which allows wired telephones to be used as intercoms.
Figure 9. Actual model for placing voice and video calls using only the Asterisk Server
The Public Switched Telephone Network (PSTN), also known as Plain Old Telephone Service (POTS), is the wired phone system over which landline telephone calls are made. The PSTN relies on circuit switching. To connect one phone to another, the phone call is routed through numerous switches operating on a local, regional, national or international level.
Figure 10. Actual model for placing voice and video calls using only the Asterisk Server Support for regular POTS is provided by Digium’s series of analog telephony cards. Digium’s analog cards utilize separate capability modules for Foreign Exchange (FXO) trunk lines and Foreign Exchange Station (FXS) telephone sets. The analog cards are provided in four (4), eight (8), and twenty-four (24) modular port varieties for both PCI and PCI-Express slot types. An optional DSP module provides hardware-based echo cancellation for Digium’s analog cards.
Support for Robbed Bit Signaling (RBS), Basic Rate ISDN (BRI) and Primary Rate ISDN (PRI) lines are provided by Digium’s series of digital telephony cards. The digital cards are provided in one (1), two (2), and four (4) port varieties for both PCI and PCI-Express slot types. An optional DSP module provides hardware-based echo cancellation for Digium’s digital PRI cards; digital BRI cards include on-board DSP-based echo cancellation [9].