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MAC Protocol Used in Wireless Sensor Networks

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Pre-requisites: Classification of MAC Protocols

In Wireless Sensor Networks (WSNs), the Medium Access Control (MAC) protocol is a set of guidelines that dictate how each node should transmit data over the shared wireless medium. The primary objective of the MAC protocol is to minimize the occurrence of idle listening, over-hearing, and collisions of data packets. By efficiently managing access to the wireless medium, the MAC protocol helps to reduce energy consumption and optimize the use of network resources.

MAC Protocol Categories 

  • Contention based MAC
  • Scheduled based MAC
  • Hybrid MAC
  • Cross-Layer MAC

Contention-based MAC 

Contention-based MAC protocol is also known as a random access MAC protocol. It allows all nodes to transmit data on the shared medium, but they have to compete with each other to access the medium. One example of contention-based MAC is CSMA/CA.

In CSMA/CA, each node senses the medium before transmitting the data. If the medium is idle, the node can transmit data immediately. However, if the channel is busy the node has to wait for a random time also known as back-off time. This back-off time reduces the chances of collisions.

Contention-based MAC Used in Wireless Sensor Networks

Sensor MAC (SMAC) is a contention-based MAC protocol that is specifically designed for wireless sensor networks. The primary objective of SMAC is to minimize idle listening, over-hearing, and collisions of data packets. To achieve this goal, SMAC adopts a duty-cycle approach, also known as a sleep-wakeup cycle. In this approach, each node alternates between a fixed length of active and sleeping periods based on its schedule.

To prevent collisions among packets, SMAC utilizes the Request to Send (RTS) and Clear to Send (CTS) packets before transmitting data packets. This helps to ensure that only one node is transmitting data at a time, reducing the likelihood of collisions and improving overall network efficiency.

Scheduled-based MAC 

Scheduled-based MAC is also known as a deterministic MAC protocol. Where each node follows a predetermined schedule and transmits the data according to its given time slot. The data collision is completely nullified in scheduled-based MAC. An example of Scheduled based MAC is TDMA(Time Division Multiple Access).

In TDMA the time is divided into fixed slots and each node is allocated a specific time frame in which they can transmit the data. During this time slot, other nodes remain silent.

Scheduled-based MAC Used in Wireless Sensor Networks 

LEACH (Low Energy Adaptive Clustering Hierarchy) is a TDMA-based protocol that utilizes a clustering mechanism in wireless sensor networks. A cluster comprises sensor nodes grouped together, with one node designated as the cluster head and the others serving as members. The cluster head is selected based on a probabilistic algorithm, which ensures that power consumption is evenly distributed among the nodes. 

Once the cluster is formed, a schedule is created for nodes to transmit data within the cluster. Additionally, to mitigate inter-cluster interference, each cluster head assigns a unique CDMA code to its cluster.

Hybrid MAC

Hybrid MAC is a combination of different protocols such as contention-based MAC and scheduled-based MAC to optimize the performance of wireless sensor networks. For example, contention-based MAC protocols, such as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), allow nodes to access the medium based on a random backoff interval, which reduces collisions but may result in inefficient utilization of the medium. On the other hand, scheduled-based MAC protocols, such as TDMA (Time Division Multiple Access), divide the medium into time slots and assign them to different nodes, which can achieve high utilization but may not be flexible enough to adapt to changing network conditions. Hybrid MAC solved the issue by using other MAC protocols, During transmission of data if the channel is idle or the channel has low traffic then Hybrid MAC switches to contention-based MAC. If the traffic in the channel increases then it is switched to scheduled-based MAC such as TDMA.

Hybrid MAC Used in Wireless Sensor Networks

The IEEE developed 802.15.4 as a standard for low-rate WPANs, which outlines the PHY and MAC layers for low-power wireless communication in the 2.4 GHz ISM band. It was specifically created for applications that require low data rates, low power consumption, and cost-effectivenesses, such as sensor networks, home automation, and industrial automation. 

The physical layer employs DSSS modulation with a data rate of 250 kbps and works in the 2.4 GHz ISM band that has 16 channels with 5 MHz bandwidth. Additionally, it uses FHSS to prevent interference from other wireless devices. 

On the other hand, the media access control layer implements a CSMA-CA protocol to avoid device collisions. It supports different data packet sizes, ranging from 9 to 127 bytes, and also offers error detection and correction mechanisms.

Cross-Layer MAC

Cross-layer MAC allows the different layers in the protocol stack, typically including physical, MAC, and network layers, to interact and share information with one another. Firstly MAC layers gather information about the state of the channel whether the channel is busy or not. This information will be further used to control the other parameters such as data transmission rate, packet loss rate, and delay.

Once the parameters have been determined, the MAC layer sends the data packets to the PHY layer for transmission over the wireless channel. After the data transmission, the PHY layer sends feedback to the MAC layer about the success or failure of the transmission. If the transmission was unsuccessful. Based on the feedback MAC layer repeats the transmission

Overall, the working of Cross-Layer MAC involves the interaction between the MAC and PHY layers to improve the efficiency of data transmission and energy consumption in WSNs. By optimizing the transmission parameters

Cross-Layer MAC Used in Wireless Sensor Networks

The IEEE 802.11e standard expands on the existing IEEE 802.11 WLAN standard by incorporating Quality of Service (QoS) support. It utilizes a cross-layer approach, allowing the MAC layer to collaborate with higher layers such as the network and application layers, to provide specific services based on the application’s needs.

On the other hand, IEEE 802.16, or WiMAX, is intended for broadband wireless access and utilizes a cross-layer design as well. This design allows the MAC layer to communicate with the physical layer to adjust to the changing channel conditions, such as interference, noise, and fading.


Last Updated : 28 Mar, 2023
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