# A New Intra-Cluster Scheduling Scheme for Real-Time Flows in Wireless Sensor Networks
## 1 Introduction
Therefore, data must reach the monitoring stations before the expiration of the deadline [17]. In real-time wireless sensor networks, the media access control (MAC) layer plays an important role in determining the channel access delay. The techniques to handle the channel access in WSN are mainly divided into contention-based and TDMA-based schemes. In a contention-based MAC--due to the distributed and random back-off nature--it is difficult to provide deterministic channel access guarantees, resulting in unpredictable delays which are inappropriate for real-time system. Compared to contention-based schemes, TDMA-based MAC schemes provide a deterministic medium access delay through time slot scheduling. Therefore, many research works focus on the TDMA-based MAC approach for real-time communications in WSNs [18; 19]. In WSNs, real-time communication is more difficult to achieve due to the natural property of node interference in broadcast wireless communication. The communication between pair of nodes is affected by other transmitting nodes within the interference, thus, interference-aware scheme becomes important in WSN. Especially, when it comes to guaranteeing real-time delivery, therefore contention-free scheme such as time division multiple access (TDMA) is preferred due to deterministic operation [20; 21]. In adaptive DRAND [22] the scheduler assign slot to node across 2-hop neighborhood to avoid the interference while in [23] assign colors to node different from 2-hop neighbor node. The mentioned scheme assigns slots to nodes across 2-hop or 3-hop away to avoid the interference. That consider that if the time slots are not used in 2-hop or 3-hop neighborhood then that time slots can be reused. However, these schemes do not consider the concurrent transmission of nodes in the network. This is due to cumulative nature of interference, where trans- missions of other nodes that are n-hop away can interference with a transmission as they transmit simultaneously. Real-time communication in WSNs have been addressed previously in [24; 25; 26; 27; 28]. Real-time query services are scheduled in [16] where data are routed from many-to-one communication by avoiding interference using the conflict graph. In [24], under the Wireless Hart model, real-time packets are scheduled by avoiding the interference using multiple channels, therefore allowing one transmission in each channel in a time slot. In [25], a conflict-free real-time scheduling algorithm for periodic real-timeflows is provided with consideration of interference in the network. Furthermore, cluster-based real-time communication scheme characterized interference by using graph theory, while graph coloring is used to avoid the collision. Similarly, in [26], three cluster-based TDMA scheduling algorithms are proposed for real-time flows. The representative approach has been described to schedule real-time data; however, some schemes have centralized approach and therefore are not scalable. Cluster-based coloring schemes for real-time flow increase the schedule length and hence are not suitable for a real-time system. The cluster-based real-time flows are scheduled in three different time slots, namely IntraSend, InterComm and IntraRecv slot to avoid interference [27]. However, this scheme uses static scheduling and does not consider the incoming and outgoing flows in the intra-cluster scheduling therefore some flows cannot utilize the available empty time slots. Several TDMA scheduling algorithms have been proposed to transmit data from source to destination using multi-hop [25; 28; 29]. Two centralized, node-based and level-based TDMA scheduling algorithms are proposed in [25]. In both algorithms, base station assigns slots to nodes by considering interference relation. These algorithms attempt to find TDMA schedule that minimize the number of time slots. In [28; 29], conflict- free TDMA scheduling algorithms for multihop intra- and inter- cluster are provided.