Wide-area wired communications and network-access services are rapidly being augmented by more flexible wireless solutions
David A. Harvey and Richard Santalesa
The year 1921 marks the first recorded use of mobile wireless in the form of two-way radio communications by the Detroit Police Department. A quarter of a century later, in 1946, the first mobile telephone rang through the air. Until recently, however, ubiquitous wireless connectivity seemed as fanciful as the communicators used on Star Trek. Not any more.
Today, wide-area wireless communications span the radio spectrum from the low kilohertz bands to the ultrahigh microwave frequencies (see the figure "Frequencies for Wireless Data Communications"). Even Orbit, a proposed telecommunications device from Bell Northern, is supposed to affix (in the b
est Captain Picard style) to your clothing. The wired ties that bind you to your wide-area communications, data transmission, and network-access services are rapidly being augmented by more flexible wireless solutions.
Catch the Wave
Since our last look at wireless mobile communications (February 1993 BYTE), the wireless market has evolved dramatically. CDPD (cellular digital packet data), which rides atop the existing analog cellular infrastructure to provide a digital data capability, has moved from the drawing board into the market with the backing of industry big guns such as AT&T, Bell Atlantic Mobile Systems, Nynex, GTE Mobilnet, PacTel, Southwestern Bell, McCaw Cellular, and Sprint. In response, RAM Mobile Data (Woodbridge, NJ), a data-packet-radio joint venture of BellSouth Mobile Systems and RAM Broadcasting, announced in early March flat monthly rates of $25 for up to 100 KB of messaging, $75 for up to 400 KB with additional messaging at 20 cents per KB--approximately 10 cents per packet--
and $135 for unlimited messaging. The effects of such competition will be to bring wireless more quickly within the reach of more users.
In addition to wireless service providers, a slew of value-added companies are riding the wireless wave to double-digit growth. RadioMail, which provides links between user-wired E-mail services and the Ardis or RAM Mobile Data networks, and Wireless Telecom, a wireless hardware and gateway distributor, have experienced rapid expansion. In fact, RadioMail announcements of additional services and alliances are almost a weekly phenomena; its latest announcement introduced a wireless fax service complete with customized cover sheets at 99 cents per domestic page.
Further fueling the wireless marketplace is an increasingly broad range of wireless- enabled hardware and software products. On the hardware side, Dell Computer's new line of Latitude notebooks ship with a comprehensive communications package called CommWorks from Traveling Software, as well as a free sub
scription to RadioMail. Also, IBM's recent ThinkPads offer wireless connectivity, and AST's newly released PalmPad lists similar capabilities as an option.
On the software side, nearly every major LAN E-mail package and numerous stand-alone communications packages have added or are scrambling to add wireless support. Lotus cc:Mail, Microsoft Mail, WordPerfect Office, Da Vinci eMail, CE Software's QuickMail, and Connect Software's E-Mail Connection 2.0 all work with wireless services. E-mail is quickly moving beyond the LAN.
At the Speed of Light
Wireless technology breaks down into one-way and two-way communications. One-way communications, best typified by paging services, allows broadcasts of information to single or multiple receivers. Perfect for dispatching messages, news updates, pricing information, and routing assignments, one-way communication is a cost-effective method for delivering information to those who are able to easily respond through terrestrial means. For example, a number
of financial institutions are using EMBARC's (Boynton, FL) one-way messaging service to post up-to-the-minute mortgage rates to all their branches and field-loan officers. The most interesting developments in one-way messaging come from products like Motorola's NewsStream PCMCIA cards coupled with providers like EMBARC to deliver information from various news services anytime, anyplace.
While one-way broadcasts play an important role--their lower price and one-to-many broadcast abilities make them extremely useful for information delivery--two-way communications will shape your untethered working future by allowing connection to LANs, wireless pay phones, real-time conferencing, and more. The three most important two-way technologies are analog cellular, RF packet radio, and CDPD.
Signals and Packets
One of the simplest forms of wireless communication is analog cellular. Designed originally for voice communications, analog cellular operates much like a land-line-based telephone--indeed a cel
lular call travels for most of its distance via land lines. Transmitting data over analog cellular requires a modem that links to your cellular service or phone.
Cellular gets its name from the hexagonal cells (which are roughly 8 miles in diameter but smaller in densely populated areas with heavy traffic) served by a single base station (i.e., a setup common to most wireless services). Moving between cells requires that your signal be handed off between adjacent cells. This is no problem for voice signals filled with gaps and pauses; however, add in data, and without special protocols or equipment, every handoff is a potential fouled transmission. Indeed, the problems with data transmission over analog cellular are manifold and are so inherently related to the nature of analog that in the long run, packet-based communications will be the way to go.
Packet-based communications represents the next step up the ladder of complexity. The basis for services such as RAM Mobile Data and Ardis and for t
echnologies such as CDPD, packet-based communications is far better suited for data transmission than analog. Even voice is moving to packets. In the future, voice cellular will take the packet route using CDMA (code division multiple access) and TDMA (time division multiple access). Both of these technologies increase the cellular capacity: TDMA by a factor of three and CDMA by 10 times over existing analog cellular systems. In part, this increase is due to their packet nature.
The advantages that packet transmission has over conventional circuit switching include the following:
-- Robustness: Carrier loss between packets is typically not a problem.
-- Security: Encryption on small packets is easily handled.
-- Per-packet billing: Short messages, which encompass the majority of E-mail
traffic, can be sent inexpensively, charging only for transmission air time.
The logic behind packet use is irrefutable. Radio is synonymous with interference, drop-outs, and static. This, i
n turn, dictates that wireless data transmission (where every bit counts) include robust error correction, either via cellular protocols--such as Microcom's MNP10, AT&T Paradyne's ETC (enhanced throughput cellular), and USRobotics' HST Cellular--or through the actual transmission mechanism (i.e., CDPD's TCP/IP or the Mobitex packet-based transmission architecture that RAM Mobile Data uses).
Analog radio transmission, particularly voice transmissions, maintain one advantage over pure digital wireless. Analog radios degrade more gracefully than digital radios as determined by a concept known as fade margins. Fade margin refers to the tolerated level of signal decay before communications cease. With digital, the distance between a completely valid signal and one that's totally useless is small and abrupt.
Using RF Packet Radio
Unlike analog cellular, packet communications is not connection-oriented; with RAM Mobile Data, for example, your radio modem is connected as long as it's turned on.
The message path from RAM Mobile Data to CompuServe is typical of packet wireless operation. Once powered on, the wireless modem identifies itself to the local base station, each of which provides up to 16 separate radio channels. If you need additional capacity, more than one base station can serve service areas known as MSAs (Metropolitan Statistical Areas). According to RAM Mobile Data, each channel can handle between 1500 and 5000 packets per hour.
To communicate, existing RAM Mobile Data-compatible modems, such as the Mobidem (Ericcson GE Mobile Communications), broadcast at 896 to 902 MHz and receive at 935 to 941 MHz. The modem establishes a link with your computer, and as the message text arrives from your system, the modem breaks it into packets with a maximum size of 512 bytes. Each packet is preceded by a header that can be up to 33 bytes long. The header contains a 3-byte sender code, a 3-byte addressee code, a 1-byte flag, a 1-byte packet-type marker, a 22-byte space for other addressees
, and finally, a 3-byte network time stamp.
To ensure against multipath interference, the modem applies a Gaussian Minimum Shift Keying, or GMSK, modulation to the signal. In addition, it levels 16-bit CCITT standard CRC (cyclic redundancy check) error detection and correction on top of the Hamming error-correction coding applied to each byte. Before transmission, the modem interleaves the data blocks. According to RAM Mobile Data, the interleaving means that a fade-induced error burst of 20-bit duration results in no more than 20 correctable single-bit errors in the 20 Hamming-encoded data words; therefore, no retransmission is required.
As it assembles the packet, the modem checks signal strength, verifies the connection to the base station, and transmits at 8 Kbps. At the base station, the received packet is verified, and a receipt verification is transmitted back to the Mobidem. From the base station, the header information is analyzed, and the data is relayed to a local switch--usually over
a wired network or system. From there, it passes to a long-distance carrier switch that confers with a national control center. Here, RadioMail's system tallies billing charges and routes the complete final message through its Internet gateway to CompuServe's Internet gateway for delivery. At present, RAM Mobile Data transmissions are limited to text-only messages delivered by means of the RAM Mobile Data-compatible modems and services.
CDPD Hops
Compared to the packet-radio networks, the nascent implementations of CDPD are more flexible, allowing a CDPD-compliant modem to serve both packet-based data and analog voice communications. CDPD is a clever hybrid of packet-based transmission and channel hopping that takes advantage of the natural pauses within voice transmissions. Like the RAM Mobile Data and Ardis schemata, CDPD breaks data into packets. (It uses GMSK modulation and Reed-Solomon forward error-correction code.) Also, as with packet networks, CDPD sends data on one set of frequencies and
receives it on another. What differs is that CDPD was designed to work on top of the same bandwidth as analog voice transmissions. This feat of packetized contortion is achieved by CDPD's channel-hopping protocol.
A CDPD device works by "listening" for the idle time within voice calls. When the device detects a sufficient idle interval, it sends a packet using the full 30-kHz bandwidth of the channel. If the channel is full, a CDPD device "hops" to another channel and repeats the listening/sending/hopping cycle. Corporate connectivity is greatly facilitated through CDPD's built-in support of TCP/IP, allowing interconnection of LANs, WANs, the Internet, and other information services.
CDPD-compliant devices, just beginning to roll off the assembly lines, allow users access to both traditional voice cellular, as well as packet-switched CDPD transmission. This is a needed step, especially for personal communication devices that need to incorporate existing wireless phones to make a move toward ubiq
uity.
The only problem with CDPD is that it has limited coverage. At the time of this writing, all that exists are test networks in Las Vegas and California. Although promised for early 1994, current estimates see most providers (including McCaw, GTE, and Bell Atlantic) rolling back start-up dates to the second or third quarter of this year.
Packets to PDAs
An added boost to wireless transmission is the newly formed wireless modem standards committee of the Portable Computer and Communications Association. By the end of the year, it will approve a standard interface for wireless modems based on the same Hayes AT modem command set that wired modems currently use. This will greatly ease the problems of software developers and make it easy to interoperate wired and wireless modems.
The interface standard will merge nicely with the current move away from 1-pound wireless modems, exemplified by the Ericcson GE Mobidem and Motorola's InfoTac, to newer PCMCIA form-factor wireless modems. Toda
y, the range of wireless PCMCIA devices available is restricted to paging cards and two-way wireless LAN cards. You can buy a Hewlett-Packard 100LX with a Mobidem wireless modem, but imagine how much more attractive the bundle would be if the modem were a low-power PCMCIA card.
The introduction of wide-area wireless PCMCIA devices will drive the young PDA market. Anywhere, anytime communications is the ultimate killer application for PDAs, a lesson that many companies have taken to heart seeing the disappointing sales of the Apple Newton. One of those paying attention has been Motorola, which introduced its Envoy communicator in March. Built around General Magic's Magic Cap operating system and Telescript communications language, the $1500, 1.6-pound Envoy boasts a wide range of software and hardware telecommunications capabilities, both wireless and wired. It includes subscriptions to America Online, AT&T PersonaLink, and RadioMail. With Telescript support in AT&T's PersonaLink, you can use the Envoy
to send intelligent agents to other users or service providers (see "Agents Away" on page 113).
Of main interest to wireless communications is the Envoy's integrated 4800-bps MDC wireless packet modem that taps into the Ardis network. Combined with a range of applications powered by a 32-bit processor, the Envoy is an advanced look at what the wireless future storming its way into the business world holds. Motorola plans to introduce additional wireless PDAs using Newton Intelligence and Microsoft's WinPad operating systems, both of which it licensed.
While Apple's Newton MessagePad and Sharp's Expert Pad lack built-in two-way wireless ability, third parties are stepping in to fill the gap. ETE offers the ETE Communicator for Sharp's Expert Pad. Pricing starts at $500 for a version sporting a cellular phone with a wired 14.4-Kbps data/fax modem. An optional wireless modem ties the Expert Pad into the RAM Mobile Data network.
Even though the wireless world still has much work ahead of it i
n laying the final touches on a nationwide infrastructure that reaches even rural areas, the moving curves of falling prices, increased wireless competition, new pocket-size devices, and the competitive business advantage that wireless brings points to one conclusion: The end of the decade will mark a communications landscape where wireless is mainstream.
Figure: Frequencies for Wireless Data Communications
The FCC has divided the available spectrum to support a variety of radio-based services. It is responsible for defining the limits of each class of service.
Photograph: Unlike earlier assays in PDAs, the Motorola Envoy is built from the ground up to support ubiquitous wireless connectivity.
David A. Harvey and Richard Santalesa are the publishers of PDA and Wireless World, an industry newsletter. They can be reached on the Internet or BIX at
daharvey@bix.com
or
editors@bix.com
,
respectively, or on MCI Mail at 405-6117.
Wireless Gets Real
