HMARS: A MAC Protocol for Integration ofRadio-over-Fiber and Wireless Sensor Networks

Tiago P. C. de Andrade, Leonardo B. Oliveira, Nelson L. S. da FonsecaState University of Campinas

Institute of ComputingCampinas, Brazil

tiagoandrade@lrc.ic.unicamp.br, leob@ic.unicamp.br, nfonseca@ic.unicamp.br

Omar C. BranquinhoPontifical Catholic University of Campinas

Campinas, Brazilbranquinho@puc-campinas.edu.br

Abstract—Radio-over-Fiber (RoF) has been employed in net-work infrastructures due to its large capacity, low attenuation,low operational costs and enlarged network coverage. WirelessSensor Networks (WSNs) has also been broadly deployed. Theintegration of these two powerful technologies leads to newinfrastructures. In this work, we introduce a new approach forintegrating of RoF and WSNs, and present HMARS, a MACprotocol proposed exclusively for this new type of integration.Our results indicate that HMARS performs considerable betterthan others MAC protocols in this integration.

Index Terms—Radio-over-Fiber; Hybrid Systems; WirelessSensor Network; Medium Access Control Protocols; HMARS.

I. INTRODUCTION

Wireless Sensor Networks (WSNs) are nowadays a promis-ing and powerful novelty in the scenario of industrial commu-nications. They provide distributed sensing features heavinginteresting properties with respect to wired networks, suchas the absence of infrastructures, low cost, scalability andflexibility. WSNs are ad-hoc networks comprised mainly ofsmall sensor nodes with limited resources and one or morebase stations (BSs), which connect the sensor nodes to the restof the world [1][2]. WSNs are used for monitoring purposes,providing information about the monitored area to the restof the system. Application areas range from battlefield recon-naissance and emergency rescue operations to surveillance andenvironmental protection.

WSNs are a unique tool for integration of monitoring andcontrol applications. Among them, those tailored to environ-mental monitoring, which could not be monitored by any othertechnology, are of especial interest. For instance, ubiquitousCity (u-City) [3], which has been conceived to monitor theenvironmental or vehicular traffic. In u-City, virtually all in-formation systems are linked to an information system throughWSNs.

Radio-over-Fiber (RoF) technology has been the focus ofinterest due to its low attenuation [4] [5] [6] and increasedradio coverage. In RoF, radio signals are transmitted in opticalfiber by Remote Antenna Unit (RAU) while the complicatedsignal processing is carried out in Base Stations Controller(BSC). As a result, operational costs are reduced and coveragearea enlarged. In addition, it also implies on access networkswith fine capillarity. Recently, the RoF technology has beenemployed in cellular networks, broadband systems, Road Ve-hicle Communication (RVC) systems and IEEE 802.11 homenetworks.

The RoF technology has great potentiality to be used ininfrastructures due to the availability of large amount of opticalfibers in the world, especially in Brazil, in with around 80%of deployed fibers are not used. This underutilization of theoptical fibers is caused by the wide deployment of this typeof infrastructure in the past years, and the development ofthe Wave Division Multiplexing technology to optimize theuse of the fiber. The utilization of these idle resources tosupport non-traditional applications is a desirable and of greatpotentiality of the use of fiber underutilization. Examples ofthis use is monitoring of gas and oil pipes, highways, andelectrical transmission lines.

In WSN, RoF RAUs can act as cluster-head sensor nodes(CHSNs), commonly a heterogeneous node. A hybrid archi-tecture of WSN based on RoF, or RWSN for short, comprisessensor node (SN), a CH, and a Data Center (DC) (whereis the Base Station Controller). It is expected that in thisintegration, most of the conventional solutions adopted intraditional WSNs can be used to RWSNs.

An important issue in RoF integrated networks is the addi-tional delay that optical fiber introduces in the network. If thisdelay is too long, then it can degrade the system performance.Existing MAC protocols do not consider this delay and in turnare not adequate to RWSNs.

In this work, we present a new MAC protocol calledHMARS, a MAC protocol tailored to this new type of ar-978-1-4673-0279-1/11/$26.00 c© 2011 IEEE

chitecture.The remainder of this work is organized as follows. In

Section II, we discuss related work. In Section III, we presentthe new architecture concept. In Section IV, we presenthow HMARS works. We describe simulation setup and showsimulation results in Section V. Finally, we conclude our workin Section VI.

II. RELATED WORK

The number of studies on WSNs MAC protocols hasgrown significantly [7] [8] [9] [10] [11] [12]. Amongthem, S-MAC (Sensor MAC) [7] provides a tunable periodicactive/sleep cycle for conservation of energy. During sleepperiods, nodes turn off radio to conserve energy and dur-ing active periods, nodes turn on radio to Tx/Rx messages.Neighboring nodes form virtual clusters set up a commonsleep schedule. A drawback of S-MAC is the possibilityof following two different schedules, which results in moreenergy consumption via idle listening and overhearing. As S-MAC uses the CSMA/CA to reduce collisions, control packetsRTS/CTS are exchanged prior to data transmission.

The T-MAC (Timeout MAC) protocol [8] follows up on thebasic idea introduced by S-MAC, however, T-MAC alleviatesS-MAC’s rigidity by proposing an adaptive duty-cycle inwhich the duration of active periods is no longer fixed butvaries according to the traffic. The key idea of T-MAC consistson making a node predict channel activity during an activeperiod so that it can switch its radio off before the activeperiod ends, in case it does not expect any traffic.

The B-MAC (Berkeley MAC) protocol [10] proposes atechnique based on outliers detection to improve the qualityof Clear Channel Assessment (CCA) comparing with IEEE802.15.4 [13] standard. In this technique, a node searches foroutliers in the received signal such that the channel energyis significantly below the noise floor. If the node detects anoutlier during sampling of the channel, then it declares thechannel is clear. If the node does not find any outliers withinfives samples, then it declares the channel to be busy.

The aim of the D-MAC (Data-gathering MAC) protocol [9]is to achieves very low latency, but still to be energy efficient.It could be summarized as an improved Slotted ALOHAalgorithm where slots are assigned to the sets of nodes basedon a gathering tree. Hence, during the period of a nodereceives, all of its children nodes have transmission periods.Low latency is achieved by assigning subsequent slots to thenodes that are successive in the data transmission path.

In another vein, works that consider the utilization ofRoF [14] [15] demonstrate the potential of RoF as a backhaulfor radio applications.

A potential application of the integration between WSN andRoF is presented in [16]. This work presents a WSN to monitorcathodic protection in oil pipes. The WSN was implementedwith a strategy of repetition, in which each node transmitsthe data through the air interface. The network formed leadsto high latency and low availability, which increases withthe number of nodes. There are advantages for this type of

application has advantages if the WSN uses a RoF backhaulsince, instead of transmitting the information through theair interface, the information is transmitted through the RoFsystem, with low latency and high availability.

A proposal of hybrid architecture of RoF and WSN asshowed in Figure 1 was presented in [17]. The architecture isadequate to monitor temperature, humidity, gas and to locateemployees in environment such as mine. In this scenariothe radio technology is not feasible, considering the highattenuation of radio signals. The work proposes the utilizationof small coverage areas with WSN adhoc network that isconnected with the central station by the RoF system.

Figure 1. Hybrid RWSN architecture [17]

A proposal that shows the advantage of WSN connected byRoF links in metropolitan areas considering radius of around10 km to create a smart environment for monitoring [3]. Inthis application, without the RoF as a backhaul, the ad-hocnetwork needs several hops to transmit from distant nodes tothe central station. Large cells implies on high latencies, highcollision probabilities and low availability.

III. WSNS AND ARCHITECTURE PROPOSAL

WSNs can be organized in different ways. In flatWSNs [18], all nodes play similar roles in sensing, data pro-cessing, and routing. In particular, nodes operate with limitedradio transmission range to decrease energy consumption, andthe communication Node-to-BS is multi-hop, with ordinarynodes working as routers. In hierarchical WSNs [1], thenetwork is typically organized into clusters, with ordinarycluster members and the cluster heads (CHs) playing differentroles. While ordinary cluster members are responsible forsensing, the CHs are responsible for additional tasks such ascollecting and processing data from their cluster members, andforwarding results towards the BS.

The proposed architecture considers hierarchical WSNs, inwhich the RAUs are CHs. Optical fibers are used to connectWSNs and to enlarge the coverage areas. The proposal differsfrom the proposals presented in Section II. Figure 2 shows

. . .

Cluster 1

Data Center

RAU RAU

RAU RAU

RAU

RAU

Cluster 3 Cluster N-1

Cluster 2 Cluster N-2 Cluster N

Figure 2. Our Architecture Proposal

the architecture proposal. All nodes in the coverage area forma cluster having the RAU as the CH. Clusters are distributedalong an optical fiber and the SNs in each cluster transmitonly to the Base Station Controller. Thus, all clusters are inthe same collision domain.

In this architecture, the MAC protocol needs to deal withthe challenge in contention of the wireless and of the opticalchannel contention.

IV. PROTOCOL OVERVIEW

In this section, we introduce the HMARS (Hybrid MediumAccess Control Protocol for Hybrid Radio over Fiber WirelessSensor Network Architecture), a single-hop MAC protocolspecific for hybrid Radio-Over-Fiber Wireless Sensor NetworkArchitecture.

By combining reservation and contention approaches, it usesTime Division Multiple Access (TDMA) to avoid collisionsin the optical channel and Carrier Sense Medium Access(CSMA) non persistent to avoid collisions in the wirelesschannel.

The reservation-based approach requires knowledge of thenetwork topology and network synchronization to establisha schedule that allows each cluster to access the opticalchannel and communicate with the BSC. In the contention-based approach, sensor nodes compete for the use of thewireless channel and only the winner of this competition isallowed to access to the channel and transmit.

In TDMA, time is divided into frames and each frame isdivided into slots. During a frame, each cluster is assigneda unique slot during which it has the right to transmit. As aconsequence, transmissions of different clusters do not collide.In CSMA, a sensor node having backlogged frames first sensesthe wireless channel before actually transmitting the frames. Incase the node finds the wireless channel busy, it postpones itstransmission to avoid interfering with ongoing transmission.In case the node finds the wireless channel idle, it transmitsthe frame (after possibly having waited a period with randomduration).

HMARS is a Basic Access Method, therefore, it suppressesthe RTS/CTS exchange and employs a random backoff toreduce the overhead as well as collisions. It also suppressacknowledgment frames.

Figure 3 shows the HMARS’s frame format. The FrameControl field contains information defining the frame type andother control flags. The Sequence Number field specifies thesequence identifier for the frame. The Source Cluster fieldcontains the id of the source node cluster. The Source Nodefield contains the id of the source node. The Target Clusterfield contains the id of the target node cluster. The TargetNode field contains the id of the target node. The FCS fieldcontains a 16-bit CRC and it is calculated over the MHR andpayload field of the frame.

The Preamble field is used by the transceiver to obtain chipand symbol synchronization with an incoming message. Itindicates the end of the SHR and the start of data. The FrameLength specifies the total number of octets contained in thePHY payload.

8bits

16bits

8bits

8bits

8bits

8bits

16bits

0 - 400bits

ControlSequenceNumber

SourceCluster

SourceNode

TargetCluster

TargetNode Data Payload FCS

40bits

PreambleFrameLength

8bits

72 - 472bits

PHY Payload

DATA FRAME

PHY HEADER

DestinationAddress

SourceAddress

Figure 3. The HMARS’s Frame format

TABLE IACRONYM SUMMARY

RoF Radio-over-FiberWSN Wireless Sensor NetworkSN Sensor NodeDC Data CenterBSC Base Station ControllerRAU Remote Antenna UnitCCA Clear Channel AssessmentCW Contention WindowNA Number of AttemptsMAXAtempt Maximum Number of AttemptsRWSN Hybrid Architecture of RoF and WSNSCHSN Cluster-Head Sensor NodeFCS Frame Check Sequence

A. CSMA access to the Wireless Channel

Monitoring time and contention window are fixed to avoidstarvation and collisions. Tree variables are maintained: CW,NA, and MAXAttempt. CW is the contention window size,which defines the number of backoff periods without chan-nel activity before transmission can commence; its value isinitialized to 2 before each transmission attempt and resetto 2 each time the channel is busy. NA is the number ofattempts of transmission; its value is initialized to 2 beforeeach transmission. MAXAttempt is the maximum number oftransmission attempt.

The MAC sublayer delays transmission for a random num-ber of complete backoff periods in the range 0 to 2 (Figure 4,Step 2) and then request the physical layer to perform a ClearChannel Assessment (CCA) (Figure 4, Step 3).

If the channel is busy, NA is incremented by one and CWset to two (Figure 4, Step 5). If NA is less than or equal toMAXAttempt, a new transmission is attempted after 0 to 2backoff periods later (Figure 4, Step 2). If NA is greater thanMAXAttempt, the MAC sublayer reports a transmission errorto the layer above.

If the channel is idle, the MAC protocol makes sure thecontention windows has expired. To do that, CW is decre-mented monotonically (Figure 4, Step 4) and, subsequently, itis checked whether its value is zero. If so, the MAC sublayerimmediately requires a new CCA to the physical layer. Ifnot so, the transmission is restarted. By doing that, we limitthe number of simultaneous transmissions and decrease thenumber of intra-cluster collisions.

B. TDMA access to the Optical Channel

Algorithm 1 Centralized Scheduling of TDMARequirement: all clocks of the network synchronizedEnsure: time slots synchronized correctly in the network

for each cluster ∈ (CHSN) doBSC broadcast to cluster a Synchronization frame(SYNC)

end forfor each node ∈ (SN) do

if node received SYNC frame thennode compute the Synchronization

end ifend for

(CHSN) : set of clusters order by distance to BSC(SN) : set of sensors nodes

We assume that the Base Station Controller knows thenetwork topology and the delays to each cluster, which arequite common to be known in operational networks

As the optical channel is the common collision domain toall nodes in the clusters, the TDMA protocol tries to avoidcollisions among themselves.

Time is divided into frames and each frame is divided intoslots. The number of slots is the number of clusters in the

CW := 2NA := 0

Perform CCA

CW := 2NA := NA + 1

CW := CW - 1

Channelidle?

NA >MAXAttempt?

CW = 0?

Failure

Success

Y

N

N

N

Y

Y

Delay forrandom(0-2) unitbackoff periods

(1)

(2)

(3)

(4)

(5)

(6)

(7)

Figure 4. The HMARS’s CSMA algorithm

network and each slot has the same duration. The number offrames depends on the synchronization interval.

During a frame, each cluster is assigned a unique slotaccording to its location along the optical fiber (Figure 2),during which the clusters has the right to transmit. As aconsequence, collisions among clusters are avoided, whichguarantees finite and predictable scheduling delays and alsoincreases the overall network throughput under highly loadednetworks.

Scheduling is the crucial part of the protocol. As it requiresgreat computational power, it is carried out by the Base StationController. Algorithm 1 shows how the schedule is done.

V. SIMULATIONS

A. Setup

We implemented the HMARS protocol in the NS-2 networksimulator making adaptation to include the RoF nodes. Forcomparison, we implemented three other protocols: ALOHAwithout acknowledgment, hybrid TDMA/ALOHA without ac-knowledgment and CSMA/ARC [19] p-persistent. We also usedthe version of S-MAC (RTS/CTS Access Method) imple-mented in the NS-2 with the periodic sleep disabled so thateach node runs in full active mode. However, to save energy,

each node goes to sleep only when there is another nodetransmitting.

In the ALOHA protocol, when the sender is ready totransmit, it delays the transmission for a random period. Thesender does not sense the medium and all clusters can transmitat any time.

In the TDMA/ALOHA protocol, each cluster has a specifictime slot to transmit.

In the CSMA/ARC p-persistent protocol, when the sender isready to transmit, it checks continually if the medium is busy.When medium becomes idle, the sender transmits a framewith a probability p. If the sender chooses not to transmit (theprobability of this event is 1-p), the sender waits a time periodand transmits again with the same probability p. However, allcluster can transmit at any time.

We chose three metrics to evaluate the performance ofHMARS: Delivery Ratio which is the ratio of successfullydelivered frames to the total frames originating from allsources, Frame Error Rate (FER) which is the ratio of thedropped frames to the total frames originating from all sourcesand the Maximum Distance allowed between the Base StationController and the CHSNs.

Table II shows the parameters used in the simulations.

TABLE IIFIXED MODEL PARAMETERS

PHY Module ParametersRadio Bandwidth 250kbpsFrequency 915 MHzTransmitter Power 10 dBmCarrier Sense Sensitivity -104 dBm

Traffic Module ParametersTraffic Type PoissonRate 10 frames/second

Propagation ModelModel ShadowingAttenuation Factor (β) 3.41Standard Deviation (σ) 7Reference Distance (d0) 1

In the simulations, each cluster contains 15 sensors nodesdistant 15 meters of the RAU. The clusters are distributedalong the optical fiber and separated by 10 km (Figure 2).

All simulations are run independently using 5 differentseeds. All sources generate frames according to a PoissonTraffic Process with a rate of 10 frames per seconds.

B. Results

When the optical fibers are inserted into the system eachframe will go through the fiber delay.

In this section, simulation results are presented and dis-cussed.

1) Frame Error Rate and Delivery Ratio: Figure 5 andFigure 6 show the performance under different number ofclusters, and consequently different number of sources. Asthe number of clusters increases, interference increases whichresults in increased FER and decreased Delivery Ratio for theALOHA and for the CSMA/ARC p-persistent. HMARS and

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30 35 40 45 50

FE

R

Number of Clusters

HMARShybrid TDMA/ALOHA

CSMA/ARC 0.85−persistentALOHAS−MAC

Figure 5. Frame Error Rate

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30 35 40 45 50

Del

iver

y R

atio

Number of Clusters

HMARShybrid TDMA/ALOHA

CSMA/ARC 0.85−persistentALOHAS−MAC

Figure 6. Delivery Ratio

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

Del

iver

y R

atio

Fiber Length (km)

HMARS

S−MAC with RTS−CTS

Figure 7. Maximum Distance

TDMA/ALOHA, however, can still maintain a good deliveryratio and FER, having HMARS better performance.

As all clusters can transmit at the same time in the ALOHAand in the CSMA/ARC p-persistent, the number of collisionsincreases with the increase of number of clusters. The numberof intra-clusters collisions is lower in CSMA/ARC p-persistentthan in ALOHA.

According Figure 5 and Figure 6, the increased delay due tothe inserted fiber is more severe in RTS/CTS Access Method.It happens because the wait time for the CTS frame oversbefore of the CTS frame arrives, doing the sensor nodes donot transmit their data.

2) Maximum Distance: Figure 2 shows the clusters dis-tributed along optical fiber. Indeed, the distance betweenthe cluster and Base Station Controller affects the networkperformance. Thus, we analyzed the network behavior for onecluster with one sensor node and varied distances betweenthis cluster and Base Station Controller from 10 to 300 km.In this analysis, only the HMARS and S-MAC were used inthe simulations and no physical impairment were considered.

S-MAC adopts the RTS/CTS message exchange, so thestations using the RTS/CTS frames have the power to controlthe use of the medium between them. This reserves the sharedmedium for the time needed to transfer the actual frame priorto its transmission. During this period, all stations in thereserved area are restricted from transmission even though thechannel is idle.

When the optical fiber is inserted into the system eachframe will suffer additional delay. According to Figure 7,the increased delay due to the inserted fiber is more severein the RTS/CTS Access Method than in the Basic AccessMethod. Hence, the S-MAC covers a maximum of 100 kmwhile HMARS is not limit to such distance.

VI. CONCLUSION

Radio-over-Fiber (RoF) has been employed in networkinfrastructure because of its large capacity and low attenuation.It makes system setup easier, decrease operational costs, thus ithas been used to enhance network coverage. WSNs have alsobeen widely spread as a consequence of the integration of thesetwo powerful technologies has been the focus of attention.

In this work, we present a new hybrid architecture in whichoptical fibers are used to connect RAUs to one DC using theRoF system. To our knowledge, the HMARS is the first MACprotocol proposed exclusively with this architecture in mind.Our results indicate that HMARS performs considerable betterthan the others protocols tested in this architecture.

Future work will focus on the analysis of energy consump-tion of HMARS. We will also implement fiber attenuation inour simulations, in order to simulate more realistic scenarios.Another point is extending HMARS to the multi-hop case, toincrease even further the coverage area of clusters.

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