What is MIME type "audio/t38"?

A MIME type is a string that tells browsers and other tools how to handle a particular kind of file.

audio/t38 is a MIME type used for fax transmissions over IP networks. It adheres to the T.38 standard defined by the ITU. It enables fax machines to send and receive fax data without relying on traditional phone lines. Instead, fax signals are converted into digital packets that travel over internet networks and are reassembled at the destination.

Fax over IP: Facilitates the transmission of fax data via VoIP systems.
Real-Time Conversion: Converts analog fax tones into digital packets for reliable data transfer.
Integration: Often used with protocols like SIP to seamlessly incorporate fax services into modern communication systems.

Files associated with this protocol typically use the T38 file extension. For more detailed technical insights, you can read about the T.38 fax standard.

Associated file extensions

.t38

Usage Examples

HTTP Header

When serving content with this MIME type, set the Content-Type header:


    Content-Type: audio/t38

HTML

In HTML, you can specify the MIME type in various elements:


    <a href="file.dat" type="audio/t38">Download file</a>

Server-side (Node.js)

Setting the Content-Type header in Node.js:


    const http = require('http');    
    
    http.createServer((req, res) => {    
      res.setHeader('Content-Type', 'audio/t38');    
      res.end('Content here');    
    }).listen(3000);

Associated file extensions

.t38

FAQs

What is the primary purpose of the audio/t38 MIME type?

The audio/t38 MIME type is used for Fax over IP (FoIP) transmissions, adhering to the ITU-T T.38 recommendation. Unlike standard audio files, this type represents a stream of fax data sent over a VoIP network, ensuring faxes are delivered reliably even over unstable internet connections. It is distinct from standard audio formats found on mime-type.com.

Can I play a file with the .t38 extension in a media player?

No, you cannot play .t38 files in standard media players like VLC or Windows Media Player. These files contain raw fax data packets or network captures intended for protocol analysis or fax server processing, not for human listening. To view the content, you typically need network analysis tools or specialized fax software.

How do I configure my web server to serve .t38 files correctly?

If you are hosting T.38 capture logs or data files, you should map the extension to the correct MIME type. For Apache, add AddType audio/t38 .t38 to your configuration. For Nginx, include audio/t38 t38; in your mime.types file to ensure clients identify the file correctly as T38 data.

Why use audio/t38 instead of standard audio codecs for faxing?

Standard audio codecs (like G.711) are designed for voice and can distort the precise analog tones required for fax machines due to compression or network jitter. The T.38 standard converts the fax signal into digital data packets, making it much more robust against packet loss and latency common in VoIP environments.

Is audio/t38 supported by modern web browsers?

No, web browsers do not natively support the audio/t38 MIME type or the T.38 protocol. This technology is used primarily between VoIP gateways, Analog Telephone Adapters (ATAs), and fax servers. Browser-based fax solutions typically rely on backend servers to handle the T.38 transmission and present the result as a PDF or image.

What does a '415 Unsupported Media Type' error mean with T.38?

In the context of SIP and VoIP, this error often occurs if one endpoint tries to switch to audio/t38 for a fax transmission but the receiving server or gateway does not support it. You should verify that T.38 Fax Relay is enabled and supported on both the sender and receiver devices.

General FAQ

What is a MIME type?

A MIME (Multipurpose Internet Mail Extensions) type is a standard that indicates the nature and format of a document, file, or assortment of bytes. MIME types are defined and standardized in IETF's RFC 6838.

MIME types are important because they help browsers and servers understand how to process a file. When a browser receives a file from a server, it uses the MIME type to determine how to display or handle the content, whether it's an image to display, a PDF to open in a viewer, or a video to play.

MIME types consist of a type and a subtype, separated by a slash (e.g., text/html, image/jpeg, application/pdf). Some MIME types also include optional parameters.

How do I find the MIME type for a file?

You can check the file extension or use a file identification tool such as file --mime-type on the command line. Many programming languages also provide libraries to detect MIME types.

Why are multiple MIME types listed for one extension?

Different applications and historical conventions may use alternative MIME identifiers for the same kind of file. Showing them all helps ensure compatibility across systems.