# HCLE: Hybrid Clustering Approach for Extending WBAN Lifetime
## 2 Related Work
Despite the advantages mentioned earlier, some challenges of cross-layer design include the increased complexity of the protocol stack and conflicts between different layers due to the exchange of information. Examples of cross-layer routing protocols in WBANs include WASP (Wireless Autonomous Spanning Tree Protocol) [39], CICADA (Cascading Information of Controlling Access and Distributed Slot Assignment) [40], TICOSS (Time-Zone Coordinated Sleeping Scheme) [34], Biocomm (a communication protocol for biomedical sensor networks) [41], and AMR (Adaptive Multihop Tree-Based Routing) [42]. Postural routing protocols leverage the specific structure and posture of the human body, such as the relative positions and orientations of the sensors on the body, to improve the efficiency of data transmission [26; 27; 43; 44]. It uses this information to make routing decisions that minimize the energy consumption of the sensor nodes and improve the reliability of the network. For example, postural routing protocols can improve the network's overall reliability and energy efficiency by selecting routes that avoid interference caused by body movement or selecting nodes with stronger signals due to their position on the body. However, postural routing protocols may face some limitations, such as the need for accurate posture sensing and interference from body movement. Examples of a postural routing protocol in WBANs are Opportunistic (opportunistic routing based on posture prediction) [45], DVRPLC (Distance-Vector Routing Protocol with Postural Link Costs) [46], and OBSFR (On-body Store and Flood Routing Protocol) [14]. Quality of Service (QoS) is important in WBANs since some applications require real-time data transmission and reliable communication. QoS-aware routing protocols consider the QoS requirements of different applications, such as delay, reliability, and energy efficiency, and prioritize them for routing decisions accordingly [47]. For example, for applications that require low latency and high reliability, QoS-aware routing protocols can prioritize routes with low delay and high link quality. Conversely, for applications that are more tolerant to delay but require low energy consumption, QoS-aware routing protocols can prioritize routes with low energy consumption and choose one irrespective of how long it takes. QoS-aware routing protocols can improve overall performance by providing differentiated services to different types of traffic and by meeting the QoS requirements of each application. However, it may increase the complexity of the routing protocol and the QoS conflicts between different applications. Examples of QoS-aware routing protocols in WBANs include DMQoS (Data-Centric Multi-Objective QoS-based routing protocol) [48], QPRD (delay-sensitive data QoS-aware routing protocol) [49], EPR (Energy-Aware Peering routing protocol) [50], TMQoS (Thermal-Aware Multi-Constrained Intrabody QoS routing protocol) [51], and DARE (Distance-Aware Relaying Energy-Efficient System) [52]. The thermal-aware routing protocol uses the temperature information from sensor nodes to make routing decisions. It aims to balance the trade-off between energy and temperature control to ensure the reliable operation of the sensors. Temperature information is used to estimate energy consumption and predict the lifetime of sensor nodes [53; 54]. Thermal-aware routing protocols can reduce energy consumption and prevent sensor nodes from overheating by avoiding routes that pass through nodes with high temperatures. Furthermore, these protocols can increase the lifetime of the sensor nodes by distributing the traffic load across the network. However, there are some challenges, such as the need for accurate temperature sensing and the complexity of the routing algorithm.