# Design and Analysis of a Low Latency Deterministic Network MAC for Wireless Sensor Networks
## 1 Introduction  
The IEEE 802.15.4 [1] standard is popular for short-range and low-power wireless networks, which supports sixteen channels in 2.4-GHz and ten channels in 902-/928-MHz Industrial, Scientific and Medical (ISM) band. It specifies the physical and medium access control (MAC) layer and defines the format of the data handling. ZigBee is an enhancement to the IEEE 802.15.4 that supports the network to application layer. Integration of ZigBee with IEEE 802.15.4 is popular because of its low power consumption, low bandwidth, low cost and ease to implement. It is expected that these ZigBee sensors will play a major role in different applications of IoT. However, the critical requirements in the industrial/commercial IoT applications such as high reliability and low energy consumption in the industrial environment have not been addressed adequately in the IEEE 802.15.4 standard. The IEEE 802.15.4 standard defines the slotted and unslotted channel access mechanism through carrier sense multiple access with collision avoidance (CSMA/CA) for communication. To start a transmission, each IEEE 802.15.4-enabled node first performs clear channel assessment (CCA) to ensure that the channel is idle. The IEEE 802.15.4e [2] working group has redesigned the existing IEEE 802.15.4 MAC protocol to overcome the limitations such as low latency and robustness to meet the critical requirements of IoT applications. They define a low-power multi-hop MAC protocol, which is capable of addressing the emerging needs of different IoT applications. The final standard of the IEEE 802.15.4e MAC enhancement protocol adopts the ideas like slotted access, multi-channel communications and frequency hopping from WirelessHART [3] and International Society of Automation (ISA) 100.11.a [4]. IEEE 802.15.4e supports low latency deterministic network (LLDN) MAC, which can be applied to different applications like fall detection, factory automation, robots, automated dispensations, airport logistic and many emergent automated applications. The major requirements of such applications are to reduce the delay and energy consumption. It is to be noted that the sensing data of applications are typically of a few bytes, which make the time slot size very small. As a result, the sensing data from nodes can be accommodated in one superframe due to the small slot size. Allocating a dedicated time slot for each LLDN node in the superframe provides a deterministic system. Due to the strict requirements of such low-latency applications, LLDN uses a star topology with a minimal number of superframe structures. The star topology has a special node called the coordinator that manages communications among the nodes in a personal area network (PAN). Various applications of wireless sensor networks (WSN) require low latency data transmissions. Therefore, if any sensor node fails to transmit its packet, a retransmission for the failed packet should be arranged within the current superframe to meet the low latency requirement. All retransmissions within an LLDN superframe are possible whenever the numbers of failed packet are less than or equal to the available retransmission slots. However, there is a problem that the numbers of failed packets are greater than the available retransmission slots. The PAN coordinator fixes the number of retransmission slots for a superframe and cannot be changed dynamically. Note that there is no provision of retransmissions in IEEE 802.15.4e LLDN in the case of the non-availability of retransmission slots, and as a result, the performance of the network in terms of delay, throughput and packet drop rate is degraded. Two types of slot allocations (dedicated or shared) are made for nodes attached to the coordinator in the LLDN superframe. In the case of shared slots, when a slot is not used by the slot owner, it can be used by other nodes associated with the slot through the CSMA/CA mechanism.