Miracast
Miracast是由Wi-Fi聯盟於2012年所制定，以Wi-Fi直連（Wi-Fi Direct）為基礎的無線顯示標準。支援此標準的3C裝置可透過無線方式分享視訊畫面，例如手機可透過Miracast將影片或相片直接在電視或其他裝置播放而無需任何連接線，也不需透過無線熱點（AP，Access Point）。
Miracast™ enables seamless display of multimedia content between Miracast® devices.
Miracast can connect two devices using network infrastructure or Wi-Fi Direct®.
Miracast™ Highlights
Wireless display of high-definition content for screens everywhere
How to use Miracast:- Power on the source and display devices
- Enable Miracast on both devices, if not enabled by default
- From the source device, request to discover compatible Miracast devices
- Select the display device to be paired from the list
User may choose content to share or stream from source device to display device.
Underlying technology
After initially pairing two Wi-Fi devices through Miracast, users can stream content or mirror a display from one device (source) to a second device (display). Connections can be established through network infrastructure or Wi-Fi Direct ®, without the need of a Wi-Fi access point or network.
Miracast™ Technical Overview (2017)
Miracast may be used on devices without embedded Wi-Fi ® through the use of a Miracast-certified adapter that supports an interface such as HDMI( High-Definition Multimedia Interface ).
To attain Miracast certification, devices must also be certified for the following:
- Wi-Fi Direct ® – device-to-device connectivity
- Wi-Fi Protected Setup – easy setup of security protections
- WPA2 security
- Wi-Fi MultimediaTM (WMM ® ) – multimedia traffic management
- Core Wi-Fi functionality: Wi-Fi CERTIFIED ac or Wi-Fi CERTIFIED n
Connectivity overview
Miracast connections may be peer-to-peer without requiring an access point (AP) acting as an intermediary, or negotiated through existing Wi-Fi network infrastructure.
Peer-to-peer connection
A direct link between devices may be established through Wi-Fi Direct based on the Wi-Fi Peer-to-Peer (P2P) Specification.
All Miracast devices are required to support Wi-Fi Direct.
Wi-Fi Direct® is a certification mark for devices supporting a technology that enables Wi-Fi devices to connect directly.
Connecting Wi-Fi Direct-certified devices is easy and simple, with the push of a button, tapping two NFC-capable devices together, or entering a PIN. Moreover, all Wi-Fi Direct connections are protected by WPA2. With Wi-Fi Direct, you do not need an access point or internet connection
A set of optional pre-defined services let devices send files, print documents, play media and display screens between and among devices easily, no matter the vendor.
You only need one Wi-Fi Direct device to form a group and all the Wi-Fi devices you have can be connected.
Wi-Fi Direct devices emit a signal to other devices in the area, letting them know a connection can be made.
Infrastructure connection
Operation and supported formats
A Miracast session starts with a request from the user either from the source or from the display device.
Once the content is available for transmission, the source device identifies available display devices and their respective capabilities, and asks the user to select which device should act as the display.
Once the connection is established, the source device encodes the content, taking into account display device capabilities and channel conditions to optimize transmission over the Wi-Fi interface.
Miracast supports a wide range of audio and video formats wthout transcoding, extending the battery life in mobile devices.
- Display resolution Mandatory: 1280 x 720p30 (HD)
- Video Mandatory: ITU-T H.264 (Advanced Video Coding [AVC]) for HD and Ultra HD video; supports several profiles in transcoding and non-transcoding modes, including Constrained Baseline Profile (CBP), at levels ranging from 3.1 to 5.2
- Audio Mandated audio codec: Linear Pulse-Code Modulation (LPCM) 16 bits, 48 kHz sampling, 2 channels.
Principal mechanisms
The session management stages in Miracast:
- Device discovery Source and display devices discover each other prior to connection setup. The Device discovery mechanism is defined in the Wi-Fi Peer-to-Peer Specification or through existing AP connections as defined in the Wi-Fi Display Technical Specification 2.0. Device discovery over an existing infrastructure connection utilizes mDNS/DNS-SD.
- Service discovery (optional) Service discovery is only used for infrastructure connections and in conjunction with device discovery utilizing mDNS/DNS–SD.
- Device selection A remote device is selected for connection setup. User input and local policies may be used to decide which device is a display and which is a source.
- Connection setup Connection setup selects a method to manage the connections, either through P2P or network AP connection. Upon the establishment of connectivity between the source and display devices, the display initiates a TCP connection, with a control port using Real-Time Streaming Protocol (RTSP) to create and manage the sessions between source and display devices.
- Capability negotiation Source and display devices determine the parameters for the Miracast session.
- Content protection setup(optional) If the devices support content protection and are streaming content requiring protection, session keys for link content protection are derived using HDCP(High-bandwidth digital contect protection). HDCP session keys are established before the RTP(Real-Time Protocol) session is initiated.
- Session establishment and streaming Upon completion of capability negotiation, the source and display devices set up the Miracast session prior to streaming content. The audio and video content available on the source device is packetized using MPEG2-TS coding and encapsulated by RTP over UDP. Switching from UDP to TCP is supported to provide dynamic network optimization.
- User input back channel setup (optional) A User Interface Back Channel (UIBC) for transmitting control and data information related to user interaction with the user interface is set up. User inputs at a display are packetized using a UIBC packet header and transported using TCP/IP.
- Payload control When the payload transfer starts, devices may adapt transmission parameters on the basis of channel conditions and power consumption:
- Compression ratio change and macroblock skipping
- Frame skipping
- Format change
- Display session teardown Either the source or the display terminates the Miracast session.
Certification program testing summary
Miracast may be added to existing devices that do not have embedded Wi-Fi. This can be done by using a Miracast-certified dongle that supports HDMI or other interfaces to connect a rendering device. In this scenario, the dongle manages the Miracast pairing and sessions, and it is treated by the device as if it were a
cabled input source.
TDLS: Easy-to-Use, Security-Protected Direct Links to Improve Performance of Wi-Fi® Devices (2012)
TDLS (Tunneled Direct Link Setup) optimizes performance by establishing a direct link between
devices so that they can more efficiently complete tasks such as streaming multimedia, transferring files, or backing up data.
TDLS is based on the IEEE 802.11z standard amendment. TDLS is characterized by the use of set-up frames that are encapsulated in data frames that can be transmitted through the access point (AP) transparently. This is known as 'tunneling' the frame through the AP. The result of this tunneling action is that direct connections can be formed in a Wi-Fi network without a requirement for special functionality in the AP.
Key Benefits from TDLS
- makes data transmission more efficient In a typical infrastructure Wi-Fi network, packets are forwarded by AP from the source to the destination. A direct link can reduce by half the number of packet transmissions.
- devices can perform at the highest level of shared capabilities, regardless of the capabilities of the AP.
- Even when the direct link is switched to another channel, the stations periodically switch back to the home channel to maintain the connectivity with the Wi-Fi network.
- the TDLS direct link always employs WPA2 encryption The keys are automatically exchanged during the TDLS setup procedure so no user password is required.
How does TDLS work?
- Discovery The TDLS Discovery process begins when one TDLS device sends a TDLS Discovery request to another device, via the AP or GO. Then the target sends a TDLS discovery response directly to the initiator, providing information on its capabilities, including supported rates and channels.
- TDLS Setup and Teardown The initiating device first sends a TDLS Setup request, tunneled through the AP, to the target device. The target device then responds with the TDLS Setup response, also tunneled through the AP. If the Setup Request is accepted, the initiating device then sends the TDLS Confirm Frame via the AP. At this point the devices begin to communicate directly. Either the requesting or accepting device may send a TDLS Teardown Frame directly to the other device or, if unreachable, this frame can be tunneled via the AP.
- TDLS Power Save The TDLS Peer Power Save scheme is similar to WMM®-Power Save, also known as WMM U-APSD,
- TDLS Channel Switch TDLS devices can negotiate to move to another channel.
- TDLS Security Features TDLS capability can be disabled by a system administrator. A system administrator may wish to exclude direct links and therefore a "TDLS Prohibit" bit may be set in AP beacons.
Wi-Fi Direct® in the enterprise: Evaluating peer-to-peer Wi-Fi® connectivity (2015)
Wi-Fi Direct ® is a certification program for products implementing peer-to-peer Wi-Fi ® connections between devices, without the need to establish a link to an access point.
Four pre-defined services:
- Wi-Fi Direct Send
- Wi-Fi Direct Print
- Wi-Fi Direct for DLNA®
- Miracast
Introduction: Wi-Fi Direct matters to the enterprise
Wi-Fi Direct expands the functionality of Wi-Fi to enable devices to directly connect to each other using Wi-Fi peer-to-peer technology, while preserving their ability to also connect to access points (APs).
Wi-Fi Direct can also be used to provide basic Wi-Fi connectivity between devices in environments where there is no Wi-Fi infrastructure.
Wi-Fi Peer-to-Peer (P2P) Technical Specification
This document is the specification for the Wi-Fi Direct ® program, which allows Wi-Fi client devices to connect directly without the use of an access point.
1. Introduction
1.5 Abbreviations and acronyms
- DMG Directional Multi-Gigabit. The IEEE 802.11ad, which is also called DMG (Directional Multi-Gigabit in IEEE 802.11 standard), provides data throughput speeds of up to 8 Gbps with low latency, using the 60 GHz ISM band to achieve the required levels of bandwidth and ensure reduced interference levels.
- WSC Wi-Fi Simple Configuration
2 Architectural overview
2.1 P2P components
- P2P Device
- Supports both P2P Group Owner and P2P Client roles.
- Negotiates P2P Group Owner or P2P Client role.
- Supports WSC and P2P Discovery mechanism.
- May support WLAN and P2P concurrent operation.
- P2P Group Owner role
- “AP-like” entity that provides BSS functionality and services for associated Clients (P2P Clients or Legacy Clients) when not operating within DMG, or a PCP that provides PBSS functionality and services for Clients (P2P Clients) when operating within DMG.
- Provides WSC Internal Registrar functionality.
- May provide communication between associated Clients.
- May provide access to a simultaneous WLAN connection for its associated Clients.
- P2P Client role
- Implements non-AP STA functionality.
- Provides WSC Enrollee functionality.
2.2 P2P topology
The P2P Topology is 1:n where multiple Clients are connected to one Group Owner. Such a set of connected devices is called a P2P Group.
A P2P Group has a single SSID and provides one security domain.
2.3 Concurrent operation
A P2P Device can operate concurrently with a WLAN (infrastructure network).
Such a device is considered a P2P Concurrent Device. The concurrent operation requires a device to support multiple MAC entities. Implementation of multiple MAC functionality is out of scope of this specification. For ex., two virtual MAC entities over one PHY entity.
2.4 Functions and services
3 Functional description and procedures
3.1 P2P discovery
P2P Discovery consists of the following major components:
- Device Discovery Device Discovery consists of two major phases: Scan and Find.
- a P2P Group Owner
- in the Listen State
- in the Search State and the P2P Device operates within DMG
- a P2P Device associated with an infrastructure AP on the channel on which the Probe Request was sent — in which case the P2P Device may respond provided it is not already a member of a P2P Group
- a P2P Client supporting Peer-to-Peer services (P2Ps), having a Service Advertiser with a Service Hash matching the hash value in the incoming Probe Request
- Service Discovery is
- Group Formation determine which device will be the P2P Group Owner and form a new P2P Group.
In-band Device Discovery uses Probe Request and Probe Response frames to exchange device information.
A P2P Device shall not respond to Probe Request frames unless it is in one of the following situation:
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