A Survey: MAC Layer Protocol for Wireless Sensor Networks

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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 3, Issue 9, September 2013)

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A Survey: MAC Layer Protocol for Wireless Sensor

Networks

Prof. Urmila A. Patil

1

, Smita V. Modi

2

, Suma B. J.

3

1

Associate Professor, DYPIET, Pune University, pune.

2,3 _{Student, DYPIET, Pune University, pune.}

Abstract—Wireless sensor networks provide broad area for Researchers due to their wide range of applications in Areas such as target detection, tracking and Monitoring. The MAC protocols have gained a lot of importance in the last few years because of their influence on the lifetime of sensor nodes of wireless sensor networks. As sensors operate on batteries, replacement of which is often difficult. A lot of work has been done to minimize the energy expenditure and increase the sensor lifetime through energy efficient protocols, across layers. This paper introduces different types of MAC protocols used for WSN; A modification of the protocol is proposed to improve the energy efficiency, latency and throughput of WSN. Later it has been discussed some typical energy saving protocols in the MAC layer and in the network layer and compare their performances. By comparing the advantages and disadvantages of these protocols, it gives direction to some new research towards energy strategy and provides a reference for the further application of WSNs.

Keywords- MAC protocol, sensor node, WSN.

I. INTRODUCTION

Improvements in hardware technology have resulted in low-cost sensor nodes which are composed of a single chip with embedded memory, processor, and transceiver. Low power capacities lead to limited coverage and communication range for sensor nodes compared to other mobile devices. Hence, for example in target tracking and border surveillance applications, sensor networks must include a large number of nodes, to cover the target area successfully. Unlike other wireless networks, it is generally hard (or impractical) to charge/replace the exhausted battery, which gives way to the primary objective of maximizing node/network lifetime, leaving the other performance metrics as secondary objectives. Since the communication of sensor nodes will be more energy-consuming than their computation, it is a primary concern that the communication is minimized while achieving the desired network operation. However, the medium access decision within a dense network composed of nodes with low duty-cycles is a hard problem that must be solved in an energy-efficient manner. The paper emphasizes the peculiar features of sensor networks including reasons of potential energy wastes at medium access communication.

Then, this paper gives brief definitions for the key MAC protocols proposed for sensor networks listing their advantages and disadvantages. It concludes the survey on MAC protocols with a comparison of investigated protocols and provides a future direction to researchers for open issues that have not been studied thoroughly.

Maximizing the network lifetime is a common objective of sensor network research, since sensor nodes are assumed to be disposed when they are out of battery. Under these circumstances, the proposed MAC protocol must be energy efficient by reducing the potential energy wastes presented. Types of communication patterns that are observed in sensor network applications should be investigated since these patterns are used to extract the behavior of the sensor network traffic that has to be handled by a given MAC protocol. Categorization of the possible communication patterns is outlined. Afterwards, the properties that must be possessed by a MAC protocol to suit a sensor network environment are presented.

A. Need Of MAC

MAC specifies

1. Which nodes in the network need to talk?

2. How the nodes need to talk?

3. When and how long are the nodes going to talk?

4.

If there is no MAC then all the nodes might try

to talk at the same time, resulting in collisions.

B. MAC Protocol Design Challenges

The medium access control protocols for the wireless sensor network have to achieve two objectives. The first objective is the creation of the sensor network infrastructure. A large number of sensor nodes are deployed and the MAC scheme must establish the communication link between the sensor nodes. The second objective is to share the communication medium fairly and efficiently.

C. Attributes of a Good MAC Protocol

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1: Energy Efficiency: The first is the energy efficiency. The sensor nodes are battery powered and it is often very difficult to change or recharge batteries for these sensor nodes. Sometimes it is beneficial to replace the sensor node rather than recharging them.

2: Latency: The second is latency. Latency requirement basically depends on the application. In the sensor network applications, the detected events must be reported to the sink node in real time so that the appropriate action could be taken immediately.

3: Throughput: Throughput requirement also varies with different applications. Some of the sensor network application requires sampling the information with fine temporal resolution. In such sensor applications it is better that sink node receives more data.

4: Fairness: In many sensor network applications when bandwidth is limited, it is necessary to ensure that the sink node receives information from all sensor nodes fairly. However among all of the above aspects the energy efficiency and throughput are the major aspects. Energy efficiency can be increased by minimizing the energy wastage.

D. Major Sources of Energy Wastes

Major sources of energy waste in wireless sensor network are basically of four types [1] [2]

1: Collision: The first one is the collision. When a transmitted packet is corrupted due to interference, it has to be discarded and the follow on retransmissions increase energy consumption. Collision increases latency also.

2: Overhearing: The second is overhearing, meaning that a node picks up packets that are destined to other nodes.

3: Packet Overhead: The third source is control packet overhead. Sending and receiving control Packets consume energy too and less useful data packets can be transmitted.

4: Idle listening: The last major source of inefficiency is idle listening i.e., listening to receive possible traffic that is not sent. This is especially true in many sensor network applications. If nothing is sensed, the sensor node will be in idle state for most of the time. The main goal of any MAC protocol for sensor network is to minimize the energy waste due to idle listening, overhearing and collision.

E.MAC Performance Matrices

In order to evaluate and compare the performance of energy conscious MAC protocols, the following matrices are being used by the research community.

1: Energy Consumption per bit: - The energy efficiency of the sensor nodes can be defined as the total energy consumed / total bits transmitted.

The unit of energy efficiency is joules/bit. The lesser the number, the better is the efficiency of a protocol in transmitting the information in the network. This performance matrices gets affected by all the major sources of energy waste in wireless sensor network such as idle listening, collisions, control packet overhead and overhearing.

2: Average Delivery Ratio: - The average packet delivery ratio is the number of packets received to the number of packets sent averaged over all the nodes.

3: Average Packet Latency: - The average packet latency is the average time taken by the packets to reach to the sink node.

4: Network Throughput:-The network throughput is defined as the total number of packets delivered at the sink node per time unit.

II. PROPOSED MAC PROTOCOLS

The medium access control protocols for the wireless sensor networks can be classified broadlyinto two categories: Contention based and Schedule based.

The schedule based protocol can avoid collisions, overhearing and idle listening by scheduling transmit & listen periods but have strict time synchronization requirements. Scheduled protocols are very attractive for applications in sensor networks because of their energy efficiency. Since slots are pre-allocated to individual nodes, they are collision-free. These protocols are characterized by a duty cycle built-in with the inherent

collision-free nature that ensures low energy

consumption. On the other side, the complexity of the design is high due to problems of synchronization. In general, they are not flexible to changes in node density or movement, and lack of peer-to-peer communication [3].

The representative schedule-based protocols are: • Time Division Multiple Access (TDMA): it allows

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• Frequency Division Multiple Accesses (FDMA): it allocates users with different carrier frequencies of the radio spectrum. It is another scheme that offers a collision-free medium, but it requires additional hardware to dynamically communicate with different radio channels. This increases the cost of the sensor nodes, which is in contrast with the philosophy of sensor network systems.

• Code Division Multiple Access (CDMA): it

employs spread spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. It also offers a

collision-free medium, but its high computational

requirement is a major obstacle for the minimum energy consumption objective in WSNs.

A. Contention-based Protocols:

Contention schemes differ in principle from scheduled schemes since a transmitting user is not guaranteed to be successful. Unlike scheduled protocols, contention protocols do not divide the channel into sub-channels or pre-allocate the channel for each node to use. Instead, a common channel is shared by all nodes and it is allocated on demand. At any moment, a contention mechanism is employed to decide which node has the right to access the channel. Contention protocols have several advantages compared to scheduled protocols. First, because contention protocols allocate resources on demand, they can scale more easily across changes in node density or traffic load. Second, contention protocols can be more flexible as topologies change.

There is no requirement to form communication clusters, and peer-to-peer communication is directly supported. Finally, contention protocols do not require fine-grained time synchronization as in TDMA protocols. The major disadvantage of a contention protocol is its inefficient usage of energy. The resolution process does consume resources. If the probability of interference is small, such as might be the case with burst users, taking the chance of having to resolve the interference compensates for the resources that have to be expanded to ensure freedom of conflicts. Moreover, in most conflict-free protocols, idle users do consume a portion of the channel resources; this portion becomes major when the number of potential users in the system is very large to the extent that conflict-free schemes are impractical. In contention schemes idle users do not transmit and thus do not consume any portion of the channel resources.

The representative contention-based protocols are: • ALOHA: a node simply transmits a packet when it is generated (pure ALOHA) or at the next available slot (slotted ALOHA).

Should the transmission be unsuccessful, every colliding user, independently of the others, schedules its retransmission to a random time in the future. This randomness is required to ensure that the same set of packets does not continue to collide indefinitely.

• Carrier Sense Multiple Access (CSMA): when a user generates a new packet the channel is sensed and if found idle the packet is transmitted. When a collision takes

place every transmitting user reschedules a

retransmission of the collided packet to some other time in the future (chosen randomly) when the same operation will be repeated. In accordance with common networking lore, CSMA methods have a lower delay and promising throughput potential at lower traffic loads, which generally happens to be the case in WSNs. However, additional collision avoidance or collision detection methods should be employed.

[image:3.595.335.547.346.476.2]

Fig. 1. Classification Of MAC Protocols for WSN

 S-MAC - The basic concept behind the Sensor-MAC

(S-MAC) protocol is the locally managed

synchronization and the periodic sleep–listen schedules [4]. Basically built in a contention-based fashion, S-MAC strives to retain the flexibility of contention-based protocols while improving energy efficiency in multi-hop networks. S-MAC includes approaches to reduce energy consumption from all the major sources of energy waste: idle listening, collision, over-hearing and control overhead. Neighboring nodes form virtual clusters so as to set up a common sleep schedule. If two neighboring nodes reside in two different virtual clusters, they wake up at the listen periods of both clusters. Schedule exchanges are accomplished by periodic SYNC packet broadcasts to immediate neighbors. The period for each node to send a packet is called the synchronization period. Collision avoidance is achieved by a carrier sense. Furthermore, RTS/CTS packet exchanges are used for uncast-type data packets. Periodic sleep may result in high latency, especially for multi-hop routing algorithms, since all intermediate nodes have their own sleep schedules. The latency caused by periodic sleeping is called sleep delay.

MAC Protocol’s

Contentio

n Based

Protocols

Schedule

Based

Protocols

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The adaptive listening technique is proposed to improve the sleep delay and thus the overall latency. In that technique, the node that overhears its neighbor‟s transmissions wakes up for a short time at the end of the transmission. Hence, if the node is the next-hop node, its neighbor could pass data immediately. The end of the transmissions is known by the duration field of the RTS/CTS packets. The energy waste caused by idle listening is reduced by sleep schedules in S-MAC. In

addition to its implementation simplicity, time

synchronization overhead may be prevented by sleep schedule announcements. However broadcast data packets do not use RTS/CTS, which increases collision probability. Adaptive listening incurs overhearing or idle listening if the packet is not destined to the listening node. Sleep and listen periods are predefined and constant, which decreases the efficiency of the algorithm under variable traffic load.

[image:4.595.333.542.140.264.2]

Sensor-MAC (S-MAC) locally managed synchronizations and periodic sleep listen schedules based on these synchronizations form the basic idea behind the Sensor-MAC (S-MAC) protocol [5]. Neighboring nodes form virtual clusters to set up a common sleep schedule. If two neighboring nodes reside in two different virtual clusters, they wake up at listen periods of both clusters. A drawback of S-MAC algorithm is this possibility of following two different schedules, which results in more energy consumption via idle listening and overhearing. Schedule exchanges are accomplished by periodical SYNC packet broadcasts to immediate neighbors. The period for each node to send a SYNC packet is called the synchronization period.

Figure 1 represents a sample sender-receiver

communication. Collision avoidance is achieved by a carrier sense, which is represented as CS in the figure. Furthermore, RTS/CTS packet exchanges are used for uncast type data packets. An important feature of S-MAC is the concept of message-passing where long messages are divided into frames and sent in a burst. With this technique, one may achieve energy savings by minimizing communication overhead at the expense of unfairness in medium access. Periodic sleep may result in high latency especially for multi-hop routing algorithms, since all immediate nodes have their own sleep schedules. The latency caused by periodic sleeping is called sleep delay in [1]. Adaptive listening technique is proposed to improve the sleep delay, and thus the overall latency. In that technique, the node who overhears its neighbor‟s transmissions wakes up for a short time at the end of the transmission. Hence, if the node is the next-hop node, its neighbor could pass data immediately. The end of the transmissions is known by the duration field of RTS/CTS packets.

Figure 1 S-MAC Messaging Scenario

Advantages: The energy waste caused by idle listening is reduced by sleep schedules. In addition to its

implementation simplicity, time synchronization

overhead may be prevented with sleep schedule announcements.

Disadvantages: Broadcast data packets do not use RTS/CTS which increases collision probability. Adaptive listening incurs overhearing or idle listening if the packet is not destined to the listening node. Sleep and listen periods are predefined and constant, which decreases the efficiency of the algorithm under variable traffic load.

 Timeout – MAC (T – MAC)- Static sleep-listen periods of S-MAC result in high latency and lower throughput as indicated earlier. Timeout MAC (T-MAC) is proposed to enhance the poor results of S-MAC protocol under variable traffic load. In T-MAC, listen period ends when no activation event has occurred for a time threshold TA. The decision for TA is presented along with some solutions to the early sleeping problem defined in [6]. Variable load in sensor networks are expected, since the nodes that are closer to the sink must relay more traffic. Although T-MAC gives better results under these variable loads, the synchronization of the listen periods within virtual clusters is broken. This is one of the

reasons for the early sleeping problem. Timeout T-MAC

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Figure 2: Basic T-MAC Scheme

Berkeley MAC (B-MAC)- B-MAC is highly configurable and can be implemented with a small code and memory size. B-MAC consists of: clear channel assessment

(CCA), packet back-off and link layer

acknowledgements. For CCA, B-MAC uses a weighted moving average of samples when the channel is idle in order to assess the background noise and to better be able to detect valid packets and collisions. The packet back-off time is configurable and is chosen from a linear range as opposed to an exponential back-off scheme typically used in other distributed systems. This reduces delay and works because of the typical communication patterns found in a WSN. B-MAC also supports a packet by packet link layer acknowledgement. In this way only important Packets need to pay the extra cost. A low power listening scheme is employed where a node cycles between awake and sleep cycles. While awake, it listens for a long enough preamble to assess if it needs to stay awake or can return to sleep mode. This scheme saves significant amounts of energy. Many MAC protocols use a request to send (RTS) and clear to send (CTS) style of interaction. This works well for ad hoc mesh networks where packet sizes are large (1000s of bytes). However, the overhead of RTS-CTS packets to set up a packet transmission is not acceptable in WSNs where packet sizes are on the order of 50 bytes. B-MAC, therefore, does not use a RTS-CTS scheme.

 IEEE 802.11- IEEE 802.11 is the first wireless LAN (WLAN) standard proposed in 1997 [7].The medium access mechanism, called the Distributed Coordination Function, is basically a Carrier Sense Multiple Access with Collision Avoidance mechanism (CSMA/CA). A station wanting to transmit senses the medium. If the medium is busy then it defers. If the medium is free for a specified time (called Distributed Inter Frame Space, DIFS in the standard), then the station is allowed to transmit. The receiving station checks the CRC of the received packet and sends an acknowledgment packet. If the sender does not receive the ACK, then it retransmits the frame until it receives ACK or is thrown away after a given number of retransmissions. According to the standard, a maximum of seven retransmissions are allowed before the frame drops.

In order to reduce the probability of two stations colliding due to not hearing each other, which is well-known as the “hidden node problem”, the standard defines a Virtual Carrier Sense mechanism: a station wanting to transmit a packet first transmits a short control packet called RTS (Request To Send), which includes the source, destination, and the duration of the intended packet and ACK transaction. The destination station responds (if the medium is free) with a response control packet called CTS (Clear to Send), which includes the same duration information. Obviously, collisions are still possible because the efficiency of CSMA/CA depends on the sensing range of each node and the presence of a hidden station. In general, the performances of CSMA/CA are strictly related to the network topology and the nodes density: the more nodes can hear each other the better quality of communication can be achieved avoiding collisions. Inevitably, large latency times affect the efficiency of the system, because before transmitting each station has to wait an unpredictable amount of time that mainly depends on the demands of users and topology of the network.

B.Schedule based MAC Protocols

 TRAMA (Traffic Adaptive Medium Access Protocol) -The traffic adaptive medium access (TRAMA) [8] is a TDMA based protocol that has been designed for energy efficient collision free channel in WSNs. In this protocol the power consumption has been reduced by ensuring collision free transmission and by switching the nodes to low power idle state when they are not transmitting or receiving. This protocol consists of three main parts:

a)The Neighbor Protocol is for collecting the

information about the neighboring nodes

b)The Schedule Exchange Protocol is for exchanging

the two-hop neighbor information and their schedule

c)The Adaptive Election Algorithm decides the

transmitting and receiving nodes for the current time slot using the neighborhood and schedule information. The other nodes in the same time slot are switched to low power mode.

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[image:6.595.61.260.293.427.2]

 D-MAC- Converge cast is the mostly observed communication pattern within sensor networks. These unidirectional paths from possible sources to the sink could be represented as data gathering trees. The principal aim of DMAC [10] is to achieve very low latency, but still to be energy efficient. DMAC could be summarized as an improved Slotted Aloha algorithm where slots are assigned to the sets of nodes based on a data gathering tree as shown in Figure 3. Hence, during the receive period of a node, all of its child nodes has transmit periods and contend for the medium. Low latency is achieved by assigning subsequent slots to the nodes that are successive in the data transmission path.

Fig. 3. A data gathering tree and its DMAC implementation

Advantages: DMAC achieves very good latency compared to other sleep/listen period assignment methods. The latency of the network is crucial for certain scenarios, in which DMAC could be a strong candidate. Disadvantages: Collision avoidance methods are not utilized, hence when a number of nodes that has the same schedule (same level in the tree) try to send to the same node, collisions will occur. This is a possible scenario in event-triggered sensor networks. Besides, the data transmission paths may not be known in advance, which precludes the formation of the data gathering tree.

 WISE-MAC- Spatial TDMA and CSMA with Preamble Sampling protocol are proposed in [11] where all sensor nodes are defined to have two communication channels. Data channel is accessed with TDMA method, whereas the control channel is accessed with CSMA method. Proposed Wise MAC protocol which is similar to Hoiydi et al.‟s work [11] but requires only a single-channel. Wise MAC protocol uses non-persistent CSMA (np-CSMA) with preamble sampling as in [11] to decrease idle listening. In the preamble sampling technique, a preamble precedes each data packet for alerting the receiving node. All nodes in a network sample the medium with a common period, but their relative schedule offsets are independent.

If a node finds the medium busy after it wakes up and samples the medium, it continues to listen until it receives a data packet or the medium becomes idle again. The size of the preamble is initially set to be equal to the sampling period. However, the receiver may not be ready at the end of the preamble, due to reasons like interference, which causes the possibility of over emitting type energy waste. Moreover, over emitting is increased with the length of the preamble and the data packet, since no handshake is done with the intended receiver. To reduce the power consumption incurred by the predetermined fixed-length preamble, Wise MAC offers a method to dynamically determine the length of the preamble. That method uses the knowledge of the sleep schedules of the transmitter node‟s direct neighbors. The nodes learn and refresh their neighbor‟s sleep schedule

during every data exchange as part of the

acknowledgement message. In that way, every node keeps a table of sleep schedules of its neighbors. Based on neighbors‟ sleep schedule table, Wise MAC schedules transmissions so that the destination node‟s sampling time corresponds to the middle of the sender‟s preamble. To decrease the possibility of collisions caused by that specific start time of up preamble, a random wake-up preamble is advised. Another parameter affecting the choice of the wake-up preamble length is the potential clock drift between the source and the destination. A lower bound for the preamble length is calculated as the minimum of destination‟s sampling period, Two, and the potential clock drift with the destination which is a multiple of the time since the last ACK packet arrival. Considering this lower bound, a preamble length, Tp, is chosen randomly.

Figure 4 Wise MAC concepts.

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However, this redundant transmission will result in higher latency and power consumption. In addition, the hidden terminal problem comes along with Wise MAC model as in the Spatial TDMA and CSMA with Preamble Sampling algorithm. That is because Wise MAC is also based on non-persistent CSMA. This problem will result in collisions when one node starts to transmit the preamble to a node that is already receiving another node‟s transmission where the preamble sender is not within the range.

C. Hybrid MAC Protocols:

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A two-level semi-random scheme is implemented at MAC layer (see Fig.5):

Figure 5: Hybrid MAC representation

• A deterministic MAC with a weighted TDMA Scheme: it regulates channel access among clusters. The main advantages of using this approach are the robustness to collision and the reduced energy consumption. During a TDMA-cycle, each cluster is allowed to transmit for number of TDMA-slots that is proportional to the amount of traffic it has to forward. A node has to be awake only when it is in its listening TDMA-slot or its transmitting TDMA-slot if it has a packet to send.

• A random based MAC with a p-persistent CSMA Scheme within a single TDMA-slot: it manages the communication between the nodes of the transmitting cluster and the nodes of the receiving cluster within a single TDMA-slot. It offers flexibility to the introduction of new nodes and robustness to node failures. In SERAN the flexibility is obtained by having the transmitting nodes access the channel in a p-persistent slotted CSMA fashion [12]. The time granularity of this level is the CSMA-slot. The packet is sent in multi-cast over all nodes of the receiving cluster; then the receiving nodes implement a random acknowledgment contention scheme to prevent duplication of the packets.

The algorithm is the following:

1.Each of the nodes in the transmitting cluster that has a packet to send senses the channel at the first CSMA-slot with probability p. If the channel is clean, the node tries to multi-cast the packet to the nodes of the receiving cluster. If clear channel assessment (CCA) is supported, a node performs collision avoidance (CA) with a random back off time. If another transmission is detected, the node aborts the current trial to avoid collisions.

2.At the receiving cluster, if a node has successfully received a single packet, it starts a back-off time Tack before transmitting an acknowledgment. The back-off time is a random variable uniformly distributed between 0 and a maximum value called Tmaxack. If in the interval between 0 and Tack, it hears an acknowledgment coming from another node of the same cluster, the node discards the packet and does not send the acknowledgment. 3.At the transmitting side, if no acknowledgment is

received, the node assumes the packet transmission was not successful and it multi-casts the packet at the next CSMA-slot again with probability p. The procedure is repeated until transmission succeeds or the TDMA-slot ends.

 Zebra MAC (Z-MAC)

Z-MAC is a hybrid MAC scheme for sensor networks that combines the strengths of TDMA and CSMA while offsetting their weaknesses [13]. The main feature of Z-MAC is its adaptability to the level of contention in the network so that under low contention, it behaves like CSMA, and under high contention, like TDMA. By mixing CSMA and TDMA, Z-MAC becomes more

robust to timing failures, time-varying channel

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Optimal choice of MAC protocols is determined by application specified goals such as accuracy, latency, and energy efficiency. However, B-MAC protocol is widely used because it has good results even with default parameters and it performs better than the other protocols.

III. FUTURE RESEARCH DIRECTIONS

In the recent years a large number of medium access control (MAC) protocols for the wireless sensor network have been published by the researchers. Most of the work on the MAC focuses primarily on the energy efficiency in the sensor network However; still a lot of work has to done in the other areas at the MAC layer such as:

A. Network Security: - Sensor network security at MAC layer to protect against eavesdropping and malicious behavior has to be studied further. Karlofet al. in TinySec have proposed secure MAC protocol based on shared key but still more advanced schemes needs to be developed.

B. Nodes Mobility: - The nodes in the wireless sensor network were originally assumed to be static. Recently there has been increasing interest in medical care and disaster response applications where the mobile sensors can be attached to the patient, doctor or first responder. The mobility at the MAC layer has been considered in MMAC , still there is a lot of scope for future research in this area.

C. Evaluation on Sensor Platforms: - Most of the protocols for the wireless sensor network have been evaluated through the simulations. However, the performance of the MAC protocol needs to be evaluated on the actual sensor system. The researchers should focus on experimenting on the real sensor platforms.

D. Real Time Systems: - Energy efficiency is the main design objective of the sensor network but the reliable delivery of data in the real time is essential for certain time critical applications. This is also a promising research area which needs to be studied more extensively.

IV. OPEN ISSUES AND CONCLUSIONS

Table I represents a comparison of MAC protocols

investigated. Time Synchronization needed column

indicates whether the protocol assumes that the time synchronization is achieved externally. Adaptively to Changes means ability to handle topology changes.

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The two S-MAC variants, namely, T-MAC and DSMAC, have the same features with S-MAC given in Table I.

Table I.

Comparison of Mac Protocols

Although there are various MAC layer protocols proposed for sensor networks, there is no protocol accepted as a standard. One of the reasons behind this is the MAC protocol choice will, in general, be application-dependent, which means that there will not be one standard MAC for sensor networks. Another reason is the lack of standardization at lower layers (physical layer) and the (physical) sensor hardware. TDMA has a natural advantage of collision-free medium access.

However, it includes clock drift problems and decreased throughput at low traffic loads due to idle slots. The difficulty with TDMA systems are the synchronization of the nodes and adaptation to topology changes where these changes are caused by insertion of new nodes, exhaustion of battery capacities, broken links because of interference, sleep schedules of relay nodes, scheduling caused by clustering algorithms. The slot assignments, therefore, should be done regarding such possibilities.

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In pursuit of low computational cost requirements of wireless CDMA sensor networks, there has been limited effort to investigate source and modulation schemes, particular signature waveforms, designing simple receiver models, and other signal synchronization problems. If it is shown that the high computational complexity of CDMA could be traded with its collision avoidance feature, CDMA protocols could also be considered as candidate solutions for sensor networks. Lack of comparison of TDMA, CSMA or other medium access protocols in a common framework is a crucial deficiency of the literature. Common wireless networking experience also suggests that link-level performance alone may provide misleading conclusions about the system performance. Similar conclusion can be drawn for upper layers as well. Hence, the more layers contributing to the decision, the more efficient the system can be. For instance, the routing path could be chosen depending on the collision information from the medium access layer. Moreover, layering of the network protocols creates overheads for each layer which causes more energy consumption for each packet. Therefore, integration of the layers is also a promising research area which has to be studied more extensively.

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