Knowledge Base

What is T.38 and how does it work?

The T.30 fax protocol was designed for use over analog phone lines (circuit-switched network). T.38 is an ITU protocol that dictates how fax audio traverses a packet-switched network, such as the Internet or a company’s LAN/WAN, reliably.

In order for fax audio to traverse a packet-switched network like the Internet, the analog signal used by fax machines must be converted into digital packets. The real challenge, though, is reliable fax over IP.

Voice over IP (VoIP) networks or services are often optimized for voice calls. Compression is used to reduce the amount of bandwidth required to carry the call. Because we are human, we can tolerate poor connections where packet loss or latency may cause gaps in the voice stream, or a “choppy call” experience. But fax is not nearly as forgiving. In order to fax reliably over an IP network, the network must have minimal latency, jitter, and other elements that contribute to packet loss. Thus the T.38 protocol was created to provide reliable real-time faxing over IP (FoIP).

handSIP T.38 Fax SIP Trunking is designed and optimized for faxing. While the service can carry voice calls, it doesn’t. We have a separate handSIP VoIP SIP Service for voice calling. handSIP T.38 Fax SIP Trunking carries fax calls only, thus we can tweak and massage the network to provide the most reliable faxing experience possible.