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Use the Cisco SPA112 adapter to send / receive FAXes

The use of FXS adapters for sending and receiving FAXes may have a higher failure rate than a traditional line, but in cases where you cannot connect your fax machine directly to a telephone line, it is possible configure the Cisco SPA112 adapter to connect a FAX machine.

Attention: the main requirement for using FAX with the adapter is to have an ISDN socket with an additional telephone number to the main one and dedicated to the fax, other than the voice number. In this way, incoming calls for the fax number can be directly routed to the FAX extension (to be configured as a PBX user). If you have an analog line (with a single voice / fax number), the most reliable solution is to connect the fax machine to the telephone line, while the SIP / FXO adapter for the PBX must be connected to the Auxiliary port of the FAX called Aux or Tel, which is used to connect an analog telephone.
It is also understood that the adapter must be used in the same network in which the PBX resides: adapters connected remotely via data connections are absolutely not recommended.

To configure the Cisco adapter it is necessary to create an extension of the PBX, which will be configured in the FXS port where the FAX will be connected.

Optimize fax transmission / reception

The transmission of faxes on IP networks can present some problems also using the T.38 standard. It is possible to adjust various settings on the adapter to optimize the success rate of sending / receiving faxes.

Normally the T.38 protocol is more reliable. If you intend to use it, you must activate it both on the Cisco and on the line adapter (eg. Mediatrix). To enable the T.38 protocol, simply set the FAX Enable T38 parameter to Yes

The following settings are suggested values, but may vary in specific cases.

  1. Make sure you have enough bandwidth available for reception and transmission:
    • for G.711, at least 100 kbps is recommended;
    • for T.38, at least 50 kbps.
  2. Click on the Voice menu, and then Line1 or Line 2
  3. In the Network Settings section, enter the following values:
    • Network Jitter Level: very high
    • Jitter Buffer Adjustment: no
  4. In the Supplementary Service Subscription section, enter the following values:
    • Call Waiting Serv: no
    • Three Way Call Serv: no
  5. In the Audio Configuration section, enter the following values ​​to support T.38:
    • Preferred Codec: G.711a
    • Use pref. codec only: yes
    • Silence Supp Enable: no
    • Echo Canc Enable: no
    • FAX Passthru Method: ReINVITE
  6. Click Submit to save.