DVB Interaction System

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The Digital Video Broadcasting Project (DVB) includes over 200 well known organisations in more than 30 countries world-wide. Members include broadcasters, manufacturers, network operators and regulatory bodies, committed to designing a global family of standards for the delivery of digital television. [10] The DVB interaction system is based on the DAVIC specification described in the previous chapter.In the DVB interaction system, two channels are established between the service provider and the user [6]:

  • Broadcast Channel (BC): Unidirectional broadcasting of audio, video and data from the service provider to the service consumers. This channel may include the Forward Interaction path.
  • Interaction Channel (IC): Bi-directional transmission of interaction data between the service provider and the user. The IC is formed by:
  • Return Interaction path (Return Channel): This interaction channel is used to make requests from the user to the service provider (upstream transmission).
  • Forward Interaction path: This interaction path is used to provide information from the service provider to the user (downstream transmission). This interaction path can be embedded in the Broadcast Channel. Therefore this interaction path is not required in some simple implementations.

Figure 13 shows the reference model for an interactive cable television system.


Figure 13 - Reference model for an interactive cable television system.

To have different users share the transmission media, a multiple access scheme is used. The different channels are dedicated to different regions in frequency using the technique of Frequency Division Multiple Access (FDMA). Within one upstream channel, users send packets using the technique of Time Division Multiple Access (TDMA).

Downstream transmission from the Interactive Network Adapter (INA) to the Network Interface Unit (NIU) is used to provide synchronisation and information to all NIUs, allowing them to send synchronised information upstream. One downstream channel can synchronise up to eight upstream channels. Synchronization information must at least be sent in every period of 3 ms. In the simulations for this thesis, only the downstream information has been considered.


Figure 14 - Spectrum allocation for DVB interactive systems.

The interactive system is based on either Out Of Band (OOB) or In Band (IB) downstream signalling. In OOB downstream signalling the interactivity data is sent in a Forward Interaction path at a rate of 1,544 Mbit/s or 3,088 Mbit/s using a bandwidth of 1 MHz or 2 MHz. When using IB downstream signaling, the interactivity data is embedded into the MPEG-2 TS of a DVB cable channel. No constraints for the downstream IB channels exists, but a guideline is to use rates multiples of 8 kbps. For upstream signaling, bit rates of 3,088 Mbps, 1,544 Mbps or 256 kbps are indicated by the INA using bandwidths of 2 MHz, 1 MHz or 200 kHz respectively.

To allow the INA to send information to one particular user, an address assignment exists for each user. Two addresses are stored in the STBs to identify users on the network [6]:

  • MAC address: A 48-bit value representing the unique MAC address of the NIU. This address indicates the frequencies to be used for upstream transmission.
  • NSAP address: A160-bit value representing a network address intended for ATM interconnection setup [7]. This is an interface between the Session and Transport Service Layer and the Network Service Layer objects through which the server objects provide services to the client objects [1].

The block diagrams for the lower physical layer of the head-end transmitter for In Band and Out Of Band downstream signaling can be seen in Appendix B.