s-to-landline telephone path. specific area and provides its telephone number in case anybody is calling it. We will have a lot more to say about the wireless telephone number and incoming calls in Sec. 12.5.5. Call initiation in wireless telephone systems is done through preorigination dialing. A subscriber enters the entire telephone number to be called and presses a special key usually marked SEND. 1 Preorigination dialing was conceived when the market for wire- less was vehicular. The driver could enter two or three digits at a time with breaks to look up and check traffic. When the entire telephone number had been entered, the driver could press SEND to start the radio signaling that initiates a telephone call. Once the telephone initiates or receives a call, it establishes a duplex radio channel supporting two-way voice communications with signaling. Whether this uses a pair of radio frequencies [frame division multiple access (FDMA), Advanced Mobile Phone Ser- vice (AMPS)], time slots in a larger channel (TDMA, GSM), or spread-spectrum com- ponents (CDMA) is not important here. The important thing is that the user terminal and the system are in continuous two-way, full-duplex voice-link communication. As each party talks, his or her voice is modulated, transmitted, received, demodulated, and reproduced in sound at the other end. Both speakers can talk at the same time, with both voices being reproduced at each other's telephones just like a regular land- line-to-landline call, and there is some mechanism built in to carry signaling informa- tion. We call the duplex channel for voice plus the signaling for one call a connection. The other end of the air interface from the user terminal is an antenna configuration atop a base station. In the AMPS trial system in Chicago in 1982, even the mobile tele- phones had two antennas for diversity. This was dropped in the commercial system for three reasons: First, the mobile telephone was already expensive enough without having an extra piece of wire. Second, most mo