Wireless WANs

How many times have you rushed through the airport, notebook smacking against your leg, trying to find a pay phone with a phone jack so you can dial in for messages between flights ? For the most part, airport pay phones don't have jacks , and you can't use your wired modem.

And how many times have you checked into a hotel room, only to discover that you can't plug your notebook's modem into a jack in the room? The jack and wires are fused, or the jacks are nonstandard. "DataPorts" on hotel phones are few and far between, though they are very welcome.

You're trying to mate a mobile technology notebooks with a stationary conceptdial-up. With your notebook, you can compute anywhere , but the need to communicate with your office or with your customers ties you back down to land.

Then you eyeball your portable cellular phone, sitting there in your briefcase, right next to your landlocked notebook. You're not the only one. Wireless technologies will be able to provide communication for laptops, notebooks, Personal Digital Assistants (PDAs), and even less intelligent devices, such as soda machines.

Talk of wireless networking has accompanied the arrival of notebooks and the talk of PDAs. To be successful, PDAs will have to be wireless. Notebooks are likely to exist in a quasi-wired world; they could go either way. Most of the loudly discussed wireless technologies are for in-building, or local, usage.

Companies such as Motorola, NCR, and Windata are building wireless network products that are designed to be used locally. Even the companies coming out with wireless PCMCIA cards, such as Xircom and Proxim, are using spread-spectrum technology, which is designed for local transmission.

But as the universe of roaming computer users grows, they'll roam farther. And they'll need universal connectivitywireless connectivity.

Unwiring The Wan

Five types of wireless technologies may serve: packet radio, packet cellular, circuit cellular, satellite, and paging. Packet radio, packet cellular, and circuit cellular are the most important.

• Packet radio. Mobile users can use the existing packet radio networks, such as RAM Mobile Data and ARDIS, to connect their computing devices to their networks.

• Packet cellular. Data communication can also take place over the US cellular network.

• Circuit cellular. Data can be carried over cellular voice channels today, in a manner similar to the way data is carried over analog lines by conventional modems. Modem vendors , including AT&T, PowerTek, and Vital, will play a big role in this market.

• Satellite. Low earth orbit satellites will carry voice and packet data. Motorola is building its Iridium network to provide worldwide voice coverage. Satellite bandwidth will be too small to serve the needs of most LAN users.

• Paging. The same networks that serve beepers may apply, despite low bandwidth.

Four Factors

Which technology or technologies take hold depends on their coverage, cost, performance, and adapter.

Service coverage must be nationwide and penetrate buildings. Many current wireless LANs fail in that they are localand often can't cross buildings or even floors.

For nomadic computing to take off, communication coverage must be ubiquitous. Both packet cellular and circuit cellular will offer greater geographical coverage than the packet radio networks, because a huge cellular infrastructure already exists to handle voice calls. Advantage in for packet and circuit cellular.

What is the market price of convenience? Dialing out from hotel rooms and racing from pay phone to pay phone in airports will be cheaper than using wireless communication. According to Forrester Research (Cambridge, MA), circuit cellular costs $.50 per minute, while packet radio and packet cellular charge $.04 per packet and up. Advantage in for packet radio and packet cellular. Cost and performance are a delicate balance. Although packet technologies are lower cost, they cannot elegantly handle fax. Packet cellular has been demonstrated at 19.2Kbps, which is a far cry from the speed LAN users are accustomed to. Fax service, because of the nature of the transmission, requires a circuit connection. For those on the road, faxing is as important as dialing back into the company network. Ad in for circuit cellular.

The adapter is critical. The different technologies' adapters vary in size, weight, cost, and power consumption. The new PCMCIA cards will help reduce the footprint of these adapters from the size of a brick to the size of a credit card.

Initial PCMCIA cards will use spread-spectrum technology (for LAN usage), as well as circuit cellular (for WAN usage). The adapter should be a fraction of the notebook or PDA's costanywhere from 10 percent to 25 percent. The adapter must also be simple to insert and remove. Make the adapter too big or difficult to install, and no one will want to transport it or figure out how to connect it.

Power consumption is key. The adapters draw power from the notebooks, placing yet another demand on the notebook's already short-lived battery.

Wireless vendors are acutely aware of the power crunch and are working to manage the power consumption. A packet cellular adapter can be combined with a cellular phone and use the same radio and antenna. The circuit cellular modem cards, as mentioned, will be implemented as PCMCIA cards.

A Look At Packet Cellular

Forrester predicts that by 1997, packet cellular will have an installed base of 1.4 million users in the US, which will be more than half the wireless installed base.

CelluPlaN, in particular, is projected to be the leader. It is a consortium composed of IBM and these nine cellular carriers : Ameritech Mobile Communications, Bell Atlantic Mobile Systems, Contel Cellular, GTE Mobile Communications, McCaw Cellular Communications, Nynex Mobile Communications, PacTel Cellular, Southwestern Bell Mobile Systems, and US West NewVector Group. Together, they propose a cellular packet technology called Cellular Digital Packet Technology (CDPD).

Packet cellular has a number of advantages. Because the technology relies on the existing cellular voice network (with some modifications), current cellular users will be familiar with it: they'll see cellular packet as an extension of their existing voice service. They'll feel comfortable buying and using it.

Packet cellular used the existing cellular infrastructure, with some modifications. The initial investment in CelluPlaN was $400 million added to the $9 billion cellular partners can use to subsidize their data service investment off their substantial voice revenues .

Also key to packet cellular's future success is its use of existing cellular radios and antennae. The cellular companies have the infrastructure in placethey can simply add a few more components . Most of the carrier and end- user components will cost less because they're already manufactured in such high volume.

ARDIS and RAM, the packet radio networks currently in use, cannot match packet cellular on a number of points. ARDIS and RAM have fewer users, smaller geographic coverage, lower bandwidth (from a LAN point of view), and require specialized equipment.

The Tech Behind The Curtain

The technology behind CelluPlaN is CDPD, which allows data transmissions to be overlaid onto the existing analog voice networks. With CDPD, data is transmitted in short bursts and only when the system detects an idle time between voice calls. Data can be sent without needing a dedicated channel, so data transmission will not impact the system's ability to carry its bread-and-buttervoice.

CDPD sends packets (just like on LANs) across one or more channels as the idle time occurs. Even during peak times, channels are idle between the time when a call is terminated or handed off and the time when the channel is reassigned to a new call.

A technique called channel hopping is used to select the channel that is least likely to be used for a voice call. This technology exists in today's cellular systems to hand calls off to new cells .

Cellular poses a few unique problems for data. When talking over a cellular phone, you can (impatiently) wait out the clicks, blackouts, and detect that the control and user-identity information, and user-authentication procedures are also in place. When a user connects, the system assigns a temporary ID for addressing. With nationwide cellular handing calls from cell to cell, it will be harder to physically eavesdropyou would have to be sure you hit the right cell control site. The advent of digital cellular will make it harder still. But data encryption seems like the sensible thing to do.

CDPD proponents are publishing their specifications, which will allow multiple vendors to implement the technology. The proponents, except for IBM, are service providers and therefore benefit from other companies manufacturing adapters, modems, and other equipment. The CDPD organizers are also working with the Cellular Telecommunications Industry Association and the Telecommunications Industry Association to make it a standard.

Beyond The Usual Applications

The target users of wireless WANs are no surprise: people in sales, service, or those who travel frequently. By giving salespeople a "nomadic office," they're forced to spend more time with customers and less time at their desk. With networks, they can get more immediate access to information, such as inventory and pricing. Service providers can also use wireless WANs. Typical users may be rental car agencies, waitresses, baggage handlers, and loading dock workers. Traveling managers can stay in touch with their offices via e-mail and wireless connectivity.

But some applications may be unique. Consider for example, the soda vending machine that calls a central computer when it runs low on change or out of Mr. Pibb. Gas and electric meters could be imbued with intelligence so they notify the utility company how much gas or electricity the customer has used, and meter readers no longer knock on doors. Parking meters could do the same thing, I suppose.

This tutorial, number 52, by Patricia Schnaidt, was originally published in the December 1992 issue of LAN Magazine/Network Magazine.