Live TV Broadcast in the DLNA/DVB-HN home network
Watching TV on the game console? Using a WLAN enabled mobile phone as a universal remote control? And everything without any configuration effort?
Thanks to media transmission based on the Internet Protocol (IP) this is not a dream of the future. With respect to the guidelines of the Digital Living Network Alliance (DLNA1) there are already many devices available that support similar scenarios. However, the integration of the classical media broadcast in high quality is still missing, even though the translation of the already digitally available radio and TV content into the home network is possible without reasonable effort. A suitable standardization has been published recently under the DVB-HN standard.
The Problem …
… comes along with the nature of the live media broadcast: Unlike stored media, a strictly time constrained transmission is required in order to avoid delays during channel selection and to preserve the live character of the media, etc. The error-free synchronization of the rendering devices to the live data stream is important and strongly related to the real-time requirements.
- UPnP AV3 Device Architecture 1.0
Typical scenarios in a DLNA based network are the 2-box pull scenario and the 3-box pull scenario. The first one consists of a Digital Media Server (DMS) and a Digital Media Player (DMP). The second describes the interaction of three physically separated devices: The DMP is replaced by a Digital Media Renderer (DMR). The renderer is controlled via a Digital Media Controller (DMC).
- DLNA Guidelines 1.5
In order to ensure a seamless interaction of DLNA compliant devices, a detailed system of rules was de- signed which concerns the whole protocol stack. From the device discovery up to the actual media transport it is based on open stan- dards and protocols. The DLNA Guidelines are based on the devices and mechanisms defined in UPnP AV 1.0.
- DVB-HN Reference Model
The DVB-HN Reference Model is the basis of the liaison between DVB and DLNA. DVB-HN specifies in- stances which are derived from the DLNA devices, i. e. components and functions that are mandatory in the DLNA Guidelines are inherited. According to the requirements of DVB-HN, Device and Service Discovery are purely carried out via the methods specified by UPnP AV. The transport of the media data is based on the Real-time Transport Protocol (RTP).
- RTP Transport
Digital media transmission is actually a time critical application. In the area of IP based transmission the service providers vie with each other for providing a television experience at highest possible quality. The Hypertext Transfer Protocol (HTTP) is a mandatory component of the DLNA protocol stack based on the Transmission Control Protocol (TCP). TCP is not useful for real-time transmission. In compliance with DVB-HN, RTP based on the User Datagram Protocol (UDP) should be used. RTP is optional in the DLNA Guidelines.
- Error Correction
UDP does not implement any error correction. Consequently, it is not able to recover from packet losses which are very common in wireless networks. The Hybrid Error Correction (HEC) applied here combines Automatic Repeat Request (ARQ) and Forward Error Correction (FEC) mechanisms under strict time constraints. Depending on the packet loss rate of the network, an erasure block code operates at packet level. Unless the receiver is able to recover the whole block of packets using the appended redundancy information, it is allowed to request additional redundancy packets via negative acknowledgements during several retransmission cycles. The number of cycles is limited by the time constraint of the real-time application. The error correction is applied backwards compatibly based on RTP.
- PCR Synchronization
DVB receivers use the Program Clock Reference (PCR) available in the digital video stream in order to synchronize their internal oscillator. Based on a Digital Phase Locked Loop (DPLL) running in software this process is imitated in the software implementation of the DMR. However, packetization and network jitter affect the precision of the PCR. Therefore, a longer process of digital filtering is required compared to a conventional DVB receiver. In this way a stable time basis is created which is synchronous to the clock of the sender.
And that’s how it works:
- A DLNA Digital Media Server (DMS) is equipped with a digital tuner. The tuner is able to receive broadcast signals compliant to the DVB standard.
- The DMS shares information about the available TV and radio stations to the network in terms of a UPnP AV Content Directory. DLNA compliant Digital Media Controllers (DMC) are able to request the Content Directory and to display it in a user-friendly way.
- Digital Media Renderers (DMR) in the home network that support rendering of MPEG-2 video may be used to display the TV programs. The DMR is controlled remotely by the DMC.
- After a program selection by the user a connection between DMR and DMS is initiated with the Real-time Streaming Protocol (RTSP). This causes the actual program selection at the DMS and the negotiation of the transport protocol.
- The DMS adjusts the DVB tuner to the selected channel and encapsulates the digital TV signal into a packet stream based on the Real-time Transport Protocol (RTP).
- Depending on the number of tuners and the service offerings of the DVB transponders a selection of several TV and radio channels might be delivered to different renderers.