# A Novel IEEE 802.15.4e DSME MAC for Wireless Sensor Networks
## 1 Introduction
### Motivation  
In a WSN, normally nodes are deployed densely around the coordinator and several nodes may compete with each other to transmit data simultaneously. Though, IEEE 802.15.4e has its own collision avoidance mechanism, there is no provision to reduce the probability of collisions by restricting the number of devices to be associated with the coordinator at the same time. Hence, it is essential to restrict large numbers of devices to be associated with the coordinator or none of devices can get benefits to transmit data. In IEEE 802.15.4e medium access mechanism, a node has to go for clear channel assessment (CCA) after its random backoff period is over. It senses the channel busy if any other node within its transmission range is transmitting either data or ACK and therefore it increases the delay and consume more power due to another longer backoff. It is necessary to devise some mechanism by which unnecessary backoffs can be avoided. Hence, we design a new carrier sensing mechanism by which a node can avoid sensing the channel busy situation when the exchange of ACK is going on by other node.  
The group ACK option given in DSME MAC of IEEE 802.15.4e provides a retransmission opportunity to the nodes within the same superframe. Accordingly, an additional DSME GTS is allocated for Retransmission (GTSR) to the coordinator. However, these allocated slots become useless, if a device is succeeded to transmit data in its first attempt and has no new data to transmit. Therefore, we design here one dynamic GTS allocation scheme by which the retransmission delay can be reduced and the network throughput can be improved. In the beacon enabled IEEE 802.15.4e network, devices first discover the coordinator by scanning the channels. The easiest way for coordinator discovery is to scan all channels sequentially with maximum beacon interval, which is approximately 250 s. Hence, discovery of a beacon by scanning 16 channels would take an average of 33 min (250 s \(\times\) 16/2). In order to mitigate this delay, we design a novel MAC mechanism for the IEEE 802.15.4e enabled WSNs and analyze its performance. The main contributions of our work can be summarized as follows.  
* A new contention channel access scheme is designed for wireless sensors to enhance the throughput, reliability and to reduce the power consumption.
* A new dynamic GTS allocation scheme is designed to reduce the delay for the devices who have failed to transmit data.
* A new beaconing scheme is designed to reduce the association delay for orphan mobile devices.
* A Markov chain model is designed for the performance analysis of the system in terms of delay, throughput, reliability and energy consumption due to data transmission.  
Rest of the paper is organized as follows. Related works of the existing IEEE 802.15.4e standard are given in Section 2. Network model and proposed methods are presented in Section 3. Analytical models are given in Section 4. Performance analysis of different network parameters based on our models are given in Section 5. Simulation results are presented in Section 6 and concluding remarks are made in Section 7.