Статьи 5 семестр / Wireless networks (3)
.docforward E-mail, one line at a time, to your alphanumeric pager via satellite. Artificial-speech chips are now sophisticated and inexpensive enough that they could be put into telephones, so that they would read incoming text aloud. But more versatile devices will require displays and reasonably fast transmission rates to handle graphics and text.
Analog network technology limits data communications via modem to relatively slow speeds: 14.4 kilobits per second or less. Digital networking will help somewhat by eliminating conversions between analog and digital formats; it will not necessarily increase data rates for standard service beyond the equivalent of a telephone line.
Eventually, even the boost from the present digital standards and compression techniques will prove insufficient to provide enough room to pass data-intensive messages among hundreds of millions of customers. By the turn of the century, wireless faxes will be common, and video mail could be widely used throughout wireless as well as wire-line networks. If spectrum is allocated to accommodate the resulting deluge of images, it will probably be at frequencies of around 30 to 40 gigahertz, although some video services may be available at 2.5 gigahertz. Because radio signals at these short wavelengths behave like light, buildings and even foliage block them, and coverage is essentially limited to line of sight. So wireless broadband services would be fixed rather than mobile, and service providers would need to shrink cells further to serve large numbers of subscribers.
Air interface standards will also probably evolve to accommodate broadband data and video transmission. The first generation of digital standards presumes a circuit like connection between two devices for an entire call. That arrangement is better suited to telephone calls than to surfing the Net. The Internet uses standards based on routing individually addressed packets of data.
Telephone companies designing fiber optic networks to carry interactive video services plan to connect them with broadband packet switches—using a technology called asynchronous transfer mode, or ATM—that can shunt packets of data, voice and video along the appropriate paths at extremely fast rates. Wireless networks will follow this trend when it becomes the most efficient way to combine voice and multimedia services. It is possible, in fact, that wireless service providers and cable TV companies may lead the shift to ATM. The timing will depend on existing network assets, technological plans and investment strategies.
Intelligence in Motion
As portable telephones grow in popularity, they will also tend to offer more features. Ten years from now you will almost certainly still be able to buy a simple wireless device that is limited to voice calls; it will probably be quite inexpensive. More common, however, will be devices that can handle faxes and video and run software applications. Doubtless there will be many styles with widely varying capabilities. This could cause problems. The smarter a device must be, the greater the risk that its complexity will baffle the user. And different models will work differ-
Wireless Telephony for Developing Countries
The same technologies that provide flip-phone service to people on the go can provide basic dial tone to regions where it has never been heard before. This is much of the planet: about half the people alive today have never made a telephone call. A number of developing countries have expressed a desire to leapfrog over a generation or two of network technology, using wireless infrastructure to jump-start telephone service.
These countries see two advantages in wireless telephone systems. The first is cost. Many of the facilities necessary for a conventional wire-line access network are typically engineered for 20 to 30 years of service expansion. As a result, the cost of building such a network where no infrastructure exists may be prohibitively high.
With a "fixed wireless" network geared for access rather than mobility, a service provider can cover a large region with base stations and a single unit for switching and control—a significantly smaller investment. Subscribers can then connect to the global network using portable telephones, wireless public telephones, or terminals mounted on buildings and wired to conventional telephones. As the number of subscribers grows, the service provider can easily add more base stations to split the area covered by the network into smaller segments.
The second benefit is time. Wireless networks can be installed in months rather than the years required to install copper wires. Argentina, for example, announced in February 1994 that it was awarding licenses for the entire country to CTI, a CTE-led consortium. By May an 800-cell fixed wireless network that AT&T built for CTI was up and running. It can serve as many as 160,000 subscribers.
Telecommunications, many have observed, constitutes the main infrastructure for the global economy. Where there is a potential for revenue, wireless technology could offer a bootstrap to participation in that market for many regions of the world that might otherwise be excluded.