The application of wireless sensor networks (WSNs) technology in monitoring systems is demanding more efficient services to fulfill the requirements of the monitoring task. For this purpose, the simultaneous presence of features such as different communication mediums (air and water) used by nodes and various sizes of data generated by heterogeneous nodes are the key obstacles to build a communication protocol, which can ensure the reliable data delivery. This work terms such WSNs as mixed wireless sensor networks (MWSNs) which contains the aforementioned features. In this paper, we introduce a new cross-layer protocol for mixed wireless sensor network (XMSN) which can adapt these features. The proposed cross layer protocol XMSN for such mixed environment is implemented and analyzed extensively in Castalia simulator. The performance of XMSN is compared with composition of well-known protocols, namely, CTP plus BoX-MAC-2. The result shows that XMSN has better efficiency in terms of end-to-end delay, energy consumption, and goodput than that of CTP plus BoX-MAC-2 protocol. KEYWORDS cross-layer, delay, energy consumption, mixed wireless sensor network (MWSN), sensor Int J Commun Syst. 2019;32:e3946. wileyonlinelibrary.com/journal/dacGenerally, the energy consumption rate for sensors and end-to-end delay vary greatly based on the protocols used in the different layers like media access control (MAC) and routing. MAC layer directly accesses the physical layer and controls the radio; and network layer finds an energy-efficient path by selecting the suitable parent after knowing neighbor nodes' network performance. Thus, energy-efficient protocols envisioned to have longer network lifetime are heavily dependent on MAC layer and network layer. Most traditional MAC 7-11 and routing protocols 12-16 for WSN are based on the single layer approach. The existing MAC and routing protocols provide a certain level of efficiency in terms of energy, throughput, and end-to-end delay. However, in such individual layer approaches, overall optimization is not possible. On the other hand, cross-layer protocols provide better energy efficiency and network performance in terms of high throughput, low latency, and better quality of service (QoS) support. Since, cross-layer protocol breaks the virtual boundary between (or among) two (or more) layers by exchanging information, 17 it becomes easier to manage the resources like energy and bandwidth. The network performance of future WSN-based applications will be enhanced by cross-layer design between radio, MAC, network, and higher layer protocols. In this section, we summarize some existing works on cross-layer protocols for WSNs from the perspectives of performance metrics like energy efficiency, delay, and QoS.