Wireless technology may breed mobility, but AT&T saw in its sizable acquisition of digital cellular spectrum an opportunity to create a new stationary communications network that will mimic some of the transmission modes of local area networks.
"People expected we would use the PCS (personal communications system) spectrum as a mobile network," said Nick Kauser, chief technology officer for AT&T Wireless. "What we've done is not cellular, and it's not mobile. It's fixed."
On Tuesday, AT&T announced it has devised a way to convert analog voice and data transmissions from household phone lines to digital information and send that data over its PCS wireless network, bypassing most of the local telephone networks.
Although it will still be accessing part of the local network, AT&T officials believe no real access charge will be needed. Instead, they will negotiate "bill and keep" agreements under which AT&T and the local phone companies grant each other access to the respective networks to complete calls for no charge. For example, AT&T would not be billed for its customers' accessing a Bell's local network; likewise, a Bell company would not be billed for its customers' accessing AT&T's local network.
The development gives AT&T a way to build its own local phone networks more cheaply, and more quickly, than if it were to lay new fiber in the ground.
The technology, the fixed wireless local loop, takes a slice of radio frequency, the 10 MHz sections of PCS AT&T purchased in government auctions, and crams into it high-capacity data transmission "almost as wide as the spectrum itself," said AT&T Wireless president John Walter.
To achieve this, AT&T combined three different digital cellular technologies - time division multiple access (TDMA), spread spectrum, and smart antenna, each of which multiplies the capacity available, but through different means.
TDMA divides transmissions into discrete movielike frames. Like a film, a band of TDMA transmissions represents a complete conversation or data connection. An additional transmission can be woven into the spaces between these frames, doubling the capacity, said Kauser.
Spread spectrum divides the band into multiple narrow segments, each of which represents an independent transmission channel. Each segment that materializes is not diminished in its ability to transmit data by the presence of other segments.
Smart antenna narrows the field of transmission to one area, allowing for the same frequency to be used simultaneously when broadcast in a direction different from the first transmission.
All three of these technologies are combined into the electronics of a "pizza-sized" box that will be installed on the outside of a house. That box will have a flat antenna on the front and a connection to household phone lines on the back. Customers will enjoy transmissions at a rate of 128 Kpbs. By contrast, the maximum transmission rate of an ordinary phone line is 64 Kpbs.
Data transmissions will only use the bandwidth they need. That's because data will be sent across the wireless path via packet switching, but only when a customer is actively downloading or sending files or accessing the Web. The capacity will otherwise be available for other services or other users.
"Usually, a connection isn't usable once you've dialed up an ISP. This gives you capacity on demand," said Kauser.
