ADSL extends base network

It's all connected by "a very large, robust, fiber-optic intelligent hub
network," said Frank Dawson, chief of Fort Bragg's Automation Division, and the
Synchronous Optical Network backbone operates at 155 megabits/sec.


"It does almost all of our connectivity, logistical to financial to
personnel," Dawson said.


But it made no sense to run such a big fiber-optic pipeline to the base's 48 scattered
point-of-sale (POS) terminals, which use primarily text applications to exchange sales and
inventory figures with a central computer. Somehow the old copper telephone wiring to the
terminals would have to carry these occasional data bursts.


In December, Dawson began installing Atium GateWays from AG Communications Systems of
Phoenix to provide Asymmetric Digital Subscriber Line service to the POS terminals.


Over existing twisted-pair lines, the terminals send data upstream at 76 kilobits/sec
and receive at 1.5 megabits/sec. For all practical purposes, users can't tell the
difference between those connections and Ethernet direct to the backbone, Dawson said.


"They have full access to the network," he said. "There is probably a
small speed degradation in getting information back, but it's insignificant."


Fort Bragg officials were so impressed at the way ADSL economically extended the base
network to thinly populated areas that they plan to buy up to 1,000 more Atium GateWays.
Wherever there's a small group of one to four users in a building, "this is what
we're proposing to put in," Dawson said.


ADSL, originally designed for home video-on-demand services over installed unshielded
twisted-pair wiring, could not compete with Blockbuster Video and satellite television,
said Steve Quibell, senior product planning manager for AG Communications. But the demand
for higher-bandwidth Internet and intranet access has created a new ADSL market.


ADSL not only transmits high-speed data in one direction, it also leaves a channel open
for voice transmissions, so an extra telephone line is unnecessary.


"Your phone can ring while you're passing high-speed data, and you can talk at the
same time," Quibell said.


ADSL modems compress signals to achieve their high speeds. The Atium GateWay product
has a carrierless amplitude modulation/phase modulation chip set from GlobeSpan
Technologies Inc. of Red Bank, N.J., to do the compression.


The Atium GateWay consists of an ADSL modem with a built-in 10Base-T Ethernet hub at
the user end, between telephone and terminal. Also built in is a high-pass telephone
filter that lets through high-frequency data transmissions. A low-pass filter outside the
modem directs the low-frequency voice traffic to the telephone network.


At the telephone central office, an ADSL modem card fits into the carrier's switch, and
a second set of filters separate voice from data traffic. The card's 10Base-T or
asynchronous transfer mode hub sends data to an Ethernet or ATM network switch, while
voice traffic goes on to the switched telephone network.


The maximum range for this type of configuration is 18,000 feet-about 312 miles-from
user to central telephone office. For longer distances, a standalone ADSL box can be
installed within 18,000 feet of the user to route the signal to an intelligent hub on the
ATM backbone.


Fort Bragg had to buy a few of the standalone boxes, but most connections lay within
17,000 feet of one of the base's central-office switches.


"We're right at the edge of the maximum distance, and we're working fine,"
said John Pruitt, chief of the Communications Division.


Fort Bragg is updating its telephone infrastructure and has state-of-the art switching
equipment.


About the Author

William Jackson is a Maryland-based freelance writer.

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