A Survey on TDMA-based MAC Protocols for Wireless Sensor Network

8/10/2019 A Survey on TDMA-based MAC Protocols for Wireless Sensor Network

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal,Volume 4, Issue 6, June 2014)

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A Survey on TDMA-based MAC Protocols for WirelessSensor Network

Pijus Kumar Pal1, Punyasha Chatterjee2

School of Mobile Computing and Communication, Jadavpur University, Kolkata, India

Abstract Recent advancement in micro-electro-mechanical systems (mems) have enabled the development of

Wireless sensor Network (WSN) which is gaining popularity

day by day and is used in wide range of applications. The

sensor nodes that constitute WSN have several constraints like

limited battery power, memory constraint, limited bandwidth

etc. Therefore designing an efficient MAC layer protocol is a

challenging task. TDMA-based MAC protocols can avoid

collisions, overhearing and idle listening and therefore energy

efficient. In this paper, we firstly describe a brief account of

the factors influencing WSN MAC design. Then, we describe

several TDMA-based MAC protocols both centralized and

distributed which are proposed so far for wireless sensor

network.

Keywords MAC Layer, Scheduling, Time Division

Multiple Access (TDMA),

Wireless Sensor Network (WSN).

I. INTRODUCTION

Recent technological advances have enabled the

development of low cost, low power & multifunctional

sensor devices. These autonomous devices with integrated

sensing, processing, and communication capabilities are

called sensor nodes. A sensor node is an electronic device

that is capable of detecting environmental conditions such

as temperature, sound, or the presence of certain objects.

Wireless Sensor Network (WSN) typically consists of a

large number of spatially distributed autonomous sensors to

monitor certain environmental and physical phenomenon

and cooperate with each other to perform the designated

task and send the information to the Base Station (BS) or

the Access Point (AP).

Medium access control (MAC) is one of the critical

issues in the design of wireless sensor networks. As in

most wireless networks, collision, which is caused by two

nodes sending data at the same time over the sametransmission medium, is a great concern in WSNs. To

address this problem, a sensor network must employ a

MAC protocol to arbitrate access to the shared medium in

order to avoid data collision from different nodes and at the

same time to fairly and efficiently share the bandwidth

resources among multiple sensor nodes.

Therefore, a MAC protocol plays an important role in

enabling efficient network operation and achieving good

network performance.

There are many types of MAC protocols, designed for

WSN so far. In TDMA based MAC protocols, time are

divided into time-frames and each time-frame is further

divided into a fixed number of time-slots as shown in

Figure 1. Each node is allocated a time-slot in a time-frameand is allowed to transmit only in the allocated time-slot.

Furthermore, a node depending on the schedules of its

neighboring nodes may remain in the sleep mode when it is

neither to transmit, nor to receive, i.e. can switch off their

transceiver conserving appreciable amount of energy [23].

In this paper we are giving the overview of several TDMA

based MAC protocols developed for WSN.

Rest of the paper is organized as follows: Section II

describes the different categories of WSN MAC protocols;

Section III describes the factors that influence sensor

network MAC design. Section IV presents various TDMA

based MAC protocols both centralized and distributed and

finally Section V concludes the paper.

Figure 1. TDMA Frame Structure

II.

CATEGORIES OF WSNMACPROTOCOLS

MAC protocols for wireless sensor network can be

broadly classified into two categories: Contention-based

MAC protocols and Schedule-based MAC protocols.

A.

Contention-based MAC ProtocolsIn contention-based MAC protocols, all nodes share a

common medium and contend for the medium for

transmission. Thus, collision may occur during the

contention process. To avoid collision, shared channel

access can be arbitrated through some probabilistic

coordination.


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The sender listens to the shared medium before

transmission, waits a random period of time if the mediumis busy and then tries again. It is better for networks, where

the contention is low and burst traffic is expected.

Both ALOHA (Additive Link On-line Hawaii System)

and CSMA (Carrier Sense Multiple Access) are the most

typical examples of contention-based MAC protocols [2].

B.

Schedule-based MAC Protocols

In schedule-based MAC protocol, nodes' access to the

shared medium is divided in respect to either time (Time

Division Multiple Access) or frequency (Frequency

Division Multiple Access) or orthogonal pseudo - noise

codes (Code Division Multiple Access) [2]. This allows

different nodes to access the shared medium without

interfering with each other and thus effectively avoids

collisions.

Contention-based MAC protocols consume more energy

as compared to Schedule-based MAC protocols specially

TDMA-based protocols, because they waste energy in

collisions [20] and idle listening in the network. They also

cant provide delay guarantees [19]. A good schedule notonly avoids collisions by silencing the interferers of every

receiver node in each time slot but also minimizes the

frame-size hence the end-to-end communication latency.

Apart from all the positive parts of TDMA, it has some

negative impacts too. Firstly, TDMA uses topology

information for scheduling purpose which comes in the

form of neighboring and interference relationships amongnodes. But it is very difficult to precisely capture these

interference relationships because of interference range

irregularity [21]. Secondly, it has limited scalability and

adaptability to network changes. Thirdly, TDMA requires

strict time synchronization for the time-slots [18].

However, a guaranteed packet delivery and bounded

latency are highly desirable in real-time applications, which

can be ensured in TDMA.

III.

FACTORS INFLUENCING WSNMACDESIGN

According to [22], to design an efficient MAC protocol

for WSN, attributes such as Energy-efficiency, Scalability

and Adaptivity, Latency, Channel utilization, Throughput,

Fairness etc. are to be considered. In this section, brief

description of each of the attributes will be given.

A.

Energy-efficiency

As the sensor nodes are battery powered, it is almost

impossible to change or recharge the batteries of the nodes.

The radio is the major consumer of energy in many

hardware platforms.

So, the MAC layer needs to consider this issue as it

directly controls radio activities in order to prolonging thenetwork lifetime.

B. Scalability and Adaptivity

The number of sensor nodes deployed in studying a

phenomenon may vary. Depending on the application, it

can be in the order of hundreds, thousands or the number

may reach an extreme value of millions. Some nodes in the

network may die, some new nodes may join or due to

mobility some nodes may move to new location. A good

MAC protocol should accommodate such changes

gracefully.

C.

Latency

Latency refers to the time-delay between the time whena packet is sent by the sender and the time when that packet

is successfully received by the receiver. In case of sensor

network application with stringent latency requirements (e.

g., real - time monitoring of bush fires), the detected event

must be reported to the sink node in real time so that the

appropriate action could be taken.

D.

Channel Utilization

This reflects how well the entire bandwidth of the

channel is utilized in communication. Bandwidth is a

valuable resource in wireless communication. So, the MAC

protocols designed for WSN should maximize the

utilization of this scared resource.E.

Throughput

Throughput refers to the amount of data successfully

transferred from a sender to a receiver in a given time. This

is usually measured in bits or bytes per second. Similar to

latency, the importance of throughput depends on different

applications.

F.Fairness

Fairness refers to the ability of different sensor nodes in

the network to equally share a common transmission

channel among them. As the nodes in WSN cooperate with

each other to accomplish a single common task, it is

important not to achieve per-node fairness, but to ensurethe quality of service for the whole task.

IV.

TDMA-BASED MACPROTOCOLS

In TDMA-based MAC protocol, the total time duration

of communication is divided into a fixed number of time-

slots. TDMA configures these time-slots into time-frames

that repeat periodically.


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Each node in the sensor network is allocated a fixed

number of time-slots and is allowed to transmit only in theallocated time-slots in each frame. In this section, a number

of TDMA-based MAC protocols developed for wireless

sensor network are described in brief. Based on the nature

of the algorithms, we have classified the protocols in two

groups: Centralized protocols and Distributed protocols.

A.

Centralized TDMA Protocols

In Centralized TDMA protocols, the Base Station (BS)

or the Cluster-Head (CH) (in case of Hierarchical WSN)

centrally schedules different slots to different nodes in the

network. Every node uses these time-slots for data

communication. In this sub-section, a number of

centralized TDMA protocols are described.

1) Bit-map-assisted MAC Protocol: Bit-map-assisted

(BMA) MAC Protocol [3] is an intra-cluster

communication MAC protocol for large-scale cluster-based

WSNs. BMA is intended for event-driven applications

where sensor nodes transmit data only when significant

events are observed.

BMA operation is divided into rounds. An example of a

complete round is depicted in Figure 2. Each round is

divided into cluster set-up phase and steady-state phase.

During cluster set-up phase, cluster-head is determined

based on nodes energy levels. This is done using non-

persistent CSMA and elected node broadcasts an

advertisement message claiming to be the new cluster-

head. The steady-state phase is divided into k-sessions with

fixed duration. Each session consists of a contention-

period, a data-transmission period and an idle period.

During each contention period, all nodes keep their radios

on. The contention period follows a TDMA-like schedule:

each node is assigned a specific slot and the nodes transmit

a 1-bit control message to the cluster-head during its

scheduled slot if it has data to transmit; otherwise, its

scheduled slot remains empty. After the contention period

is over, the cluster head has complete knowledge about the

nodes in the network.

Figure 2. Illustration of a Single Round in BMA [3]

The cluster head sets up and broadcasts a transmission

schedule for the source nodes, the nodes that wish to sendits data. Then the system enters into the data transmission

period. During the data transmission period, each source

node turns on its radio and sends its data to the cluster-head

over its allocated slot-time, and keeps its radio off at all

other times. All the other nodes (non-source nodes) keep

their radios off during the data transmission period. When a

session finishes, the next session begins with a contention

period and the same procedure is repeated. The cluster head

collects the data from all the source nodes and forwards the

aggregated and compressed data to the base station .

Advantages- Significant energy savings is possible in

BMA. The nodes have average packet latency and utilize

the bandwidth efficiently.

Disadvantages-The disadvantage of this protocol is that

it is superior for only the cases of low and medium

traffic loads.

2) Self- organized TDMA Protocol for WSN: Self-

Organized TDMA protocol (SOTP) [4] is a cross layer

protocol to serve the application-specific and data-centric

nature of WSN. SOTP applies across-layer design to

achieve superior energy efficiency and lower transmission

delay via a TDMA MAC scheme. In SOTP the

transmission range of base station is much larger than that

of the other sensors in the network. It is assumed that the

base station can broadcast data to all sensor nodes while a

sensor node can reach their immediate neighbors via one

hop. Here, time is divided into frames and each frame is

divided into slots. It is assumed that the number of slots in

one frame is larger than the number of sensor nodes in

whole sensor network.

The operation of SOTP is divided into frames. Each

frame consists of five types of time slot: Broadcasting slot

(BR), Carrier Sensing slot (CS), Transmitting slot (TX),

Receiving slot (RX) and Idle slot (ID). A BR slot is always

the first slot in the frame and a CS slot is the second. Each

node runs in one of the three states: searching,

synchronized, or registered. A node when boots up, it is in

searching state (except for base station, it boots up in

registered state). The base station periodically broadcastsslot allocation packets (SAP). A node (in searching state),

when detects and receives the broadcast SAP from the base

station, it moves into synchronized state and picks up one

unoccupied slot as its TX slot, selects one of its neighbor

registered nodes as its father node, and sends out a register

packet (REG). After the base station receives the REG

packet, it allocates the unoccupied slot and sends this

information in the next SAP packet.


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A node when receives this, it moves to the registered

state. A node in the registered state can have one father nodeand several child nodes. Every node sets the TX slots of its

child nodes as its own RX slots.

Advantages- SOTP is energy efficient due to its pure

TDMA and non-clustering architecture. It reduces

transmission delay efficiently.

Disadvantages-This protocol assumes the transmission

range of the base station is such that it can cover all the

other sensor nodes in the network. Data aggregation and

compression is left to the upper layers and it is assumed

that such aggregation and compression will not add to the

delay of multi-hop transmission.

3) Event Driven TDMA Protocol:Event Driven TDMA

Protocol (ED-TDMA) [5] is an energy efficient TDMA

protocol for event driven application in wireless sensor

network. ED-TDMA improves channel utilization by

changing the length of TDMA frame according to the

number of source nodes and saves energy with a

bitmap-assisted TDMA schedule. ED-TDMA also

employs intra-cluster coverage to prolong network lifetime

and to improve system scalability.

The operation of ED-TDMA protocol is divided into

rounds. Each round begins with a set-up phase followed by

a steady phase. The Set-up phase includes clustering and

time synchronization. An example of TDMA frame

structure of ED-TDMA is shown in Figure 3. It begins with

a reservation phase, followed by a TDMA schedule anddata transmission.

The reservation phase consists of m mini-slot where m is

the number of members in the cluster. The members

occupy the mini-slot according to their ID. Node with

maximum ID occupies the first mini-slot while node with

minimum ID occupies the last mini-slot, and so on. If a

node has data to send in the current frame it sends a 1-bit

RSV (Reservation) message to the cluster head. The length

of the reservation phase is m bit.

Figure 3. Frame structure of ED-TDMA [5]

In TDMA schedule phase, the cluster head broadcasts a

schedule packet according to the RSV packet received in

the reservation phase. The schedule packet format is shown

in Figure 4. This frame consists of two parts.

The first k bit parts represent piggybacking reservation

of the previous frame. The second m-bit part represents thereservation in the current frame.

Figure 4. TDMA schedule packet of ED-TDMA [5]

In the transmission phase a node sends data to the cluster

head during its data slot. If it has more data to send in next

frame, it can book a data slot in the next frame by

piggybacking a flag in the data packet.

Advantages-The energy consumption is reduced in each

node thereby network lifetime is prolonged. This protocolperforms better for event-driven application with high-

density deployment and under low traffic in wireless sensor

network.

Disadvantages- Energy consumption of ED-TDMA

depends on the monitoring area. As this is cluster-based

protocol, there would be larger overheads such as cluster

management, time synchronization under large monitoring

area. So, energy utility efficiency of ED-TDMA decreases

drastically with the enlargement of monitoring area.

4) Mobility Tolerant TDMA-based MAC Protocol:

Mobility tolerant TDMA-based MAC Protocol [6] is a new

TDMA based MAC protocol which can be used in mobile

wireless sensor network. In this protocol it is assumed thatthe network is static during its set-up phase, cluster-head

has less mobility with more battery power and the

synchronization is done automatically. The other nodes in

the cluster have same capability.

According to this protocol, whole network is divided

into clusters. Each cluster is owned by a cluster-head. The

time is divided into frames (N) and in turn into time slots as

shown in Figure 5.

Before every time frame, the Cluster-head allocates a

time slot to every node (n) in the cluster and x slots are

left free for the nodes that join the cluster in later part of

the time frame. This x is generally considered to be 10% of

number of nodes in a cluster. The node which holds thecurrent slot is known as owner node. Whenever a new node

comes and joins the cluster, the Cluster-head allocates half

of the free time slots (x/2 slots) to this new node. If another

comes and joins the cluster, it is allocated half of the current

free slots (x/4). Therefore a cluster will have some empty

slot at anytime in the time frame to allocate to a new node

that joins the cluster.


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In order to improve channel utilization, each time slot is

further divided into 3 sections: Communication Request(CR), Channel Allocation (CA) and Data Section (DS). In

communication request (CR) section, owner node collects

the information about the surrounding nodes. All those

nodes that have data to transmit will put requests to the

owner node for the grant of slot for data transmission. If the

owner node doesnt have any data to transmit then it

will calculate the Priority Index (PI) of each node that

has put a request for grant of a time slot. Among the

slot requested nodes, the node with less PI is given

more priority.

Figure 5. Schematic of Mobility tolerant TDMA Based MAC Protocol

[6]

In Channel Allocation (CA) section, the node decides

whether to send the data and keep the channel to it or allots

the channel to a cluster node which has requested for a slot

and has the highest priority. Packets are transmitted or

received in this Data Section. After the communication

process is over the node goes into sleep as often as possible

in order to save the battery power.

Advantages- It is very good energy conserving MAC

protocol and another important observation is that it has

relatively very less delay as compared to other traditional

TDMA-based MAC protocol

Disadvantages-This protocol assumes the network to be

static during its setup phase and can tolerate less mobility

of the cluster heads.B.

Distributed TDMA Protocols

In Distributed TDMA protocols, scheduling is done by

the nodes themselves based on the local information that

they have. There is no need of any centralized entity (BS or

CH). So, here the message communication is very less

compared to the centralized algorithms and hence it is more

energy efficient. In this sub-section, a number of

distributed TDMA protocols for WSN are discussed.

1) Self- Organizing Medium Access Control: Self-

organizing medium access control for sensor networks(SMACS) [7] is a distributed MAC protocol which enables

a collection of nodes to discover their neighbors and

establish schedules for communicating with them without

the need for any local or global master nodes. In SMACS,

each sensor node is able to turn its radio on and off, and

tune the carrier frequency to different bands as per

requirements.

The number of available bands is relatively large. In

order to form a flat topology, the neighbor discovery and

channel assignment phases are combined. A channel is

assigned to a link immediately once the existence of the

link is discovered. Therefore, by the time all nodes hear

from all their neighbors, they will have formed a connectednetwork, where there is at least one multi-hop path between

any two distinct nodes. In SMACS, only partial

information about radio connectivity in the vicinity of a

node is used to assign timeslots to each links. Each node

maintains a TDMA-like frame called super-frame, in which

it schedules different timeslots to communicate with its

known neighbors. In each timeslot, a node only

communicates with one neighbor. Each link operates on a

different frequency, which is chosen randomly from a large

pool of frequencies when the links are established. This

reduces the probability of collision during channel

assignment. After a link is established, a node knows when

to turn on its transceiver to communicate with another node

and will turn off when there is no communication.

Advantages: In SMACS, link assignment is done

without a need for collecting global connectivity

information or even connectivity information that reaches

farther than one hop away. Hence, significant energy

savings can be achieved.

Disadvantages: The drawback of SMACS is its low

bandwidth utilization. For example, if a node only has

packets to be sent to one neighbor, it cannot reuse the

timeslots scheduled for other neighbors.

2) Power Aware Clustered TDMA: Power Aware

Clustered TDMA (PACT) [8] is an energy-efficient

TDMA-based MAC protocol for a large population ofsensors interconnected by a wireless multi-hop network.

The key features of PACT are listed below:

PACT uses passive clustering that allows nodes to

take turns to become the communication backbone

nodes. The role of the cluster heads and the gateways

are rotated to save energy.


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PACT adapts duty cycles of the nodes to user traffic.

In other words, the radios are turned off if thenetwork is inactive.

A simple selection scheme is used to limit the number

of active gateways between neighboring cluster heads.

PACT provides a clustered structure which can be

utilized by high level protocols to reduce global

communication overhead. This helps PACT to be an

efficient protocol for large sensor networks.

In PACT, there are two common slot assignment

schemes usually referred to as node activation and link

activation. For node activation schemes, each node is

assigned a single time slot in each TDMA frame and the

node can use this slot to transmit to any neighbor. On the

other hand, a link allocation scheme assigns time slotsbased on each directed links. A node can transmit a

packet to its neighbor only during the time slot that is

assigned to the directed link for that neighbor.

Figure 6. PACT TDMA Structure [8]

Figure 6 shows the general TDMA structure of PACT.

Each frame consists of control mini slots and data slots.

All nodes turn their radio on during the control mini slots.

Every node broadcasts the allocation of the data slots

assignments to its neighbors using its assigned mini slot.

Each node learns the data slot assignment status from these

control packets and choose non-conflicting transmission

allocations. Each node shuts its radio off during those time

slots in which it does not send or receive any packet.

Advantages- It is the first TDMA protocol that uses

clustering to take advantages of the dense topology to

preserve energy and prolong the network lifetime. It adapts

energy consumption to user traffic.

Disadvantages- As clustering is unavoidable in PACT,

an amount of overhead, it may be very less, is still there

due to clustering.

3) Distributed Energy-Aware MAC Protocol: The

distributed energy -aware MAC (DE - MAC) [9] protocolis a TDMA based MAC protocol for addressing the energy

management problem in WSNs. The DE-MAC protocol

exploits the inherent features of TDMA to avoid energy

waste caused by collision and control overhead, and

employs a periodical listening and sleeping mechanism to

avoid idle listening and overhearing. Unlike some existing

MAC protocols that treat all nodes equally with respect to

energy conservation, DE-MAC treats those critical nodes

(i.e., with lower energy) differently by using them less

frequently to achieve load balancing among all nodes. A

group of neighbor nodes periodically perform a local

election process based on their energy levels to elect the

worst-off node(s) as the winner(s) and let the winner(s)sleep more than its (or their) neighbor nodes. The protocol

initially assigns the same number of transmission slots to

each node in a TDMA frame. A node can independently

decide to initiate an election if its current energy level is

below a threshold value. Once an election is initiated, each

node sends its energy level to all of its neighbors, which is

included to its regularly scheduled transmission packet

during its scheduled timeslot. To receive the energy level

information from other nodes, all nodes listen to all

transmitted packets. There are no sleeping nodes when

other nodes are transmitting. This is to enable the

integration of leader-election with regular TDMA

transmission and thus save bandwidth. At the end of the

election process, the node with the minimum energy level

is elected as a winner. Once one or more winners are

elected, all the losers reduce the number of their timeslots

by a constant factor (e.g., two) and the winners have

timeslots twice the number of the losers.

Advantages: In DE-MAC, the idling listening time of

those critical nodes are reduced, leading to more energy

savings in the critical nodes thereby increases network

lifetime.

Disadvantages:As the low energy nodes sleep for more

time (they perform only sensing activity), this introduces

end-to-end delay to be more as compared to other TDMA

based MAC protocols.4) Traffic Adaptive MAC Protocol:The traffic-adaptive

medium access (TRAMA) [10] protocol is a TDMA based

MAC protocol to provide energy-efficient collision-free

channel access in WSNs while maintaining good

throughput, acceptable latency, and fairness. In TRAMA,

nodes switch to a low-power idle state when they are not

transmitting or receiving, thereby TRAMA saves

significant amount of energy.


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Time is divided into a series of random-access periods

and scheduled-access periods (Figure 7), which alternatewith each other. A random-access period, also referred to

as a signaling slot, is further divided into smaller signaling

slots and a scheduled-access period, also referred to as a

transmission slot, into smaller transmission slots. The

TRAMA protocol starts with a random access period where

each node randomly selects a timeslot and then transmits.

A node can only join the network during a random access

period.

The TRAMA protocol consists of three components: the

neighbor protocol (NP), the schedule exchange protocol

(SEP), and the adaptive election algorithm (AEA). Both the

NP and the SEP allow nodes to exchange 2 - hop

neighborhood information and their schedules. The AEAuses the neighbor and schedule information to select

transmitters and receivers for the current timeslot, allowing

all other nodes to switch to a low - power mode.

Figure 7. Time Slot Structure in TRAMA [10]

Advantages- In this algorithm, less collision probability

is achieved. Higher percentage of sleep time helps in

significant energy savings and throughput is higher as

compared to Contention -based protocols.

Disadvantages- TRAMA has a higher delay than

contention - based protocols due to a higher percentage of

sleep time and thus is suitable for applications that are not

delay sensitive, but require high delivery throughput and

energy efficiency.5) On- demand TDMA Scheduling for Energy

Conservation in Sensor Network: On-demand TDMA

scheduling (ODS) [11] represents two variants of TDMA-

based MAC protocol, specifically designed for sensor

networks. They attempt to reduce energy consumption

while providing efficient delivery of sensor data to the

sinks. The two variants are Busy Tone On-Demand

Scheduling (BTODS) and On-Demand Scheduling (ODS).

The goal is to schedule sensor-to-sink flows by

scheduling a slot for transmission and reception at each hopalong the path that does not interfere with the existing

flows.

In BTDOS, the channel is divided into two sub channels,

one for data and one control channels for busy tones. Busy

tones do not need to be demodulated, but neighbors must

be able to detect the presence of the busy tone signal on the

control channel. It is assumed that the transmission ranges

of control channel and data channel are chosen as such that

the interference becomes sufficiently low. Thus, busy tones

are used to avoid the hidden terminal because receivers can

emit a busy tone while they are receiving data. This allows

the neighbors to know that a transmission would interfere

with the receivers packet reception.In BTODS, a node will transmit a busy tone under the

following conditions:

It is receiving on the data channel.

It overhears another node sending on the data

channel.

Figure 8 shows the timing diagrams for BTODS slots

with unicast data. In this figure (Figure 8), T rand_1 is chosen

uniformly at random from (Tmin , T rand) and Trand_1 + T rand_2

= Trand to keep the slots aligned. T min is chosen to be long

enough to allow a nodes busy tone to be propagated to its

two-hop neighbors. Every slot begins with an idle listening

period of to account for synchronization errors. Data that

has been previously scheduled in a slot will receive priorityover data which is attempting to be scheduled in the slot.

ODS is similar to BTODS, but does not use busy tones.

The slot structure of ODS is shown in Figure 9. Here nodes

use extra periods to indicate they will be busy sending or

receiving in the current slot. These slots are T TX_busy and

TRX_busy. When a node is scheduled to send data in the

current slot, it will transmit on the data channel during the

TTX_busy period. A node will transmit on the data channel

during the TRX_busyperiod if either it is scheduled to receive

data in the current slot or it heard another node transmit

during the TTX_busy period. The latter case indicates another

node is already scheduled to transmit in the current slot.

Thus, a node which detects the data channel busy duringthe TTX_busy period must indicate that it will not be able to

receive in the current slot to all potential senders, which it

does during the TRX_busyperiod.

a) Previously scheduled data


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b) Attempting to schedule data

Figure 8. Timing diagram for slots in BTODS [11]

a) Previously scheduled data

b) Attempting to schedule dataFigure 9. Timing diagram for slots in ODS [11]

Advantages- Both protocols are designed to schedule

sensor to sink flows while reducing energy consumption

due to collision, overhearing, idle listening and control

overhead. Both allow nodes to find slots which do not

interfere with existing flows in their vicinity.

Disadvantages- BTODS requires the hardwarecapability to provide two non-interfering channels, either

by splitting a channel or using two separate radios. If the

channel is split, then the bit rate of the data channel may be

slightly reduced and the busy tone may be more susceptible

to the effects of fading [12]. For the latter case, twice as

much bandwidth is needed and energy will be consumed by

both radios. ODS requires longer time-slot and less time is

devoted to data transmission. This also has more control

overhead in terms of time and transmission.

6) Lightweight Medium Access Control Protocol:

Lightweight Medium Access Protocol (LMAC) [13] is an

energy-efficient TDMA-based MAC protocol designed for

wireless sensor network. It takes into account the physicallayer properties. The intention of the LMAC protocol is to

minimize the number of transceiver to make the sleep

interval of sensor nodes adaptive to the amount of data

traffic in the network.

In LMAC, time is divided into time slots, which nodes

can use to transfer data without having to content for the

medium. Thus it reduces energy wastage due to collisions.

Each node is assigned one time slot and the node has the

control over this time slot. A frame consists of several timeslots. After a frame length, a node again has a period of

time reserved for it. Time slots can be reused at a non-

interfering distance. This time slots are assigned using a

distributed algorithm. During its assigned time slot, a node

can only transmit a message which consists of two parts:

control message and a data unit. As a time slot is

controlled by a single node, this node can communicate in a

collision free manner. The control message carries the ID

of the time slot controller, it indicates the distance of the

node to the gateway in hops for simple routing, it addresses

the intended receiver and reports the length of the data unit.

The control data will also be used to maintain

synchronization between the nodes. The nodes alsotransmit the sequence number of their time slot in the

frame. All the neighboring nodes will put effort in

receiving the control messages of their neighboring nodes.

When a node is not addressed in that message the node will

switch off its power consuming transceivers only to wake

at the next time slot. If a node is addressed, it will listen to

the data unit which might not fill the entire remainder of

the time slot. Both transmitter and receiver(s) turn off their

transceivers after the message transfer has completed. In

this protocol, a node can only transmit a single message per

frame.

Advantages-Significant energy savings prolongs the

network lifetime. Nodes in the network can communicatewith each other in a collision free manner.

Disadvantages-The main drawback of LMAC scheme is

that it increases idle-listening overhead since nodes must

always listen to the control sections of all slots in a frame,

to allow nodes to receive data and to allow new nodes to

join the network anytime. As the slots are computed only

once in LMAC, this protocol is not suitable for mobile

sensor networks, where nodes can enter and leave other

nodes radio neighborhood at any time.

7) Self Organization and Energy Efficient TDMA MAC

Protocol by Wake Up: Self Organization and Energy

Efficient TDMA MAC Protocol by Wake Up (TDMA-W)

[14] is a self organizing TDMA based MAC protocol forWSN with collision free communication and maintenance

simplicity. Time is divided into TDMA frames and each

TDMA frame is further divided into time slots. Each node

is assigned two slots in a TDMA frame. One is

Transmit/Send slot (s-slot) and the other is Wakeup slot (w-

slot). A node always listens to the channel during the w-slot

and transmits in the s-slot, if necessary.


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In this protocol, a sensor utilizes its assigned slot only

when it is sending or receiving information from othernodes in the network, otherwise its receiver and transmitter

are turned off to avoid unnecessary neighbor listening

which consumes extra power. In order to avoid

overhearing, only destination nodes need to listen to the

transmitter. Other neighbors can turn-off their RF circuits

to save energy. To activate a node, the source node sends a

wakeup packet to the destination node in the w-slot

associated with the destination node. After receiving the

wakeup packet from source node, the destination node

identifies the source node and starts listening during the s-

slot associated with the source node.

Advantages- The power consumption of TDMA-W

protocol is only 1.5%-15% as much as SMAC. In otherwords, the lifetime of TDMA-W networks is 6-67 times

longer than 10% S-MAC.

Disadvantages- The problem with this protocol is that

the slot assignment scheme sometimes cant detect one hop

collisions. Another problem that needs to be taken care of

is deadlock.

8) Flexible TDMA based MAC Protocol: Flexible

TDMA-based MAC Protocol (FlexiMAC) [15] is a novel

TDMA-based protocol for efficient data gathering in

wireless sensor networks that provides end-to-end

guarantees on data delivery. This protocol also takes into

account the severe energy and memory constraints of

wireless sensor networks and this is designed for periodicdata gathering application. In FlexiMAC, nodes only

transmit and receive packets during their own time slot(s)

then sleep until their slots turn up again. Initially,

FlexiMAC builds a data gathering tree and nodes'

schedules. This initial network setup is a one-off phase.

Once the schedule is built, nodes then maintain their

schedules throughout their lifetime in the network. During

the initial network setup, Flexi-MAC uses CSMA/CA for

packet transmission and so nodes' receivers are always on

(i.e., in the listen mode), and also uses a token passing

scheme. Nodes avoid collisions by allowing a node to

execute a specific procedure only when it holds a token.

After the initial network setup finishes, nodes performregular data gathering tasks using their TDMA schedules.

They also can modify their schedules when the network

topology changes means addition of new nodes or failure of

some nodes.

An example of look up table of nodes in Flexi-MAC is

shown in Figure 10. The schedule only represents a list of

slots when a node should be active and so the slots are not

contiguous in time (discrete).

Figure 10. FlexiMAC nodes' schedule (lookup table) [15]

In FlexiMAC, the Fault Tolerant-Listening Slot (FTS) is

simply a short CSMA period where all nodes in the

network are in the listen mode. The Base Station generates

a Time Slot Assignment token (TSA_token) and sends thetoken to the nodes using DFS technique. This allows nodes

to claim a slot and allows them to adjust themselves

according to their local neighborhood state. Nodes switch

to the receive mode for their scheduled Receive Slot List

(RSL), transmit mode for scheduled Transmit Slot List

(TSL), or else they switch to the sleep mode. A Multi-

Function Slot (MFS) is used for local time synchronization

and local repair. This helps the network to be fault tolerant.

The Conflict Slot List (CSL) which records slots that are

used by a node's first-level and second-level neighbors,

allows the slot to be reused in other nodes.

Advantages-As the nodes are active in only their TDMA

slots or else they sleep, resulting in high energy savings.Nodes also can continue to perform properly irrespective of

the failure of individual nodes by performing a local repair

in FTS. So, this is fault tolerant, energy efficient and

guarantees end-to-end data delivery while achieving energy

and memory efficiency for different network configuration.

Disadvantages- FlexiMAC considers only energy &

memory constraints with end-to-end delivery guarantee.

But the end-to-end delay is one of the most important

parameters for wsn, is not considered for assessment. This

is the main problem with FlexiMAC.

9) Sensor Network TDMA Scheduling with Adaptive

Slot-stealing And Parallelism: Sensor Network TDMA

Scheduling with Adaptive Slot-stealing And Parallelism(TDMA-ASAP) [16] is a modified TDMA protocol that

will allow for same efficiency of TDMA, while allowing

the network to conserve energy during times of low load.

This is an efficient MAC protocol that allows the network

to adapt to changing conditions and to balance end-to-end

transmission time and power consumption. This adds the

following techniques to TDMA: (a) the use of parallel

transmissions; (b) slot-stealing; and (c) adaptive sleeping

between transmissions.


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A conventional TDMA scheme will assign one time slot

to each sensor node. However, the sensors outside of eachothers range can transmit simultaneously. There by slot

reuse is possible, that reduces the schedule length.

For WSN, the scheduling problem has two additional

constraints: (1) No child is to be scheduled after its parent

due to the use of data aggregation; and (2) Because of

possible collision at the receiver, nodes sending to a

common parent must be scheduled in different time slots,

although they might be outside each others range (hidden

nodes).

In the TDMA schedule each node is assigned a time slot

to transmit. In the case of a high traffic load each node will

transmit its data in its assigned slot, and hence, this

guarantees the minimum end-to-end delay. In case of lighttraffic load, a technique called, slot stealing, is used on top

of the TDMA schedule. Every node is assigned a slot. The

node is called the owner of the slot. If the slot owner has no

data to send in its assigned slot, some other node can take

over this slot in a controlled way and send its data.

Advantages-TDMA-ASAP allows the network to adapt

to the changing conditions and to balance end-to-end

transmission time and energy consumption. It allows for

quick response time from sensor queries during bursts of

activity and conserves energy during periods of light load.

Disadvantages- The disadvantage of TDMA-ASAP is

that this assumes the sensor network to be lightly loaded.

10) Distributed TDMA Scheduling Protocol based onColoring Algorithm: Distributed TDMA Scheduling

Protocol based on Coloring Algorithm (TDMA-CA) [17] is

a distributed TDMA scheduling protocol which uses spatial

reuse of transmission channel.

Here the whole sensor network is defined to be a

spanning tree rooted at Access Point (AP). The tree is

represented by a graph G(V,E) where V is the set of nodes

in the network and E is the set of edges, namely the

communication link between nodes. The ID of the AP is 0.

A sample example of this protocol with eight nodes is

shown in Figure 11. Here, G represents the spanning tree of

the sensor network, G shows the collision among nodes

with a dashed line, GC represents the conflict graph, anytwo nodes associated with the solid line are conflict.

Figure 11. Conflict graph [17]

Every data packet is forwarded by a node to its parent

and until it reaches the AP. TDMA-CA uses a distributed

coloring algorithm to allocate different colors to theconflicting nodes (nodes within the 2-hop distance of a

node) and arranges distinct slots for data transmission for

each color by TDMA scheduling. Distributed Coloring

algorithm is divided into two stages. In first stage, each

node of graph G picks one slot for transmission in the

order of the traversal of the depth first search (DFS).

In the second stage, the DFS is repeated and now each node

picks as many of the remaining colors as it can for

transmission. At both stages, the nodes send this

information to their one-hop and two-hop neighbors in G'

so that all their interferers in GC learn about the

assignment. The result of both stages is shown in

Figure 12.

Figure 12. The result of both stages [17]

Advantages- This is conflict-free TDMA algorithm

which is energy efficient and has low latency.

Disadvantages- This protocol assumes strict time

synchronization among sensor nodes and the isomorphic

nature of sensor nodes.


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TABLE I

COMPARISON OF TDMA PROTOCOLS

List Of

Protocols

Distributed Delay Topology Schedule

BMA

MAC [3]

No Average Clustered Dynamic

SOTP [4] No Less Flat Static

ED-TDMA[5]

No-

Clustered Dynamic

Mobility

TolerantTDMA [6]

No Less Clustered Dynamic

SMACS

[7]

Yes

-

Flat Static

PACT [8] Yes

-

Clustered Dynamic

DE-MAC

[9]

Yes

-

Flat Dynamic

TRAMA

[10]

Yes Significant Flat Dynamic

BTDOS /

ODS [11]

Yes

-

Flat Dynamic

L-MAC

[13]

Yes

-

Flat Static

TDMA-W

[14]

Yes Significant

(depends

on traffic

load)

Flat Static

Flexi-

MAC [15]

Yes

-

Flat Static

TDMA-

ASAP [16]

Yes Guaranteed

Minimum

end-to-end

delay

Flat Dynamic

TDMA-

CA [17]

Yes Significant

(depends

on trafficload)

Flat Static

V. CONCLUSION

Wireless sensor networks are energy constraint and to

enhance the lifetime of the network an energy efficient

MAC protocol is required. TDMA-based MAC protocols

can be a solution which divides the time span into time-

slots and allocate the slots to different nodes in a WSN.

The nodes can use the allocated time-slots for data

transfer whenever required. TDMA has the naturaladvantage of eliminating collision and bounding the delay,

thereby more energy-efficient. Nodes in the network can

conserve more energy by entering into inactive states when

they are not transmitting or receiving. This paper gives a

description of several TDMA-based MAC protocols both

centralized and distributed for the wireless sensor network.

Advantages and disadvantages of each protocol is

mentioned. A comparison study of different protocols

which are investigated in this paper is also given. Apart

from all the positive sides of TDMA, it has some

drawbacks too like limited scalability and adaptability to

network changes, strict time synchronization etc. With all

these positive and negative sides keeping in mind, futureresearch areas are trying to find a distributed energy

efficient standard MAC protocol suitable for wireless

sensor network.

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A Survey on TDMA-based MAC Protocols for Wireless Sensor Network

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