T.30 protocol
T.30: the conversation two fax machines hold before exchanging anything
Before a single page travels, the two machines hold a coded dialogue. That dialogue, standardised as T.30, is what makes a fax verifiable.
Short answer
T.30 is the ITU recommendation describing the negotiation between two fax machines: mutual identification, agreement on a speed and a correction mode, page-by-page transmission, then confirmation of each page. That confirmation is what feeds the transmission report.
Five phases, always in the same order
The exchange follows strict phases. The caller sets up the link, the called party announces what it supports — admissible speeds, page widths, whether error correction is available. The caller picks from that list, sends a training sequence, then transmits. After each page, the called party states whether it arrived correctly. The link closes with an announced disconnection, never with a bare hang-up.
Identification, and its limit
Each machine announces an identification string, often its own number. It appears on the report and in the printed header. It is worth knowing that this string is declarative: it is configured by the machine's owner, not assigned by the network. It therefore identifies a machine that presents itself as such, which is useful but is not authentication.
Error correction changes what the report means
Without correction, a page damaged by line noise arrives damaged and the receiver confirms it anyway. With the correction mode, data travels in verified blocks that are resent on error: a confirmed page is then an intact page. That is the difference between “something arrived” and “exactly this arrived”.
Frequently asked questions
- Does T.30 encrypt the document?
- No. The standard organises negotiation and integrity checking, not confidentiality. Protection comes from the medium: a switched telephone link is a closed channel, which is not the same thing as an encrypted one.
- Why does the speed drop during an exchange?
- When too many errors appear, the machines renegotiate downward rather than give up. A long transmission on a noisy line is therefore normal: the protocol prefers slowness to failure.
Send a fax now
The form fits on one page: a PDF, a recipient and a return address. No account, no card.
Open the form