S-MAC Protocol of Wireless Sensor Network

Jiang Xue

（Nanjing University of Posts and Telecommunications， Nanjing 210003， China）

Abstra c t:

W ireless SensorNetwork（WSN）usually contains a large quantity ofself-organizing distributed nodes.Since its networking is simp le,flexib le and unconstrained by w ired networks,itcan be w idely app lied in special cases such as emergency searching,d isastersalvation and m ilitary app lications.Therefore,WSN has a p rom ising future.However,the energy ofsensornodes inWSN is lim ited.Energy efficiency according ly becomes the p rimary issue forMed ia Access Control（MAC）p rotocoldesign inWSN.Itcan be seen in Reference[1]thatSensorMed ia Access Control（S-MAC）p rotocolis designed just tomeet the energy efficiency demand ofWSN.Periodic sleep,adap tive listen,overhearing avoidance and message passing mechanism smake the S-MAC p retty excellent in network energy-consum ing and latency performance,but there's stilla long way to go to satisfy all the requirements ofp racticalapp lications.

1 Introduction of S-MAC Protocol

S-MAC is p roposed based on 802.11 p rotocols,and itsmajor design targetis to reduce energy consum p tion and to p rovide good scalability.Itsmainmechanism s inc lude period ic listen and sleep,overhearing avoidance,message passing and traffic-based adap tive listen.

1.1 Critical Technology to Implement S-MAC Protocol

（1）Nesting Structure ofData Packet

Figure1 shows the nesting struc ture of the S-MAC p rotocol packet.In S-MAC p rotocol,the upper layerpacketcontains the information of lower layer packet.Each layeronly assumes the responsibility to dealw ith the information belonging to itwhen a packetis received.

▲Figure1. Data packet format of S-MAC protocol.

（2）Structure and Function ofS-MAC Stack

In S-MAC p rotocolstack,when the MAC layerofa node receives a data packet from its upper layer,the node w illstart carrier listening.If the resultshows the MAC layeras id le,the node willsend the packet to the Physical Layer；if the MAC layer is busy,the node w illenter the sleeping state,waiting for the nextid le time,and the data w illbe resentby then.When the MAC layer receives a packet from the PhysicalLayerand finds that there is no errorafter Cyc lic Redundancy Check（CRC）,the MAC layerw illsend this packet to its upper layer.The network modelis shown in Figure 2.

（3）Selecting and Maintaining Scheduling Tab le

Before starting period ic listen and sleep,each node needs to select theirsleep schedulingmechanism whichmustbe consistentw ith thatof theirneighbornodes.How do nodes select theirschedules and keep the consistency？The follow ing are threeways ofselec ting and keep ing themechanism:

a.In listening time,ifa node does notd iscover the sleep scheduling mechanism s ofothernodes,itw illselecta sleep scheduling mechanism immed iately；

b.Ifa node receives a sleepmechanism the neighbornodes broadcastbefore selecting and dec laring its own scheduling mechanism,itw illuse the neighbor's sleep scheduling mechanism；

c.Ifa node receives severald ifferentsleep mechanisms after selecting and broadcasting its own sleep scheduling mechanism,the case should be considered in two ways:ifone node does nothave a neighbornode,itw illd iscard current sleep scheduling mechanism of its own and adop t the latest received one；if this node have one ormore neighbornodes,it w illadop tseverald ifferentsleep scheduling mechanism s simultaneously.

（4）Clock Synchronization

In S-MAC p rotocol,a node and its neighbornodes should keep c lock synchronization for listening and sleeping simultaneously.S-MAC p rotocoladop ts relative,butnot absolute,timestam ps,and italso lets listening timemuch g reater than c lock errorand floating so as to reduce synchronizing errors.Besides,the node w illupdate its own c lock in accordance w ith its neighbornodes to keep them synchronized.

（5）Carrier Sense Multip le Access/Collision Avoidance（CSMA/CA）

The basic mechanism ofCSMA/CA is to estab lish a handshakemechanism to transfer data between the senderand the receiver.This handshakemechanism works as follows:

a.A sender sends a Request-to-Send（RTS）packet to its receiver,and the receiver rep lies a Clear-to-Send（CTS）packet to the sender.

b.After receiving this CTS packet,the senderbegins to send data packets to the receiver.The handshake between the RTS and the CTS ensures that the neighbornodes know that the senderand the receiverare transm itting data.With that,the transm ission collision can be g reatly reduced.

（6）Network Allocation Vector（NAV）

In S-MAC p rotocol,each nodemaintains an NAV to ind icate the ac tive time of its neighbornodes,and each packetof S-MAC p rotocolcontains a continuous time duration ind icator,which ind icates the duration of currentcommunication.When a neighbornode receives the packetsentby the senderor the receiver to othernodes,the neighbornode w illknow how long it should sleep,thatis,to update NAV value according to the time duration in the packet.When the NAV value is notzero,the node should entera sleep ing state to avoid overhearing.As soon as the NAV value turns zero,the node w illwake up atonce and get ready for communication.

1.2 Network Performance of S-MAC Protocol

（1）Energy

The factors ofenergy-consum ing inWSN inc lude id le listening,competition collision,overhearing,and overhead control.The S-MAC p rotocolhas formulated corresponding mechanisms based on these energy-consum ing factors.The regular listen and sleep mechanism is used tomake nodes entera sleeping state periodically in order to reduce energy consump tion for listening id le nodes.The overhearing avoidancemechanism is used tomake the nodes in communication entera sleep ing state to reduce energy consump tion foroverhearing.Themessage passingmechanism is adop ted to reduce RTS/CTS controlpackets so as to decrease the energy foroverhead control.

（2）Latency

In S-MAC p rotocol,the introduction ofperiod ic sleep mechanism b rings extra latency.However,adap tive listen mechanism is ab le to im p rove its latency feature.With the adap tive listenmechanism,a node does notentera sleep ing state immed iately after the communication ends；instead,itw ill keep on listening fora period of time.And if the node receives the RTS or CTS controlpacketsent to itw ithin this period,itw ill immed iately get ready for communication,w ithoutwaiting for the next listen time.In the follow ing,there are analyses and com parisons of network latency among three types ofS-MAC p rotocols:S-MAC p rotocolw ith neither sleep noradap tive listen mechanisms,S-MAC p rotocolw ith sleep mechanism but w ithoutadap tive listenmechanism,and S-MAC p rotocolw ith both sleep and adap tive listenmechanisms.

The following definitions aremade first:

Tnothingrefers to average latency in the network based on the S-MAC p rotocolwith neither sleep noradap tive listen mechanism s；

Tsleeprefers to average latency in the network based on the S-MAC p rotocolw ith sleep mechanism butw ithoutadap tive listenmechanism；

Tallrefers to average latency in the network based on the S-MAC p rotocolwith both sleep and adap tive listen mechanism s.

Conc lusions are ob tained from the analysis ofReference[2]:

▲Figure2. Network model of S-MAC protocol stack.

Among which,tcsis the size ofCom petitionWindow（CW）,ttxis transm ission latency,Tfis one frame time,and N is the sum of transm ission hops ofa packet.In a cond ition that the duty cyc le is not large,Tfismuch greater than tcsand ttx.According to Formulas（1）,（2）and（3）,Tsleepand Tall,w ith the increasing ofhops,augmentmore than Tnothing,and the increase of Tsleepis app roximately halfof thatof Tall.Itind icates that,in the S-MAC p rotocolw ith the sleepmechanism,the latency increasing rate in the network w ith adap tive listen mechanism is reduced to halfof thatw ithout the adap tive listenmechanism.Anyhow,both of them havemore latency than the S-MAC p rotocolw ithout the sleep mechanism,so neither is idealin latency performance.

2 Im provement of S-MAC Protocol

There aremainly twomethods to imp rove S-MAC p rotocol:dynam ic ad justment,and sending controland data packets underd ifferent transportconditions.

2.1 The Enhanced S-MAC Protocol Based on Dynam ic Adjustment

In S-MAC p rotocol,dynam ic ad justment is fixed,that is,the time durations for periodic listen and sleep are not flexib le.The follow ing p rotocols are aim ing at this d isadvantage to dynam ically ad justand save energy.

TimeoutMAC（T-MAC）[3]p rotocolconduc ts dynam ic ad justmentbased on five events and Time Ac tive（TA）.The five events are:

·Frame-leng th timeouts.·Node has received data.

·Data Transm ission collision occurs.

·Node data acknow ledges thatsend ing is successful.

·A neighbornode comp letes the data exchange.

IfRad io Frequency（RF）module doesn'tdetectany of those five events w ithin TA via listening,itw illconsider that the signal channelis id le,and the node w ill then shutdown the RFmodule and entera sleeping state.

In Pattern MAC（P-MAC）p rotocol[4],a node and its neighbor nodes ind icate their sleep/wake p lan for future severalslots by a bitstring,which is called“samp le”in this p rotocol.The node is ab le to adap tively ad just relying on its own“sam p le”as wellas thatofneighbornodes and finally form its own scheduling tab le.In Traffic Aware Energy EfficientMAC（TEEM）p rotocol[5],there are two categories ofsynchronization packet（SYNC）:SYNCdata,which ind icates there are data packets queuing and ready for send ing,and SYNCnodata,which indicates there's no data packet forsending.When a node receives SYNCnodata,itw ill sleep at the time when it is originally supposed to send data packets.When a node receives SYNCdata,itw illrep ly CTS data packetdirectly and starts communicating.

A variab le U is defined in ParameterUMAC（U-MAC）p rotocol[6].An equation is shown below:

Among which Trxstands for the totalsend ing time,and Tidlestands for the totalid le time.

The dynam ic ad justmentis done in accordance w ith parameter U from the above equation.If U is g reater than the upper threshold of traffic（Uhigh）,it im p lies the current communication traffic is beyond what the currentscheduling mechanism can bear.This case asks to increase the dynam ic ad justmentvalue.If U is less than the lower threshold of traffic（Ulow）,it ind icates the communication traffic is very small,and that the dynam ic ad justmentvalue should be smaller to p rolong the sleep ing time and save energy.

Adap tive MAC（AC-MAC）p rotocol[7]uses the quantity of data packets queuing in MAC Layer to rep resent the volume of traffic,recorded as Ni.Itd ivides the original large period into several Tframe,the smallerperiods,via the utility function,Ri=f（Ni）,and the formula,stands for the frame time,Riis themapp ing of Niusing the algorithm of the above utility func tion.This givesmore opportunities in send ing data than the originaldynam ic ad justment.

2.2 Sending Control and Data Packets under Different Transport Conditions

▼Table 1. Comparison of protocol performances

In S-MAC p rotocol,the conditions,such as send ing powerand channelcond itions,fora node to send controlpackets RTS/CTS/ACK and data packets DATA are consistent.However,in p rac tice,the length of controlpackets ismuch shorter than thatofdata packets,and the information in controlpackets is not so im portant.PowerControlMAC（PCSMAC）p rotocoland Doub le ChannelMAC（DCMA/AP）p rotocolcan send control and data packets under different transportcond itions,and,in this way,save energy.

In PCSMAC p rotocol,the powers fora node to send control packetRTS/CTS/ACK and data packetDATA are d ifferent.

The node scheduling tab le adds them inimum transm ission power required for communication between one node and its neighbornodes.Therefore,the node willselect themaximum power to send RTS/CTS/ACK,and select the p roper power enough to send DATA to its destination.

In DCMA/AP p rotocol,the channels fora node to send controlpackets RTS/CTS/ACK and data packets DATA are different,that is to say,the p rotocoladop ts doub le channels for transm ission.The channelused for transferring RTS/CTS/ACK controlpackets is the controlchannel,and the otherone used for transferring DATA packets is data channel.Tab le 1 summarizes the com parison ofperformances between the above revised p rotocols and the originalS-MAC p rotocol.

From the analysis above,the research on the com petition-based MAC p rotocols should be focused on the follow ing aspects in the future:

（1）Granted that the energy efficiency is guaranteed,various performance ind ices should be com p rom ised.Ifenergy efficiency is fulfilled,latency or throughputperformancemustbe sacrificed in order to save energy-under the cond ition that latency and throughputare ensured.

（2）The p rotocols guarantee Quality ofService（QoS）better.With the developmentofvarious app lications,itbecomesmore and more important to p rovide d ifferentQuality ofService to differentservices.Further research is necessary for the MAC p rotocolof the WSN thatsupports QoS.

（3）Inter-layer p rotocols are relatively comp licated for imp lementation.However,the inter-layerdesign ofp rotocols enab les the network to perform atits bestin every aspec t.

3 Conclusions

Sensornetwork is one of the latesthot top ics in the communications industry.Ithas a p rom ising app lication p rospect.Through the researches allover recentyears,peop le know more about the design requirements for Sensor Network MAC（S-MAC）p rotocol[8-9].

Naturally,it's not realistic to design a MAC p rotocolto satisfy allrequirements ofWSN.So,app rop riate MAC p rotocols should be designed according to d ifferentapp lication requirements,and using d ifferentmethods.