2IN95 MAC Protocols

8/3/2019 2IN95 MAC Protocols

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Survey wireless sensor network MAC protocols

Anton BilosTechnical University of Eindhoven

Eindhoven, The Netherlands

David HardyTechnical University of Eindhoven

Eindhoven, The Netherlands

ABSTRACTIn this paper we look into various MAC protocols and givea short explanation about their function. We compare theseMAC protocols and explain the advantages/disadvantagesof various power reduction schemes. We conclude by cate-gorizing the MAC protocols to various applications.

General TermsSurvey MAC protocols

1. INTRODUCTIONWireless sensor networks is a fast developing eld that haslead to the proliferation of various protocols. In this paperwe will look primarily at MAC protocols that manages howthe wireless communication is handled.

But what are MAC protocols? MAC stands for Media Ac-cess Control, where in the case of wireless sensor networksthe media stands for the wireless frequency band. There isspecialized control needed to control access to this mediumto ensure correct operation. There are a few general aspects

that all MAC protocols (try) to address:

• Collisions, two transmitters sending at the same timeand corrupting the transmission.

• Overhearing avoidance, preventing overhearing of mes-sages not meant for you.

• Fairness, all participants in the network get their shareof bandwidth without degrading the performance of others.

• Power efficiency.

Our focus is on energy usage and conservation as wirelesssensor networks are usually battery operated or extract en-ergy from their environment. Our research questions are:

• What inuence has power consumption on the choiceof a MAC protocol?

• To save power there are several options, like clusteringand sleep times for nodes. What are the disadvantagesof these power saving options?

• What MAC protocols are suitable for what applica-tions?

We start out by presenting the classication of the MACprotocols we have treated, each class is highlighted by itsproperties and how it is built up. The classes for the MACprotocols are:

• Contention based protocols: These protocols have nonotion of time and it uses the medium when it is avail-able.

• Time based protocols: Assignment of the medium isdone by timing, hence, time based protocols.

• Hybrid: A few protocols combine parts of both proto-cols.

We then treat a few MAC protocols, describe their func-tion and add our own analysis to the protocol. We treatwhat power saving options have been implemented and itsinuence on other parameters.

In the conclusion we compare these various classes and seehow they perform with respect to power usage and powerefficiency. We will treat advantages and disadvantages of theprotocols and see how that maps on various applications.

2. MAC PROTOCOL CLASSIFICATIONBefore we can start by treating various MAC protocols werst create a classication of these protocols. There are twomain classes and a third hybrid class. The classicationis made on the method the protocols use when accessingthe wireless medium. This distinction splits the protocolsin two camps, contention based protocols and timing basedprotocols.

2.1 Contention protocolsThe Contention based protocols are protocols that deal withthe medium on a rst rst serve basis. There are various



methods and schemes that are used in determining or opti-mizing the medium access to ensure fairness and reducingcollisions. This method is vulnerable to collisions in generaland there are various schemes used to prevent these collisionsand wasting bandwidth and reducing errors. PAMAS andIEEE802.11 are the primary example for contention basedprotocols. PAMAS is treated in section 5 as it is one of the more heavily cited protocols in literature and 802.11

is treated in section 4 used a lot in experimental compar-isons with various protocols. We will treat both protocols indepth. As we will see especially for this protocols in combi-nation with sleep it is hard to determine when to wake up,since sleeping is in the time domain and the communicationis not.

2.2 Timing protocolsThe timing based protocols use periods to determine mediumaccess where in each period there is exactly one transmit-ter, and one or multiple receivers. The activation times of receiver and transmitter is governed by a schedule that isrepeated periodically. This schedule can be modied duringusage of the network depending on external events or a given

time period. Example protocols we choose are TRAMA,which is handled in section 8 and ER-MAC treated in sec-tion 7. These protocols usually have a long (contentionbased) initialization phase, in which the schedules are made.Once these schedules are complete sleeping of nodes is rela-tive easy.

2.3 Hybrid protocolsThe hybrid protocols combine various aspects of time andcontention based protocols to x or at least reduce the dis-advantages of each protocol. As there are many possiblehybrid mixes of protocols there are large amount of possi-ble MAC protocols, we chose S-MAC which is explained insection 6.

3. TAXONOMY OF APPLICATIONSAs there are three groups of MAC protocols there are alsoa few groups of applications possible. This makes it easy tosee what protocols are suitable for each application.

• Static monitoring

• Dynamic monitoring

• Dynamic tracking

3.1 Static monitoringIn static monitoring the assumption is made that we have anetwork where the nodes do not move and they monitor theirenvironment. This application is used in monitoring variouslarge objects where it would be impractical or infeasible tomonitor the object by people.

Monitoring a bridge is one such example, this is a very largeobject with many hard to access locations. Sending outpeople to monitor various parts of the bridge is expensiveand requires dedicated expertise. A wireless sensor networkis a ideal replacement, the nodes can monitor various aspectsof the bridge with sensors and report back to the primary

monitoring station. We speak of only one sink, but multiplesinks are possible.

One example is the implementation described in Paek etal [1].

This application has some requirements with respect to theMAC protocol. The nodes are generally not replaceable so

in general lifetime is the most important aspect.

3.2 Dynamic monitoringIn dynamic monitoring the assumption is made that we havea network where the nodes do move around and they monitortheir environment. This application is used in monitoringmobile units like equipment or animals.

Monitoring a eld of cows is one possible application. Cowshave nodes attached to their legs to monitor step rates. Thecows move trough the eld and in and out of the stablewhen they want thus changing the topology of the network.A wireless sensor network ts well in this application as itreduces periodic checkups on the cows and when somethingis wrong it can be detected right away.

This application has some requirements with respect to theMAC protocol. Nodes are replaceable but a long run timeis expected, you can not expect the farmer replacing nodesevery day on cow legs. The dynamic network introducesmany complexities in the MAC protocol.

3.3 Dynamic trackingIn dynamic tracking the network is mobile but instead of monitoring the environment it tracks the nodes to trackmovement. This is also known under the concept of ”ac-tive RFID”.

Tracking visitors in a conference/fair is one application. Vis-itors to a conference are given a badge with a unique numberattached to their name and a button to signal that the visi-tor has seen something interesting. The visitors move troughthe terrain and each patch of terrain is monitored by a basestation. This base station monitors what nodes are nearbyand records this data with a time stamp. For a practicalimplementation see the openbeacon.org project [3].

This application has some requirements with respect to theMAC protocol. Nodes are usually replaceable, or have re-placeable batteries. Power optimization is important as youdo not want to replace the batteries or tags every few months.The protocol is usually centered around ”access points” or”base stations” that collect data sent by the nodes.

4. IEEE 802.11 BASED MACThe IEEE 802.11 MAC for wireless networking is widelyused for wireless networking in a home and business com-puting environment. Many studies and experiments com-pare their protocol other wireless sensor network MAC’s butalso use 802.11 networking as a baseline. The 802.11 MACprotocol is not optimized for power optimization but moreon throughput and bandwidth efficiency.



4.1 OperationThe IEEE 802.11 standard is optimized for wireless network-ing applications and not for wireless sensor networks. There-for most of its specialized features are centered in preventingbandwidth waste and improving utilization.

4.1.1 Collision avoidanceThe 802.11 MAC protocol uses RTS/CTS pairs in achieving

collision avoidance, a RTS is sent to the node that is theend point for the data. The node itself transmits a CTS tothe sender of the RTS. All the other nodes in range of thetransmitters of the CTS and RTS receive a RTS and/or CTSand refrain from transmitting until the transmission is over.This is the basic protocol behind the CSMA/CA scheme(for a more thorough explanation see [6]). The transmis-sion also holds a duration eld that describes how long thistransmission takes. This way other nodes know when themedium becomes available. This is done as the transceiversin 802.11 can not detect when a collision occurs so a vir-tual carrier sense is created. This scheme is also called theNetwork Allocation Vector (for more information see [8]).

4.2 DesignThis protocol assumes that there are base stations called”access points”. They provide arbitration and clustering of the various nodes.

4.3 ResultsWe have only included 802.11 for completeness, most MAC’sfor wireless sensor networks compare 802.11 with their ownMAC, so will not show any experimental results.

5. PAMAS PROTOCOLThe Power Aware Multi-Access protocol with Signalling (PA-MAS) MAC is one of the basic MAC’s of which many otherMAC continued from. It is created by Suresh Singh et al [5].

5.1 OperationThis protocol tries to minimize power usage, by letting nodessleep as much as possible. To do so it relies on the followingmechanisms.

• PAMAS State machine.

• Power off scheme.

• Probe Protocol.

Also we discus an alternative for the Probe Protocol.

5.1.1 PAMAS State machineTo be able to use PAMAS the nodes need to have two sepa-rate channels. One is used for data traffic, the other is usedfor signalling.

As can be seen in Figure 1 there are 6 different states a nodecan be at. Initially a node is in the Idle state. With use of CTS and RTS, which are communicated over the signallingchannel, a note can communicate. When node B receivesan RTS from node A (assuming that currently there are noother nodes communicating), node B will reply with a CTS.

As soon as node A sends the RTS it goes into the AwaitCTS state . When node B sends the CTS node B goes tothe Await Packet State . If for some reason the CTS is notreceived by node A it will go to the Binary ExponentialBackoff (BEB) state after 1 time step and it will resend aRTS after a random time. If CTS is not received by node A,then node B will leave the Await Packet Sate after 1 timestep and return to Idle state. When the CTS is received by

node A it will go to the Transmit Packet state, ignoring allcommunication on the signalling channel and sending dataon the data channel. When node B starts receiving data itwill go to the Receive Packet state and will send out a busytone on the signalling channel, so that other nodes know thatnode B is receiving data on the data channel (this to avoidcollisions). When the transmission is complete both nodeswill return to the idle state. This busy tone is send in sucha period that a RTS or a CTS cannot be send completely,so it will always collide, thus preventing additional nodessending over the data channel.

During the states Await Packet , Await CTS and BEB, nodescan still receive a (new) RTS and go to the Await Packetby replying with a CTS.

If a node is sending a busy tone no other nodes that receivethis busy tone communicate over the data channel to avoidcollisions. When a collision occurs on the signalling channel,for example two nodes try to send a RTS, both nodes willenter the BEB state after 1 time step and will try again arandom time later.

5.1.2 Power off To extend the lifetime of a node powering off the radio isimportant. But this should be done with care. PAMAS hastwo rules to power off the radio:

1. A node has no data to send and a neighbor is trans-mitting (to another node).

2. A neighbor is receiving, even if the node itself has datato send.

The second rule is the easiest to explain. The receiving nodeoccupies the signalling channel with its busy tone. There-fore the rst node cannot receive any RTS or CTS and thuscannot do anything. The rst rule is less obvious, becausea node should only power down if it has no data to send.When it has no data and a neighbor is transmitting the nodecannot receive anything without collision, because the trans-mitting node occupies the data channel. But when a nodewants to send data and no neighbors are receiving, the sig-nalling channel is free and the data channel has no collisionat the intended receiver side. This because the intended re-ceiver and the sending neighbor node cannot see each other.The collsion that occurs at both senders does not matter,because they are not receivers. This powering down schemedoes not prevent idle listening.

When a node is powered down and another node tries tosend a RTS no errors will occur. Because to the sendingnode it just looks like an collision occurred and the nodewill enter the BEB state. To know how long a node shouldstay powered off is handled by the Probe Protocol.



Figure 1: The PAMAS State Machine, Suresh Singh et al [5]

5.1.3 Probe ProtocolTo know how long a node must power itself down it usesthe Probe Protocol. When a node decides to power downit does so because there is a transmission which does notrequire the node to be active. From this transmission the

node can determine how long this transmission is going tolast. This information is either in the data header (rstpacket), or in the busy tone. So the node powers down forthis duration. When the node is powered up again there isa possibility that another node started sending while it wasasleep. Now the node has missed the header and there is apossibility that it hears no busy tone, so the node does notknow how long it should power down.

Now the node will send a probe packet over the signallingchannels. Node which hear this probe packet respond witha probe response . This contains the duration for the on-going transmission, so the node knows for how long it cansleep. When it does not hear a reply, a node will try to senda probe packet again.

5.1.4 Alternative for the Probe ProtocolNormally we assume that a node shuts down both channels.But if instead the node does not disable the signalling chan-nel it can still overhear the transmission durations which aresend.

5.2 DesignPAMAS does not solve routing, so the application has toprovide for this. Because PAMAS does not have routing itis exible in use in respect to dynamic nodes (as most con-

tention based protocols). The only thing that PAMAS needsare two separate radios and that nodes can communicate bi-directional, so base stations that can send to all nodes arenot supported by PAMAS. Also no clock synchronization isneeded because times is only used for waking up and this is

relative time.

5.3 ResultsThe PAMAS protocol only has results based on simulations,taken from the paper of Suresh Singh et al [5]. For the sim-ulation the following settings were used: The packet sizeis 512 bytes, the CTS and RTS 32 bytes each. The busytone lasts twice as long as the sending of 1 CTS/RTS. Thebandwidth is 12.8Kbps. Sending 32 bytes or receiving 64bytes takes 1 unit of energy. Other operations cost no en-ergy. Nodes can buffer 2n messages, were n is the number of nodes. To measure power saving the amount of bytes sendand received were calculated, as well as the total numbersof packets send. Nodes generate packets by a poisson pro-cess and determine there destination uniformly. Messagesare routed via the shortest path.

Depending on the structure of the network different improve-ments were measured. For a fully connected network at lowloads a gain of more then 50% was found, in power sav-ing. For higher loads it converges to 50% (see Figure 2). Innetworks which have more of a line structure (Figure 3) alower improvement was measured. At low loads only 20%was measured and higher loads converge to 10%. This dif-ference in respect to a fully connected network is becausenow more transmission can be executed in parallel. Nodes



Figure 2: Power saved in a fully connected network,Suresh Singh et al [5]

in the far left can independently communicate form nodes atthe far right. The third setup is a random distributed net-work. Figure 4 shows that the more connected the networkbecomes, the more power is saved. Because when a node isconnected to more other nodes, it more frequently overhearstransmissions and will go to sleep.

6. S-MAC PROTOCOLThe S-MAC protocol (Short for Sensor MAC) is a protocolspecically designed for usage in Wireless sensor networks

and is designed by Wei Ye et al [9].

6.1 OperationThe S-MAC protocol focuses a lot on keeping ”on air time”of the transceiver to a minimum. It accomplishes this invarious ways:

• Scheduling.

• Collision avoidance.

• Overhearing avoidance.

• Message passing.

6.1.1 SchedulingThe S-MAC protocol uses periodic listening en sleeping tokeep the transceiver and processor sleeping as much as possi-ble, thus conserving power. Each node creates its own sched-ule and communicates this to other nodes so they know whenthe node is active. These schedules are resend from timeto time to account for clock drift with the network nodes.Before nodes start with their schedule a discovery phase isinitiated to discover the other nodes schedules. This is doneby the following phases.

Figure 3: Power saved in a line network, SureshSingh et al [5]

1. The node listens for a certain amount of time, when itdoes not hear a schedule from another node it choosesa random time to go to sleep and broadcasts its sched-ule. A node that has no own schedule but broadcastsits own schedule is known as a ”synchronizer” becauseit chooses its own schedule and other nodes will syn-chronize with it.

2. If the node receives a schedule before it chooses its ownit will follow this schedule by setting his own scheduleto the received schedule. Such a node is called a ”fol-

lower”.

As the nodes copy schedules from other nodes the wholenetwork have usually the same schedule or ”islands” thatmay have the same schedule.

The method of listening and sleeping periodically requiresperiodic updates with neighboring nodes because of clockinaccuracies. This is done by periodically sending out syn-chronization packets to rebroadcast the schedule.

6.1.2 Collision AvoidanceThe S-MAC protocol uses the RTS/CTS system to realizecollision avoidance. Each transmitted packet has a addi-tional eld that indicates how long the transmission willtake. This way nodes that overhear the packets know howlong the transmission is going to take, this is updated ina internal counter. This way the overhearing node knowswhen and how long the medium is going to be busy.

6.1.3 Overhearing avoidanceThis continual listening to data and RTS/CTS used in col-lision avoidance is not energy efficient. In wireless sensornetworks receiving data only consumes a bit less power thentransmitting. In S-MAC overhearing avoidance is accom-plished by letting nodes go to sleep when a RTS or CTS



Figure 4: Power saved in a random network with 10nodes, Suresh Singh et al [5]

packets is received. Each node maintains the counter thatindicates how long the medium is going to be busy.

6.1.4 Message passingS-MAC has efficient methods in passing large messages aroundin the network. Large messages that are transmitted as onelarge packet are inefficient as this also has a higher proba-bility of failure in noisy environments. If packets are frag-mented the chances of failure are smaller, but this increasesthe overhead as each packet needs a RTS CTS packet. S-MAC works around this by sending ”bursts” of data usingonly one RTS CTS pair. Each data packet received is ac-knowledged by the receiver and if not received is retrans-mitted immediately. The reason to acknowledge each datapacket is to prevent the hidden terminal problem (See [7]).It is possible that a new node comes online or into rangeand might disrupt the current transmission. The new nodeoverhears the transmission in progress, by the data packetsif its in range of the transmitter or the ACK packets if it isin range of the receiver. This way the node knows that atransmission is in progress and will remain silent. The trans-mitter and receiver pair may extend the transmission timedue to errors. Nodes that wake up expecting a clear mediumwill detect this condition of time extension by detecting theacknowledges or data fragments (these both contain dura-tion information).

6.2 DesignThe S-MAC protocol deals only with node to node commu-nications and does not have any routing provisions. Thenetwork is strictly ad-hoc and there is no mention of us-ing base stations or access points in the network. The re-quirements of the hardware for the S-MAC protocol is atransceiver and a reasonable accurate clock (not more thena few ms jitter/inaccuracy). The clock is needed for internaltimekeeping and to determine when to wake up from sleepafter a time period has expired.

A

Source 1

B

Source 2

C

E

D

Sink 2

Sink 1

Figure 5: S-MAC experiment setup, Wei Ye et al. [9]

6.3 ResultsThe paper of Wei Ye et al [9] also has in eld measurementresults of the S-MAC protocol.

6.3.1 PlatformThe hardware platform used in the experiments are the ReneMotes. These nodes consist of a AT90LS8535 microcon-troller from atmel with 8KiB ash and 0.5KiB RAM. Theradio transceiver is the TR1000 from RF Monolithics, Inc.

6.3.2 Experiment SetupThe software running on the platform is TinyOS with a fewadaptations to implement S-MAC. They implemented threeprotocol types on these nodes:

• Simplied IEEE 802.11 DCF

• Message passing with overhearing avoidance

• The complete S-MAC

The IEEE 802.11 DCF has support for: physical and virtualcarrier sense, back off and retry, RTS/CTS/DATA/ACKpacket exchange and fragmentation support. The experi-mental setup consists of simple network of ve nodes. Thissmall network is sufficient to show the basic characteristicsof the protocols and their performance, see Figure 5. Data istransmitted from sources to sinks and various performancemetrics are recorded like power consumption and latency.The arrival time of data is varied to show the advantage of

the sleep schedules.

6.3.3 Numbers and AnalysisFigure 6 shows the power consumption of the three pro-tocols under various loads of the network. S-MAC has adistinct advantage under higher load conditions and savespower with overhearing avoidance and efficient transmissionof long data packets. At lighter loads the S-MAC advantagecontinues to grow as idle listening rarely happens while inwith IEEE 802.11 the receiver is operated continually. Atlight loads the periodic sleep starts to pay off and shows thescalability of the protocol.



Figure 6: S-MAC power consumption compared to

other MAC protocols, image taken from the paperof Wei Ye et al [9]

7. ER-MACThe ER-MAC protocol is a extension on the S-MAC protocoland has been developed by Rajgopal Kannan et al [2]. Itsmain extensions are that it takes into account the powerusage distribution of the nodes in the network. In S-MAC, afew nodes that route many messages throughout the networkmay get depleted much faster as they consume power at amuch higher rate then other nodes. This may create holesor lead to premature network failure if a lot of critical nodesfail.

7.1 OperationThe ER-MAC protocol extends the following aspects of theS-MAC.

• Collision avoidance

• Power aware participation

First, each node computes its energy level criticality, this isthe amount of energy the node has left. Some nodes con-sume more energy in the network then others, so this en-ergy level is communicated to the other nodes. This waythe nodes can elect the node with the highest power level,and thus distribute power usage across the network. Theprotocol starts with a selection of a local leader for a groupof nodes. This leader election is done by the nodes thattransmit ”energy-level” messages. This leader election pro-cess can be restarted at any time.

7.1.1 Collision avoidanceIn this MAC protocol there are by design no collisions, allthe nodes are allocated a set of TDMA slots that do notinterfere with the nodes around them. The leaders usuallyget more slots as their energy level will allow this extra power

usage. Each node knows the TDMA schedule of all its localnodes and thus can transmit and receive to each node thatwants to send or receive data.

Another property of using TDMA slotting of the mediumis that no contention issues occur as all the nodes in thenetwork get their fair share of bandwidth. The leaders havemore energy to consume so are allocated more TDMA slots

to use this energy.

7.1.2 Power awarenessNodes record their own energy state by calculate their en-ergy level criticality and the node also knows the energylevels of the nodes around him. When the energy level fallsbelow a certain threshold (dependant on the energy levelsof its neighbors) and it is the current leader, it may optto restart the leader election process as its more depletedthen its neighbors. This way all the nodes get equal powerutilization.

7.2 DesignER-MAC does not deal with any routing, and purely handlescommunications between neighbors. Because the protocoldepends on time slotting a accurate clock is needed to keepsynchronization with neighbors. This clock must have a low jitter and drift to keep periodic synchronization to a mini-mum. The node itself needs to monitor its own power leveland must be aware how much energy it contains. This canbe done by knowing what capacity the battery has and us-ing a energy gauge (coulomb counter) to measure how muchpower is being consumed.

7.3 ResultsThe paper of Rajgopal Kannan et al [2] contains some sim-ulation results from the MAC protocol they developed.

7.3.1 Experimental setupThe experimental setup is done in simulation. In the simu-lation 100 moving nodes where distributed on a grid of 1000by 1000 meters.

7.3.2 Numbers and analysisThe experiments compares the ER-MAC protocol to ba-sic TDMA, unfortunately no comparison is made to 802.11or its predecessor S-MAC. Figure 7 shows the differenceof power consumption between ER-MAC and TDMA-MACprotocols over a period of time. While gure 8 shows howtime is spent sleeping and being awake when given a amountof time-slots. Since no comparisons are made in respect to

any other protocol no metrics can be given.

8. TRAMAThe TRaffic Adaptive Medium Acces Protocol (TRAMAProtocol) tries to minimize power consumption by synchro-nizing the transmitter as well as the receiver. It is createdby Rajendran et al [4].

8.1 OperationTo be as energy efficient as possible TRAMA let nodes sleepnot only when no node is sending, but also when a node isnot the intended receiver of the sender. Also, when a node



Figure 7: Difference in energy of minimum energynode under ERMAC versus basic TDMA. Figuretaken from the research of Rajgopal Kannan et al [2]

has no information to send, it will give up its slot, so othernodes may send. To be able to do this TRAMA relies on 3protocols.

• Neighbor Protocol (NP).

• Schedule Exchange Protocol (SEP).

• Adaptive Election Algorithm (AEA).

8.1.1 Neighbor ProtocolTo counter the hidden terminal effect each node in the net-work has to know its one- and two-hop neighbors. So itknows who the neighbors of his neighbors are. This protocolis prone to collision and to allow all nodes to be discovered asufficient long time should be taken. Only during this phasenew nodes can be added to the network. Depending of thetype of network this phase has to occur more or less often.Since every node is able to send during this phase a nodehas to be receiving when not sending. Therefore this phaseis rather power consuming.

During this phase a node picks a random slot to send in-formation about his one-hop neighborhood. The node willmention which nodes are no longer in its reach and whichnodes are new. If a node has no new information to send itwill still send an empty message, so that other nodes knowthat this node is still alive.

8.1.2 Schedule Exchange ProtocolAfter the NP phase the time slotted performance starts. Thenodes will start making a schedule to send their data col-lision free. Each node calculates a schedule interval, whichis based on the overall rate on which packets are producedby the higher level application. Then for that interval thenode will calculate for which slots it has the highest prior-ity among all its two-hop neighbors, these slots are called”winning slots”. For each winning slot the node will selectintended receivers. Depending on the message queue of anode a node will not necessary need all winning slots. These

Figure 8: Average number of slots a node is awakeand is asleep. Figure taken from the research of

Rajgopal Kannan et al [2]

Figure 9: Hidden three-hop terminal, Rajendran etal [4]

vacant slots can be used by other nodes, a vacant slot is rep-resented by selecting no receivers. The last winning slot isalways used to announce the new schedule, and all one-hopnodes are intended receivers. The vacant slots can be usedby other node as though the node that discarded them hadlower priority, during this slot.

8.1.3 Adaptive Election AlgorithmThis algorithm will change the state of a node. A node hasthree states in which it can be. These are either sending,when the node has highest priority among all nodes thatneed to send data. Or receiving, when it is the intendedreceiver of the sending node. Or it is sleeping when it is notin one of the two states.

As can be seen in Figure 9 node B’s absolute winner for aparticular time slot is node D. This means that node B can-not send. However it can still receive message from node A,which is not affected by node D’s higher priority, because it



is three hops away. Therefore each node also has to calculatewhich node can possibly send although it is not the highestpriority node in its two hop vicinity.

8.2 DesignTRAMA has little support for dynamic nodes. This is be-cause the nodes dene a (local) schedule among each otherwhich they continue to use for a long period. This period

should be long, because this is the main advantage over col-lision based protocols. When the NP is executed more oftenthe advantage of TRAMA is gone. Also, because nodes needto exchange neighbors, for scheduling, base station are outthe question. Nodes need to be able to communicate to eachother, bi-directional traffic.

When a certain node has more data to send, burst like, thisshould be accounted for in advance. The priority of thisnode should be high, so it will get more timeslots. Whennodes are distributed random over a eld it is often notknown which nodes will be prone to sending bursts of data.The application which drives TRAMA should increase thenode’s priority and schedule interval. The schedule interval

can be changed every time it expires. The priority of a nodeis a function of the nodes identity number (each node needsa number) and the number of the time slot, this ways eachnode is able to get highest priority. This function can becommunicated over the network, when it needs to change.

Since TRAMA heavily relies on timing the clocks of eachnode should not drift. Depending on how fast the systemshould operate small drifts are allowed. These small driftscan be contained by use of time stamping. Each node willcommunicate to each neighbors at least once each scheduleinterval, so clocks will not drift to much.

8.3 ResultsTRAMA is put to the test in respect to other protocols,S-MAC, CSMA and IEEE 802.11. This is done using simu-lations, as described by Rajendran et al [4].

8.3.1 Simulation setupThe simulations uses the simulation platform Qualnet. Theradio simulated is based on a common radio in sensor net-works, TR1000. The power consumptions for transmitting is24.75mW, for receiving is 13.5mW and for sleeping is 15 µ W.The nodes have on average 6 one-hop and 17 two-hop neigh-bors, the schedule interval is set to 100 for all nodes.

8.3.2 Simulation resultsTests were conducted with several netwerk structures (seeFigure 10, but always is one sink collection data from severalsources. In average TRAMA delivers more and more packetssuccessfully when the load goes up, in respect to the otherprotocols, which become congested. But it comes with aprice on the delay, buffers ll and messages are delayed afactor 1000. The energy saving for TRAMA are a lot betteron the other hand. As can be seen in Figure 11 in theaverage of the three scenarios TRAMA sleeps more thenS-MAC when the load goes up. Where as S-MAC staysconstant at 80% sleep time TRAMA averages on 85%. Thesleep duration of S-MAC is on average 50ms, TRAMA getson average 500ms, a factor 10 improvement.

Figure 10: 3 Different simulated networks, Rajen-dran et al [4]

In case that delay is not a problem and network lifetime andcorrect delivery have priority TRAMA is a good choice. Ascan be seen in Figure 11 depending on the structure of thenetwork nodes can sleep up to 90% of the time, which isbetter then S-MAC with 80%.

9. DISCUSSION AND COMPARISONThe MAC protocols treated here can be divided into threemain groups:

• Contention based protocols: These protocols have nonotion of time and it uses the medium when it is avail-able.

• Time based protocols: Assignment of the medium isdone by timing, hence, time based protocols.

• Hybrid: A combination of parts from timing and con-tention based protocols.

9.1 Contention Based protocolsContention based protocols attempt to use the medium whenit is available. There are various methods of declaring thatthe medium is busy, like the CTS/RTS scheme used in the802.11. Many protocols reuse this scheme to solve the hid-den terminal problem. Any node can use the medium atany time, this can introduce a few problems: one node canoccupy the medium for a long time while other nodes needto wait until the medium is released. There are methodsby circumventing this by either using placing bounds on themaximum transmission length and by introducing QoS rules.

The contention based protocols have some advantages anddisadvantages:

+ Ease of implementation in code and memory.

+ Full bandwidth is available to any node that needs it.

+ Handles dynamic networks well.

- Fairness is not ensured as one node can occupy the mediumfor a long time.

- Receiver needs to be active for the virtual carrier sensemechanism.



Figure 11: Percentage Energy Savings, Rajendran et al [4]

Type Congest E aware sleep802.11 Contention No No No

PAMAS Contention No No YesS-MAC Hybrid No No Yes

ER-MAC Time slot Yes Yes YesTRAMA Time slot No No Yes

Table 1: Feature comparison of MAC protocols

9.2 Timing Based protocolsTiming based protocols use time based scheduling to control

what node has access to the medium. The usual methodis using TDMA time slotting as a schedule to access themedium.

The Timing based protocols have some advantages and dis-advantages:

+ Collision prevention due to time slotting.

+ Transceivers only have to operate during designated timeslots, and can sleep the rest of the time.

- Complex implementation with schedule creation.

- Does not handle dynamic and mobile networks well.

9.3 Hybrid protocolsThere are some protocols that combine some aspects of tim-ing and contention based protocols. There are a lot of differ-ent combinations so no general advantages or disadvantagescan be given.

9.4 ComparisonTable 1 shows to what classes various MAC protocols belongto. We have compared these protocols in their operationand have especially focused on power consumption, but also

we have looked on how various power optimizations can bemade in these protocols.

IEEE 802.11 is not suitable for wireless sensor networks,it assumed that the receiver is in continual operation. Thisdrains the batteries of a node quickly and limit its usefulness.

The S-MAC protocol [9] and PAMAS [5] try to work aroundby reducing the amount of time spent in idle reception. S-MAC does this by creating a sleeping schedule that it syn-chronizes with its neighbors, so all the nodes sleep and awakeat the same time. During the active period S-MAC acts like

a contention based protocol. This method has the advan-tage that if the full bandwidth of the medium is required theschedule can be adapted to accommodate this. The reason-ing behind S-MAC is that bandwidth is traded for energyconsumption. PAMAS does this by turning a node off-linewhen it overhears a transmission not intended for the node.

The ER-MAC protocol [2] and TRAMA protocol [4] are thetiming based protocols we have treated. The ER-MAC pro-tocol has some adaptations that make the protocol awareto the energy level at each node. Whilst TRAMA dependsmore on a static priority assignments, which is made moreexible by analyzing the application above.

The comparisons made here only reect on power usage byone individual node, we do not take into account networktopology, density and error rates. The experiments con-ducted by each of the creators of the protocols differ greatlyso no good operational comparison can be made.

10. CONCLUSIONTiming based MAC protocols are more energy efficient asthey avoid idle listening that is prevalent in contention basedprotocols. There are various optimizations possible for boththe contention based MAC protocols and the timing basedprotocols



Methods of improving the bandwidth allocation of timingbased MAC protocols can be improved by using Quality of Service metrics for the data, combined with adaptive timingsof the schedule according to bandwidth requirements.

For mobile and dynamic networks contention based proto-cols are more suitable as there is no setup and schedule cre-ation phase that new nodes are not aware of. There are

ways to x this problem in timing based protocols as peri-odic rescheduling with the network.

For static and low bandwidth applications timing based pro-tocols are recommended while for highly dynamic and orhigh bandwidth applications contention based protocols aremore suitable.

When evaluating MAC protocols for your own applicationpay close attention to how large the factor of improvementis. For example: the baseline ooding protocol with nooptimization has a endurance of 1 day, and one improvedprotocol has a 100% improvement. This only improves theendurance by one day. When evaluating protocols improve-ments of a factor 10 or 100 are more desirable.

11. ACKNOWLEDGMENTSWe would like to thank professor Johan J. Lukkien for givingus insights into the world of wireless sensor networks.

12. REFERENCES[1] R. G. J. C. S. M. Jeongyeup Paek,

Krishna Chintalapudi. A wireless sensor network forstructural health monitoring: Performance andexperience. 2004.

[2] R. Kannan, R. Kalidindi, S. S. Iyengar, and V. Kumar.Energy and rate based mac protocol for wireless sensornetworks. SIGMOD Rec. , 32(4):60–65, 2003.

[3] H. W. Milosch Meriac and B. Meriac. openbeacon.org afree active 2.4ghz beacon design, 2009.

[4] V. Rajendran, K. Obraczka, and J. J.Garcia-Luna-Aceves. Energy-efficient collision-freemedium access control for wireless sensor networks. InSenSys ’03: Proceedings of the 1st international conference on Embedded networked sensor systems ,pages 181–192, New York, NY, USA, 2003. ACM.

[5] S. Singh and C. S. Raghavendra. Pamas: Power awaremulti-access protocol with signalling for ad hocnetworks. ACM Computer Communication Review ,28:5–26, 1998.

[6] Wikipedia. Carrier sense multiple access with collisionavoidance — wikipedia, the free encyclopedia, 2008.[Online; accessed 16-October-2008].

[7] Wikipedia. Hidden node problem — wikipedia, the freeencyclopedia, 2008. [Online; accessed 9-October-2008].

[8] Wikipedia. Network allocation vector — wikipedia, thefree encyclopedia, 2008. [Online; accessed16-October-2008].

[9] W. Ye, J. Heidemann, and D. Estrin. Anenergy-efficient mac protocol for wireless sensornetworks. In Proceedings of the IEEE Infocom , pages1567–1576, New York, NY, USA, June 2002.USC/Information Sciences Institute, IEEE.

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