  This tutorial paper analyzes the traffic characteristics of immersive
experiences with extended reality (XR) technologies, including Augmented
reality (AR), virtual reality (VR), and mixed reality (MR). The current trend
in XR applications is to offload the computation and rendering to an external
server and use wireless communications between the XR head-mounted display
(HMD) and the access points. This paradigm becomes essential owing to (1) its
high flexibility (in terms of user mobility) compared to remote rendering
through a wired connection, and (2) the high computing power available on the
server compared to local rendering (on HMD). The requirements to facilitate a
pleasant XR experience are analyzed in three aspects: capacity (throughput),
latency, and reliability. For capacity, two VR experiences are analyzed: a
human eye-like experience and an experience with the Oculus Quest 2 HMD. For
latency, the key components of the motion-to-photon (MTP) delay are discussed.
For reliability, the maximum packet loss rate (or the minimum packet delivery
rate) is studied for different XR scenarios. Specifically, the paper reviews
optimization techniques that were proposed to reduce the latency, conserve the
bandwidth, extend the scalability, and/or increase the reliability to satisfy
the stringent requirements of the emerging XR applications.
