Published March 2000

ROAMING ABOUT

Wireless Connectivity for Mobile PCs
by Vinit Nijhawan

Start • Local-Area Wireless • Mobile Computer Technology • Applications • Sources and PDF

Wireless communications is one of the world’s fastest growing industries, matched only by the growth of the Internet. The wireless industry has many segments, including wireless LAN, wireless WAN, cellular voice, LMDS, satellite, SMR, and telemetry. The majority of the wireless industry’s growth is attributed to cellular voice technologies (see Figure 1). This article focuses on public wireless network technologies (those available to anyone for a fee) rather than private radio networks (SMR and telemetry).

Figure 1—Worldwide wireless phone subscribers and the growth of digital cellular technology. Japan’s numbers are through June 1999 (source—New York Times, July 1999).

WIDE-AREA WIRELESS

The use of two-way radios began just before World War II with the advent of AM radios used by the military. However, these radios were difficult to adapt to widespread operation in the trunks of cars and were sensitive to noise and interference. Pre-war AM radios gave way to FM two-way radios, occupying 11 channels in the 40-MHz band. Two improved systems, occupying 11 and 12 radio channels in the 152-MHz and 454-MHz bands, followed the initial FM radios.

In the early ’60s, as FM radio technology improved, the channel bandwidth was required to transmit a voice signal reduced from 120 kHz to 30 kHz. These early systems allowed only one conversation per channel, and users had to scan channels manually to find a free channel. In the mid ’60s, a new system called trunking was developed, providing automatic channel selection for each call and eliminating the need of push-to-talk operation.

In the late ’60s, the Federal Communications Commission (FCC) allocated the 800-MHz band for mobile telephony. In 1971, AT&T Bell Laboratories proposed a cellular advanced mobile phone system (AMPS) concept. AMPS was essentially a number of trunking radio cells with radio channels being controlled by a central trunking controller over a dedicated control channel. By reducing each cell’s coverage, frequencies could be reused in different cells.

Variations of the AMPS architecture were deployed around the world in the 1980s, but with no defining worldwide standard. These were termed first-generation analog cellular systems, which used narrowband-FM, 10- to 30-kHz channels.

Even though AT&T developed AMPS technology, the government restricted the company from producing the equipment because of AT&T’s landline phone monopoly. Between 1974 and 1981, AT&T Bell Labs worked with all other cellular terminal vendors to develop cellular phones, enabling consumers to have quality products that connected to the cellular network. After AT&T’s divestiture in 1981, Western Electric was permitted to manufacture cellular terminals, as well as the network equipment.

These early systems were capacity-limited, as existing mobile radio operators were reluctant to give up bandwidth for cellular systems. As a result, airtime prices remained high, and the user base was limited. In the early ’90s, digital transmission radio technologies appeared, primarily to increase capacity over limited channel bandwidth.

Voice codecs (coder-decoders) are used to digitize and compress voice to multiplex voice conversations on a radio channel. Second-generation cellular systems utilized two forms of digital transmission: time- and code-division multiple access (TDMA and CDMA). TDMA is used by most second-generation cellular systems with the exception of IS-95, which uses CDMA.

U.S. companies wanted to preserve their investment in AMPS infrastructure, so they developed a second-generation standard (IS-136) that allowed dual-mode AMPS/TDMA service. Europe decided to adopt a new second-generation TDMA architecture called Global System for Mobile Communication (GSM), which operates in the 900-MHz band. And, Japan deployed a second-generation digital system called Personal Digital Communication (PDC).

In the mid ’90s, the FCC auctioned radio channels in the 1900-MHz band for digital-only Personal Communications Systems (PCS). The CDMA or IS-95 standard was a big winner as the choice architecture for PCS services in the U.S. The CDMA standard is now vying with GSM as the standard for the rest of the world, with GSM currently enjoying a significant lead.

The major difference among GSM and the IS-136/IS-95 systems is in the way roaming is handled. GSM phones use a plug-in subscriber identity module (SIM) that can be carried from phone-to-phone. The SIM has all the information needed to identify the user to the network for voice, data, and billing. IS-136 and IS-95 systems are identified by the phone.

The U.S., Japan, and Europe are attempting to agree on a single standard for third-generation cellular systems, which will allow phones to interoperate throughout the globe. Most likely, the signaling scheme will be the W-CDMA (wideband CDMA). The disagreement lies in the system architecture.

IS-95 operators would like to use the CDMA2000 standard that can be built on existing CDMAOne systems. GSM operators would like a GSM-like W-CDMA system, which allows operators to build on existing systems. It is likely that more than one third-generation cellular standard will be adopted. A worldwide standard will likely have to wait for a fourth generation. The Japanese will deploy a third generation in 2001, the Europeans in 2003, and the U.S. in 2005.

The ability to transfer data over first- and second-generation cellular networks was established soon after voice capability was developed. Companies were also able to transmit both data and voice over two-way radio channels. In some networks, this capability was dedicated for data only.

Data on these networks was transferred in one of two methods: circuit-switched or packet-switched. Circuit-switched is just like dial-up modems used on landline connections. The radio first establishes a connection after dialing the recipient’s computer modem, and then data transfer can take place. Packet-switched networks transfer data at any time and are data-only networks.

Motorola developed signaling schemes allowing data to travel over their voice networks, first the MDC4800 (4800 bps), followed by RD-LAP (19,200 bps). The RD-LAP protocol was used to develop data-only networks as well, including ARDIS in the U.S. and Datatac elsewhere. Ericsson developed the Mobitex packet-switched network (9800 bps) that has been deployed in many countries around the world.

Table 1 lists only terrestrial radio networks. Satellite radio networks are slowly proliferating but are not covered in this article.

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