## 2 Architecture and Protocol
### Protocol Design  
The VRCL should encapsulate interpolation and frame rate information, using a VR video frame in an aggregated MAC protocol data unit (A-MPDU). The VRCL would control the UL and DL frame arrival rate for WLAN systems based on the frame rate of the original VR video rate and the wireless environment. Owing to VRCL, a VR AP and VR devices could utilize IEEE 802.11 family standards  
Figure 4: Distributed coordination function (DCF) and the proposed delay-oriented VR mode operation for wireless VR system. (**a**) Wireless multiuser VR with conventional wireless local area network (WLAN); (**b**) Wireless multiuser VR with the proposed delay oriented VR mode WLAN. that provide MAC and PHY layers without standard modification. VRCL is the only additional layer for a multi-user VR network interface. VR applications do not require any additional network control features, and are required only to generate proper VR video frames, based on information delivered by the VRCL. The VRCL provides its UL control information rates and the required DL video frame to the VR application layer. Because the VRCL would discard VR frames that could cause large network delays on VR devices, the VR application does not need to waste its resources on unnecessary video frames. For this reason, the VRCL and VR AP need to perform the function of a VR system controller. Not only does the VRCL provide information that can be used to control VR video frames to the VR application layer, but VRCL also controls its network operations based on information that is provided from the SME. The packet loss rate, received signal strength indication (RSSI), and modulation and coding scheme (MCS) index information are the representative information observed by VRCL. Based on the observed information, VRCL could control the MCS level, channel bandwidth, frame arrival rate, number of users supported by the VR AP, and so on. The 60-GHz and 5-GHz standards may be utilized by VRCL, because of its efficient multi-user VR operation. The main purpose of the 60-GHz wireless link, in this paper, is the delivery of a high-resolution VR video frame. Because that delivery requires a large bandwidth, the use of the 60-GHz standard is inevitable. A 5-GHz standard is employed to accommodate the multi-user UL frame, by utilizing the multi-user UL OFDMA procedure. Although the required data rate of multi-user uplink sensing data is not very high, the frame arrival rate of the sensing data frames is relatively high. Usually, the video frame rate ranges from 90 Hz to 120 Hz, but the motion sensing rate is 1000 Hz in current-generation, wired VR devices. Because of this, a small-size UL data frame could spoil the overall wireless VR system, in spite of its small required data rate. This causes a large channel access delay on the DL VR video frame, unless the DL and UL are separated. By separating the UL sensing data transmission from the 60-GHz standard, DL VR video frames do not contend with other frames from VR devices. If a VR video frame requires additional transmission time, owing to its poor channel condition or the increasing number of VR users, the VRCL can control the video frame rate based on its packet loss rate. The VRCL in a VR AP encapsulates the video frame rate information that is used to generate VR video frames. The VRCL in VR devices extracts the information from the received frames and delivers the information to the VR application, in order to generate a video interpolation frame based on its received video frames and motion-sensing information. Unless the channel has high coexistence issues, the DL frame may not have large channel access interference. However, in the WLAN multi-user VR scenario, UL frames always suffer large contention delays, owing to frequent channel access with the contention-based channel access method. In order to solve the problem, a VR AP should support the UL frame accommodation by transmitting a trigger frame, which is defined in the IEEE 802.11 ax standard.