Digital television technology enables asymmetrical network configurations.
The NATO Consultation, Command and Control Agency is exploring the use of digital video broadcast technology in both satellite and terrestrial versions. The technology would support the organization’s requirement for a system that can distribute large volumes of information to strategic, deployed and mobile nodes simultaneously at very high transfer rates.
In recent years, digital video broadcast (DVB) technology has developed into a prominent international standard, fostering technologies that enable efficient, cost-effective and interoperable digital data broadcasting over different broadband media. Multivendor DVB technology provides flexible data multiplexing into one transport stream, using Moving Picture Experts Group-2 (MPEG-2) cells as data containers. Lightweight submeter receive antennas and inexpensive integrated receivers and decoders based on commercial digital television technology make DVB bearers ideal for asymmetric network access configurations, using separate networks to carry return traffic upstream.
The NATO Consultation, Command and Control Agency (NC3A) satellite broadcast system (SBS), examined during the U.S.-led Joint Warrior Interoperability Demonstration (JWID) 2001, is a technology demonstrator that employs MPEG-2 and DVB. It provides a secure high-speed Internet protocol (IP) networking overlay over military and commercial satellite transponders. Transmit and receive satellite gateways, which encompass DVB, IP encryption and standard routing devices, are attached to terrestrial wide area network access nodes to create the satellite broadband overlay. Unicast and multicast IP flows are encapsulated into an MPEG-2 transport stream and transmitted as a broadcast to any number of receive sites within the satellite beam coverage. DVB operates over military super high frequency or extremely high frequency channels as well as over high-capacity commercial Ku- or Ka-band transponders, enabling the use of smaller and lighter receive dishes.
The SBS demonstrator uses only commercial hardware and software that is configured to military requirements. Type-1 IP encryption devices and standard Internet-over-satellite system components are bound together to secure the unidirectional satellite link in both point-to-point and point-to-multipoint broadband data transfers. Throughout JWID 2001, the SBS demonstrator injected a test DVB-satellite (DVB-S) carrier over a super high frequency military transponder. The injection point was located at The Hague in the Netherlands, and a transportable satellite receive node was deployed at the Supreme Headquarters Allied Powers Europe in Mons, Belgium, to feed the event’s coalition wide area network (CWAN). A terrestrial extension of the system also was demonstrated.
SBS is a hybrid, asymmetric communications system. Full-duplex communications are achieved over forward and return channels differing in bandwidth, latency, encryption, transmission medium and supporting technology. Organic secure communications channels that encompass narrowband terrestrial, satellite and wireless links can be used to carry return traffic from deployed and mobile information consumers to strategic sources. Asymmetric networking techniques, based on Internet Engineering Task Force standards recently implemented on leading router operating systems, make the split-path configuration transparent to the upper protocol layers. This technique enables Internet-like services using transmission control protocol/IP, reliable data dissemination, real-time video streaming and channel subscription using IP multicast. Ultimately, mobile return links can be established over narrowband secure personal communications systems, affiliating to different networks as the user platform moves across the broadcast coverage area.
The system combines attributes of a push/pull data delivery method with smart IP routing techniques. By diverting the most bandwidth-intensive streams over the satellite path, terrestrial networks are relieved of congestion caused by large file transfers and multimedia applications. While some products such as satellite imagery and intelligence data can be specifically requested and downloaded through Web-enabled interfaces, others are pushed over thematic multicast channels following predefined time lines. Part of this process is the mirroring and/or caching of Web sites and databases across multiple sites using reliable multicast protocols. Those protocols use the return channel for reporting about lost data blocks that must be rebroadcast.
Multicasting in this context offers several advantages. Multicast streams can be tunneled and routed from sources widely dispersed across a wide area network and converge at the uplink injection point. There they can be selectively encrypted with different keys, encapsulated and beamed up. On one hand, multicast enables broadcast streams received by a satellite node to be relayed forward autonomously and selectively to users scattered throughout the terrestrial wide area network. On the other hand, it contributes to the efficient use of the available satellite bandwidth because traffic that has no audience at a given time is dropped by the access router and not transmitted over the satellite. In addition, IP multicast allows end users to select only the channels that carry information relevant to their mission.
During last year’s demonstration, SBS services were made available to workstations across the JWID CWAN. At the injection point in The Hague, streaming video and still imagery products were selectively grouped, announced and pushed over multicast channels to which national hosts could dynamically subscribe. The SBS carried scenario-related unmanned aerial vehicle (UAV) video streams, high-resolution UAV and satellite imagery, and mapping and meteorological data over a number of multicast channels.
Multicasting of real-time and pre-recorded UAV video streams employed Windows Media technology built into Microsoft’s Windows 2000 servers. The NC3A used technology from Lariat Incorporated, Seattle, to add a broadcast management layer and to provide a centralized program guide through their StationManager product. The technology enabled the SBS administrator to build, schedule and announce video programs remotely over 10 multicast channels, including live commercial television feeds such as CNN.
The NC3A has explored the use of DVB-terrestrial (DVB-T) technology to provide a horizontal augmentation to SBS for mobile users. Because satellite and terrestrial DVB share MPEG-2 as the data transport layer, both technologies can be easily bound into homogeneous and highly efficient transport architectures. The objective is to forward the satellite broadcast streams selectively to mobile users such as scattered land-based users, ships in a battle group or low flying aircraft that are near a satellite receive station. In the maritime scenario, DVB-T carriers could relay the satellite broadcast from the command ship’s DVB-S stabilized antenna front-end to the battle group components. In turn, a broadcast relay station could bundle the return traffic flows from multiple mobile users and transmit them to the satellite injection point.
The European DVB-T standard is based on the coded orthogonal frequency division multiplex (COFDM) modulation. This very recently developed standard includes a large number of transmission modes that cover a wide range of data broadcasting scenarios. Among other strengths, the COFDM intrinsic resilience during multipath propagation allows the building of networks of scattered transmitters that operate on the same frequency channel and are slaved to a common global positioning system reference. Large areas can then be covered with cells fed by multiple transmitters receiving a common satellite broadcast carrier.
Field trials and laboratory tests recently performed in several countries demonstrate that mobile reception of a DVB-T signal on rural and urban propagation environments is viable. The robustness offered by some DVB-T transmission modes can be exploited to broadcast up to 15 megabits per second to mobile receivers, even when they are moving at several hundred kilometers per hour. Small ultrahigh frequency (UHF) omnidirectional or low-gain antennas can be used on both transmit and receive sides of the link. Moreover, DVB-T features hierarchical modulation techniques that allow the splitting of the radio frequency channel into two virtual circuits, each carrying a dedicated MPEG-2 multiplex with a specific level of protection. This facilitates the differentiation of user groups based on their requirements for mobility and information delivery assurance. As in DVB-S, content is carried over encrypted IP streams that are encapsulated in MPEG-2 cells. Channels in the analog television UHF band or in the 2.4-gigahertz band can carry the terrestrial broadcast.
In close cooperation with Rohde & Schwarz, Munich, Germany, the JWID SBS demonstrator presented the use of DVB-T over UHF channels to relay the satellite broadcast to mobile receive suites. A DVB-T receive terminal mounted on a vehicle could receive the MPEG-2 satellite broadcast augmented with data streams injected from workstations in the CWAN. Standard global system mobile rooftop antennas received the broadcast in the vehicle while in motion at nearly 50 miles per hour. Return links into the injection point in The Hague were established over Inmarsat telephones and other personal mobile communications systems.
Rohde & Schwarz provided the DVB-T modulator and UHF transmit front-end equipment as well as their advanced IP-inserter technology. IP-inserters selectively replace streams in the satellite broadcast by others carrying locally generated content. Because the information rate is normally not constant, bandwidth of opportunity can serve the same purpose. Content gathered by a deployed satellite receive node can then be added to an MPEG-2 satellite broadcast stream before it is forwarded to a DVB-T transmit front-end. Terrestrial links inject that content into the satellite terrestrial relay cell using encrypted IP transport. In turn, airborne sensors use DVB-T lightweight front-ends and MPEG-2 transport for wireless transmission of real-time streaming video over IP, down to the multiplexer in the relay cell. Mobile users then gain access to strategic sources of content as well as to real-time feeds from in-theater sensors.
The SBS demonstrator was primarily aimed at presenting a split-path communications architecture that supports IP services exhibiting asymmetric bandwidth requirements. However, the strengths of SBS are in its ability to support services specifically tailored to the broadcast and the scalable nature of the overlay. Those may involve complex data gathering, management, and injection scheduling schemes as well as off-line management and prioritization of user requests. The SBS as such is not intended to offer end-to-end information services, but to emulate a reliable bi-directional communications channel that is capable of supporting different information and broadcast management overlays.
Ramon Segura is a senior scientist in the Communications and Information Systems Division of the NATO Consultation, Command and Control Agency.
Details on how to request information about satellite broadcast system-related activities at the NATO Consultation, Command and Control Agency are available on the World Wide Web at www.nc3a.nato.int.