Fax Glossary

Terms & definitions

Group 3 FaxG3

Group 3 is the ITU-T T.4 standard that has defined ordinary fax since 1980. It digitizes a page, compresses the scan lines, and sends them over an ordinary phone line in roughly a minute, replacing the slower analog Group 1 and Group 2 machines that came before it. Almost every fax machine and fax service in use today is a Group 3 device.

Online faxing: An online fax service speaks the same G3 language on the phone-network side, so a fax you send from a browser arrives on a recipient's standalone machine exactly as if it came from another machine.

Super G3SG3 / V.34

Super G3 is a faster profile of Group 3 fax that uses the V.34 modem standard to negotiate speeds up to 33.6 kbit/s, roughly double the 14.4 kbit/s ceiling of the older V.17 standard. Pages that once took a minute can transfer in a few seconds when both ends support it. It is fully backward compatible and falls back to slower modulations on poor lines.

Online faxing: V.34 negotiation is sensitive to the jitter of packetized voice paths, which is one reason fax over IP networks often pins sessions to a slower, sturdier modulation for reliability.

T.30 ProtocolT.30

T.30 is the ITU-T recommendation that governs a fax call from start to finish: the opening handshake, the exchange of each side's capabilities, modem training, page transmission, end-of-page confirmation, and disconnect. It is the session-control layer that sits on top of the T.4 image coding. Two fax endpoints that both implement T.30 can interoperate regardless of who made them.

Online faxing: When a cloud fax provider connects a call, its media gateway runs the full T.30 procedure on the customer's behalf, which is why you only see a simple upload-and-send screen.

Fax over IPFoIP

Fax over IP is the general practice of carrying fax traffic across an IP network instead of a traditional copper phone line. It covers two broad approaches: passing the fax tones through a voice codec, and the purpose-built T.38 method that relays the fax data as packets. FoIP is how nearly all modern fax infrastructure connects to the public phone network.

Online faxing: Online faxing is FoIP end to end on the provider's side; your document never touches a physical phone line until the provider's gateway places the call.

T.38T.38

T.38 is the ITU-T recommendation for relaying Group 3 fax in real time over IP networks. Rather than trying to squeeze fragile fax tones through a voice codec, a T.38 gateway demodulates the fax at the edge, carries the underlying T.30 data as IP packets with redundancy against loss, and remodulates it at the far end. This makes fax far more reliable across VoIP links.

Online faxing: Reputable cloud fax carriers use T.38 between their gateways and the phone network, which is a key reason internet faxing is dependable where a consumer VoIP line with a fax machine often is not.

Error Correction ModeECM

Error Correction Mode is an optional T.30 feature that divides each page into small frames and lets the receiver ask the sender to retransmit any frame that arrives corrupted. Without it, line noise shows up as smeared or missing bands on the printed page; with it, the page either arrives clean or the call retries. ECM trades a little speed for accuracy.

Online faxing: Cloud fax gateways typically run with ECM enabled, so noisy-line artifacts that plagued old machines are largely corrected before the page is ever delivered.

Fax Compression (MH, MR, MMR)MH / MR / MMR

Fax pages are compressed before transmission using run-length and reference-line schemes defined in T.4 and T.6. Modified Huffman (MH) codes each scan line on its own; Modified READ (MR) and Modified Modified READ (MMR, also called T.6) also encode the difference from the line above, achieving tighter compression on typical documents. Higher schemes shrink transmission time but need clean lines or ECM to stay safe.

Online faxing: A fax service converts your uploaded PDF or photo into one of these black-and-white coded formats behind the scenes, which is why color and fine gray shading do not survive a fax.

Fax ResolutionDPI

Standard fax resolution is about 204 by 98 dots per inch, and fine mode roughly doubles the vertical detail to 204 by 196. Fax is fundamentally a one-bit, black-or-white medium with no grayscale, so photographs and light text reproduce poorly compared to crisp printed type. Resolution is negotiated during the T.30 handshake based on what both ends support.

Online faxing: Sending a high-contrast PDF rather than a phone photo gives the cleanest result, because the service still has to reduce whatever you upload to fax's coarse black-and-white grid.

Call Subscriber IDCSID / TSI

The CSID is the identifying string a fax machine transmits during the handshake and that the receiving machine prints across the top of each page, usually a phone number and a short name. The transmitting side's value is formally called the Transmitting Subscriber Identification (TSI) in T.30. It is self-declared by the sender, not verified by the network.

Online faxing: Online fax dashboards let you set the header line that appears as your CSID, so the recipient sees your business name and fax number even though no machine of yours is involved.

Direct Inward DialingDID

Direct Inward Dialing is a carrier service that assigns individual phone numbers that route directly to endpoints behind a single trunk or system, instead of requiring an operator or extension. A DID number can terminate on a phone, a PBX extension, or a virtual service. It is the mechanism that gives each user or line its own dialable number.

Online faxing: A dedicated online fax number is a DID provisioned to the provider's fax platform, so incoming faxes to that number are captured and delivered to you as files rather than printed paper.

North American Numbering PlanNANP

The NANP is the telephone numbering scheme for the United States, Canada, and many Caribbean nations, all sharing country code +1. Numbers follow a 10-digit NPA-NXX-XXXX pattern: a three-digit area code (NPA), a three-digit exchange (NXX), and a four-digit line number. The plan is administered to keep numbers unique across the participating countries.

Online faxing: When you pick a local fax number, you are choosing an available NPA-NXX combination in the area code you want, which lets you present a local presence in a city where you have no office.

Toll-Free Number & RespOrgRespOrg

Toll-free numbers use the 800, 888, 877, 866, 855, 844, and 833 codes and bill the called party rather than the caller. Each number's routing record lives in a shared national database, and the Responsible Organization (RespOrg) is the entity authorized to reserve and manage that record. Moving a toll-free number between providers means changing its RespOrg.

Online faxing: A toll-free fax number lets clients and partners fax you without a long-distance charge, and a cloud provider acts as the RespOrg that points the number at its fax platform.

Analog Telephone AdapterATA

An ATA is a small device that converts the analog signals of an ordinary phone or fax machine into VoIP packets, letting legacy equipment work over an internet connection. It presents a standard phone jack on one side and an Ethernet connection on the other. For fax, an ATA usually has to be configured carefully, often forcing T.38 or a low speed, to pass calls reliably.

Online faxing: Internet faxing skips the ATA entirely: instead of feeding a physical machine through an adapter, you send the document straight from software, removing the most common point of failure.

Handshake & Training

Before any page is sent, two fax endpoints negotiate. The calling side emits a CNG tone, the answering side replies with a CED tone, and the machines exchange T.30 capability frames to agree on speed, resolution, and compression. Modem training then sends a known signal so the receiver can tune itself to the line. Only after a successful handshake does page data flow.

Online faxing: Most failed faxes die during the handshake on a noisy or mismatched line, so a provider's hardened gateways and T.38 relays improve the odds of a clean connection on the first try.

Store-and-Forward Fax

Store-and-forward describes a fax workflow where a message is fully received and stored by an intermediary, then delivered onward to its final destination, rather than passing through in real time. The classic example is the T.37 standard, which carries a fax as an email attachment. It decouples the sending and receiving ends in time.

Online faxing: Inbound cloud faxing is a store-and-forward model: the provider answers the call, captures the full document, and then forwards it to your email or dashboard as a PDF.

Fax Server

A fax server is a system that sends and receives faxes on behalf of an organization's whole network, letting users fax from their computers without individual machines or lines. Traditionally it was an on-premises box with fax modem boards connected to phone trunks. The cloud fax model replaces that hardware with a hosted service reached over the internet.

Online faxing: A cloud fax provider is effectively a shared, fully managed fax server, so a small business gets the same desktop-to-fax capability without buying, housing, or maintaining the equipment.

Optical Character RecognitionOCR

OCR is the technology that analyzes a scanned image and converts the pictures of letters into machine-readable, searchable text. Because a received fax is just a bitmap image of a page, OCR is what turns it into copyable, indexable content. Accuracy depends heavily on the resolution and contrast of the original scan.

Online faxing: Faxes that arrive as PDFs can be run through OCR so the text inside them becomes searchable in your records, which is useful for retrieving a specific document later.

DTMF SignalingDTMF

Dual-Tone Multi-Frequency is the touch-tone system that represents each keypad digit as a pair of audio tones, one from a low group and one from a high group. It is how dialed digits and menu choices travel down a phone line as sound. DTMF is distinct from the fax handshake tones, though both share the same audio channel.

Online faxing: When a cloud platform dials a recipient's fax number, the destination digits are conveyed by DTMF or its digital equivalent before the two fax endpoints begin their handshake.