The following figure shows the external and internal interfaces of the fax interface module. There are three external interface ports, labeled FXS, FAX and PSTN. Normally, the FXS port is connected over the internal voice/fax channel to the Passport network, and the FAX port is connected to the PSTN port. The PSTN connection is the published 7-digit fax number accessible to any fax user on the PSTN.
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