RETURN Protocol Information


ITU Facsimile Standards

CCITT (now the ITU) has published many recommended standards for facsimile (FAX) transmission. This document describes the FAX transmission modes and outlines the structure of these recommendations.

FAX Groups

CCITT Recommendation T0 describes four FAX apparatus groups; Groups 1 through 4.

  • CCITT Recommendation T2 describes a Group 1 FAX apparatus capable of transmitting a page (“A4” size document) in about 6 minutes.
  • CCITT Recommendation T3 describes a Group 2 FAX apparatus capable of transmitting a page (A4) in about 3 minutes.
  • CCITT Recommendation T4 describes a Group 3 FAX apparatus capable of transmitting a page (A4) in about 1 minute.Group 4 FAX further compresses the document signal for transmission over PDNs (Public Data Networks). Recommendations that are applicable are: T.6, T.60, T.61, T.62, T.62bis, T.70, T.72, T.73, T.503, T.521, T.563, and F.161.

FAX Test Charts

Standardized test charts (used to check FAX quality) are available for purchase from the CCITT. Recommendation T.20 describes Test Chart #1 (both First and Second editions of #1).

Recommendation T.21 describes Test Chart #2 and #3. Test Chart #2 tests for general alignment and text legibility, while Test Chart #3 can be used for precise measurements of alignment, contrast, and shading.

Document Transmission Procedures

CCITT Recommendation T.30 describes the equipment Interoperability processes and operation over the PSTN. This applies to Group 1, Group 2, and Group 3 equipment.

Five phases of transmission are discussed:

  • Setup/Call Establishment
  • Pre-Message Procedures
  • Message Transmission
  • Post-Message Procedures
  • Disconnect Processes

Additionally, two different signaling systems are described:

  • Tonal (using single frequency tones – Group 1 and Group 2).
  • Binary (300 BPS synchronous HDLC protocol; may optionally employ Error-Correction).

Facsimile Option Processes

CCITT Recommendation T.35 describes the “Country Codes” allocated to member countries, Administrations, or RPOAs (Recognized Private Operating Agency). These codes are used in an optional “CIG” message that is sent from the Sender to Receiver. Additionally, when a FAX machine is capable of providing optional NSF (Non-Standard Facilities) functions, the use of “Country Codes” in the NSF, NSS, and NSC messages is mandatory.