Broadband Multimedia Satellite Systems

What is Broadband Multimedia?

The term Broadband litterally means large capacity, but ist has comne to mean much more than that in the communications industry. The most spoken about broadband systems were those designed based on the Asynchronous Transfer Mode (ATM), or cell-baszed switching. These systems were designed to support multiple services and have become synominous with multimedia capability.

Multimedia is a term which may be applied to digital media-rich content, digital platforms, and networks. Multimedia Networks enable a numbe of services to be sent simultanesously over the same network. Examples include support for different classes of Internet customers (Home, Buisness, High Availability), Video on Demans, Voice over IP, Videoconference, etc. In the case of ATM, multimedia is linked to the ability to control the Quality of Service (QoS) of each class of service, and in some cases to specify individual Service Level Agreements. More recently, the full range of multimedia services have been demonstrated over next generation IP networks, leading to questioning as to the merits of introducing a unified ATM network.

Types of multimedia service:

Most moderm broadband systems also permit some level of Interactivity. This is true of most current content: TV, Radio, etc, but especially true of the new digital services, where users often wish to sort, manipulate or participate based on the recived content. Although it is possible to conceive a one way Internet service, one-way routed Internet (e.g. supplying supplementary capacity to network service providers requiring bandwidth exceeding their purchased terrestrail caapcity), most services will require some form of return channel to allow two way packet flow.

Multi-Platform delivery is also a key concept for many people thinking of Broadband Multimedia. This is the ability to deliver content (digital video, web pages, etc) to a range of network devices - TV sets, personal PCs, Multimedia devices, web-enabled telephones (e.g. WAP) and wireless devices. To date the very different user interfaces, display capabilities and human factors considerations has made multi-platform delivery difficult. Many proposed networks instead will restructure information for each type of display. Some suggest this is a short-term solution, and future generations will have to converge on a common technology (presumably Internet-based for data applications).

There are five key players in a typical broadband multimedia system:

Broadband Multimedia Content - Is Content King?

The key issue in bringing broadband multimedia networks to reality was once thought of as Content. Finding the right content was seen as a key part of planning a broadband system. Early content (well suited) news, sports, weather, financial data - mainly text based.One way to generate additional content for a new format is to use content gateways (which rewrite content across HTML, for OpenTV, for WAP, etc), although such gateways may seem attractive, they are unlikely to provide effective long-term use for access to rapidly evolving Internet content.

Convergence is the term given to the perception that many platforms now have (or soon will have) common capabilities. Although specific platforms are optimised for specific types of content some capabilities are now becomming common. The TV set can receive and send email (as can a wireless personal computer, mobile phone, etc), networked PCs can receive broadcast TV, digital music can be downloaded from any digital appliance, and digital camera pictures uploaded. A multiservice network shows a similar convergence in network capability.

One advantage of convergence is the ability to access the same information from a TV set as from a PC. Particularly in Europe, many households do not have their own Internet PC (estimates place the proportion of European households with a PC to be 50% less than in the USA). In contrast to what is seen as the complexity of a PC, most potential broadband customers perceive the TV as a less difficult to use device. Never-the-less to evolve the set-top-box will need to acquire much more sophictication, and may never prove the ideal device for personal access. An advantage of such systems is that the service provider can leverage their existing (often loyal) customer based for subscriber TV.

There is growing perception that content alone is not the solution. There is a need for a sound commercial model for the new service. Traditionaly few Internet users have been willing to pay more than modest charges for their data services.There are also legal issues on the distribution (and redistribution) of content, both from the viewpoint of national laws and the ownership of copyright / loyalties.

DVB Satellite Return Channel (DVB-RCS)

The DVB Return Channel System via Satellite (DVB-RCS) was specified by an ad-hoc ETSI technical group founded in 1999. This tracked developments by key satellite operators and followed a number of pilot projects organised by the European Space Agency (ESA). The DVB-RCS system is specified in ETSI EN 301 790 . This specifies a satellite terminal (sometimes known as a Satellite Interactive Terminal (SIT) or Return Channel Satellite Terminal (RCST)) supporting a two-way DVB satellite system. The use of standard system components provides a simple approach and should reduce time to market. (There is also a related guideline document for use of EN 301 790.)

The satellite user terminal receives a standard DVB-S transmission generated by a satellite hub station. Packet data may be sent over this forward link in the usual way (e.g. MPE, data streaming, etc).

In addition, DVB-RCS provides transmit capability from the user site via the same antenna. The transmit capability uses a Multi-Frequency Time Division Multiple Access (MF-TDMA) access scheme to share the capacity available for transmission by the user terminal. The return channel is coded using rate 1/2 convolutional FEC and Reed Solomon coding. The standard is designed to be frequency independent - it does not specify the frequency band (or bands) to be used - thereby allowing a wide variety of systems to be constructed. The data to be transported may be encapsulated in Asynchronus Transfer Mode (ATM) cells, using ATM Adaption Layer 5 (AAL-5), or use a native IP encapulation over MPEG-2 transport. It also includes a number of security mechanisms.

One arrangment of the User Terminal Protocol Stack

DVB-RCS terminals require a two-way microwave feed arrangement / antenna system, able to transmit and receive in the appropriate satellite frequency bands. These are typically connected via a cable (or group of cables) to an indoor unit. This unit could be a Set Top Box (STB) with a network interface, could be integrated in a PC peripheral (e.g. a USB or FireWire device), or may be integrated in a PC expansion card. A key goal of DVB equipment suppliers is to reduce equipment costs. Since cost is likely to be (at least initially) dependent on the terminal transmit capability (that is the rated transmit power), a number of different classes of terminal are envisaged, supporting a range of transmit bit rates. Suitable target prices would probably be 1000- 50000 euros, depending on transmit capability of the terminal.

Terminal Operation

A Return Channel Satellite Terminal (RCST), once power on, will start to receive general network information from the DVB-RCS Network Control Centre (NCC). The NCC provides monitoring & control functions and generates the control and timing messages required for operation of the satellite network. All messages from the NCC are sent using the MPEG-2 TS using private data sections (DVB SI tables) . These are transmitted over the forward link. Actually the DVB-RCS specifcation calls for two forward links - one for interaction control, and anotehr for data transmission. Both links can be provided using the same DVB-S transport multiplex.

The term forward link, refers to the link from the hub station which is received by the user terminal. DVB-RCS allows this communication to use the same transmission path as used for data (that is the DVB-S receive path), or an alternate interaction path. Conversely the return link is the link from the user terminal to the hub station using the DVB interaction channel. The control messages received over the forward link also provide the Network Clock Reference (NCR). The NCR contains a 27 MHz clock reference and reception of the NCR is used by user terminals to adjust the transmit frequency of each user terminal to ensure a common reference for the MF-TDMA transmissions.

All transmissions by a user terminal are controlled by the NCC. Before a terminal can send data, it must first join the network by communicating (logon) with the NCC describing its configuration. The logon message is sent using a frequency channel also specified in the control messages. This channel is shared between user terminals wishing to join the network using the slotted ALOHA access protocol.

Simplified Transponder Usage by a Group of DVB-RCS User Terminals

After receiving a logon message from a valid terminal, the NCC returns a series of tables including the Terminal Burst Time Plan (TBTP) for the user terminal. The MF-TDMA burst time plan (TBTP) allows the terminal to communicate at specific time intervals using specific assigned carrier frequencies at an assigned transmit power. The terminal transmits a group of ATM cells (or MPEG-TS packets). This block of informatiom may be encoded in one of several ways (using convolutional coding, RS/convolutional coding or Turbo-coding). The block is prefixed by a preamble (and optional control data) and followed by a postamble (to flush the convolutional encoder). The complete burst is sent using QPSK modulation. Before each terminal can use its allocated capacity, it must first achieve physical layer synchronisation (of time, power, and frequency), a process completed with the assistance of special synchronisation messages sent over the satellite channel.

A terminal normally logs off the system when it has completed its communication. Alternately, if there is a need, the NCC may force a terminal to log off.

One of the strengths of the system is the extreme flexibility this provides to configuring individual transmission capabilities. Some also criticise this as a weakness: The current standard allows a range of implementations, and therefore does not promote equipment interoperability between different systems.

Broadband Interactive Service (BBI)

BBI is an example of a satellite system built according to the DVB-RCS standard. The system has been designed for Societe Europeene des Satellites (SES), the company operating the ASTRA Satellite network. BBI uses ASTRAs existing Ku-Band system (14 GHz) for transmission from the hub station to the SIT, and Ka-Band capacity (29.5-30.0 GHz) for the return channel from the SIT to the hub station using a 20 MHz MF-TDMA channel.

The BBI system supports three classes of SIT, each with similar receive capabilities, but different transmit capabilities:

Antenna 0.75 m 0.95 m 1.2 m
SIT Transmit EIRP (estimated) 40 dBW 45 dBW 50 dBW
Return Transmission Speed 144 kbps 384 kbps 2 Mbps
Forward Link Speed (DVB-S) 38 Mbps 38 Mbps 38 Mbps

See also :

Minor rev Apr 2001.