Design Feature: December 21, 1995

[Figure 1] You’re in a meeting when your two-way pager receives a message. You glance at the display and see an urgent query from a customer: "Where’s my replacement module?" You push a couple of buttons, and the customer has his answer: "On its way." The entire exchange takes only about 30 sec.

Pagers are getting sophisticated. Two-way paging is already available, and voice paging will soon put a tiny, wireless answering machine right in your pocket. The latest pagers have come so far from traditional pagers that the term "paging" is hardly even appropriate anymore. Get ready for the far-reaching implications of wireless messaging.

Wireless messaging moves paging out of the realm of stand-alone devices and into the realm of system components. It has already linked pagers with portable computers, and soon it will merge pagers with pocket organizers and cellular phones. The technology is transferable, too. Chips designed for pocket pagers are just as useful in vending machines and cars and for applications such as remote control and telemetry.

Pagers with two-way and voice capabilities are new and different (see box, "Beyond beepers: the new world of paging"). Motorola’s Tango pager, used with SkyTel’s two-way paging service, has a four-line display and several buttons for responding to received messages. An RS-232C connection lets you use a computer to compose and send messages of your own, using Tango as a wireless modem. Motorola’s Tenor pager will be available late this spring along with the inauguration of PageNet’s voice-paging service and will contain a speaker for playing back voice messages. Tenor stores 4 minutes of voice messages and provides access to more messages through PageNet’s virtual-memory system.

New frequencies from the FCC

[Figure 2] Paging’s two-way and voice capabilities come on the heels of newly available frequency bands and high-speed paging protocols. The new frequencies became available for personal-communications systems with last year’s auction of spectrum space in the 900-MHz range by the Federal Communications Commission. The paging protocols—Reflex for two-way paging and Inflexion for voice paging—are extensions of Motorola’s Flex protocol for one-way paging, which was introduced in 1993.

A 12.5-kHz response channel in the 901- to 902-MHz band provides the foundation for two-way and voice paging. The response channel augments a 25- or 50-kHz message channel in the 929- to 932-MHz band or in the 940- to 941-MHz band. Voice paging incorporates additional subchannels in the 930- to 931-MHz band or in the 940- to 941-MHz band. Two 25-kHz subchannels provide control, and seven 6.25-kHz subchannels carry voice data.

The Flex family of paging protocols speeds message delivery through the new narrowband channels. Flex uses four-level frequency-shift-keying (FSK) modulation for data-transfer rates as high as 6400 bps. In contrast, the predominant paging protocol, POCSAG (Post Office Code System Advisory Group), uses two-level FSK for a maximum rate of 2400 bps. Because Flex’s higher data rate allows more pager users per channel, the new protocol has met with rapid acceptance in the booming paging markets of North America and Asia.

For pager users, Flex’s most obvious advantage is extended battery life. Because Flex is a synchronous, rather than an asynchronous, protocol, a Flex pager must "wake up" to check for messages only at periodic intervals. If a Flex pager doesn’t find a message within a scheduled 1.875-sec data frame, it can then go back to sleep for as long as 4 minutes. Motorola claims this approach extends pager battery life as much as five times more than pagers using the asynchronous POCSAG protocol.

Flex’s synchronous operation does require precise timing, however. All Flex pagers must be synchronized to within 5 µsec of each other in a simulcast area where multiple transmitters provide overlapped coverage. The paging system’s transmitters update the pagers’ internal time references at least once every 4 minutes, except when the pager is out of range. If a Flex pager gets out of synch (as most do, occasionally, because they sometimes are out of transmission range for extended periods of time), it automatically performs an elaborate procedure to resynchronize. The transmitter sites themselves maintain accurate time via Global Positioning System receivers.

A combination of several different approaches helps Flex provide reliable message delivery. At the most basic level, the Flex protocol requires specification of message length, so a pager can tell if it hasn’t received a message in its entirety. To guard against burst errors and signal fading, Flex implements a fairly elaborate scheme of error detection and correction. By interleaving the bits of eight data words during transmission, Flex minimizes the effects of brief signal degradations on each individual word. When degradations do cause data errors, the use of 11 bits for checksum validation and parity in each 32-bit word usually allows error correction. As a result, Flex works reliably even through 10-msec fades, according to Motorola.

Flex’s two-way capabilities add other reliability features. For example, a two-way paging system can query a pager for its location. If the pager doesn’t respond—when it’s turned off or out of range, for example—the system holds the messages until the pager is available again.

Frequency reuse—the simultaneous transfer of multiple pager messages on the same paging channel—is another benefit of two-way paging. Because two-way and voice pagers can tell paging systems where the pagers are, the system can use only a few localized antennae to communicate with a pager. By directing a transmission to a specific area, rather than everywhere, the system can use antennae in other areas to send messages to other pagers—at exactly the same time and on the same frequencies.

The key to frequency reuse is a procedure known as zone registration, in which a two-way or voice pager informs a paging system of its location. When a message is available for a pager, the paging system notifies the pager and expects a response. When the pager responds, its transmission is picked up by several system antennae within the pager’s transmission range. Those antennae then denote a zone where the system can concentrate its transmission to deliver the pager’s message. The system needs to "light up" only the transmission antennae in that particular zone, leaving antennae in other areas available for delivering other messages.

Extra antennae needed

Two-way paging does require additional system antennae, however. Compared to a one-way paging system that can only transmit, a two-way system must also have receiving antennae. In addition, because pagers transmit at low power, a paging system’s receiving antennae must be closely spaced. Motorola’s Tango pager, for example, puts only 1W of power into the patch-type antenna inside its cover. In comparison, a transmitting antenna in a paging system typically puts out 300 to 400W.

On average, two-way and voice paging systems need three to five times as many antennae for receiving as for transmitting, according to Motorola. Transmitting antennae are typically six to 16 miles apart, depending on terrain and other factors. For receiving antennae the spacing must be less—typically, between one and eight miles.

With the infrastructure for two-way and voice paging now being established, new kinds of messaging applications are also evolving. SkyTel’s Palmtop Messenger, for example, a $49.95 package that includes messaging software and a cable, links SkyTel’s two-way pager with the address book in Hewlett-Packard’s LX palmtop computer. Similar consumer products are in the works from other paging companies and from other manufacturers of palmtops and pocket organizers. Paging technology is also going into embedded systems (see box, "Paging ahead: wireless messaging systems").

A confluence of hardware and software developments is simplifying and speeding the development of new messaging applications. Windows 95, for example, makes software development easier with built-in messaging capabilities. Already, software from SkyTel is working with Windows 95’s e-mail box, called Exchange, to provide seamless, integrated access to SkyTel services. For hardware design, new ICs specifically target pagers and other personal-communications devices.

Chips implement protocol

Motorola, seeking to hasten the acceptance of Flex, is backing the development of chips that implement parts of the Flex protocol. The company is even lining up other manufacturers to make Flex ICs to speed the protocol’s proliferation. Texas Instruments will be the first manufacturer of Flex chips, with production quantities anticipated by mid-1996. Motorola and other companies will follow with chips of their own. Customers who buy the chips will automatically be eligible to use the Flex protocol without licensing. A similar arrangement applies to the purchase of Flex core logic, which is available from Motorola.

The first Flex IC, TI’s TLV5591 signal processor, acts as a decoder for the Flex protocol signal, translating digital signals between a pager’s receiver and its host µP. The chip includes a flexible interface for controlling off-the-shelf paging receivers, plus a serial peripheral interface (SPI) for communicating with off-the-shelf µPs.

No chips currently implement Flex’s two-way and voice protocols, however. Motorola says chips for those protocols will be introduced next year. In the meantime, more general-purpose devices are being put to use, and some are being tailored for wireless messaging, if not for the Flex protocol specifically. The DSP1615 signal processor from AT&T Microelectronics is an example.

Claiming that paging protocols are still in flux, AT&T has not aligned itself with Flex. Accordingly, it calls its new DSP1615 a "piece-of-mind processor" (Pomp for short) that can work with any protocol. AT&T says it designed the device specifically for "personal messaging units" and that the device is optimized for two-way messaging. Based on the DSP1600 core, the chip integrates only those functions—codec, memory, clock generator, and controller peripherals—that AT&T says are necessary for its target application.

Although paging protocols are indeed unsettled, Motorola’s Flex protocol family seems well-positioned for success. One-way Flex already has dozens of backers in the United States and Asia, and its two-way and voice extensions currently have no competition. Reflex has four US backers, and Inflexion has one (PageNet).

More protocols coming

Still, Flex’s successes in the United States and Asia are not guaranteed in Europe, where Flex faces the established ERMES (European Radio Message System) protocol, which has data rates that nearly match those of Flex. ERMES currently provides only one-way paging, however; its two-way capabilities will not debut until late 1996. In the meantime, AT&T Wireless Services and communications-equipment manufacturer Ericsson are reportedly leading the development of another two-way paging protocol.

However the protocol battles end, paging will be dramatically changed. Indeed, paging will no longer be purely paging, but wireless messaging in myriad forms. It will continue to merge with computing, telephony, and embedded systems—and with applications such as voice mail, e-mail, and telemetry—in ways that only time will reveal.