https://en.wikipedia.org/wiki/PSTN_network_topology
https://en.wikipedia.org/wiki/Class-4_telephone_switch https://en.wikipedia.org/wiki/No._4_Electronic_Switching_System https://en.wikipedia.org/wiki/5ESS_Switching_System https://en.wikipedia.org/wiki/Audichron https://upload.wikimedia.org/wikipedia/commons/4/4e/4ESS_Switch_Emergency_Action_Interface_%28EAI%29_Display_Page.png https://en.wikipedia.org/wiki/Bell_Telephone_Company https://en.wikipedia.org/wiki/Bell_System https://en.wikipedia.org/wiki/Western_Electric
