Multi-screen interactive / wireless projection technology -Miracast Version 2 modification points

Miracast/WIDI (Wi-Fi Display Technical Specification), one of the standard protocols for multi-screen interactive/wireless projection, released version 2.0 (Release 2 final version) on 2017/4/21, and released 2.1 on 2017/7/31. version

This revision has added a lot of content, this article will sort it out.

Change 1: Added a new way of discovering.

The original receiving end is looking for the receiving end to be discovered through WIFI-P2P. The Version 2 version adds the mDNS discovery method, and the mDNS bottom layer needs the BSS (AP)/IP network support. The mDNS has applications in many fields, among which Airplay discovers. The process is the mDNS used.

The mechanism of mDNS is not detailed here. Interested students can visit https://blog.csdn.net/wirelessdisplay/article/details/78088992

Change 2: Connection establishment process

In version 1, the establishment of RTSP TCP is established by the SINK side, and in version 2, if the mDNS/IP mode is adopted, the establishment of the TCP is initiated by the source, and the TCP SYN packet is sent to the SINK, and the negotiation is performed. RTSP TCP link mode, then the SINK side initiates RTSP TCP link

Change 3: Media transmission method

In version 2, RTP carrying audio and video can be transmitted over TCP and UDP, and can be switched at any time, and for the transmission quality of audio and video, several RTSP signalings are specially added to adjust the buffer buffer, which are:

Wfd2-transport-switch/wfd2-buffer-length/wfd2-audio-playback-status/wfd2-video-playback-status

Change 4: Media Negotiation

In version 2, a lot of media formats have been added, and all negotiation process changes are more complicated.

Change 5: Add video format

In version 1, the video size supports 1080P/H264, and in version 2, the support is up to 4Kp60, and the format is increased by H265.

Change 6: Increase the auxiliary stream

There is no auxiliary stream option in version 1, and the auxiliary stream format mainly includes PNG/JPEG/H264.

Change 7: Add audio format

Add CTA Audio/AAC-ELDv2 format

Change 8: Add Commands

Added a lot of commands, one of which is wfd2-direct-streaming-mode is more interesting, he is sent from the source to the sink, used to indicate whether the mainstream is active. This command has not been measured, the actual effect does not know how

In general, the main purpose of the modification of the new version of Miracast/WIDI is to increase the quality of audio and video transmission. The increase of mDNS and TCP mainly draws on Airplay. The performance of P2P is too dependent on the quality of the network card, which leads to poor P2P transmission quality in some scenarios.

The flow control aspect command is also added, and the transmission code stream is adjusted when the network condition is poor.

Of course, it is more important to support H265. At present, under the same quality, H265 is the only means to significantly reduce the code stream.