# A Novel IEEE 802.15.4e DSME MAC for Wireless Sensor Networks
## 1 Introduction  
In Wireless Sensor Network (WSNs), devices are spatially distributed to monitor the physical or environmental conditions such as temperature, sound, pressure etc. Nowadays, WSNs are used in many industrial and consumer applications to monitor and control the industrial process, machine health, air pollution and many others. ZigBee devices are suitable to play major roles in different applications of Internet of Things (IoT). Large numbers of devices are deployed for different applications in IoT. Hence, many devices compete with each other to acquire the channel for data communications. IEEE 802.15.4 [1] is a popular standard for the medium access control (MAC) scheme of low-powered, low data rate wireless sensors. However, in order to meet the critical requirements such as low latency in industrial and commercial applications, IEEE 802.15.4e Working Group has redesigned the existing 802.15.4 MAC scheme. IEEE 802.15.4e [2] standard defines the slotted medium access control scheme for the short-range communication devices with low data rates. There are many MAC schemes for different applications defined by IEEE 802.15.4e such as Time Slotted Channel Hopping (TSCH), Low Latency Deterministic Networks (LLDN), Deterministic and Synchronous Multi channel Extension (DSME), Radio Frequency Identification blink (RFID), and Asynchronous Multi-Channel Adaptation (AMCA). Out of these, DSME mode proposed in IEEE 802.15.4e standard [2] is designed for the habitat monitoring, process automation, factory automation, smart metering, home automation, smart building, entertainment,patient monitoring and telemedicine applications. The MAC scheme of DSME superframe structure extends the number of GTS slots and the single channel operation of IEEE 802.15.4 standard to multichannel operation for data transmission. DSME superframe structure supports the star topology based WSNs, where collisions occur either due to hidden terminal problem or simultaneous channel assessment for data transmission. Hidden terminal problem can be avoided by allocating a common Contention Access Period (CAP) in the superframe to the devices present within the same communication range. CSMA/CA scheme avoids the collision during the transmission of other devices, which are not hidden. In WSN, large numbers of devices are deployed for different applications in which many devices attempt to send frames simultaneously resulting significant collisions in the system. The probability of collisions is related directly to the number of devices in the network and this issue is significant, when large number of devices are associated with one coordinator. To fulfill the present need of different applications, the coordinator needs to support more devices, which may result higher rate of collisions. The performance of CAP is deteriorated when large numbers of devices contend with each other. In IEEE 802.15.4e MAC, nodes perform slotted carrier sense multiple access with collision avoidance (CSMA/CA) mechanism to access the channel and uses a random backoff algorithm to reduce the collision probability. To save power consumption, the devices use power saving mode during the backoff duration and perform carrier sensing after the backoff duration is over. The channel access scheme suggested by IEEE 802.15.4e standard simply adopts a random backoff with clear channel assessment (CCA). The channel busy occurs either due to data or acknowledgement transmission. However, the current scheme proposed in IEEE 802.15.4e increases delay as well as energy consumption and reduces the throughput of the network due to inefficient backoff and carrier sensing management. Hence, this problem needs to be addressed to reduce unnecessary backoff and carrier sensing. The DSME MAC supports a group acknowledgement option to provide a retransmission opportunity within the same superframe for any data frame, which is failed in its DSME-GTS transmission.