Selective signalling for portable applications

The selective call has numerous signalling and control applications in portable communications, but has perhaps seen i ts most extensive development for metropolitan paging. In the past, many of these applications were handled using a single audio tone and a narrow band filter for detection. The need for more addres ses and higher throughput to accommodate the metropolitan paging requirements has resulted in the development of various multi-sequential tone systems, Sequent ia l tone systems feature good sensit ivity, adequate false alarm protection, short (. 2 second per address ) t ransmiss ion t imes , and code capac i t ies in excess of 100,000 addresses.