## 4 Multiple Edge Computing Platforms Enabled Air-Ground Network Architecture for Vehicular Scenarios
Jointly both TN and NTN based EC platforms serve VUs with their available computation and communication resources. As shown in Figure 6, the considered network infrastructure can be split into several layers of ground-based and areal networking infrastructures. The TN is constituted by several connected VUs in Layer 1 which can form a small vehicular cloud, RSUs equipped  
Figure 5: Multiple EC enabled VN. with the EC servers in Layer 2, and multiple BS with EC facilities in Layer 3. The NTN has two layers of LAPs and HAPs belonging to the areal networks. LAP nodes are located at a relatively low distance from the ground compared with the HAPs and have limited computation and communication resources. Though HAPs are at a long distance from the VUs layer they can serve large coverage areas. Below we describe each of these networking layers in detail. **Layer 1--**Connected VUs layer  
Several connected VUs having communication and computation capabilities are grouped into Layer 1. Each VU can communicate with its neighboring VUs for possible information sharing through V2V communication technologies and use V2I links for interacting with other EC layers. VUs can communicate over a limited distance to share important information for enabling several safety-related, traffic flow management services that make drivers life easy on the road. Through V2I communication, VUs can share their workloads towards EC servers in proximity for enabling latency-critical and data-intensive applications. VUs often generate different tasks requests with specific requirements of computation, communication, and storage resources. These task requests often come with additional requirements (i.e., critical latency requirements), for which VUs often need assistance from the EC platforms in the proximity. **Layer 2--**RSU-Edge Computing (RSU-EC) Layer  
For enabling the EC facilities into VNs, several RSUs have been deployed alongside road infrastructures. RSUs can have communication technologies installed for communicating with VUs and other higher lever networking layers (i.e., cellular BSs, cloud infrastructures, etc.). EC servers having limited computation and storage resources can be deployed alongside RSUs to integrate the EC services into VNs. Thus, RSUs equipped with EC servers and communication technologies constitute an EC layer in the proximity of VUs for providing low latency services. However, with their limited resources and coverage, RSUs can serve a limited number of VUs. Furthermore, VUs mobility often restricts them from accessing RSU services for longer periods of time. **Layer 3--**5G Base Station (BS) Layer  
5G base stations (5G-gNB) have integrated modern communication and computation technologies that can be exploited as an EC platform. With larger coverage areas, they can serve a higher number of VUs. However, the BS-EC platform can have additional transmission delays due to longer communication distance compared with RSUs. Figure 6: The Multi-EC framework for the vehicular scenario. Layer 4 Low Altitude Platform (LAP) Layer  
LAP platforms such as UAVs equipped with EC servers can add several benefits in terms of a reduced transmission time (less than 1 msec), reduced deployment and maintenance time, line of sight communications with better channel quality, etc. They can also act as a relay node for allowing VUs to transmit their information towards higher air networking layers such as HAP with reduced latency and energy costs. With the limited size and reduced flight times, UAVs can only have limited communication and computation resources onboard. Layer 5 High Altitude Platform (HAP) Layer  
A HAP network constituted by several nodes able to communicate amongst themselves and other infrastructures is forming another EC layer in the air network. Each HAP node can have a powerful EC server and the communication resources for serving VUs under its coverage. Better coverage, additional renewable energy sources, better stability are some of the HAPs main advantages over LAPs.