# A Review on Mixed Reality: Current Trends, Challenges and Prospects
## 4 Architecture of MR Systems
### Architecture Issues  
Different architectural structures have been used in previous studies for MR applications. Ref. [85] proposed a client-server architecture for collaborative design environments. This architecture can support multiple users, who can simultaneously edit features in a 3D environment. Ref. [86] proposed the client-server architecture for MR mobile platforms, which comprises four features: the MR server, stream server, client, and the thin client. In this architecture, after the content is received by the client, the mediator is responsible for managing and updating the system. In addition, the stream server provides rendering capabilities along with user tracking, and the client provides the user with the rendering service, which changes based on the stream server. Ref. [87] proposed a resource-oriented architecture to create a restful web service platform. This platform can support geospatial multimedia, such as 3D objects, point cloud, and street panorama, as well as a social network to support mobile clients. Ref. [88] suggested a cloud client-server architecture, which is multimodal and supports various tools and virtual models in a collaborative environment. In this architecture, the servers \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline \hline
**Security and Privacy Approach** & **Approaches** \\
**Categories for MR Application** & \\ \hline \multirow{3}{*}{Input protection approach} & Darkly, Context-based sanitization, PlaceAvoider, 3D humanoids replace humans OpenFace/RTFace, Capture-resistant spaces, See-through vision, World-driven I-pic, Privacy Camera, Cardea, Marklt, PrePose, Recognizers \\ \hline \multirow{3}{*}{Data protection approach} & Rappor, SemaDroid, HE-sift, Leveled HE-sift, CryptoImg, SecSift, P3, Cloth try-on, PDV, OpenPDS, DataBox \\ \hline \multirow{3}{*}{Output protection approach} & Arya, SurroundWeb, EyeGuide, VR Codes, Psycho-visual, Visual cryptography using AR Secret messages using AR, Mobile visual cryptography \\ \hline \multirow{2}{*}{User interaction protection} & [PERSON], Kinect Conferences, SecSpace, Candid Interactions, Pip, TouchSpace, BragFish, Looks Good To me, HoloPair \\ \hline \multirow{3}{*}{Device protection} & Seamless and secure VR, Mid-air authentication gestures, Head and blinking gestures, HeadBanger, PSKA, SkullConduct, Facial multi-modal authentication, GazeTouchPass, Polarization, Taps Widget, Chameleon-like Eye Decrypt, Preventing keystroke inference \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Security and privacy approach for MR [6]. provide consistency between different clients to simultaneously provide access to visualization or editing virtual models. Ref. [89] proposed a service-oriented (SOA) web architecture that provides a runtime to support distributed services. This architecture contains a lookup discovery mechanism to provide healthcare services on the web platform. Ref. [90] proposed an innovative architecture that was an atomic model between peer-to-peer and the client-server to create a distributed MR. This architecture manages data collection from a sensor installed in real space and manages the resources and communication of the sensor node using an agent-based structure. In this architecture, the GUI agent displays the interface, the sensor agent collects data, and the rendering agent shares the scene with the MR device. The behavior agent tools request new data. Such an architecture plays an important role in using the system for a dynamic environment. Ref. [91] proposed an architecture by which to use MR on the Internet of Things (IoT), based on the integration of OneM2M as a standard architecture for IoT platforms and three different use cases for the MR device, i.e., sensor data visualization, actuator control, and MR-sensor data transmission. This architecture combines Mobius as one M2M platform for IoT and Hololens as an MR device. Ref.