Cellular Priority System Begins Operation

March 2003
By Robert K. Ackerman
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Mobile calling for emergency response is up and running.

Emergency responders now can count on priority cellular access in a pinch as the U.S. government establishes a wireless version of its Government Emergency Telecommunications Service. Known as the Wireless Priority System, or WPS, the new cellular system promises connectivity in a shirt pocket for authorized users ranging from the president down to a local fire chief.

Access privileges are tied to a particular cellular telephone rather than a user’s identification. When a user keys an emergency authorization code into that telephone, it is granted predetermined priority access to the commercial cellular network. This priority access is achieved while still maintaining considerable capacity for the general public to use the same cell during an emergency.

The first WPS element stood up in January through the commercial carrier T-Mobile. Service originally was available in the eastern United States through the global system mobile (GSM) carrier. Throughout this year, the service will be expanded to cover the entire contiguous United States. Other GSM carriers will be added to WPS, and plans are in the works for including code division multiple access (CDMA) cellular systems as well.

John Graves is the director of the Government Emergency Telecommunications Service (GETS) for the National Communications System (NCS). He also is the engineering and development lead for WPS. Graves explains that the push for a wireless priority service began well before the terrorist attacks of September 11, 2001. From its origins early in the 1990s, the program took several years before the Federal Communications Commission issued a report outlining its permissible parameters in the summer of 2000.

This report provided several guidelines: The wireless priority service must be invoked on a call-by-call basis; like GETS, it must be available for the national security and emergency preparedness (NS/EP) user community; it must provide five levels of prioritization; it must be voluntary on the part of the carriers; and these carriers must provide reasonable capacity for the public even though they are providing a priority service for the government.

WPS shares some similarity with GETS (SIGNAL, April 2002, page 17), but it is a different system, Graves notes. A cellular telephone call begins as a wireless connection to the public switched telephone network. The caller connects through a cellular tower to a telephone switch, after which the call proceeds through the public network as would any other call. In effect, WPS serves as an extension of the priority service for NS/EP to 135 million more telephones than provided by GETS, Graves explains.

The two systems differ in how the user makes the call. With GETS, the user carries a calling card that enables the priority access from any landline telephone. Because this card features a personal identification number (PIN) for GETS access, the priority is invoked only in emergencies. For WPS, the user’s access is his or her cellular telephone unit. This eliminates the need for a PIN, so the user need only use an access code that identifies the call as a priority call for the WPS.

Both WPS and GETS use similar exemption from congestion controls, so that a carrier’s efforts to restrict service in the face of excessive demand will not hamper the priority calls. Both systems also use call queuing to increase the probability of call completion. This becomes more vital in the cellular world, as its bottlenecks are smaller—the capacity of the local cell.

Graves explains that these WPS cellular calls are given different levels of priority, based on the caller’s occupational position, and are placed in a queue to await the first open channel in a cell. If a cell is not busy, the call goes through immediately. If a cell is full, the first available opening goes to the highest priority WPS call. As mandated by the Communications Act of 1934, routine commercial users cannot be bumped or pre-empted from the cell.

If a WPS user is moving, a call can be continued into a new cell only if that cell has an available channel. If the caller crosses into a full cell, it cannot take a call handoff regardless of the priority. The call will be dropped, and the WPS caller must redial and enter a new queue in the newly entered cell. Graves explains that arranging for automatic queuing upon entry into a full cell could be engineered, but it would be too technically difficult and would take too much time to be incorporated currently.

WPS’ multilevel priority system employs five levels of service. Graves explains that WPS needs five priority levels because its capacity per cell is limited to ensure continued public access. The potential range and scope of disasters eliminates the ability to adopt a one-size-fits-all approach. Deciding which callers are more important than others touches upon “Solomonic issues,” Graves allows. This approach is not needed for GETS because the bottlenecks in the commercial wireline network are large enough to handle all potential GETS calls in various scenarios, he notes.

A limited form of WPS service has been operational in the Washington, D.C., and New York City areas since May 2002. The expanding WPS system builds on this service, which began on T-Mobile. Graves notes that T-Mobile uses switches from different companies such as Nokia, Ericsson and Nortel. Ericsson also interconnects with base station subsystems from Nokia and Nortel.

These switches are the drivers for determining how soon WPS can provide service in different parts of the T-Mobile network, Graves explains. WPS engineers are working with these companies to develop enabling software, but Graves notes that all three companies are not delivering this software at the same time, depending on the availability of patches or generic releases. WPS now can deliver the capability with the part of the T-Mobile network that uses Ericsson switches, and the remainder of the network should be working fully by the end of the year.

“It’s a very complex set of activities that goes on before we can say, ‘yes, we’re ready to support a user’ in any given geographic area or market,” Graves says.

The January WPS launch added the metropolitan areas surrounding Atlanta; Birmingham, Alabama; Boston; Jacksonville, Florida; Louisville, Kentucky; Memphis, Tennessee; Miami; Mobile, Alabama; Nashville, Tennessee; New Orleans; Norfolk, Virginia; Philadelphia; and Richmond, Virginia, to the Washington and New York coverage. The Chicago and St. Louis regions are the next to receive coverage, with some southwestern areas joining the system in the summer.

Ironically, the East Coast coverage actually began one month before formal implementation in January, when the contract with T-Mobile actually was signed. Graves explains that the WPS program features concurrency of development, fielding and user support. “All the things that we normally would do sequentially in a well-structured program, we are doing concurrently because of the urgency of the program,” he relates. “T-Mobile knew it was going to get a contract with us, so we deployed the capability—a technical activity—tested it, had it in place, and then finally we inked the contract with all the terms and conditions.” The cost was known before then, he adds.

Graves continues that WPS is working to sign three other GSM cellular providers—AT&T Wireless, Cingular and Nextel—to contracts for coverage. Both AT&T and Cingular currently are converting their systems from time division multiple access (TDMA) to GSM. WPS will try to bring in these systems “in a more rational way” than was done with the T-Mobile contract, he emphasizes. Once their contracts are signed, WPS wants to implement them into the system by year’s end, although complete AT&T and Cingular coverage is dependent on the full GSM conversion of their networks. This may take up to four years.

The next step will be to add CDMA carriers such as Verizon Wireless and Sprint PCS. “Our objective is to make sure that we address substantially all of the capacity of the wireless industry, just like we do the wireline industry with GETS,” Graves says. “However, we cannot do that without CDMA.”

There are pros and cons to both GSM and CDMA, Graves notes. The short run favors CDMA in capacity. But, in the long run, the conversion of AT&T and Cingular to GSM will boost its capacity past that of CDMA. This applies directly to queuing issues.

Adding CDMA carriers to WPS probably will cost an additional $165 million, Graves offers. This touches on one of the biggest challenges facing the program—funding. “Finding money is very hard,” Graves declares. “One of our current difficulties is that we have no program funds in FY [fiscal year] 03.” The Defense Department funding for WPS in its last FY 03 appropriation was zeroed. The program is still proceeding because it received $101 million from the Defense Emergency Response Fund, or DERF, which was not assigned to any fiscal year. The NCS is transitioning to the Department of Homeland Security, so the funding responsibility will transfer to the new department.

WPS originally received approval for a $208 million nationwide program, with $10 million provided for the early capability in Washington and New York as well as coverage for the Salt Lake City Olympics in 2002. The original Defense Department funding for FY 03 was set at $73 million, and Graves emphasizes that WPS still must receive that money to operate, albeit from the Department of Homeland Security. There is a degree of time-sensitivity to receiving this funding, he adds. “We are an up-and-running program that is burning money on contracts as we go through this process. This is not future years’ money that we need—this is last year’s money for contracts and work underway,” he declares.

Funding is not the only challenge facing WPS planners. The reserve capacity assurance for the public, or RCAP, involves software that ensures that WPS users do not utilize more than 25 percent of a cellular site’s capacity during a period of congestion. This guarantees that public users have access to a cell site even if its use is sought by a large number of WPS customers.

While Graves relates that RCAP sounds simple in principle, achieving it is fairly complex. Engineers cannot simply reserve 25 percent of a cell’s capacity for WPS users because that capacity would go to waste if WPS usage on that cell is low during an emergency. The approach being taken is to ensure that a congested cell is used fully by all users in an emergency, even if only 5 percent of its capacity is accessed by WPS customers. Public customers would consume the remaining 95 percent, with that number dropping down gradually to no less than 75 percent as the number of WPS customers using that cell increased. WPS users would be at the head of the queue until their 25 percent limit is reached, after which they would form their own priority-driven queue for access when channels in that 25 percent block become available.

Another engineering challenge arose with the establishment and management of queues. The very concept of a queue had to be built into many of the switches’ software, Graves relates. Changes in the billing system also needed to be built into the system, especially for international travel. This was complicated by the variety of subscription plans and free call minutes available to cellular users.

The next big challenge facing WPS is to convert the original users in New York and Washington to accounts in their departments and agencies under the overall system umbrella. Concurrent with this is an awareness campaign for potential users in the new areas that were added in January. Many local emergency responders in those metropolitan areas, along with officials of vital infrastructure elements, are not yet aware of the new WPS availability. This includes military personnel at installations throughout the coverage areas. “We have a message to carry to the potential user base that this is a capability that they need to consider in their emergency kit bag,” Graves allows.

“In the case of the Defense Department, no responsible communicator or command and control individual relies on one source of communications,” Graves warrants. “Even in command centers where you have [all kinds] of connectivity, if you need to use the public network, this is one way to do it on a priority basis. We need to put [WPS] in the hands of those people who need it before the next disaster,” he declares.

Additional information on the Wireless Priority System is available on the World Wide Web at  http://wps.ncs.gov.

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