Vol 11, No 1 (2014)

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 67

Survey of MAC Protocols for

Heterogeneous Traffic in Wireless

Sensor Networks

Sridevi S.

Associate Professor, Department of Computer Science and Engineering, Sona College of Technology,

Salem, India

Priyadharshini R.

PG Scholar, Department of Computer Science and Engineering, Sona College of Technology,

Salem, India

Usha M.

Professor & Dean, Department of Computer Science and Engineering, Sona College of Technology,

Salem, India

ABSTRACT

Wireless Sensor Networks (WSNs) consists of multiple sensor nodes, which are deployed randomly to collect periodic data, processes the data and forward it to the sink node. The main challenges that WSN faces are severe energy constraints, robustness, responsiveness, self-configuration, etc… Among this the main challenge is the energy efficiency. In order to tackle all these challenges, new protocols in all the layers of communication stack need to be designed. Designing a MAC protocol is of crucial importance because it influences the transceiver unit of the sensor node. The Quality of Service (QoS) at the MAC layer matters as it rules medium sharing and supports reliable communication. In WSNs nodes generate heterogeneous traffic which have different QoS requirements like reliability and delay deadline with different priority requirements that vary according to the application. In this work, a variety of MAC protocols for WSNs are surveyed, with a special focus on traffic classification and priority assignment. As in the existing TDMA based MAC protocols, only one timeslot is allocated to all the sensor nodes in each frame. But our work is to classify the sensed data according to its priority first and allocate slots variably based on its requirement to be sent to the sink node to perform faster rescue operations. A comparison of different MAC protocols with various parameters and future research directions are also included.

Keywords:

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 68 1 INTRODUCTION

Wireless Sensor Networks (WSNs) are becoming more popular and they are used in numerous applications like industry, academia, military, forest fire, medical and health and so on. In all these kinds of applications requires data delivery with QoS as opposed to best-effort-performance in classical monitoring applications. Reliable and real-time delivery of collected data is important in the sensor network operation.

A sensor node has limited battery capacity of < 0.5Ah. With this capacity itself, it plays the role of both data originator as well as data router. Sensing, communicating and processing of data consume battery power. But communication consumes 100 times more power than sensing and processing. [1] So, optimization of energy consumption is required in WSNs to improve the network lifetime.

1.1.Medium Access Control (MAC)

MAC is responsible for providing communication link between large numbers of sensor nodes and shares the medium fairly and efficiently. [2] Let us discuss some of the attributes of good MAC protocol. The first is the energy efficiency. Instead of recharging the battery, it is better to replace the sensor nodes. To get access to the channel, many sensor nodes will compete with each other. The MAC protocol should be able to avoid collisions among these nodes.

MAC layer is responsible for correcting the errors occurred at the physical layer. It also performs some activities like framing, physical addressing, and flow and error controls. It resolves the channel access conflicts among different nodes. It also addresses issues like mobility of nodes and unreliable time varying channel [3].

As already said MAC is responsible for controlling the transceiver unit of a sensor node, this has its effect on the other side also. If sensor node turns off its radio, it cannot communicate with other sensor nodes. If it switches to listen state, it must wait for other nodes also to switch to listen state [4]. In order to save energy, the node usually goes to sleep mode and wakes up according to its planned schedule [5]. This is called duty cycling or sleep scheduling.

1.2.Types Of MAC Schemes

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 69

1.2.1. CSMA VS. TDMA SCHEME

TDMA-based MAC schemes are contention-free. It divides the time into slots and allocates slots to all the nodes. The node then communicates in the allocated slots without any collision. This provides correct sleep-listen schedule for all the nodes to save energy. TDMA protocols require proper synchronization of nodes.

CSMA-based MAC schemes are contention-based. It does not require any additional information about the network. As the node does not follow any transmission schedule, it can able to handle busty and sporadic traffic. Here collision is possible which gives rise to extra delivery latency and retransmissions. Techniques like RTS/CTS are required to provide certain level of service quality.

1.2.2. SENDER-DRIVEN VS. RECEIVER-DRIVER MAC

In sender-driven TDMA-based MAC scheme, the owner of the timeslots are sender nodes. Sender nodes sends control message to inform the intended receiver to wake up and receive data at specified slots. Each node is assigned two slots: 1) transmit slot for data transmission, 2) wake-up slot to receive control packets. The sender node will send control packets in “wake-up” slot of intended receiver to inform it to wake-up in “transmit” slot of sender node in the next frame. It assigns slots to the nodes for message transmission. It eliminates collision of data messages but energy is wasted due to message overhearing.

In receiver-driven TDMA-based MAC scheme, the owner of the timeslots are receiver nodes. It assigns slots to the nodes for message reception. Schedule of the timeslots in which the receiver nodes must wake-up is constructed and exchanged between neighbor nodes. Receiver nodes have to wake-up on their own timeslots. Neighbors of the slot owners have to contend for the medium. It eliminates message overhearing. Contention overhead and packet collision among sender nodes are the main drawbacks of this scheme [6].

The remaining part of the paper is organized as follows: in section 2, we present the various MAC protocols existing in the literature, in section 3, we compare the surveyed MAC protocols based on certain performance criteria and in section 4, we discuss possible directions for further research and conclude the paper.

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 70 2. CLASSIFICATION OF MAC PROTOCOLS

The MAC protocol classification includes QoS-based MAC protocol, Cross-layer MAC protocol, sender-driven MAC, receiver-driven MAC protocol and various other kinds of MAC protocols. Let us discuss this classification one by one.

2.1. QoS-based MAC Protocols

Although all the layers of OSI are responsible for QoS provisioning, MAC layer is of particular importance as it solves the problem of medium sharing and supports reliable communication. MAC also handles additional challenges like severe energy constraints by duty cycling and unpredictable environmental conditions by retransmission [7]. Performance of a QoS-based MAC protocols totally depends upon the requirements of application. The designed MAC protocol must be energy efficient, scalable, must have good memory and processing capability and no bandwidth scarcity.

2.1.1. PQ-MAC

Hoon Kim et. al [4] have designed PQ-MAC (Priority-based QOS MAC) protocol to maintain the energy efficiency and solve the transmission latency problem simultaneously. This also provides data type classification and scheduling scheme for fast transmission of event data. This fast transmission is provided by additional listen time and priority queue scheduling. First the data is classified into four priority levels. Level 0 has the highest priority and this mainly focuses on transmission delay rather than energy efficiency.

Level 1 has the second highest priority and this may have more delay than level 0. Periodic event data belongs to the Level 2, which has low importance and fault tolerant characteristics. Level 3 has the lowest priority data and may have delay tolerant scheduling. Each sensor node has two priority queues, one for higher priority and another for lower priority. It sends high priority data from high priority queue first. If the queue is empty, then it sends low priority data from low priority queue. The priority queue guarantees faster transmission of high-priority data compared to low-priority data.

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 71 In normal MAC protocols, the sensor node wakes up in the middle of sleep time to receive data and energy is wasted. If the probability of data reception is known well in advance, the energy wastage can be reduced. In advanced wake-up scheme, additional filed is added in RTS/CTS message to know the probability of receiving high priority data. Dynamic Priority Listen (DPL) scheme changes the listen/sleep periods according to network traffic conditions. This scheme suits well for dynamic traffic environment.

Parameters considered:

The simulation is done in ns-2. The simulation results show that the protocol manages scheduling by adaptively controlling network traffic and priority level. High priority data is given less waiting time. It reduces latency and has good energy efficiency.

2.1.2. Diff-MAC

M. Aykut Yigitel et. al [8] proposed Diff-MAC which aims to increase the channel utilization by differentiating the traffic and provide fast delivery of data. In case of MAC failures, delivering the video frames as a single packet is expensive. So Diff-MAC divides the video frame into many small fragments. All the video frame fragments are sending as a burst once the medium is reserved.

Diff MAC monitors the network periodically and calculates the probability of collision as the ratio of number of collisions to the total number of transmission attempts. To provide service differentiation, the size of contention window is increased for lower priority traffic and decreased for higher priority traffic. To give precedence to higher priority traffic, to improve the throughput and decrease the latency the authors set CWRT < CWNRT < CWBE. Here, RT, NRT, BE represents real-time multimedia traffic, non real-time traffic and best effort traffic respectively.

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The protocol is first simulated and then implemented on crossbow Imote-2 platform. The result shows that the protocol provides fast delivery of data and has less collision and packet latency.

2.1.3. AMPH MAC

M.Souil et. al [9] proposed AMPH MAC which is an adaptive MAC protocol for Heterogeneous wireless sensor networks. This follows hybrid channel access method for achieving high channel utilization. The hybrid behavior is the combination of both contention-based and schedule-based techniques. This hybrid method allows slot-stealing and also adapts to variable traffic loads. For meeting the needs of real-time traffic, it uses a prioritization scheme.

It is based on TDMA mechanism and in order to increase the channel utilization and reduce the latency, the nodes can transmit during any timeslot. It follows fixed timeslot allocation. Nodes which are two-hop neighbors to each other are not assigned to the same slot. Owners are the name given to the nodes which are assigned to the given slot. Otherwise, they are called non owners. The nodes with same priority level have equal chances of stealing unused slots.

The transmission process has totally three states. They are init, wait and backoff states. The node is in init state during the setup phase. When the node reaches the end of a slot, it is in wait state. If the node has packets to send at the beginning of each slot, it enters into backoff state. To improve the channel utilization, more packets can be sent to the timeslot.

This paper uses a backoff mechanism and a strict priority scheduler which always favor real time traffic. This may also result in starvation for the best effort traffic. Nodes having best effort traffic have higher priority than nodes having real time traffic as backoff values are less for best effort than real time. This can be implemented only in high data rate continuous real time traffic networks.

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 73 2.2. Cross-Layer MAC Protocols

The cross-layer interaction is defined as back-and-forth information flows, merging of adjacent layers, design coupling without a common interface, and vertical calibration across layers. The implementations for cross-layer interactions include explicit interfaces between different layers, shared database, and heap organization. [2]

Advantages of Cross-Layered Protocols:

Both the information and the functionalities of traditional communication layers are melted into a single protocol. It provides informed scheduling decisions, reflecting the current network status, and dynamically optimized scheduling. [10]

2.2.1. CL-MAC

Mohamed S. Hefeida et. al [11] proposed CL-MAC to efficiently handle multi-hop and multi-flow traffic patterns for heterogeneous wireless sensor networks. The multi-flow traffic is generated by sensor nodes having different sensors each sensing different type of data.

It is useful for both homogenous and heterogeneous traffic. It uses a unique Flow setup packet (FSP) scheme which schedules multiple packets over multiple multi-hop flows in a single cycle whereas other MAC protocols support only multi-hop flows. Each FSP can operate as an RTS up to K different destinations. Before setting up a flow, CL-MAC scheduling considers pending packets in routing layer buffer and pending flow setup requests.

The advantages of this set up is that it allows the nodes to make better flow set up decisions, scheduling optimization, minimizes control overhead per data packet and detects the traffic load variations and provides information about the current network status. It follows dynamic timeslot allocation. It accommodates more FSP requests by adopting early acknowledgement scheme.

The protocol is simulated in ns2 and results prove that CL-MAC makes good scheduling decisions and reduces end-to-end latency. It detects traffic load variations by monitoring current network status. It minimizes control overhead by having FSP.

2.2.2. XLP

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 74 of initiative determination which is the node’s willingness to participate in the communication. This enables receiver-based contention, initiative-based forwarding and distributed duty-cycle operation.

Each node initiates data transmission by listening the channel. If the channel is idle, the node sends RTS packet which also serves as link quality indicator. The neighbor receiving this packet checks the source and destination. If neighbors are closer to the sink, then it is a feasible region and performs initiative determination and receiver based contention.

Receiver-based contention is performed only if the initiative determination is 1. It forwards a packet based on routing level of each node. The routing level is based on progress of the packet. Nodes with longer progress have highest priority. If two nodes want to send RTS packet to the same node at the same time, the one with longer progress is allowed. A node does not receives RTS packet for 3 reasons: 1) if initiative determination is not 1, 2) if no feasible region, and 3) due to collision of CTS packets.

Distributed duty-cycle operation is to control the transceiver unit of the node to save energy. Here, the buffer occupancy of the node is build-up when they sleep. XLP has good network performance and less implementation complexity.

XLP protocol is simulated on ns-2 and the result show that it performs uniform energy consumption throughout the network. Each node performs distributed duty cycle operation, which helps to improve network performance and energy consumption.

2.2.3. EEDS PROTOCOL

Tayseer Alkhdour et. al [13] proposed an ILP (Integer Linear Programming) model for EEDS (Energy Efficient Distributed Schedule-based) protocol for constructing a routing tree and a TDMA schedule to maximize the network lifetime. EEDS protocol will reduce the energy consumed at idle listening state and it is mainly designed for periodic data collection applications. It considers both data link and routing layers.

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 75 EEDS timeframes are divided into rounds. Each round has three phases: 1) Building the tree, 2) Building the Schedule and 3) Data transmission phase. In the first phase, tree routed at sink is built. In the second phase, based on the tree, the TDMA schedule is built. The parent node prepares a schedule and broadcast to its children. The children data are aggregated and forwarded to the parent. The parent node does not transmit soon after receiving the packets from a single child as this eliminates data aggregation.

In the third phase, data is transmitted from the source node to the sink. Different frequencies are used to avoid interferences. Every node will be ON in their own slots. Leaf node is ON for one slot where as non-leaf node for its own and also for its children slots. This phase can be repeated many times but till the nodes have required energy.

ILP problem is solved by using LINGO solver tool. EEDS simulations and ILP model are compared and the results prove better in terms of throughput, energy consumption and transmission range. Deployment of sensor nodes and resource scarcity are the main drawbacks of this paper.

2.3. Other MAC Protocols

2.3.1. RMAC

Wee Lum Tan et. al [6] proposed Receiver-driven MAC (RMAC) protocol which mainly focuses on Timeslot Stealing and Timeslot re-assignment mechanism for optimizing channel utilization and handling traffic load variations. For every time slot a pair of nodes are assigned one as primary sender node and another as secondary sender node.

In Timeslot Stealing, the lightly loaded sensor nodes are called primary sensor nodes and they does not fully utilize the timeslots assigned to it. The heavily loaded sensor nodes are called secondary sensor nodes and they do not have necessary timeslots. So by using stealing mechanism, the secondary sensor nodes can steal the unused timeslots of primary sensor nodes. It enhances the protocol throughput for varying traffic loads. It also handles shorter timescale changes in traffic patterns.

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 76 In Timeslot re-assignment procedure, the timeslots are redistributed among the sender nodes according to the traffic load. This mechanism assigns one slot for lightly loaded sender nodes and multiple timeslots for heavily loaded sender node. The number of packets backlogged in the buffer is listened by sender nodes and number of times the timeslot is not utilized is listened by receiver nodes.

The protocol is simulated in ns2 and the results show that energy consumption is less and channel utilization is high. It eliminates message overhearing but has contention overhead and packet collisions among sender nodes.

2.3.2. DSA

Hoon Oh et.al proposed DSA [14], which allocates timeslots based on the bandwidth demand of each node. It allocates timeslots in a sequence of receiving slots and then sequence of sending slots in a disjoint manner which increases the bandwidth by removing wasted slots, reduces power consumption at the lower depths as switching between states is less, and provides better data aggregation and filtering.

Also RTS/CTS messages are exchanged between parent and child within a slot which removes link breakages and supports reliable data transmission and updates synchronization time. Before forwarding the packets to the parent, each node filters and aggregates the received packets. DSA handles clock drift problem by considering SYNC_DELAY parameter. The clock speed is not always constant at all the times. It varies by its quality and power. This is called clock drift problem.

The slot demand is the number of slots needed to send its own packets and also its children’s packets. Sink starts slot scheduling and allocates slots to its children but no slot in the super frame is wasted here. The slot demand is calculated using the equation (1) given below,

D_ip(i)= k∈ch (i)D_ki + |T i | (1)

where, D_ip(i) is the slot demand of node i with respect to its ancestor j, |T(i)| is the set of nodes that belong to the tree the originates from node i

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 77

DSA protocol is implemented using ns2 and result show better performance in terms of network lifetime, energy efficiency, reliability, bandwidth utilization, balanced power consumption.

3. SUMMARY OF MAC PROTOCOLS

The summary shows that many authors have taken the parameters like energy efficiency, throughput, bandwidth utilization, latency, scheduling efficiency and traffic load adaptivity. They have also mentioned what type of MAC used such as TDMA or CSMA or Hybrid and also mentioned the timeslot size as fixed or variable slots.

Table 1. Comparison of the surveyed MAC protocols

SCHEME QoS-based MAC Cross-layer MAC Other MAC

DIFFMAC [8]

PQMAC [4]

AMPH [9]

XLP [11]

CL-MAC [10]

EEDS [12]

DSA [13]

RMAC [6]

Energy Efficiency

√ √ √ √ √ √ √

Priority √ √ √

Throughput √ √ √ √ √

Reliability √ √

Bandwidth utilization

√ √ √ √

SYNC_ DELAY

√ √

Latency √ √ √ √ √

Scheduling efficiency

√ √ √

Type CSMA/CA Hybrid Hybrid CDMA TDMA TDMA Hybrid TDMA

Traffic load adaptivity

√ √ √

Timeslot size

Fixed Variable Fixed Fixed Fixed

Fairness √

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 78 4. CONCLUSION AND FUTURE RESEARCH DIRECTIONS

In this paper, we have surveyed various MAC protocols for WSN and a comparison is also presented with different parameters. Majority of the protocols follows traffic classification to classify the data according to their type and the packets are treated according to their requirements. There are also certain protocols which differentiate MAC parameters according to the networking conditions and provide QoS support indirectly. Finally, we have decided to take priority as a main parameter as it is considered by few authors only. Each node will select its neighbor to forward the data sensed. The traffic classifier will differentiate various types of traffic and according to the QoS requirement, priority is assigned to all the sensed data. With this traffic classification and priority assignment, the prioritized data will reach the destination faster to perform the rescue operations as soon as possible.

REFERENCES

[1] Ranjana Thalore, Jyoti Sharma, Manju Khurana, M.K. Jha “QoS evaluation of energy-efficient ML-MAC proocol for wireless sensor networks”, International Journal of Electronics and Communications (AEÜ), Elsevier, pp. 1-6, June 2013.

[2] Rajesh Yadav, Shrishu Varma, N. Malaviya “A SURVEY OF MAC PROTOCOLS FOR WIRELESS SENSOR NETWORKS”, UbiCC Journal, Volume 4, Number 3, pp. 827-833, August 2009.

[3] Sunil Kumar, Vineet S. Raghavan, Jing Deng, “Medium Access Control protocols for ad hoc wireless networks: a survey” Ad Hoc networks, Elsevier, pp. 1-33, 2004. [4] Hoon Kim and Sung-Gi Min, “Priority-based QoS MAC Protocol for Wireless sensor

Networks”, IPDPS ’09: Proceedings of the 2009 IEEE International Symposium on Parallel& Distributed Processing, IEEE Computer Society, Washington, DC, USA, pp. 1- 8, Dec 2009.

[5] GholamHossein Ekbatanifard, Reza monsefi, Mohammad H. Yaghmaee M., Seyed Amin Hosseini S., “Queen-MAC: A quorum based energy-efficient medium access control protocol for wireless sensor networks”, Computer Networks, Elsevier, pp. 2221-2236, 2012.

[6] Wee Lum Tan, Wing Cheong Lau, On Ching Yue, “Performance analysis of an adaptive, energy-efficient MAC protocol for wireless sensor networks”, Journal of parallel and Distributed Computing, Elsevier, pp. 504-514, Feb 2012.

[7] M. Aykut Yigitel , Ozlem Durmaz Incel, Cem Ersoy, “QoS- aware MAC protocols for wireless sensor networks: A survey”, computer networks, Elsevier, pp.1982-2004, Feb. 2011.

[8] M. Aykut Yigitel , Ozlem Durmaz Incel, Cem Ersoy “Design and implementation of a QoS-aware MAC protocol for Wireless Multimedia Sensor Networks”, Computer Communications, Elsevier, pp. 1991-2001, June 2011.

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ISSN: 1694-2108 | Vol. 11, No. 1. MARCH 2014 79 [10]Christophe J. Merlin, “Adaptability in Wireless Sensor Networks Through Cross-Layer

Protocols and Architectures”, Ph.D. Thesis, University of Rochester, Rochester, New York, 2009.

[11]Mohamed S. Hefeida , Turkmen Canli , Ashfaq Khokhar “CL-MAC: A Cross Layer MAC Protocol for Heterogeneous Wireless Sensor Networks”, Ad Hoc Networks, Elsevier, pp. 213-225, May 2013.

[12]Mehmet C. Vuran, Ian F. Akyildiz, “XLP: A Cross-Layer Protocol for Efficient Communication in Wireless Sensor Networks”, IEEE TRANSACTIONS ON MOBILE

COMPUTING, Jan 2010.

[13]Tayseer Alkhdoura, Uthman Baroudib, Elhadi Shakshukic, Shokri Selimb “An Optimal Cross-Layer Scheduling for Periodic WSN Applications”, The 4th International Conference on Ambient Systems, Networks and Technologies, Elsevier, pp. 88-97, June 2013.

[14]Hoon Oh and Trung-Dinh Han “A demand-based slot assignment algorithm for energy- aware reliable data transmission in wireless sensor networks”, Springer, pp. 523–534, Feb. 2012.

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