CDPD: Does It Work Yet?

When Cellular Digital Packet Data (CDPD) service first appeared in 1994, it was heralded as a panacea to the cellular modem blues. It promised lower connect-time charges and the error-handling and reliability that conventional cellular modems just couldn't deliver. Three years later, CDPD is available in almost all the major phone markets, and a growing number of modems and other supporting products are now shipping.

A host of law enforcement agencies, including Philadelphia's police department, are embracing CDPD technology as a way of providing up-to-the-minute information to officers in the field. The officers need to query databases at the motor vehicles department and the National Crime Information Center. These short, sporadic transactions are an ideal application for CDPD, which also prevents others, including suspects, from eavesdropping on conversations. That's why many police departments are deploying CDPD-equipped computers in patrol cars. But CDPD is good for more than just law enforcement applications. It can also be used by anyone who wants to get e-mail or Internet access on the road. For government buyers still on the fence about CDPD technology, the most important question is: Will it work? After extensive testing of the technology, we believe the answer is a resounding yes.

We looked at several CDPD products, including two modems (one with a Global Positioning System device) and a software utility. These products put CDPD to work in a range of platforms, including handheld computers, notebooks and vehicle-based solutions. We were delighted with how well these CDPD systems worked. And the potential savings over traditional cellular modems make CDPD a compelling solution for many mobile users.

CDPD works over the existing cellular network but adds a few twists. In a conventional cellular call, your modem connects to the nearest cellular antenna and stays connected to the network until you hang up. You "own" the connection from the moment you dial until you hang up, and you'll pay for connect time for every minute you're on the line. This scheme is known as a circuit-switched connection. CDPD, on the other hand, is a packet-switched technology. That means that you broadcast data in packets rather than owning a particular frequency at a given time.

How Much Will You Pay?

Packet switching has two advantages. Because you're not hogging a frequency, you're only paying for the packets you send and receive, rather than paying for a modem circuit that you're not using all the time. How much will you pay? Costs vary by provider, but plans with a high monthly minimum get as low as 4 cents per kilobyte. Occasional users may opt for the low monthly rates but pay as much as 15 cents per kilobyte.Sending packets of data only when necessary also helps conserve battery life-a precious commodity in many mobile scenarios. Packet switching is only part of the CDPD picture. Because CDPD is a data network, your modem doesn't get a phone number; it gets an Internet Protocol address. That means the equipment at the phone company's office routes your CDPD traffic-in the form of IP packets-out to the Internet or to some other data network. In essence, you're subscribing to a wireless Internet service provider.

CDPD also offers some signaling advantages over a simple analog connection. Your CDPD device can recognize when a frequency is busy and delay transmission for a moment. Also, the receiving modem sends an acknowledgment of each packet so that the transmitter knows when to retransmit.

The CDPD approach has some obvious advantages. First, you can forget the modem pool that you'd need for servicing calls from the field. They can all flow through a single network gateway. Second, any network application that can use an IP network will work without modification in a CDPD network. You can browse the Web or send electronic mail to database queries, and it won't require special software.

CDPD: It's Not for Everyone

That doesn't mean, however, that CDPD makes sense for all network applications. Applications such as e-mail that send short, sporadic bursts of data tend to fit CDPD well. CDPD's nominal 19.2 kilobit/sec speed means that you can send a typical e-mail message in a few seconds. Posting and retrieving transactions to or from a database also make sense for CDPD. Assuming that your mobile system doesn't need to look up a lot of information in order to create a transaction, you can post a transaction pretty quickly in just a few kilobytes. Of course, you may need to retool your database front end to minimize wireless traffic. Likewise, if you're querying a database, you won't want to browse through records. A query that returns a lot of data costs more in terms of time and money. But querying DMV records based on a unique value, such as a driver's license number, should prove quick and inexpensive (assuming the subject of your query isn't a scofflaw).

Remote monitoring applications make a lot of sense too. If a piece of equipment needs only occasional attention, it can send a short packet of data for just a few cents. Unlike notebooks and handheld computers, however, few electric meters have the smarts to interact with an IP network, so the cost of enabling the equipment for CDPD may deter you. On the other hand, interactive applications are not an appropriate application for CDPD. Given that you'll pay around $100 for 2M of data transmission, Web browsing could prove a very expensive proposition.

Traveling outside your CDPD vendor's coverage area may still prove problematic, although it's a lot better than it was a year ago. Unlike conventional cellular phone service, where you can expect to roam anywhere in the country without much hassle, your CDPD provider may not have a reciprocal agreement with the CDPD operators in other parts of the country. You'll need to evaluate what out-of-area coverage your provider can deliver if you expect to use your CDPD modem elsewhere.

We recommend all three of the products that we evaluated: the Sierra Wireless Corp. AirCard modem; the AirLink Communications Inc. PinPoint modem/GPS device; and bSquare Development Corp.'s bMobile software utility.

Sierra Wireless AirCard

The Sierra Wireless AirCard offers a remarkably flexible package for mobile workers. The package consists of a pair of Type II PC Cards: One card serves as a 33.6 kilobit/sec modem, and the other handles cellular radio transmissions. This combo lets you easily plug into land-line phone jacks when they're available or rely on either circuit-switched cellular or CDPD, depending on available connections.

We took the AirCard for a spin in our Panasonic Personal Computer Co. CF-25 Mark II ruggedized notebook. You'll need two PC Card slots in your notebook to accommodate the AirCards. (Our Panasonic notebook had three slots, so we had room for another card.) The two AirCards connect through a small external plug. It's a little inconvenient, given that we had to remove the little plug to close the door to the PC Card bay. Of course, when you're making a wireless connection, you'll need to attach the small cellular antenna, but it's big enough that you're unlikely to lose it.

Windows 95 recognized the cards and loaded Sierra Wireless' drivers and utilities. Sierra Wireless includes a handy little toolbar-size application that monitors your connections. Through it, we could easily switch from a land line to either of the cellular services and monitor the strength of our radio signals.

The whole process was remarkably painless, but if you're unfamiliar with setting up a dial-up IP connection, you may be intimidated by the host of settings you have to deal with.

If you've ever used a conventional cellular modem, you'll notice a couple of advantages of CDPD right from the start. First, the connection is almost automatic and practically instantaneous. You no longer have to go through the conventional dial-up routine while you connect to the host's modem. The process is more like using e-mail when you're connected to a LAN, only slower. Our e-mails took only a few seconds to post, so the delay isn't bothersome. Downloading larger files, however, never reached the nominal CDPD speed of 19.2 kilobit/sec. Our 115K file took more than a minute and half to transfer, which works out to about 1 kilobyte/sec. On the plus side, CDPD makes dropped connections easier to endure. If you lose a packet, the modem automatically retransmits it, so you don't have to re-establish the connection by hand. At $895, the AirCard isn't cheap. But mobile workers who can use land lines at times but need the flexibility of cellular service couldn't ask for a better product. We'd like to see Sierra Wireless improve the documentation-which doesn't give you much trouble-shooting advice and lapses into pretty technical descriptions at times-but if you need to be connected no matter what, the AirCard is unbeatable.

AirLink PinPoint

The AirLink PinPoint combines a CDPD modem with a Global Positioning System (GPS) receiver for a solution well-suited to vehicle-based applications. This ruggedized-and hefty-external CDPD modem connects to your mobile computer over a conventional serial cable and requires an external 12VDC power supply. We didn't actually install the unit in a vehicle, so we relied on the AC adapter that came with it. The combination of a CDPD modem and GPS receiver lets you stay in touch with your fleet and know where they are at all times. Of course, the system assumes that you are running a vertical application that knows how to integrate this information. The AirLink PinPoint isn't an end-user solution. The documentation-ours was still preliminary-does little to guide users through the configuration and use of the modem. It does, however, list all the specifications you'll need to configure the unit, which is done through extensions to the modem's AT command set. The serial connection to your mobile computer is most likely to use a SLIP or PPP interface, both of which the modem supports. AirLink doesn't provide any software tools either, so you'll need to rely on your operating system's TCP/IP (or UDP) services to communicate with the modem.

After a little trial and error, we were able to get the unit working with our notebook. Our applications (e-mail and FTP) worked just fine. Obviously, neither application used the GPS interface. How easily you can retrieve position information depends on your GPS application.

Our transfer speeds were similar to the other CDPD modems we looked at-just over 1 kilobyte/sec for a 115K file. As with any application suited to CDPD technology, you'll probably work in short bursts of data of only a few kilobytes. Therefore, you'll seldom wait more than 10 seconds for the data to wend its way back from the host.

AirLink also offers the same package without the GPS receiver, the AirLink Raven. If you're developing a wireless application for your vehicle fleet or for remote equipment, both products are worth a look. You'll need some engineering talent to make the products mesh with your applications, but the interface is straightforward-at least to a programmer.

Square Mobile

CDPD and handheld computers make a very interesting pair. CDPD is well-suited to short, sporadic communications. Handheld computers work best with small, text-oriented documents, such as forms. The problem is that handheld computers tend not to have the IP and communications software that we take for granted on notebooks and larger, full-fledged systems. Enter bSquare. The company sells a variety of connectivity tools for handhelds, including an IP stack called bMobile.

To test Mobile, we used an NEC Technologies Inc. MobilePro 450 H/PC. This handheld unit worked great, and its 480-by-240-pixel touch-screen was remarkably easy to read. The MobilePro joins the growing ranks of small computing devices that run Windows CE, a Microsoft Corp. operating system designed for small systems-everything from handhelds to digital pagers and Web TVs. It sports an interface that's very similar to Windows 95, but it handles menus and similar items a little differently to accommodate smaller screen sizes and different input devices. We added a Motorola Inc. PM 100C for the CDPD connection.

For our handheld unit, we needed a host computer (a Windows 95 desktop in our case) to load new software. The two systems connect via Windows 95 Direct Cable Connection. And you can browse the contents of your MobilePro using an Explorer interface. This is where you'll start with the bMobile installation. We launched the installation routine from our desktop. The software automatically loaded onto the notebook, and we were impressed with just how easy it was. Once we restarted the MobilePro, we walked through the configuration. bMobile presents many options that need to be configured, but nothing out of the ordinary for a CDPD and IP setup. A little more explanation of what each setting does would help novices, but if you're familiar with IP connections, you'll be fine.

Once the system was configured, establishing an IP connection was as simple as double-clicking on our Dial-Up Networking icon and launching the Pocket Internet Explorer that comes with Windows CE. bMobile has a nice status indicator to let you know the condition of your connection. We don't usually recommend buying an IP stack for most applications, given that the Microsoft stack does a good job in most situations. Reconfiguring a handheld computer, however, can be a trying experience. A lot of the configuration tools and control panel applets may not be included in your handheld. The bMobile software remedies that by giving you nearly one-stop shopping for your IP and modem configuration.

For just $29.95, we found bMobile a worthwhile investment. Our only complaint is the short list of CDPD modems it'll work with; at the moment, you'll need a Motorola Personal Messenger.

-- Gregory S. Smith is a free-lance writer and network consultant based in San Francisco.

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Report Card

AirCardSierra Wireless Corp.(604)

Price and Availability: $895 retail. Government discounts available for volume purchases.

Remarks: This is a clever combination of two PC Cards that serves all your mobile connection needs from land line to CDPD. The cards work well and make a painless transition from wireless to conventional modem.

Final Score: Very Good

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Report Card

PinPointAirLink Communications Inc.(408)

Price and Availability: $1,000 retail.

Remarks: This rugged external CDPD modem also sports a GPS receiver, although you can buy a version without the GPS option. Ideal for vehicle fleet-based applications, this unit requires an engineer -or two-to install and integrate it.

Final Score: Good

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Report Card

Mobile Square Development Corp.(206)

Price and Availability: $29.95 retail.

Remarks: This is a handy Internet stack that works with any Internet application, including the Pocket Internet Explorer. bMobile may not be Earth-shattering technology, but it simplifies connection management and is definitely worth the modest price.

Final Score: Good

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Report Card

Visit for more information on CDPD, including:* Alexandria, Va., police CDPD application* Gloss on CDPD* Use of CDPD in public safety* Public Safety Wireless Advisory Committee home page* Explanation of our testing methodology.


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