# A Delay-Aware and Reliable Data Aggregation for Cyber-Physical Sensing
## 2 Related Work  
Sensing is a key component of cyber-physical sensing. Meanwhile, data collection is a key function of sensing. Various kinds of data sensed from cyber-physical objects including machines, equipment or vehicles need to be collected to gain valuable information for further processing and operation, such as machine control, failure prediction, and so forth. To support cyber-physical sensing, wireless sensor networks are frequently applied. Collecting the gathered information efficiently is a key issue for wireless sensor networks. A great deal of work has been devoted to this field [16; 17; 18; 19; 20; 21]. [PERSON] et al. [22] studied the time complexity, message complexity, and energy cost complexity of some data collection, data aggregation, and queries for a multi-hop wireless sensor network of \(n\) nodes. Network delay and reliable communication have an important influence on WSN applications and need to be considered in many WSN studies. There are many research efforts devoted to providing a reliable transmission service because of the unreliable links in WSNs [7; 8; 9; 23; 24; 25]. [PERSON] et al. [9] pointed out that there are mainly two categories, including packet-loss avoidance and packet-loss recovery. Packet-loss avoidance (e.g., [12]) attempts to reduce the occurrence of packet loss and packet-loss recovery (e.g., [25]) tries to recover from the packet loss when it happens. Because packet-loss avoidance methods need to pay a high price and from the cost consideration the most widely used mechanism in networks is based on packet-loss recovery.  
The tiered structure has appeared in the literature for Low Energy Adaptive Tier Clustering Hierarchy [26]. In the scheme, a two-level hierarchical approach has been proposed to organize a sensor network into a set of clusters and every cluster is divided into smaller clusters called mini clusters. As the way the clusters are organized, for each mini cluster a mini cluster-head is defined. The mini cluster-head communicates with the cluster-head directly, and it aggregates its mini-cluster information. [PERSON] et al. [3] developed a new network with tiered structure for in-network computation for a class of generalized maximum functions, which focuses on the delay performance of the function computation subject to reliability constraints in lossy wireless environments. They showed that aggregation using wireless broadcast technology can greatly reduce the delay while meeting the reliability requirement. [PERSON] et al. [27] suggested a novel variable width tiered structure routing (WWTSR) scheme, wherein the network is divided into circular tiers with different widths and each tier is further partitioned into cells. Those cells that do not interfere with each other could simultaneously finish data aggregation by unicast and retransmission within each cell.  
However, the proposed WTSR scheme is suitable for flat networks. Moreover, inspired by [28], which has proposed an energy-efficient and delay-aware wireless computing system to satisfy an acceptable delay and low power consumption for data collection, the main focus of the paper is to study the efficient data collection scheme taking reliability and delay into consideration for cyber-physical sensing with spherical networks.