## 1 Introduction  
Internet of Things (IoT) is an emerging technology that has been highly attracted due to its wide utility in diverse applications since last decade. IoT is used to connect physical objects with the Internet and has plenty of applications for sensor data monitoring and information analytic [1; 2; 3]. In critical scenarios such as natural disasters like floods, earthquakes, and tsunamis, etc., the consequences are catastrophic and need some reliable, timely and coordinated response to overcome the damage. In this regard, the IEEE 802.15.4 based IoT plays a vital role to provide the connectivity among IoT nodes [4; 5; 6; 7]. The above mentioned critical applications demand better throughput, reduced power consumption, and guaranteed data delivery with permitted latency [8]. Physical and Medium Access Control (MAC) layers protocols in addition to routing protocols are designed to meet critical Quality of Service (QoS) requirements of IoT applications [9; 10]. IoT is a key part of future smart cities and Wireless Sensor Networks (WNSs) are its core components, which are deployed in different applications to create a Wireless Personal Area Network(WPAN). To facilitate the transmissions from various IoT nodes, it is vital to design an efficient MAC protocol. Various MAC protocols traditionally used for WSNs such as LoRaWAN [11; 12], and Symphony Link [13] can be used for IoT applications but they suffer from long delay in the presence of adaptive data traffic. Energy is one of the major constraint in WSNs. Institute of Electrical and Electronics Engineering (IEEE) has developed 802.15.4 standard for low data rate and low power Wireless Personal Area Networks (WPAN). IEEE 802.15.4 standard offers a very low duty cycle even less than 0.1% and is highly attracted by such devices having low power constraints such as WSNs [14]. IEEE 802.15.4 operates at three different frequency bands such as 868 MHz, 915 MHz, and 2.4 GHz. The standard works either in a _Beacon_ enabled or _Non-Beacon_ enabled mode. The Beacon enabled mode is divided into two main sections, _active_ and _inactive_ period, as shown in Figure 1. All WSN nodes communicate during an active period and remain in sleep mode during the later inactive period to conserve energy. The active period of Beacon enabled mode consists of Contention Access Period (CAP) and optional Contention Free Period (CFP). Each Superframe in this mode is divided into 16 equal duration time slots. One or more slots are reserved for the Beacon frame because its size may vary due to several remaining data frames for the associated nodes. The Beacon frame is generated by the PAN coordinator and contains information about frame structure, next Beacon, network, and sending messages. The CAP consists of maximum of 16 or minimum of 9 slots. In CAP, nodes contend to access medium by following the slotted CSMA/CA mechanism. On the other hand, the maximum number of slots in CFP can be up to 7 and are known as Guaranteed Time Slots (GTS). Nodes having critical data requests are allocated Guaranteed Time Slot (GTS) by the coordinator. The nodes that are allocated GTS can explicitly carry out communication during their allocated period to the PAN coordinator. However, this standard has some limitations for GTS allocations. 1. The cumulative delay from GTS allocation till transferring of data causes a significant delay, which is appropriate for time-sensitive WSN applications. 2. Due to a limited number of CAP time slots maximum of 7 nodes can be allocated GTS. These constraints along with the performance of the standard are evaluated and analyzed by many researchers in different prospects of application scenarios. CAP performance of the superframe structure of the standard is evaluated in different prospects on all frequency bands [15; 16]. CFP performance of the standard is also analyzed and some new scenarios are proposed to improve the performance of the standard in different prospects.