FAX MACHINE OVERVIEW (VoIP)

14.1
At a high level, the facsimile terminal device (fax) consists of a paper input device (scanner) paper output device (printer); accessories like telephone keypad, display, handset, and controller [McConnell et al. (1999), URL (Maine), URL (ATIS)]; and a fax modem as shown in Fig. 14.2. The fax modem is connected to the phone line of the PSTN. Once a call is established between two fax machines on the PSTN line, the PSTN infrastructure is transparent without keeping any distinction between voice or fax calls. A functional representation of a PSTN-based fax call is shown in Fig. 14.2.

Figure 14.2. Functional representations of G3 fax equipment (G3FE) internal modules.
A fax machine scans a page and electrically breaks up a document into picture elements or pixels or pels. The most common type of scanning method used in fax machines is the flat bed type, and in this method, a document is moved across an optical unit one line at a time in the vertical direction and then the optical device scans the line in the horizontal direction. The size of each picture element or pixel is different based on the resolution of the image being scanned. A smaller pixel size generates a higher resolution of the document, but it increases the amount of picture data sent over the phone lines. These pixels generate a binary bit pattern. The scanned picture elements are coded using modified Huffman (MH), modified read (MR), or modified MR (MMR) coding methods. The compression, also called the coder block, represented in Fig. 14.2 generates code words containing the picture information in a compressed format. The compressed picture elements are modulated using amplitude, phase, and frequency variations depending on the selected modulation. These modulations appearing as samples are converted into analog signals that can be sent on a TIP-RING interface and on telephone lines. The group-3 facsimile devices use V.21, V.27ter, V.29, and V.17 modems for modulation and demodulation of fax digital data/bits. SG3 facsimile devices use a V.34 modem that supports up to a 33.6-kbps rate. End fax machines distinguish between various modes of operation and rates. The users of fax machines may not be distinguishing the fax machines by G3 and SG3, but the speed of operation in sending pages can give some idea.
The receiving fax machine assembles the picture elements together as it receives them, until a copy of the original is made. During reception, the receiver modem demodulates the received analog signal and regenerates the digital signal. The decoder or decompression block expands the facsimile
digital data to picture elements. The picture elements as electrical signals are sent to a printing device that converts the bit stream into a copy of the original page. Typically, the printing device would be a thermal print head, bubble jet print head, or laser printer. The operation of the fax machine is specified by the T.30 [ITU-T-T.30 (2005)] recommendations. All group-3 facsimile devices operate in a half-duplex mode that can either send or receive at one time.
When a page is transmitted with ECM, the compressed coded data are buffered and embedded into high-level data link control (HDLC) frames with error correction before sending it for modulation block. Data of up to 256 HDLC frames constitute one partial page. More details on ECM functionality are given in Section 14.4.3. The overhead of the HDLC frame structure for each frame that includes flags, address, control, and frame check sequences will create an extra time of 2 to 3 percentage (%) as given in Annex A of T.30 [ITU-T-T.30 (1996)]. At the receiver fax machine, the receiver modem decodes the received analog HDLC frames and buffers the data. The raw payload is extracted from the buffered HDLC frame and expanded as pixel information for printing.