethernet). Several 999 tests were performed in order to: 1000 • Demonstrate that network self-configuration works. Each node detects the others and connects to the most suitable one. • Demonstrate that, in case a node goes down, surrounding nodes automatically look for a new AP. • Demonstrate that the dynamic routing protocol works. • Demonstrate that if an Internet gateway appears, every node can obtain Internet access. • Demonstrate that if the antenna orientation changes, the network topology is automatically modified in the most suitable way. • Prove other services like Voice over IP or videosurveillance. Once each component was validated separately, it was time to prove the whole system. For that, a network with

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015

JID: ADHOC

ARTICLE IN PRESS I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

[m3Gdc;September 14, 2015;11:56] 15

Fig. 14. Empirical validation with an outdoor testbed.

1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031

1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047

four dual nodes was implemented. Three of these nodes were placed in laboratory environment and powered by PoE, while the other one was placed on a roof terrace and connected to a surveillance camera. These tests aimed to validate both hardware and software integration and to check that devices were woken up and shut down remotely. The dual nodes were working with this configuration for more than a month without detecting communication problems. Finally, the last scenario was composed by a network with four dual nodes. Three nodes were placed on flat roofs of separate buildings in the Rey Juan Carlos University. The gateway, which provided Internet access, was placed on the windowsill of one the buildings pointing to the node in the roof (Fig. 14). The network was operating continuously for a month and demonstrated a high performance: IP layer throughput over 20Mbps, 10ms of delay and 5ms of jitter. 8. Conclusions This paper presents a dual architecture for wireless multihop networks called dualWireless. Each node contains a IEEE 802.15–4 mote always powered and connected to neighbors, and an embedded computer with two IEEE 802.11n interfaces that is switched on and off on demand to save energy. The 802.15–4 network acts as control network that enables the network operator to send commands to appropriate nodes at any time. The systems include mechanisms to recognize the neighbors and self-configure the network. The proposed dual network is very well suited for situations in which there are many periods without traffic and it becomes reasonable to save energy by switching off the nodes while they are not being used. The architecture has been extensively tested. Ten nodes have been built and the resulting testbed has been tested in

laboratory and in an outdoor testbed. Both testbeds have exhibited a correct behavior during tests. The energy consumption measurements have shown that the proposed systems can save energy in many different scenarios. Two case studies have been used to illustrate this: a remote video surveillance camera and a set of remote meteorological stations, obtaining important energy savings in both cases. The results allow us to think of the many other scenarios in which this proposal would be more energy-efficient than a common multi-hop wireless network. The dualWireless proposal has also been compared with other solutions proposed in the scientific literature, showing that dualWireless is advantageous under low traffic load conditions. Future works that include more variables in the power consumption model and that compare our proposals with new ones would be of great interest. In addition, it is also interesting to consider the IEEE 802.15–4 network not only for control purposes but also for low throughput data traffic. Finally, it would be important to study the implications of a mesh topology in the control network, and how to ensure the compatibility of routes in both layers in that case. Regarding the development of prototypes, there is also room for important improvements using faster hardware that could be powered on an off almost transparently for users.

1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073

Acknowledgment

1074

This work has been performed in the framework of the FP7 project TUCAN3G IST-601102 STP, which is funded by the European Community. The authors would like to acknowledge the contributions of the colleagues from TUCAN3G Consortium (http://www.ict-tucan3g.eu).

1075

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

1076 1077 1078 1079

JID: ADHOC 16

ARTICLE IN PRESS

[m3Gdc;September 14, 2015;11:56]

I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

1080

References

1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157

[1] International Telecommunications Union, “Green Solutions to Power Problems (Solar and Solar- Wind Hybrid Systems) for Telecom Infrastructure,” www.itu-apt.org/gtas11/green-solutions.pdf, Retrieved 1/2015. [2] J. Simó-Reigadas, A. Martínez-Fernández, P. Osuna-García, S. LafuenteSanz, J. Seoane-Pascual, The design of a wireless solar-powered router for rural environments isolated from health facilities, IEEE Wireless Commun. Mag. 15 (3) (2008) 24–30. [3] C. Rey-Moreno, I. Bebea-González, I. Foche-Pérez, R. Quispe-Tacas, L.L. nán Benítez, J. Simó-Reigadas, A telemedicine WiFi network optimized for long distances in the Amazonian jungle of Perú, in: Proc. of 3rd Extreme Conference on Communication - The Amazon Expedition, Manaus, September, 2011. [4] J. Simo-Reigadas, E. Municio, E. Morgado, E.M. Castro, A. Martinez, L.F. Solorzano, I. Prieto-Egido, Sharing low-cost wireless infrastructures with telecommunications operators to bring 3G services to rural communities, Communications Networks, 2015.(In Press). [5] J. Li, C. Blake, D.S.J. de Couto, H.I. Lee, R. Morris, Capacity of Ad Hoc wireless networks, in: Proceedings of the of MobiCom’01, Rome, July, 2001. [6] Green-wifi, Prototype description, Dec. 2013, http://www.green-wifi. org/solutions/technology/, Accessed 10. [7] R. Patra, S. Nedevschi, S. Surana, A. Sheth, L. Subramanian, E. Brewer, WILDnet: design and implementation of high performance WiFi based long distance networks, in: Proceedings of the USENIX NSDI, 2007. [8] S. Surana, R. Patra, S. Nedevschi, E. Brewer, Deploying a rural wireless telemedicine system: experiences in sustainability, IEEE Comput. 41 (6) (June 2008) 48–56. [9] K. Lee, J. Young, W. Hyun, H. Seong, An energy-efficient contentionbased MAC protocol for wireless Ad Hoc networks, in: Proceedings of the IEEE Vehicular Technology Conference, Melbourne, May, 2006. [10] Y. Shi, T. Aaron, An energy-efficient MAC protocol for mobile Ad hoc networks, in: Proceedings of the 4th Annual Communication Networks and Services Research Conference, Moncton, May, 2006. [11] S. Datta, S. Biswas, Energy savings by intelligent interface Idling in 802.11 based wireless networks, in: Proceedings of the IEEE International Conference on Electro Information Technology, Lincoln, May, 2005. [12] X. Zhang, S. Member, K.G. Shin, E-MiLi: energy-minimizing idle listening in wireless networks, IEEE Trans. Mob. Comput. 11 (9) (Sept. 2012) 1441–1454. [13] E. Rozner, NAPman: network-assisted power management for WiFi devices, in: Proceedings of the 8th International Conference on Mobile Systems, Applications, and Services, San Francisco, June, 2010. [14] F.R. Dogar, P. Steenkiste, Catnap: exploiting high bandwidth wireless interfaces to save energy for mobile devices, in: Proceedings of the 8th International Conference on Mobile Systems, Applications, and Services, San Francisco, June, 2010. [15] V. Vukadinovic, I. Glaropoulos, S. Mangold, Enhanced power saving mode for low-latency communication in multi-Hop 802.11 networks, Ad Hoc Netw. 23 (Dec. 2014) 18–33. [16] S. Nedevschi, L. Popa, G. Iannaccone, S. Ratnasamy, D. Wetherall, Reducing network energy consumption via rate-adaptation and sleeping, Technical report, Electrical Engineering and Computer Sciences University of California at Berkeley, 2007. [17] D. Lymberopoulos, N.B. Priyantha, M. Goraczko, F. Zhao, Towards energy efficient design of multi-radio platforms for wireless sensor networks, in: Proceedings of the International Conference on Information Processing in Sensor Networks, St. Louis, May, 2008. [18] L. Zhong, Micro power management of active 802.11 interfaces, in: Proceedings of the 6th International Conference on Mobile Systems, Applications, and Services, New York, June, 2008. J. Liu. [19] L. Xu, L. Haixia, Y. Mingqiang, S. Mei, G. Wei, Research and analysis of topology control in NS-2 for Ad-hoc wireless network, in: Proceedings of the International Conference on Complex, Intelligent and Software Intensive Systems, Barcelona, March, 2008. [20] D. Fang, N. Huang, J. Steck, Experiments with Energy Saving Dynamic Source Routing, Technical Report, Oct. 2013. http://cseweb.ucsd.edu/ ∼jlbradle/fang-huang-steck-report.pdf, Accessed 13. [21] S. Arumugam, G.M. JansiRani, Performance analysis of power efficient hierarchical extension of source routing protocol for mobile Ad Hoc networks, in: Proceedings of the International Conference on Computer and Communication Engineering, Kuala Lumpur, May, 2008. [22] H. Qin, W. Zhang, ZigBee-assisted power saving management for mobile devices, in: Proceedings of IEEE 9th Int. Conf. Mob. Ad-Hoc Sens. Syst., Las Vegas, Oct., 2012.

[23] T. Jin, G. Noubir, B. Sheng, WiZi-Cloud: application-transparent dual ZigBee-WiFi radios for low power internet access, in: Proceedings of the IEEE INFOCOM, Shanghai, April, 2011. [24] R. Zhou, Y. Xiong, G. Xing, L. Sun, J. Ma, ZiFi: wireless LAN discovery via ZigBee interference signatures, in: Proceedings of the 16th Annual International Conference on Mobile Computing and Networking, Chicago, Sept., 2010. [25] Y. Agarwal, R. Chandra, A. Wolman, P. Bahl, K. Chin, R. Gupta, Wireless wakeups revisited: energy management for Voip over WiFi smartphones, in: Proceedings of the 5th International Conference on Mobile Systems, Applications and Services, San Juan, June, 2007. [26] H. Qin, W. Zhang, ZigBee-assisted power saving for more efficient and sustainable Ad Hoc networks, IEEE Trans. Wirel. Commun. 12 (12) (Dec. 2013) 6180–6193. [27] A. Jardosh, G. Iannaccone, K. Papagiannaki, B. Vinnakota, Towards an energy-Star WLAN infrastructure, in: Proceedings of the 8th IEEE Workshop on Mobile Computing Systems and Applications, Tucson, March, 2007. [28] A. Jardosh, K. Papagiannaki, E. Belding, K. Almeroth, G. Iannaccone, B. Vinnakota, TÃtulo, Green WLANs: On-Demand WLAN Infrastructures 14 (6) (Dec. 2009) 798–814. [29] M. Ajmone-Marsan, L. Chiaraviglio, D. Ciullo, M. Meo, A simple analytical model for the energy-efficient activation of access points in dense WLANs, in: Proceedings of the 1st International Conference on EnergyEfficient Computing and Networking, 2010. [30] M. Ohmori, K. Okamura, Implementation of green wireless LAN system by on-demand power supply, in: Proceedings of the International Conference on Information Networking, Barcelona, Jan., 2011. [31] E. Shih, P. Bahl, M.J. Sinclair, Wake on wireless: an event driven energy saving strategy for battery operated devices, in: Proceedings of the 8th Annual International Conference on Mobile Computing and Networking, Atlanta, Sept., 2002. [32] B. Raman, K. Chebrolu, Experiences in using WiFi for rural internet in India, IEEE Commun. Mag. 45 (1) (Jan. 2007) 104–110. [33] N. Mishra, K. Chebrolu, B. Raman, A. Pathak, Wake-on-WLAN, in: Proceedings of the 15th International Conference on World Wide Web, Edinburgh, May, 2006. [34] N. Mishra, D. Golcha, A. Bhadauria, B. Raman, K. Chebrolu, S-WOW: signature based wake-on-WLAN, in: Proceedings of the 2nd International Conference on Communications Systems Software and Middleware, Bangalore, Jan., 2007. [35] M. Doering, S. Rottmann, L. Wolf, Demonstration of a low-power energy management module with emergency reserve for solar powered DTN-nodes, in: Proceedings of the 3rd Extreme Conference of Communication, Manaus, Sept., 2011. [36] S. Nedevschi, L. Popa, G. Iannaccone, S. Ratnasamy, D. Wetherall, Reducing network energy consumption via sleeping and rate-adaptation, in: Proceedings of the 5th USENIX Symposium on Networked Systems Design and Implementation, April, 2008.

1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207

Ignacio Foche-Pérez Javier received the Telecommunications Engineering degree and the Ph.D degree from Polytechnic University of Madrid, Spain, in 1997 and 2007 respectively. He was a researcher with the EHAS Foundation between 2003 and 2005 in the field of rural broadband networks for developing countries. Since 2005, he is an Associate Professor with the Department of Signal Theory and Communications with Rey Juan Carlos University. His main fields of research are broadband wireless rural networks supporting multimedia services.

1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219

Javier Simó-Reigadas Dr. Eduardo Morgado studied Telecommunication Engineering (BSc and MSc) at Universidad Carlos III in Madrid (1996– 2004). He read for a Ph.D. in Telecommunication Engineering at Universidad Rey Juan Carlos in Madrid (2009). He is currently an Associate Professor in the Department of Signal Theory and Communications at Universidad Rey Juan Carlos. His research interests include signal processing for wireless communications with applications to wireless ad-hoc networks.

1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

JID: ADHOC

ARTICLE IN PRESS I. Foche-Pérez et al. / Ad Hoc Networks xxx (2015) xxx–xxx

1231 1232 1233 1234 1235 1236 1237 1238 1239 1240

Ignacio Prieto-Egido Telecommunications Engineer from the Universidad Politécnica de Madrid (13-XI-1994) with average rating of Excellent. Ph.D. from the Polytechnic University of Madrid, within the Program of Biomedical Engineering and Health Technology. Special award doctorate from the Polytechnic University of Madrid from 2002–2003 academic year. Specialist in Quantitative Research Methodology: Techniques “by the Polytechnic University of Madrid in 2005.

1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257

Eduardo Morgado Ignacio Prieto-Egido received his degree in Telecommunication Engineering from Carlos III University in Madrid. He worked for Albentia Systems as Field Applications Engineer designing and testing WIMAX networks. Later on he participated in the UNV University Volunteers Programme providing ITC support and working on capacity building for UNV and FAO Representations in Cambodia. His last job was as as researcher in ICAM of Castilla-La Mancha University, where he participated in R&D projects funded by Spanish public institutions. Nowadays, he is a Vising Lecturer Rey Juan Carlos University and also works as Project Director of the EHAS Foundation, researching on applications of broadband wireless technologies to connect rural and remote areas of developing countries, and coordinating several cooperation projects on Latin America.

[m3Gdc;September 14, 2015;11:56] 17

Andrés Martínez-Fernández Ignacio obtained his Telecommunications Engineer degree in the Carlos III University of Madrid. He holds also a M. Sc. in Telecommunication Networks for Developing Countries from the Rey Juan Carlos University of Madrid, where he has been also teacher for the same Master. He has been a member of the Communications and Signal Theory Department for 4 years at that university. He has a wide experience in engineering telecommunications projects for isolated regions in developing countries (with the NGO Engineers without Borders and the EHAS foundation). Within these organizations, he has been in charge of interdisciplinary projects for creating robust and energy efficient systems for telemedicine purposes and telecommunication infrastructure. He has also worked in the field of energy efficiency in telecommunication networks. Ignacio worked for the Spanish National Biotechnology Center (CNB-CSIC) in the field of electron microscopy image processing as part of the team that developes Xmipp(1) and Scipion (2) softwares. He is now dedicated full time to the London-based EyeSeeTea LTD company where he is also a co-founder director, making software and designing ICT for the NGO field.

Please cite this article as: I. Foche-Pérez et al., A dual IEEE 802.11 and IEEE 802.15–4 network architecture for energy-efficient communications with low-demanding applications, Ad Hoc Networks (2015), http://dx.doi.org/10.1016/j.adhoc.2015.08.028

1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279