Codecs & Media

T.38 (Fax over IP)

T.38 is the ITU-T standard for transporting real-time fax over IP networks. It does not transmit fax audio — it relays the underlying T.30 fax protocol over UDPTL or TCP, which is far more reliable than trying to send fax tones through an audio codec.

Why fax over G.711 fails

Fax is a modem protocol carried as audio tones (V.27ter, V.29, V.17). G.711 is lossy enough at 1-2% packet loss that fax modulation desynchronizes. Even small jitter causes page errors. T.38 sidesteps this by encoding the fax data as packets, with redundancy and retransmission built in.

How it works

1. Fax call begins with a regular SIP INVITE offering G.711.
2. When the receiving fax machine emits the CED tone (~3 seconds in), the gateway sends a re-INVITE switching to image/t38 media.
3. The remaining fax exchange runs over T.38 UDPTL (UDP with Forward Error Correction).

DIDHub T.38

Enabled per-trunk in dashboard → SIP Trunks → Codecs. T.38 only works on PSTN routes that originate as TDM. For voice-app traffic, fax is unsupported.

Related terms