By Wilson Dizard III
WASHINGTON - During a pivotal global exercise last year, the modern military`s perennial struggle with the shortage of communications bandwidth was a constant obstacle that threatened tactical electronic mail routing and other communications paths.
Limited bandwidth, in fact, was a substantial issue in command and control systems almost from the opening moments of the operation called Joint Warrior Interoperability Demonstration, or JWID-97, a testing ground for computer and communications technology.
For starters, a glitch in e-mail software set up for JWID-97 generated an unusual outpouring of message traffic that swamped the command and control system set up for the exercise. "It got clogged up seriously." Worse yet, the e-mail system`s automatic attempts to resend returned messages repeatedly worsened the problem," explains Darryl Henry, chief of the Defense Information Systems JWID Project Office. "Once we created the problem, we couldn`t get rid of it for three days. Finally we say, `time out, stop this, turn off the e-mail system and restart the process.`"
Exacerbating the e-mail meltdown in JWID-97 was the extra burden placed on communications links by bandwidth-hungry battlefield awareness applications such as the Common Operational Picture, a distributed system for coordinating coalition operations. The shortages arose even though military officials increased bandwidth capacity for JWID-97 above normal levels so they could test several bandwidth-hungry communications systems simultaneously.
"When we tell telecommunications engineers develop a newer system to provide more and more bandwidth, the users always find a way to fill it up with more and more and better and better [traffic]," Henry explains.
More help is on the way, however. Operational Army units now are receiving UHF satellite communications transceivers that use demand-assigned multiple-access (DAMA) technology - a technical advance that, while it may not solve the perennial bandwidth shortage, has already helped systems designers exploit existing bandwidth more efficiently than they could before.
"DAMA technology allows more user access by multiplexing several calls together over a single channel," explains U.S. Army Lt. Col. Kevin Dietrick, director of the UHF Satellite Communication Terminal Procurement Office at the Army Communications-Electronics Command at Fort Monmouth, N.J.
"Consider the example of a single 25 KHz channel," says the Army`s Dietrick. "In the past, the single channel was assigned to a single user. If that same channel was converted to DAMA, four unique opportunities for narrowband voice communications would exist concurrently, plus a capability for data as well." Dietrick says DAMA allows 400 percent greater efficiency in channel use than the UHF satcom systems it is replacing.
While Pentagon leaders are working with several multiplexing strategies and standards, DAMA-compliant equipment promises to become one of the most widely deployed and useful varieties. DAMA squeezes several channels onto a narrow slice of bandwidth by interleaving traffic from several users. It divides traffic into segments divided by four divisions of time and by further subdivision of spectrum space. A central DAMA control station allots the time slots based on user data rate, priority, and availability. System users share any unused DAMA capacity.
DAMA technology represents a refinement and improvement of time-division multiple-access (TDMA) multiplexing that became possible in the 1970s with the advent of digitizing and encoding technologies. In DAMA systems, a channel divides into units measured by millionths of a second.
The basic protocols for DAMA-compliant systems are in three military standards: Mil-Std 188-182 for 5 KHz DAMA communications; Mil-Std 188-183 for 25 KHz channels; and Mil-Std 188-185 for network controllers.
When DOD officials established the standards in 1991, members of the Joint Chiefs of Staff mandated DAMA-compliant terminals for all military UHF satcom users by 30 Sept. 1996. Budget restrictions, however, have slipped the schedule, and now specialists in the field expect the gear to become nearly universal for Pentagon UHF satcom radios by the turn of the century. Other military frequency bands, such as SHF and EHF, also are in line for conversion to the DAMA standards. Existing DAMA terminals uplink to the Pentagon`s nine-satellite UHF Follow-On constellation.
As JWID-97 progressed, DAMA gear from Titan Corp. of San Diego installed on the attack submarine USS Scranton showed that efficient bandwidth use can improve command and control. Submarines, while treasured for their stealth, have limited communications capability, Henry explains. In JWID 97, an AN/USC-42(V)2 mini-DAMA terminal from Titan`s Linkabit division enabled crew members aboard the Scranton to participate in a command and control video conference, Henry says.
Army, Air Force, and Marine units also are in line to receive DAMA-compliant equipment from Titan as well as from other manufacturers such as Motorola Inc. of Schaumberg, Ill., whose engineers produce the LST-5-D man-portable DAMA terminal.
Dietrick says U.S. Army special operations forces now are receiving DAMA- compliant AN/SPC-5 Spitfire radios from Hughes Defense Communications of Fort Wayne, Ind. Army leaders plan to buy 2,397 AN/PSC-5 units.
He adds that the Spitfire, like the Titan DAMA radios, takes advantage of commercial off-the-shelf technology to cut costs and bolster performance. "In this acquisition we specified things like reliability and environmental extremes, while leaving the detailed process/component selection to industry," Dietrick says. "As part of the solution, many commercial products are embedded in the terminal design, along with standard software engines for the DSP (digital signal processor) and CPU (central processing unit) components. Not all DAMA terminal acquisition activities have chosen this route, but we felt it would meet the needs of ground users while reducing the cost of the terminal and complying with DOD policy."
So far, the Army alone has spent approximately $8 million for research on DAMA technology, and procured DAMA radios worth about $65 million. The Air Force is approaching DAMA compliance by purchasing modems that add the capability to existing radios - the ARC 210 modem manufactured at the Rockwell International Corp. Collins Avionics & Communications Division in Cedar Rapids, Iowa.
Across the military, approximately 10,000 DAMA-compliant terminals likely will be deployed by 2000; by 2005, as many as 15,000 units could be fielded, says Tom Trimble, vice president of business development for Titan Linkabit. That could bring the Pentagon`s total expenditure on DAMA systems to $300 million to $400 million, Trimble says.
Future steps in DAMA technology development likely will include the transmission of data formatted for the widely used TCP/IP (Transmission Control Protocol/Internet Protocol) on half- and full-duplex channels, Trimble says. That improvement will enable DAMA to double the number of users on a single satellite channel from four to eight, Trimble says.
Despite DAMA`s increase in bandwidth efficiency, bandwidth shortages likely will continue, agree Trimble, Dietrick, and Henry.
"We all know that there will never be enough bandwidth capability," Dietrick says. "Users have an insatiable thirst for more data in real time, hence more bandwidth to pass the data. DAMA will improve the equation significantly, but more and more users are demanding access on a daily basis. Available channels will continue to be rationed out among the warfighting CINCS (Commanders-in Chief), who will prioritize their use."
Henry says, "You find that because memory is cheap, because processors are cheap, effective utilization of the communications facilities is not a priority with system developers or system programmers." He adds that officials have two ways of dealing with the problem: "Either increase the available bandwidth with technology, which we do regularly, or provide an administrative throttle on how much bandwidth these systems use. That`s a real unpopular tactic."
Shortages of military satellite capacity have become so severe, in fact, that defense communication planners expect Pentagon leaders to depend on commercial satellite capacity should another regional conflict like the Persian Gulf War arise.
"Many more users and platforms believe they can have satellite communications capability" since the Gulf War, Trimble says. "The only system that can do that is UHF. From going to a few hundred in a theater, the military has gone to several thousand satellite terminals. Operational forces of 2,000 to 6,000 don`t strain the system. But with hundreds of thousands of people, you would have to limit access at the lower echelons. The satellites are sitting there and capacity is as great as its ever going to be. Demand arises for use of channels almost as quickly as terminals are distributed."
The DAMA shipboard terminal pictured above is part of a growing military trend that uses this technology to make efficient use of limited radio frequency spectrum.