Medium Access Control in Ad hoc and Sensor Networks

Medium Access Control in

Ad hoc and Sensor Networks

MAC allows multiple users to share a common channel. MAC allows multiple users to share a common channel.

Conflict-free protocolsConflict-free protocols ensure successful transmission. Channel can be allocated to ensure successful transmission. Channel can be allocated to users statically or dynamically.users statically or dynamically.

Only static conflict-free protocols are used in cellular mobile communicationsOnly static conflict-free protocols are used in cellular mobile communications

- - Frequency Division Multiple Access (FDMA): provides a fraction of the frequency (FDMA): provides a fraction of the frequency range to each user for all the timerange to each user for all the time

- - Time Division Multiple Access (TDMA)(TDMA) : : The entire frequency band is allocated to a The entire frequency band is allocated to a single user for a fraction of timesingle user for a fraction of time

- - Code Division Multiple Access (CDMA) : provides every user a portion of bandwidth (CDMA) : provides every user a portion of bandwidth for a fraction of timefor a fraction of time

Contention based protocolsContention based protocols must prescribe ways to resolve conflicts must prescribe ways to resolve conflicts- Static Conflict Resolution: Carrier Sense Multiple Access (CSMA) - Static Conflict Resolution: Carrier Sense Multiple Access (CSMA)

- Dynamic Conflict Resolution: the Ethernet, which keeps track of various system - Dynamic Conflict Resolution: the Ethernet, which keeps track of various system parameters, ordering the users accordinglyparameters, ordering the users accordingly

Multiple Access Control (MAC) Protocols

Channels are assigned to the user for the duration of a call. No other user can Channels are assigned to the user for the duration of a call. No other user can access the channel during that time. When call terminates, the same channel access the channel during that time. When call terminates, the same channel can be re-assigned to another usercan be re-assigned to another user

FDMA is used in nearly all first generation mobile communication systems, like FDMA is used in nearly all first generation mobile communication systems, like AMPS (30 KHz channels)AMPS (30 KHz channels)

Number of channels required to support a user population depends on the Number of channels required to support a user population depends on the average number of calls generated, average duration of a call and the required average number of calls generated, average duration of a call and the required quality of service (e.g. percentage of blocked calls)quality of service (e.g. percentage of blocked calls)

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Frequency Division Multiple Access (FDMA)

The whole channel is assigned to each user, but the users are multiplexed in The whole channel is assigned to each user, but the users are multiplexed in time during communication. Each communicating user is assigned a particular time during communication. Each communicating user is assigned a particular time slot, during which it communicates using the entire frequency spectrumtime slot, during which it communicates using the entire frequency spectrum

The data rate of the channel is the sum of the data rates of all the multiplexed The data rate of the channel is the sum of the data rates of all the multiplexed transmissionstransmissions

There is always channel interference between transmission in two adjacent There is always channel interference between transmission in two adjacent slots because transmissions tend to overlap in time. This interference limits slots because transmissions tend to overlap in time. This interference limits the number of users that can share the channelthe number of users that can share the channel

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Time Division Multiple Access (TDMA)

CDMA, a type of a CDMA, a type of a spread-spectrumspread-spectrum technique, allows multiple users to share the technique, allows multiple users to share the same channel by multiplexing their transmissions in code space. Different signals same channel by multiplexing their transmissions in code space. Different signals from different users are encoded by different codes (keys) and coexist both in time from different users are encoded by different codes (keys) and coexist both in time and frequency domainsand frequency domains

A code is represented by a wideband pseudo noise (PN) signalA code is represented by a wideband pseudo noise (PN) signal

When decoding a transmitted signal at the receiver, because of low cross-When decoding a transmitted signal at the receiver, because of low cross-correlation of different codes, other transmissions appear as noise. This property correlation of different codes, other transmissions appear as noise. This property enables the multiplexing of a number of transmissions on the same channel with enables the multiplexing of a number of transmissions on the same channel with minimal interferenceminimal interference

The maximum allowable interference (from other transmissions) limits the number The maximum allowable interference (from other transmissions) limits the number of simultaneous transmissions on the same channelof simultaneous transmissions on the same channel

All channels share bandwidthAll channels share bandwidth

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Code Division Multiple Access (CDMA)

Spreading of the signal bandwidth can be performed usingSpreading of the signal bandwidth can be performed using

Direct SequenceDirect Sequence (DS): (DS):

The narrow band signal representing digital data is multiplied by a wideband The narrow band signal representing digital data is multiplied by a wideband pseudo noise (PN) signal representing the code. Multiplication in the time domain pseudo noise (PN) signal representing the code. Multiplication in the time domain translates to convolution in the spectral domain. Thus the resulting signal is translates to convolution in the spectral domain. Thus the resulting signal is widebandwideband

Frequency HoppingFrequency Hopping (FH): (FH):

Carrier frequency rapidly hops among a large set of possible frequencies Carrier frequency rapidly hops among a large set of possible frequencies according to some pseudo random sequence (the code). The set of frequencies according to some pseudo random sequence (the code). The set of frequencies spans a large bandwidth. Thus the bandwidth of the transmitted signal appears as spans a large bandwidth. Thus the bandwidth of the transmitted signal appears as largely spreadlargely spread

Code Division Multiple Access (CDMA)

– S- MAC S- MAC protocol designed specifically for sensor networks to reduce protocol designed specifically for sensor networks to reduce

energy consumption while achieving good scalability and collision energy consumption while achieving good scalability and collision

avoidance by utilizing a combined scheduling and contention schemeavoidance by utilizing a combined scheduling and contention scheme

– The major sources of energy waste are:The major sources of energy waste are:

1.1. collisioncollision

2.2. overhearingoverhearing

3.3. control packet overheadcontrol packet overhead

4.4. idle listeningidle listening

– S-MAC reduce the waste of energy from all the sources mentioned in S-MAC reduce the waste of energy from all the sources mentioned in

exchange of some reduction in both per-hop fairness and latencyexchange of some reduction in both per-hop fairness and latency

An Energy-Efficient MAC Protocol for Wireless Sensor Networks (S-MAC) [Ye+ 2002]

– S- MAC S- MAC protocol consist of three major components:protocol consist of three major components:

1.1. periodic listen and sleepperiodic listen and sleep

2.2. collision and overhearing avoidancecollision and overhearing avoidance

3.3. Message passingMessage passing

– Contributions of S-MAC are:Contributions of S-MAC are:

The scheme of periodic listen and sleep helps in reducing energy The scheme of periodic listen and sleep helps in reducing energy

consumption by avoiding idle listening. The use of synchronization to form consumption by avoiding idle listening. The use of synchronization to form

virtual clusters of nodes on the same sleep schedulevirtual clusters of nodes on the same sleep schedule

In-channel signaling puts each node to sleep when its neighbor is In-channel signaling puts each node to sleep when its neighbor is

transmitting to another node (solves the overhearing problem and does not transmitting to another node (solves the overhearing problem and does not

require additional channel)require additional channel)

Message passing technique to reduce application-perceived latency and Message passing technique to reduce application-perceived latency and

control overhead (per-node fragment level fairness is reduced)control overhead (per-node fragment level fairness is reduced)

Evaluating an implementation of S-MAC over sensor-net specific hardwareEvaluating an implementation of S-MAC over sensor-net specific hardware

(S-MAC)

– A power control MACA power control MAC protocol allows nodes to vary transmit power level on a protocol allows nodes to vary transmit power level on a

per-packet basisper-packet basis

– Earlier work has used different power levels for RTS-CTS and DATA-ACK, Earlier work has used different power levels for RTS-CTS and DATA-ACK,

specifically, maximum transmit power is used for RTS-CTS and minimum specifically, maximum transmit power is used for RTS-CTS and minimum

required transmit power is used for DATA-ACK transmissionsrequired transmit power is used for DATA-ACK transmissions

– These protocols may increase collisions, degrade network throughput and result These protocols may increase collisions, degrade network throughput and result

in higher energy consumption than when using IEEE 802.11 without power in higher energy consumption than when using IEEE 802.11 without power

controlcontrol

– Power saving mechanismsPower saving mechanisms allow nodes to enter a allow nodes to enter a doze statedoze state by powering off by powering off

its wireless network interface whenever possibleits wireless network interface whenever possible

– Power control schemesPower control schemes vary transmit power to reduce energy consumption vary transmit power to reduce energy consumption

A Power Control MAC (PCM) Protocol for Ad hoc Networks [Jung+ 2002]

IEEE 802.11 MAC ProtocolIEEE 802.11 MAC Protocol

– Specifies two MAC protocols:Specifies two MAC protocols:

Point Coordination Function (PCF) Point Coordination Function (PCF) centralized centralized

Distributed Coordination Function (DCF) Distributed Coordination Function (DCF) distributeddistributed

Transmission range:Transmission range:

When a node is in transmission range of a sender node, it can receive andWhen a node is in transmission range of a sender node, it can receive andcorrectly decode packets from sender node.correctly decode packets from sender node.

Carrier Sensing Range:Carrier Sensing Range:

Nodes in carrier sensing range can sense the sender’s transmission. It is generally Nodes in carrier sensing range can sense the sender’s transmission. It is generally

larger than transmission range. Both carrier sensing range and transmission rangelarger than transmission range. Both carrier sensing range and transmission range

Depends on the transmit power level.Depends on the transmit power level.

Power Control MAC (PCM)


Carrier Sensing Zone:Carrier Sensing Zone:

Nodes can sense the signal, but cannot decode it correctly. The carrier sensing zone Nodes can sense the signal, but cannot decode it correctly. The carrier sensing zone

does not include transmission rangedoes not include transmission range

[Figure adapted from Jung+ 2002]



– DCF in IEEE 802.11 is based on CSMA/CS (Carrier Sense Multiple Access with DCF in IEEE 802.11 is based on CSMA/CS (Carrier Sense Multiple Access with

Collision Avoidance)Collision Avoidance)

– Each node in IEEE 802.11 maintains a NAV (Network Allocation Vector) that Each node in IEEE 802.11 maintains a NAV (Network Allocation Vector) that

indicates the remaining time of the on-going transmission sessionsindicates the remaining time of the on-going transmission sessions

– Carrier sensing is performed using physical carrier sensing (by air interface) and Carrier sensing is performed using physical carrier sensing (by air interface) and

virtual carrier sensing (uses the duration of the packet transmission that is virtual carrier sensing (uses the duration of the packet transmission that is

included in the header of RTS, CTS and DATA frames)included in the header of RTS, CTS and DATA frames)

– Using the duration information in RTS, CTS and DATA packets, nodes update Using the duration information in RTS, CTS and DATA packets, nodes update

their NAVs whenever they receive a packettheir NAVs whenever they receive a packet

– The channel is considered busy if either physical or virtual carrier sensing The channel is considered busy if either physical or virtual carrier sensing

indicates that channel is busyindicates that channel is busy

– Figure 2 shows how nodes in transmission range and the carrier sensing zone Figure 2 shows how nodes in transmission range and the carrier sensing zone

adjust their NAVs during RTS-CTS-DATA-ACK transmission adjust their NAVs during RTS-CTS-DATA-ACK transmission






– IFS is the time interval between frames and IEEE 802.11 defines four IFSs which IFS is the time interval between frames and IEEE 802.11 defines four IFSs which

provide priority levels for accessing the channel provide priority levels for accessing the channel

SIFS (short interframe space)SIFS (short interframe space)

PIFS (Point Coordination Function interframe space)PIFS (Point Coordination Function interframe space)

DIFS (Distributed Coordination Function interframe space)DIFS (Distributed Coordination Function interframe space)

EIFS (extended interframe space)EIFS (extended interframe space)

– SIFS is the shortest and is used after RTS, CTS, and DATA frames to give the SIFS is the shortest and is used after RTS, CTS, and DATA frames to give the

highest priority to CTS, DATA and ACK respectivelyhighest priority to CTS, DATA and ACK respectively

– In DCF, when the channel is idle, a node waits for DIFS duration before transmittingIn DCF, when the channel is idle, a node waits for DIFS duration before transmitting

– Nodes in the transmission range correctly set their NAVs when receiving RTS/CTSNodes in the transmission range correctly set their NAVs when receiving RTS/CTS

– Since nodes in carrier sensing zone cannot decode the packet, they do not know Since nodes in carrier sensing zone cannot decode the packet, they do not know

the duration of the packet transmission. So, they set their NAVs for the EIFS the duration of the packet transmission. So, they set their NAVs for the EIFS

duration to avoid collision with the ACK reception at the source nodeduration to avoid collision with the ACK reception at the source node



– The intuition behind EIFS is to provide enough time for a source node to receive the The intuition behind EIFS is to provide enough time for a source node to receive the

ACK frame, meaning that duration of EIFS is longer than that of ACK transmissionACK frame, meaning that duration of EIFS is longer than that of ACK transmission

– In PCM, nodes in the carrier sensing zone use EIFS whenever they can sense the In PCM, nodes in the carrier sensing zone use EIFS whenever they can sense the

signal but cannot decode itsignal but cannot decode it

– IEEE 802.11 does not completely prevent collisions due to the IEEE 802.11 does not completely prevent collisions due to the hidden terminalhidden terminal

problem (nodes in the receiver’s carrier sensing zone, but not in the sender’s carrier problem (nodes in the receiver’s carrier sensing zone, but not in the sender’s carrier

sensing zone or transmission range, can cause a collision with the reception of a sensing zone or transmission range, can cause a collision with the reception of a

DATA packet at the receiverDATA packet at the receiver

– In Figure 3, suppose node C transmits packet to node DIn Figure 3, suppose node C transmits packet to node D

– When C and D transmit an RTS and CTS respectively, A and F set their NAVs for When C and D transmit an RTS and CTS respectively, A and F set their NAVs for

EIFS durationEIFS duration

– During C’s data transmission, A defers its transmission due to sensing C’s During C’s data transmission, A defers its transmission due to sensing C’s

transmission. However, since node F does not sense any signal during C’s transmission. However, since node F does not sense any signal during C’s

transmission, it considers channel to be idle (F is in D’s carrier sensing zone, but not transmission, it considers channel to be idle (F is in D’s carrier sensing zone, but not

in D’s)in D’s)




D’s carrier sensing rangeC’s carrier sensing range



– When F starts a new transmission, it can cause a collision with the reception of When F starts a new transmission, it can cause a collision with the reception of

DATA at DDATA at D

– Since F is outside of D’s transmission range, D may be outside of F’s transmission Since F is outside of D’s transmission range, D may be outside of F’s transmission

range; however, since F is in D’s carrier sensing zone, F can provide interference at range; however, since F is in D’s carrier sensing zone, F can provide interference at

node D to cause collision with DATA being received at Dnode D to cause collision with DATA being received at D


BASIC Power Control ProtocolBASIC Power Control Protocol

– Power control can reduce energy consumptionPower control can reduce energy consumption

– Power control may bring different transmit power levels at different hosts, creating Power control may bring different transmit power levels at different hosts, creating

an asymmetric scenarios where a node A can reach node B, but node B cannot an asymmetric scenarios where a node A can reach node B, but node B cannot

reach node A and collisions may also increase a resultreach node A and collisions may also increase a result

– In Figure 4, suppose nodes A and B use lower power level than nodes C and DIn Figure 4, suppose nodes A and B use lower power level than nodes C and D

– When A is transmitting to B, C and D may not sense the transmissionWhen A is transmitting to B, C and D may not sense the transmission

– When C and D transmit to each other using higher power, their transmission may When C and D transmit to each other using higher power, their transmission may

collide with the on-going transmission from A to Bcollide with the on-going transmission from A to B




– As a solution to this problem, RTS-CTS are transmitted at the highest possible As a solution to this problem, RTS-CTS are transmitted at the highest possible

power level but DATA and ACK at the minimum power level necessary to power level but DATA and ACK at the minimum power level necessary to

communicatecommunicate

– In Figure 5, nodes A and B send RTS and CTS respectively with highest power In Figure 5, nodes A and B send RTS and CTS respectively with highest power

level such that node C receives the CTS and defers its transmissionlevel such that node C receives the CTS and defers its transmission

– By using a lower power level for DATA and ACK packets, nodes can save energyBy using a lower power level for DATA and ACK packets, nodes can save energy




– In the BASIC scheme, RTS-CTS handshake is used to decide the transmission In the BASIC scheme, RTS-CTS handshake is used to decide the transmission

power for subsequent DATA and ACK packets which can be achieved in two power for subsequent DATA and ACK packets which can be achieved in two

different waysdifferent ways

Suppose node A wants to send a packet to node B. Node A transmit RTS at Suppose node A wants to send a packet to node B. Node A transmit RTS at

power level power level ppmaxmax (maximum possible). When B receives the RTS from A with (maximum possible). When B receives the RTS from A with

signal level signal level ppr,r, B calculates the minimum necessary transmission power level, B calculates the minimum necessary transmission power level,

ppdesireddesired. For the DATA packet based on received power level, . For the DATA packet based on received power level, pprr, transmitted , transmitted

power level, power level, ppmaxmax, and noise level at the receiver B. Node B specifies , and noise level at the receiver B. Node B specifies ppdesired desired in in

its CTS to node A. After receiving CTS, node A sends DATA using power level its CTS to node A. After receiving CTS, node A sends DATA using power level

ppdesired.desired.

When a destination node receives an RTS, it responds by sending a CTS (at When a destination node receives an RTS, it responds by sending a CTS (at

power level power level ppmaxmax). When source node receives CTS, it calculates ). When source node receives CTS, it calculates ppdesireddesired based based

on received power level, on received power level, pprr, and transmitted power level (, and transmitted power level (ppmaxmax) as) as

PPdesired desired = = (p(pmaxmax / / pprr) x Rx) x Rxthreshthresh x c x c

where where RxRxthreshthresh is minimum necessary received signal strength and c is constant is minimum necessary received signal strength and c is constant



– The second alternative makes two assumptions:The second alternative makes two assumptions:

Signal attenuation between source and destination nodes is assumed to be the Signal attenuation between source and destination nodes is assumed to be the

same in both directionssame in both directions

Noise level at the receiver is assumed to be below some predefined threshold Noise level at the receiver is assumed to be below some predefined threshold

Deficiency of the BASIC ProtocolDeficiency of the BASIC Protocol

– In Figure 6, suppose node D wants to transmit to node EIn Figure 6, suppose node D wants to transmit to node E

– When nodes D and E transmits RTS and CTS respectively, B and C receives RTS When nodes D and E transmits RTS and CTS respectively, B and C receives RTS

and F and G receives CTS, therefore, these nodes defer their transmissionsand F and G receives CTS, therefore, these nodes defer their transmissions

– Since node A is in carrier sensing zone of node D, it sets its NAV for EIFS durationSince node A is in carrier sensing zone of node D, it sets its NAV for EIFS duration

– Similarly node H sets its NAV for EIFS duration when it senses transmission from ESimilarly node H sets its NAV for EIFS duration when it senses transmission from E

– When source and destination decide to reduce the transmit power for DATA-ACK, When source and destination decide to reduce the transmit power for DATA-ACK,

not only transmission range for DATA-ACK but also carrier sensing zone is also not only transmission range for DATA-ACK but also carrier sensing zone is also

smaller than RTS-CTS smaller than RTS-CTS


Deficiency of the BASIC ProtocolDeficiency of the BASIC Protocol

– Thus, only C and F correctly Thus, only C and F correctly

receives DATA and ACK packetsreceives DATA and ACK packets

– Since nodes A and H cannot Since nodes A and H cannot

sense the transmissions, they sense the transmissions, they

consider channel is idle and start consider channel is idle and start

transmitting at high power level transmitting at high power level

which will cause collision with the which will cause collision with the

ACK packet at D and DATA packet ACK packet at D and DATA packet

at Eat E

– This results in throughput This results in throughput

degradation and higher energy degradation and higher energy

consumption (due to consumption (due to

retransmissions)retransmissions)



Proposed Power Control MAC ProtocolProposed Power Control MAC Protocol

– Proposed Power Control MAC (PCM) is similar to BASIC scheme such that it uses Proposed Power Control MAC (PCM) is similar to BASIC scheme such that it uses

power level, power level, ppmaxmax, for RTS-CTS and the minimum necessary transmit power for , for RTS-CTS and the minimum necessary transmit power for

DATA-ACK transmissionsDATA-ACK transmissions

– Procedure of PCM is as follows:Procedure of PCM is as follows:

1.1. Source and destination nodes transmit the RTS and CTS using Source and destination nodes transmit the RTS and CTS using ppmax.max. Nodes in Nodes in

the carrier sensing zone set their NAVs for EIFS durationthe carrier sensing zone set their NAVs for EIFS duration

2.2. The source may transmit DATA using a lower power level The source may transmit DATA using a lower power level

3.3. Source transmits DATA at level of Source transmits DATA at level of ppmaxmax, periodically, for enough time so that , periodically, for enough time so that

nodes in the carrier sensing zone can sense it and this would avoid collision nodes in the carrier sensing zone can sense it and this would avoid collision

with the ACK packetswith the ACK packets

4.4. The destination node transmits an ACK using the minimum required power to The destination node transmits an ACK using the minimum required power to

reach the source nodereach the source node

– Figure 7 presents how the transmit power level changes during the sequence of Figure 7 presents how the transmit power level changes during the sequence of

RTS-CTS-DATA-ACK transmission RTS-CTS-DATA-ACK transmission


Proposed Power Control MAC ProtocolProposed Power Control MAC Protocol

– The difference between PCM and BASIC scheme is that PCM periodically increases The difference between PCM and BASIC scheme is that PCM periodically increases

the transmit power to the transmit power to ppmaxmax during the DATA packet transmission. Nodes that can during the DATA packet transmission. Nodes that can

interfere with the reception of ACK at the sender will periodically sense the channel interfere with the reception of ACK at the sender will periodically sense the channel

is busy and defer their own transmission. Since nodes reside in the carrier sensing is busy and defer their own transmission. Since nodes reside in the carrier sensing

zone defer for EIFS duration, the transmit power for DATA is increased once every zone defer for EIFS duration, the transmit power for DATA is increased once every

EIFS durationEIFS duration

– PCM solves the problem posed with BASIC scheme and can achieve throughput PCM solves the problem posed with BASIC scheme and can achieve throughput

comparable to 802.11 by using less energycomparable to 802.11 by using less energy

– PCM, like 802.11, does not prevent collisions completelyPCM, like 802.11, does not prevent collisions completely



– Why STUDY MAC protocols in sensor networks?Why STUDY MAC protocols in sensor networks?

Application behavior in sensor networks leads to very different traffic Application behavior in sensor networks leads to very different traffic

characteristics from that found in conventional computer networkscharacteristics from that found in conventional computer networks

Highly constrained resources and functionalityHighly constrained resources and functionality

Small packet sizeSmall packet size

Deep multi-hop dynamic topologiesDeep multi-hop dynamic topologies

The network tends to operate as a collective structure, rather than The network tends to operate as a collective structure, rather than

supporting many independent point-to-point flowssupporting many independent point-to-point flows

Traffic tends to be variable and highly correlatedTraffic tends to be variable and highly correlated

Little or no activity/traffic for longer periods and intense traffic over Little or no activity/traffic for longer periods and intense traffic over

shorter periods shorter periods

A Transmission Control Scheme for Media Access in Sensor Networks [Woo+, 2003]

– Major factors to be considered in the design of MAC:Major factors to be considered in the design of MAC:

Communication efficiency in terms of energy consumed per each packetCommunication efficiency in terms of energy consumed per each packet

o Communication by radio channel consumes the highest energy Communication by radio channel consumes the highest energy

o Transmit , receive and idle consume roughly the same amount of energyTransmit , receive and idle consume roughly the same amount of energy

Fairness of the bandwidth allocated to each node for end to end data Fairness of the bandwidth allocated to each node for end to end data

delivery to sinkdelivery to sink

o Each node acts as a router as well as data originator resulting in two Each node acts as a router as well as data originator resulting in two

kinds of traffickinds of traffic

o The traffics compete for the same upstream bandwidthThe traffics compete for the same upstream bandwidth

o Hidden nodesHidden nodes

Contention at the upstream node may not be detected and results Contention at the upstream node may not be detected and results

in significant loss ratein significant loss rate Efficient channel utilizationEfficient channel utilization


– Major factors to be considered in the design of MAC:Major factors to be considered in the design of MAC:

The routing distance and degree of intermediate competition varies The routing distance and degree of intermediate competition varies

widely across the networkwidely across the network

The cost of dropping a packet varies with place and the packet The cost of dropping a packet varies with place and the packet

– Contribution of this paper are as follows:Contribution of this paper are as follows:

Listening mechanism:Listening mechanism:

o Listening is effective when there are no hidden nodesListening is effective when there are no hidden nodes

o It comes at an expense of energy cost as the radio must be on to listenIt comes at an expense of energy cost as the radio must be on to listen

o Many protocols such as IEEE 802.11 require sensing the channel even Many protocols such as IEEE 802.11 require sensing the channel even

during backoffduring backoff

o Shorten the length of carrier sensing and power off the node during Shorten the length of carrier sensing and power off the node during

backoffbackoff

o Highly synchronized nature of the traffic causes no packet transfer at all Highly synchronized nature of the traffic causes no packet transfer at all

in the absence of collision detection hardwarein the absence of collision detection hardware

o Introduce random delay for transmission to unsynchronized the nodesIntroduce random delay for transmission to unsynchronized the nodes


Backoff Mechanism:Backoff Mechanism:

o Used to reduce the contention among the nodesUsed to reduce the contention among the nodes

o In the sensor networks, traffic is a superposition of different periodic In the sensor networks, traffic is a superposition of different periodic

streamsstreams

o Apply back off as a phase shift to the periodicity of the application so Apply back off as a phase shift to the periodicity of the application so

that the synchronization among periodic streams of traffic can be brokenthat the synchronization among periodic streams of traffic can be broken

Contention-based MechanismContention-based Mechanism

o Explicit control packets like RTS and CTS are used to avoid contentionExplicit control packets like RTS and CTS are used to avoid contention

o ACKS indicate lack of collisionACKS indicate lack of collision

o Use of lot of control packets reduces bandwidth efficiencyUse of lot of control packets reduces bandwidth efficiency

o ACKS can be eliminated by hearing the packet transmission from its ACKS can be eliminated by hearing the packet transmission from its

parent to its upstream which serves as an ACK for the downstream nodeparent to its upstream which serves as an ACK for the downstream node


Rate Control MechanismRate Control Mechanism

o The competition between originating traffic and route-thru traffic has a The competition between originating traffic and route-thru traffic has a

direct impact in achieving the fairness goal.direct impact in achieving the fairness goal.

o MAC should control the rate of originating data of a node in order to MAC should control the rate of originating data of a node in order to

allow route-thru traffic to access the channel and reach the base allow route-thru traffic to access the channel and reach the base

station and some kind of progressive signaling for route-thru traffic station and some kind of progressive signaling for route-thru traffic

such the rate is controlled at the origin. such the rate is controlled at the origin.

o A passive implicit mechanism is used to control the rate of A passive implicit mechanism is used to control the rate of

transmission of both trafficstransmission of both traffics


Multi-hop Hidden Node problem:Multi-hop Hidden Node problem:

o It avoid the hidden node problem by constantly tuning the It avoid the hidden node problem by constantly tuning the

transmission rate and performing phase changes so that the transmission rate and performing phase changes so that the

aggregate traffic will not repeatedly collide with each other.aggregate traffic will not repeatedly collide with each other.

o A child can reduce a potential hidden node problem with its grand A child can reduce a potential hidden node problem with its grand

parent by not sending packets for t+ x+ packet time at the end of parent by not sending packets for t+ x+ packet time at the end of

packet transmission t by its parent packet transmission t by its parent


Advantages:Advantages:

– The amount of computation for this scheme is small and within The amount of computation for this scheme is small and within

networked sensor’s computation capabilitynetworked sensor’s computation capability

– The scheme is totally computational which is much cheaper in energy The scheme is totally computational which is much cheaper in energy

cost than on the radio cost than on the radio

– The control packet overhead is reduced The control packet overhead is reduced


Disadvantages:Disadvantages:

– The MAC protocol developed here takes into consideration the The MAC protocol developed here takes into consideration the

periodicity of the originating traffic which doesn’t help for non periodic periodicity of the originating traffic which doesn’t help for non periodic

traffictraffic


Suggestions/Improvements/Future Work:Suggestions/Improvements/Future Work:


– T-MAC T-MAC is a contention based Medium Access Control Protocolis a contention based Medium Access Control Protocol

– Energy consumption is reduced by introducing an active/sleep duty cycleEnergy consumption is reduced by introducing an active/sleep duty cycle

– Handles the load variations in time and location by introducing an Handles the load variations in time and location by introducing an

adaptive duty cycleadaptive duty cycle

It reduces the amount of energy wasted on idle listening by dynamically It reduces the amount of energy wasted on idle listening by dynamically

ending the active part of itending the active part of it

– In T-MAC, nodes communicate using RTS, CTS, Data and ACK pkts In T-MAC, nodes communicate using RTS, CTS, Data and ACK pkts

which provides collision avoidance and reliable transmissionwhich provides collision avoidance and reliable transmission

– When a node senses the medium idle for TA amount of time it When a node senses the medium idle for TA amount of time it

immediately switches to sleepimmediately switches to sleep

– TA determines the minimal amount of idle listening time per frameTA determines the minimal amount of idle listening time per frame

– The incoming messages between two active states are bufferedThe incoming messages between two active states are buffered

An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Networks [Van dam+, 2003]

– The buffer capacity determines an upper bound on the maximum frame The buffer capacity determines an upper bound on the maximum frame

timetime

– Frame synchronization in T-MAC follows the scheme of virtual clustering Frame synchronization in T-MAC follows the scheme of virtual clustering

as in S-MACas in S-MAC

– The RTS transmission in T-MAC starts by waiting and listening for a The RTS transmission in T-MAC starts by waiting and listening for a

random time within a fixed contention interval at the beginning of the each random time within a fixed contention interval at the beginning of the each

active state active state

– The TA time is obtained using TA > C + R + TThe TA time is obtained using TA > C + R + T

– T-MAC suffers from early sleeping problemT-MAC suffers from early sleeping problem

– Its overcome by sending Future request to send or taking priority on full Its overcome by sending Future request to send or taking priority on full

buffers buffers


Advantages:Advantages:

– The T-MAC protocol is designed particularly for wireless sensor The T-MAC protocol is designed particularly for wireless sensor

networks and hence energy consumption constraints are taken into networks and hence energy consumption constraints are taken into

accountaccount

– The T-MAC protocol tries to reduce idle listening by transmitting all The T-MAC protocol tries to reduce idle listening by transmitting all

messages in bursts of variable lengths and sleeping between burst messages in bursts of variable lengths and sleeping between burst

– T-MAC facilitates collision avoidance and overhearing -- nodes transmit T-MAC facilitates collision avoidance and overhearing -- nodes transmit

their data in a single burst and thus do not require additional RTS/CTS their data in a single burst and thus do not require additional RTS/CTS

control packets.control packets.

– By stressing on RTS retries, T-MAC gives the receiving nodes enough By stressing on RTS retries, T-MAC gives the receiving nodes enough

chance to listen and reply before it actually goes to sleep -- this chance to listen and reply before it actually goes to sleep -- this

increases the throughput in the long runincreases the throughput in the long run


Disadvantages:Disadvantages:

– The authors do not outline how a sender node would sense a FRTS packet The authors do not outline how a sender node would sense a FRTS packet

and enable it to send a DS packet and enable it to send a DS packet

– Also sending a DS packet increases the overhead.Also sending a DS packet increases the overhead.

– The network topology in the simulation considers that the locations of the The network topology in the simulation considers that the locations of the

nodes are knownnodes are known

– T-MAC has been observed to have a high message loss phenomenonT-MAC has been observed to have a high message loss phenomenon

– T-MAC suffers from early sleeping problem for event based local unicastT-MAC suffers from early sleeping problem for event based local unicast


Suggestions/Improvements/Future Work:Suggestions/Improvements/Future Work:

– If a buffer is full there would be a lot of dropped packets decreasing the If a buffer is full there would be a lot of dropped packets decreasing the

throughput. A method to overcome this drawback is that we could have throughput. A method to overcome this drawback is that we could have

the node with its buffer 75% full broadcast a special packet Buffer Full the node with its buffer 75% full broadcast a special packet Buffer Full

PacketPacket

– MAC Virtual Clustering technique needs to be further investigatedMAC Virtual Clustering technique needs to be further investigated

– An adaptive election algorithm can be incorporated where the schedule An adaptive election algorithm can be incorporated where the schedule

and neighborhood information is used to select the transmitter and and neighborhood information is used to select the transmitter and

receivers for the current time slot, hence avoiding collision and receivers for the current time slot, hence avoiding collision and

increasing energy conservationincreasing energy conservation


[Jung+ 2002] E.-S. Jung and N.H. Vaidya, A Power Control MAC Protocol for Ad hoc Networks, Proceedings of ACM MOBICOM 2002, Atlanta, Georgia, September 23-28, 2002.

[Ye+ 2002] W. Yei, J. Heidemann and D. Estrin, Energy-Efficient MAC Protocol for Wireless Sensor Networks, Proceedings of the Twenty First International Annual Joint Conference of the IEEE Computer and Communications Societies (INFOCOM 2002), New York, NY, USA, June 23-27

2002.

[Woo+ 2003] A. Woo and D. Culler, A Transmission Control Scheme for Media Access in Sensor Networks, Proceedings of the ACM/IEEE International Conference on Mobile Computing and Networking, Rome, Italy, July 2001, pp. 221-235.

[Van Dam+ 2003] T. V. Dam and K. Langendoen, An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Networks, ACM SenSys, Los Angeles, CA, November, 2003.

References

Documents

Medium Access Control in Ad hoc and Sensor Networks