Facsimile, or fax, is an electronic system for transmitting graphic information by wire or radio. Facsimile is used to send printed material by scanning it and converting it to electronic signals that modulate a carrier to be transmitted over telephone lines. Since modulation is involved, fax transmission can also take place by radio. With facsimile, documents such as letters, photographs, line drawings, or any printed information can be converted to an electric signal and transmitted with conventional communication techniques. The components of a fax system are illustrated in Fig. 18-14.
Although facsimile is used to transmit pictures, it is not TV because it does not transmit sound messages or live scenes and motion. However, it does use scanning techniques that are generally similar to those used in TV. A scanning process is used to break a printed document up into many horizontal scan lines that can be transmitted and reproduced serially.
How Facsimile Works
The early facsimile machines scanned the document to be transmitted with a light and Nphotocell arrangement. A scanning head consists of a light source and a photocell. A light source, focused to a tiny point with a lens system, was used to scan the document.
The lens was also used to focus the reflected light from on the document onto the photocell. As the light scanned the letters and numbers in a typed or printed document or the grayscale in a photograph, the photocell produced a varying electronic signal whose output amplitude was proportional to the amount of reflected light. This baseband signal was then used to amplitude- or frequency-modulate a carrier in the audio frequency range. This permitted the signal to be transmitted over the telephone lines.
Fig. 18-15 shows how a printed letter might have been scanned. Assume that the letter F is black on a white background. The output of a photodetector as it scans across the line a is shown in Fig. 18-15(a). The output voltage is high for white and low for black. The output of the photodetector is also shown for scan lines b and c. The output of the photodetector is used to modulate a carrier, and the resulting signal is put on the telephone line.
The resolution of the transmission is determined by the number of scan lines per vertical inch. The greater the number of lines scanned, the finer the detail transmitted and the higher the quality of reproduction. Older systems had a resolution of 96 lines per inch (LPI), and the new systems have 200 LPI.
On the receiving end, a demodulator recovered the original signal information, whicH was then applied to a stylus. The purpose of the stylus was to redraw the original information on a blank sheet of paper. A typical stylus converted the electric signal to heat variations that burned the image into heat-sensitive paper. Other types of printing mechanisms were used.
Today’s modern fax machine is a high-tech electrooptical machine. Scanning is done electronically, and the scanned signal is converted to a binary signal. Then digital transmission with standard modem techniques is used.
Fig. 18-16 is a block diagram of a modern fax machine. The transmission process begins with an image scanner that converts the document to hundreds of horizontal scan lines. Many different techniques are used, but they all incorporate a photo- (light-) sensitive device to convert light variations along one scanned line into an electrical voltage. The resulting signal is then processed in various ways to make the data smaller and thus faster to transmit. The resulting signal is sent to a modem where it modulates a carrier set to the middle of the telephone voice spectrum bandwidth. The signal is then transmitted to the receiving fax machine over the public switched telephone network.
The receiving fax machine’s modem demodulates the signal that is then processed to recover the original data. The data is decompressed and then sent to a printer, which reproduces the document. Because all fax machines can transmit as well as receive, they are referred to as transceivers. The transmission is half-duplex because only one machine may transmit or receive at a time.
Most fax machines have a built-in telephone, and the printer can also be used as a copy machine. An embedded microcomputer handles all control and operation, including paper handling.
Most fax machines use charge-coupled devices (CCDs) for scanning. A CCD is a light-sensitive semiconductor device that converts varying light amplitudes to an electric signal. The typical CCD is made up of many tiny reverse-biased diodes that act as capacitors, which are manufactured in a matrix on a silicon chip (see Fig. 18-17). The base forms one large plate of a capacitor that is electrically separated by a dielectric from many thousands of tiny capacitor plates, as shown. When the CCD is exposed to light, the CCD capacitors charge to a value proportional to the light intensity. The capacitors are then scanned or sampled electronically to determine their charge. This
creates an analog output signal that accurately depicts the image focused on the CCD.
A CCD is actually a device that breaks up any scene or picture into individual picture elements, or pixels. The greater the number of CCD capacitors, or pixels, the higher the resolution, and the more faithfully a scene, photograph, or document can be reproduced. CCDs are available with a matrix of many thousands of pixels, thereby permitting very high-resolution picture transmission. CCDs are widely used in modern video cameras in place of the more delicate and more expensive vidicon tubes. In the video camera (camcorder), the lens focuses the entire scene on a CCD matrix. This same approach is used in some fax machines. In one type of fax machine, the document to be transmitted is placed face down as it might be in a copy machine. The document is then illuminated with brilliant light from a xenon or fluorescent bulb. A lens system focuses the reflected light on a CCD. The CCD is then scanned, and the resulting output is an analog signal whose amplitude is proportional to the amplitude of the reflected light.
In most desktop fax machines, the entire document is not focused on a single CCD. Instead, only a narrow portion of the document is lighted and examined as it is moved through the fax machine with rollers. A complex system of mirrors is used to focus the lighted area on the CCD (see Fig. 18-18).
The more modern fax machines use another type of scanning mechanism that does not use lenses. The scanning mechanism is an assembly made up of an LED array and a CCD array. These are arranged so that the entire width of a standard 81⁄2 3 11 in page is scanned simultaneously one line at a time. The LED array illuminates a narrow portion of the document. The reflected light is picked up by the CCD scanner. A typical scanner has 2048 light sensors forming one scan line. Fig. 18-19 shows a side view of the scanning mechanism. The 2048 pixels of light are converted to voltages proportional to the slight variations on one scanned line. These voltages are converted from a parallel format to a serial voltage signal. The resulting analog signal is amplified and sent to an AGC circuit and an S/H amplifier. The signal is then sent to an A/D converter where the light signals are translated to binary data words for transmission.
An enormous amount of data is generated by scanning one page of a document. A typical 81⁄2 x 11 in page represents about 40,000 bytes of data. This can be shortened by a factor of 10 or more with data compression techniques. Furthermore, because of the narrow bandwidth of telephone lines, data rates are limited. That is why it takes so long to transmit one page of data. Developments in high-speed modems have helped reduce the transmission time, but the most important developments are data compression techniques that reduce the overall amount of data, which signifi cantly decreases the transmission time and telephone charges.
Data compression is a digital data processing technique that looks for redundancy in the transmitted signal. White space or continuous segments of the page that are the same shade produce continuous strings of data words that are the same. These can be eliminated and transmitted as a special digital code that is significantly faster to transmit. Other forms of data compression use various mathematical algorithms to reduce the amount of data to be transmitted.
The data compression is carried out by a digital signal processing (DSP) chip. This is a high-speed microprocessor with embedded ROM containing the compression program. The digital data from the A/D converter is passed through the DSP chip, from which comes a significantly shorter string of data that represents the scanned image. This is what is transmitted, and in far less time than the original data could be transmitted.
At the receiving end, the demodulated signal is decompressed. Again, this is done through a DSP chip especially programmed for this function. The original data signal is recovered and sent to the printer.
Every fax machine contains a built-in modem that is similar to a conventional data modem for computers. These modems are optimized for fax transmission and reception. And they follow international standards so that any fax machine can communicate with any other fax machine.
A number of different modulation schemes are used in fax systems. Analog fax systems use AM or FM. Digital fax uses PSK or QAM. To ensure compatibility between fax machines of different manufacturers, facsimile standards have been developed for speed, modulation methods, and resolution by the International Telegraph and Telephone Consultative Committee, better known by its French abbreviation, CCITT. The CCITT is now known as the ITU-T or International Telecommunications Union. The ITU-T fax standards are divided into four groups:
- Group 1 (G1 or GI): Analog transmission using frequency modulation where white is 1300 Hz and black is 2100 Hz. Most North American equipment uses 1500 Hz for white and 2300 Hz for black. The scanning resolution is 96 lines per inch (LPI). The average transmission speed is 6 minutes per page (81⁄2 3 11 in or A4 metric size, which is slightly longer than 11 in).
- Group 2 (G2 or GII): Analog transmission using FM or vestigial sideband AM. The vestigial sideband AM uses a 2100-Hz carrier. The lower sideband and part of the upper sideband are transmitted. Resolution is 96 LPI. Transmission speed is 3 min or less for an 81⁄2 3 11 in or A4 page.
- Group 3 (G3 or GIII): Digital transmission using PCM black and white only or up to 32 shades of gray. PSK or QAM to achieve transmission speeds of up to 9600 Bd. Resolution’s 200 LPI. Transmission speed is less than 1 minute per page, with 15 to 30 s is typical.
- Group 4 (G4 or GIV): Digital transmission, 56 kbps, resolution up to 400 LPI, and speed of transmission less than 5 s.
The older G1 and G2 machines are no longer used. The most common configuration is group 3. Most G3 machines can also read the G2 format.
The G4 machines are not yet widely used. They are designed to use digital transmission only with no modem over very wideband dedicated digital-grade telephone lines. Both G3 and G4 formats also employ digital data compression methods that shorten the binary data stream considerably, thereby speeding up page transmission. This is important because shorter transmission times cut long-distance telephone charges and reduce operating costs.
Fax Machine Operation
Fig. 18-20 is a simplified block diagram of the transmitting circuits in a modern G3 fax transceiver. The analog output from the CCD array is serialized and fed to an A/D converter that translates the continuously varying light intensity into a stream of binary numbers. Sixteen grayscale values between white and black are typical. The binary data is sent to a DSP digital data compression circuit as described earlier. The binary output in serial data format is used to modulate a carrier that is transmitted over the telephone lines. The techniques are similar to those employed in modems. Speeds of 2400/4800 and 7200/9600 Bd are common. Most systems use some form of PSK or QAM to achieve very high data rates on voice-grade lines.
In the receiving portion of the fax machine, the received signal is demodulated and then sent to DSP circuits, where the data compression is removed and the binary signals are restored to their original form. The signal is then applied to a printing mechanism. The most common fax printer today is an inkjet printer like those popularly used with PCs. In the high-priced machines, laser scanning of an electrosensitive drum, similar to the drum used in laser printers, produces output copies by using the proven techniques of xerography.
The control logic in Fig. 18-20 is usually an embedded microcomputer. Besides all the internal control functions it implements, it is used for “handshaking” between the two machines that will communicate. This ensures compatibility. Handshaking is usually carried out by exchanging different audio tones. The called machine responds with tones designating its capability. The calling machine compares this to its own standards and then either initiate the transmission or terminates it because of incompatibility. If the transmission proceeds, the calling machine sends synchronizing signals to ensure that both machines start at the same time. The called machine acknowledges the receipt of the sync signal, and transmission begins. All the protocols for establishing communication and sending and receiving the data are standardized by the ITU-T. Transmission is half-duplex.
As improvements have been made in picture resolution quality, transmission speed, and cost, facsimile machines have become much more popular. The units can be easily attached with standard RJ-11 modular connectors to any telephone system. In most business applications, the fax machine is typically dedicated to a single line. Most fax machines feature fully automatic operation with a microprocessor-based control. A document can be sent to a fax machine automatically. The sending machine simply dials the receiving machine and initiates the transmission. The receiving machine answers the initial call and then reproduces the document before hanging up.
Most fax machines have a built-in telephone and are designed to share a single line with a conventional voice transmission. The built-in telephone usually features TouchTone dialing and number memory plus automatic redial and other modern telephone features. Most fax machines also have automatic send and receive features for fully unattended operation.
Fax machines are slowly fading away as technology changes. Today, most computer printers incorporate a scanner and a printer. The fax function including a data-only telephone with RJ-11 connection is built into the printer. A scanned document the digitized and sent using the fax procedures described earlier.
Internet telephony, also called Internet Protocol (IP) telephony or Voice over Internet Protocol (VoIP), uses the Internet to carry digital voice telephone calls. VoIP, in effect, for the most part, bypasses the existing telephone system, but not completely. It has been in development for over a decade, but only recently has it become practical and popular. VoIP is a highly complex digital voice system that relies on high-speed Internet connections from cable TV companies, phone companies supplying DSL, and other broadband systems including wireless. It uses the Internet’s vast fiber-optic cabling network to carry phone calls without phone company charges. This new telephony system is slowly replacing traditional phones, especially in large companies. It offers the benefits of lower long-distance calling charges and reduces the amount of new equipment needed because phone service is essentially provided over the same local-area network (LAN) that interconnects the PCs in an organization. VoIP is rapidly growing in use and in the future is expected to replace standard phones in many companies and homes. While the legacy PSTN will virtually never go away, over time it will play a smaller and smaller role as VoIP is more widely adopted or as more and more individuals choose a cell phone as their main telephone service.
There is two basic parts to an IP phone call: the “dialing” process, which establishes an initial connection, and the voice signal flow.
Voice Signal Flow
Fig. 18-21 shows the signal flow and major operations that take place during an IP phone call. The voice signal is first amplified and digitized by an analog-to-digital converter (ADC) that is part of a coder-decoder (codec) circuit, which also includes a digital-to-analog converter (DAC). The ADC usually samples the voice signal at 8 kHz and produces an 8-bit word for each sample. These samples occur one after another serially and therefore produce a 64-kbps digital signal. A relatively side bandwidth is needed to transmit this bitstream (64 kHz or more). To reduce the data rate and the need for bandwidth, the bitstream is processed by a voice encoder that compresses the voice signal. This compression is usually done by DSP either in a separate DSP processor chip or as hardwired logic on a larger chip. The output is at a greatly reduced serial digital data rate.
The type of compression used is determined by International Telecommunications Union standards. Various mathematical algorithms beyond the scope of this text are used.
The 64-kbps digital signal is designated as standard G.711 and is better known as pulse-code modulation (PCM), covered earlier in this book. Standard G.729a is probably the most common compression standard used and results in an 8-kbps digital voice signal. Ano her popular standard is G.723, which produces an even more highly compressed 5.3-kbps signal at the expense of some voice quality. Numerous other compression standards are used, and they are selected based upon the application. Most VoIP phones contain all the common compression standard algorithms in the DSP memory for use as called for. The signal is also processed in the DSP to provide echo cancellation, a problem in digital telephony.
The resulting serial digital signal is put into a special packet by a microcomputer processor running a VoIP protocol and then transmitted by Ethernet over a LAN or via a high-speed Internet connection such as is available from a cable TV company or on DSL. From there the signal travels over standard available Internet connections using TCP/IP through multiple servers and routers until it comes to the desired location. At the receiving phone, the process is reversed. The Internet signal gets converted back to Ethernet, and then the VoIP processor recovers the original packet. From there, the compressed data is extracted, decompressed by a DSP, and sent to the DAC in the codec where the original voice is heard.
One of the main problems with VoIP is that it takes a relatively long time to transmit the voice data over the Internet. The packets may take different routes through the Internet. They all do eventually arrive at their intended destination, but often the packets are out of sequence. The receiving phone must put them back together in the correct sequence. This takes time.
Furthermore, even though the signals traverse the high-speed optical Internet lines at gigabit speeds, the packets pass through numerous routers and servers, each adding transit time or latency. Latency is the delay between the time the signal is transmitted and the time it is received. It has been determined that the maximum acceptable latency is about 150 ms. Any longer time is noticeable by the user. One party may have to wait a short time before responding to avoid talking while the signal is still be received. This annoying wait is unacceptable to most. Keeping the latency below 150 ms minimizes this problem.
In the PSTN, the dialing process initiates multiple levels of switching that literally connects the calling phone to the called phone. That link is maintained for the duration of the call because the switches stay in place and the electronic paths stay dedicated to the call. In Internet telephony, no such temporary dedicated link is established because of the packetized nature of the system. Yet some methods must be used to get the voice data to the desired phone. This is taken care of by a special protocol developed for this purpose. The initial protocol used was the ITU H.323. Today, however, a newer protocol established by the Internet Engineering Task Force (IETF) called the session initiation protocol (SIP) has been adopted as the de facto standard. In both cases, the protocol sets up the call and then makes sure that the voice packets produced by the calling phone get sent to the receiving phone in a timely manner.
Internet Phone Systems
There are two basic types of IP phones: those used in the home and those used in larger organizations. The concepts as described above are the same for both, but the details are slightly different.
To establish IP phone service in the home, the subscriber must have some form of high-speed Internet service. Cable TV provides this service in most homes, but it can also be provided over the standard POTS local loop with DSL. In addition, the subscriber must have a VoIP interface. This is called different things by the different service providers. A common example is the Analog Terminal Adapter (ATA). This device connects the standard home telephone to the existing broadband Internet modem. Another configuration is a VoIP gateway that contains the ATA circuitry as well as the broadband modem.
A general block diagram of an ATA is shown in Fig. 18-22. Notice that the ATA allows standard telephones and cordless phones to attach to the ATA via the usual RJ-11 modular plug. In fact, the input to the ATA is the phone wiring in the home.
The home wiring is disconnected from the subscriber interface at the connection provided outside the home by the phone company. In this way, any of the available home phones can be used with the ATA over installed wiring. Note in the figure that because standard phones are used, they must be provided with SLIC BORSCHT functions. The SLIC circuitry is usually packaged in a single IC chip, and often the codec is also contained on this chip.
The codec inputs and outputs go to one or more processors where the H.323 or SIP protocol is implemented and where the DSP functions for compression and decompression reside. An Ethernet interface is also provided. The Ethernet signal connects to the broadband modem for cable TV service or DSL. If the cable modem is used, the POTS and last-mile local loop are simply not used. However, if the DSL service provides a broadband connection, the POTS connection is used for the DSL modem. The home phone wiring must be disconnected from the POTS line as described earlier.
Enterprise IP Phones
IP phones in companies or large organizations are specially designed for VoIP service. The telephone set contains all the ATA circuitry except for the SLIC and connects directly to the available Ethernet connection usually supplied to each desk. No broadband modem is needed. Since most employees will also have a PC connected to the LAN, a two-port Ethernet switch in the phone or PC provides a single Ethernet connection to the LAN that the phone and PC share.
A major benefit of IP phones is that they may also use wireless Ethernet connections. Wireless Ethernet, generally called Wi-Fi or the IEEE standard designation 802.11, is widely used to extend the LANs in most companies. If the IP phone is equipped with a wireless Ethernet transceiver, then no wired connection is needed. Already some cell phone manufacturers are including Wi-Fi VoIP in some models. In this way, a person’s cell phone works outside the company with the standard cell site service but also serves as the person’s company phone with a wireless Ethernet connection when inside the company. Wireless systems such as this are covered.
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