# T.38 fax connections overview

The ITU T.38 standard describes a mechanism for transferring facsimile documents in real-time between standard Group 3 facsimile terminals or Internet Aware Facsimile (IAF) devices over the Internet or other networks using IP protocols. It also specifies a protocol by which IP fax gateways or IAF devices exchange messages and data over an IP network.

This topic describes:

- T.38 fax transmission model

- T.38 fax relay

- Fusion T.38 fax channels

Note: Fusion does not currently support IPv6 for T.38 fax connections.

## T.38 fax transmission model

The process of transferring facsimile documents by IP fax gateways includes the following steps:

- Demodulating incoming T.30 fax signals at the sending or emitting gateway.

- Translating T.30 fax signals into T.38 Internet fax protocol (IFP) packets.

- Exchanging IFP packets between emitting and receiving T.38 gateways.

- Translating T.38 IFP packets back into T.30 signals at the receiving gateway

- Modulating T.30 signals and transferring them to the receiving fax machine and vice versa.

Gateways that support T.38 fax are usually transparent to the fax machine user. The following illustration shows an overview of the basic model for Fusion T.38 fax transmission:

## T.38 fax relay

Fax relay transmissions occur in real-time. A calling fax machine places a PSTN call and is connected through a PSTN line to a gateway system. The emitting gateway demodulates the fax data and then relays the data to another receiving gateway (in packet form).

When it receives the IP packets, the receiving gateway recovers the fax data, re-modulates the data, and transfers the data to the called fax machine over a PSTN line. Under normal conditions, the packet network connection is transparent to the fax machines and they operate as if they were connected directly over a PSTN. Although T.38 fax relay offers the advantage of real-time fax transmission, it is sensitive to network parameters such as network latency, jitter, and packet loss.

## T.38 fax standard

The ITU-T T.38 standard describes a fax relay oriented protocol and specifies the messages exchanged between gateways while transferring fax data over IP networks. The following items are specified by this standard:

- The order of messages transported across the packet network

- The message format required

- Error correction techniques

- IP call setup mechanisms (in the Annexes section)

The ITU-T T.38 standard specifies how data is transmitted from one gateway to the another. It does not specify how the sending gateway demodulates fax data from the calling fax machine or how fax data is re-modulated to the called fax machine.

## Fusion T.38 fax channels

Fusion provides the following two types of T.38 fax components:

Channel type | Description
T.38 fax full duplex channel | General purpose full duplex T.38 fax channel that transfers T.38 data between a DS0 and T38UDP endpoint. The channel also demodulates or re-modulates the fax data stream depending on the direction (towards the IP or PSTN interface) that the data is moving.
T38UDP full duplex endpoint | Provides an entry point and an exit point for T.38 fax data transferred through the system. Also performs processing tasks such as UDPTL encoding and decoding, and IFP encoding and decoding with the data that flows through the endpoint. When the T38UDP endpoint is enabled, it listens for T.38 fax packets on the incoming data stream. When it detects the first valid T.38 packet, it returns an MSPEVN_T38_PACKET_DETECTED unsolicited event to the application. Disabling and then re-enabling the T38UDP endpoint resets the endpoint.