Understanding Fax Technology

• Elements of a Fax Call
• Establishing the Call (Phase A)
• Pre-Message Procedure (Phase B)
• In-Message Procedure and Message Transmission (Phase C)
• Post-Message Procedure (Phase D)
• Releasing the Call (Phase E)
• Compression
• Resolution
• Routing
• DTMF Routing
• Direct Inward Dialing
• T.30 Subaddressing

The major elements of fax technology include:

• Elements of a fax call
• Compression
• Resolution
• Routing

Elements of a Fax Call

The fax call elements consist of five phases:

• Establishing the call
• Pre-message procedure
• In-message procedure and message transmission
• Post-message procedure
• Releasing the call

Establishing the Call (Phase A)

The first stage occurs when the transmitting and receiving units connect over the telephone line, recognizing each other as fax machines.

Pre-Message Procedure (Phase B)

In this stage, the answering machine identifies itself in a burst of digital information packed in frames conforming to the High-Level Data-Link Control (HDLC) standard. The caller then responds with information about itself.

In-Message Procedure and Message Transmission (Phase C)

This is the actual fax transmission part of the procedure. The in-message procedure and message transmission occur simultaneously. The in-message procedure deals with synchronization, line monitoring, and problem detection. Message transmission is the actual data transmission. Once a page/file has been transmitted, the next phase begins.

Post-Message Procedure (Phase D)

After a page has been transmitted, the sender and receiver revert to the pre-message procedure modulation rate. If the sender has more pages to transmit, the in-message procedure and message transmission (Phase C) begins again for the next page. After the last page is sent, the sender transmits either an End of Message (EOM) frame, or an End Of Procedure (EOP) frame to show it is ready to end the call. The receiver then sends a confirmation.

Releasing the Call (Phase E)

Once the call is complete, the side that transmitted the last message sends a Disconnect (DCN) frame and hangs up without waiting for a response.

Compression

One of the most important components of a successful modern fax call is compression technology. The various compression encoding schemes used for fax remove redundancy from scanned material and restore the data at the receiving end. Using any of the compression schemes shortens transmission times and reduces errors.

Resolution

Fax images are made up of dots. Resolution refers to the size and density of the dots used to portray an image. There are two resolutions widely used by fax machines and fax boards: standard and fine. Fine mode contains twice as many dots per inch than the standard mode, which means the quality of the image on the receiving end is clearer. However, with more dots per inch, the file is bigger, and it takes longer to transmit the file across the telephone line.

Routing

Recent enhancements such as fax security and private mailboxes have created a need for a routing mechanism. Three types of routing mechanisms include:

• Dual-Tone Multi-Frequency (DTMF)
• Direct Inward Dialing (DID)
• T.30 subaddress

DTMF Routing

DTMF routing uses the buttons on a touch-tone telephone. The disadvantage of DTMF is that the sender needs an extension number as well as a telephone number.

Direct Inward Dialing

DID is generally considered the most foolproof, transparent routing alternative. All the sender does is dial a single telephone number, and the fax is sent directly to the recipient's workstation.

T.30 Subaddressing

A subaddress encodes a numeric string identifying the recipient into the information exchange that occurs in Phase B of the fax call.

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