A Fax server is usually a network device that will be attached to both the telephone network and the internal LAN. Computers on the LAN can send documents to the Fax Server, it will queue them before making the call and then handling the transmission of the document. The most important job of the fax server is to handle the conversion of the digital document into an analogue signal capable of being transmitted over the PSTN. The fax server will also have to understand and replicate all the required fax signaling and handle the return analogue to digital conversion when it receives facsimiles.
However they are also a feature of an IP/PBX whereby the fax server feature handles the transmission of Facsimile over the VoIP network. Fax machines cannot work over a VoIP network as that is a digital medium and Fax machines are designed to transmit over the analogue PSTN. Therefore the IP/PBX must handle the transmission of facsimile differently than other digital voice calls. Typically the IP/PBX server which is a normal server running PBX software over a standard operating system such as Linux or Windows, will handle the facsimile through its print spooling or print server functions. The IP/PBX server will accept the document to be transmitted via a fax queue, which is similar to a typical servers print queue. The PBX will queue each document whilst it makes the required digital to analogue conversion. It will then make the fax connection over its own PSTN bridge line and transmit to the facsimile at the other end over an analogue telephone line.
Some of the features of standard fax servers are they can send and receive faxes from email or web interfaces. They can also store and archive faxes and track and report on sent faxes.