MAC Protocols In Sensor Networks

MAC Protocols In 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 ensure successful transmission. Channel can be allocated to users statically or dynamically.allocated to 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 (FDMA): provides a fraction of the frequency range to each user for all the timefrequency range to each user for all the time

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

- - Code Division Multiple Access (CDMA) : provides every user a portion of (CDMA) : provides every user a portion of bandwidth for a fraction of timebandwidth for 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 - Dynamic Conflict Resolution: the Ethernet, which keeps track of various system parameters, ordering the users accordinglysystem parameters, ordering the users accordingly

Multiple Access Control (MAC) Protocols

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

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

Number of channels required to support a user population depends on Number of channels required to support a user population depends on the average number of calls generated, average duration of a call and the the average number of calls generated, average duration of a call and the required qualityrequired quality of service (e.g. percentage of blocked calls) 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 The whole channel is assigned to each user, but the users are multiplexed in time during communication. Each communicating user multiplexed in time during communication. Each communicating user is assigned a particular time slot, during which it communicates using is assigned a particular time slot, during which it communicates using the entire frequency spectrumthe entire frequency spectrum

The data rate of the channel is the sum of the data rates of all the The data rate of the channel is the sum of the data rates of all the multiplexed transmissionsmultiplexed transmissions

There is always channel interference between transmission in two There is always channel interference between transmission in two adjacent slots because transmissions tend to overlap in time. This adjacent slots because transmissions tend to overlap in time. This interference limits the number of users that can share the channelinterference limits the 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 technique, allows multiple users to share the same channel by multiplexing their transmissions in code space. share the same channel by multiplexing their transmissions in code space. Different signals from different users are encoded by different codes Different signals from different users are encoded by different codes (keys) and coexist both in time and frequency domains(keys) and coexist both in time and 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 correlation of different codes, other transmissions appear as noise. This property enables the multiplexing of a number of transmissions on the property enables the multiplexing of a number of transmissions on the same channel with minimal interferencesame channel with minimal interference

The maximum allowable interference (from other transmissions) limits the The maximum allowable interference (from other transmissions) limits the number of simultaneous transmissions on the same channelnumber of 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): the narrow band signal representing digital data (DS): the narrow band signal representing digital data is multiplied by a wideband pseudo noise (PN) signal representing the is multiplied by a wideband pseudo noise (PN) signal representing the code. Multiplication in the time domain translates to convolution in the code. Multiplication in the time domain translates to convolution in the spectral domain. Thus the resulting signal is widebandspectral domain. Thus the resulting signal is wideband

- - Frequency HoppingFrequency Hopping (FH): carrier frequency rapidly hops among a large (FH): carrier frequency rapidly hops among a large set of possible frequencies according to some pseudo random sequence set of possible frequencies according to some pseudo random sequence (the code). The set of frequencies spans a large bandwidth. Thus the (the code). The set of frequencies spans a large bandwidth. Thus the bandwidth of the transmitted signal appears as largely spreadbandwidth of the transmitted signal appears as largely 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) [Ye+ 2002]

– 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


[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

MAC Protocols In Sensor Networks