e download. wireless LAN hot spots. Smart, or phased array, antennas Software Radio Software Radio Figure 1.0-5 The ideal software radio (lower diagram) is much simpler and more flexible than the traditional radio (upper diagram), with its hardware tightly coupled to a specific signal type. CHAPTER 1.0 Introduction may be considered an extreme form of antenna diversity. home area code) while roaming to a different market An antenna array is composed of a collection of individual with a different area code. This is not true today in data antenna elements, each of which is capable of signal re- networking, in which a computer's unique identifier (IP ception (or transmission). A signal arriving at the array address) is tied to a specific "location" on the network will typically be picked up at each element, offset slightly (i.e., a specific router). The situation is addressed by in time. By internally adjusting the phase (time offset) of assigning a new IP address as the user reattaches through each received signal and then summing, the antenna a different router. This presents limitations to some pattern is effectively focused on the remote transmitter. applications, constraining their ability to maintain a con- Signals arriving from other directions do not benefit from nection as their supporting device moves. the summing and sink into the noise. The same technique A set of features collectively known as mobile IP offers may be used to benefit the transmitted signal as well as a solution to this problem, providing services to the the received signal. One of the challenges of the tech- mobile data use that is comparable to those provided by nology is to dynamically adjust the offsets to maintain the mobile telephony infrastructure. focus on a remote mobile unit. Figure 1.0-6 shows As mentioned earlier, security is one potential weak- a simplified model of smart antenna operation. ness of wireless systems. First generation cellular net- works were vulnerable to theft of services as well as Advanced networking eavesd