Review and Classification of Multichannel MAC Protocols for Low-Power and Lossy …homepages.vub.ac.be/~ndeligia/pubs/CARLOS_Classification... · 2020-02-20 · At the network layer,

Received June 28, 2017, accepted August 6, 2017, date of publication September 1, 2017, date of current version October 12, 2017.

Digital Object Identifier 10.1109/ACCESS.2017.2748178

Review and Classification of Multichannel MACProtocols for Low-Power and Lossy NetworksCARLOS M. GARCÍA ALGORA1,2, (Student Member, IEEE), VITALIO ALFONSO REGUERA2,NIKOS DELIGIANNIS1,3, (Member, IEEE), AND KRIS STEENHAUT1,3,4, (Member, IEEE)1Department of Electronics and Informatics, Vrije Universiteit Brussel, B–1050 Brussels, Belgium2Department of Telecommunications and Electronics, Universidad Central ‘‘Marta Abreu’’ de Las Villas, Santa Clara 54834, Cuba3imec, B–3001 Leuven, Belgium4Department of Engineering Technology, Vrije Universiteit Brussel, B–1050 Brussels, Belgium

Corresponding author: Carlos Manuel García Algora ([email protected])

The work of N. Deligiannis was supported by Fonds Wetenschappelijk Onderzoek under Grant G084117, Grant G0A2617N, and GrantG025615.

ABSTRACT The so-called Internet of Things (IoT) aims at connecting every single object to the Internetwith the purpose to automate every aspect of daily life. The IoT relies heavily on wireless low-power andlossy networks (LLNs) that collect information from the physical world and send the measurements todata aggregation and processing nodes. Most LLNs operate in the non-licensed industrial, scientific, andmedical radio band, which is shared by a considerable number of systems. Coexisting wireless systemscause interference to each other, limiting their achievable performance.Multichannel communications enablefrequency diversity, which in turn provides robustness against interference as well as increased networkcapacity. There is a considerable interest in multichannel medium access control (MAC) protocols for LLNs,including an evolving standard for theMAC layer of LLNs. In this paper, we review the latest advances in thetopic and introduce a new classification framework for multichannel MAC protocols for LLNs. While ourframework builds on previous review and classification studies, it adds aspects of aMAC protocol that reflectits interactions with the surrounding network stack. Seeing the resource constraints of the LLN devices, thestudy of such interactions—which is missing in prior classification efforts—can be the key for improvingfuture designs. Relevant protocols published since 2006 are discussed and classified using the presentedframework, including the recent multichannel MAC protocols for LLNs, such as the latest version of theIEEE 802.15.4 standard for time slotted channel hopping.

INDEX TERMS LLN, multichannel MAC protocols, Internet of Things, classification framework.

I. INTRODUCTIONThe Internet of Things (IoT) refers to ‘‘things having identi-ties and virtual personalities operating in smart spaces usingintelligent interfaces to connect and communicate withinsocial, environmental, and user contexts via the Internet’’ [1].The IoT is expected to provide advanced connectivity ofdevices, systems and services, and covers a variety of pro-tocols and applications in several domains [2]. Industrialcontrol and monitoring, home automation and the field ofconsumer electronics, environmental and health monitoring,security and military sensing, asset tracking and supply chainmanagement, and intelligent agriculture are just some exam-ples of application domains that could be disrupted by the riseof IoT [3]–[8].

The practical realization of the IoT is based on multi-ple technologies, such as IEEE 802.11 (WiFi) [9], [10],

Bluetooth [11], [12], telephone data networks, wireless adhoc networks and low-power and lossy networks. Low-powerand Lossy Networks (LLNs) [13] are composed of (a largenumber of) low-end energy-constrained devices that com-municate in a multi-hop fashion at low data rate over lossyunreliable links using small frames. Though most of themare deployed as wireless ad hoc networks [3], LLNs showresource constraints that set them apart from other traditionalwireless ad hoc networks, such as vehicular (VANET) [14]and mobile ad hoc networks (MANET) [15]–[17], as shownin Table 1.

MAC protocols proposed for traditional wireless ad hocnetworks [18]–[22] keep the radio always on, requiremultipletransceivers at each node, or generate high medium accesscontrol overhead [23]–[25]. On the other hand, LLN devicesuse single half-duplex transceivers and turn off the radio

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C. M. García Algora et al.: Review and Classification of Multichannel MAC Protocols for LLNs

TABLE 1. Feature comparison of LLNs vs wireless ad hoc networks.

periodically to save energy, this operation is known as RadioDuty Cycling (RDC). Moreover, the control overhead is sig-nificant when compared to the small size of the data packetsinvolved [26]. For all the above reasons, MAC protocolsspecifically devoted to LLNs have been designed, such as theones reviewed in [27]–[32].

FIGURE 1. Channels defined in the standards IEEE 802.11 (WiFi) andIEEE 802.15.4 (LLNs).

The vast majority of IoT networks operate in the non-licensed ISM band and cause interference to surroundingsystems using the same band [10], [33]–[36]. Fig. 1 showsthe overlap of the channels defined in the standard IEEE802.11 for local area networking and the ones specified forlow-power wireless networks in the IEEE 802.15.4 stan-dard for the physical layer of LLNs. In the struggle for theavailable spectrum, LLNs’ resource-constrained devices areamong the weakest in the wireless arena, when it comes toradio output power. Therefore, mechanisms that enable thecoexistence of LLNs with other systems operating in the ISMband are required. In addition to the coexistence problem, therequirements for LLNs in terms of throughput, latency androbustness tend to increase, as new applications emerge.

A potential solution to the issues raised above, is the use ofmultiple radio channels [37]. Multichannel operation createsfrequency diversity, which combats the effect of the inter-ference caused by external and internal sources, increasesthe network capacity and reduces the contention for themedium [32]. Most LLN hardware supports multichanneloperation in its half-duplex radio transceiver. As an example,the well-known CC2420 radio transceiver from Texas Instru-ments [36], can hop between 16 channels. The support forthis multichannel operation is also present in the LLNs’ oper-ating systems (OSs), such as Contiki [38] and TinyOS [39].As a result, many multichannel MAC protocols have beenproposed [26], [28], [32], [40].

This paper introduces a new classification framework formultichannel MAC protocols for LLNs, that builds on priorreview studies [28], [29], [32], but extends them with a morein-depth analysis. Its main contributions are:• A new classification framework, including new method-ologies used in the field and stressing the interactions ofthe MAC layer with the lower and upper layers of thenetwork stack.

• A classification of multichannel MAC protocols forLLNs, following the proposed framework, includingmore than 30 protocols proposed from 2006 until thebeginning of 2017. A short but to-the-point explana-tion is provided to motivate the classification of eachprotocol.

The attention devoted to the interactions between theMAClayer and the other layers of the network stack is moti-vated by the well-accepted idea that the protocol design forresource-constrained devices must take advantage of cross-layer interactions. Thanks to those interactions, the (tradition-ally independent) layers of the network stack can exchangeinformation to coordinate their behavior. This coordinationcan help, for example, to save energy.

II. BACKGROUND ON LOW-POWER ANDLOSSY NETWORKSThe application domain of LLNs includes militaryapplications, for surveillance [41], self-defense and survivalpurposes [42]; environmental monitoring, such as volcanicactivity monitoring [43] and the ZebraNet project [44]; bodyarea networks for patient monitoring [45], [46] and emer-gency response [47]; smart metering, such as the NAWMSproject [48]; and farming and equipment monitoring [3]. Thevast majority of LLNs, such as the ones just mentioned, sharethe following characteristics:• Large number of low-end devices: up to several hundredsof devices with 16-bit processors andRAM/ROMcapac-ities in the order of kilobytes (KB).

• Unreliable lossy links with low data rates: the devicesinclude single half-duplex radio transceivers with maxi-mum data rate of 250 Kbps and maximum output powerof 0 dBm.

• Multihop communications: covering large areas withlow-power radios requires multihop communications.

• Small frame size: the typical frame size is 127 bytes.

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TABLE 2. Challenges vs. required mechanisms in LLNs [3].

FIGURE 2. The most important standardized protocols stacks for LLNs.

FIGURE 3. Traffic patterns in WSNs: Convergecast (3a), Multicast (3b),Flooding (3c), and Local Gossip (3d).

• Energy scarcity: battery-powered devices with littlehuman intervention for recharging or replacing the bat-teries, and expected network lifetime in the order ofmonths or years. Optionally, LLNs can include energyscavenging mechanisms [49]–[52].

LLNs offer several advantages in terms of scalability, cost,flexibility, accuracy and ease of deployment when comparedto traditional wireless ad hoc networks in the aforementionedapplications. However, their unique characteristics and limi-tations pose challenges in terms of hardware, protocols andapplications design, as summarized in Table 2 [3].

Most multichannel MAC protocols reviewed in this paper,have been designed for WSNs. In most WSN applications,the sensor nodes send the data to one or more collectionpoints (the sinks) using multi-hop communications [53]. Thepredominant traffic pattern in these applications is therefore

multipoint-to-point. This traffic pattern, depicted in Fig. 3a,is known as convergecast. However, other traffic patternsappear, such as the ones shown in Fig. 3. Point-to-multipointcommunications can be required to update configurationparameters at the nodes, trigger a global repair of the routingtree, or send queries to sensors. Point-to-multipoint commu-nications can rely on multicast transmissions, to reach morethan one node with the same packet (nodes 2, 3 and 7 inFig. 3b), or flooding (broadcast), in which all nodes in thenetwork should receive the information, as shown in Fig. 3c.In addition, communication between neighbors is useful forcontrol information exchange and data aggregation purposes.This point-to-point-based local traffic is known as local gos-sip, see example in Fig. 3d.

The integration of LLNs in the Internet and the WorldWide Web, has given rise to several standards (cf. Fig. 2) forthe various OSI layers. At the application layer, the InternetEngineering Task Force (IETF) has defined the ConstrainedApplication Protocol (CoAP) [54], as a lightweight alterna-tive to the Hypertext Transfer Protocol (HTTP) [55], [56].Though both the Transmission Control Protocol (TCP) [57]and the User Datagram Protocol (UDP) [58] could be used inLLNs, most applications rely on UDP traffic (e.g. CoAP), inorder to avoid the high control overhead generated by TCP.At the network layer, the IPv6-compatible Routing Protocolfor Low-Power and Lossy Networks (RPL) [59]–[61] is thede facto routing standard. RPL creates a routing tree towardsthe sink in the form of a destination-oriented directed acyclicgraph (DODAG), thus defining parent-children relationshipsalong the gathering tree (e.g. in Fig. 3a, nodes 5 and 6are node 3’s children, and node 7’s potential parents). Thestudy in [62] summarizes RPL and highlights some ofits weaknesses for its future exploitation. The IPv6 overLow-Power Wireless Personal Area Networks (6LoWPAN)[63], [64] allows encapsulation and header compression forIPv6 packets such that they can be sent using small IEEE802.15.4 frames.

The IEEE 802.15.4 standard [33] defines the MAC andphysical (PHY) layers for Low-Rate Wireless Personal AreaNetworks (WPANs) as part of the IEEE standardization effortfor LLNs. Besides the open standards for LLNs proposedby the IETF and the IEEE, other proprietary standards havebeen proposed that do not follow the stack proposed by theIETF, as shown in Fig. 2. Among them, ZigBee [34] andWirelessHART [35] stand out. Both of them are based on

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the IEEE 802.15.4 standard PHY layer specification. ZigBeeadopts its MAC specifications from the IEEE 802.15.4 butWirelessHART defines its own multichannel MAC layer tofulfill the requirements of industrial applications [65], [66].From there on, ZigBee and WirelessHART build their ownnetwork stack up to the application layer; the former orientedto general LLN applications, and WirelessHART specificallydesigned for process measurement and control applicationsusing LLNs.

The classification in this paper is oriented to MAC-layerprotocols, on top of the IEEE 802.15.4 PHY specification,that use multiple channels for communication in multihopnetworks. Star-based solutions, such as LoRaWAN [67] andSigFox [68], which are low-throughput long-range solutions,are out of the scope of this classification.

III. PREVIOUS REVIEW AND CLASSIFICATION WORKMany MAC protocols for operating on the IEEE 802.15.4PHY layer have been proposed over the years andvarious efforts have been carried out to classify them [27],[30], [40], [69]. The variety of protocols and survey paperson the subject is so vast that meta-surveys, such as [70],have been published to organize the review efforts. In thearea ofmultichannel protocols, in particular, the reviewworkspublished in [28], [29], and [32] stand out.

The work in [28] includes a capacity analysis of mul-tichannel communications in LLNs and the proposed tax-onomy includes the main characteristics of multichannelMAC protocols. The study in [29] reviews various protocolsin a detailed way and extracts their main characteristics,but does not follow a particular classification framework.Nevertheless, it shows a table that summarizes some impor-tant aspects and parameters of the presented protocols.EkbataniFard and Monsefi [29] also contribute by pinpoint-ing the sources of energy loss caused by the MAC protocol,including the impact of multichannel operation.

The classification in [32] focuses on channel assignmentmethods and compares them with the ones used in cellu-lar and wireless mesh networks. The authors emphasize theexistence (yes or no) of explicit negotiation for assigningchannels. The study in [32] also provides a summary ofthe characteristics and benefits of multichannel communica-tion in WSNs. Three network architectures for multichannelcommunication are proposed, based on convergecast traffictowards the sink. The first architecture only includes sen-sor nodes and the sink, the second one includes aggregatornodes, and the third one allows communication betweenaggregators.

Table 3 shows the features involved in the multichannelMAC protocol classification and in which of the three relatedreview works—namely, [28], [29], [32]—they appear (notethat the exact naming can differ from one author to the other).The detailed explanation of the depicted classifiers, as wellas their relationship with the current overview paper, is givenin Section V.

TABLE 3. Summary of previous classification works.

IV. MULTICHANNEL COMMUNICATIONSCHALLENGES IN WSNsMultichannel communication can increase robustness againstinterference, as well as bandwidth availability in LLNs, atthe expense of a higher complexity for organizing communi-cation between the constrained nodes, also known as motes.The main challenges for multichannel communications in thecontext of LLNs are listed below:

1) INTERNAL (INTRA-NETWORK) INTERFERENCEWhen two nodes transmit at the same time on non-orthogonalchannels (or actually the same channel), interference can becaused at their respective intended receivers. Using chan-nels that are spectrally distant enough from each other canavoid this problem at the expense of less channels to choosefrom. This can limit the maximum number of concurrenttransmissions. A detailed discussion of this subject can befound in [28].

2) EXTERNAL INTERFERENCEIEEE 802.11 networks, Bluetooth devices, and othermachines, such as microwave ovens, pollute the frequencybands in which LLNs operate [10], [11], [71], [72]. Multi-channel communication can be a key tool to overcome theseproblems by using interference-aware channel assignmentmechanisms and/or channel blacklisting [73].

3) MULTICHANNEL HIDDEN TERMINAL PROBLEMThis appears when a node misses a RTS/CTS exchange inthe common control channel (CCC) while communicating insome other channel. In Fig. 4, node C andD are on channelC1

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FIGURE 4. Schematic representation of the hidden terminal problem.

while A and B exchange a RTS/CTS pair of messages inthe CCC and agree to use channel C2 for the data exchange.When C and D go back to the CCC, they are unaware of thereservation made by A and B, so a collision may occur if theydecide to use C2 while A and B are still using it.There are two additional problems linked to this situation,

as explained in [28]. The first problem is the traditionalhidden terminal problem in multi-hop networks, in whichtwo nodes outside their respective coverage areas transmitsimultaneously to a node in between them, causing a collision[if C (or D) transmit to A simultaneously with B because theycannot hear the transmission from B]. The second problemis the sleep hidden terminal problem, as defined in [74]. Theauthors describe a situation inwhich a nodemisses the controlinformation exchange because of being asleep. This situationis similar to the multichannel hidden terminal problem, butC andD are in sleepmode instead of in another channel, whileA and B exchange the RTS/CTS.

4) MULTICHANNEL DEAF NODEThe multichannel deaf node problem [75] occurs when nodeA mistakenly assumes that node B is unreachable after tryingto get a response from B, while A and B are inside theirmutual coverage area but on different channels.

5) CONTROL CHANNEL BOTTLENECK (CCB)This problem is associated to protocols that require a CCC.It arises when the data exchange cannot be coordinatedbecause the CCC is not available [76]. The unavailabilityof the CCC can be caused by contention or problems withinterference, jamming or noise.

6) BROADCAST SUPPORTIn WSNs, the so-called link-layer broadcast—meaning thata frame at the link layer will carry a MAC-layer broad-cast address indicating that all the receivers should pro-cess that frame—is of great use at the discovery phase ofthe network [13], [59]. In this phase, motes need to getto know each other and, in particular, know their directneighbors, i.e. reachable through a single radio transmission.Efficiently organizing neighbor discovery via link-layerbroadcast frames in a multichannel and sleep/awake networkis a challenge. Note that link-layer broadcasting can also be

used to disseminate information to all neighbors of a givennode, although the cost of sending link-layer broadcasts in aduty-cycled network should be carefully analyzed. Actually,the work in [77]–[79] points out that in RDC-enabled LLNslink-layer broadcast is only viable under certain situations.

7) JOINING THE NETWORKNew nodes must be able to join the network, preferablywithout jeopardizing its correct functioning or requiring acomplete reorganization of the channel assignment [29].In addition, a node joining the network should be able to find asuitable channel for communicating within a short time lapse,spending few energy.

8) NETWORK PARTITIONSSome protocols [80]–[83] divide the network in clusters,assigning a channel to each of them for intra-cluster commu-nication. The clusters must be connected to the sink so thatevery node can reach the sink through the cluster it belongsto. This can be an advantage if nodes in different clusters donot need to communicate with each other but care must betaken during the partitioning of the network to keep the nodesconnected while keeping the energy constraints in mind [83].

9) CHANNEL SWITCHINGChannel switching consumes energy and the time spent on itadds up to the end-to-end delay of the packets. On the otherhand, static channel assignments lead to network partitions,whose consequences have been explained in Section IV-.8.

FIGURE 5. Categories of MAC protocol features considered in theproposed classification framework for multichannel MAC protocolsfor LLNs.

V. PROPOSED CLASSIFICATION FRAMEWORKMultichannel MAC protocols for LLNs show that a widevariety of approaches and multiple aspects influence theirdesign. Therefore, classifying them is a challenging task.The proposed framework organizes the classification fea-tures (or attributes) in 5 main categories, as shown inFig. 5. Multichannel MAC protocols have two major com-ponents: the Channel Assignment and the Medium Access.Channel assignment is the process of (statically, semi-dynamically or dynamically) attributing frequency bands to

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FIGURE 6. Group view of the proposed classification framework for multichannel MAC protocols for LLNs.

nodes, at given times, to allow them to communicate.Mediumaccess takes care of how, and especially when, each node canuse its currently assigned frequency band. The functionali-ties of multichannel MAC protocols for LLNs often involvethese two components, whose design tends to be coupled, asdiscussed in [73] and [84]–[86].

Our classification framework indicates how well the MAClayer can fulfill the service requests coming from the upperlayers, denoted in Fig. 5 as Upper-Layer Interactions. Forexample, broadcast traffic is needed by upper-layer proto-cols to exchange control information, so it is important topoint to what degree the MAC layer can provide supportfor it.

The Physical Layer Management category aims at indi-cating how well the MAC protocol can manage the physicallayer. For example, alleviating the hidden terminal problemcan save energy by avoiding radio collisions. In addition tothe interaction with the other layers of the network stack,the MAC layer needs functionalities for itself, for examplelink-layer synchronization, which are put into theMAC LayerCoordination category.Note that this classification framework is particularly use-

ful for multichannel MAC protocol designers as it includesthe interactions between the MAC layer and the surroundingnetwork stack. The depiction of the classification frame-work, shown in Fig. 5, is expanded in Fig. 6 by men-tioning the actual classification features for each category.In the following, we explain the classification frameworkin detail.

A. CHANNEL ASSIGNMENTChannel assignment can be performed deploying one of threebasic strategies [32]:

1) STATIC CHANNEL ASSIGNMENTIt means that nodes stay on the same channel for all theiractions (listen or transmit) after the network initialization,unless a very unlikely network update occurs [80]–[83]. Thisimplies that the multichannel operation will partition thenetwork in clusters, each one using a particular channel. Themain advantage of this approach is that no energy is wastedon channel switching, so the protocol overhead and compu-tational load tend to be low in the long run. The downsidesof static assignment are the low adaptability to changes in theradio environment and its sensitivity to interference.

2) SEMI-DYNAMIC ASSIGNMENTThese protocols assign a fixed channel to each node, or groupof nodes, but the nodes can switch channels to reach othernodes or to react to changes in the network or the communica-tion environment [87]–[91]. Doing so, the network partitionscan be eliminated but a (typically) complex coordinationmechanism is required for making nodes meet in the samechannel. Also, this approach introduces multichannel hiddenterminal and deafness problems and the support for broadcasttraffic is not as straightforward as in static assignments.

3) DYNAMIC ASSIGNMENTIt is used in protocols on which the channel selection processis done very frequently, e.g. once per wake-up [73], [84].

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As in semi-dynamic approaches, the multichannel hiddenterminal, deafness and broadcast support problems must betaken into account and some coordination mechanism is oftenneeded to avoid them.On the other hand, dynamic assignmentcan alleviate the effect of interference, internal or external,and contribute to the adaptability of the network. Dynamicchannel assignment mechanisms can be further classifiedaccording to the following coordination methods:

a: Common Control Channel (CCC)A CCC channel, statically or dynamically assigned, is usedto exchange control information to agree on when and onwhich channel the transmitter and the receiver will meetfor the data frame exchange [74], [92]–[94]. The use of aCCC can reduce, and potentially eliminate, the problems ofoverhearing, over-emitting andmultichannel hidden terminal,and can be a vehicle for broadcast support. One disadvantageof CCC is the high protocol overhead on account of thecoordination, which wastes energy and time, and can createa CCB. In addition, the CCC constitutes a single point offailure, which can cause a network breakdown in case thecommunication in that specific channel becomes impossi-ble due to jamming or low signal-to-interference-plus-noiseratio (SINR).

b: Split Phase (SP)In SP, time is divided in two periods, namely, a control periodand a data exchange period [95]–[97]. During the controlperiod, nodes use control frames to coordinate a ‘‘meetingpoint’’ to exchange the data frame during the followingperiod. This mitigates the multichannel hidden terminal anddeafness problems but requires synchronization. Moreover,the channel switching time strongly influences the perfor-mance of the protocol. Furthermore, during the control phasethe data channels remain idle, leading to bandwidth waste.High contention during the control period can cause a prob-lem analogous to CCB under high traffic loads.

c: Channel Hopping (CH)In CH, nodes switch between the available channels fol-lowing a given sequence [73], [84]–[86], [98]. By hoppingthrough the available channels, nodes avoid staying in low-SINR channels, hence improving the robustness against inter-ference. Moreover, CH schemes tend to decrease the protocoloverhead by reducing the amount of coordination frames.Broadcast support must be addressed carefully and the chan-nel switching cost, in terms of energy and time, determinesthe applicability of the protocol. Moreover, storing or calcu-lating the hopping sequence of a node’s neighbors may becostly in time, memory and energy.

B. MEDIUM ACCESS STRATEGYThemedium access strategy can be divided in three categoriesas follows:

1) CONTENTION-BASEDContention-based protocols make nodes compete forthe medium using back-off-based schemes [73], [80], [87],[88], [99]. A typical example of a single-channel contentionprotocol is CSMA [100]. Contention-based methods do notrequire stringent synchronization, can easily adapt to trafficfluctuations and, and joining the network is usually easierthan for time-slotted methods. Under light to medium trafficloads, contention-based protocols have lower delays andbetter potential throughput. However, when the load becomeshigh, collisions and back-offs cause increased bandwidthwaste, latency and packet loss.

2) TIME-SLOTTEDThese protocols partition the time in slots that conformschedules in which nodes are assigned channel-and-timeslotcombinations [89], [91], [98], [101], [102]. They requiretight synchronization across the network and tend to lackadaptability. Moreover, a node joining the network can leadto global or local reconstruction of the schedules, which cancost time and energy. On the other hand, time-slotted pro-tocols aim at providing deterministic end-to-end delays andcollision-free communication, and show good performanceunder high traffic loads.

3) HYBRID APPROACHESSome protocols aim at taking advantage of the benefits ofthe aforementioned strategies by combining aspects of both,contention-based and time-slotted approaches [84], [85],[94], [103].

C. UPPER LAYERS INTERACTIONThis category of classification features (attributes) involvesthe support that the MAC protocol provides to the upperlayers of the network stack. The MAC layer gives support todifferent traffic patterns that are generated by the applicationand/or network layer protocols.

1) BROADCAST SUPPORTThe support provided for broadcast traffic plays an importantrole as a service provided to the upper layers [28], [74].Intra-network processing functionalities and higher-layerprotocols [3], [28], [59] rely on broadcast traffic for signal-ing/control purposes. In addition, link-layer broadcast framescan be useful for joining the network and for channel negotia-tion [33], [95]. Most approaches to provide broadcast supportfall in one of the following four classes:

a: Cluster/treeBroadcast frames can only be sent inside the tree or cluster towhich the sending node is associated [83], [101], [104].

b: One-by-oneFrames with a broadcast address are sent to each neighborindividually as unicast frames [73], [88], [91], [98], [103].

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c: Dedicated channel or timeslotA channel or timeslot/channel combination is dedicatedexclusively to broadcast traffic, so all the nodes in a neigh-borhood must listen to the same channel for broadcasts [74],[85], [89], [105].

2) UPPER-LAYER ORIENTED CHANNEL ASSIGNMENTIn resource-constrained devices, cross-layer design has beenrecognized as a good approach for saving energy and compu-tational resources. Therefore, some multichannel MAC pro-tocols use information from the upper layers in the channelassignment process, e.g. [89], [90], [105], [106]. A central-ized entity can allocate channels (and timeslots if applicable)and send the allocation schemes to the nodes; or a distributedalgorithm runs at the nodes to allow them to agree on channeland timeslot allocation by communicating with each other.In general, two forms of upper-layer-oriented channel assign-ment can be identified:

a: Collection-oriented assignmentWhen theMACprotocol uses information about the gatheringtree to optimize the performance of data collection appli-cations, e.g. in terms of delay and/or throughput [89], [94],[103], [104].

b: Topology-based assignmentMostly applied in protocols with static or semi-dynamicchannel assignment that use graph-coloring techniques [107]in order to balance the traffic load and/or avoid intra-networkinterference [80]–[82], [87]. It requires preliminary knowl-edge of the network topology.

3) OPTIMIZATION FOR TRAFFIC PATTERN SUPPORTSince convergecast is the most common traffic patternin WSNs [26], some multichannel MAC protocols havebeen optimized for it [80], [81], [89], [94], [101], [102],[104], [105]. Though other traffic patterns are present, asexplained in Section II, protocols are seldom optimized for atraffic pattern other than convergecast. With the classificationfeature mentioned above, we specify whether the protocolsare optimized for convergecast or not.

D. MAC LAYER COORDINATIONThe MAC Layer Coordination category includes the organi-zation of link-layer communication, the choice of the channelswitching frequency, and the support for synchronizationbetween the nodes.

1) LINK-LAYER COMMUNICATIONIt can be organized in one of the following ways:

a: Receiver-orientedReceiving nodes are responsible for initiating the communi-cation by sending beacons to announce that they are ready tolisten to a sending node [73], [74], [83], [91]. Hence, potential

transmitters must go to the receiver’s channel and listen for itsbeacon. Those beacons are not needed in some time-slottedor hybrid protocols in which the transmitters know the slots inwhich the receiver will be waiting for incoming transmissions(as opposed to the slots in which it will be in sleep mode orsending) [84], [85], [108].

b: Sender-orientedCommunication is initiated by the senders. Therefore,in time-slotted protocols, potential receivers must wakeup at every timeslot in which a neighbor could send[94], [96], [101]. In contention-based protocols, the transmit-ter lets the receiver know that it is ready to transmit usingpreambles or RTS/CTS exchange [87].

c: Channel-orientedChannels are assigned to a set of communication links andall the nodes in the same channel receive and send frames inthat channel. Typical examples of this category are protocolsthat partition the network into multiple sub-trees, each oneconstituting a set of links to the sink that always use the samechannel [80]–[82]. These protocols do not specify a RDCmechanism, so transmitters just send their frames withoutprevious communication.

d: Exclusive channel-and-timeslot assignmentSome time-slotted MAC protocols assign channel-and-timeslot combinations to specific communication linksbetween specific nodes [35], [86], [89], [102]. In this case,there is no need to preliminary exchange information betweenthe transmitter and the receiver(s), since the purpose of theslot is implicitly known from the schedule. More than onenode can receive in some other node’s transmission slot(e.g. a parent sending a frame to its children) and, likewise,some specific transmitters can be allowed to send in a receiveslot of a node (e.g. dedicated receive slot of a parent to listenfor incoming frames from any of its children).

2) CHANNEL SWITCHING FREQUENCYIt refers to how often the nodes switch channel. A widerange of channel switching frequencies can be distinguished,such as:

• No channel switching in protocols with static channelassignment methods [80]–[82].

• Twice per data frame in CCC-based protocols, onceto go to the CCC to coordinate the communicationand once to go to the data channel agreed upon [74],[92]–[94], [109].

• Once per wake-up in CH protocols, in which nodesswitch channel (almost) at every wake-up [73], [84],[85], [105].

Channel switching consumes energy and takes time butit is useful to avoid collisions and to provide resilience tointerference.

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3) SYNCHRONIZATIONIt contributes to the realization of collision-free communica-tion, reduces idle-listening, and allows the use of coordinationmechanisms for improving throughput and delay. Unfortu-nately, the control traffic required to provide synchronizationconsumes network resources, which can reduce the network’soverall performance and lifetime. Three categories for orga-nizing synchronization are considered:

a: Built-inA synchronization mechanism is included in the protocoldesign, as in [73], [84], [85], [91], and [98].

b: ExternalThe MAC protocol needs synchronization but does not orga-nize it itself [87], [89], [101], [103].

c: Not requiredThe MAC protocol does not require synchronization for itsoperation [74], [83], [93], [99], [106].

E. PHYSICAL LAYER MANAGEMENTThis category includes the RDC mechanism, managementof the Preferable Channel List (PCL) and interferenceavoidance.

1) PREFERABLE CHANNEL LIST (PCL) MANAGEMENTThe PCL is defined in [110] as the set of channels availablefor the node to choose from.

a: Inclusion in the PCLThe way channels are included in the PCL are classified as:• Default inclusion: At network deployment, nodes areconfigured to use a fixed set of channels, denotedcounter-based in [32]. The set can include all the chan-nels available in the operation band, or a selection oforthogonal channels among the ones available [85], [87],[97], [101], [104].

• Channel quality indicator: Channel quality is measuredusing a given metric and only the channels with themetric above a given threshold are included. Orthogo-nal channels are assumed to be ‘‘good’’ channels and,therefore, included by default in the PCL [32]. Changesin the channel quality during network deployment maylead to exclusion of channels from the PCL.

b: Selection from the PCLIt defines how each node chooses the next channel to switchto. In the classification framework proposed, the selection canbe performed using one of the following principles, discussedin [32]:• Round-robin [85], [89], [101]• Least chosen [91], [95], [97]• Least loaded [103], [106]• Probabilistic or Pseudo-random [73], [83], [84]• Interference-aware selection [82], [90], [99], [102]

2) INTERFERENCE AVOIDANCEThesemechanisms [111] are useful for providing resilience tointernal and/or external interference. Interference avoidancemechanisms can be classified as:

a: Active avoidanceProtocols that provide a mechanism to actively avoid theinterference, for example, by reacting to changes in the chan-nel conditions [88], and doing dynamic channel blacklist-ing [73], [84].

b: ImplicitThe channel selection policy is implicitly robust against inter-ference, such as in CH schemes [33], [86] which preventnodes from staying on the same channel. As such, if one chan-nel is hit by interference, the communication between twonodes can take place when they meet in some other channelsoon after the failed attempt on the interfered channel.

c: Intra-network interference avoidanceSome protocols [80], [81], [90], [91], [101] try to avoid theinterference caused by simultaneous transmissions inside thenetwork (intra-network interference) by preventing nodes thatare in each others radio range (or two hops from each other)to simultaneously transmit in the same channel.

3) RADIO DUTY CYCLING (RDC) MECHANISMRDC [112] is a technique used in WSNs to save energy bykeeping the radio off (sleep mode) most of the time and,nevertheless, wake it up periodically to listen for incomingtransmissions or wake it up to send frames. In IEEE 802.15.4radios, such as the very popular CC2420 [36], the powerconsumption is almost the same whenever the radio is on.This means that receiving and idle listening are roughly aspower consuming as transmitting. This classifier pinpointswhether an RDCmechanism is included in theMAC protocoldesign or not. The interested reader can find more details onRDC mechanisms in [30] and [40].

Some protocols, such as [80], [81], [102], [106], and [108],do not specify a RDCmechanism. The protocols can be appli-cable in solutions in which nodes are main-powered [113].In addition, time-slotted protocols that do not explicitly spec-ify slots for the nodes’ radios to go to sleep, could be con-figured to allow nodes to sleep during inactive slots, i.e. slotsnot used for transmission or reception. The IEEE 802.15.4standard [33] requires a scheduling algorithm to build theschedule that will be managed by TSCH [98], which takescare of putting the nodes to sleep during inactive slots.

VI. MULTICHANNEL MAC PROTOCOLS FOR WSNsIn this section we review more than 30 multichannel MACprotocols for LLNs, highlighting their functionalities andmechanisms according to the proposed classification frame-work.

A. CHANNEL ASSIGNMENTMultichannel MAC protocols can be classified accordingto the channel assignment strategy, which can be static,

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semi-dynamic and dynamic. The latter class can be fur-ther divided with respect to the use of CCC, SP orCH mechanisms. We refer to Table 3 for a depiction of theclassification regarding this feature.

1) STATIC CHANNEL ASSIGNMENTThis approach, applied by the TMCP [80], [81], PMC [83]and MCRT [82] protocols, involves constructing vertex-disjoint trees and assigning a channel to each convergecasttree. The goal is to reduce intra-network interference by creat-ing clusters, in the form of trees, to group nodes that commu-nicate very frequently with each other or share a convergecastpath to the sink. In TMCP and MCRT, the algorithms areexecuted by the sink, and aim at partitioning the network,based on graph-coloring theory [107], [114]. Afterwards, theassignment is distributed across the network. This causesprotocol overhead at the network set-up phase and hampersnodes to join the network in the future. Creating isolatedclusters has the inherent disadvantage that nodes that mayrequire to directly communicate in the future, e.g. becauseof topology changes, would have to do so through the sink.An example of channel assignment using TMCP is shownin Fig. 7.

FIGURE 7. Depiction of the concept behind the TMCP protocol [80].

PMC clusters nodes and assigns a different communicationchannel to each cluster in order to combat internal interfer-ence [83]. Clusters are chosen by the nodes in a distributedway such that the need for inter-cluster communication isminimized. Nodes are allowed to switch to a different channelbut the clustering should guarantee that this is rarely needed.Per cluster, the congestion of the channel is minimized bydiminishing the number of nodes in the cluster.

2) SEMI-DYNAMIC CHANNEL ASSIGNMENTA considerable portion of the reviewed protocols performssemi-dynamic channel assignment. Being a broad category,its diversity is apparent. For example, the authors of MMSN(Multi-Frequency Media Access Control for Wireless SensorNetworks) [87] propose four different receiver-oriented chan-nel assignment strategies, whose applicability depends on therelation between the number of available channels and thenumber of nodes composing the network. All the proposedmethods can be executed in a distributed way and result in

TABLE 4. Classification of several MAC protocols according to thechannel assignment strategy and localization.

a balanced number of nodes per channel, if no exclusivechannel assignment per node is possible. A channel is solelydedicated to broadcast traffic and must be checked at the startof every access period.

One of the channel assignment algorithms proposed inMMSN is applied by TACA (Traffic-Aware Channel Assign-ment) [106], for which the authors added a traffic aware-ness mechanism. MMSN and TACA perform the channelassignment only once, which limits the adaptability of theseprotocols and their ability to allow new nodes to join thenetwork.

In ARCH [88], nodes are assigned a receiving channel butthey switch to another channel if the Estimated TransmissionCount (ETX) exceeds a given threshold. This mechanismimplies notifying the children of the switching node whena channel switch is made, which affects the communicationreliability during the transition. A contribution of ARCH isthat it takes into account that spectrally neighboring channelstend to suffer similar conditions of interference and noise,so the channel selection policy promotes switching to spec-trally distant channels.

Routing information can be used as an extra input for chan-nel assignments algorithms. In this regard, Yen and Lin [115]model the channel assignment task as the Channel Con-strained Data Aggregation Routing problem and proposefour algorithms to solve it. DeTAS [89] proposes a time-slotted protocol that uses routing information from RPL to

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construct a schedule designed to be managed by the MACprotocol specified in the IEEE 802.15.4e standard [116],known as TSCH [98].Whereas the algorithms in [115] followa centralized approach, DeTAS constructs the schedule ina distributed way. The main disadvantage of the solutionin [115] and DeTAS [89] is that it uses routing informa-tion, which must be gathered during network initialization;the routing information can vary frequently in IoTapplications.

In JFTSS [104], MODESA [102] and WAVE [101], threetime-slotted protocols with semi-dynamic channel assign-ment are proposed, with the goal of minimizing the datagathering time in IoT applications. JFTSS aims to eliminatethe interfering links in the convergecast tree by assigning thechannels targeting at delivering packets to the sink as quicklyas possible. Having complete knowledge of the interferinglinks in a deployed IoT application can be very difficult andresource consuming. Moreover, the network topology mayvary due to changes in the radio environment or due to nodemobility.

Soua et al. proposed MODESA [102], a multichannelassignment for raw-data convergecast that uses a centralizedalgorithm tominimize the gathering end-to-end delaywithoutaffecting the throughput. In 2014, the same authors proposedWAVE [101], a channel-and-timeslot assignment scheme tobe managed by the IEEE 802.15.4e standard [116], with thesame goal as MODESA but including a distributed version ofthe algorithm. These three protocols, i.e. JFTSS, MODESAand WAVE, build their schedules based on knowledge thatcan be costly to acquire in LLNs, e.g. routing topology andtraffic requirements of the nodes.

3) DYNAMIC CHANNEL ASSIGNMENTIt is mostly done in a distributed way either by using aCommon Control Channel, as in A-MAC [92], ARM [74],CR-WSN [109] and OMA [93]; or by using Split-phase-based approaches, as in MC-LMAC [95] and TMMAC [96];or through Channel Hopping schemes, as in Y-MAC [85],ARCH [88], EM-MAC [73] and MuChMAC [84].

a: CCCA-MAC [92] originates from a receiver-initiated single-channel protocol extended with a CCC-based multichanneloptimization. A mechanism similar to RTS/CTS [100] isimplemented using probes and hardware ACK frames. Thedesign shows an attractive strategy for ACK transmissionwith non-destructive collisions.

ARM [74] is an asynchronous protocol that uses a CCCand aims to solve three problems: the CCB (called CCS bythe authors of ARM) problem, the hidden terminal problem,and the low reliability of broadcast problem. The protocoluses opportunistic access to the CCC to alleviate the CCBproblem. To improve the reliability of broadcast traffic, a ded-icated channel is used for broadcasting. The other channelsare used for data exchange and are assigned on a per-framebasis through coordination via the CCC.

A cognitive radio perspective is used in CR-WSN [109],which includes frequent sensing of the data channels to avoidinterference from/to the primary users (i.e. term borrowedfrom cognitive radio systems to refer to systems outside theLLN). In CR-WSN, nodes sense the status of each chan-nel and keep the results in a ‘‘channel availability vector’’.When two nodes coordinate a data exchange, they share theirrespective channel availability vectors to agree on a datachannel available for both of them. The protocol leaves roomto implement dynamic assignment of the CCC, in order to beable to use different channels for the CCC, be it one at thetime.

In general, CCC approaches are susceptible to interferencein the control channel, can suffer from the CCB problem, andthe overhead of RTS/CTS-like frames exchanged in the CCCcan be too high for IEEE 802.15.4 radios using 127-byte dataframes. However, using a CCC facilitates the coordinationof the data exchange, often improves the adaptability, andshould reduce the energy waste caused by idle listening,over-emitting and overhearing. Using dynamic selection ofthe CCC improves the robustness of the protocol againstinterference and jamming.

b: Split-phase (SP)MC-LMAC [95] proposes a multichannel version of a dis-tributed single-channel MAC protocol proposed by the sameauthors, i.e. LMAC [117]. In MC-LMAC, nodes negotiateunique channel-and-timeslot combinations in the two-hopneighborhood, eliminating the hidden terminal problem andcollisions. MC-LMAC also includes a blacklisting mecha-nism to avoid highly interfered channels.

Although TMMAC [96] is proposed as aMAC protocol forad hoc networks, the presence of RDC and the assumptionof single-radio half-duplex transceivers in the design make itsuitable for LLNs. During the control period of the SP schemein TMMAC, links between nodes are assigned a channel andtimeslot to be used during the data exchange.

SP-based protocols require synchronization for knowingthe boundaries of the control and data periods. Despite thefact that their control overhead is similar to the one neededfor time-slotted protocols, they do not benefit from having aschedule. Moreover, they are vulnerable to congestion in thecontrol channel when data traffic load is high. Also, controlframes can still collide because of the hidden terminal prob-lem. On the other hand, SP approaches tend to provide goodtraffic adaptability and scalability, and interference avoidancemechanisms can be used for improved robustness.

c: Channel hopping (CH)Protocols that perform CH, usually follow a distributedapproach and can bring benefits like adaptability, reducedcontrol overhead and low delay under low/medium trafficload. For instance, Y-MAC [48] uses CH for traffic adapt-ability, such that when a node successfully receives a framein its assigned reception slot, it jumps to the next channel in around-robin fashion to wait for any potential transmitter that

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FIGURE 8. Illustration of the concept of channel hopping within theY-MAC protocol [85].

lost the contention in the previous slot. This mechanism ispresented in Fig. 8.

DynMAC [86], PMC [83] and ARCH [88] use CH toavoid bad channels due to external interference and noise.In DynMAC, all nodes use the same channel as in a singlechannel protocol, but they can be asked to all switch toanother channel if the Packet Error Rate (PER) in the currentchannel exceeds a given threshold. The channel switch isdecided by the sink, based on reports received from the nodeswith their respective PER values. The main disadvantage ofthis design is that concurrent transmissions in the same radiorange are not possible, which is one of the main motivationsfor using multiple channels.

In EM-MAC [73] and MuChMAC [84], nodes follow anapproach based on slotted seeded channel hopping (SSCH)rendezvous [118], [119] with pseudo-random CH. Nodesfollow independent channel hopping sequences but, whenthey need to send a frame, they can predict their neighbors’wake-up times and corresponding channels. An advantage ofMuChMAC is that it includes a broadcast slot in the sequence,in which all nodes switch to the same dynamically-selectedchannel.

In RL-MMAC [105], nodes use routing information toidentify their parents and children, predict the channel usedby their parents at any timeslot and use a decentralized rein-forcement learning algorithm to select the best action for eachslot (i.e. transmit to a parent, receive from a child or sleep).

B. MEDIUM ACCESSThe classification framework defines three classes for themedium access strategy: contention-based, time-slotted andhybrid, being a combination of the aforementioned strategies,as shown in Table 5.

1) CONTENTION-BASED PROTOCOLSFor these protocols, the resulting contention level willstrongly depend on the channel assignment strategy. Forexample, since TMCP [80], [81] and MCRT [82] assign

TABLE 5. Classification of several MAC protocols according to themedium access strategy.

channels to sub-trees, the collision domain is limited to thenodes in the same sub-tree located inside the coverage areaof the sender.

Nodes can be assigned channels for reception suchthat senders will have to contend for the medium(e.g. MMSN [87]). Under high traffic load, this becomes aproblemwhenmany children want to send frames to the sameparent. Because of the spatial traffic variations in the network,this problem becomes more severe when the receiver is closerto the sink. The collision probability increases when MMSNis not able to achieve exclusive channel allocation in thenetwork and multiple nodes in a contention area are supposedto receive frames on the same channel. The alternative ofassigning channels for transmission implies that nodes mustvisit all the channels to check if there are frames for them,which would waste energy on idle listening and overhearing.

In most protocols that use a CCC, such as A-MAC, ARM,CR-WSN and OMA, the access to the control channel isbased on contention, creating CCB during periods of hightraffic load. Therefore, some protocols propose strategies tomitigate this problem. In ARM and OMA, the access to theCCC follows an opportunistic scheme. In OMA, nodes decidewhether attempting to transmit with probability p or to putthe radio in sleep mode; and, then, choose whether actuallyaccessing the CCC with probability q or to put the radio insleep mode. In ARM, only the first probability (p) is used.These mechanisms allow nodes to go to sleep unless they

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have a good chance of achieving successful communication,which mitigates the CCB problem. When the probability ofsuccessful transmission in the CCC decreases (e.g. because ofcontention or interference), nodes refrain from transmittingwhich can force nodes to drop frames because of a fulltransmission queue.

Other contention-based protocols use the results ofmedium access attempts as a measure of the success ofthe ongoing strategy and try to improve it. RMCA [99]and GBCA [90] follow game-theoretic approaches that usethe result of the contention for adaptability and robustness.In PMC, control theory is used to regulate the contention,which is measured directly from the ratio of gained channelaccesses to the total number of channel access attempts.In ARCH, nodes switch their channel when the ETX exceedsa given threshold. By doing so, not only collisions are takeninto account but also the effect of interference, jamming andany other event that may cause a transmission to fail.

Most contention-based protocols provide great potentialfor traffic adaptability and self-configuration, incurring arelatively low complexity. In general, tight synchronizationbetween nodes is not required and low delay can be achievedunder low-to-medium traffic conditions [95]. The downside isthat they cannot provide deterministic delays that are vital formany applications. Moreover, high traffic loads cause band-width waste because of back-offs and collisions. Problemssuch as over-emitting, overhearing and idle listening oftenoccur in contention-based protocol, reducing their energyefficiency.

2) TIME-SLOTTED PROTOCOLSScheduling nodes to transmit/receive/sleep using a time-slotted approach can provide deterministic end-to-end delaysand collision-free medium access, while reducing overhear-ing and idle listening. These advantages come at the expenseof higher delay, energy waste and protocol overhead causedby synchronization algorithms, in particular under low traf-fic loads. However, time-slotted solutions are promising forapplications that include synchronization for other purposesthan MAC coordination, as well as for applications in whichthe data is gathered on a periodic basis.

In this regard, somemultichannelMACprotocols for LLNshave followed time-slotted approaches, most of them orientedto periodic data collection applications. A particular featureof MC-LMAC [95], is that nodes build the schedule in adistributed way, by selecting a unique channel-and-timeslotcombination on which they are allowed to transmit. Eachtimeslot starts with a control period, during which all thepotential receivers must listen, causing an inherent source ofenergy waste for transmitter-oriented time-slotted protocols.

Decentralized time-synchronized channel swapp-ing (DT-SCS) [91], [120] uses a distributed algorithmto create a time-slotted schedule based on the conceptof pulse-coupled oscillators [121]–[123]. The mechanismachieves time-frequency multiple access scheduling bymeans of intra-channel desynchronization and cross-channel

synchronization [124], [125]. As such, DT-SCS succeeds tohave a balanced number of nodes per channel and evenly dis-seminates them in time on each channel. Nodes can ‘‘swap’’channel to reach intended receivers, because of applicationrequirements or to avoid interference.

DynMAC [86] divides the schedule into upstream anddownstream slots, which are used for sending towards thesink and receiving from it, respectively. From the set of slots,each leaf node is assigned one upstream slot for transmissionto his parent, whereas each parent must be assigned oneupstream slot per child to relay the received frames towardsthe sink, and one upstream slot to transmit its own frames.Note that there is no data aggregation in this reasoning.In the downstream, the sink requires at least one slot per nodein the network and each node requires one slot per child totransmit to them. Since inWSNs, nodes usually communicateonly with their preferred parent, DynMACwouldmake nodeswaste time and energy on slots for any other parent. Thisaggravates a usual problem of time-slotted protocols, namelythe allocation of slots that are not actually used by the nodes.

To avoid allocating more slots than needed by the nodes,WAVE, MODESA and DeTAS build the schedule at thesink (centralized way), based on the traffic conditions in thenodes. In MODESA and the centralized version of WAVE,the sink node has an a priori knowledge of the nodes’ traffic,whereas in DeTAS and the distributed version of WAVE, theactual traffic and routing information is collected from thenodes. Nodes build their own local schedules (called micro-schedules) to communicate with their neighbors, and sendthe micro-schedules to the sink. The overhead of sending theinformation to the sink and of distributing the schedule is adisadvantage. Also, it is not clear how and in which channelnodes communicate with each other to set up the network(e.g. a common control channel during the set-up phase). Themain advantages of these protocols are throughput maximiza-tion, data-gathering timeminimization, and traffic awareness.Their applicability is reduced to some periodic reportingapplications for which the individual traffic demands of thenodes can be determined in advance, or are not likely tochange once the schedule is built. Nonetheless, even in thosecases, changes in topology and environmental conditionsmayvary the amount of traffic sent by the nodes, degrading thenetwork’s performance until a new schedule is constructed.

Incel et al. propose JFTSS [104] for aggregated traffic inapplications based on periodical reporting or raw-data one-shot collection. JFTSS is based on algorithms able to achievelower bounds on the schedule length, once the interferinglinks are completely eliminated through the use of multiplechannels.

Typical application scenarios for time-slotted MAC pro-tocols are found in industrial environments because of thestrict requirements of those applications in terms of latencyand reliability. WirelessHART [35], the de facto standardfor the MAC layer in industrial LLN applications, meetsthose requirements and adds security. It handles routing andbuilds communication schedules accordingly, with additional

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mechanisms for providing robustness and encryption. Thestandard includes channel blacklisting, channel hopping,network-wide synchronization and AES-128 ciphers andkeys. WirelessHART’s high MAC-layer protocol overheadis justified in order to meet the requirements of the strictlyplanned industrial deployments.

3) HYBRID CHANNEL ASSIGNMENT PROTOCOLSSome authors have proposed hybrid schemes that com-bine time-slotted and contention-based medium access. Forinstance, MuChMAC [84] is time-slotted in the sense thatnodes use timeslot boundaries to switch channels but poten-tial transmitters do not follow a schedule but just contendfor the use of the channel in each slot. Each node pseudo-randomly selects a channel on which it will receive duringeach timeslot. To reduce the probability of collision, a TDMAoptimization is proposed to spread transmissions over thelength of each slot by using sub-slots that will be randomlyselected by the receivers. Although MuChMAC does notrequire tight synchronization, the notion of slot is providedby a built-in synchronization mechanism.

Similar hybrid approaches are followed by TFMAC [108]and TMMAC [96], for which time is divided in time-slottedframes composed of a contention-based period, for con-trol message exchange, and a contention-free period, duringwhich the actual data communication takes place. In TFMAC,the contention-based period occurs only once and nodes useit to coordinate the schedules for operating in the subsequentcontention-free mode. On the other hand, in TMMAC thecontention-based period is repeated periodically and it is usedto agree on a channel-and-timeslot combination to exchangedata frames during the contention-free period. Since allnodes are aware of the arrangements made during thecontention-based period, collision-free data communicationis guaranteed.

In HyMAC [103], time-slotted frames are also dividedinto contention-free slots and contention-based slots. Thecontention-based slots are used by the nodes to send HELLOmessages to the sink to inform it about their neighbors.Afterwards, the sink computes the schedule, which is sentto all nodes in the network during the control slots. Thisschedule assigns channel-and-timeslot combinations to thenodes for sending datamessages to their parents, with the goalof maximizing the network throughput and minimizing theend-to-end delay. These HELLO messages create protocoloverhead and consume energy, but they simplify the processof joining the network.

The hybrid approach of Y-MAC [85] assigns a timeslot inthe base channel (f 1 in Fig. 8) to each node. Each node wakesup to receive during its slot, and potential senders contendfor the medium at the beginning of each timeslot. Sinceonly one sender can win the contention, the receiving nodeswitches to the next channel during the following timeslot,to give a chance to the contention losers to transmit theirframes.

C. UPPER LAYERS INTERACTION1) BROADCAST SUPPORTIn protocols that assign channels to disjoint trees/clusters,such as TMCP [80], [81] and MCRT [82], broadcast supportis limited to nodes in the coverage area that belong to the sametree. Awide variety of protocols provides a dedicated channelor timeslot for broadcast. For instance, in MMSN [87], thereis a broadcast channel to which every node listens at thestart of every timeslot. Each broadcast frame is transmittedonly once, but collisions may occur. An analogous strategy isfollowed by Y-MAC [85], which introduces a slotted broad-cast period during which nodes can send broadcasts in thecorresponding slots, before the unicast period of the time-frame starts.

In ARM [74], on the other hand, the broadcast channelmust be periodically visited by each node and the senderof a MAC broadcast frame must transmit it repeatedly dur-ing the maximum time between consecutive visits. Doingso consumes a lot of energy at the sender. The broadcasttraffic in DeTAS [89] and RL-MMAC [105] is exchangedthrough a common dedicated channel that is also used forcontrol information. A very interesting approach is proposedby MuChMAC [84], which introduces broadcast slots in thehopping sequence and uses a CH sequence for broadcast slotsthat is common to all nodes and independent from the oneused for the unicast slots.

Some protocols do not handle upper-layers broadcast pack-ets through link-layer broadcast frames. Instead, they relyon a node to send unicast frames to each of its neighbors,one by one, putting the link-layer broadcast address in thedestination address field. Examples of this behavior can befound in A-MAC [92], RMCA [99], GBCA [90], ARCH [88],EM-MAC [73] and CR-WSN [109]. The medium accesscontrol mechanism determines the cost in terms of energy andtime.

2) UPPER-LAYER ORIENTED CHANNEL ASSIGNMENTMost MAC protocols with data collection-oriented channelassignment use information from the routing protocol or havepreliminary knowledge of the network topology. For exam-ple, DeTAS [89] is a distributed time-slotted protocol, whichuses routing information from RPL to create a collision-freeoptimal time-slotted schedule for data collection applications.RL-MMAC [105] creates a DODAGon the basis of hop countand uses reinforcement learning to select the best parent in aspecific timeslot and to create the schedule in a distributedway. A potential downside of the RL-MAC design is thatnodes can only receive from their children or transmit to theirparents along the routing tree.

JFTSS [104] uses two algorithms oriented to buildtime-slotted schedules for two extreme cases in WSNs:aggregated convergecast in continuous data collection,and raw-data convergecast in one-shot data collection.The proposed algorithms require information about therouting tree and the interfering links in the network.

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TABLE 6. Classification of several MAC protocols according to the upper layers interaction.

The routing tree can be obtained from a routing protocol, suchas RPL.

In iQueue-MAC [94], nodes acting as routers (i.e. parents)are responsible for the schedules of their children. The routingstructure must be obtained from a routing protocol, such asRPL and LEACH. HyMAC [103] and the protocol proposedin [115] do not rely on any routing protocol but they constructtheir own collection tree. HyMAC creates it using BreathFirst Search (BFS) for the channel-and-timeslot assignment,while checking for possible conflicts with neighboring nodes,whereas the protocol in [115] uses different algorithms tocreate and manage the routing tree.

Protocols in the class of topology-based assignment useinformation about the (full or partial) network topology toassign channels and timeslots. Here, TMCP [81],MCRT [82],MMSN [87], PMC [83], TACA [106] and DT-SCS [91] areincluded. In TMCP and MCRT, the network is partitioned indisjoint trees rooted at the sink by applying graph-coloringtechniques, in which the channels are considered colors.In the case of PMC, a clustering heuristic is applied to solvethe K-way cut problem [126] targeting at the assignment of ahome channel to each node while minimizing the inter-clustercommunication.

MMSN and TACA [106] aim at assigning a channel toeach node that is exclusive in its two-hop neighborhood.

Therefore, the assignment can be done in a distributed wayand only topology information of the two-hop neighborhoodis needed. If it is not possible to assign an exclusive homechannel to each node, MMSN tries to balance the number ofnodes per channel, whereas TACA uses the traffic demandsof the nodes to balance the traffic load on the channels. TACAis better in terms of load balancing and traffic-awareness thanMMSN, but it requires knowledge about the future trafficdemands of the nodes and control packets to exchange thatinformation.

3) OPTIMIZATION FOR TRAFFIC PATTERN SUPPORTAnother relevant aspect of a MAC protocol in LLNs iswhether it is optimized for some specific traffic pattern or not,and the level of support it provides to other traffic patterns.As collecting sensor data is one of the main applications ofLLNs, a considerable number of protocols are optimized forconvergecast traffic. Table 6 summarizes the classification ofthe multichannelMAC protocols reviewed in the scope of thispaper, according to their optimization for convergecast traffic.

It is worth pointing out that TFMAC and DynMAC are notexplicitly oriented to a specific traffic pattern. Nevertheless,the tests performed by the authors of TMMAC assume onlylocal gossip traffic, whereas DynMAC restricts the nodes tocommunicate only with their parent and children, not with

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TABLE 7. Classification of several MAC protocols according to the MAC layer coordination.

an arbitrary node in the radio range, limiting the protocol toconvergecast and flooding traffic patterns. Promoting somespecific traffic pattern tends to improve the performanceof a protocol in applications relying on that traffic pattern,but it brings down the efficiency of the protocol for otherapplications.

D. MAC LAYER COORDINATIONIn theMAC Layer Coordination category of the classificationframework three aspects appear: link-layer communication,channel switching frequency, and synchronization, as shownin Table 7.

1) LINK-LAYER COMMUNICATIONIn this category, protocols fall in one of four groups: sender-oriented, receiver-oriented, channel-oriented and exclusivechannel-and-timeslot assignment.

a: Sender-orientedDesigns such as HyMAC, WAVE and MC-MAC, assign theresources (channels and timeslots) to senders, therefore allpotential receivers must visit the channel of every transmitteror coordinate the data exchange using a CCC or SP strat-egy. OMA, CR-WSN, MC-LMAC and TMMAC are typi-cal examples of sender-initiated protocols. In MMSN, the

sender initiates the communication using a technique calledtoggle snooping and toggle transmission. In a first phase,nodes alternatively listen to their home channel and to theintended receiver’s channel before starting their own trans-mission (Fig. 9a). If no signal is detected on any of the twochannels, the sender starts transmitting small preambles onboth channels alternatively, to mitigate the hidden terminalproblem, as shown in Fig. 9b. This technique needs a highchannel switching frequency and care should be taken thatthe channel switching time does not jeopardize the correctfunctioning of the protocol.

b: Receiver-orientedThese protocols assign resources to receivers, implyingthat potential transmitters must contend for the medium orcoordinate their transmissions using the CCC or SP. Mostreceiver-oriented protocols initiate the data exchange througha beacon sent by the receiver, or through a RTS/CTS-likeexchange, but some other approaches have been proposed.An interesting example of receiver-oriented protocol is EM-MAC [73]. Thanks to its ACK/Beacon technique, it allowspotential transmitters to use the ACK sent after the data frameas a beacon for a new round of contention. This techniqueallows nodes to receive more than one frame in one wake-up,which improves the protocol performance in terms of traffic

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FIGURE 9. Toggle snooping (9a) and toggle transmission (9b) for unicastframe transmission in MMSN. f0: broadcast channel. fself : home channel.fdest : intended receiver’s home channel [87].

adaptability, potential throughput, delay and energyefficiency.

Another interesting technique is proposed by A-MAC [92]through which non-destructive ACKs are sent by all potentialtransmitters after receiving the beacon from the intendedreceiver. Hardware ACKs are used, so all the ACK framesshould be sent at the same time, containing the same infor-mation, so that they should not collide but strengthen eachother instead. This behavior has been called ConstructiveInterference (CI) and it has been proven in [127] that there isno significant correlation between the number of transmitters,the received signal strength indicator (RSSI) and the packetreception ratio. Moreover, a receiver would be able to benefitfrom CI only under very strict requirements of synchroniza-tion among transmitters, equal distances from transmitters tothe receiver and very similar values of RSSI (otherwise, thegain is mostly due to the capture effect, according to [128]).

c: Channel-orientedProtocols such as TMCP and MCRT use channels to groupnodes, and all the nodes in the same channel can communicatewith each other using some medium access mechanism, suchas ALOHA and CSMA.

d: Exclusive channel-and-timeslot assignmentMainly used by time-slotted protocols that assign slots tolinks between specific nodes; and the ones that build theschedule based on paths to the sink. Typical example of theformer group are DynMAC and RL-MAC, in which slots areassigned to exchange frames between parents and childrenalong the gathering tree. In the latter group, JFTSS, DeTAS,

WirelessHART andMODESA all build their schedules basedon links along the collection tree.

2) CHANNEL SWITCHING FREQUENCYThe selection of the channel switching frequency is a trade-off between flexibility, robustness against interference andenergy consumption. Nodes that stay on the same channelfor long periods of time (forever, in the extreme case of staticchannel assignment, as in TMCP [80], [81] and MCRT [82])could suffer critical performance degradation if/when thechannel gets interfered or jammed by external sources. In thatsense, frequency agility would improve the robustness of thenetwork.

On the other hand, switching channels can cost time andenergy because of the time needed by the oscillators of thelow-end radio interfaces used in LLN devices to stabilize.Also, protocols with channel hopping strategies, such asEM-MAC [73] andMuChMAC [84], require mechanisms fortransmitters to determine the channel in which their potentialreceiver(s) are. DynMAC [86] tries to find a balance bykeeping all the nodes in the same channel at any given time,and making them all switch to a different channel when theone in use experiences performance degradation.

3) SYNCHRONIZATIONAlthough most multichannel MAC protocols proposed forLLNs either assume the existence of synchronization,e.g. [87], [89], [103], or do not require synchronization atall, e.g. [74], [81], [93], [97], some authors have proposedtheir own algorithms, as shown in Table 7. For instance,Y-MAC [85] provides synchronization by correcting the localclock to the average between the local time and the value inthe time synchronization frames received from its neighbors.

EM-MAC [73] nodes, on the other hand, do not attemptto have a common clock but they use timestamps to create alinear timemodel of the clock differences with each neighbor,where the slope accounts for the clock drift. Then, a node canpredict the next wake-up of a neighbor using the informationof its last wake-up and its parameters for the time model.

In other protocols, such as A-MAC, nodes store the sched-ule of neighbors they have already communicated with.A hierarchical scheme is used by MuChMAC, MC-LMACand RL-MMAC, where nodes synchronize with their parents.MuChMAC does not require tight synchronization amongnodes and the authors demonstrate that neighboring nodesshould at least communicate every 52minutes to keep in sync,given a clock drift of ±40 ppm. Nonetheless, they proposea hierarchical synchronization mechanism based on a powergradient from the sink to the leaf nodes. In RL-MMAC,network-wide synchronization is initiated by the sink byflooding the network with synchronization frames [129].

Protocols that assume synchronization or rely on an exter-nal protocol to provide it, may be useful in systems involvingnodes with highly accurate clocks or for which the synchro-nization is carried out at another layer, so there is no needto spend resources on it at the MAC layer. On the contrary,

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TABLE 8. Classification of several MAC protocols according to the physical layer management.

protocols providing their own synchronization mechanismcan adapt the amount of resources invested on it to their ownactual needs, as is for example the case of MuChMAC.

Protocols that do not implement synchronization are proneto energy wastage for idle listening and over-hearing butthey do not spend resources on achieving synchroniza-tion, since synchronizing can be complex and resourceconsuming [130], [131].

E. PHYSICAL LAYER MANAGEMENTIn the physical layer management, the PCL management, theRDCmechanism and the interference avoidance are included,as shown in Table 8.

1) PREFERABLE CHANNEL LIST (PCL) MANAGEMENTa: Inclusion in the PCLMost protocols use a static default list of channels. Otherprotocols manage the PCL by including and excluding chan-nels based on some quality measure, such as ETX and PER.A typical mechanism is the use of blacklists for low-qualitychannels. Blacklists can be created in a distributed way,as in ARCH and EM-MAC, or in a centralized way, as inDynMAC, in which nodes collect the channel quality infor-mation and send it to the sink which decides the best channel

to be used. Each protocol uses a different metric: ARCHuses ETX, DynMAC uses RSSI, MC-LMAC uses the numberof packets lost, MuChMAC blacklists a channel when nopackets are received on it, and EM-MAC uses a ‘‘badness’’metric that varies according to the outcome of the CCAs per-formed by the nodes and the number of transmission attempts.TMCP performs channel quality checks before the channelassignment and only includes in the PCL the channels with aquality above a given threshold.

b: Selection from the PCLProtocols such as MMSN, TACA and MC-LMAC try tobalance the channel occupancy by selecting the least chosenor the least loaded ones. Note that the least chosen chan-nel is not necessarily the least loaded. Round-robin selec-tion can reduce the protocol complexity without negativeimpact on the performance, as inDeTAS,Y-MAC,WAVE andMODESA. On the other hand, some protocols use pseudo-random selection of channels so that no fixed pattern isfollowed, such as TFMAC, ARM, TMMAC,MuChMAC andEM-MAC. As explained in the previous subsection, protocolslike ARCH, JFTSS and GBCA select communication chan-nels based on interference metrics. Some protocols, such asEM-MAC and WirelessHART, use pseudo-random channel

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selection but include some aspects of interference-awarenessthrough their blacklisting mechanisms.

2) INTERFERENCE AVOIDANCEInterference avoidance mechanisms have been split in theclassification framework in three groups: intra-network,implicit and active interference avoidance.

a: Intra-network interference avoidanceSome protocols try to find interfering links and partition thenetwork in accordance. In the case of TMCP and MCRT,the algorithms actually partition the network in sub-trees andassign different channels to them. DeTAS,WAVE,MODESAand JFTSS aim to discriminate interfering links along thegathering tree and distribute channels and timeslots in sucha way that communications do not interfere. On the otherhand, GBCA models the network as a game in which theobjective is to minimize the intra-network interference, calledtotal interference by the authors of GBCA.1

b: Implicit interference avoidanceThemechanisms included in RMCA andDynMAC implicitlyavoid interference. In RMCA, the algorithm is based on aregret function [132] that combines the average packet trans-fer delay (ATD) and the valid packet reception ratio (VRR),which is defined as the ratio between the number of validpackets received by a node and the number of packets it hassensed. Channels with high ATD and low VRR will receive ahigh regret value from the algorithm, preventing nodes fromvisiting them. In DynMAC, whose primary objective is toreduce the effects of adjacent and co-channel interference,nodes inform the sink when the PER rises above a giventhreshold, signaling that the interference levels are too highto stay in that channel.

c: Active interference avoidanceA common solution to combat external interference is touse a blacklisting mechanism that avoids the use of highlyinterfered channels. EM-MAC, MuChMAC, ARCH, Wire-lessHART and MC-LMAC employ this technique based ondifferent metrics, e.g. channel quality, ETX and other vari-ables based on the success/failure of transmissions. ARCHtakes into account the fact that adjacent channels experiencesimilar conditions of interference, so the algorithm promotesthe use of channels spectrally far from the interfered ones.CR-WSN approaches the problem from the perspective ofcognitive radio [133]–[135] by modeling the external inter-ference as external (to the LLN) users that should not bedisturbed by the LLN. Therefore, nodes sense the channelconditions and decide whether to access them according tothe level of activity perceived from external sources.

3) RDC MECHANISMIn contention-based sender-oriented protocols, such asMMSN and CR-WSN, nodes wake up periodically and listen

1The term total interference is used as the interference suffered by areceiver because of the links it can hear.

for an incoming transmission until the time left in the trans-mission window is not enough to send a full-length frame.If no incoming transmission is detected, nodes go back tosleep. This mechanism is very inefficient in lightly-loadednetworks because of excessive idle listening. In contention-based receiver-oriented protocols, such as A-MAC, ARM,EM-MAC and OMA, receivers send a beacon to signal thatthey are awake and wait for potential transmitters to act. If nosignal from potential transmitters is detected, the receiverscan go to sleep. In both cases, sender- and receiver-orientedprotocols, transmitting nodes that loose the contention for themedium can go to sleep and follow their own periodic wake-up behavior until another transmission attempt is scheduled.An exception to this is EM-MAC, because the losers of thecontention can use the ACK of the data frame as a beaconto start a new contention during the same wake-up of thereceiver.

In the time-slotted approaches, TFMAC [108] andTMMAC [96] allow nodes to turn off the radio during theslots in which they are not scheduled to transmit or receive;whereas nodes using RL-MMAC sleep during the slots inwhich the probability of successful transmission or receptionis lower than a threshold. DeTAS, leaves that decision tothe schedule manager included in the recent IEEE 802.15.4standard. In MC-LMAC, nodes are assigned slots for trans-mitting. Therefore, potential receivers must wake up in thetransmission slots of their neighbors to check if there is aframe addressed to them. If this is not the case, they can backgo to sleep.

HyMAC [103] and Y-MAC [85] are examples of hybridapproaches. HyMAC, a sender-oriented protocol, appliesRDC during the contention slots and, during the slots inwhich a scheduler is active, it keeps the node awake onlyif receptions or transmissions are scheduled. In Y-MAC,a receiver-oriented protocol, nodes only wake up during theirassigned slots and the consequent slots after successful dataexchanges, unless they have a frame to transmit. In that case,potential transmitters contend during the contention periodbut they sleep during the back-off. A receiver that owns a slot,wakes up at the end of the contention period to receive thedata frame.

F. NON-FUNCTIONAL CONSIDERATIONSBesides all the functions performed by multichannel MACprotocols, there are some additional considerations that areimportant during the design and implementation process.These issues can be related to design principles (e.g. maingoal of multichannel operation), consequences of the design(e.g. protocol overhead), or practical matters (e.g. the level ofimplementationmaturity). A summary of these aspects for theprotocols included in the classification is shown in Table 9.In the following, a brief discussion on these aspects ispresented.

1) MAIN GOAL OF MULTICHANNEL OPERATIONThe design of the protocol is determined by the goals.Although the variety of secondary goals is quite broad, the

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TABLE 9. Classification of several MAC protocols according to the non-functional considerations.

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main goals of most multichannel MAC protocols for WSNsare related to improving network performance in terms ofthroughput, delay and robustness against interference whilereducing the energy consumption as much as possible, as itis accurately identified by [32]. For example, time-slottedprotocols that aim to reduce the delay from source to sinktend to create schedules in such a way that interfering linksare eliminated and children-to-parent communication is stag-gered to create a collection cycle. JFTSS and WAVE aretypical examples of this.

Some protocols try to cope with the ad hoc and unpre-dictable nature of LLNs and their environment by designingmore flexible and decentralized protocols, as in MuChMACand Y-MAC, but they fail to provide deterministic delays.EM-MAC and PMC address the robustness against inter-ference through channel hopping schemes whereas others,like TMCP, use fixed channel assignment to mitigate intra-network interference. On the other hand, ARM and OMAstand as examples of protocols that look for more specificgoals, such as mitigation of the CCB and hidden terminalproblems.

2) PROTOCOL OVERHEADThe protocol overhead has a higher impact on the networkperformance in LLNs than in most other wireless networks.The main reason is the reduced frame size used in LLN, e.g.127 bytes according to the IEEE 802.15.4 standard. The mainpotential sources of protocol overhead in multichannel LLNsare channel-and-timeslot coordination, control informationexchange and synchronization frames or header fields. Inthe scope of this classification and review study, a coarseclassification (low, medium, high) has been considered forthe protocol overhead, since this categorization comes fromthe analysis made by the authors, not on theoretical or experi-mental measurements. Nonetheless, the information providedin Table 9 can be taken into account and serve as a guide forfuture work in the field.

3) IMPLEMENTATION MATURITYBy implementation maturity, we point out to which levelthe protocol has been implemented and tested. Some of theprotocols classified here have been tested through simulationand real test-bed experiments butmost of them have used onlyone of the two possibilities. Although simulators constitutea powerful tool during the development process, simulationexperiments’ results should be validated through real test-bedexperiments as much as possible.

The second and third columns of Table 9 show the simula-tor(s) and the real testbed platform, respectively, used in theevaluation tests presented by the authors of the protocols. Forthe real testbed platform, the operating system is specified,except for TMCP, OMA and AdvMAC, for which only thehardware platform has been mentioned by their authors.

The review of the implementation maturity shown inTable 9 confirms a very important issue about the researchand development of multichannel MAC protocols for LLN:

the huge variety of simulators, operating systems and hard-ware platforms available for LLNs. Note that ContikiOS isthe dominant operating system used to evaluate the protocolson real testbeds, but others have also been used, such asTinyOS and RETOS.

As shown in Table 9, the plethora of simulatorsused by the LLNs research community [136], [137]include well-established simulators, such as COOJA [138],Castalia/OMNET++ [139] and GloMoSim [140], as well ascustom-made simulators created by the authors of some ofthe protocols [88], [89], [101]. Some studies, such as [74],[104], and [106], do not even specify the simulator used in theperformance test and comparison with other protocols, whichmakes it difficult to assess the validity of their results. Thesame happenswith the operating system or hardware platformused by WirelessHART [35] and MC-LMAC [95].

4) INTERFERENCE MODELThe interference model used in the performance evaluation ofa protocol influences the validity of the results. The interfer-ence models used in the performance evaluation of the proto-cols presented here, can be classified in graph-based modelsand realistic models. The former, which is closely relatedto simulation-based tests, refers to simulators that model thecoverage area as a circle, or sphere, in which nodes perfectlyreceive any transmitted frame (or a user-defined percentageof them) and simultaneous transmissions inside it always leadto collisions. On the other hand, realistic (or physical) modelsare related to advanced simulation environments, in which thecoverage areas are not perfect circles and can vary across timeand frequency.

Realistic models can be implemented in simulators byusing well-known propagation and channel models, and theyare already present in some simulators, such is the case ofGloMoSim. Though they provide a more accurate view of theexpected behavior of the protocols in real life scenarios, theaccuracy of these models translates into higher computationalcomplexity and, therefore, longer time needed for the simu-lations to complete. For this reason, MAC (and upper layers)protocol designers often use graph-based interference modelsand leave the complex realistic ones for researchers lookinginto issues closer to the physical layer.

VII. IEEE 802.15.4 STANDARD MAC PROTOCOLBecause of the impact the IEEE 802.15.4 standard has inthe research, development and deployment of LLNs, thissection aims to briefly describe the recent multichannel MACprotocol included in the standard. This protocol is based onthe single-channel version included in the first specificationof the standard in 2011 [141]. The single-channel MACoperation in IEEE 802.15.4 is not discussed here, see [33]for details.

In 2012, an amendment to the IEEE 802.15.4 standard wasintroduced to add multichannel operation [116]. It proposes atime-slotted multichannel MAC protocol for Low-RateWire-less Personal Area Networks (LR-WPANs), known as TSCH.

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In this mode of operation, time is divided into slots and theduration of each slot is sufficient to send a maximum lengthdata packet and receive the corresponding ACK. Slots can beshared or dedicated. In every slot, three actions are possible:transmit, receive and sleep.

Slots are grouped into time-slotted frames, called slot-frames, which repeat periodically over time. Duty-cycling isachieved by introducing sleeping slots in every slotframe. Allnodes are synchronized to a given slotframe andmultiple slot-frames can coexist. Synchronization information is includedin all packets.

Themultichannel operation is based on deterministic chan-nel hopping by assigning a channel offset to each slot. Thehopping sequence is calculated using the following equation:

f = F{(

ASN+ choffset)

mod chtotal}

(1)

where ASN is the absolute slot number of the network, choffsetis the channel offset for the current slot, and chtotal is thetotal number of available channels. Function F is a lookuptable containing the available channels. ASN keeps the countof the number of slots that have elapsed since the start ofthe network. Through Eq. (1), a channel-hopping behavioris produced with period equal to chtotal, as shown in Fig. 10.

FIGURE 10. TSCH channel hopping example.

The problem of creating the schedule for the nodes tocommunicate with each other is considered out of the scopeof the standard. Therefore, scheduling protocols orientedto be managed by the IEEE 802.15.4 MAC protocol havebeen proposed, such as the ones published in [89] and[142]–[144]. The work in [144], combined with TSCH, hasshown better performance than EM-MAC, a state-of-the-artcontention-based protocol for LLNs, according to a studypresented in [145]. The work in progress in [142], [143],and [146]–[148] aims at standardizing a sublayer in chargeof managing the schedule in TSCH-based networks.

VIII. FINAL CONSIDERATIONSIn this paper we have presented a classification frameworkand a classification and review of multichannel MAC proto-cols for LLNs. The proposed framework takes into accountthe challenges of multichannel communications in LLNsand the design aspects of a protocol including the interac-tions between a multichannel MAC protocol with the sur-rounding protocol stack. The validity and suitability of theclassification framework have been demonstrated throughthe classification and review of more than 30 multichannelMAC protocols for LLNs. The discussion presented along

the classification provides insights about themain approachesfollowed by designers and developers in this research area.

The study presented here provides guidance for futuredevelopments in multichannel MAC protocols for LLNs,which can be summarized as follows:

1) For protocol designers, this study presents the compo-nents of aMAC protocol for LLNs that need to be takeninto account when designing a multichannel MAC pro-tocol for LLNs, as well as the main approaches thathave been followed until now. Moreover, the interac-tions between theMAC layer and the other layers of theprotocol stack has been presented from the perspectiveof a multichannel MAC protocol.

2) For software developers, the analysis about the non-functional issues of the MAC protocols presents keyelements of the development process, such as the useof real testbed experiments to validate the performanceevaluation of the protocols, and the importance ofthe interference model used during simulation, and itseffect on the simulation time.

3) For application designers, the suitability of eachapproach for different types of applications has beenpresented along the paper by pointing out the weak andstrong points of each design element of the multichan-nel MAC protocols.

There are various open research and developments issuesin the area of multichannel MAC protocols for LLNs. Fromthe perspective of this study, we would like to highlight thefollowing research challenges:• The adaptability to traffic changes, robustness againstinterference and support for time-critical applicationsprovided by the existing protocols are not enough forfuture LLN applications.

• The interaction of the MAC layer with the protocolstack is not yet an integral part of the design process ofmultichannel MAC protocols for LLNs and schedulingalgorithms for TSCH.

• More realistic models with manageable computationalcost are needed to improve the validity of the simulationexperiments of MAC protocols for LLNs.

• Unified simulation and testbed setups are needed for theperformance evaluation and comparison of multichannelMAC protocols for LLNs.

• TSCH, the multichannel MAC mode for LLNs includedin the IEEE 802.15.4 standard, requires scheduling algo-rithms that take into account the requirements of theMAC layer in LLNs, as well as its interoperability withthe proposed standards for the network stack, especiallywith CoAP, UDP, RPL and 6LoWPAN.

ACKNOWLEDGMENTThe authors would like to thank the Belgian DevelopmentCooperation through VLIR-UOS for supporting this workunder the ICT Network Project between VLIR (VlaamseInter Universitaire Raad, Flemish Interuniversity Council,Belgium) and Five Cuban universities.

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CARLOS M. GARCÍA ALGORA received theB.Eng. degree in telecommunications and elec-tronics engineering and the master’s degree intelematics from the Universidad Central ‘‘MartaAbreu’’ de Las Villas (UCLV), Cuba, in 2011and 2014, respectively. He is currently pursuingthe Ph.D. degree in a joint program with theDepartment of Electronics and Informatics, VrijeUniversiteit Brussel, and the Department ofTelecommunications and Electronics, UCLV. His

primary research interests include the design, implementation, and eval-uation of multichannel MAC protocols for low-power and lossy wirelessnetworks.

VITALIO ALFONSO REGUERA received theB.Eng. degree in telecommunications and elec-tronics engineering, the M.Sc. degree in telecom-munications engineering, and the Ph.D. degree inelectrical engineering from the Universidad Cen-tral ‘‘Marta Abreu’’ de Las Villas (UCLV), Cuba,in 2000 and 2007, respectively. He is currently aProfessor with the Department of Telecommunica-tions and Electronics, UCLV. His current researchinterests include cognitive radio, communication

protocols, quality of service, and wireless networks.

NIKOS DELIGIANNIS received the Diplomadegree in electrical and computer engineeringfrom the University of Patras, Greece, in 2006,and the Ph.D. degree (Hons.) in applied sciencesfrom Vrije Universiteit Brussel (VUB), Belgium,in 2012. From 2012 to 2013, he was a Post-Doctoral Researcher with the Department of Elec-tronics and Informatics, VUB. From 2013 to 2015,he was a Senior Researcher with the Department ofElectronic and Electrical Engineering, University

College London, U.K., and also a Technical Consultant on Big Visual DataTechnologies with the British Academy of Film and Television Arts, U.K.He is currently an Assistant Professor with the Department of Electronicsand Informatics, VUB. He has authored over 80 journal and conferencepublications, book chapters, and two patent applications (one owned byiMinds, Belgium and the other by BAFTA, U.K.). His current research inter-ests include big data processing and analysis, machine learning, Internet-of-Things networks, and distributed signal processing. He was a recipient of the2011 ACM/IEEE International Conference on Distributed Smart CamerasBest Paper Award, the 2013 Scientific Prize FWO-IBM Belgium, and the2017 EURASIP Best Ph.D. Award.

KRIS STEENHAUT received the master’s degreein engineering sciences in 1984, the master’sdegree in applied computer sciences in 1986,and the Ph.D. degree in engineering sciencesfrom Vrije Universiteit Brussel (VUB), Belgium,in 1995. She is currently a Professor with theDepartment of Electronics and Informatics and theDepartment of Engineering Technology, Facultyof Engineering Sciences, VUB. She has authoredover 150 journal and conference publications,

including book chapters. Her research interests focus on the design, imple-mentation and evaluation of (wireless) sensor network protocols for buildingautomation, environmental monitoring, and smart grids.

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Review and Classification of Multichannel MAC Protocols for Low-Power and Lossy …homepages.vub.ac.be/~ndeligia/pubs/CARLOS_Classification... · 2020-02-20 · At the network layer,