Fax Modems
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- What is a modem?
- What is a fax modem?
- What are the differences between the Digital
Signal Processor (DSP) based and the Application-specific Integrated Circuit
(ASIC) based fax/modem cards? Which type is more popular in today's market?
- What do the "V.DOT" standards specify (V.32,
V.42bis)?
- What does "Group III" mean?
- What is the difference between baud versus Bits
Per Second (bps)?
- What does "throughput" mean?
- What does MNP stand for?
- How does the fax/modem work?
- What are the three elements of modem
communications?
- What is asynchronous communications?
- What is the start/stop bit?
- What is the parity bit?
- Explain Half/Full Duplex.
What is a modem?
A modem is a device that enables a computer to transmit data over telephone
lines. Computer information is stored digitally, whereas information transmitted
over telephone lines is transmitted in the form of analog waves. A modem
converts between these two forms.
There is one standard interface for connecting external modems to computers
called RS-232. Consequently, any external modem can be attached to any computer
that has an RS-232 port, which almost all personal computers have. There are
also modems that come as an expansion board that you can insert into a vacant
expansion slot. These are sometimes called internal modems.
The following characteristics distinguish one modem from another:
Bps: How fast the modem can transmit and receive data. At slow rates,
modems are measured in terms of baud rates. The slowest rate is 300 baud (about
25cps). At higher speeds, modems are measured in terms of bits per second (bps).
The fastest modems download at 56,000bps*, although they can achieve even higher
data transfer rates by compressing the data. The faster the transmission rates,
the faster you can send and receive data.
*Actual speeds may vary depending on line connections. Due to FCC
limitations, speeds in the U.S. are less than 56Kbps.
voice/data: Many modems support a switch to change between voice and data
modes. In data mode, the modem acts like a regular modem. In voice mode, the
modem acts like a regular telephone. Modems that support a voice/data switch
have a built-in loudspeaker and microphone for voice communications.
auto-answer: An auto-answer modem enables your computer to receive calls
in your absence. This is only necessary if you are offering some type of
computer service that people can call in to use.
data compression: Some modems perform data compression, which enables
them to send data at faster rates. However, the modem at the receiving end must
be able to decompress the data using the same compression technique.
ASVD: Analog Simultaneous Voice and Data, or Analog SVD modems can
transmit both voice and data at the same line. In the near future, fax/modems
will come with Analog SVD capability. Applications of Analog SVD interactive
computer games and remote presentations.
What is a fax modem?
A fax modem is a device you can attach to a personal computer that enables
you to transmit and receive electronic documents as faxes. A fax modem is like a
regular modem except that it is designed to transmit documents to a fax machine
or another fax modem. Some, but not all, fax modems do double duty as regular
modems. As with regular modems, fax modems can either be internal or external.
Internal fax modems are often called fax modem boards.
Documents sent through a fax modem must already be in an electronic form
(that is, a computer file), and the documents you receive are likewise stored in
files on your disk. To create fax documents from images on paper, you need to
connect an optical scanner to the fax modem.
Fax modems come with communications software similar to communications
software for regular modems. This software can give the fax modem many
capabilities that are not available with stand-alone fax machines. For example,
you can broadcast a fax document to several sites at once.
What are the differences between the Digital Signal Processor
(DSP) based and the Application-specific Integrated Circuit (ASIC) based
fax/modem cards?
Which type is more popular in today’s market?
The table below lists the differences.
| DSP-based |
ASIC-based |
| Utilizes CPU for fax/modem
processes Performances will be affected by the programs that are running
in the system |
Utilizes chipset for fax/modem
processes No consumption of CPU MIPS |
| Can be easily upgraded to a
higher speed without adding or changing the chipsets. |
Cannot be upgraded without
adding or changing the chipsets. |
| Cannot handle multiple tasks. |
Can handle multiple tasks. |
ASIC-based fax/modem cards can manage more concurrent tasks than DSP-based
cards, and, therefore, are more popular in today’s market.
What do the "V.DOT" standards specify (V.32, V.42bis)?
The international standards for telecommunications, telex, and fax are
established by a United Nations committee called the International Consultative
for Telephony and Telegraphy. The name for this group is abbreviated CCITT.
CCITT developed standards for ensuring clean modem to modem communications at
high speeds. These standards are known as the "V.DOT" standards because the name
for each protocol developed under these standards consists of a "V" followed by
a dot and a numeric extension. V.DOT protocols provide error checking and data
compression for a variety of modem speeds.
The latest V.DOT protocol to become popular is the V.42bis standard, which
specifies error correction and data compression that yields transfer speeds of
up to 38,400bps.
What does "Group III" mean?
Over the years, the CCITT has set international standards for fax
transmission. This has led to the grouping of faxes into one of four groups.
Each group (I through IV) uses different technology and standards for
transmitting and receiving faxes. Groups II and I are relatively slow and
provide results that are unacceptable by today’s standards. Group II is the
standard in use today by virtually all fax machines, including those combined
with modems.
A Group III fax usually operates at about 9600 baud and is the backbone of
modern office fax technology. Group IV fax is a newer standard, which offers
greater compression but is at present in limited use. A good number of
plain-paper faxes use the newer Group IV standard.
What is the difference between baud versus Bits Per Second (bps)?
Baud rate and bit rates often are confused in discussions about modems. Baud
rate is the rate at which a signal between two devices changes in one second. If
a signal between two modems can change frequency or phase at a rate of 300 times
per second, for example, that device is said to communicate at 300 baud.
Sometimes a single modulation change is used to carry a single bit. In that
case, 300 baud also equals 300 bits per second (bps). If the modem could signal
two bit values for each signal change, the bit per second rate would be twice
the baud rate, or 600 bps at 300 baud. Most modems transmit several bits per
baud, so that the actual baud rate is much slower than the bit per second rate,
which is the true gauge of communications speed.
What does "throughput" mean?
Throughput is defined as the amount of user data transmitted per second
without the overhead of protocol information such as start and stop bits or
frame headers and trailers. Performance on this measurement is an indication of
a modem’s ability to process commands and transfer data, irrespective of the
effect communication protocols may have on its performance.
What does MNP stand for?
MNP, or Microcom Networking Protocol, is a communications protocol developed
by Microcom, Inc., which is used by many high-speed modems. MNP supports several
different classes of communication, each higher class providing additional
features. Modems can support one or more classes. Class 4 provides error
detection and automatically varies the transmission speed based on the quality
of the line. Class 5 provides data compression. Class 6 attempts to detect the
highest transmission speed of the modem at the other end of the connection and
transmit at that speed.
The most common levels of MNP support are Class 4 and Class 5, frequently
called MNP-4 and MNP-5. Using the data compression techniques provided by MNP-5,
devices can double normal transmission speeds.
Because MNP is built into the modem hardware, it affects all data
transmission. In contrast, software protocols, such as Xmodem and Kermit, affect
only file transfer operations.
How does the fax/modem work?
The telephone system is an analog system, designed before digital electronics
was widespread to transmit the diverse sound and tone of the human voice. The
modem is the bridge between the computer’s digital data and the phone system’s
analog signal. It converts on and off digital data into an analog signal by
varying, or modulating, the frequency of an electronics wave, a process similar
to that used by FM stations. On the receiving end of the phone connection, a
modem does the opposite: it demodulates the analog signals back to digital code.
These two terms, Modulate and DEModulate, are the basis for the word Modem.
What are the three elements of modem communications?
- Serial ports
- Modem Commands
- Communication Software
What is asynchronous communications?
Not synchronized; that is, not occurring at predetermined or regular
intervals. The term asynchronous is usually used to describe communications in
which data can be transmitted intermittently rather than in a steady stream; for
example, a telephone conversation is asynchronous because both parties can talk
whenever they like. If the communication were synchronous, each party would be
required to wait a specified interval before speaking.
The difficulty with asynchronous communications is that the receiver must
have a way to distinguish between valid data and noise. In computer
communications, this is accomplished through a special start bit and stop bit at
the beginning and end, respectively, of each piece of data. For this reason,
asynchronous communication is sometimes called start-stop transmission.
What is the start/stop bit?
In asynchronous communications, the bit that signals the receiver that data
is coming: a stop bit indicates that a byte has just been transmitted. Every
byte of data is preceded by a start bit and followed by a stop bit.
What is the parity bit?
The parity bit is a part of the parity checking routine used to detect errors
occurring in transmissions. Parity checking is the process of using parity bits
to check whether data has been transmitted accurately. Parity checking is the
most basic form of error detection in communications. The parity bit is added to
every seven bits that are transmitted. The parity bit for each byte (seven data
bits plus a parity bit) is set so that all bytes have either an odd number or an
even number of set bits.
As an example of parity checking, consider the situation where two devices
are communicating with even parity (the most common form of parity checking). As
the transmitting device sends data, it counts the number of set bits in each
group of seven bits. If the number of set bits is even, it sets the parity bit
to 0; if the number of set bits is odd, it sets the parity bit to 1. In this
way, every byte has an even number of set bits. On the receiving side, the
device checks each byte to make sure that it has an even number of set bits. If
it finds an odd number of set bits, the receiver knows there was an error during
transmission. Although parity checking is the most basic form of error detection
it cannot detect situations in which two or four consecutive bits are changed
due to electrical noise. There are many other more sophisticated protocols for
error checking and correction, such as MNP and CCIT V.42.
Explain Half/Full Duplex.
Half duplex refers to the transmission of data in just one direction at a
time. For example, a walkie-talkie is a half-duplex device because it only one
party can talk at a time. In contrast, a telephone is a full-duplex device
because both parties can talk simultaneously.
Modems contain a switch that lets you select between half-duplex and
full-duplex modes. The correct choice depends on which program you are using to
transmit data through the modem. In half-duplex mode, each character transmitted
is immediately displayed on your screen. (For this reason, it is sometimes
called local echo – characters are echoed by the local device.) In full-duplex
mode, the other device does not display transmitted data on your monitor until
it has been received and returned (remotely echoed). If you are running a
communications program and every character appears twice, it probably means that
your modem is in half-duplex mode when it should be in full-duplex mode, and
every character is being both locally and remotely.
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