Data’s Workhorse

CDPD solves problems without causing many.

A lone police officer stops a suspicious vehicle on a desolate road at the edge of town. How does he get accurate, instantaneous registration information, warrant notifications, and even photography, at high speed?

It’s likely the information arrives via a CDPD network. Although CDPD networks don’t carry with them the glamour of newer, faster data networks, they do carry a lot of data.

CDPD, designed for use over analog networks, can be a mature solution for those with focused needs or a transitional solution for those looking eventually to enable quick data transfer or streaming video.

Several North American carriers are providing CDPD networks to enable public-safety departments to communicate with single police officers or to multicast important updates to the entire field. Devices range from in-car equipment to laptops to even small units built for bicycle patrols.

Milt Decol, Telus Mobility (www.telusmobility.com) director of major accounts for wireless data, said within the past three years, more than 850 public-safety units in the Telus coverage area have moved to CDPD. Verizon Wireless (www.verizonwireless.com) contracted to provide statewide CDPD for public safety in Illinois, completing a 4-phase roll-out over two years, finishing in late 2000. It uses existing infrastructure, so there were no delays in siting towers or base stations.

"It was designed and deployed with off-the-shelf products," said Tim Wong, Verizon Wireless data group manager of technical services. Beyond public safety, CDPD frequently is used for monitoring.

"Typical city, water and sewer systems are now moving to the CDPD network as a more cost-effective way to support their information and control capabilities to devices in the field,” Decol said.

Water, for example, can be routed remotely around a main break using CDPD control.

Credit-card authorization at point of sale is another area where CDPD’s mobility makes it handy for temporary events such as flea markets. It can be applied to telemetry applications, such as monitoring the stock in vending machines to providing efficient support, and is gaining acceptance as an enterprise application, offering remote access to sales and customer databases.

Predictable Technology
A particular advantage of CDPD technology is that it is relatively straightforward, said Andy Willett, AT&T Wireless Data Services (www.attws.com ) vice president. AT&T's involvement with the technology stretches back nearly a decade.

Problems that crop up in CDPD networks tend to fit into a few basic categories. The on-again, off-again nature of the technology (connections are only maintained when in use) sometimes can result in synchronization errors as the mobile unit and central server renew acquaintances for each transmission.

Other issues are not unique to CDPD but crop up in the networks that carry it. Network and terminal must cooperate to lessen multipath interference. This can be accomplished by robust software on the receiving end, as well as thoughtful placement of cell sites, especially sites placed outside of a carrier's existing network.

Fading can be an issue at the fringes of coverage, dependent on conditions and on the frequency and speed at which the signal is carried.

These, however, are issues carriers mostly have learned to deal with.

"The things that we need to be conscious of are applications that generate high volumes of traffic," Willett said.

AT&T Wireless is confident of its underlying backbone. Monitoring and telemetry devices may transmit as little as 1kb per day, while other applications might require 10Mb. If an application does not fit network availability, compression software can be employed to reduce the strain.

Most troubleshooting deals in deployment from the application side, in integrating CDPD with legacy systems, Willett said. Non IP-based databases, or older firewalls, can require some client-specific engineering. AT&T Wireless has developed its Data Solutions Program, in which it has partnerships with more than 600 companies that can build interfaces and create links between the client's existing systems and the CDPD network. In the case of a new public-safety application, database access, vehicle installation and officer training can take 4 to 6 months to implement; although, if the stars align, a point-of-sale system can be up within days.

Ensuring It Works
Although Willett radiates confidence, companies such as AirLink (www.airlink.com) provide equipment and software to identify shortcomings in the strength or reliability of CDPD networks. In fact, AirLink has made a business out of CDPD. Along with test equipment, it offers a CDPD vehicle-tracking system and CDPD modems. (Venezuela’s Movilnet (www.movilnet.com.ve/main.htm) recently bought AirLink’s CDPD modems to connect lottery machines rather than wait for landline connections.)

To diagnose and measure problems in the field, the company’s CDPD Cell Wizard notes registration failures, adjacent-channel interference, ERP delta discrepancies and problems in broadcast well configuration messages. In addition, performance data from the Windows-based AirLink Cell Wizard can be cross-referenced to GPS data as the Wizard moves through the coverage area, reporting holes in coverage in real time to a central server. This tool will save engineers time, according to Ray Fasnacht, AirLink president, by using algorithms to troubleshoot the causes of disruptions, rather than just identifying the disruptions themselves.

The company’s CDPD Protocol Analyzer is designed to collect real-time coverage and performance data. It monitors the entire protocol stack, from raw bits up to TCP/IP, Fasnacht said.

Other firms have introduced products geared toward testing the performance of CDPD modems. IFR Systems (www.ifrsys.com) recently announced a new software suite to improve the performance of its 2959 Advanced Multi-Mode Cellular Test Set, geared to test issues such as power level, frequency stability and error rate. The 2959 simulates base-station functions to test the response of CDPD modems or other handsets.

At the end of the day, CDPD carriers and test systems will ensure that the lone police officer gets his crucial information quickly and reliably. Of course, finding juice in the vending machine at the end of a long trail isn’t bad either.