## 1 Introduction  
Various applications of Wireless Sensor Network (WSN) in the IoT environment generate heterogeneous data which require differentiated service in terms of bandwidth, latency and reliability. WSNs comprise low power IoT devices which provide the essential monitoring functionality to various emerging applications [1; 2; 3; 4]. Examples of such applications include healthcare [5], industrial automation and control [3], smart homes [1], smart cities [6], tracking victims and disaster management [7]. In most of these scenarios, the periodic monitoring data could be of low priority and can deal with jitter and long delay; however, there also exists high priority data such as alarms, events or commands [8], which need to be transmitted immediately. Therefore, the non-delay-tolerant traffic should be prioritized, due to its need for urgent channel access [9; 10]. Typical WSNs were developed assuming that these networks would mostly be used for periodic monitoring applications and no strict time-constraints would be involved. However, the IoT applications which integrate WSNs have emerged as the game-changer. In time critical IoT applicationssuch as healthcare [11], smart homes [1], and vehicle monitoring [12], even a delay on order of milliseconds could result in an accident or disaster. In this regard, a range of priority-based MAC protocols has been proposed by previous researchers which include EARS [13], TAP-MAC [5], WA-MAC [14], Urg-MAC [15], CoR-MAC [16], TraPy-MAC [17], Duo-MAC [18], WirArb [19], L-CSMA [20], MEM-MAC [9], PA-MAC [21] and PriorityMAC [22]. However, none of these schemes allow the high priority data to transmit in case the channel is occupied by low priority or less urgent data. In case a priority scheme or a WSN to deal with heterogeneous traffic is not implemented efficiently, the high priority real time data may starve for access due to the continuous transmission of long low priority packets. This implies the importance of pre-emptive priority, which refers to interrupting ongoing transmissions for ensuring quick channel access for high priority traffic. It is to be noted that most of the previous authors such as [13; 16; 23] have claimed to assign preemptive priority to the high priority traffic only by providing them a chance of transmission by holding the lower priority packets in the queue; however, no mechanism has been developed to interrupt the ongoing transmission of lower priority traffic. Clearly, in the existing protocols, the high priority traffic must wait for the channel to become free before it could begin transmission. In this paper, we propose a simple yet novel fragmentation scheme (FROG-MAC) for transmission of heterogeneous data in WSNs; we define heterogeneous traffic as traffic which has different arrival rates. Furthermore, we assign different priorities to each traffic type. It is proposed to transmit the low priority data by splitting the packets into multiple fragments. The medium would be idle between the transmission of fragments for a certain duration (termed as 'interruptible period'). This interruptible period could be used by the high priority traffic to begin its transmission. In this work, we present the high-level design of MAC protocol FROG-MAC and validate the performance improvement brought by the suggested scheme using MATLAB simulations. The major contributions of this work are as follows:  
1. We propose a high-level design of fragmentation scheme (FROG-MAC) for heterogeneous traffic in WSNs which allows interruption of ongoing transmissions by high priority traffic. 2. We develop a high-level MATLAB implementation of the proposed scheme and validate the advantages in terms of delay and PDR for the high priority traffic.