Table of Contents:

Definition and Overview

1. Introduction

2. Why Fax Testing Is Necessary

3. Fax Operation Overview

4. Basic Fax System Structure and Process

5. Message Repetitions and Time-Outs

6. Fax System Test Issues

7. Network Test Issues

8. Common Deviations from ITU Standards

9. Fax Testing Customer Types

10. Fax Testing Application Areas

11. Conclusion

Self-Test

Glossary

PDF of this tutorial

Fax communications takes place through complex networks that are subject to impairments of various kinds. These impairments (for example, noise, delay, lost data packets) can result in corrupted or lost messages. In order to compensate for the effects of possible impairments, the fax protocol specifies that handshake messages expecting to receive a response will be repeated automatically if no response is received within three seconds.

The handshake messages used in simple fax calls are as follows:

• digital identification signal (DIS)—sent by answering unit, specifies unit capabilities
• digital transmit command (DTS)—sent by calling unit, polling request to ask for document from answering unit
• digital command signal (DCS)—specifies transmit parameters

• confirmation to receive (CFR)—valid TCF test pattern received acknowledgment
• failure to train (FTT)—TCF test pattern unacceptable, request for retraining

• end of message (EOM)—end of fax transmission, request turnaround or new parameters
• multipage signal (MPS)—end of fax page, more pages to follow
• end of procedure (EOP)—end of fax message, disconnect after confirmation

• message confirmation (MCF)—valid image received, ready for more pages
• retrain positive (RTP)—valid image received but retraining required before next page
• retrain negative (RTN)—bad image received, retrain and resend image

• disconnect (DCN)—line disconnect to follow, no response required

These message types can be classified as command messages and response messages as shown below:

• capabilities message (DIS)—expects DCS or DTC as response
• polling request (DTC)—expects DCS as response
• configuration message (DCS/TCF)—expects CFR or FTT as response
• image completion message (EOP, MPS, EOM)—expect MCF, RTP, or RTN as response

• configuration message (DCS/TCF)—response to DIS or DTC
• polling request (DTC)—response to DIS
• CFR, FTT—test pattern acknowledgement response to DCS/TCF sequence
• MCF, RTP, RTN—image reception response to EOP, MPS, or EOM, etc.

Unacknowledged command messages are automatically repeated after listening for a response for three seconds. Commands are sent up to three times or until acknowledged, whichever comes first. Unacknowledged DIS messages are repeated every three seconds until the end of the T1 time-out, which is 35 seconds. These timing and automatic message retransmission constraints can result in a wide variety of error conditions and error recovery sequences, all of which must be tested.