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Extending Fax Line over Network with two SPA3102's

We have setup two SPA3102's in a lab environment to test the ability to extend a fax line from our main office to a remote office over VOIP.

PSTN --------> (FXO) SPA3102#1 -------------LAN---------------> SPA3102#2 (FXS) --------------> FAX

The two SPA3102's are setup with a dial plan between each other so that an incoming call on the SPA1 FXO rings on the FXS port on SPA2. When the FXS station is picked up on SPA2 it automatically dials and picks up the FXO line on SPA1.

Receiving faxes seems to work fine, however when sending faxes the machine gets a fast busy. The only way to get the fax to transmit is to manually dial the fax number VERY slowly on the machine manually.

Right now the setup is over our LAN so quality between the two SPA units should be excellent. Is there any adjustments that should be made?

Rising star

This is reported on Linksys guide:

Using a FAX Machine (SPA2102, SPA3102 or SPA8000)

T.38 Fax is only supported on the SPA2102, SPA3102, and the SPA8000. The SPA2102 and SPA3102 support a single connection, while the SPA8000 supports one connection for each pair of ports (1/2, 3/4, 5/6, and 7/8) for a maximum of four connections.

To optimize fax completion rates, complete the following steps:

     1. Upgrade the Linksys ATA firmware to the latest version

     2. Ensure that you have enough bandwidth for uplink and downlink.

          • For G.711 fallback, it is recommend to have approximately 100Kbps.

          • For T.38, allocate at least 50 kbps.

     3. To optimize G.711 fallback fax completion rates, set the following on the Line tab of your Linksys ATA device:

         Network Jitter Buffer: very high

         Jitter buffer adjustment: disable

         Call Waiting: no

         3 Way Calling: no

        Echo Canceller: no

         Silence suppression: no

         Preferred Codec: G.711


Fax Troubleshooting

If have problems sending or receiving faxes, complete the following steps:

     1. Verify that your fax machine is set to a speed between 7200 and 14400.

     (in my experience the fax must be set to 9600 bps with error correction ECM turned off)

     2. Send a test fax in a controlled environment between two Linksys ATAs.

     3. Determine the success rate.

     4. Monitor the network and record the following statistics:

          • Jitter

          • Loss

          • Delay