In the Companion, neural networks analyze torrents of broadcast and sensory data and alert other components when something inter esting comes along. Serial logic systems are like today's familiar com puters. They complement the neural networks by being precise and reliable. They handle internal housekeeping, library, security, and other functions. The computers in the Companion are tiny: depending on requirements, hundreds to thousands of them may be employed. Nanotechnology will enable a gigantic library to fit into a tiny space. Data storage schemes, such as coded polymer chains, may be able to achieve storage densities of a billion bytes per cubic micron, or one million trillion bytes per cubic millimeter.1 This storage den sity is roughly comparable to DNA, which packs about 100 mega-

The Companion 111 bytes into a small part of a cell. The Companion's design assumes a one-cubic-millimeter block of such memory, however, if this isn't enough, the frames have room for several cubic millimeters. This data storage capacity is astronomical by today's standards, exceeding the present combined storage capacity of every computer in the world. The problem with capacity advances, however, is that the marvelous gains have been temporary; people are quick to find new uses for expanded memory capacity and fill it quickly. The li braries and software described here are based on present-day trends; the needs and wants of twenty-first century society will doubtless make these projections seem naive. The components described will occupy less than a tenth of one percent of the Companion's memory, leaving the rest free for a lifetime of use.2

A self-integrity system consisting of dedicated computers and software keeps tabs on the Companion's battery condition, structural integrity, available memory capacity, and security status. It will au tomatically alert a network-based emergency center if it or its owner comes to harm. If it is lost or stolen, the personal data contained inside can be automatically erased or transferred to another Companion.

Communications The Companion uses both optical and radio channels of communi cation. The outside surfaces of the frames are lined with dozens of optical send-receive units; a radio antenna 20 centimeters long is imbedded within the frames. The light-based link is by far the faster medium, capable of sending and receiving several billion bits per second, but it will be limited to wherever you can find an optical outlet. Radio is more widely accessible, but slower. Because of noise, power, and distance trade-offs, radio channels will have a much smaller bandwidth for transmitting, less than 500 kilobits per second. This will be sufficient for voice transmissions or small data files. In the next ten to twenty years, fiber optic outlets will likely re place conventional telephone and cable TV in most homes and of fices. If you're at home or in a public building, your Companion will