Cordless key telephone system for repeating attempts to alert stations located in interference-affected zones

摘要

In a cordless key telephone system where adjacent service zones (Z1, Z3; Z2, Z4) having the same control channel are overlapped, forming an unavoidable interference-affected overlapping zone (Z13, Z24), a main unit (1) initially selects an access unit (21-28) from each service zone in response to an incoming call from a network and transmits a first alert signal to that unit where it is broadcast to elicit an acknowledgment (ACK) signal from cordless stations. On receiving it, the access unit selects a speech channel, broadcasts a first channel assignment signal on the control channel, and then a first ringing signal on the selected speech channel. The main unit determines whether there is a cordless station (36) failing to respond to the first alert signal. If such a station exists, the main unit selects a first, idle access unit (22; 28) which is located in one of the adjacent service zones. A second alert signal is then broadcast on a control channel from the first access unit. Following receipt of an ACK signal from a cordless station (36), a second channel assignment signal is broadcast from the first access unit (28), signalling the identification of a speech channel (S3) previously selected by a second, initially selected access unit (27) located in the adjacent service zones. A second ringing signal is broadcast from the second access unit (27) on the previously selected speech channel (S3).