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Miracast – Wireless Devices to Seamless Video Display

Miracast is a Wi-Fi display certification program announced by Wi-Fi Alliance for seamlessly displaying video between devices. The intersection of wireless connectivity and streamed audio/video content can be termed as miracast. This solution enables seamless mirroring of entire displays across devices or sharing of any type of content that a source could display using Wi-Fi Direct.
Miracast Advantages:

 

  • Content support: Miracast can support any type of content that the source device can display, with no restriction of applications or file formats.
  • Ease of installation: WPS (Wi-Fi Protected Setup) helps users to automatically configure Wi-Fi networks, enable WPA2 security, and add new devices.
  • Battery life: WMM Power Save extends the battery life of mobile devices like smartphones or tablets by minimizing the time the device is actively connected to the AP during idle time. Power save mechanisms in Wi-Fi Direct provides similar benefits when connecting devices without an AP.
  • Cost effectiveness: Miracast uses the Wi-Fi functionality designed into connected devices. It does not require implementation of an additional wireless technology, or of ports for cabled solutions, in order to provide connectivity.
  • Demand-driven connectivity: Connectivity between source and display devices can be established through the user’s normal interaction with an application or service. This creates an environment where users only need to choose what content to display, without having to establish device-to-device connectivity as a preliminary step.
  • Ease of use: Users can share video content between devices seamlessly, without a complex setup procedure.
  • Strong Wi-Fi foundation: Miracast delivers Wi-Fi CERTIFIED 802.11n performance and Wi-Fi Direct, enabling connectivity across devices even where there is no Wi-Fi access point.
  • Proven interoperability: Miracast works between any source device and display device, regardless of vendor, as long as they are both certified for Miracast.
  • Traffic management: Miracast uses channel selection mechanisms from Wi-Fi Direct and TDLS to select an optimal channel, avoiding congestion from nearby networks that might impact video transfer. Quality of service mechanisms enhances the viewing experience by prioritizing latency sensitive video traffic.
  • Trusted security:  Wi-Fi Protected Access (WPA2) security protects users’ privacy, as it does in all Wi-Fi CERTIFIED networks.

The below figure shows Miracast benefits from multiple Wi-Fi Alliance certification programs.

 

Miracast devices may support as an option Tunneled Direct Link Setup (TDLS), a Wi-Fi Alliance certification program that enables a direct link between client devices connected to the same Wi-Fi access point so they can communicate directly.

Miracast Topologies:

Miracast does not require a typical Wi-Fi infrastructure network, though many devices will take advantage of network connectivity to access content. The direct link between devices is established either through Wi-Fi Direct, a feature that all Miracast devices are required to support, or through TDLS, an optional feature. When two devices connect with each other directly, one fulfills its role as the source (the transmitting device) and the other functions as a display (the device receiving and rendering the content to the user).

The below figure shows the four topologies supported by miracast.


Factor Deciding Source or Display:

Devices can be act as a source in some usage contexts and a display in others, their capabilities will largely determine their role. Devices that can generate, transcode, or store content, such as laptops, smartphones, and cameras, are more likely to be sources; devices such as digital TVs (DTVs) and projectors are more likely to be displays. Devices like tablets may act as both sources and displays, depending on whether they are used to generate, transcode or store content (source) or to present content (display) transmitted wirelessly by the source.

Miracast approach to wireless streaming of multimedia content:

A Miracast session starts with a request from the user either from the source or from the display device. The content must be present on the source device, and may be acquired through streaming, copying or downloading the content, or generated by the source device itself, as in the case of screen mirroring, business applications or gaming.
Once the content is available for transmitting, the source device identifies available display devices and their respective capabilities, and asks the user to select which device should act as the display. At this point, the source device establishes a link with the chosen display device in preparation for transmission.

The below figure shows the wireless streaming in Miracast.

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. Finally, the display device receives the content, decodes it, and renders it.
The below table show the miracast supported formats:



Miracast: Supported formats

Display Resolution

 

  1. 17 Consumer Electronics Association (CEA) formats, from 640 x 480 up to 1920 x 1080 pixels, and from 24 to 60 frames per second6 (fps)
  2. 29 Video Electronics Standards Association (VESA) formats, from 800 x 600 up to 1920 x 1200 pixels, and from 30 to 60 fps.
  3. 12 handheld formats, from 640 x 360 up to 960 x 540 pixels, and from 30 to 60 fps

 

Video

 

  1. ITU-T H.264 (also known as Advanced Video Coding [AVC]) for high-definition (HD) video, supporting the Constrained Baseline Profile (CBP) and the Miracast-specific Constrained High Profile (CHP), at levels ranging from 3.1 to 4.2.

 

Audio

 

  1. Mandated codec: Linear Pulse-Code Modulation (LPCM) 16 bits, 48 kHz sampling, 2 channels
  2. Optional audio codecs, based on:
  • LPCM mode 16 bits, 44.1 kHz sampling, 2 channels.
  •  Advanced Audio Coding (AAC) modes
  • Dolby Advanced Codec 3 (AC3) modes

 

Miracast Principal Mechanisms (Miracast session management):

  1. Device Discovery: Source and display devices discover each other prior to connection setup.
  2. Service Discovery (optional): Source and display devices discover each other’s Miracast capabilities prior to connection setup.
  3. 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.
  4. Connection setup: Connection setup selects a method (Wi-Fi Direct or TDLS) to manage the connection. Upon the establishment of connectivity between the source and display devices,the display initiates a Transmission Control Protocol (TCP) connection, with a control port using Real-Time Streaming Protocol (RTSP) to create and manage the sessions between source and display devices.
  5. Capability negotiation: Source and display devices determine the parameters for the Miracast session.
  6. 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 High-bandwidth Digital Content   Protection (HDCP) 2.0/2.1. HDCP session keys are established before the RTP session is initiated.
  7. Session establishment and streaming: Upon completion of capability negotiation, the source and display devices setup the Miracast session prior to streaming content. The audio and video content available on the source device is packetized using Moving Picture Experts Group 2 Transport Stream (MPEG2-TS) coding and encapsulated by Real-Time Protocol (RTP) User Datagram Protocol (UDP) and Internet Protocol (IP).
  8. 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. Two user input categories are available, i.e., Human Interface Device Class (HIDC) and Generic.
  9. Payload control : When the payload transfer starts, devices may adapt transmission parameters on the basis of channel conditions and power consumption
  10. Display session teardown: Either the source or the display terminates the Miracast session.

 

Miracast simplifies the process of forming direct wireless connections, freeing users from spending time and effort connecting cables or manually establishing a wireless link every time they want to share content between devices. With Miracast, devices identify and connect to each other, manage their connection, and optimize the transmission of content on the basis of their capabilities and network conditions. Miracast brings a user experience that is functionally similar to a wired connection, but with the added advantage of portability within the Wi-Fi coverage area.

Miracast user experience:

To start using Miracast, the user has to:

  1. Power on both the source (transmitting) and display (receiving) devices, and enable Miracast if not enabled by default
  2. If using the user interface on a display device:
    1. Request the device to discover all the compatible source devices.
    2.  Browse the source devices that have been discovered or previously paired
    3. Select the source device to be paired
  3. If using the user interface in a source device:
    1. Request the device to discover all the compatible display devices.
    2. Browse the display devices that have been discovered or previously paired.
    3. Select the display device to be paired.
  4. Begin playing the content on the source.

Wi-Fi Direct is used to complete these initial steps to connect the two devices. In subsequent sessions, the two devices recognize that they are paired without repeating these steps. The user can initiate a new session from either the source device or the display device. A source device and a display device can be involved in only one active session at a time. The session can be ended by either device.

Dhanya JR – Software Engineer
iWave Systems Technologies