mission picked up by the base transceiver stations, and depending on the signal level, they can determine whether the unit is well within the cell or transitioning out of that cell. If the unit's power levels start to weaken and it appears that the unit is leaving the cell, an alert is raised that queries the surrounding base transceiver stations to see which one is picking up a strong signal coming in; as the unit crosses the cell perimeter, it is handed over to an adjacent frequency in that incoming cell. The mobile unit cannot stay on the same frequency in between adjacent cells because that would create co-channel interference (i.e., interfer- ence between cells). 1G Wireless Data Networks Early wireless data networking methods included CDPD and packet radio. These technologies are not generally part of new deployments today because better solu- tions are now available. However, the following sections cover them briefly because you may encounter the language in your studies and should know what it means. CDPD is a packet data protocol designed to work over AMPS or as a protocol for TDMA. The initial concept of CDPD was publicly introduced in 1992. CDPD was envisioned as a wide area mobile data network that could be deployed as an over- lay to existing analog systems, a common standard to take advantage of unused bandwidth in the cellular airlink (i.e., the wireless connection between the service provider and the mobile subscriber). (Unused bandwidth is a result of silence conversations as well as the moments in time when a call is undergoing handover between cells. These periods of no activity can be used to carry data packets and therefore take advantage of the unused bandwidth.) That was the main objective, and it was backed mostly by large cellular network operators. CDPD is defined as a connectionless, multiprotocol network service that pro- vides peer network wireless extension to the Internet. CDPD's greatest advantage is that it was designed to operate as an extension of existing networks (i.e., IP net