# Switching from voice to fax

Fusion gateways configured for voice/fax switchover initially treat all calls as voice transmissions. The following table shows how Fusion gateways switch from voice to fax processing mode in the presence of fax data:

Step | Fax station 1 (calling machine) | Emitting gateway | Receiving gateway | Fax station 2 (called machine)
1 | Sends CNG tone | Transfers CNG tone over a voice connection. | Transfers CNG tone over a voice connection. | Receives the CNG tone Sends a CED tone
2 | Receives CED tone | Receives the T.38 packet: T38UDP endpoint detects the first valid T.38 packet and sends an event to the application. Application disables the voice channel and enables the fax channel. Application sends a command to DS0 endpoint, switching it from voice to fax transmission mode. Fax channel transfers fax data to PSTN. | Receives the CED tone: DS0 endpoint detects the CED tone and sends an event to the application. Application disables the voice channel and enables the fax channel. Application sends a command to DS0 endpoint, switching it from voice to fax transmission mode. Fax channel starts sending T.38 packets. |
3 | Fax transmission | Fax transmission | Fax transmission | Fax transmission
4 | Disconnects | Ends fax transmission (or disconnects) and switches DS0 endpoint back to voice mode. | Disconnects and switches DS0 endpoint back to voice mode. | Disconnects

After the fax connection is disabled, you can reuse the connection after disabling and then re-enabling the T38UDP endpoint.

For information about MSPP service functions and information about MSPP service T.38 filters and filter commands, refer to the Dialogic® NaturalAccess™ Media Stream Protocol Processing API Developer's Manual.