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DiviComTM
How Digital Video Networks.
Digital Video and Data
Mike Perkins, DiviCom
International Cable Magazine, May 1998
Cable networks are capable of carrying high speed data both to and from a customer's home. If the headend contains data servers, or is connected to the internet, sophisticated data services can be offered to subscribers possessing advanced settop boxes. These services can be two-way interactive, such as web browsing, or virtual interactive, such as navigating through an electronic program guide. Two-way interactive services require a backchannel, whereas virtual interactive services require a settop application that can navigate through a repetitively transmitted data stream broadcast to many settop boxes.
Some interesting data services require a settop box to simultaneously receive both video information and data. These services can be broadly classified into two categories based on whether or not the video and data are integrated with each other. One example of an integrated video / data service is the bundled transmission of a sports match with statistics on the major players. The user could browse through information on the player of his choice while watching the game. An example where the video and data are not integrated is when a customer pays to have certain data continually downloaded to his settop, such as stock quotes or email, regardless of the video channel to which he is currently tuned.
If the data is carried together with the video information we say that it is sent in-band, whereas if the data is transmitted in a separate channel it is referred to as out-of-band. As the examples above show, the capability to receive and transmit out-of-band data enables a broad class of applications. This is because out-of-band channels enable uninterrupted data communication between a settop and the headend even when the user is channel surfing or tuned to an analog video channel. On the other hand, in-band downstream digital channels are capable of carrying data at a much higher rate than out-of-band downstream channels. Whether downstream data should be sent in-band or out-of-band depends on the service.
Several organizations are working to standardize out-of-band data paths for settop box communication. The Digital Audio Visual Council (DAVIC) effort, which boasts participation from vendors and service providers around the globe, has defined an out-of-band communication path based on QPSK transmission upstream and downstream. In Europe, the Digital Video Broadcasting effort has largely adopted the DAVIC specification, albeit with some modifications. In the United States, the OpenCable initiative is considering both DAVIC and proprietary approaches. All of these efforts are currently ongoing.
Cable modem systems employ technology very similar to that used in out-of-band settop systems. In the case of cable modems, data is sent downstream using methods similar to those used to transmit digital video. The upstream path is provided using techniques essentially identical to those used by settops. A consortium of U.S. cable companies is backing the Multimedia Cable Network System (MCNS) cable modem standard. MCNS may ultimately prove appealing outside the United States as well, although this remains to be seen. Alternatives based on DAVIC techniques are also being developed by some organizations.
The most challenging aspect of building out-of-band systems is implementing the return path. There are three major reasons for this. First, the return path funnels noise to the headend receiver in the same way that a river concentrates the water of its tributaries. The receiver must therefore operate well in the presence of noise, and the network must be designed to minimize noise ingress. Second, the headend receiver must operate in a burst decoding mode. The upstream channel is allocated first, to one settop, then to another, and so forth, thousands of times per second. The receiver must quickly lock onto the transmission of a settop, demodulate its signal, and then prepare for a new transmission from a different settop. Finally, the received power level of each settop will vary, as will the exact timing of its transmision burst. All-in-all, a challenging operation for the headend receiver!
In 1998 and early 1999 several cable operators plan to rollout settops that support out-of-band services. The experiences of these operators will be closely watched by the cable industry, and will undoubtedly affect the nature and pace of future deployments |
