# Using T.38 fax full duplex channels

There are several ways to implement T.38 fax on Fusion systems so that the application can switch from voice transmission to fax transmission during the course of a phone call.

One way to implement this switch is to create consecutive MSPP voice encoding and decoding connections and T.38 fax full duplex channels on Fusion systems for transferring data across the IP network. When the application switches from voice data transmission to fax data transmission (either based on some external call control indication or the detection of a CED tone through the ADI
service), it first tears down the voice channel and then sets up a fax channel to transfer the fax data.

When the receiving gateway detects the start of a T.30 fax transmission (for example, detects a CED tone via ADI service tone detection):

Step | Action
1 | The receiving gateway disables the voice connection. It then creates and enables a T.38 fax full duplex connection using the same DS0 endpoint, and a new T38UDP endpoint and a new T.38 fax channel. The application also sends a MSPP filter command to the DS0 endpoint that switches the endpoint's transmission mode from voice to fax.
2 | The receiving gateway notifies the emitting gateway (through a reliable transport medium such as TCP or a call control protocol such as SIP, MEGACO or H.248) that fax data is forthcoming.
3 | The emitting gateway performs the same tasks as the receiving gateway to switch from voice to fax transmission, and can begin the fax transmission.

When the application disables the voice channel and creates a new T.38 fax full duplex connection, it can assign the UDP port number formerly used by the voice channel's RTP IPv4 endpoint to the new T38UDP endpoint.

The configuration shown in the following illustration conserves UDP ports by using the same UDP port for the voice connections and the fax connections (active RTP IPv4 and T38UDP endpoints cannot use the same UDP ports at the same time). However, this configuration requires considerable application interaction, not only to set up and tear down MSPP channels, but also to send fax data notification messages across the network at the appropriate times.