Technology Enables Multichannel Satellite Links

April 2002
By Robert K. Ackerman
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Spread spectrum, digital processing allow more bytes for the buck.

The Internet protocol revolution is reaching satellite video communications with a new system that permits transmitting tens of thousands of channels over a single orbital transponder. Users can leapfrog existing satellite video limitations with two-way virtual private networks that can carry streaming video without a hitch.

Multichannel satellite technology previously available only to high-end users now is opening up to individual users, including small businesses and residences. Companies can operate thousands of video channels for training, education and corporate news. Civil government can establish and maintain a network covering thousands of diverse sites for homeland security or disaster relief. Military forces dropped into a distant theater of operations can rapidly build a multicast network that moves streaming video among all regional commanders and their headquarters in the United States. And, homeowners finally may realize the dream of full multimedia access.

The new system employs spread spectrum and digital video broadcasting (DVB)-satellite technology, and it includes special software that eliminates the latency effect from satellite video transmissions. The spread spectrum technology even provides a measure of stealthy operation when used in congested or hostile environments.

Known as Digio, the new system is built by ViaCast Networks Incorporated, Ijamsville, Maryland. This system evolved from the company’s technologies that provided high-speed, Internet protocol (IP) multicast greater than 1 megabit per second to many locations via satellite. Many of these technologies are in use by large corporations, including several Fortune 500 companies. Originally known as Intelligent Devices Incorporated, the company took the name ViaCast in late 1999 as an end-to-end provider of IP networking.

Jay Yass, vice president, business development, ViaCast, notes that the company leveraged its one-way multicast product by adding a return channel to create Digio. He describes the new technology as disruptive in that it can provide a seamless network of high-speed IP links, which can be rapidly deployed with different levels of service that can be moved and recommissioned rapidly. Component setup time can be as short as 30 minutes, depending on conditions. A ruggedized complete mobile system can establish links in less than five minutes.

“Instant infrastructure” is how he defines it.

Frank Huebner, chief scientist at ViaCast, observes that existing very small aperture terminal (VSAT) technology is about 15 to 20 years old. The company essentially dissected that technology and enhanced each aspect to come up with the DVB and spread spectrum technologies used for Digio. “This solution is much more robust and cost-effective, and it will be around for the next 15 to 20 years,” he claims.

ViaCast Executive Vice President and Chief Technical Officer Michael J. Beeler allows that Digio is revolutionary in that it was designed from a clean slate instead of built upon legacy systems. Its components include state-of-the-art digital signal processors and field programmable gate arrays, and engineers can change the system architecture as needed while adapting the technology to enable new capabilities.

While Beeler uses the term revolutionary, Huebner offers that Digio also is evolutionary. Within the next year, engineers will develop a baseline system that will permit the evolution of different application-specific configurations. These may include tactical military systems focusing on antijam capabilities and security, commercial systems that replace VSATs by providing increased efficiency and security, and home systems that enable long-awaited multimedia gateways.

Michael J. Scott, the company’s vice president of engineering, observes that the existing satellite communications infrastructure was not designed for an IP-centric environment. The legacy technology, when modified for IP, still cannot overcome significant limitations. The new technology is designed for those applications.

Richard Clewer, senior principal engineer at ViaCast, describes Digio as efficient because it is small, it provides more channels per transponder, it uses much less power to reach a satellite and it allows users to obtain their information at a higher data rate in less time.

With Digio, as much as 80 megabits per second can be allocated to a specific task at any given time. The return channel can move data at 2 megabits per second. Users can employ dial-up, frame relay, fractional T-1, cable or digital subscriber line (DSL) for satellite links. An organization can have thousands of networked terminals always online. Any of its users can broadcast from a remote back to the hub, which will rebroadcast it instantly. Digio works with Ka, Ku, C and X bands.

Beeler explains that the main problem facing most satellite customers is the density of users per transponder. The currently available architecture tends to enable up to 8,000 users per transponder. Digio, on the other hand, raises the bar by at least a factor of two. The new technology can permit a minimum of 25,000 users on a single 36-megahertz transponder. For a single 54-megahertz transponder, Digio can allow 40,000 users.

Scott notes that this permits users to save considerable sums of money on recurring operating costs. In addition, the system’s terminal equipment is less expensive than comparable gear. The architecture is scalable enough to be effective for an enterprise network as small as fewer than 10 users or a consumer network as large as hundreds of thousands of users.

For the military, current satellite technology features small slices of bandwidth allocated to different groups. That works well for small messaging traffic, but it becomes a problem for visual files such as imagery. Similarly, these bandwidth assignments do not permit the flexibility that would enable more efficient use of satellite communications assets.

A Digio system allows bandwidth to be reallocated as needed—“pretty much bandwidth on demand,” Huebner states. The system permits users to capture all the bandwidth they need to move data into theater as rapidly as possible. Bandwidth dedicated to another function can be prioritized to provide automatic reallocation to the original user when needed. The communications pipe can be choked so that the theater commander still can receive his needed real-time data.

Digio’s flexibility opens its use to a number of different types of customers. Civil government users, for example, can share the same seamless infrastructure by employing individual program identifications. These diverse users each would have individual private subnetworks, but they all could tune into the same type of information on certain multicast channels.

Disaster recovery forces can count on its operation independently of relays or ground systems. Its private-network nature also allows greater security that is almost hack-proof, Yass states. One goal of the new Office of Homeland Security is to bring 38,000 law enforcement locations at different levels of government into a single network that provides access to the same information.

Commercial applications may emerge from a multitude of directions. Corporations may employ it for distance learning and training. Direct commercial uses involve point-of-sale and kiosk applications. File distributions or multimedia-centric video with little interaction also are likely applications.

“The chief driver is just broadband access,” declares J. Mitchell Robinson, president, chief executive officer and chairman of ViaCast. “Being able to channelize the bandwidth, being able to have efficient multicast, being able to marry demographic databases together to make a multicast—that is the killer app[lication].”

Robinson believes that traditional satellite direct video and audio technology “is headed for a cliff. The reason is that it needs to be IP-based. It needs to be high bandwidth and channelized.

“Forget about 400 channels—talk about 12,000 channels,” he offers. An IP stream uses less bandwidth than traditional audio and video broadcasts, and being able to run it at much lower data rates can enable sending streaming video at 400 kilobits per second that is not discernable from video sent at the traditional 1.5 megabits per second.

One of the system’s keys is spread spectrum technology. Clewer describes it as a natural multiple access technology that allows multiple users to share the same channels simultaneously. Other technologies such as time division multiple access (TDMA) cannot achieve this capability without tightly controlling users. Digio uses spread spectrum technology on its return link to achieve the multichannel characteristics.

This technology allows the user to employ a lower-power radio, which also saves money. The hub and gateway can be significantly smaller, which allows rapid deployment. Yass estimates that 90 percent of the continental United States would be suitable for a hub antenna of 2.4 meters. A desktop rack of equipment, as opposed to floor-to-ceiling rack-mounted gear, would be all that is necessary for the indoor station. “This is almost suitcase deployable,” Yass contends.

Spread spectrum technology also provides a greater measure of inherent security. The signal bursts are so small that the signals from a single site would be virtually undetectable from noise flow. There are no signal spikes that can be detected by adversaries seeking to locate a transmitter, Yass maintains. A short-burst signal can be transmitted before anyone even knows the system is operating. The system’s wideband nature and high frequencies will require substantial power to jam its return channel, especially when an adversary must be in its line of sight.

Clewer notes that spread spectrum technology’s smaller aperture also allows smaller antennas. A Ka-band antenna, for example, can be as small as 50 centimeters, while a Ku-band antenna can perform with a diameter of only 75 centimeters.

The forward path from the hub through the users employs DVB technology. Beeler notes that it is based on quadrature phase shift keying (QPSK) access technology. It also features eight phase shift keying in the forward path that includes turbo product codes, which enables an additional 50 percent pickup on bandwidth.

The spread spectrum slotted access technology used for the return channels enables inbound data rates as high as 2 megabits per second. This can be achieved without significant power consumption or large antennas, Clewer explains.

The system is scalable in that the smallest terminal can be scaled up to higher data rates. However, enterprise-scale technology will require more powerful amplifiers and a slightly larger dish antenna.

Digio is built as an embedded product so that users are not required to adapt existing technologies or platforms. Functions can be installed on a personal computer, for example, but this may require modifications to enable it to operate with satellite terminal devices. The Digio system provides its own interface functionality.  

A vital element of Digio is a transmission control protocol (TCP) acceleration software known as Pronto. The software, which consists of two packages, operates in tandem with gateway and satellite receivers to speed streaming video and other large files smoothly through the channels.

Conventional satellite links feature propagation delay from the long distance between the earth station and the satellite in geosynchronous orbit. Receiving an acknowledgment after moving data will take at least a half of a second, and this propagation delay causes problems with protocols such as TCP/IP. Beeler explains that Pronto mitigates this propagation delay by making the sender and the server believe that they are communicating locally.

Without Pronto, streaming video can appear to be halting and jerky as the latency causes download delays. With Pronto on, a streaming video runs smoothly and without hesitation, and its operation is transparent to the user. The software effectively keeps the user ahead of the movie’s playing speed. Robinson explains that Pronto, which is controlled by each system’s hub, suppresses protocol acknowledgments at a rate of 50 to one.

Digio users may employ their own security systems, but the company also provides its own security product. Known as Keyspan, it is a managed digital encryption standard (DES) security architecture that provides a secure channel for both the forward and return paths.

The three-tier key-transport-level security system is based on a 56-bit DES engine. The keys are unique to each receiver/router, and an auto-discovery mode automatically configures Keyspan enabled remotes and locks out unauthorized users. Keys can be rolled from once a month to once an hour or by packet, Beeler states. “We are able to roll our keys so quickly that it would be nearly impossible to hack the system,” he claims.

The biggest hurdle in developing Digio has been lowering the cost and power consumption of the transmit radios, company engineers agree. The firm currently is subcontracting these radios, but it intends to bring their development in-house. Another ongoing goal is to make system setup as easy as possible. Engineers are looking at improving the physical link between indoor and outdoor units. “There is no reason why you need to have copper between the two units,” Scott says.

Engineers also are examining how the indoor Digio unit can be a network hub. This might involve adding wireless and other routing capabilities that would empower both enterprise and consumer applications.