Filling the communications gap
- By Paul Korzeniowski
- Mar 24, 2002
More than 30 U.S. Postal Service branch offices in the New York City area were open for business in the days following the Sept. 11 terrorist attacks, despite the destruction of the terrestrial telephone network. Satellite links filled the gaps.
"Without the satellite services, our offices would have been out of commission for a week, maybe more," said Warren Schwartz, acting manager of wireless solutions at the agency's Raleigh, N.C., office.
All 12,000 USPS branch offices use satellite connections as either their primary mode of communications or as a backup, a decision made three years ago. But although satellite data networks have been available for decades as alternatives, or complements, to traditional terrestrial networks, they have garnered only niche acceptance.
C.E. Unterberg, Towbin Inc., a financial services and market research firm, found that 97 percent of telecommunications traffic travels via landlines and only 3 percent via satellite links.
In the past, satellite communications have done well mostly in sales applications, where only small bits of data flow back and forth, and in areas where companies have offices that cannot be reached by terrestrial lines, experts say.
But the percentages could change now that interest in satellite services has risen dramatically since Sept. 11, said Fritz Stolzenbach, director of marketing at Spacenet Inc., a satellite service provider.
"Government agencies now need disaster recovery plans that take into account entire central offices being wiped out," he said. "In that case, the traditional technique of relying on alternative [telecommunications] carriers doesn't work, but satellite links do."
Satellite communications, though, do not mirror standard telecom performance. The technology has become more attractive in recent years — especially with TCP/IP's emergence as the network protocol of choice — but it's still not necessarily a plug-and-play proposition, because of underlying problems that don't have easy solutions.
"Every major satellite services vendor now offers TCP/IP capabilities, and just about every new installation relies on it rather than legacy protocols," said Chris Baugh, an industry analyst with Northern Sky Research, a market research firm that focuses on satellite links.
However, the pairing of IP and satellite networks is unnatural and sometimes slows transmission rates to the point that users can notice the difference.
A satellite link takes approximately one-quarter of a second. A data packet travels from a user's desktop through a network switch to a very small-aperture terminal — basically a satellite dish that exchanges information with a spacecraft orbiting the earth 22,300 miles away — and back down to a receiving dish.
The framing, queuing and switching that TCP/IP relies on to ensure that packets arrive at the proper destination can increase the link time to as much as half a second.
That doesn't sound like much, but because TCP/IP was designed for networks where delays are measured in milliseconds rather than tenths of seconds, the delays introduced by the satellite communication can disrupt the time-sensitive acknowledgments and "handshaking" at the core of the network protocol.
"Whenever a line becomes garbled, TCP/IP adapts by slowing down the transmission rate," said Jeremy Guralnick, senior vice president of strategic marketing at Tachyon Inc., a satellite network provider. "In a satellite network, that can mean that transmissions slow to a crawl."
Satellite vendors also have to take the weather into account. Heavy rain or snow can slow transmissions and increase the number of dropped packets — much like how bad weather can disrupt FM or AM radio transmissions.
Vendors devised various workarounds to address these problems. One solution, packet spoofing, fools a sending system into thinking it has received acknowledgments for shipped packets so that it continues to transmit data steadily.
However, these techniques often add overhead, or extra processing time, to data transmissions. This exacerbates another problem with satellite networks: lack of bandwidth.
Terrestrial carriers are upgrading their fiber-optic backbones to OC-192 links — based on the Optical Carrier speed levels set by the International Telecommunication Union — which transmit data at up to 10 gigabits/sec. Satellite lines, however, typically top out at 45 megabits/sec. This is slower than OC-1, the lowest Optical Carrier level.
The bandwidth constraints are most evident when companies attempt to upload information. Unlike most terrestrial services, satellite services support different transmission rates for uploads and downloads.
When first setting up their networks, satellite vendors optimized them for video transmissions. In such situations, complex images are shipped to tens — or even hundreds — of endpoints, requiring large bandwidth on the download. Little to no data is then sent back from those end stations. As a result, satellite download speeds can be 10 to 20 times faster than upload rates, instead of the standard transmission rate for terrestrial networks.
Landline carriers also rely on less costly equipment, possess more network capacity to spread out their operating costs and have larger customer bases. This gives them greater revenues than satellite suppliers, so a satellite link may cost two to 10 times as much as a terrestrial line.
Satellite service suppliers have developed various encryption techniques to keep intruders from tapping into their networks, something users are demanding even more in the post-Sept. 11 environment. However, these safeguards also add overhead to network transmissions, which in turn reduces available bandwidth.
Performance concerns convinced the Federal Emergency Management Agency to limit encryption with its mobile services. "We constantly have to balance the trade-offs between increased security and available bandwidth," noted Ron Miller, chief information officer at the agency. "For our applications, we opted for less security and more bandwidth."
Agencies such as the Defense Department require airtight security. Traditionally, this has not been an issue, because they relied on closed networks, launching and maintaining satellite systems themselves. Now, to reduce costs, agencies are beginning to explore buying time from commercial satellite services.
Defense agencies will only use the commercial market if they can encrypt information from a sending device all the way through a network to a receiving system, industry experts say. Layering an agency's encryption scheme on top of a satellite network will reduce throughput and tax an information technology staff's technical capabilities, because such work is extremely complex, they say.
Just like terrestrial carriers, satellite suppliers have been building out their networks with the expectation of never-ending, double- and triple-digit increases in network bandwidth. "A couple of years ago, no
one anticipated the magnitude of dot-com fallout," admitted Tachyon's Guralnick.
But this may be to users' advantage, because it leaves a satellite bandwidth glut, experts say.
"Satellite bandwidth utilization rates are in the 60 [percent] to 80 percent range, which means there is enough capacity to handle the growth anticipated in the next few years," said Northern Sky's Baugh.
Despite this apparent surplus, a handful of satellite launches are scheduled during that time:
Baugh does not expect all of the planned launches to take place. "There just isn't that much demand for more bandwidth," he said.
But the glut could entice more government agencies to examine satellite services.
"Whenever there is a bandwidth glut, pricing comes down, and we are seeing that scenario unfold in the satellite services market," he said. "The lower pricing, coupled with the need for various types of backup plans, is resulting in growing interest in satellite network services."
Korzeniowski is a freelance writer in Sudbury, Mass., who specializes in technology issues. He can be reached at firstname.lastname@example.org.