Mention mobile communications, and most people’s thought turn immediately to cellular telephony. It’s not surprising – the huge surge of consumer enthusiasm for mobile phones during the 1990s means that in many countries the cellphone has become as commonplace as its fixed-line counterpart, and much more ubiquitous.

But while cellular telephony continues to win millions of converts each year in countries all around the world, mobile communications comes in many other flavours, each optimized to fulfill a specialized role, and each also experiencing rapid growth as the traditional wired world rapidly evolves into an unwired planet. Some of these technologies fulfill the needs of important niche markets – others are only beginning to realize their potential as mainstream mobile alternatives.

One of the earliest forms of mobile communications – and still one of the simplest – is radio-based paging. Initially developed in the 1950s as ‘doctor alert’ systems for use within hospitals, the technology rapidly grew beyond the in-building inductive loop systems to encompass wide area communications. Early systems were "bleep-only" receivers linked to their own phone numbers, which alerted users by way of a number of differentiated bleeps corresponding to particular messages – for example, one bleep might mean "call the office", while a string of rapid bleeps could tell the user to come back to base.

The popularity of paging, along with strong competition in most major markets, spurred the introduction of numeric pagers in the mid-1980s. These systems had the advantage of being able to transmit a sequence of digits – a useful way of communicating important telephone numbers as well as for expanding and refining message codes. The 1990s saw the advent today’s popular alphanumeric systems, which can handle a mix of digits and characters and transmit longer messages of up to 40 characters at a time.

While cellular mobile, and especially Short Message Service (SMS) capability, has stolen a little of the limelight of late, markets for paging services around the world continue to show strong growth. The reasons are not difficult to guess. Lightweight and ultra-compact, pagers provide instant, non-intrusive communications extremely cost-effectively. If their convenience has made them a favourite business tool in markets like the US, where one in three workers routinely uses a pager, their low cost has found favour in developing markets like Asia. Pager penetration rates in Hong Kong SAR and Taiwan-China are already well over 30%, while the Republic of Korea, one of Asia’s fastest-growing economies, now boasts a staggering 70%-plus penetration. Price has also been an important lure for the penetration of paging into niche market segments, particularly the youth market. In the UK, for example, some 40% of 15-25 year olds now own a pager.

Future developments in paging technology will see the development of two-way paging networks, which make more efficient use of radio spectrum while enhancing utility by extending service to applications like mobile e-mail and even voice paging. At the same time, a new service known as "guaranteed paging" will make pagers more reliable by ensuring messages are stored on central network servers and re-sent periodically until the network receives a signal indicating that the message has been delivered successfully. Pagers will also form an important mobile link with the Internet, via a range of simple text-based services that are delivered directly to users’ pagers at very low cost.

The advent of new kinds of store-and-forward satellite systems known as Global Mobile Personal Communications by Satellite (GMPCS) is also opening up the possibility of low-cost satellite-based wide area paging. A number of new Low Earth Orbit (LEO) mobile satellite constellations, particularly the systems popularly known as "Little LEOs", are now in development with plans to launch commercial service over the next three years or so. Optimized for a range of remote messaging functions, the ability of these satellite-based communication services to deliver relatively inexpensive messaging to remote communities not normally served by communications networks could soon represent an important step forward in efforts to bring basic communications to populations in developing countries.

If simple paging doesn’t meet your communication needs, there’s also a viable voice-based alternative to cellular systems. DECT – the European standard for Digital Enhanced Cordless Telecommunications – can provide voice, data, fax and even multimedia communications over a radio-based system now in use in more than 100 countries around the world. While DECT systems can’t offer the wide-area mobility of wireless technologies like cellular and paging, they nonetheless represent a relatively low-cost, flexible solution for in-building and local outdoor communications. Long a favoured technology for cordless telephones, DECT is compatible with GSM and ISDN standards, and is now gaining a new lease of life through the development of dual-mode mobile phones which use cheaper DECT services when in reach of a base station, switching automatically to the cellular network when signal strength drops below and acceptable level.

As a technology, DECT has a number of advantages which are hard to beat. In a business environment, for example, the convenience of cordless telephony is complemented by huge potential savings, since calls between affiliated DECT handsets are free of charge. DECT’s ability to handle very high user densities means a multi-cell DECT PBX system can provide voice and high-speed data services to several thousand users on one or more linked sites – a boon for any business wanting to streamline efficiency while keeping costs low. DECT can also serve as an effective base for new Wireless Local Loop networks.

Trunked radio is a robust communications solution most often used by businesses with a large fleet of vehicles to manage, such as distribution and courier services, taxis, ambulance, police, fire and rescue organizations and public transport. Designed around a central base station and a number of mobile receivers, the system shares a bundle – or trunk – of radio channels among a pool of users on an as-needed basis. Traditionally, the base station – typically the dispatch centre or corporate headquarters – transmits on one channel, called the base channel, while the mobile units transmit on another, linked channel, called the mobile channel. Thanks to a system of repeaters, communications from the base station can typically be heard by all mobile units, enhancing mobile-to-mobile communications and, ultimately, a businesses operational efficiency. On the downside, once all channels are in use, mobile units have no choice but to wait for a clear channel before they can make contact with the base.

Digital trunked radio systems can boast multi-functional capabilities that allow it to be used to simultaneously carry voice and data at rates of up to 28.8kbps. Popular trunked radio standards include TETRA, Tetrapol, APCO 25, EDACS and iDEN. Within the next two years, these standards are expected to support a market for trunked radio services worth more than US$5 billion worldwide.

Perhaps the most glamorous of the new breed of mobile services, personal satellite communications systems were pioneered by Inmarsat, a UK-based intergovernmental treaty organization which corporatized in 1998. The launch of the Iridium constellation back in October 1998 spurred great interest in the potential of such services to deliver voice and data services seamlessly to the growing number of highly mobile business travellers, as well as to deliver Internet and other multimedia services via satellite to PC or other fixed device. While third generation cellular systems will soon strip mobile satellite systems of one of their strongest selling points – seamless interconnection across continents – more than 30 systems are still in development, with most due for launch within the next four years.

With the exception of the Inmarsat system, which delivers personal mobile satellite services over the same kind of geostationary satellites that also deliver TV and radio broadcasting, most new systems are based around compact satellites which orbit in Low-Earth Orbit (LEO), Medium Earth Orbit (MEO) or what’s known as Highly Elliptical Orbit (HEO). Unlike standard geostationary satellites which remain in a fixed position relative to the surface of the Earth, these smaller satellite systems are in constant motion, forming a moving constellation which provides users with cellular-quality voice and data communications from just about anywhere on the face of the planet.

While mobile satellite systems have the potential to bring communications to corners of the world still unserved by fixed-line and cellular network, their high equipment and access costs have tended to be an appealing proposition for the top-end business market. Until their access charges drop as they gain the favour of an increasing subscriber base, mobile satellite services will remain as far out of reach as the satellites that deliver them for the millions of people in the developing world still waiting for a dial tone.

Cost is the main constraint which prevents mobile cellular from being a viable alternative for first-time users in developing countries.

One alternative is to strip away some of the functionality of a mobile cellular system to reduce its price. This has given rise to a variety of technologies which were originally referred to as "fixed cellular" but have come to be known as "wireless local loop" (WLL).

WLL provides an alternative to the traditional way of connecting subscribers to the local telephone exchange using copper wire. With the declining cost of WLL over time, it is becoming price competitive with copper wire. As a critical mass of users is reached, the cost of WLL systems should fall even faster. WLL’s low implementation and operating cost promises to significantly alter the expense equation of building telephone networks, resulting in lower tariffs and enhanced affordability for potential subscribers.

While not strictly mobile, they are able to offer users a limited amount of mobility within range of a base station – which can range up to 50 kilometres if they have mobile handsets – making them a potentially useful local mobile technology for smaller communities.

Used essentially for basic telephony, WLL effectively uses radio signals to substitute for the copper wire connection that links callers to the PSTN.

Speed of deployment – a typical network can be built from scratch in as little as 120 days – combined with low access charges, reliability, low maintenance and an ability to cover areas of difficult terrain – make WLL particularly well suited for the needs of developing countries, many of which struggle to maintain ageing and expensive copper networks which only serve a fraction of the population. One trend is the take-up of WLL by new network operators in developing countries whch have been mandated to install a specific number of lines in a relatively short time period under a licensing agreement.

The key factors though are scale, standards and spectrum.

There are no global standards and most WLL systems are based around distinct technologies such as DECT, CDMA and PHS. They also tend to suffer attenuation where there is a heavy rainfall or where there is extensive foliage – conditions often encountered in developing countries. In addition, competition for bandwidth with mobile systems can be a drawback. The lack of an international frequency allocation also means that manufacturers have not been able to optimize economies of scale because different equipment must be produced according to different frequencies used in each country. Finally, operators need to apply for licenses which can make service financially unattractive in rural areas.

According to ITU forecasts however, the number of WLL lines will climb to 340 million by 2002, the majority of which will be installed in developing countries. That’s good news for potential users in countries like Honduras, Lesotho or Sudan, where the waiting period for standard phone service remains at over 10 years.

Global Positioning System (GPS) technology is not something most people associate with mobile communications. Yet this system is playing an increasingly important role in telematics systems, where its highly accurate positioning data is being integrated with mapping and other applications to provide services such as mobile in-car guidance and information systems. Based around an operational constellation of typically 24 satellites which orbit the earth every 12 hours or so, GPS systems send a constant stream of radio signals to a network of monitoring stations and receivers on the ground. The monitoring stations perform computer adjustments for orbit and other factors, and send this to receivers which convert the information into precise measurements of position, velocity and time. GPS measurements are so precise that a high-end receiver can determine position with an accuracy of well under one metre.

In addition to the US GPS and the Russian GLONASS, a third system – Europe’s Galileo – is expected to go into operation in 2008.