CSID vs TSID: Explained

CSID and TSID are the fax standard's two identification fields: one for the answering machine, one for the machine sending the fax.

Sarah Mitchell

Published on

September 9, 2026

Last updated on

September 11, 2026

TSID (Transmitting Station ID) identifies the fax machine or service sending a fax. CSID (Called Subscriber ID) identifies the one receiving it.
Both fields come from the T.30 fax standard and are typically limited to around 20 characters, usually a fax number or business name.
Fax.Plus's Custom Fax Header setting serves the same purpose as a TSID: it's the identifying text and number printed at the top of every outgoing fax.

What's the Difference Between CSID and TSID?

TSID (Transmitting Station ID) and CSID (Called Subscriber ID) are two identification fields defined by the T.30 fax standard. TSID identifies the machine or service sending the fax; CSID identifies the machine or service receiving it. Both are short text strings, usually a fax number or business name, and both are typically capped around 20 characters. Unlike automatic number identification (ANI), which the network captures automatically, CSID and TSID are values a fax machine or service sets itself and sends as part of the T.30 handshake.

Where This Shows Up in Practice

On a traditional fax machine, the CSID and TSID are often set to the same value, since most machines both send and receive. Fax.Plus's Custom Fax Header setting covers the same practical ground as a TSID: it's the sender identification, along with the date, time, and page count, printed at the top of every outgoing fax. The receiving side's equivalent concept is what a fax header calls its Station ID field.

A Common Case: Confirming Which Office a Fax Came From

A company with several branch fax lines receives a document and needs to know exactly which office sent it, not just that it arrived. The TSID printed at the top of the page answers that, since it's set by the sending machine or service itself rather than guessed from the call. Comparing it against the CSID confirms which line on the receiving side picked it up.

FAQs

Can the CSID and TSID be the same value?

Yes. On most fax machines and services, both fields carry the same information, typically the fax number or business name, since the same device is usually capable of both sending and receiving faxes.

Are CSID and TSID the same as ANI?

No. ANI is captured automatically by the phone network during call setup. CSID and TSID are values a fax machine or service sets itself and sends as part of the T.30 handshake.

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