A Survey on Mac Protocols for Wireless Sensor Networks

— Wireless Sensor Network (WSN) is an infra-structure-less wireless network of nodes that can sense the environment or physical conditions and relay the data to a sink or a gateway possibly through multiple hops. The primary task of Medium Access Control (MAC) protocols in such a network is to synchronize the task of data communication between the nodes with energy efficiency being the prime consideration due to power constraints in sensor nodes. So the most significant performance specifications for MAC protocols in WSN are throughput, efficiency, stability, fairness, low access delay, low transmission delay and low overhead. In this paper we have surveyed significant classes of MAC protocols used in WSN and reviewed the merits and demerits of those protocols based on the aforementioned specifications.


I. INTRODUCTION
Wireless Sensor Network is a collection of sensor nodes or eotes which is used to sense the data and forward it to base station(BS) [1], [2].WSN is widely used in eany applications like Military, Environeental, Forest fire detection, Flood detection, Health applications Etc.[6].Nodes in WSN has lieited resources, eeeory and energy.Hence, energy efficiency is an ieportant factor in WSN sensor nodes and it eust be achieved at both node level and network level [1], [4].
MAC protocols are one of the prieary protocols in a network where the participating systees/nodes share a coeeon coeeunication eediue.Conventional MAC protocols are designed to provide high throughput and QoS through better utilisation of the eediue.In WSN, energy consueption of the resource constraint nodes is a key design factor but the MAC protocols designed for conventional networks are less energy efficient due to one or eore of the following factors [5].
• Idle listening : A node is ready to receive but is not being sent with data • Collisions : When two or eore source nodes transfer data to the saee node • Overhearing : Wasted effort in receiving a packet destined for another node • Protocol Overhead : MAC related control fraee structure which are non-application bytes To counter the above factors, in WSN specific MAC protocols, researchers have proposed different variations of active/sleep eechanise coebined with other techniques aieed at ieproving the following key attributes [7].
• Energy efficiency: Sensor nodes are eostly battery operated and are difficult to charge/change • Latency: Tiee between detection of an event by sensor nodes till it reaches sink node • Throughput: Requireeent of throughput depends on specific application Such MAC protocols can be broadly classified as follows.

Contention Based
The Contention based protocols eanage the shared eediue access by defining the events that eust occur when two or eore nodes atteept to sieultaneously access the eediue and by iepleeenting rules by which a transeitting node provides scope for other nodes to transeit.They also define eethods for initiating new transeissions, detereining the state of the eediue and eanaging retranseissions in the event of occupied eediue.Carrier Sense Multiple Access (CSMA)/ (CSMA/CA) [3] are eost coeeon contention based MAC protocols.In CSMA/CA, the transeitter sends an RTS packet and the receiver, upon receiving it, replies with a CTS packet which refrains other nodes in the receiver's vicinity froe transeitting.Although this eethod efficiently reduces collisions in traditional networks, in WSN, the use of RTS/CTS increases the energy consueption and supports only unicast transeissions.Hence, several variants of these contention based protocols like T-MAC [10], S-MAC [8], WiseMAC [15] were proposed for WSN.

Reservation Based
In Reservation based protocols, each node is given a guaranteed periodic access to the shared medium by segmenting the channel into superframes and a global synchronization between nodes is assumed.A slot is reserved to each real-time node and the node uses the same slot in subsequent superframes.Time division multiple access (TDMA) [3] is a well known reservation based MAC protocol.They are more energy efficient since nodes in the network can be inactive until their allocated time slots.But the latency is directly proportional to the number of time slots and networks with large number of nodes like WSN requires a higher data rate and higher energy consumption to satisfy a deadline.Hence, several WSN specific TDMA-based MAC protocols like W-MAC [20], D-MAC [13], LL-MAC [16] were proposed.

Hybrid
The Hybrid MAC protocols combine the advantages of the TDMA and CSMA.Control packets are transmitted by random access and the data packets are transmitted in the scheduled channel.In comparison to CSMA and TDMA, the hybrid MAC protocols are energy efficient, has better scalability and improves flexibility.Some of the hybrid MAC protocols are A-MAC, IHMAC [22], IEEE 802.15.4 and Z-MAC [19].

Cross Layer
The Cross Layer MAC protocols exploits the potential synergies of the interaction among different network layers to improve the energy consumption.The B-MAC [11] and CLMAC are few examples of such protocols.
The rest of the paper presents a brief survey on few of the WSN specific MAC protocols.Section II discusses various protocols, Section III presents analysis and comparative study, Section IV discusses open issues and finally, Section V concludes the paper.

II LITERATURE REVIEW
The Sensor-MAC (S-MAC) [8] is a CSMA based protocol in which every node follows a periodic sleep and listen time for energy efficiency.Neighboring nodes within a virtual clusters follow the same sleep/listen schedule and the neighboring nodes in two different virtual clusters follow the periods of both clusters.

Fig 1: S-MAC with periodic sleep/listen time
Dynamic Sensor MAC (DSMAC) [9] aims to improve the latency time of S-MAC by adjusting the duty cycle of node based on the traffic and energy conditions dynamically.In DSMAC all nodes have the same duty cycle value and shared one-hop latency values in the SYNC period.When a receiver node detects the average one-hop latency value to be high, it shortens its sleep time and announces it within the SYNC period.And the sender node doubles its duty cycle after receiving this sleep period decrement signal.The latency observed with DSMAC is better than S-MAC.

Fig 2: DSMAC with duty-cycle doubling
Timeout MAC (T-MAC) [10] improves the energy efficiency of S-MAC by reducing the listening period of sensor node during variable traffic conditions, as the nodes closer to the sink must relay more traffic.Accordingly, a node ends its listen period when no activation event has occurred for a time threshold TA.

Fig 3: T-MAC with adaptive active times
In Berkeley Media Access Control (B-MAC) [11] a node self-regulates the wakeup and sleep time.The sum of awake and sleep time period is called a check interval.The sender node sends a wake-up preamble, which is not a packet but a physical layer RF pulse, greater than the check interval followed by data packet.When the receiver node wakes up, it senses the medium and if it detects the preamble, it waits for the preamble to end.If the data packet is for the node itself, it receives it otherwise goes to sleep.
Pattern MAC (PMAC) [12] is CSMA based protocol.In PMAC the wakeup and sleep time of nodes are changed dynamically based on the its own traffic pattern and that of the neighbor's.X-MAC [14] uses the technique of strobed preamble where the sender node sends a series of short preamble packets which contain the receiver node`s address.This allows the target node to interrupt the short preambles by sending acknowledgement, thus saving energy and reducing latency.
Data gathering MAC (D-MAC) [13] is a TDMA based protocol.It is an improved Slotted Aloha protocol where slots are assigned to the nodes based on a data gathering tree.During the receive period of a node, all of its child nodes has transmit periods and subsequent slots are assigned to the nodes that are successive in the data transmission path resulting in low latency.It also uses MTS (more to send) control packets to avoid interference between different branches.

Fig 4: D-MAC and data gathering tree
Low Latency MAC (LL-MAC) [16] is TDMA based protocol designed with low latency as the primary goal.The data interval is divided into X divisions which in turn is divided into Y time slot subdivisions.Each node communicates to its parent in the time slot subdivision within the assigned division corresponding to the hop number it is in and the parent aggregates the data until its turn to communicate.
WiseMAC [15] proposes a short wakeup preamble by using the knowledge of sampling schedule of direct neighbours of the sender node.Funneling MAC [17] is a hybrid TDMA/CSMA scheme proposed to be used in the intensity region, under the control of the sink for small intensity region depths of one or two hops.

Fig 6: Concept of Funneling MAC
Traffic-adaptive MAC protocol (TRAMA) [18] is a TDMA based protocol and it uses an election algorithm to select one sender within two-hop neighborhood.In TRAMA time is divided into random-access and scheduled-access periods.The random-access period is used to establish two-hop topology information.In scheduled-access period, each node exchange its transmission schedule to its neighbours.The election algorithm is used to select the sender and receiver for the current time slot.
Zebra MAC(Z-MAC) [19] is a hybrid MAC protocol.It is a traffic adaptive protocol; in low contention it behaves like CSMA to achieve high channel utilisation and low delay, and in high contention it behaves like TDMA to achieve high channel utilisation and less collision.The efficient scheduling adjustment method is used to tolerate the network topology and data traffic variation.
Energy efficient and Quality of service aware MAC (EQ-MAC) [21] is a Hybrid MAC protocol.It differentiates the long and short messages and it uses the priority techniques for higher priority data.It uses schedule and non schedule techniques for data transmission for greater performance.

III ANALYSIS
In this section we have analysed some of the MAC protocols discussed in the previous section for their merits and demerits.
In S-MAC [8], idle listening is reduced by periodic sleep.Bus neighboring nodes across two different clusters suffer from overhearing as they follow the sleep/wakeup cycle of both the clusters.Also the periodic sleep increases latency as most WSN routing algorithms are multi hop. Figure 8 shows the effect of number of hops on latency and throughput with and without sleep cycles.Although T-MAC [10] improves energy efficiency under variable traffic conditions, the synchronization of the listen periods within a virtual cluster is broken resulting in early sleeping problem. Figure 9 shows that T-MAC uses less energy than S-MAC for linear topology network PMAC [12] is good for relatively stable traffic conditions and performs better than S-MAC.But it is not suitable for convergecast,broadcast and point-to-point network.
B-MAC [11] is efficient at both low and high data rates and is reconfigurable by upper layers.It is also scalable to large number of nodes.But hidden terminal and multi-packet mechanisms are not provided and should be implemented by higher layers.Figure 10 shows the throughput of B-MAC against S-MAC with increased number of nodes.

Fig 10: Throughput of B-MAC and S-MAC with different network size
For B-MAC, with a data rate of 60 seconds per packet, the average throughput was 78% for 15 nodes and 49% for 46 nodes whereas for S-MAC with 10% duty cycle and more than 15 nodes in the network, the performance degraded.The following table lists the comparison of various key attributes of each of the protocols discussed above along with its key merits and demerits.With respect to specific protocols discussed in this paper, in S-MAC [8], adaptability to the changes in network topology requires more work.TMAC discusses virtual clustering but it is not clearly described.PMAC [12] is not suitable for point to point converge gate and broadcast based network.In D-MAC [13] the sensor nodes are fixed based on assumption and strength of sensor nodes are not considered.In X-MAC [14] only few number of nodes can be used but latency can be measured with more data points, so the research can be carried out with more number of nodes.In W-MAC [20] transmissions are not carried-out simultaneously, but if we reuse the same time slot again we can support concurrent transmissions.

V CONCLUSION
This paper presents the study of various WSN specific MAC protocols based on various design factors.It must be highlighted that there is no one protocol accepted as a universal standard.The prime reason is that the choice of the MAC protocol in WSN will be application specific based on the requirement of the key attributes specific to that application.Another reason is that the lower layers lack standardisation and similar conclusion can be drawn for upper layers as well.Hence, a cross-layer design approach is still feasible as attempted in few of the protocols discussed in this paper and it seems to be a promising research area which has to be studied more extensively.

Fig 8 :
Fig 8: Latency and Throughput in S-MAC This was based on a simple linear topology network with 11 nodes which are put in a 1-m space and are configured to send in the minimum transmission power with the source generating 20 messages, each of 100 bytes.

Fig 9 :
Fig 9: Energy use of T-MAC and S-MAC D-MAC[13] fares better in terms of latency.But no collision avoidance methods were proposed for scenarios where the nodes have same schedule time.