Most people would be surprised to learn that facsimile is one of the oldest
forms of electrical information transmission, following close behind the telegraph.
The first facsimile was a crude device consisting of two pens connected to two
pendulums, joined by a wire. The apparatus was able to reproduce writing on an
electrically conductive surface. The year was 1843, the same year that Congress
appropriated $30,000 to build an experimental telegraph line from Washington to
Baltimore. The telegraph expanded quickly, but facsimile languished for more
than a century before it became a mainstream product. In a prophetic statement in
1872, the U.S. Postmaster General, Jonathan Creswell said, “The facsimile systems
and the countless other applications of electricity to the transmission of intelligence
yet to be made must eventually interfere with the transportation of letters
by the slower means of post.”
News media used facsimile to send wire photos and the military used it widely
in World War II. These devices scanned the image and converted it to an analog
tone—one tone for white and another for black. Some used photosensitive receiving
media, which required developing the image like a photo. Other devices converted
the analog tone to an electrical current that varied the heat of a stylus, which literally
burned the image into paper. The receiving paper mounted on a rotating drum similar
to a lathe, and as the image burned, a thin trickle of smoke rose from the machine.
Facsimile is one of the first and best illustrations of the potency of international
standards. Fax technology improved steadily as solid-state electronics
advanced through the second half of the twentieth century, but the standards were
proprietary. Since fax transmission requires matched pairs of machines, this limited
its application to communication between organizations that had deliberately
chosen compatible machines. The first fax standards, Group 1, were published in
1968. These machines, now obsolete, required about 6 min to transmit a page.
Long-distance telephone at the time cost about 40 cents per minute, so fax was
535
neither fast nor cheap. Group 2 fax, introduced in 1976, cut the transmission time
in half. Group 3 standards, published in 1980, were the turning point that caused
fax to become indispensable. ITU published the final standards, Group 4, in 1984.
Group 4 is intended for transmitting high-quality documents over ISDN lines.
Today every organization and many individuals have G3 fax capability, either
through a machine or over the Internet, but G4 has never caught on.
Group 3 facsimile is digital, making it a natural adjunct to the computer. Fax-ondemand
(FoD) marries fax and IVR technology to enable callers to request technical
documents and product information and have them sent to a fax machine. The Web
is a more effective way of providing such information today, but many companies
still offer FoD service. Most desktop computer modems can be used interchangeably
for either data or fax, bringing fax capability down to the desktop. Many companies
have internal servers to make facsimile available to everyone on the company network.
Users can send and receive faxes without leaving their desks, and without the
extra cost of a dedicated business line or an analog station port on the PBX. Fax documents
can be converted to text with optical character recognition (OCR) software. It
is not quite as convenient as receiving a document attached to an e-mail message, but
it is fast, particularly when it comes to sending documents that originate on paper
instead of a computer file. Fulfilling Mr. Creswell’s prophecy, a facsimile machine can
send a letter in a fraction of a minute for less than the cost of postage.
The Internet has changed the face of facsimile and will continue to do so.
Besides eliminating much of the need for FoD, users find it easier to send documents
as an e-mail attachment than to fax them. One of the main drawbacks of fax
servers is the difficulty of delivering incoming fax to the desktop, which is not a
problem with e-mail. Even if the original document is not stored in a computer
file, scanners have become so inexpensive that it is often easier to scan and e-mail
a document than to carry it to the office fax machine. Furthermore, for fax to be
effective, it requires access to a phone line, or at least a switching device that recognizes
the fax tone. If a document can be sent over the Internet, its incremental
cost is essentially zero.
Even with the Internet and e-mail as alternatives, fax still has several advantages.
You never have to think about compatibility. The documents cannot be
edited, which is important for contracts, invoices, and such documents. It is easy
to send paper documents, while retaining the original format and any graphics. In
this chapter we explain how facsimile works and discuss the alternatives to a regular
fax machine including computer fax boards, fax over IP (FoIP), and service
bureaus that provide you with a fax number and send and receive faxes worldwide
on your behalf.
FACSIMILE TECHNOLOGY
Afacsimile machine has four major elements as diagrammed in Figure 30-1: scanner,
printer, controller, and communications facilities. The scanner sweeps across
536 PART 4 Customer Premise Systems
a page, segmenting it into multiple lines in much the same way that a television
camera scans an image. The scanner output converts the image to a binary code.
The digital signal is compressed and applied directly to a digital circuit in the case
of G4, or through a modem to an analog circuit. Control circuitry directs the scanning
rate and compresses solid expanses of black or white to reduce transmission
time. At the receiving end the incoming signal is demodulated and drives a print
mechanism that reproduces the incoming image on paper.
Facsimile Machine Characteristics
G3 equipment is categorized by modulation method, speed, resolution, and transmission
rate. Digital machines produce a binary signal that is applied to the circuit
through a modem. The digital transmission speed varies from as low as 2400 bps to
64 Kbps. The time required to send a page depends on circuit quality and the
amount of information encoded. Typically, a single-spaced page of text takes a
minute or less depending on factors that we will discuss later. Resolution is measured
in lines per inch (lpi) and varies from slightly less than 100 lpi to nearly 400 lpi.
G3 transmits at a maximum data rate of 14.4 Kbps, and falls back in steps to
a sustainable rate. On a noisy line, the rate may be as low as 2.4 Kbps. The original
standard had a maximum speed of 9.6 Kbps, but the V.17 protocol increased
the speed to 14.4 Kbps. Some products can transmit as high as 33.6 Kbps under the
right conditions. The latter are known as “super group 3.” Group 3 faxes use T.30
CHAPTER 30 Facsimile Systems 537
Scanner
Image Processor
and Compressor
T.30 Protocol
Processor
Image Processor
and
Decompressor
Printer
Fax Modem
F I G U R E 30-1
Block Diagram of a Facsimile Machine
protocol to negotiate the connection and manage the details of the session. The T.4
protocol controls the coding scheme, resolution, page size, and transmission time.
Group 3 fax has three different vertical resolutions:
_ Standard: 98 lpi (3.85 lines/mm)
_ Fine: 196 lpi (7.7 lines/mm)
_ Superfine: 391 lpi (15.4 lines/mm)
The superfine resolution is not an official standard, but many machines
support it. The horizontal resolution is 203 picture elements (PELs) per inch
(8 PELs/mm). A PEL is analogous to a pixel in video transmission. Digital facsimile
machines produce a binary signal that is either on (black) or off (white) for
each PEL. The number of PELs and lpi determines the resolution and transmission
time of digital facsimile.
G4 fax has a resolution of up to 400 lpi. It is intended for transmission of documents
needing laser printer quality, but it is not popular except for special applications.
One reason for its lack of acceptance is the fact that it requires an ISDN
line to support its 56/64 Kbps transmission speed, and ISDN lines are rare, at least
in North America. The second reason is that G3 fax quality is good enough for
most applications. If high quality is required, the document can be converted
at the receiving end with OCR, and if an image such as an original signature is
required, the document can be scanned at the source, converted to a PDF file, and
sent as an e-mail attachment.
The ITU-T standards specify the protocol, scanning rate, phasing, scans per
millimeter, synchronization, and modulation method. Phasing is the process of
starting the printer and the scanner at the same position on the page at the beginning
of a transmission. Synchronization keeps the scanner and printer aligned for
the duration of the transmission.
Scanners
Most machines use a flatbed scanner that converts an image to binary form. As the
document moves through the feeder, the scanner detects light from the source and
emits a 1 or 0 pulse, depending on whether the reflected light is above or below
the threshold. A single line can be represented in 1728 bits or PELs. In standard
resolution there are 1145 lines in the document, which equates to about 2 Mb
of information. This amount of data would take nearly 2.5 min to transmit at
14.4 Kbps, so it must be compressed to reduce transmission time.
Transmission times vary with the line quality and the density of the information
on a page. Many documents have expanses of white or black that are compressed
by a process called run-length encoding. Instead of transmitting a string of
zeros or ones corresponding to a long stretch of white or black, the length of the
run is encoded into a short data message. By using run-length encoding, a digital
facsimile machine compresses data into approximately one-eighth of the number
538 PART 4 Customer Premise Systems
of nonencoded bits, but the amount of compression depends on the character of
the document. For example, a document with a border around it cannot be compressed
vertically. The average typewritten document is compressed by a factor of
at least 10:1.
The speed of a fax machine is often quoted in pages per minute. ITU-T publishes
a reference page known as Test Chart # 1 that is standard for quoting transmission
speed. Four percent of the standard page is black, and the rest is white.
A page with more black content will take longer to transmit.
Facsimile Handshake
When fax machines connect, they go through a handshake process to resolve such
variables as the speed of transmission, compression method, and special features
the machines may have. The initial handshake takes about 16 s for fax machines up
to 14,400 bps. They also pass messages at the end of each page to verify transmission,
a process known as retraining. This function takes about four seconds per
page. The G3 standard uses a compression method known as Modified Huffman
(MH). Two other methods provide faster image transmission through additional
compression. These are modified read (MR) and modified modified read (MMR).
MMR is an advanced data encoding and compression scheme similar to that used
in G4. Both of these still require 16 s for the initial handshake at lower speeds, but
with V.34 modems the time is cut in half. The retraining interval is cut from 4 to
0.25 s. The handshaking time has a significant effect on facsimile transmission
speed. As the modem speed increases, the transmission time does not decrease in
direct proportion because the handshake remains fixed. Only when the faster
MMR protocol is employed does improved transmission speed become significant.
When the receiving fax machine answers the line, the transmitting machine
sends a CNG (calling) tone to signal the receiving machine that a fax machine is
calling. Fax line-sharing devices recognize this tone and switch the line to the fax
machine. The receiving fax sends a CED (called) tone to signal the transmitter that
it is connected to a fax machine. At this point, the two machines send a series of
signals to communicate such variables as paper size, modem speed, compression
method, and resolution. The sending fax transmits its transmitting subscriber
identification, without which the receiving machine refuses the session. These
signals up to this point are sent at 300 bps.
Next, the machines exchange training signals, starting at 14.4 Kbps and
gradually reducing speed if necessary. At 7.2 and 9.6 Kbps, the data is modulated
on an analog carrier with QAM. Trellis modulation is used at 9.6 and 14.4 Kbps.
When the transmitting machine receives confirmation that the session has been set
up, it begins sending the image. The receiver checks each line of data for errors
with an error correction mode. At the end of the page it slows down to 300 bps
and either sends an end of message or a multipage signal to indicate that more is
to come. At the end of the session, the transmitter and receiver hang up the line.
CHAPTER 30 Facsimile Systems 539
Printing
Fax machines either use plain paper or coated electro-sensitive paper. Coated fax
paper comes in a roll, and is less desirable than plain paper fax. Its major disadvantages
are that it curls up, and it discolors, but it is durable and the machine
is cheap to manufacture. Coated paper deteriorates over time, so it must be run
through a copier to make it permanent.
Plain-paper fax creates documents that have the same degree of permanence
as printed output. Plain-paper fax machines are more expensive, but supplies are
less costly, so for high-volume applications they are the preferable method. The
principal plain-paper fax printing technologies are thermal film, laser, and inkjet.
Athermal-film machine uses a page-width ribbon that melts onto the paper. These
machines are also inexpensive, but the ribbon is good only for a single use. Laser
and inkjet printers are identical to those used in regular office copiers and printers.
Many products combine fax, copying, scanning, and printing into a single
device.
FACSIMILE FEATURES
Some facsimile machines have features that make them complete document-communications
centers, designed for attended or unattended operation. For example,
stations can be equipped with polling features so a master can interrogate
slave stations and retrieve messages from queue. In some machines the master
polls the remote, after which the remote redials the master to send the document.
Other machines have a feature called reverse polling, which enables the receiving
machine to transmit a document on the initial poll.
Most fax machines have automatic digital terminal identification capability
and apply a time and date stamp to transmitted and received documents.
Machines can be equipped with document feeders and stackers to enable them to
send and receive documents while unattended. Machines using coated paper
often include cutters to separate the document at each page. Some digital facsimile
machines contain memory to store digitized messages and route them to designated
addressees on either a selective or a broadcast basis. Memory also enables
fax machines to scan and store documents while they are dialing out, and receive
documents before the printer is ready to start or when they are out of printing
supplies. Most facsimile machines handle only standard letter or legal size paper,
but some machines have a larger bed for handling oversized paper.
Besides the above, the following features are available as options on fax
machines and servers:
_ Automatic dial directory. This feature is the same as speed dialing
on a telephone. Direct speed-dialing buttons are available in some
machines. In others the directory is stored in memory and is recalled
from a list.
540 PART 4 Customer Premise Systems
_ Automatic redial. When a busy signal is encountered the document is
retained in memory or held in the document feeder. The machine
continues to dial until the transmission goes through.
_ Confidential transmission. The document is sent into memory in the
receiving machine, and can be printed only by entering a code.
_ Custom cover sheets. This feature is common with PC fax boards and
with fax servers. The system creates a cover sheet with a few
keystrokes by the operator and automatically fills in details such as
sender’s name, number of pages, return fax and telephone number, and
custom graphics.
_ Distinctive ring detection. The fax machine can be set up to respond to
different ringing codes sent from the central office.
_ Document verification. The system stamps each page transmitted to
enable the operator to detect misfeeds.
_ Duplex transmission. The system can send and receive simultaneously
over the same connection.
_ Dynamic port allocation. This feature limits fax broadcasting to certain
ports so incoming ports will not be tied up with outgoing calls.
_ E-mail gateway. This feature is normally found only in fax servers. The
server links to an electronic mail system and uses it to receive and
deliver fax messages.
_ Forwarding. The system can receive a document and forward to a different
machine without the loss of resolution that results from forwarding
a fax document that has been printed.
_ Group fax. This feature enables the user to send the same fax to an
address list.
_ Halftone capability. Enables the system to transmit photographs.
_ Message logging. The system logs both failed and correctly received
messages. The date, time of day, received telephone number, and other
significant information are logged. The log can be interrogated or
printed automatically.
_ Priority queuing. In a fax server certain stations can be designated as high
priority. Their transmissions take precedence over those of lower
priority.
_ Receive alerts. When a fax message is received, an alert pops up on the
computer screen or telephone display to notify the user.
_ Transmission scheduling. The system can schedule faxes for delivery at
night when rates are lower. This feature is particularly important when
large documents are sent to multiple locations. The system also can
reschedule when delivery fails for some reason such as busy or no
answer.
CHAPTER 30 Facsimile Systems 541
FACSIMILE ALTERNATIVES
The Group 3 fax machine is the most common way of faxing, but other alternatives
are available. Nearly every modem available for or built into a laptop or
desktop computer has fax capability. Accompanying software displays the fax on
the screen, and it can be sent to a printer if required. Fax servers, FoD systems, and
voice mail are all convenient ways of handling faxes. One of the most intriguing
is Internet fax, largely because it costs little or nothing.
Facsimile over IP
As the price of fax machines has continued to drop, one drawback remains: the
need to either dedicate a phone line to the fax or share it through distinctive ringing
or a fax switcher. Analog phone lines can be eliminated completely by sending
the faxes over an IP network, which can either be the Internet or private. ITU-T
and the IETF worked together to devise two different standards for FoIP. Simple
pass-through FoIP is covered by T.37 standards. Real-time FoIP, also known as fax
relay, is covered in T.38.
The major difficulty with inserting an IP network between two fax machines
is timing. Machines must exchange the various handshake signals within controlled
time intervals or else the session fails. The fax pass-through mode is also
known in ITU terms as voice-band data (VBD), which is the transmission of fax or
modem signals over a packet network. The gateways must detect that the call is
from a fax machine, but they treat it like a voice call. Low bit-rate codecs cannot be
used for fax transmission. The fax gateways must ensure that they use the G.711
(full 64 Kbps) algorithm with no voice activity detection or echo cancellation. This
method is simple, but it may not work if delay or jitter disrupts the timing.
A more reliable method is the fax relay, illustrated in Figure 30-2. In this
method, the gateway carries on a T.30 dialog with the sending fax machine over
the PSTN. The message is then packetized and sent over a VoIP network using
RTP for the transport to the other end. There the process is reversed with the
receiving gateway communicating with the receiving fax machine using T.30.
542 PART 4 Customer Premise Systems
Fax Fax
T.30 VoIP T.30
Gateway Gateway
IP Network
with QoS
F I G U R E 30-2
Fax over IP
The gateways spoof the two machines into thinking they have a PSTN connection.
As with the pass-through mode, the call must be identified as a fax call and QoS
must be enabled to ensure sufficient bandwidth.
Facsimile via Internet E-mail
Fee-based fax-by-e-mail service is an alternative to IP-based fax, which is usually
associated with VoIP. Several services use a Web portal to send and deliver
faxes. The service provides you with a personal PSTN fax number that can be
reached from anywhere in the world. In the receiving direction, the fax is converted
to a graphic file and delivered as an e-mail attachment or the recipient can
view it on the Web.
To send a fax you can send an e-mail to the service bureau, which converts it
to a document and transmits it. You can also compose the fax on the service
bureau’s Web site. Electronic mail services can transmit documents with embedded
graphics that are stored in their library. For example, a sender can store a logo and
signature block and insert them in the transmitted document. Many of these services
also offer fax broadcasting, in which documents are sent to a directory of users.
Fax Boards and Modems
A board plugged into an expansion slot in a desktop computer or a fax feature in
a modem allows a computer to emulate a facsimile machine. Fax-enabled application
software can transmit a document via fax by treating the fax board or modem
as a printer. The user does not have to go through any special steps to convert the
document for transmission. The application software takes care of the conversion
and dials the recipient without any special action on the user’s part except selecting
the addressee.
Fax software offers features that are available only on high-end facsimile
machines. Since the file to be transmitted is retrieved from a disk, a facsimile
board can transmit a document or a list of documents to a list of different telephone
numbers with minimal effort. The fax board can store an almost unlimited
number of pages. Signatures, logos, and other graphics may be integrated from a
separate file, but authentication requires additional steps. PC facsimile is also useful
as a relay device. With conventional facsimile, each time a received document
is rescanned and transmitted, it loses clarity. Since a PC facsimile is received and
sent from a file, it can be relayed indefinitely without loss of detail.
Although a PC board is a satisfactory means of transmitting fax, receiving is
another matter. Not all fax boards operate in background mode, and those that do
may slow down the computer while a document is being received. Received documents
can be displayed on either the screen or printer. Since smaller monitors are
incapable of displaying an entire page in readable form, it is necessary to scroll the
document, and the resolution may be too low to read the document without
expanding it.
CHAPTER 30 Facsimile Systems 543
Although the cost of a fax board is less than a full-featured fax machine, it is
less convenient for users to share, more time consuming for transmitting graphics,
and less than adequate for receiving documents. It also requires access to an
analog phone line, which is becoming less prevalent as more organizations move
such applications to the LAN.
Fax Servers
In most offices everyone needs access to fax, which may result in queues at the fax
machine. Furthermore, when many outgoing faxes are being sent the number
rings busy to incoming callers. The solution to the busy fax machine is more lines
and machines, or some form of fax store-and-forward capability. The latter is
provided by LECs as a service, and it is available in fax servers and fax-enabled
voice-mail systems.
Fax servers offer the following advantages over standalone fax machines:
_ It is unnecessary to print documents before faxing them, which saves
cost and time.
_ The quality of received documents is higher because it is unnecessary to
scan them if they are machine-readable.
_ It eliminates human queues at the fax machine, and therefore increases
productivity.
_ Automatic delivery of received documents may be possible with one of
the methods discussed later.
_ The system automatically redials busy numbers and those that do not
answer.
_ Fax lines can be left free to answer incoming calls.
_ Fax documents can be sent over the company network to file in another
server, eliminating the need to go outside the system, and saving
transmission costs.
_ Fax can originate from within fax-enabled applications. Word
processors, spreadsheets, and other applications treat fax servers
and modems as printers.
Fax servers have some drawbacks that limit their application. Some of the
principal disadvantages of a server compared to a standalone fax machine are:
_ If the document is not already in a computer file, it must be converted
with a scanner.
_ Signatures are difficult to control and authenticate. Scanned signatures
can be added to documents, but it is easy for someone other than the
signer to add it.
544 PART 4 Customer Premise Systems
_ Bit-mapped fax files are slower to print than text. They can be converted
to text with OCR, but the conversion process is time consuming.
_ Automatic routing of incoming faxes is problematic in many
applications.
The choice of modems to use with fax servers is important. Inexpensive fax
modems are readily available, but they lack the features of more expensive
modems. TIA/EIA has developed standards for fax modems known as Class 1
and Class 2 standards. The common fax modems that are available everywhere
are Class 1 modems (TIA/EIA 578). They assume that most of the transmission
features are built into the application software. Class 2 modems (TIA/EIA 592)
build communication features into hardware. Class 2 modems have flow control
capability to enable the receiving modem to control the transmission rate. The
initial handshake between the two modems, which complies with ITU T.30
standards, establishes their ability to regulate transmission rates.
Class 1 modems assume that fax-timing specifications will be implemented
in software. Class 2 modems handle timing from an on-board processor. The Class
2 standards provide copy quality checking, which is absent from the Class 1 specification.
These and other features make Class 2 modems preferable for heavy fax
server applications, but some devices the server communicates with may be
unable to comply with Class 2 standards.
Inbound Fax Delivery
The delivery of incoming faxes from a server is a problem with few ideal solutions.
With standalone machines paper output must be delivered manually, which
is time-consuming and often results in delays. Theoretically, fax servers can
deliver faxes directly to the desktop by one of three methods. The most reliable is
direct inward dialing, in which each user has his or her own DID fax number. This
method is common with voice-mail systems. It is the most reliable method, but
it adds expense for DID numbers plus the additional cost of the hardware and
software in the voice mail.
A second method is to scan the cover page with OCR software and pick the
recipient’s name out of the resulting file. This method is fine in theory, but reliability
is low because of different ways the recipient’s name may be written. Users
report about 75 percent reliability with this method. The third method is using
additional DTMF digits for routing the document. This method is reliable, but it
requires the sender to know the receiver’s personal code.
Group 4 Facsimile
ITU Group 4 facsimile standards support a high-speed, high-resolution digital
system operating at speeds up to 64 Kbps using either switched 56 or BRI service.
Resolution is from 200 to 400 lpi using high compression. G3 fax machines code
CHAPTER 30 Facsimile Systems 545
each line individually, but lines are coded in G4 fax based on the coding of the previous
line. Each line becomes a reference line for the succeeding line and only the
changes are transmitted. The G4 coding standard is T.6. It achieves compression
more than double that of G3 machines. G4 assumes a digital medium so it omits
error checking. As discussed earlier, G4 fax is not a popular application because
other alternatives are available, but where high-quality images are important, it
provides better results than G3. For example, law enforcement agencies use it for
picture and fingerprint transmission.
The G4 standard separates fax machines into three classes. All three classes
support 100 PEL per inch horizontal resolution. Classes 2 and 3 have 300 PEL per
inch resolution with options of 240 and 400 lpi. Table 30-1 lists the characteristics
of the three classes of Group 4 machines. The quality of Group 4 fax is about
as good as an office laser printer, which makes it suitable for transmitting letterquality
documents.
In addition to offering the advantages of high speed and improved resolution,
G4 supports communications between word processors. Communicating
word processors transfer data between their memories using a feature known as
teletex (not to be confused with teletext, which is the transmission of information
during the vertical blanking interval of a video signal). Group 4 facsimile standards
make it possible to integrate facsimile and communicating word processors.
To overcome the inherent disadvantages of each type of system, Class 2 and 3
machines send textual information in alphanumeric form and graphic information
in facsimile form. Terminals capable of this form of communication are called
mixed mode. Page memory is required for memory-to-memory transmission.
Facsimile Line-Sharing Devices
It is awkward to receive fax messages if the machine is not connected to a dedicated
telephone line. Several methods of sharing the line manually are used, but
546 PART 4 Customer Premise Systems
TA B L E 30-1
ITU-T Group 4 Facsimile Characteristics
Class
Service 1 2 3
Facsimile Transmit/receive Transmit/receive Transmit/receive
Teletex Receive Create/transmit/receive
Mixed Mode Receive Create/transmit/receive
Resolution (lines/inch)
Standard 200 200/300 200/300
Optional 240/300/400 240/400 240/400
none are foolproof. One way is to let the phone ring and answer it if the fax
does not pick it up. Another way is to listen for the CNG tone and connect the fax
if one is heard. Some devices answer the telephone with synthesized voice that
instructs the caller to press a DTMF key to direct the call to a person. Otherwise,
the device connects to the fax machine to initiate a facsimile session. More reliable
are two different types of device that enable line sharing among fax, voice, and
modem.
Some line-sharing devices monitor the line for a CNG tone and switch to the
fax port if one is detected. Otherwise, the phone rings until answered. A more
effective method is distinctive ringing, which most LECs offer, and which many
fax machines support. Distinctive ringing assigns more than one telephone number
to the same line, with each number assigned a different ringing code. Adecoding
box recognizes the ring and switches it to telephone, fax, or modem as
appropriate. Most fax machines can recognize distinctive rings.
FACSIMILE APPLICATION ISSUES
Fax is undoubtedly the largest single data transmission application on the PSTN.
The criteria discussed below apply primarily to the telecommunication aspects of
facsimile. A discussion of the technical requirements and features of facsimile
machines is beyond the scope of this book.
Document Characteristics
The primary factors in determining which group of facsimile equipment to select
are the document volume and resolution required. Plain-paper fax offers superior
quality compared to coated paper. Of the plain-paper devices, laser printers offer
somewhat better quality than ink jet machines, although the difference is unimportant
in most companies.
Many machines offer a high-resolution mode, which produces quality close
to that of a letter-quality printer. If copy quality is important, a high-resolution
machine should be selected. The need for halftones in documents should also be
considered. Facsimile machines vary widely in their ability to handle halftones,
with the level of gray scale varying from 0 to 64 shades of gray.
Labor-saving features such as automatic document feed, paper cutter, automatic
dial and answer, document storage, and document routing should be considered.
If confidential information is being transmitted, encryption should be
considered.
Compatibility
When special features such as halftone transmission, networking, and polling are
required the systems should be from the same manufacturer or fully tested with
CHAPTER 30 Facsimile Systems 547
systems of other manufacturer. Compatibility between machines for basic sending
and receiving is generally not a problem.
Document Memory
Most high-end fax machines offer memory for document storage. This feature is
useful for unattended operation. If the machine runs out of paper, received documents
can be stored in memory. Multipage documents can be sent without fear of
paper jams. Documents can be sent to address lists that are recalled from memory.
Transmission time is often reduced because the machine receives into memory so
that transmission is not paced by the speed of the printer.
Fax Server or Fax Machine
The question of whether to use a fax server or fax machines may arise in some
organizations. As a practical matter, a fax server will not replace all fax machines,
but a server has several advantages that weigh in its favor:
_ Faxes can be sent with a click of a button instead of printing the
document and taking it to a fax machine.
_ Faxes can be sent to groups quickly and with less manual effort.
_ Lost time waiting for busy fax numbers is eliminated.
_ Fax numbers are stored in one central directory, reducing personal fax
directories.
_ Faxes are of higher quality because they are sent directly from a highresolution
computer file.
_ Faxes can easily be archived.
_ If incoming faxes are distributed automatically, they can be kept private.
_ Costs can be distributed using a standard call-accounting system.
_ Faxes can be sent at off-peak times.
A fax server has disadvantages in addition to the cost that may outweigh the
advantages in some organizations.
_ A fax server can fax only computer files so paper documents must be
scanned.
_ Routing automatically to recipients is difficult and may be expensive.
_ Some standalone fax machines will still be needed to print faxes.
