CTM4-fax.pdf

Fax Basics

Transmission

Reception

Fax Circuit Update

Compression Techniques

Modulation Techniques

Protocol

Faxing From a PC

Fax Troubleshooting

Common Fax Features

Facsimile Processes

Fax Basics

What is a Fax Machine?

Facsimile, commonly known as fax, is used for

sending written, printed, or graphic information from

one location to another. The communication can be

across the room or across the world. A simple block

diagram of fax operation is on the right.

Facsimile machines combine scanner and printer

technology with telephone equipment to send copies

to a remote location. There are three basic steps in a

facsimile transmission.

RECEIVED

FAX MESSAGE

ORIGINAL

--- ---- -- ---- -

------ --- ---- --

-------------

-- -- -

-----

-- -- -

-----

-- ---- -- --- --

--------------

------ --- ---- --

---- --- - - --- -

-- -- -

-----

-- -- -

-----

-- ---- -- --- --

A light source scans the writing and drawings

on the original, and converts the information

into an electrical signal.

This signal is sent over the telephone line to a

receiving fax machine.

SCANNING

PRI NTI NG

The receiving machine converts the incoming

signal into a copy of the original.

CONVERSION

CONVERSI ON

TRANSMITTING

FAX MACHINE

RECEIVING

FAX MACHI NE

TELEPHONE LINE

Faxintro.wmf

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Group 3 Fax Communication

Fax Basics

Facsimile machines can transmit alphanumeric and graphic characters. Anything that can be put on

paper, from handwritten notes to photographs, can be sent by fax.

Mechanical Processes

A fax machine contains a scanner and a printer. The mechanisms are basically the same as those

used in copiers.

Data Path

This section outlines the path of data through the machine in transmission and reception modes.

Components

The following components of the fax video data path are not used in copiers.

SAF Memory (Store And Forward Memory)

When a user stores a fax message in the memory for sending later or transmission to more than one

destination, the message goes into the SAF memory. Also, incoming confidential messages and

’substitute’ receptions (data coming in when the printer is not working) are held in this memory.

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Group 3 Fax Communication

Fax Basics

Line Buffer

This memory buffer ensures synchronization of video data transfer between different components of

the circuit. If the line buffer is too small, the scanner mechanism will have to keep stopping and

starting, leading to excessive noise. A line buffer size of four or eight lines is typical.

Data Compressor and Reconstructor (DCR)

This circuit compresses the data before sending it out over the telephone line. It also reconstructs

compressed data coming in from the telephone line

ECM Memory (Error Correction Mode Memory)

ECM is an optional extension to Group 3 protocol that provides a more reliable way to send data over

noisy lines. Using ECM, data is assembled into protocol frames. ECM requires RAM for assembly

into and extraction from protocol frames. This memory is the ECM memory.

With ECM, image data is arranged in protocol frames (256 bytes in each frame), and transmitted in

blocks made up of 256 frames. Therefore, each block is 64 kbytes. Normally, one page of

compressed image data can be sent in one block, but more than one may be needed if halftone

mode was used when scanning the original.

Single Buffer and Double Buffer ECM Memory

Single Buffer (64 kbytes): This memory is only large enough to hold one block. After receiving a block

of compressed facsimile data from the remote terminal into ECM memory, the data is reconstructed

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Group 3 Fax Communication

Fax Basics

and printed. The next block cannot be received until printing has finished. The first block is deleted

from memory after printing.

Double Buffer (128 kbytes): This memory can hold two blocks. While one block is being printed, the

next block can be received into the other half of the double buffer memory. This ensures continuous

operation, saving time and overall communication charges. In some machines with a single buffer

only, SAF memory or hard disk memory are used to provide double buffer capability.

ECM is explained in more detail in the Protocol section. For full details, see ITU-T recommendation

T.30.

FIFO Memory (First-In First-Out Memory)

The FIFO synchronizes the transfer of video data to the modem (transmission) or from the modem

(reception). It also acts as a buffer, ensuring that there is always some data for the modem to pick

up, modulate, and send out.

The FIFO also has some unique functions, in addition to synchronizing data transfer from the cpu to

the modem, as explained in the Transmission section (see the part entitled Data Transmission).

Modem

During transmission, the modem converts the data into a form that can be sent out on the telephone

line in accordance with the appropriate ITU-T V-series recommendations; this process is called

Modulation.

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Group 3 Fax Communication

Fax Basics

During reception, the modem converts incoming data into a form that the machine can work with; this

process is called Demodulation.

The term ’modem’ is derived from these two processes; MO dulation/ DEM odulation. The different

types of modulation encountered in fax machines are described in the Modulation section (for full

details see the ITU-T V-series recommendations).

Network Interface Circuits

The filters, relays, attenuators and other components in these circuits interface the machine with the

public telephone network. These circuits ensure that the machine connects to the line and that it

dials in the correct way. They also ensure that the machine and the network equipment do not

damage each other. Some of the components are included in an assembly called the “Hybrid IC”

(HIC).

Voice Message Processor

This converts recorded voice messages from analog (audio) to digital for storage in the memory. It

also retrieves the message for memory when it is needed for sending out over the telephone line.

Transmission

This section explains the path of data through the machine during transmission. The main

description is for memory transmission with ECM. Following this main description are diagrams

showing the differences between memory transmission with ECM and the following modes:

Memory transmission without ECM

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