# A Delay-Aware and Reliable Data Aggregation for Cyber-Physical Sensing
## 1 Introduction  
Sensing is one of the essential components of a cyber-physical system (CPS). Different types of sensors are being deployed to collect a large amount of sensing data about the environment (temperature, humidity), transportation, terminals, and even social activities. Wireless sensor networks (WSNs) are an essential way to realize CPS sensing. Wireless sensor networks have been widely recognized as a ubiquitous and general approach with extensive applications in agriculture, industry, and unattended environments, such as habitat monitoring, surveillance and tracking for the military. Wireless sensor networks can provide industrial, agricultural, medical and vehicular Internet of Things or CPS service. They also make smart factories, e-healthcare and intelligent transportation real. Wireless sensor networks consist of a large number of low-cost and low-power sensor nodes that obtain information of physical objects, which can collaborate with each other to collect data and disseminate it to sinks (or gateway nodes) through an established routing path [1; 2]. Due to the fact they provide fine-grained spatial-temporal sensing, communication and computation at a low premium of cost and power, wireless sensor networks are a way to support CPS. In the paper, we focus on the surface information collection of spherical objects in CPS such as machinery, buildings (where sensor nodes are deployed on a TV tower to collect information as shown in Figure 1), etc. In the data collection procedure, time is slotted. In order to avoid transmission interference, nodes are assigned different time slots to complete data packet processingand transmission. Nodes without interference could transmit simultaneously. Besides, multi-hop communication is used to relay the information from the source node to the sink for further data management or operation. In order to achieve significant performance improvements in energy consumption, memory usage, bandwidth, and delay, distributed in-network aggregation is applied to improve the communication efficiency of the system [3,4]. Meanwhile, in wireless communication, due to the complexity of the transmission channel and the harsh environment, reliable communications are essential for most WSNs applications because of the unreliable links [5, 6, 7, 8, 9, 10]. Although many applications require that each data packet be successfully delivered to sink with statistical probability \(\delta<1\), such as 60%-95%, some reports reveal that the wireless link packet loss rate may be up to 70% in real WSNs, which is far from being satisfactory [11, 12, 13]. Thus, it is more important to ensure reliability in communication. Moreover, in WSNs, delays also play an important role [14]. For data collection, delay is generally defined as the sensed data transmission time from source nodes to sink, named transport delay or end-to-end (E2E) delay [15]. For information processes, being beyond of the time limit may cause information loss or affect the next series of data processing. Besides, in many safety-critical industry applications, missing urgent information may cause severe property loss and casualties, which are often not acceptable. Therefore, in order to rapidly respond to the event and satisfy an acceptable delay, the detected information should be transported quickly to the sink for further manipulation. Consequently, the delay for data collection and data transmission reliability should be considered in data collection of CPS. To address these issues, a novel delay-aware and reliable data aggregation scheme based on tiered structure routing, named Tiered Structure Routing-based Delay-aware and Reliable Data Aggregation (TSR-DARDA), is proposed in this paper for spherical physical objects. TSR-DARDA takes both data reliability and transport delays into consideration. Our main contributions can be summarized as follows:  
1. A novel TSR-DARDA scheme is proposed to achieve delay performance subject to reliability constraint for data collection of spherical physical objects.