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Reprinted courtesy of
Ericsson Inc.
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On October 28, 1998, winds and tidal surges from Hurricane Georges brought destruction to homes on Dauphin Island, just south of Mobile Bay.
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Mobile County's radio communications withstand nature's toughest blows
FOR MOST, the threat of a hurricane is either nonexistent or pretty remote. But what about those who live in hurricane prone coastal areas? Some decide it is safer to flee, while others make the choice to ride out the storm. But there is one group that cannot evacuate -- public safety officials. Whether it is the police, fire, EMS, public works, or communications division, these agencies' skills and services are needed most during these times. How do they cope? How do they handle the huge evacuations that are necessary? How do they communicate with one another in the midst of the hurricane? For Eric Linsley, Mobile County (Alabama) electronics systems analyst, it is simple. You design an effective communications system and you rely on that system to perform when you need it most.
Mobile County first implemented its 800 MHz, five site, 24 channel radio system in 1992. To better understand how they have expanded to their current six site, 39 channel system, we must first look to the beginning.
Train crash
During the early morning hours of Sept. 23, 1993, an Amtrak train derailed while crossing the bridge spanning Bayou Canot, a deep river gorge, plunging passenger cars into the river below. More than 47 people lost their lives in the wreck, and 160 were rescued.
This catastrophe was the first real test of Mobile County's new radio system. Linsley remembers the situation well. It is his job to ensure that the radio communications work, and he says that no one was disappointed with the system's performance that night.
"There were EMS personnel, the sheriff's department, Saraland and Chickasaw municipalities, state troopers, Marine police, fire departments, and dozens of other agencies on the scene and all operating on the Ericsson system, on two or three talk groups," Linsley says. "Natural disasters you can usually prepare for. But there was no preparation for something like this."
During those hours of rescue and recovery, the system logged approximately 73,000 push-to-talks (PTT's), nearly doubling the county's monthly average of 35,000 PTT's. The heroic efforts of rescuers are credited with saving many lives. For Linsley, this situation provided him with vital information about the radio system. For instance, Linsley has some interesting figures on one four channel tower site that had 150-200 radios utilizing one talk group.
"A lot of consultants think that a four channel site would be bogged down with this many calls, but that's just not the case," Linsley says. "We logged approximately 143,000 PTT's during the first two days of the incident, without any queuing at all."
Living with hurricanes
Located on the Gulf Coast, Mobile County is in a prime location to receive the brunt of most hurricanes that pass through that region. Preparedness is always the key to successful communications, especially during these trying natural occurrences.
Since the Amtrak train crash, Mobile County expanded its system by adding another site to boost its coverage and beefing up channels to 39 from the original 24. The county's average monthly PTTs jumped to about 100,000, up from about 35,000 two years previous.
Nearly two years to the day after the Amtrak train crash, Hurricane Erin began heading toward the Gulf Coast. As Linsley has already said, at least the county can prepare for something like this. And they did.
Erin skirted the Northeast region of the county, and caused extensive damage, leaving trees toppled everywhere. In fact, the road leading to one of the county's sites in the town of Stockton needed quite a lot of clean up. Officials had to call in the state forestry service to clear a path to the site. Despite the destruction to hundreds of huge pine trees that lined the road, the tower site was undamaged.
"During Hurricane Erin, our peak calls reached 135,000, nearly doubling our On October 28, 1998, winds and tidal surges from Hurricane Georges brought destruction to homes on Dauphin Island, just south of Mobile Bay. peak calls during the train crash," Linsley states.
Unfortunately, as the county was still cleaning up from Erin, Hurricane Opal struck one month later. This category three hurricane possessed maximum sustained winds of up to 150 mph. Although the brunt of the Hurricane bore down on Florida's panhandle, Mobile County and neighboring jurisdictions sustained serious damage from the immense rain and subsequent flooding.
"During Opal, we had peak usage on October 1st of about 117,000 PTT's. We had very high winds, but we didn't get as much damage as we did with Erin," Linsley says.
Mobile County went through a two-year respite without expe-riencing a severe hurricane, but when the next one hit, it made up for those two years.
"Danny was the next big hurricane to hit us," says Linsley. "We had upgraded channels on a number of our sites by this time. Danny dumped about 45 inches of rain on the Southern part of the county, and it just seemed to sit in Mobile Bay."
This torrential rain strained the resources of the public safety agencies and caused them severe problems in trying to reach victims. Linsley says the hurri-cane peaked on July 19th, and that the county had an astonishing 206,000 PTT's that day. That is nearly 100,000 more than with Hurricane Opal and almost 70,000 more than with Hurricane Erin.
"Even with this tremendous increase, the system continued to perform well," says Linsley.
Prefer microwave links
All of Mobile County's six sites are tied together using microwave links, except one, site six, which is owned by the state of Alabama. Linsley holds strong views on the use of microwave links versus using T1 (land) lines, especially during emergencies such as the hurricanes.
Site six covers the eastern portion of Mobile County and uses a leased T1 line. Although, none of the county's sites has ever gone down during operation, site six has been isolated during power outages.
"Leased T1 lines don't cut it," says Linsley. "We are dependent on our phone carrier to make repairs to the T1 lines, and they don't go out in the middle of hurricanes, which really hampered us during Danny."
"If lightning strikes a T1 line, it can go out and be out for days, and there is nothing we can do about it," states Linsley. "We can fix a microwave."
Linsley does have strong praise, however, for the remaining sites that are connected by microwave links.
"Ericsson recommended to us to use microwave on the other sites, and it was a good idea," he says. "Police want to push the button and be able to talk to all groups, and when tower six loses its T1 line, they lose that ability and can only talk to the users on that one tower site's group. Ericsson did an excellent job of engineering the microwave links and connecting them to our control site."
The county also discovered, during the most recent hurricane -- Hurricane Georges -- that T1 lines can be unreliable in any disaster. Hurricane Georges reached Southern Alabama on October 28th of last year. Mobile County logged 224,000 PTT's on that day, and recorded another 224,000 PTT's the following day.
Linsley says that the only problem during Georges was the T1 line at site six. He believes that the cost of upgrading that site to microwave will be recouped in its increased reliability.
"During all of these hurricanes, we never lost the microwave link between the sites and the controller," Linsley says. "Again, that says a lot for the engineers."
Investment, investment, investment
Linsley estimates that Mobile County has invested more than one million dollars in the original system, including the addition of 1,900 radios. By making and analyzing reports daily, Linsley has been able to assess areas where the system needed to grow, and where channels needed to be added to current sites.
"We like to think we are on the cutting edge of radio technology," says Linsley, "and we feel it is important that we stay up-to-date, and that is not going to happen without additional investment."
The system covers the 1,600 square miles of Mobile County, and users can go about 20 miles into Mississippi and still maintain radio coverage. Confidently, Linsley says, "I think we can easily cover 2,600 square miles with our system."
From his past successes with the network, no one seems to doubt him. Linsley says that Mobile County has preliminary plans to add a seventh site and 10 more channels by the year 2000.
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| Mobile County has endured frequent emergencies due primarily to hurricanes. Detailed analysis is extremely important to public safety system managers and reveals key information about system operation. For example, this comparison of peak and average traffic loading demonstrates how the peak emergency traffic has actually grown at a faster rate than the system itself -- a critical factor in assessing system performance now, and in the future. |
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