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Ad Hoc Networks 9 (2011) 73–94

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Ad Hoc Networks

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MC-LMAC: A multi-channel MAC protocol for wireless sensor networks

Ozlem Durmaz Incel a,*, Lodewijk van Hoesel b, Pierre Jansen c, Paul Havinga c

a NETLAB Research Group, Department of Computer Engineering, Bogazici University, Istanbul 34342, Turkeyb Ambient Systems B.V., Colosseum 15d, 7521 PV, Enschede, The Netherlandsc Pervasive Systems Research Group, Department of Computer Science, University of Twente, 7500 AE Enschede, The Netherlands

a r t i c l e i n f o

Article history:Received 13 October 2009Received in revised form 10 May 2010Accepted 14 May 2010Available online 4 June 2010

Keywords:Wireless sensor networksMAC protocolsMulti-channel communicationSchedule based communication

1570-8705/$ - see front matter � 2010 Elsevier B.Vdoi:10.1016/j.adhoc.2010.05.003

* Corresponding author. Tel.: +90 2163597125.E-mail addresses: [email protected]

[email protected] (L. van Hoesete.nl (P. Jansen), [email protected] (P. Havinga

a b s t r a c t

In traditional wireless sensor network (WSN) applications, energy efficiency may be con-sidered to be the most important concern whereas utilizing bandwidth and maximizingthroughput are of secondary importance. However, recent applications, such as structuralhealth monitoring, require high amounts of data to be collected at a faster rate. We presenta multi-channel MAC protocol, MC-LMAC, designed with the objective of maximizing thethroughput of WSNs by coordinating transmissions over multiple frequency channels.MC-LMAC takes advantage of interference and contention-free parallel transmissions ondifferent channels. It is based on scheduled access which eases the coordination of nodes,dynamically switching their interfaces between channels and makes the protocol operateeffectively with no collisions during peak traffic. Time is slotted and each node is assignedthe control over a time slot to transmit on a particular channel. We analyze the perfor-mance of MC-LMAC with extensive simulations in Glomosim. MC-LMAC exhibits signifi-cant bandwidth utilization and high throughput while ensuring an energy-efficientoperation. Moreover, MC-LMAC outperforms the contention-based multi-channel MMSNprotocol, a cluster-based channel assignment method, and the single-channel CSMA interms of data delivery ratio and throughput for high data rate, moderate-size networksof 100 nodes at different densities.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction packets is generated, for instance, due to a change in mon-

In typical wireless sensor network (WSN) applications, itis of interest to extend network lifetime due to battery lim-itations of the sensor devices. As an important source of en-ergy consumption, wireless communication in WSNs hasreceived a lot of attention. Especially several MAC protocols[1] have been extensively studied with the objective of en-ergy efficiency whereas throughput, bandwidth utilization,fairness and latency were considered as secondary objectives [2].

Typically, bandwidth is not a primary concern in tradi-tional low duty cycle, low data rate applications. However,it becomes crucial when sampling at high rate is required,or during certain periods of time when a large burst of

. All rights reserved.

.tr (O.D. Incel), lode-l), [email protected]).

itored conditions. For example, it has been noted that innetworked structural health monitoring, more than 500samples per second are required to efficiently detect dam-ages [3]. Multimedia WSNs [4], which are composed ofembedded cameras and microphones besides scalar sen-sors, also require high throughput and high delivery rate.Moreover, it becomes more common to use sensor nodesthat run multiple concurrent applications requiring higherdata rates.

The fundamental limitations on the achievable through-put are the limited reuse and/or wastage of bandwidth dueto interference and half-duplex operation of the radios. Ingeneral, in wireless networks multiple channels1 have been

1 A channel is defined to be a frequency range over which two nodescommunicate. We use the terms ‘‘channel” and ‘‘frequency” interchange-ably in the text.

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74 O.D. Incel et al. / Ad Hoc Networks 9 (2011) 73–94

provisioned to mitigate the effects of interference by sched-uling interfering transmissions on different frequencies.

In this paper, we investigate the use of multi-channelMAC protocols to improve the achievable throughput ofWSNs. Although the typical WSN radios operate on a lim-ited bandwidth, the operating frequency of the radios canbe adjusted over different channels. Once different chan-nels are assigned to previously interfering or contendinglinks, more concurrent transmissions can take place andmore data can be delivered to the sink node in shorterintervals.

We first present the challenges and requirements ofmulti-channel communication from the perspective ofWSNs. Next, we introduce Multi-Channel LightweightMedium Access Control (MC-LMAC), which is a schedule-based multi-channel MAC protocol that takes advantageof contention and collision-free parallel transmissions ondifferent channels. MC-LMAC is designed to provide higherthroughput over multiple channels besides meeting thetraditional requirements of WSNs, such as energy effi-ciency and scalability.

The main design is based on the single-channel Light-weight Medium Access Control (LMAC), which has beenproven to be an efficient and energy-aware MAC protocolfor WSNs [1,5]. A node selects a time slot and a channelon which it is allowed to transmit. time slot and channelselection are fully distributed and guarantee that thesame slot/channel pair are not used for conflicting trans-missions. A timeslot consists of a control period and adata transmission period. During the control period, allthe nodes switch their interfaces to a common channel.The control period is used for notifying the destinationabout the incoming packet and the channel on whichthe data transmission will take place such that the recei-ver switches its interface.

Our contribution is to present a new multi-channelMAC protocol with a fully distributed scheduling mecha-nism that does not require a centralized scheduler. In con-trast to the rich literature on scheduling, especially thoseusing offline graph-coloring approaches, nodes discoverand take control of their slots and channels in a localizedway by only exchanging information within their localneighborhood in MC-LMAC. The MC-LMAC protocol ad-dresses all the challenges and meet the requirements ofmulti-channel communication in WSNs. Moreover, we donot assume any unrealistic communication or interferencemodels, such as the graph based models where nodes areassumed to have a communication link if they are withina certain distance from each other, but we do implementthe protocol in the GlomoSim simulator using realisticphysical layer models and also on real sensor motes. Thefollowing are some of the other key highlights of this work:

� We present a review of existing multi-channel MACprotocols for WSNs and discuss the requirements andchallenges of multi-channel communication.� MC-LMAC not only supports many-to-one communica-

tion toward the sink node but also broadcasts and local-gossip operations. This can be quite challenging in amulti-channel communication environment with a sin-gle transceiver available on each node [6].

� We evaluate the performance of MC-LMAC with exten-sive simulations in Glomosim, and present a large studyof comparisons with MMSN [7], which is a recently pro-posed multi-channel MAC protocol for WSNs. Differentfrom the scheduled communication in MC-LMAC,MMSN provides contention-based channel access. Theprotocols with completely different designs allow usto study a large set of trade-offs between different per-formance metrics. Moreover, we compare the perfor-mance of MC-LMAC with single-channel CSMA, andwith a clustering mechanism where the branches ofthe convergecast routing tree are assigned differentchannels to prevent inter-branch collisions andinterference.� To show the advantages of multi-channel protocols, we

compare MC-LMAC and the above-mentioned tech-niques with an alternative where the communicationtakes place on a single-channel but over a largerbandwidth.� We implement MC-LMAC on the Ambient lNode sensor

node platform as a proof of concept of time synchroni-zation. In [8], it is argued that frequent channel switch-ing may cause potential packet losses. With thisimplementation, we aim to show that nodes can changetheir operating frequency without losing synchroniza-tion and packets while running MC-LMAC.

The remainder of the paper is organized as follows: Sec-tion 2 presents related works. Section 3 motivates the useof multiple channels in WSNs. Section 4 introduces theMC-LMAC protocol. Section 5 presents the performanceof our proposed protocol for typical WSN traffic patterns.Finally, Section 6 draws the conclusions.

2. Related work

2.1. Use of multiple channels in general wireless networks

The problem of channel assignment and multi-channelMAC protocols are well-studied topics for both cellularand wireless ad hoc networks. In cellular networks [9],base stations use different frequency domains within a cell,while clients share the time domain to access the wirelessmedium. However, this approach is either infrastructure-based or works within a single-hop neighborhood, and soit may not suitable for WSNs where multi-hop topologiesare used to cover large areas with short-range radios.

Multi-channel communication has been extensivelyused in multi-hop ad hoc networks to increase systemthroughput [10–13]. Most of these approaches are basedon IEEE 802.11; for instance, IEEE 802.11b allows 11 chan-nels that are spaced 5 MHz apart. However, the IEEE802.11 protocols are expensive in terms of energy con-sumption and may not meet the requirements of WSNsas addressed in [7]. Protocols in [14] either assume multi-ple radios on the nodes or consider radios that can listen tomultiple frequencies simultaneously. Protocols in [15,16]can operate with frequency-hopping spread spectrumwireless cards. However, in WSNs, usually nodes are

O.D. Incel et al. / Ad Hoc Networks 9 (2011) 73–94 75

equipped with much simpler radios and there is only oneavailable on each node.

Considering these differences, multi-channel protocolsthat are developed for wireless ad hoc networks may notbe directly applied to WSNs since the traditional require-ments of WSNs, such as energy efficiency and scalability,remain important concerns. On the other hand, the funda-mentals of the presented channel assignment strategiescan guide protocol designing since WSNs share the chal-lenges of single-radio wireless ad hoc networks, such asbroadcast support and avoiding network partitioning.

Single-radio, multi-channel protocols for wireless ad hocnetworks can be classified according to the following chan-nel assignment methods: (i) fixed assignment, (ii) semi-dy-namic assignment, and (iii) dynamic assignment. In fixedassignment, radios are assigned channels for permanentuse. Although the assignment of channels can be renewed,for instance due to changing interference conditions, radiosdo not change the operating frequency during communica-tion. In the semi-dynamic approach, radios are assigned con-stant channels, either for receiving or transmitting, but it ispossible to change the channel for communicating withradios that are assigned different channels. In dynamicchannel assignment method, nodes are not assigned staticchannels and can dynamically switch their interfaces fromone channel to another between successive data transmis-sions. Dynamic channel assignment is further classified intothree categories based on the methods of coordination [13]:(i) Dedicated Control Channel, (ii) Split Phase, and (iii) Fre-quency Hopping. With the dedicated control channel ap-proach [10,17], nodes synchronize by exchanging controlpackets on the dedicated control channel and negotiate forthe channel to be used for data exchanges. In split phase pro-tocols, nodes access the medium in 2 phases: a control phaseand a data exchange phase. During the control phase, all thenodes switch to a common control channel and negotiatewith their intended receivers for the channel(s) to be usedduring the data exchange phase. Usually, during the controlphase, access to the medium is contention based. Protocolsdiffer according to the channel access mechanisms they sup-port during data exchange. An example of contention-basedprotocols is Multi-Channel MAC (MMAC) [12], whereas Mul-ti-Channel Access Protocol (MAP) [18] and TMMAC [19] areexamples of protocols that are based on scheduled access.In frequency-hopping approaches [16,20], nodes switch, orin other words hop, between different channels.

Following this classification, our contribution is to pres-ent a combination of different approaches: semi-dynamicchannel assignment that benefits from the ‘‘split phase”.Different than other split phase protocols, access to themedium during the control phase is not contention-basedbut by following a schedule. This makes the protocol ro-bust against possible collisions during increased conten-tion, and in turn reduces the retransmissions and energyconsumption in the network.

2 This is actually a design decision, because the nodes that do not requireto communicate with each other may be placed in different clusters. Here,we refer to the nodes on different channels that require to communicate.

2.2. Use of multi-channel communication in WSNs

In WSNs, there are many MAC protocol proposals thatconsider single channel communication [21–27]. These

protocols exhibit good performance in terms of energy effi-ciency [1], scalability, and adaptability to changes [2].

There are other single-channel MAC protocols that aimto provide high throughput, especially with scheduledcommunication, such as Z-MAC [25] and Burst-MAC [28].While these protocols function well in single-channel sce-narios, parallel transmissions over multiple channels canfurther improve the throughput by eliminating contentionand interference.

2.2.1. Challenges and requirementsIn this section, we discuss the challenges and require-

ments for single-radio multi-channel communication andaddress how we use them in our protocol:

� Synchronization: If the channel assignment is donedynamically, i.e., the radios are switching betweenchannels instead of being fixed on one channel, adetailed coordination of channel switching is requiredbetween senders and receivers in order to communicateon the same channel at the same time. Scheduledaccess, on which our protocol is based, overcomes thiscomplexity.� Partitions: If transceivers of two nearby nodes are fixed

on different frequencies, they cannot communicatewith each other.2 MC-LMAC uses a common channel dur-ing the control period of each timeslot to let the receiversbe informed about the requests and channels on whichdata will be sent.� Joining the network: A new node joining the network

may disrupt the channel organization or may berequired to scan all the channels to find a suitable onefor transmission. In MC-LMAC, communication on acommon channel at the beginning of each timeslot letsthe new node collect full information about its neigh-borhood before starting transmission.� Broadcast support: If the nodes are switching between

channels dynamically, it might be problematic to sup-port local broadcasts. However, local broadcasts areimportant for WSN traffic, for instance, sensor nodesmay require in-network processing before they trans-mit the data toward the sink node or use broadcastsfor route discovery. In MC-LMAC, all the receivers of abroadcast are informed on the common channel at thebeginning of each slot.� Channel switching: The radio can not switch between

the channels immediately but takes some time, forinstance, around 200 ls on CC2420 [29] radios. The sizeof a timeslot in MC-LMAC is large enough to accommo-date the switching time; its associated overhead can beconsidered negligible.

Besides these challenges, the traditional challenges ofWSNs, such as energy efficiency and scalability, remainimportant concerns in designing multi-channel protocols.


2.2.2. Existing workThere exist recent proposals for multi-channel usage in

WSNs. In this section, we discuss the differences betweenprior work and our work. The performance of existing pro-tocols have been compared with single-channel protocols;here, we compare our protocol with the multi-channel pro-tocols via simulations.

The IEEE 802.15.4 protocol [30,31], which is originallydesigned for low-rate personal area networks (PAN), canbe used for WSN applications. The protocol makes use ofmulti-channel communication to reduce the effects ofinterference due to co-existing networks that share sameparts of the spectrum. The protocol has two modes ofoperation: beacon and beaconless modes. In the bea-con-enabled mode, a coordinator node, which is a fullfunction device, is responsible for adjusting the channel,on which its end-devices, i.e., members, should commu-nicate, according to the interference experienced by theconnected nodes to this coordinator. In this mode, com-munication can take place in a slotted mode of opera-tion, i.e. guaranteed timeslots can be allocated by thecoordinator, and nodes should directly communicatewith the coordinator to get the slot allocations. In thiscase, communication takes place on a single-hop net-work, such as a star. Even if a node intends to commu-nicate with a peer in its communication range, allcommunication flows via the coordinator. When the pro-tocol operates in a beaconless mode, it uses CSMA/CAand nodes operate on a fixed channel.

A multi-hop network can be constructed by linkinggroups of star formations in the beacon-enabled mode. Inthis case, the beacon message should contain the devicedepth on the tree and the timing offset, such that a nodeselects a receiving schedule different than its parent. Com-pared to the slotted mode of operation in IEEE 802.15.4,MC-LMAC does not require a central scheduler to allocatetimeslots. Due to the hierarchy in the IEEE 802.15.4 net-works, the PAN coordinator is responsible for binding ofnew nodes in the network, scheduling and routing. Addi-tionally, in IEEE 802.15.4, since all the nodes in a PAN, com-municate on the same channel, contention within thenetwork is not resolved. In MC-LMAC, nodes that contendwith each other are assigned different channels.

In [7], Zhou et al., introduced the MMSN multi-fre-quency MAC protocol especially designed for WSNs. It isa slotted CSMA protocol where at the beginning of eachtimeslot nodes need to contend for the medium beforethey can transmit. On the other hand, in the MC-LMAC pro-tocol we assume scheduled access, where each node isgranted a timeslot and performs its transmissions withinthis timeslot without contention. Contention-based proto-cols are known to have a lower delay and promisingthroughput potential at lower traffic loads, which generallyhappens to be the case in WSNs [2]. However, when thenetwork load is high, there is a higher waste of bandwidthfrom collisions and back-offs. On the other hand, schedule-based communication has the inherent advantage of colli-sion-free medium access but is less efficient when the net-work load is low.

MMSN assigns channels to the receivers. When a nodeintends to transmit a packet it has to listen for the incom-

ing packets both on its own frequency and the destina-tion’s frequency. A snooping mechanism is used to detectthe packets on different frequencies which causes thenodes to switch between channels frequently. MMSN usesa special broadcast channel for broadcast traffic, and thebeginning of each timeslot is reserved for broadcasts. Dif-ferent from MMSN, MC-LMAC does not require a dedicatedbroadcast channel. On the other hand, at the start of eachtimeslot, all nodes are required to listen on a commonchannel (different from the dedicated broadcast channel,this can be used for data exchanges as well), in order to ex-change control information, which simply adds to the pro-tocol overhead. But doing so provides many advantages[5], such as collision-free addressing, maintaining synchro-nization, allowing distributed operation of the medium ac-cess. Moreover, the control period is much smallercompared to the data period, and during the data periodthe nodes can transmit multiple packets to minimize theoverhead.

TMCP [8] is a tree-based multi-channel protocol fordata collection applications. The goal is to partition thenetwork into multiple subtrees while minimizing the in-tra-tree interference. The protocol partitions the networkinto subtrees and assigns different channels to the nodesresiding on different trees. TMCP is designed to supportconvergecast traffic, and it is difficult to have successfulbroadcasts due to the partitions. Contention inside thebranches is not resolved since the nodes communicate onthe same channel.

Similar to TMCP, the protocol in [32] uses a control-the-ory approach to assign channels to the clusters of nodes.Initially all the nodes communicate on the same channeland when a channel becomes overloaded, nodes migrateto new channels based on the feedback information fromtheir neighbors.

Y-MAC [33] is another recently proposed multi-chan-nel MAC protocol designed for WSNs that is based onscheduled access. However, timeslots are not assignedto the senders but to the receivers. At the beginning ofeach timeslot, potential senders for the same receivercontend for the medium. Each timeslot is long enoughto transmit one data message. If multiple packets needto be transmitted, then the sender and the receiver hopto a new channel according to a predetermined se-quence. Other potential senders also follow the hoppingsequence of the receiver. As we mentioned, increasedcontention especially around the sink node with highdata rate scenarios is hard to resolve with contention-based protocols.

Another multi-channel MAC protocol proposed forWSNs is HyMAC [34]. Similar to our protocol, HyMAC isalso a combination of TDMA and FDMA. However, times-lots and frequencies are assigned according to Breadth FirstSearch (BFS) order [35] on a tree topology, and there re-main open questions relating to maintaining time-syn-chronized communication, resolving collisions, newnodes joining the network, and implementing the protocolin a distributed way.

Table 1 shows a classification of the existing MAC pro-tocols and how MC-LMAC differs from these existingworks.

(a) (b)

Fig. 1. (a) Single-hop topology used in the benchmark scenario. (b) Multi-hop topology used in the benchmark scenario.


3. Motivation

Theoretically speaking, the throughput capacity of aWSN with n nodes under a many-to-one communicationpattern can not exceed W/n per node, where W is the trans-mission capacity of the radio [36]. Practically, this bound isusually not achieved due to the half-duplex nature of theradios and due to the increased amount of contentionand interference in dense deployments with multi-hoptopologies. In this section, we study a simple benchmarkscenario to show the efficiency of multiple channels.

In Fig. 1(a), we present a topology where all the sourcenodes can directly reach the sink node. Let W represent thecapacity of the shared medium. In an idealized setting,aggregate throughput would be W, and each source nodeshould transmit with a capacity of W/4. When we switchto a multi-hop scenario, which is shown in Fig. 1(b), if thereis no interference then with a suitable scheduling mecha-nism, we can achieve the W/4 throughput per node. How-ever, if all the transmissions interfere with each other, eachnode can get only W/6 capacity (nodes 1 and 2 forwardnode 3 and 4’s packets besides their own packets). Onthe other hand, if nodes can use different non-interferingchannels to transmit, then interference can be eliminatedand the nodes can reach the W/4 capacity.

In Section 5.1, we present simulation results on the effi-ciency of multi-channel communication for capacityimprovements in WSNs, using the presented topologiesin Fig. 1.

4. MC-LMAC protocol

MC-LMAC is a schedule-based multi-channel MAC pro-tocol. The main design is based on single-channel LMAC[5], which is an energy-efficient medium access protocoldesigned for WSNs. The LMAC protocol enables the com-municating entities to access the wireless medium on aschedule basis in which each node periodically uses atimeslot for transmission. The main aspects of LMAC are:

� Self-configuration: LMAC can operate in a fully-distrib-uted ad hoc manner and does not require a centralizedscheduler.� Adaptability to changes: LMAC can adapt the communi-

cation schedule according to network dynamics, forinstance, due to the topology changes, and this is doneby local decisions of the nodes without the need of acentralized scheduler.

Table 1Comparisons of multi-channel MAC protocols for WSNs.

MC-LMAC Y-MAC MMSN

Broadcast support + + +Partitions � � �Medium access Scheduled Scheduled Slotted contentionChannel

assignmentSenders Dynamic Receivers

Channel switching Once per timeslot

Once per timeslot

Multiple times perslot

Joining network Anytime Anytime At channel assignm

� Energy efficiency: Timeslot scheduling has the naturaladvantage of collision-free medium access that avoidswasting energy.

Moreover, time-scheduled communication eases thecoordination of multi-channel communication. Sincenodes switch their interfaces between different channels,a detailed coordination of channel switching is requiredbetween the senders and receivers in order to be on thesame channel at the same time. Scheduled access over-comes this complexity.

Another key aspect of time-slotted communication isthe robustness during high peak loads [37]. Alternativecarrier sense protocols may fail to successfully allocatethe medium, and thus may result in collisions when thenumber of sources or data rates increase. Scheduled com-munication has the advantage of collision-free access.Since we focus on scenarios with a high demand on themedium, we consider LMAC to be a proper choice.

4.1. Protocol organization

As in any scheduled protocol, nodes should first syn-chronize in order to send and receive messages with cor-rect timing. To access the medium and send messages,nodes select/control a timeslot together with a frequencyon which the transmissions do not conflict with the otherconcurrent transmissions. Equal number of timeslots aregrouped into frames.

Nodes transit between five different states while run-ning the protocol: initialization, synchronization, discovery,timeslot-channel selection, and medium access, as shown inFig. 2.

TMCP HyMAC [32]

No information No information No information+ � �No information Scheduled No informationClusters Senders Receivers

(home channel)time No Once per time slot If needed

ent At channelassignment

At channelassignment

Anytime(with-scanning)

Fig. 2. State diagram of a node while executing the protocol.


Nodes are in the initialization state (Fig. 2) if the networkis recently deployed or if the nodes rejoin the network, forinstance after a reset. In the initialization state, nodes sam-ple the medium for an incoming packet to synchronizewith the network. If such a packet is received, a node en-ters the synchronization state and synchronizes with thenetwork by the information about the current slot andframe number in the received packet. After the synchroni-zation, a node follows the schedule to receive packets inthe upcoming slots. If the node needs to actively join thenetwork and send packets, it selects a random wake-upframe3 to select a timeslot within a channel. Before thewake-up frame, a node enters the discovery state to get a listof conflict-free slots and channels in the neighborhood, asshown in Fig. 2. In the discovery state, a timeslot and chan-nel pair is recorded as occupied if the received signal level ofthe transmissions during the timeslot on the channel isabove a threshold, or if a neighbor node is already receivingfrom another node during the timeslot and channel. After anode collects the information for a frame duration, it entersthe timeslot-channel selection state to select the timeslot andthe frequency on which it will perform its transmissions. Atthe end of the discovery state, if the node succeeds in findingan empty slot and frequency, it transits to the medium accessstate and simply transmits packets in the selected slot on theselected frequency. During the other timeslots, a node sam-ples the medium for potential incoming packets. If a conflict,such as a collision, is reported by an intended destination ona node’s selected slot, then the node restarts the slot selec-tion process. If a node experiences synchronization error(details as discussed in Section 4.3), then it transits back tothe synchronization state.

When the network is initialized for the first time, thesink node selects a timeslot and a frequency, and startstransmitting. Accordingly, the nodes receiving from thesink synchronize and follow the state transitions. The basicstates, shown in Fig. 2, are the same for the single-channelLMAC protocol and the MC-LMAC protocol. However, inMC-LMAC, nodes not only choose a timeslot but also a fre-quency. The details of the message exchange and mediumaccess are also different.

In the following, we explain the details of the differentfive states of the protocol. First, we explain the rules forinitialization and synchronization. Then we explain the

3 Selection of random wake-up frame aims to reduce the number ofcollisions, such that the nodes do not select the same timeslot andfrequency with a neighbor node, especially at the network setup.

rules for timeslot and frequency discovery and selection,and finally those for medium access control and messageexchange together with packet formats used in theprotocol.

4.2. Initialization

As shown in Fig. 2, nodes reside in the initialization stateif the network is recently deployed or if the nodes rejointhe network, for instance after a reset or replacement ofthe batteries. In the initialization state, nodes sample themedium for an incoming packet to synchronize with thenetwork and enter the synchronization state.

4.3. Synchronization

Synchronization is achieved by a hierarchical schemesuch that every node synchronizes with its parent (everynode selects a parent node from the set of the nodes thatare closer to the sink node in terms of number of hops).Prior to data transmission, the nodes send control mes-sages which include information about the current slotand frame numbers. Upon the reception of a message dur-ing initialization, a node records the current slot and framenumbers, which are sent in the control message (as de-tailed in Section 4.5.1). As mentioned earlier, the timingscheme is started by the sink node at network initializa-tion. When the neighbors of the sink receive the transmis-sion, they synchronize their clocks with the sink’s clock.The synchronization continues hop-by-hop as each nodesynchronizes with its parent node.

Due to possible clock drifts, synchronization errors mayoccur from time to time. The nodes detect synchronizationerrors by comparing the received slot and frame numbersin the control message with their local slot and frame num-bers. If a difference is detected, nodes transit back to theinitialization state. Moreover, guard intervals are used toensure that receivers are ready to listen before the sendersstart transmitting to allow small timing differences. In [5],performance of LMAC was evaluated in terms of synchroni-zation. It was shown that, if the nodes synchronize to everyframe then a maximum drift of (+, �)2 clock ticks are ob-served. Therefore a guard interval of 2 clock ticks beforeand after the expected time of a message reception is used4

and the nodes synchronize to every frame.

4 If the crystal oscillators runs at 32.768 kHz, this translates to 61 ls.


4.4. Discovery & time slot and channel selection

In this section, we explain the methods of time slot dis-covery and selection for the LMAC protocol, and describethe extensions to the case of multiple channels for theMC-LMAC protocol.

4.4.1. Time slot discovery and selection in LMACThe localized scheduling algorithm of single-channel

LMAC is presented in [5]. The algorithm allows a node toperiodically get a time interval, called a time slot, duringwhich it is allowed to control the wireless medium andtransmit data. The nodes choose a time slot autonomously,such that a node’s transmissions in that slot does not con-flict with the transmissions of other nodes in the same slot.

If there is no conflict (we explain the causes and resolu-tion of conflicts that may cause packet losses in Sec-tion 4.5.3), a node uses the same time slot in theupcoming frames. Each frame has a fixed number of timeslots depending on deployment density. Due to the mul-ti-hop nature of WSNs, reuse of time slots or the medium,is possible. All the nodes use one time slot per frame butthe algorithm can be extended to allocate more time slots,i.e., allocate more rate, if needed [38].

Time slot selection process takes place either duringnetwork initialization or whenever a conflict occurs and anode is required to select a new time slot to eliminate aconflict. If the time slots are selected during network ini-tialization, the sink node starts the selection process bygetting the control of a time slot. When a node joins a net-work, first it has to discover a ‘‘free” time slot to transmitits data. A free slot is defined as a slot:

� during which a node is not receiving a packet or detect-ing a carrier: if a node is receiving a packet during a slot,it would not be able to exchange messages with thosenodes transmitting during this slot, and if a node isdetecting a carrier, this means the medium is busyand a transmission of the node may potentially causecollisions,� during which the potential receivers are already receiv-

ing from other senders.

To guarantee that the first constraint holds, a node thatis searching for a free time slot should exclude all timeslots during which a message is received (or a carrier is de-tected) from the list of potential slots. This means if atransmitting node is within the communication range ofa node, it should defer its transmission during that slot.The other constraint should be fulfilled by potential receiv-ers such that they should transmit a list of the time slotsduring which they are already receiving (or detecting a car-rier). This scheme is similar to the RTS/CTS exchange usedin CSMA with collision avoidance. The first constraint de-fers the potential transmissions to prevent possible colli-sions similar to the RTS transmission to reserve themedium. The second constraint is similar to the CTS trans-mission such that the receivers notify the potential sendersabout the message exchange.

The scheme lets the new node determine the list of freeslots that can be used without possible collisions. With this

information, nodes get a view of the time slot usage withintheir local neighborhood, and this lets them make a list ofpotential free slots. A node randomly selects its time slotfrom the set of free slots (for other methods of time slotselection the reader can refer to [5]). Since the number oftime slots is selected according to the expected node den-sity at the deployment phase, all the nodes are given anopportunity to select an empty slot. Together with the timeslot selection scheme described above, this guarantees thatevery node can select a slot to carry out its transmissionswithout conflicts.

Time slot selection is implemented as follows: All thenodes keep a bit vector called ‘‘occupied slot vector” witha length equal to the number of time slots. It is used forstoring the information about the slots occupied by neigh-bors, and is transmitted during the node’s time slot toshare this information with potential transmitters. Ini-tially, it is filled with 0’s, meaning all the slots are free.When a packet is received or a carrier is detected duringa time slot, the node inserts a ‘‘1” in the vector at therespective position of the vector. This guarantees that anode collects information about the occupied slots thatare used by its neighbors. To get a two-hop view of the net-work, the node should also be aware of the occupied slotsin which a neighbor node receives from its neighbors. Thisguarantees that the node does not transmit on a time slotwhere a neighbor node is already receiving, thus prevent-ing potential collisions. For that matter, nodes that alreadyselected a time slot share/transmit their list of occupiedslots and provide the necessary local information for thenodes searching for a time slot. After a complete framehas passed, the node searching for a time slot can make alist of the free slots by executing an ’OR’ operation on allthe received occupied slot vectors and the local occupiedslot vector.

Fig. 3 shows the timeslot selection. Here, the number oftimeslots per frame is 7 and the node ‘‘X” is searching for atimeslot. All the other nodes control the timeslot repre-sented by the number inside the circles. Node X receivesthe occupied slots information (the position of a bit inthe vector is the timeslot number: 1 means the timeslotis occupied and 0 means free) from the neighbors, next itexecutes the OR operation and finds timeslot 7 as freeand grabs it.

In order to keep the list of the occupied slots up-to-date,nodes clear their occupied slot vectors after transmissionsand collect new information on the usage of time slots un-til their next transmissions in the upcoming frame. Thismakes the protocol robust against variations in the wire-less links, which may cause the topology and the connec-tivity of the network change and also helps the nodeskeep an up-to-date neighbor list for routing purposes.Moreover, a new node joining the network does not dis-rupt the already established time slot organization.

4.4.2. Time slot and channel selection in MC-LMACThe number of required time slots per frame depends

on the connectivity of the network topology. If the numberof time slots is larger than what is required, the bandwidthmay get wasted during empty slots and nodes have to waitlonger before they can access the medium. On the other

Fig. 3. LMAC timeslot selection.


hand, when there are not enough slots (i.e., the local con-nectivity is higher than expected), the node remains inthe discovery state, periodically monitoring frames for anempty time slot. In single-channel LMAC, the number oftransmissions is limited by the number of time slots in aframe. However, in MC-LMAC time slots are selected withchannels.5 A node can use the same time slot that is used bya 2-hop neighbor on a different frequency so that paralleltransmissions are not disturbed at common neighbors. Con-sequently, more transmissions can take place with the samenumber of time slots.

In MC-LMAC, a node occupies slot vectors per channeland selects a time slot to be used on a particular channel.A node which is trying to get a control of a time slot, exe-cutes an ‘‘OR” operation over each occupied slot vector perchannel and discovers the free slots on different channels.Similar to single-channel LMAC, this method guaranteesthat the same ‘‘time slot/channel” pair is not used withinthe local neighborhood. Note that, the nodes do not selectthe time slots used by their neighbors on any frequencydue to the limitation of the half-duplex radio.6

Fig. 4 shows an example for time slot and channel selec-tion. The node ‘‘X” is searching for a time slot while others

5 We assume the use of only orthogonal channels. If overlappingchannels are also used, transmissions on closer channels on the spectrummay interfere with each other which depends on the distance between thetransmissions, filtering characteristics and blocking values of a giventransceiver.

6 In our design, the nodes keep a list of the free channels in theneighborhood. For other methods of channel selection mechanisms, such aspower sensing or measurement based, the reader can refer to [13].

are marked by time slot/frequency pair that they are using.The number of time slots per frame is 5 and the number offrequencies is 2. The node without a time slot receives theoccupied slot information (the position of a bit in the vec-tor is the time slot number: 1 means the time slot is occu-pied, and 0 means free) from the neighbors, executes theOR operation and finds that all the slots are occupied onF1 (frequency 1), however there is one free slot on F2.The node selects time slot 5. Here note that although slots1, 2 and 3 are not occupied by any of the neighbors on F2,the node lists those slots as occupied on all frequenciessince these are used by the neighbors on frequency F1.According to rules of MC-LMAC, a node does not select atime slot on any of the frequencies which is used by theneighbors.

4.4.3. Required number of timeslots per frameThe required number of time slots in the MAC frame is

an important parameter; it affects the possibility of colli-sions and latency of message delivery. Each node in thenetwork should be given an opportunity to use a time slotwithout causing or experiencing collisions. The requirednumber of time slots depends on the expected node den-sity of the network. In [5], a detailed analysis is presentedusing graph-coloring approaches on distance constrainedparagraphs. In single-channel LMAC, a node should notuse the same time slot which is used within the 2-hopneighborhood, i.e., slots that are sensed to be occupiedand slots reported as occupied by the nodes with which anode communicates. On the other hand, in MC-LMAC anode can use the same slot used by 2nd-hop neighbors

Fig. 4. MC-LMAC timeslot and channel selection.


on a different channel. The number of required time slots iscalculated during network initialization according to thedensity of deployment, and MC-LMAC uses the informationto tune its parameter. If new nodes are later deployed inthe network, the sink can decide to restart the schedulingand nodes follow the transition between states given inFig. 2.

Fig. 5. State diagram of a node while following the schedule.

4.5. Medium access

After a node synchronizes with the network and selectsa time slot and a channel, it stays in the medium accessstate. In the medium access state, it follows the scheduleaccording to the state diagram shown in Fig. 5 unless aconflict occurs during its transmission (we explain thecauses and resolutions of conflicts in Section 4.5.3), or un-less a synchronization error occurs. At every time slot,when the timer expires, a node first controls whether thecurrent slot is equal to its own used slot. If so, the nodetransmits during the current slot. If not, the node listensfor incoming potential packets where it is addressed as adestination. If it is addressed by any of the current trans-missions, then it receives the incoming packet. If no packetis destined to this node, then the node increases the slotnumber (if maximum slot number is reached, the slotnumber is reset and frame number is increased), sets itstimer to the beginning of the next slot, and enters into pas-sive state (sleep) to conserve energy. After the receptionand transmission of messages, other nodes also update

the slot information and set the timer to the beginning ofthe next slot.

In the following, we first explain the time slot structure,and then present the packet structures and message ex-change procedure.

4.5.1. Time slot structureA time slot consists of a common frequency (CF) phase

and a split phase. In the CF phase, all nodes switch to thecommon control channel to address their destinationsand to be informed whether they are addressed in the cur-

Fig. 6. MC-LMAC timeslot structure.


rent slot. In the split phase, senders and intended receiversor the nodes that are in the discovery state switch to thechannel on which the control message and data transmis-sion will take place.

Communication during the CF phase is also based onscheduled access and takes place in small slots called CFslots. The number of CF slots is equal to the number ofchannels, and each slot is indexed by a channel number.Since the number of channels, on which the radio can beadjusted, is usually fixed, the number of CF slots is deter-mined at the initialization of the network and does notchange over time. This means that each CF slot is reservedfor the senders of the channel whose number is equal tothe CF index (first CF slot is reserved for the senders thatselected channel 1, second is for channel 2 and so on). Asender controlling the current time slot addresses the des-tination during the CF slot which is reserved for the chan-nel number it controls. During the CF phase, the intendeddestination id (or broadcast address) and node id (sender’saddress) are transmitted.7 This enables the sender to notifythe destination node and invite it to switch its radio to thesender’s channel. The sender id is needed for the destinationnode to be able to match the sender of the CF message andthe sender of the upcoming data messages in the split phase.As mentioned earlier, the sender’s channel number is equalto the index, or CF slot number, where it notifies the desti-nation node. Therefore, no extra information is needed tobe transmitted. Fig. 6 shows the time slot structure of theprotocol.

In the split phase, the sender first sends a control mes-sage, which can be considered as a preamble packet, andthen continues with the transmission of the data message.The split phase has a fixed maximum length. Depending onthe amount of data to be transmitted, the node can sendonly a single packet or multiple packets. Even if a nodedoes not have data to transmit, it sends a control messageduring its time slot which provides neighbor discovery andfree time slot and channel discovery for the nodes that arein the discovery state. The split phase of MC-LMAC is sim-

7 It may be argued that, sending only the node id and destination id maynot be long enough, and may not be received by the receiver on time ifthere is a clock drift. In the implementation of the protocol in Glomosim,we checked the duration of sending these fields and found that it is longerthan the expected clock drift of 2 clock ticks.

ilar to the time slot structure used in the single-channelLMAC protocol.

The contents of the control message transmitted duringthe split phase are as follows:

� ID represents the node id of the sender.� Occupied Slots represents the bit vector for the occupied

slots per channel in the neighborhood.� Collision Slot represents the slot number during which a

collision has been detected.� Collision Frequency field is used to distinguish the chan-

nel on which a collision has been detected.� Current Slot and Current Frame represent the slot and

frame numbers, and are used for synchronization bythe new joining nodes.� Hop Count field lets the nodes announce their hop

distance to the sink node and it is used for synchroni-zation.� Acknowledgement Bit Vector per channel has a length

equal to the number of timeslots per frame. Nodes keeptrack of the slots and channels on which they receivedata. Initially the vector is filled with 0’s. If a messageis received, a logical 1 is inserted at the position of therespective timeslot in the acknowledgement field.

Fig. 7 shows the contents, together with their size, ofthe control message.

The split phase at each time slot can accommodate thetransmission of multiple data packets. Each time slot has afixed size. Therefore, the size of the split section changesaccording to the size of the CF section, which in turn alsochanges by the number of channels. In order to give a num-ber about the sizes, with a frame size of 1.6 s and 32 slots,the number of data messages that can be transmitted pertime slot is 18 with 1 channel, and 16 with eight channels,where size of the each data message is around 32 bytes.

4.5.2. Data transmissionThe receivers listen during the whole CF phase in order

to be informed about the intended destinations. If a recei-ver is addressed during a CF slot, it switches its transceiveron the sender’s associated frequency. If not, the nodeswitches its transceiver to standby mode by entering intopassive state for the remainder of the time slot to conserve

Fig. 7. Contents of control message according to 32 slots per frame and eight channels, sizes are in bits.


energy. Note that, the common frequency can also be usedby the nodes for data transmission, and it has the samecharacteristics as the other channels. After the CF phase,the receiver switches to the sender’s channel and the timeslot owner transmits a control message, followed by aDATA message.

If the transmission is not successful, for instance, due toa packet error or external interference on the medium, thesender should be notified for a retransmission. The receiv-ers acknowledge the correctly received packets by usingthe acknowledgement field in the control message. There-fore, a sender should listen to the control message that willbe sent during the receiver’s time slot.

Besides unicast messages, nodes can also send broad-cast messages by transmitting a broadcast address duringthe CF slot. In this case, all the nodes receiving the broad-cast request, switch to the sender’s frequency.

An example of the overall medium access coordinationis shown in Fig. 8. The initial part shows the topology: thenumbers inside the circles represent node ids. It is as-sumed that there are three channels available (representedas F1, F2, and F3), and accordingly there are three CF slots.In time slot 1, sender S1 addresses receiver R1 in the firstCF slot to communicate on channel C1, sender S2 addressesreceiver R2 in the second CF slot to communicate on C2 andsender S3 addresses receiver R3 in the third CF slot to com-municate on C3. The CF phase takes place on the controlchannel, which is Channel 1 (C1). During the split phase,the nodes tune their transceivers on the associated chan-nels: pair 1 on C1, pair 2 on C2, pair 3 on C3. In time slot2, we observe a similar schedule. Note that, due to interfer-

Fig. 8. MC-LMAC coord

ence the three parallel transmissions at each time slotwould not be possible if there was only a single channelavailable.

4.5.3. Conflict resolutionNodes always use the same time slot in each frame un-

less a conflict occurs. The causes of conflicts, i.e., packetlosses at the receiver, may be due to various factors. Colli-sions are the major causes of packet losses, when twosenders address the same receiver at the same time onthe same channel. This can happen during network setupor when network topology changes, for instance due tochanges in the routing paths. When a collision has been de-tected at a time slot, the destination node records the num-ber of the timeslot and the channel and reports theseduring its own timeslot by marking the collision fields inthe control message. If the reported numbers by a node’sdestination matches with the node’s own selected timeslotand channel of the node, the node releases its timeslot andrestarts the timeslot selection procedure. To reduce thenumber of collisions, especially at the start up, nodes waitrandom times, i.e. random number of frames, to start withthe timeslot selection.

Another cause of packet losses may be due to variationsin the link quality. In the literature, there are three distinctregions of link quality defined [39]: a nearby connected re-gion where packet reception rates are consistent and high;beyond this lies a transitional region (or a gray area),where reception rates vary much and the disconnected re-gion. The quality of the links in the connected region usu-ally exhibits a packet reception rate above 99% and does

ination scheme.

Fig. 9. Prioritization of the selection of timeslots not used by the otherchildren and parent of a parent.

8 Node N2 may have another neighbor transmitting at T2 on anotherchannel but it gets notified about node S1 during CF slots and updates itslocal occupied slot vector. Note that CF slots are clash-free.


not exhibit variations. In the transitional region, the linkshave varying qualities and can be asymmetrical.

In [39], blacklisting of links in the transitional region isgiven as a solution to deal with the unreliability of thelinks. A blacklisting mechanism can easily be incorporatedin MC-LMAC by using the control messages which carryupdated information about the neighborhood. This wouldhelp the nodes to select strong links in the connected re-gion for communication. It can be argued that still packetlosses may occur on strong links from time to time witha little percentage. In this case, if a packet loss occurs thereceiver does not need to report a collision and make thesender to change its slot and channel selection. The recei-ver can make the distinction by comparing the RSSI (Re-ceived Signal Strength value Indicator) value of thereceived packet with the SINR threshold [5]. If it is just be-low the threshold the loss is probably due to a temporalreduction in the received signal strength. In the other case,the loss is most probably due to a collision.

Another cause of packet losses may be due to interfer-ence from external networks that share the same parts ofthe spectrum. During the CF phase the nodes update thelist of their local occupied slots and channels when theydetect a carrier or receive a packet on the control channel.In certain cases, although a node does not detect a carrierfrom external networks on the control frequency, it is pos-sible that after the channel switching, excessive externalinterference on the sender’s channel may cause a packetloss. In this case, the receiver would report a collision.However, this does not prohibit the sender to select thesame channel again, since the receiver will not report theselected slot and channel as occupied. If this case persists,then the receiver concludes that the interference on a par-ticular channel prevents successful communication andblacklists the channel, i.e. report all the slots as occupiedon that channel.

In MC-LMAC, if a node is addressed by multiple sendersin the same timeslot but on different channels, it receivesthe transmission requests during the CF phase. However,it can receive from only one of them during the data trans-mission. We define this situation as a ‘‘clash”. We discussthe details and solutions for these occurrences inSection 4.6.

4.6. Resolving clashes and discussion of overheads

4.6.1. ClashesAn issue to be solved is a receiver’s response if it is ad-

dressed by multiple senders in the same timeslot but ondifferent channels, in case of a clash. An option would beto select a sender randomly or select a sender accordingto a priority mechanism. In our protocol, we use a prioritymechanism during timeslot selection by prioritizing theselection of the timeslots that are not used by the otherchildren and the parent of the parent node on the con-vergecast tree. For instance, in Fig. 9, Nodes C and D donot select the same slot with node A on any channel as longas there are free slots to select from. They can select thesame timeslot with Node E since E is not a parent or a childof node B. This efficiently reduces the probability ofclashes. For instance, according to normal timeslot selec-

tion procedure, the parent, node A in Fig. 9, and a child,node C, of a node B may select the same timeslot whichmay cause clashes and a problem for node B while decidingon which sender it should receive from. In this case, both ofthe transmissions may be important for the node. By usingthe new timeslot selection mechanism the occurrence ofclashes is reduced. In case a clash still occurs, the acknowl-edgement field in the control message is used and aretransmission is requested to inform the unselectedsenders.

Two problems may arise with clashes and we solvethem as explained in the following. If two nodes that con-trol the same slot have a common neighbor, this would notbe a problem unless the common neighbor is the addressedreceiver, i.e, the parent for a convergecast, of both of thenodes. Normally, if the nodes select their timeslots at dif-ferent times they would not select the same timeslot sincethe parent reports the occupied slots in the neighborhood.They would notice that the parent is receiving from an-other node. In case two senders of a parent node selectthe same timeslot at the same frame (which is possiblealthough the nodes wait random number of frames beforeselecting a slot), the parent can only receive from one ofthem. In this case the parent node uses the collision fieldto inform the unselected child to change its timeslotselection.

Another problem would be as follows: if a node is in thediscovery state and it has two neighbors that cause a clash,such that they control the same timeslot on different fre-quencies, the node would then be able to receive the occu-pied slots vector from one of them. As shown in Fig. 10(a),node S1 is in the discovery state and receives a messagefrom node N1 at timeslot 1 (T1) on channel 1 (C1). Sinceit cannot receive a list of occupied slots from node N2, itis not aware that N2 receives from S2 at timeslot T2 onchannel C1. If N1 selects T2 on C1 then a collision may oc-cur at node N2 as in Fig. 10(b). In Fig. 10(c), when N2 re-ports this collision during its timeslot then S2 getsnotified about the collision and starts the new slot selec-tion process. Since N2 will report (T2, C1) as occupied inits local neighborhood,8 S2 will not select the same slotand channel as shown in Fig. 10(d).

In case of clashes where one of the transmissions use abroadcast address, nodes prioritize to receive the broadcastmessage. Besides convergecasts, we also evaluate the

(a) (b) (c) (d)

Fig. 10. (a) Clash at a node in discovery state. (b) Node selects the same slot and channel, collision occurs. (c) Collision reported. (d) Selection of a newchannel.

Table 2Simulation parameters.

Terrain size 150 � 150 m2

Number of nodes 100


impact of clashes on the performance when the nodes usebroadcasts, i.e. for local-gossip operations, in Section 5.5.

4.6.2. Protocol overheadAs mentioned earlier, the main overhead of MC-LMAC is

introduced by the control messages exchanged before datatransmissions. CF phase should be kept as short as possibleto minimize the overhead of energy consumption since thenodes have to keep their transceivers on. In single-channelLMAC, nodes have to keep their transceivers on during thecontrol message section. However, in MC-LMAC keepingthe interface on during CF phase is sufficient. Moreover,in the implementation of the protocol, the idle-listeningoverhead is minimized by taking one sample of the carrierin an unused slot to sense any activity (i.e., preamble sam-pling). If there was activity, the slot is included in the occu-pied slot vector and listened to completely in the nextframe. Moreover, the control period is much smaller com-pared to the data period and during the data period thenodes can transmit multiple packets to minimize the over-head (similar to message passing in S-MAC [27]: one RTS/CTS sequence is used to reserve the medium to transmitmultiple packets). In Section 5 we show that when the net-work load is high, the CF phase does not add an overheadin terms of energy consumption compared with other pro-tocols, but it enables higher throughput at the sink node bycoordinating transmission over different channels.

If the nodes switch between channels very frequently,the channel switching may bring an overhead. However,as we mentioned the timeslot duration in MC-LMAC islarge enough to accommodate the switching time. For in-stance, if there are 32 slots in a 1 s frame and a channelswitching is around 200 ls, less than 1% of a timeslot dura-tion is spent on channel switching. Moreover, the nodes donot transmit only one packet per timeslot but are allowedto transmit multiple packets, which in turn compensatesthe amount of time spent in channel switching. We shouldalso note that, in some of the other multi-channel proto-cols designed for WSNs, nodes switch channels more fre-quently. For instance in Y-MAC [33] nodes hop betweenchannels at every message whereas MMSN requires chan-nel switching between the sender’s own channel and recei-ver’s channel a couple of times for each message.

Node placement RandomNumber of frequencies 1–10Bandwidth 250 kbpsTransmit power 1 dBmRadio model RADIO_ACCNOISERadio range 40 mMAC protocol MC-LMAC, MMSN, CSMARouting protocol GF

5. Performance analysis

In this section, we analyze the performance of the MC-LMAC protocol by extensive simulations with Glomosim[40]. Different simulation scenarios are studied accordingto four different performance metrics: aggregate through-

put, delivery ratio, latency and energy efficiency. Aggregatethroughput is calculated as the total amount of data deliv-ered to the sink node per unit time by the MAC protocol.We study the performance according to different systemloads, different source rates, different numbers of frequen-cies, different node densities and different traffic patterns.

Simulation parameters are presented in Table 2. Insteadof using a simple graph-based interference model, where itis assumed that when the nodes are in the transmissionrange of each other they cause interference, we use theRADIO_ACCNOISE model, which simulates the behavior ofthe physical interference model [41] such that interferencefrom multiple senders is captured by comparing the re-ceived signal strength to the SINR threshold. According tothe radio parameters, the transmission range of the nodesis around 40 m. The sink node is positioned in the center ofa square area. In the simulations, we assume that the pack-et losses occur due to collisions or interference from con-current packet transmissions in the same timeslot on thesame channel. Testing the performance of the protocolswith other causes of packet loss is planned as a future workon a real testbed. In most of the simulations, the Geo-graphic Forwarding (GF) [42] routing protocol, which con-structs routes according to shortest path or minimum hopcount criteria, is used but we also study the performance ofa gossip traffic pattern without GF.

Performance of MC-LMAC is compared with the MMSNprotocol [7]. Other than the protocols explained in Sec-tion 2.2.2, MMSN fulfills most of the requirements/chal-lenges about the multi-channel usage. Its performancehas been deeply studied from different aspects [7] and itis a representative of the slotted, contention-based, mul-ti-channel MAC protocols designed for WSNs.

Moreover, we simulate a previously introduced channelassignment algorithm [43] based on LMAC where eachbranch of the convergecast tree is assigned a differentchannel; in other words, each branch is clustered into


different channels. Inside the clusters, nodes communicateaccording to the single-channel LMAC protocol and we re-fer to this as clustered LMAC. The operation of clusteredLMAC is similar to TMCP [8], which was mentioned in Sec-tion 2.2.2. In TMCP the level of interference that a node cre-ates on the nodes of a branch is considered. However, inclustered LMAC, nodes join the branches according to theminimum hop count to the sink node or randomly in caseof a tie. We also compare the performance of MC-LMACwith clustered LMAC and CSMA (no RTS/CTS mechanismis used). All the results are averaged over 1000 simulationruns. We present the results with 95% confidence intervals.

5.1. Benchmark results

Before presenting the results with 100-node deploy-ments, for which the simulation parameters are shown inTable 2, in this section, we discuss the results of the bench-mark scenario. The benchmark was discussed in Section 3with the illustrated topologies in Fig. 1.

Fig. 11 shows the simulation results. The vertical axisshows the aggregate throughput in total bits per second re-ceived at the sink node, and the abbreviations are as fol-lows: SHSF: Single-hop, single-frequency, MHSF:Multi-hop, single-frequency, MHMF:Multi-hop, multi-frequency.In the multi-hop scenario, we assume all the nodes are inthe carrier sensing range of each other and nodes in the2nd level cannot directly communicate with the sink node.Therefore, parallel transmissions on different links, unlessthey are assigned different channels, may interfere witheach other. Nodes transmit 32-byte packets continuously(every 2 ms) to the sink node (effective data rate is250 kbps). The maximum aggregate throughput, i.e. totalamount of data that the sink can receive per unit time fromall sources, is approximately calculated as 104 kbps. Whenthe topology is single-hop and there is a single-channel(SHSF), slotted MC-LMAC performs close to the maximum.The only overhead is that of the control messages sent atthe beginning of timeslots. Contention-based protocolsCSMA and MMSN perform worse. MMSN performs worsethan CSMA since some part of the timeslot is spent to listenon the broadcast frequency. In the single-hop scenario,

Fig. 11. Benchmark results.

having multiple channels does not improve the resultssince senders transmit to the same sink node and have towait for each other’s transmission.

When the topology is multi-hop and there is a single-frequency (MHSF), transmissions of all the nodes interferewith each other. All the protocols perform quite poorly.However, MC-LMAC still performs better than the otherssince collisions are eliminated but it takes six timeslotsto deliver all the data compared to the four timeslots inthe single-hop scenario due to relaying of the messages.This causes the sink to stay idle (not receiving) duringtwo timeslots which are used by its 2nd-hop neighbors(grandchildren). When there are multiple frequenciesavailable (MHMF), MC-LMAC performs similar to the SHSFscenario achieving a performance very close to the maxi-mum. On the other hand, MMSN performs better than itsperformance in the MHSF scenario and better than CSMAbut cannot achieve the throughput of the SHSF scenario.

If the objective is to maximize the throughput, insteadof using multiple channels, using a more powerful radiowith a higher data rate could work better than the multi-channel scenario. In the last column of Fig. 11, we presentthe results where the nodes can transmit over a double sizeband, i.e. with an effective data rate of 500 kbps. Comparedwith the results of MHSF and MHMF, all protocols achievehigher throughput. However, most of the band is still notutilized due to the interference experienced on the samechannel. Moreover, contention-based protocols, CSMAand MMSN, over a wider band perform worse than theMC-LMAC protocol with two channels presented in theMHMF scenario. Additionally, using a radio that can trans-mit over a wider band may consume more energy which isnot desired by WSNs due to the energy constraint on thesensor nodes.

Observations from the benchmark results are two-fold:Due to the common destination problem with the many-to-one traffic pattern, aggregate throughput is limited bythe reception capacity of the sink node. However, thisthroughput is usually not achieved in multi-hop scenariosdue to contention, interference and collisions that increasewith relaying of data. Multi-channel communication cancope with interference and collisions and improve thethroughput and delivery performance. Next, we concludethat schedule-based medium access can indeed better copewith high peak loads [37] since the contention and colli-sions are eliminated.

5.2. Impact of the number of channels

In this section we analyze the impact of the number ofchannels on the network performance. All the nodes initi-ate CBR (continuous bit rate) streams towards the sinknode and each node generates a packet every 2 s (if nodestransmit more frequently, buffer overflows start to occur).This scenario can be considered as a periodic data collec-tion, which is commonly used in WSN applications.Although methods such as data aggregation can be usedto reduce the amounts of data to be delivered to the sinknode, in these simulations all the raw data from sourcesis relayed towards the sink node (we previously discussedincreasing the data collection rate of aggregated converge-

0 100 200 300 400 500 600 700 800 900

1000 1100 1200 1300 1400 1500 1600

1 2 3 4 5 6 7 8 9 10Aggregate Throuhput (bytes/sec)

Number of channels

MC-LMACMMSN

ClusterCSMA

Fig. 12. Aggregate throughput with different number of channels.


cast operations in [44]). The number of channels varies be-tween 1 and 10. The terrain size is 150 � 150 m2.

5.2.1. Aggregate throughputFig. 12 presents the results in terms of aggregate

throughput. The x-axis shows the number of availablechannels; the y-axis shows the aggregate throughput interms of the number of bytes per second received by thesink node. Maximum aggregate throughput at the sinknode is 1584 bytes/s (99 sources generate 32 byte packetsevery 2 s). Different lines present the results collected withdifferent protocols: MC-LMAC, MMSN, Clustered LMAC andCSMA. In MC-LMAC, the number of timeslots per frame is32.9 The maximum hop count for different random deploy-ments is usually two or three hops according to the density.

Aggregate throughput increases when the number ofchannels increases from 1 to 10 with all the protocols ex-cept CSMA where the number of channels is fixed to 1.Example radios used on sensor nodes may actually providemore channels. For instance CC2420 [29], can tune itsoperating frequency over 16 different channels in the2.4 GHz band and Nordic NRf905 [45] radio can operateover 512 channels in 868/915 MHz band. However, radiosignals are usually not limited to their allocated frequencyband. Therefore, channel overlaps, that may cause interfer-ence, may be examined between adjacent bands depend-ing on the filtering characteristics and the blockingvalues of the transceiver. Accordingly, the number oforthogonal channels can be limited. For instance, in [46],we experimented the impact of adjacent channel interfer-ence using Nordic Nrf905 radio and concluded that 10channels can be used as orthogonal from the 512 channels,considering the worst case. The same holds for the CC2420radio [8]. Usually all of the 16 channels cannot be used but

9 The number of timeslots is adapted according to the expected nodedensity. According to the example deployment, average number of first hopneighbors per node is around 24 nodes. Moreover, in order to have efficientforwarding during convergecasts, the nodes should not select the sametimeslot as the other children and the parent of the parent node to preventclashes. Accordingly, 32 slots per frame is an experimented, suitable valuefor the given density in the example deployment.

6-10 channels are reasonable numbers that can be used inparallel.

MC-LMAC achieves lower throughput than MMSNwith 1–3 frequencies since some of the nodes cannotget a free timeslot, i.e, 32 slots per frame is not sufficientfor all the nodes to get a timeslot according to thedeployment density, and cannot start transmissions.Especially, when there is a single-channel nodes cannotselect a timeslot that is used in the 2-hop neighborhoodand the size of the 2-hop neighborhood is larger than 32.With the given density the number of timeslots perframe should be higher than 32 slots when the numberof channels is small. A solution could be to increasethe number of timeslots. However, increasing the num-ber of timeslots would increase the throughput but can-not achieve the maximum throughput since the nodescannot be active during the slots used by 2-hop neigh-bors, as we discussed in Section 5.1.

As the number of channels increases, more nodes canget the control of a timeslot. After six channels, MC-LMACachieves the maximum throughput in most of the simula-tions. On the average, the achievable throughput is 99% ofthe maximum throughput. Loss is due to the clashes thatmay occur. As we described earlier, in the implementationof the protocol we reduced the probability of the clashes byprioritizing the selection of the timeslots that are not usedby the other children of the parent node on the converge-cast tree. Compared to CSMA the MC-LMAC achievesapproximately 1.4 times better performance and comparedto MMSN the throughput performance is 1.3 times betterbut most importantly the maximum throughput can beachieved using MC-LMAC.

Aggregate throughput with MMSN is observed to belimited and does not increase much after three channels.This is due to the failure of the nodes around the sink tosuccessfully sense the channel and prevent the collisions.The nodes should switch between the destination’s fre-quency and their own frequency to sense incoming pack-ets. However, while switching, nodes may miss some ofthe incoming packets. Additionally, sensing the destinedpackets to the receiver may not always be successful sinceno RTS/CTS mechanism is used.

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Fig. 13. Packet delivery rate with different number of channels.


Achievable throughput with clustered LMAC is ratherlimited due to the single-channel communication insidethe clusters. Nodes need to select timeslots that are notused in the 2-hop neighborhood to prevent collisions andcontention and this reduces the number of concurrenttransmissions. The performance of the clustered LMACprotocol can be improved by balancing the number ofnodes on the branches. However, due to the randomdeployment of the nodes and connectivity constraints itmay not be always possible and the interference insidethe clusters may still limit the performance.

On the average, single-channel CSMA achieves anaggregate throughput of 64% of the maximum throughput.Due to the high contention, the protocol fails to success-fully allocate the medium to the nodes. Compared toCSMA, MMSN achieves slightly lower throughput on a sin-gle channel which is due to the time spent on sampling thebroadcast channel at the beginning of each slot.

10 The selection of the timeslots that are before the parent node’s slot areprioritized. Delay per hop count is on the average half of the frame size.Considering an average number of 2 hops between a source and the sinknode, this makes around 1 frame duration.

5.2.2. Delivery ratioFig. 13 presents the results in terms of delivery ratio.

The x-axis shows the number of available channels; they-axis shows the delivery ratios. The figure has a very sim-ilar shape with the aggregate throughput graph presentedin Fig. 12. The reason for this is that, we considered onlybest effort delivery, such that the lost packets, i.e., theunacknowledged packets, are not retransmitted. AlthoughMC-LMAC provides reliability by using acknowledgementfields in the control message, MMSN and CSMA does nothave the acknowledgement mechanism which is left tothe upper layers. For fair comparisons, we did not activatethe retransmission mechanism in MC-LMAC.

With sufficient channels, MC-LMAC achieves to deliver99% of the packets on average. As we mentioned, the smallpercentage of losses is due to clashes. However, with asmaller number of channels, the delivery ratio is more lim-ited since most of the nodes cannot get a free timeslot.

On the other hand, contention-based MMSN protocolsaturates around 70% delivery ratio with an increasingnumber of channels and CSMA delivers only 64% of thepackets. As we mentioned, during peak traffic, conten-tion-based protocols cannot handle the high contention

on the medium whereas the slotted MC-LMAC guaranteesthe medium access by assigning different timeslots to thepossible contending nodes.

5.2.3. LatencyFig. 14 shows the results in terms of end-to-end latency

between the transmission of a packet at the source nodeand reception at the sink node. When we compute the la-tency values, we do take into account the time spent forexchanging control messages in MC-LMAC. Similarly, wealso include the time for listening on different channelsin MMSN prior to data transmission.

Although MC-LMAC achieves lower latency than clus-tered LMAC and CSMA, MMSN has much lower delay com-pared to the MC-LMAC protocol. Higher latency is a typicalcharacteristic of the schedule-based protocols. If a nodehas a packet to transmit it has to wait till its assigned slot.The average delay from source to the sink is equal to aframe size which is approximately 1.6 s.10

A simple solution to decrease the latency would then beto decrease the frame size. However, in that case the num-ber of packets that can be delivered per timeslot will alsodecrease and the packets will be buffered to be transmittedlater. The best option then is to assign the relaying nodesconsecutive timeslots according to their hop distance tothe sink node. Another cause of latency in MC-LMAC isthe fixed size of section in a timeslot that is reserved forthe data messages. If a node has less packets than whatcan be transmitted during a timeslot, the idle time simplyadds to the latency. Variable data sections per timeslotcould be another solution to improve the latency in MC-LMAC. The question may arise why MC-LMAC performsworse in terms of latency although it performs very goodin terms of aggregate throughput. Aggregate throughputis the amount of data delivered to the sink node dividedby the duration the sink has completed receiving packetsfrom the network. However, latency is computed per

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Fig. 14. Latency with different number of channels.


packet, which is the duration between the transmission atthe source node and reception at the sink node.

CSMA also experiences higher delay than MMSN whichis due to the exponential and higher number of back-offsduring high contention.

As we mentioned, we did not consider the retransmis-sion of the lost packets, due to collisions. MMSN achievesto deliver only 70% of the the transmitted packets. Thismeans much less packets are scheduled compared to theMC-LMAC protocol.

5.2.4. Energy efficiencyFig. 15 shows the results in terms of energy efficiency,

i.e. energy consumed per successfully delivered packet.This translates to total energy consumed that is dividedby the total number of successfully delivered packets. Weconsider both the energy spent to receive and transmit aswell as the energy spent for relaying the packet towardsthe sink node. Additionally, energy spent on sending con-trol messages are also included. Energy spent per deliveredpacket is quite high with MC-LMAC when there is only asingle-channel. This is due to the very low delivery rate.As the number of channels increases, both MC-LMAC and

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Fig. 15. Energy efficiency, energy consumption per successful

MMSN spend much less energy than CSMA. It can be ar-gued that in MC-LMAC, the duration of the CF period in-creases with more channels and this causes the nodes tospend more energy on listening for the potential incomingpackets. The same holds for CSMA where the nodes shouldalways be listening for incoming packets and with MMSNnodes always listen for a period on the broadcast channeland then listen on the destination’s frequency and itsown frequency in alternating cycles for the incoming pack-ets. On the other hand, clustered LMAC with a single chan-nel spends less time on idle-listening but the number ofattempts per transmission is much lower since some ofthe nodes cannot control a timeslot due to the interferenceand contention within the branch. That is why we pre-ferred to show energy efficiency per successfully deliveredpackets instead of the total energy consumption for faircomparisons.

5.3. Impact of load

In this section, we analyze the impact of the load on thenetwork performance. In particular, we vary the number ofsources, which in turn changes the level of contention in

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ly delivered packet, with different number of channels.

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Fig. 16. Aggregate throughput with different number of sources.


the network. Fig. 16 shows the results in terms of the ac-tive sources. We vary the number of sources from 10 to99. The number of channels for both MC-LMAC and MMSNis 8.

Since MC-LMAC assigns slots to all of the nodes,whether they are the sources or not, the performance ofMC-LMAC is close to the maximum aggregate throughputin all cases. However, MMSN and CSMA suffer from con-tention. When more sources are active, i.e. there is moreload to be forwarded to the sink, the medium access mech-anism of MMSN cannot sense the incoming packets at thedestination’s frequency, particularly around the sink node.Therefore packet losses increase. This again verifies theconclusion that schedule-based medium access can bettercope with increasing contention/load in the network. Addi-tionally, schedule-based medium access combined withmulti-channel communication enables more concurrenttransmissions and achieves a throughput close to themaximum.

In this set of simulations, the nodes generate packetsevery 2 s. We also investigated scenarios where nodes gen-

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L (Side

Fig. 17. Aggregate throughput

erate packets more frequently. In that case, all the proto-cols experience buffer overflows with higher data ratesand the achievable throughput gets much lower than themaximum.

5.4. Impact of density

In this section, in order to test the scalability of the pro-tocols we vary the density of the node deployment. Wechange the terrain size between 50 � 50 m2 and225 � 225 m2 (beyond 225 m, unconnected nodes appearwith random deployment). Fig. 17 presents the results.The x-axis shows L, the side length of the deployment areawhereas y-axis shows the aggregate throughput. The num-ber of channels is 8 for both MMSN and MC-LMAC.

Aggregate throughput with MC-LMAC is lower whenL – 150 since 32 slots per frame is lower in denser scenar-ios and higher in sparser scenarios than required. In spar-ser scenarios the sink stays idle during unused timeslots.We repeat the experiments with different numbers oftimeslots that are adjusted according to the expected den-

50 175 200 225 250

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MMSNCSMA

with different densities.


sity and the results are presented with the second linewhere the maximum throughput can be achieved. In orderto achieve maximum performance with MC-LMAC thenumber of slots per frame should be adjusted accordingto the density of the deployment.

Aggregate throughput with MMSN continues to in-crease when the network gets sparser since the contentionis lower and the nodes can successfully sense the incomingpackets. However, the performance of MMSN is still lowerthan the throughput of MC-LMAC. The same holds forCSMA. As contention decreases by the decreasing density,CSMA can deliver more packets. However, the performancedoes not change much when L P 100 and it even decreasesa bit due to the failure of the nodes sensing packets at thedestination since SINR values tend to be lower as the dis-tances between the nodes increase.

5.5. Impact of traffic patterns

In this section we evaluate the network performancewith a different traffic pattern: local gossip. We can thinkof this scenario as in-network processing such that thesource nodes exchange packets before they decide totransmit the data towards the sink node. The nodes inthe center of the terrain are assumed to be the sourcesand they exchange broadcast packets.

All the source nodes are are located in a 30 � 30 m2

(such that all source nodes are within the carrier sensingrange of each other and can receive packets from eachother) area in the center of the deployment terrain. Wevary the density by changing the terrain size and the num-ber of channels is 8 for MC-LMAC and MMSN. Fig. 18 showsthe results in terms of delivery ratios. When the network isdense, the rate of successful deliveries is low. MC-LMACsuffers from the clashes whereas CSMA and MMSN sufferfrom collisions. In the case of a clash, the receiver ran-domly selects the sender to listen in MC-LMAC. Addition-ally, the number of timeslots with MC-LMAC is 32. This islower than the required number of timeslots for the givendensity and causes some of the source nodes not to be ableto get a slot. In order to achieve higher delivery ratios in

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denser deployments, the number of timeslots should be in-creased. In MC-LMAC nodes give priority to receive thebroadcast packet during a clash. However, if the clashingtransmissions are both broadcast packets, then nodes ran-domly select a transmitter. However, in a real networkobserving a scenario where all the sources generate broad-cast packets is low. When L P 125, MC-LMAC can delivermore than 98% of the broadcast packets. In contrast, MMSNand CSMA protocols need more sparseness to mitigate theeffects of contention. The reason for CSMA to have first adecrease and then an increase in the delivery ratios is that,when the network is too dense the nodes can sense eachothers’ transmissions and backoff. When the network getssparser, the distances between the nodes increase andsensing of the parallel packets fails more. However, whenthe network gets even more sparser then the number ofnodes in the 30 � 30 m2 area decreases and contention de-creases accordingly.

5.6. Multiple sinks versus multiple channels

As we discussed in Section 3, the limiting factor is thereception capacity of the sink node. Contention-based pro-tocols fail to successfully allocate the medium during highcontention around a single sink node. In this section, as analternative to single-sink multi-channel scenario, we dis-cuss the impact of deploying more sink nodes using a sin-gle-channel on the achievable throughput.

Multiple sink nodes are randomly deployed in a150 � 150 m2 area. Source nodes transmit packets every2 s to the closest sink node. Fig. 19 shows the results. Inthis set, the nodes communicate on a single-channel (ex-cept the line titled ‘‘MMSN (F10)” in the figure). Our aimis to compare the results of n sink nodes with 1 channelwith the results of 1 sink node with n channels, which werepresented in Fig. 12. Compared to the results in Fig. 12,both CSMA and MMSN achieve higher throughput sincethe contention around the sink nodes has lower impactcompared to the contention around a single sink. Beyondfour sink nodes, MMSN starts to perform better thanMMSN with four channels and a single sink node. How-

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Fig. 19. Aggregate throughput with different number of sink nodes.


ever, around nine sink nodes the aggregate throughputstarts to saturate.

In contrast, the single-channel LMAC has a constantlower performance with a single channel and 32 timeslotssince most of the nodes cannot get a free slot on a singlechannel. However, if the number of timeslots is increasedto 48, a higher performance is achieved. Although thepacket delivery ratio with 48 timeslots is 100%, the aggre-gate throughput is on the average 75% of the maximumaggregate throughput since the nodes cannot choose thetimeslots that are used by their 2nd-hop neighbors onthe same channel and this reduces the number of paralleltransmissions. The question may arise: why does not thethroughput increase with higher number of sink nodes?As we mentioned, the maximum hop count with the givendeployment density is around three hops. Although thenodes may transmit to different sinks, according to thetimeslot selection rules they cannot get the same timeslotused in their 2-hop neighborhood. Compared with the re-sults in Fig. 12, MMSN and CSMA perform better withmultiple sinks but still they cannot achieve the perfor-mance of MC-LMAC with multiple channels which hasthe advantage of collision-free medium access over multi-ple channels.

The line named ‘‘MMSN (F10)” in the figure shows theresults when MMSN operates with multiple sink nodesand there are 10 channels available. The performance isbetter with 10 channels for a smaller number of sink nodesthan single-channel communication results given on theline titled ‘‘MMSN”. However, beyond 7 sinks there is littledifference in the performance and aggregate throughput isstill less than the achievable throughput with MC-LMACwhere there is a single sink to collect data over multiplechannels (Fig. 12). This is due to the failure of resolutionof contentions and collisions around the sink nodes anddue to the random access behavior of MMSN.

As a conclusion, multiple sink nodes can be used as acomplementary solution with MC-LMAC that can furtherimprove the achievable throughput for higher data ratescenarios.

5.7. Implementation on sensor nodes

The single-channel LMAC protocol has been imple-mented and previously tested [5] on Ambient lNodesensor platform. We added the MC-LMAC extension andperformed a simple test as a proof of concept using a sim-ple topology where two pairs of nodes are communicatingin parallel. The aim of the experiments is to investigate theimpact of channel switching on the synchronization of thenodes.

The sensor platform is equipped with Nordic Nrf905radio that can operate on the 868/915 MHz ISM band.Nodes continuously transmit 32-byte packets every 1/8 s.The conclusions of the experiments are that nodes canchange their operating frequency without loosing the syn-chronization and the protocol runs successfully on thenodes. The aggregate throughput with parallel communi-cation over different channels is doubled, as expected. Asa future work, we are interested in comparing the perfor-mance of different protocols on a large testbed by using alarger set of parameters.

6. Conclusions

We have presented MC-LMAC, designed for wirelesssensor networks with high throughput requirements.MC-LMAC takes advantage of both scheduled and multi-channel communication. Scheduled communication hasthe advantage of minimizing collisions whereas the mul-ti-channel communication overcomes the increased con-tention and interference on the limited bandwidth andimproves the throughput and bandwidth utilization. Nodescan transmit in parallel on different channels without dis-turbing each other. Simulation results show that, MC-LMAC achieves a throughput very close to the maximumwith the increased number of channels and outperformsthe MMSN protocol and the channel clustering methodfor moderate-size, 100-node networks. While MC-LMACsupports higher throughput, it also meets the typical char-


acteristics of WSNs such as energy efficiency and scalabil-ity. Besides convergecast traffic MC-LMAC supports broad-casts and local-gossip operations are performed efficiently.As a proof of concept, a simple test case of MC-LMAC dem-onstrates that nodes do not loose synchronization whileswitching between frequencies and the protocol runs suc-cessfully on the nodes.

As a future work, that would be interesting to incorpo-rate different channel selection mechanisms in MC-LMACthat can allocate not only orthogonal channels but alsooverlapping channels based on the interference conditionsdue to transmissions in closer channels or due to externalnetworks or external devices that share the same parts ofthe spectrum. Another future interest lies in testing thereliability of the protocol in case of channel errors on atestbed.

Acknowledgments

We gratefully acknowledge Dr. Gang Zhou for sharingthe source code of the MMSN protocol on Glomosim.

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Özlem Durmaz _Incel is currently a post-doc-toral researcher in the Networking Laboratory(NETLAB) of the Bogazici University, Turkey.She received her PhD in computer science fromthe University of Twente, Netherlands, inMarch 2009. Her dissertation focused on effi-cient data collection in wireless sensor net-works and was entitled as ‘‘Multi-ChannelWireless Sensor Networks: Protocols, Designand Evaluation”. She was a visiting student inthe Autonomous Networks Research Group ofthe University of Southern California as part of

her Phd studies in 2007–2008. She received both her MSc and BSc degrees incomputer engineering from the Yeditepe University, Turkey, in 2005 and
2002 respectively. Her research interests are in the design and analysis ofalgorithms/protocols for wireless networks, particularly for ad hoc andsensor networks, and in the performance evaluation of computer networks.
Lodewijk van Hoesel received his PhD in thearea of wireless sensor networks, in particularenergy-efficient communication protocolsand he has a MSc in electrical engineering. Hehas worked for the University of Twente, TheNetherlands as a post-doctoral researcher.Van Hoesel is one of the founders of AmbientSystems and is as Senior Systems Architectresponsible for many of technology advancesof Ambient. He is responsible for wirelessnetworking stack and system architecture ofcommercial wireless sensor networks and 3rd

generation active RFID applications. He is a member of the OptiWSN
standardisation committee. He has participated in many research projectsrelated to wireless sensor networks and wireless cooperating objects e.g.EYES, CoBis, e-Sense, AWARE, Sensei and FREE.
Pierre Jansen has performed research oncomputer architecture and operating systemswithin industrial and academic contexts. Hewas group leader for the design of an experi-mental high performance MIMD computerand holds a list of patents in this field. Since1982 he has a research position at the Uni-versity of Twente. His interests graduallyshifted from computer architecture to systemsupport for embedded systems. In the recentpast he co-operated in projects focusing on‘‘systems on a chip” and ‘‘wireless sensor

networks”. Currently he is retired but he is still active in the mentionedareas.

Paul Havinga is a professor in the ComputerScience department at the University ofTwente in the Netherlands, chair of the Per-vasive Systems research group and founderand CTO of Ambient Systems, in Enschede, theNetherlands. His research themes havefocussed on: wireless sensor networks,ambient intelligence, distributed systems,energy-efficient wireless communication, andmobile computing. The common theme inthese areas is on the development of large-scale, heterogeneous, wireless, distributed

systems. Research questions cover architectures, protocols, programming
paradigms, algorithms, and applications. He is the project manager ofseveral large international projects, he is involved as program committeechair, member, and reviewer for many conferences and workshops, andhe regularly serves as independent expert for reviewing and evaluation ofinternational research projects for the EU, the US, and internationalgovernment.
http://www.nvlsi.no/

ad hoc networks - kaistnslab.kaist.ac.kr/courses/2011/cs710/paperlist/1-14.pdf · and wireless ad...

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