FEATURE

Wireless Data Made To Order

by Peter Rysavy

CDPD Offerings From Software Companies

When carriers first announced intentions in 1992 to deploy a new wireless data technology called Cellular Digital Packet Data (CDPD), market reaction was enthusiastic. The primary reason: Unlike other wireless data networks that use proprietary networking protocols, CDPD would use industry-standard TCP/IP protocols. These protocols, already the foundation of the Internet, were rapidly being adopted by corporations for enterprise networks. CDPD promised numerous benefits. Customers could use exist ing TCP/IP applications over wireless connections; they could more easily connect to their servers at the back end; they could readily develop new wireless applications; and they would have a wireless connection to the Internet.

As developers and potential customers learned more about the technology and its deployment, they discovered some complications. For instance, it quickly became clear that nationwide coverage would require a large number of cellular carriers--many of whom are normally competitors--to work closely together. This, combined with the inherent difficulty of communicating data reliably over radio links, delayed deployment.

Meanwhile, software developers learned that their "off-the-shelf" TCP/IP applications would not work well over CDPD without being optimized.

Today there is encouraging progress in both deployment and application development. CDPD service is spreading. At the end of 1995, service was available in approximately 45 U.S. cities. By t he end of 1996, service is expected in some 60 cities. Customers can finally deploy applications over reasonable service areas. Meanwhile, the software industry is beginning to design products for this new environment.

Application Issues If existing TCP/IP applications are supposed to work over CDPD, users might run into problems of cost, reliability and performance if developers don't optimize their applications for wireless.

Anyone with a cellular telephone knows that wireless communications can be expensive, and wireless data is no different. A typical charge for CDPD service is about 8 cents per KB, which equates to $80 per MB. Carriers offer volume discounts, but even at half or a quarter of that rate, users don't want to receive large e-mail attachments. Users must have control over what their application transmits and receives.

Users shouldn't have to worry about reliable connections, and CDPD uses innovative measures to communicate data reliably between client and server. But the radio environment is delicate, and if a user is out of range of the base station, the radio connection can suddenly be lost. Many existing applications designed for stable LAN environments may falter when subjected to intermittent connections between mobile client and server. These applications need to be upgraded to handle intermittent connections. The same applies to TCP/IP protocol stacks and middleware solutions.

Performance is another important issue. CDPD networks have strong throughput, with a channel rate of 19.2 Kbps, and actual throughput to the user of about 10 Kbps. But a couple of factors affect overall data throughput. First, the CDPD channel is a shared channel. Multiple users in the same cell must contend for the same channel. Second, while typical round-trip times between mobile systems and a server are often under a second, these delays are still longer than other WAN connection s. Moreover, under network stress conditions, like network congestion, channel hop ping and communications errors, delays can increase to several seconds or longer, and bog down transactions.

How Vendors Are Addressing the Issues There are several strategies for developing wireless solutions. At the highest level, we can differentiate between wireless applications and wireless-enabling technologies. Wireless applications are end-user applications where the vendor has optimized the application for the wireless environment by addressing the issues we discussed.

In some cases, the wireless-enabling technology makes it easier for application developers to optimize new as well as existing applications for the wireless medium. In other cases, the wireless-enabling technology allows IS managers to take applications never designed to operate over a wireless link and to do so without changing the application.

Most software progress with CDPD has been in the area of wireless-enabling technologies. For instance, although existing TCP/IP stacks can be used ove r CDPD, users will experience better results when stack vendors have made enhancements such as having the stack's lower-level drivers automatically reconnect to the network after a connection is lost. WRQ's stack has this optimization and other innovations for reliable communication over CDPD.

Special messaging middleware optimizes the wireless link by providing a software piece for the mobile system and a matching software piece on a server back at the enterprise network. Applications on the mobile system make simple calls to the local middleware layer, which exchanges messages with the middleware residing on the network. There, the middleware acts as an "agent" on behalf of the mobile client to conduct transactions, such as sending e-mail or doing database queries. Oracle Mobile Agents is a good example of this category.

LAN access middleware fools LAN applications into thinking they are connected directly to the LAN, instead of being intermittently connected by a wireless l ink. With MobileWare/Informix MobileWare, for example, users can run cc:Mail over CDPD as if they were directly connected to their corporate LAN and its mail servers.

Finally, host access middleware lets existing host/terminal applications operate over a wireless link. It works by encapsulating the terminal traffic in IP datagrams. For example, Attachmate's Attachmate ZIP! SNA Server now supports CDPD.

In addition to these exciting software developments, standards bodies are completing standards that will prove important for CDPD applications. These include standards by the Portable Computing and Communications Association (PCCA), which has formalized an AT command set for wireless modems and worked closely with Microsoft to develop a set of wireless extensions to the NDIS. Finally, there is work to standardize how applications access wireless networks using the new WinSock 2 standard. (See Network Computing Online at http://techweb.cmp.com/nwc for a list of applications that support CDP D.)

Using Circuit-Switched Connections As effective as packet data is over cellular connections, users also have the option of using the cellular network for circuit-switched communications. Armed with "cellular-ready" modems and "data-capable" cellular phones, it is relatively straightforward for users to connect a cable directly from their PC Card modems to their cellular phones.

Since these modems use special error protocols designed specifically for cellular connections, best results occur when the modem on the other end of the connection supports the same protocols. But most modems today do not, whether they are at corporate modem pools, Internet service providers or online services. Cellular providers are solving this problem by deploying modem pools at their mobile switching centers.

Wireless packet data is best suited for short and bursty communication. Circuit data is better suited f or longer transactions, such as batch operations and fil e transfers. Users and applications developers will appreciate the flexibility. Developers should consider dual-mode applications that connect over CDPD or circuit data, depending on the type of transaction or coverage available.

Carriers are considering blending circuit data and packet data in a hybrid network referred to as Circuit Switched CDPD (CS/ CDPD). With CS/CDPD, users will use a circuit connection over the air, but most of the rest of the connection will be packet switched. CS/CDPD will offer users even more options to communicate.

Peter Rysavy is the principal of Rysavy &Associates, a consulting firm working with companies developing mobile communications solutions. He can be reached at rysavy@halcyon.com.

pACT: The Newest Wireless Son Of CDPD

While CDPD has received most of the market attention, AT&T Wireless Services and other undisclosed companies are preparing networks based on a new wireless data technology called Personal Air Communications Technology (pACT). Networks based on pACT are considered narrowband Personal Communications Services (PCSes): Advanced paging networks that use new spectrum in the 900-MHz band recently auctioned off by the FCC.

SkyTel has already launched a two-way paging service in this arena, throwing down the gauntlet in a well-publicized rollout. But carriers using pACT, running a year or two behind SkyTel, intend to leapfrog

SkyTel's technology with a highly innovative system offering significant advantages.

pACT uses industry-standard protocols, including the venerable IP protocol and Limited Size Messaging (LSM), a series of open messaging protocols supported by the CDPD Forum. Throughput will be 8 Kbps for both sending and receiving, although messages are not necessarily communicated in real time.

The fundamental limitation of narrowband PCS networks is that licenses are limited to a maximum of two 50-KHz channels. Given projected usage rates, this limitati on restricts message sizes to a practical maximum of about 500 characters. Nonetheless, this leaves the door open for many applications. On the flip side, carriers were able to obtain nationwide licenses. Companies like AT&T Wireless Services can offer a national network, which simplifies deployment immensely compared with CDPD, where multiple carriers must collaborate to offer comparable coverage.

pACT offers other advantages, including authentication and encryption, as well as a service that locates pagers to within a block or two ideal for dispatch applications.

CDPD Modems: Living Wireless, And In Style

After spending some time with CDPD modems and using the technology, we've come to a very definitive conclusion: We're impressed!

CDPD is genuinely cool technology. While these small boxes are indeed treated like modems by your laptop software and look a little bit like modems, the similarity begins and ends there. CDPD is really a digital wireless connec tion to the Internet, suited best for transaction-based applications that need to move only small amounts of data across the network because it's expensive.

The modems we tested are high-quality devices that often can be used for considerably more than just CDPD service. For general applications, we found that performance was not really the criteria upon which to base your choice of modem. Performance is affected much more by the load on the cellular network in your coverage area than by the particular modem design. Other considerations include the modem's size, additional service features, battery options and cost.

The Testing We traveled to Seattle to test CDPD modems from IBM, Sierra and PCSI with the wireless data service from AT&T Wireless Services (formerly McCaw Cellular Communications). At first glance, you can't imagine three more different products. But their performance was similar and quite good.

We performed two different sets of te sts using each mode m. The first involved bulk transfers of files using FTP to and from a laptop PC. We tested each modem under WRQ's Reflection TCP/IP software, which has been specifically enhanced to support CDPD devices.

We also tested the IBM and Sierra modems under Windows95, made possible by using INF files written by AirData's staff. We did not have an adequate INF file available for the PCSI modem. Support for Windows95 will probably be available from all of the vendors by the time you read this.

Our second set of tests attempted to discover the latency associated with sending data through the CDPD network. We simply used ICMP Ping packets of various sizes to measure network latency.

All of our tests involved timings against servers owned by AirData that were as "close" to the CDPD network as possible. A trace-route revealed that the servers used in our testing were three hops away from the CDPD modems. That's fairly close, since each additional hop probably added no more than a few milliseconds of delay to our tests. The servers against which we tested were Sun SPARCs running Solaris. Just to see how much the choice of server mattered, we did a trace-route on a server at Microsoft in nearby Redmond, from which we found ourselves 19 hops away.

The laptops used in our tests were Dell Latitude XLs with 100-MHz 486 chips or IBM ThinkPads with Pentium processors. Where serial ports were used, we attached at the highest speed that modem would accommodate.

We found the minimum round-trip latency to be in the 300-millisecond range, and the average data transfer speed to approach that of a standard 9,600-bps, land-line modem. That's not half bad. Furthermore, we found that variations in transfers were often less severe than with conventional land-line connected modems.

We also used one of the modems to read e-mail, surf the Internet with Netscape and transfer personal files. We found the service in general to be excellent, sometimes preferable to our IBM T hinkPad's built-in modem, depending on the long distance line conditions we encountered. Of course, we were basically CDPD pioneers here, so there wasn't much contention for the cell we were in.

Our testing showed that all of these modems were fine for connecting your computer to the CDPD network. The IBM and PCSI modems, however, were cheaper and had the additional flexibility of supporting voice as well as data. This earned them slightly higher marks than the Sierra modem.

The IBM Cellular/CDPD Modem

When IBM does it right, it really does it right. The really nifty thing about the IBM Cellular/CDPD Modem is that you can do voice, fax and data with the same unit. The voice stuff is very Star Trekish--the "headset" consists of nothing more than an earpiece that can pick up your voice through the vibration of your jawbone.

The modem connects to your laptop through a PC Card, and it can run off your laptop battery or with "battery assist." That's IBM's term for putting batteries in the modem. It takes four AA batteries. Physically, the IBM Cellular/CDPD Modem is about the size of three 3.5-inch floppies piled on top of each other. It comes with a kit to attach it to the back of any notebook. IBM also offers a version that fits into the ThinkPad's proprietary floppy drive slot.

In our tests, the IBM wasn't the fastest modem, but it was only slower by a cat's whisker. Both stacks that we used in our testing support "forward acknowledgments," which means that writes can be acknowledged as complete as soon as all data is transferred to the modem. As the transfers became longer, the advantage of the forward acknowledgment became less pronounced and the true capability of the modem was more obvious. Where IBM's product was faster, it benefited from the PC Card connection making its forward-acknowledged writes seem faster than those of the other modems we tested.

The IBM Cellular/CDPD Modem is also only compliant with CDPD version 1.0, while the other modems we tested were compatible with version 1.1 of the specification. While it may not be the fastest, its voice features and nifty headset will make it the right choice for those truly avante-garde mobile users.

The PCSI Ubiquity 2000

The Ubiquity 2000 is a half-inch thick and roughly the same size as a notebook computer. It is intended to be attached to the back of a laptop. If used that way, it is a fairly compact and convenient design. If you just set it next to your computer, it tends to be on the awkward side.

Like the IBM modem, it is capable of handling CDPD, AMPS cellular fax, data, voice and land-line fax, data and voice. Unlike the IBM modem, the voice option is not particularly whiz bang. Also unlike the IBM, the Ubiquity is compliant with version 1.1 of the CDPD specification, which has some advantages in congested sites.

The Ubiquity 2000 was fairly fast, particularly when writing to the network. However, latencies seemed to be a bi t higher than with the other modems. Performance was solid and predictable, as it was with the other modems. The Ubiquity was also the lowest cost device we tested.

The Sierra PocketPlus 210

The PocketPlus is a compact modem that will do CDPD as well as analog cellular and land-line data communications. The modem is about half the size of a video tape (same length, but about half as wide). It can be powered by its internal Ni-Cad Batteries or by an AC adapter.

In our tests, the PocketPlus suffered somewhat due to the fact that, at 19,200 bps, it had the slowest transfer rate between the laptop and the modem. Because of this, and the forward acknowledgments provided by our stacks, FTP writes were most significantly affected.

In our ping test, the PocketPlus did quite well, consistently ranking as one of the quickest. The ability to use land lines as well as both analog and digital cellular makes the PocketPlus a great choice for those who absolutely needs to connect to t heir data by any mea ns possible.

The PocketPlus also comes with a useful tool called The Watcher, which can report on such important measures as cellular signal strength and remaining battery life. It can also help to configure the PocketPlus. The PocketPlus is a good choice if you're a Macintosh user, since Sierra originally designed it for the Mac.

Art Wittmann is a senior editor of Network Computing and associate director of the Computer Aided Engineering Center of the University of Wisconsin-Madison. He can be reached via the Internet at wittmann@engr.wisc.edu. Network Computing would like to thank Tom Kee of AirData for sharing his time and expertise with us for this review. This guy knows CDPD.

March 11, 1996