part 19 from «The Winn Rosch HARDWARE BIBLE»
M O D E M S
-----------
[part 19 from «The Winn Rosch HARDWARE BIBLE»]
[Brady, New York, 1988 ]
© Copyright 1991 by Sergey Shulgin [SHS]. You can copy
this document for non-commercial use without changes only.
This document was printed by Sergey Shulgin from Moscow
Institute of Physics and Technology, Laser Center lab. If
you have some interest to this problem, please call me by
modem or voice.
Coord: Dolgoprudny, Moscow region, Moscow Institute
of Physics and Technology, Institutsky per, 9
Laser Center lab.
Phone: (095) 408-79-88 from 10:00 till 19:00 by voice
(095) 408-79-88 from 19:00 till 08:00 by modem
[connect to «Player's Dream» BBS and send message for me]
+-----------------------------------------------------------+
| Welcome to «Player's Dream» BBS station!!! |
| [1st BBS Station of United Group Games in Russia] |
| |
| Operating time: 19:00–08:00 Mon.-Sat. |
| 24h Sunday. |
| |
| phone: (095) 408-79-88 |
| |
| modem:up to 9600 with MNP protocol |
| |
| SysOp: Yaroslav Levashov, Sergey Shulgin |
+-----------------------------------------------------------+
INTRODUCTION
------------
For Some reason known to God and the infidels who figure
out the most profitable hardware packages to call computers,
the modem - perhaps the most desired and used computer
peripheral — remains an option in all but a random sampling
of portable computers. The modem is the one computer feature
that lets you display the personality of your personal
computer to the outside world. It puts you in touch with
on-line databases, remote computer systems, far-flung
friends, and even those who are still flinging around the
country.
The purpose and function of the modem seem almost absurd
in their simplicity. The modem merely connects your computer
to the telephone line. The need for the extra device seems so
absurd because both computer and telephone use obstensibly
the same stuff for making and moving messages — electrical
signals. Were not the giant corporations specializing in
computers and telephones (we won't name names) not such avowed
rivals, you might suspect that they were in cahoots to foist
such a contrived accessory on the computer marketplace.
Step back and look at what a modem does, however, and
you'll gain new respect for the device. In many ways, the
modern modem is a miracle worker. For instance, the best of
today's modems can squeeze more than a dozen data bits
through a cable where only one should fit. Even the least
expensive generic modem operates like a specialized time
machine that can bridge the century-wide chasm cutting
between state-of-the-art computers and stone-age telephone
technologies.
Far from the hatchlings of some plot by the
Military-industrial complex - or that even more sinister
force, the telephone company — modems are a necessary bridge
between digital and analog signals. The modern modem usually
does much more than connect. Most are boxes chocked full of
convenience features that can make using them fast, simple,
and automatic. The best of today's modems not only make and
monitor the connection but even improve it. They dial the
phone for you, even remembering the number you want and
trying again and again. They listen in until they's sure of
good contact, and only then let you transmit across the
telephone line. Some even have built in circuits to detect
and correct the inevitable errors that creep your electrical
conversations.
1. MODEM OPERATING PRINCIPLES
-----------------------------
A modem is a signal converter that mediates the
communications between a computer and the telephone network.
The very name «modem» indicates the role that it plays. The
term is a foreshortening of the words MOdulator/DEModulator.
As a modulator,the modem converts the digital, direct current
pulses used by the computer system into an analog signal
containing the same information, a process called
«modulation».
1.1 Modulating and Demodulating
-------------------------------
Modulation is necessary because the telephone system was
designed even before electronics was invented and solid-state
digital circuitry lay almost a hundred years off. The first
pained words out of Dr.Bell's speaking telegraph were analog
electrical signals, the same juice that flows through the
receiver of your own telephone. Although strictly speaking,
digital communications are older — the conventional telegraph
predates the telephone by nearly three decades (Samuel F.B.
Morse wondered what God had wrought in 1844) — current
digital technology is only a recent phenomenon.
The telephone system was designed only to handle analog
signals because that's all that speaking into a microphone
creates. Over the years, the telephone system has evolved
into an elaborate international network capable of handling
millions of these analog signals simultaneously and switching
them from one telephone set to another, anywhere in the world
and possibly beyond. Although telephone companies are
increasingly using digital signals to move trunk line
communications between switching centers, the input and
output ends of the circuit still end in conventional
analog-based telephones.
Modulation, and hence modems, are necessary because these
analog telephone connections will not allow digital, direct
current signals to pass freely — or at all. The modulation
process creates analog signals that code all the digital
information of the computer original but can be transmitted
through the voice-only channels of the telephone system.
DEMODULATION reverses the modulation process. At the
other end of the connection, the modem as a demodulator
receives that analog-coded signal and converts it back to its
original digital form while preserving its information
content.
1.2 The Carrier
---------------
The actual processor of modulation superimposes one signal on
another. The modem as modulator starts its modulation process
by generating a constant signal which is called the «carrier»
because it carries or bears the load of the modulating
information. In most systems, the carrier is a steady-state
signal of constant amplitude (strength) and frequency, and
coherent phase.
1.3 Modulation
--------------
The signal that's electrically mixed with the carrier to
modify some aspect of it is given the same name as the
process, «modulation». Changes in the modulation result in a
change in the carrier-and-modulation mix. The change in the
modulation makes a corresponding change in the carrier but
not necessarily a change in the same aspect of the carrier.
For instance, in FM or frequency modulation, a change in the
«strength» of the modulation is reflected as a change in the
«frequency» of the carrier.
Modulation brings several benefits, more than enough to
justify the complication of combining signals. Because
electronic circuits can be tuned to accept the frequency of
one carrier wave and reject others, multiple modulated
signals can be sent through a single communications medium.
This principle underlies all radio communication and
broadcasting. In addition, modulation allows digital,
direct-current-based information to be transmitted through a
medium, like the telephone system, that otherwise could not
carry direct current signals.
In demodulation, the carrier is stripped away and the
encoded information is returned to its original form.
Although logically just the complement of modulation,
demodulation usually involves entirely different circuits and
operating principles, which adds to the complexity of the
modem.
1.4 Short-Haul Modems
---------------------
Some so-called modems aren't even modems at all. The
inexpensive «short-haul modems» advertised involve minimal
circuitry, definitely not enough to modulate and demodulate
signals. So little, in fact, that often it is entirely hidden
inside the shell of a simple cable connector. All that the
short-haul modem does is convert the digital output of a
computer to another digital form that an better withstand the
rigors of a thousand feet of wire.
2. CHANNEL LIMITS
-----------------
Like all great works of art, the modem is constrainted to
work within the limits of its medium, the telephone channel.
These limits are imposed by the characteristics of analog
communications and the communications medium that's used
(primarily unshielded twisted-pair wire).
2.1 Signal Bandwidth
--------------------
All communications channels and the signals that travel
through them have a characteristic called «bandwidth».
Bandwidth merely specifies a range of frequencies from the
lowest to the highest that the channel can carry or are
present in the signal.
An unmodulated carrier wave has a nominal operating
frequency. For example, in radio broadcasting, it's the
number you dial in when you tune in your favorite station.
Without modulation, the carrier wave uses only that one
frequency and has essentially zero bandwidth.
The modulation that's added to the carrier contains
information that varies at some rate. Traditional analog
signal sources — music or voice signals, for instance -
contain a near-random mix of frequencies between 20 and
20,000 Hz. Although digital signals start off as DC, which
also has no bandwidth, every change in digital state adds a
frequency component. The faster the states change — the more
information that's squeezed down the digital channel, as
measured in its bit rate (bits per second) - the more
bandwidth the signal occupies.
2.2 Sidebands
-------------
In the simplest modulation systems, a modulated carrier
requires twice the bandwidth of the modulation signal.
Although this doubling sounds anomalous, its the direct
result of the combining of the signals. The carrier and
modulation mix together and result in «modulation products»
corresponding to the frequency of modulation both «added to»
the carrier together with the frequency of the modulation and
«subtracted from» the carrier. The added result is often
called the «upper sideband» and the subtracted result i
correspondingly called the «lower sideband».
2.3 Channel Bandwidth
---------------------
The bandwidth of a communications channel defines the
frequency limits of the signals that it can carry. This
channel bandwidth may be physically limited by the medium
used by the channel or artificially limited by communication
standards. For example, the bandwidth of radio transmissions
are limited artificially, by law, to allow more different
modulated carriers to share the air waves while preventing
interference between them.
In wire-based communications channels, bandwidth is
often limited by the wires themselves. Certain physical
characteristics of wires cause degradations in their high
frequency transmission abilities. The «capacitance» between
conductors in a cable pair, for instance, increasingly
degrades signals as their frequencies rise to the point that
a high frequency signal might not be able to traverse more
than a few centimeters of wire. «Amplifiers» or «repeaters»,
which boost signals so that they can travel longer distances,
often cannot handle very low or very high frequencies,
imposing more limits.
Most telephone channels also have an artificial bandwidth
limitation imposed by the telephone company. To get the
greatest financial potential from the capacity of their
transmissions cables, microwave systems, and satellites,
telephone carriers regularly limit the bandwidth of telephone
signals. One reason bandwidth is limited is so that many
separate telephone conversations can be stacked atop of one
another through multiplexing techniques, allowing a single
pair of wires to carry hundreds of simultaneous
conversations. Although the effects of bandwidth limitation
are obvious — that's why your phone doesn't sound as good as
your stereo — the telephone company multiplexing equipment
works so well that you are generally unaware of all the
manipulations made on the voice signals as they are squeezed
through wires.
2.4 Bandwidth Limitations
-------------------------
One of the consequences of telephone company signal
manipulations is a severe limitation in the bandwidth of an
ordinary telephone channel. Instead of the full frequency
range of a good-quality stereo system, from 20 to 20,000 Hz,
a telephone channel will only allow frequencies between 300
and 3000 Hz to freely pass. This very narrow bandwidth works
well for telephone because frequencies below 300 Hz contain
most of the power of the human voice but little of its
intelligibility. Frequencies above 3000 Hz increase the
crispness of the sound but don't add appreciably to
intelligibility.
While intelligibility is the primary concern with voice
communications (most of the time), data transfer is
principally oriented to bandwidth. The comparatively narrow
bandwidth of the standard telephone channel limits the
bandwidth of the modulated signal it can carry, which in turn
limits the amount of digital information that can be squeezed
down the phone line by a modem.
Try some simple math and you'll see the harsh constraints
faced by your modem's signals. A telephone channel typically
has a useful bandwidth of about 2700 Hz (from 300 to 3000 Hz)
At most a carrier wave at exactly the center of the telephone
channel, 1650 Hz, and burdened by both sidebands, could carry
data that varies at a rate of 1650 Hz. Such a signal would
fill the entire bandwidth of the telephone channel without
allowing for a safely margin.
2.5 Safely Margin
-----------------
A safely margin is necessary, however, because the quality of
telephone lines varies greatly, particularly when long
distance connections are involved. Because poor connections
can't handle the nominal 300 to 3000 Hz telephone bandwidth,
it's ill-advised for a modem to try to take advantage of that
entire frequency spread. If the connection is substandard,
when the data rate reaches the fringes of the bandwidth,
errors are likely to crop in.
2.6 Duplex
----------
The usable bandwidth of a data communications channel through
a modem is also limited because most modem communications are
handled in «duplex» mode. The term «duplex» - often
redundantly called «full-duplex» — describes the ability of a
communications channel to simultaneously handle two signal,
usually (but not necessarily) going in opposite directions.
Using these two channels, a full duplex modem can send and
receive information at the same time. Two carriers are used
to simultaneously transmit and receive data. Using two
carriers, of course, halves the bandwidth available to each.
2.7 Half-Duplex
---------------
The alternative to duplex communications is «half-duplex». In
half-duplex only one signal is used, and to carry on a
two-way conversation a modem must alternately send and
receive signals. It allows more of the channel bandwidth to
be put to use but in practice slows data communications
because a modem often must switch between sending and
receiving modes after every block of data crawls through the
channel.
2.8 Echoplex
------------
The term «duplex» is often, and mistakenly, used to describe
«echoplex» operation. In echoplex mode, a modem sends a
character down the phone line, and the distant modem returns
the same character thereby echoing it. The echoed character
is then displayed on the originating terminal as confirmation
the character was sent correctly. Without echoplex, the host
computer usually writes the transmitted character directly to
its monitor screen.
2.9 Guard Bands
---------------
Duplex does more than cut the bandwidth available to each
channel in half. Separating the two channels is a «guard
band», a width of unused frequencies that isolate the active
channels and prevent confusion between their separate
carriers. The safety margin is, in effect, also a guard
between the carriers and the varying limit of the bandwidth.
Once you add in the needs of duplex communication and the
guard bands, the practical bandwidth limit for modem
communications over real telephone channels that have an
innate 2700 Hz bandwidth works out to about 2400 Hz. That
leaves 1200 Hz for each of the two duplex channels.
3. MODEM MODULATION METHODS
---------------------------
For the job of making modulation, a modem has several methods
available to it much as AM and FM radio stations use
different modulation methods. The different forms of
modulation all are based on the characteristics of the
carrier wave that can be changed to encode information.
Three of the primary carrier characteristics that might
be used for modulation are its amplitude, its frequency, and
its phase.
3.1 Amplitude Modulation
------------------------
The amplitude is the strength of the signal or the loudness
of a tone carried through the telephone wire. Varying the
strength of the carrier in response to modulation to transmit
information is called «amplitude modulation».
One way that digital information could be coded with
amplitude modulation is as two discrete strengths of the
signal corresponding to the two digital states. In fact, the
most rudimentary form of amplitude modulation — which has
earned the special name «Carrier Wave» or «CW» transmission
uses the two limits of carrier strength for its code: full
power and zero power. The loudness of a telephone signal is
its most likely characteristics to vary, however, with both
changes in the telephone line and noise that might be picked
up by the line. Consequently, pure amplitude modulation is
not used by modems.
3.2 Phase Modulation
--------------------
Another state of the carrier that can be altered to encode
information is its phase. An unmodulated carrier is a
constant train of identical waves that follow one after
another precisely in step. If one wave were delayed for
exactly one wavelength, it would fit exactly atop the next
one. The peaks and troughs of the train of waves flow by at
constant intervals.
By delaying one of the waves without altering its
amplitude or frequency, a detectable state change called a
«phase shift» is created. The onset of one wave is shifted in
time compared to those that preceded it. Information can be
coded as «phase modulation» by assigning one amount of phase
shift from the constant carrier to a digital one and another
to a digital zero. Although this form of modulation is useful
in modem communications, it is most often used in combination
with other modulation techniques.
3.3 Frequency Modulation
------------------------
The other alternative modulation technique alters the
frequency of the carrier is response to the modulation. For
example, a higher amplitude of modulation might be made to
cause the carrier to shift upward in frequency. This
technique, called «frequency modulation», is commonly used in
radio broadcasting by familiar FM stations.
3.4 Frequency Shift Keying
--------------------------
In the most rudimentary digital form of frequency modulation,
a digital one would cause the carrier wave to change from one
frequency to another. In other words, one frequency would
signify a digital one and another discrete frequency a
digital zero. This form of modulation is called «frequency
shit keying» or FSK because information is encoded in (think
of it being «keyed to») the shifting of frequency. The keying
part of the name is actually left over from the days of
telegraphy when this form of modulation was used for
transmitting Morse code and the frequency shift came with the
banging of the telegraph key. Frequency shift keying is used
in the most rudimentary of popular modems, the
once-ubiquitous 300 bit per second modem that operated under
the Bell 103 standard.
3.5 Baud Rates
--------------
With such modems one bit of data causes one corresponding
change of frequency in the carrier wave. Every change of
frequency or state carries exactly one bit of information.
The unit of measurement used to describe the number of state
changes taking in place in one second is the «baud». In the
particular case of the FSK modulation, one change of state
per second - one baud - conveys exactly one bit of
information per second.
Depending on the number of states used in the
communication system, a single transition — one baud - can
convey less than or more than one bit of information. For
example, several different frequencies of tones might be used
to code information. The changing from one frequency to
another would take place at one baud, yet because of the
different possible changes that could be made, more than one
bit of information could be coded by that transition. Hence,
strictly speaking, one baud is not the same as one bit per
second, although the terms are often, and incorrectly, used
interchangeably.
The number of bits that can be coded by baud varies by
the inverse logarithm of the number of available states
)tones, voltage or phases). Most 1200 bit per second modems
operate at 600 baud with four different states available, and
most 2400 bit per second modems operate at 600 baud with 16
different states.
(In case you're interested, the term «baud» was named
after J.M.E. Baudot, a French telegraphy expert. His name is
also used to describe a 5-bit digital code used in teletype
systems).
3.6 FSK Modems
--------------
This 300 bit per second rate using the simple FSK technique
requires a bandwidth of 600 Hz. The two 300 baud carriers
(which require a 1200 Hz bandwidth, two times 600 Hz) and a
wide guard band fit comfortably within the 2700 Hz limit.
Under the Bell 103 standard, which is used by most 300
bit second modems, the two carrier frequencies are 1200 and
2200 Hz. Space modulation (logical zero) shifts the carrier
down by 150 Hz, and mark modulation pushes the carrier
frequency up by and equal amount.
Because the FSK modulation technique is relatively
simple, 300 baud modems are generally inexpensive. Because
they don't push out to the limits of available bandwidth,
they are generally reliable even with marginal connections.
Using the same simple modulation technique and exploiting
more of the 2700 Hz bandwidth of the typical telephone line,
modem speeds can be doubled to 600 baud. Beyond that rate,
however, lies the immovable bandwidth roadblock.
4. MODEMS FASTER THAN 300 Bits Per Second
-----------------------------------------
A data communications rate of 300 bits per second is slow -
slower than most folks can read text flowing across the
screen. Even the slowest PC can absorb information at least
32 times faster, limited by the maximum serial port speed
that IBM supports. Were long distance communications limited
to the 300 bit per second rate, the only people who would be
happy would be the shareholders of the various telephone
companies. Information could, at best, crawl slowly across
the continent.
By combining several modulation techniques, modern modems
can achieve much higher data rates through ordinary dial-up
telephone lines. Instead of merely manipulating the carrier
one way. they may modify two (or more) aspects of the
constant wave. For instance, today's most popular 1200 and
2400 bit per second modems combine frequency and phase
modulation to achieve faster data flow.
4.1 Quadrature Modulation
-------------------------
These more complex forms of modulation add no extra bandwidth
(remember, that's a function of the communications channel)
but they take advantage of the possibility of coding digital
data as changes between a variety of states of the carrier
wave. For example, the carrier wave can be phase modulated so
that it assumes one of four states.
In the «quadrature modulation» (a form of phase
modulation) used by most 1200 bit per second modems, each
state of the carrier differs from the unmodulated carrier
wave by a phase angle of 0,90,180, or 270 degrees - while
operating at 600 baud.
4.2 Group Coding
----------------
The four different phase states are sufficient to encode the
four different patterns of two digital bits. Each baud can
hold two bits of data, thus, a quadrature-modulated 600-baud
modem can communicate at its date rate of 1200 bits per
second. This bit-packing is the key to advanced modulation
techniques. Instead of dealing with data one bit at a time,
bits of digital code are processed as groups. Each group of
data bits is encoded as one particular state of the carrier.
The ultimate speed of the mode is determined by the
number of states that are available for coding. The
relationship is not linear, however. As the number of bits in
the code increases by a given figure (and thus the potential
speed of the modulation technique rises by the same figure),
the number of states required increases to the corresponding
power of two. Twice as fast requires four states; four times
faster requires 16 states; eight times as fast requires 256
states; and so on. Data rates of 2400 bps can be achieved by
using an even more complex modulation that yields 16 discrete
states while still operating at 600 baud. Each state encodes
one of the 16 different patterns of four digital bits. One
blip on the telephone line carries the information of four
bits going into the modem.
More complex methods of modulation allow even higher
modem speeds-dial-up modems operating at 4800 bps and beyond
are already available. Most higher speed modems - for
example, today's 9600 bps products — get an extra boost by
foregoing duplex transmission and alternate between sending
and receiving.
5. HIGH SPEED MODEMS
--------------------
Modems that operate at data rates in excess of 2400 bits per
second are generally classed as «high speed modems». The
distinction is as qualitative as it is quantitative: Above
2400 bps, squeezing more information into the confines of the
telephone line becomes increasingly difficult, requiring
inventive modulation techniques quite unlike those used at
lower rates.
According to the free lunch principle, this system of
seemingly getting something for nothing using complex
modulation must have a drawback. With high speed modems the
problem is that the quality of the telephone line becomes
increasingly critical as the data rate is increased.
Moreover, as modem speeds get faster, each phone line blip
carries more information, and a single error soon can have
devastating effects.
5.1 Leased-Line Modems
----------------------
One way to coax higher speed from a modem is to forego the
one part of the connection that imposes the severe bandwidth
limitation — the telephone line. Special high-grade circuits
can be rented from telephone companies to whisk data from
point to point at almost unbelievably high data rates (from
ten thousand to millions of bits per second). The special
lines are semi-permanently installed and stretch directly
from one location to another, never allowed to venture
through the rigors of the telephone switching system. Because
these special lines are leased by the month (or other
period), they are called «leased lines» and the modems that
use them are termed «leased-line» or «dedicated-line» modems.
They usually lack the dialing and answering features of
dial-up modems, and are meant for continuous connections.
5.2 Dial-Up Modems
------------------
In contrast, the modems that you are likely most familiar
with — the ones that tie into the telephone switching system
— are distinguished as «dial-up» modems. They face the
constraints of the telephone system and must be capable of
dealing with its special problems and shortcomings.However,
they are the most useful because they can reach nearly
anyone, anywhere - as long as the modems at the two ends of
the call are compatible with one another.
5.3 Line Compensation
---------------------
Although a long distance telephone connection may sound
unchanging to your ear, its electrical characteristics vary
by the moment. Everything from a wire swaying in the Wichita
wind to the phone company's automatic rerouting of the call
through Bangkok when the direct circuits fill up can change
the amplitude, frequency, and phase response of the circuit.
The modem then faces two challenges — not to interpret such
changes as data and to maintain the quality of the line to a
high enough standard to support its use for high speed
transmission.
5.4 Switching Modems
--------------------
Perhaps the biggest limit imposed on high speed modem
communications is the use of full duplex communications.
Because a complete duplex modem circuit is essentially two
complete channels, each can have (at most) only half the
telephone line's bandwidth available to it. Most of the time,
however, communications go only in one direction. You key in
commands to a remote access system, and only after the
commands are received does the remote system respond with the
information that you seek. While one end is sending, the
other end is more than likely to be completely idle.
To make better use of the available bandwidth so-called
«switching modems» are designed to make use of the full
bandwidth of the telephone channel, switching the direction
of the signal as each end of the line needs to send data.
Such modems are able to achieve a doubling of data rate
without adding any complexity to their modulation. In remote
mainframe access situations, where the protocol of the call
fits the mold of the two ends of the connections taking turns
using the phone line, switching modems can give a genuine
boost to the cross-country throughput of a modem system.
5.5 Asymmetrical Modems
-----------------------
Switching doesn't always work, however. The change of
direction of communication isn't instantaneous. The modem has
no way of knowing when to switch other than listening for a
pause in the data stream. Some delay to recognize such a
pause must be built into the system. Further, when the
direction of the call changes, the modem may be called to
adjust for line differences (the characteristics of a
telephone connection are not necessarily the same in both
directions because the two directions of communication may
take entirely different paths). In all, switching the
direction of the data movement can take a full second.
While a second pause may not be burdensome when you're
simply sending characters and watching a response on the
screen, it can be overwhelming when transferring a file. Most
file transfer protocols (for example, XMODEM and Kermit) are
designed to send a small block of data to the remote system,
which then checks it for accuracy and finally sends a brief
return message that the data was received intact or that it
was bad. A switching modem may require a full second or more
for each turn-around and confirmation. In that some protocols
use blocks only 256 or 512 bytes long between confirmations,
sending a file amounts to the classic hurry-up-and-wait
syndrome. The modems blast a block across the line, then
lolly around for a much longer period awaiting a confirmation.
In an attempt to get the best of both worlds,
«asymmetrical modems» cut the waiting by maintaining a
semblance of two-way duplex communications while optimizing
speed «in one direction only» by shoehorning in a low speed
(typically 300 bps) channel in addition to the high speed
one. As with a switching modem, asymmetrical modems can
flip-flop the direction of the high speed communications.
They rely on algorithms to determine which way is the best
way. Typically, the high speed channel is used for
transferring blocks of data while the confirmations trickle
back on the lower speed channel.
5.6 Fallback
------------
Most modems use, at most, two carriers for duplex
communications. These carriers are usually modulated to fill
the available bandwidth. Sometimes, however, the quality of
the telephone line is not sufficient to allow reliable
communications over the full bandwidth expected by the modem.
In such case, most high speed modems incorporate «fallback»
abilities. When the top speed does not work, they attempt to
communicate at lower speeds that are less critical of
telephone line quality. The pair of modems might first try
9600 bps and be unsuccessful; they might then try 4800, then
2400, and so on until reliable communications are established.
5.7 Multiple-Carrier Modems
---------------------------
While most modems rely on a relatively complex form of
modulation on one or two carriers to achieve high speed, a
few (notably the Telebit Trailblazer) use instead relatively
simple modulation on multiple carriers. One of the chief
advantages of this system used by these «multiple-carrier
modems» comes into play when the quality of the telephone
connection deteriorates. Instead of dropping down to the next
incremental communications rate, generally cutting data speed
in half, the multiple-carrier modems just stop using the
carriers in the doubtful regions of the bandwidth. The
communication rate may fall off only a few percent in the
adjustment. (Of course, it could dip by as much as a normal
fallback modem as well).
5.8 Data Compression
--------------------
Although there's no way of increasing the number of bits
that can cross a telephone line beyond the capacity of the
channel, the information handling ability of the modem
circuit can be increased by making each bit more meaningful.
Many of the bits that are sent through the telecommunication
channel are meaningless or redundant - they convey no
additional information. By eliminating those worthless bits,
the information content of the data stream is more intense,
and each bit is more meaningful. The process of paring the
bits is called «data compression».
The effectiveness of compression varies with the tye of
data that's being transmitted.One of the most prevalent data
compression schemes encodes repetitive data — eight
recurrences of the same byte value might be coded as two
bytes, one signifying the value, and the second the number
of repetitions.This form of compression is most effective on
graphics, which often have many blocks of repeating text.
Other compression methods may strip out start, stop, and
parity bits. Modem manufacturers often claim that their
proprietary data compression methods might reduce the number
of bits that need to be transferred by 50 percent,
effectively doubling communications speed.
5.9 Error-Checking
------------------
Because all higher speed modems operate closer to the limits
of the telephone channel, they are naturally more prone to
data errors. To better cope with such problems, nearly all
high speed modems have their own built-in «error-checking»
methods. These work like communications protocol — grouping
bytes into blocks and sending cyclical redundancy checking
information. They differ from the protocols used by
communications software in that they are implemented in the
hardware instead of your computer's software. That means
that they don't load down your computer when it's straining
at the limits of its serial ports.
It can also mean that software protocols are redundant
and a waste of time. As mentioned before in the case of
switching modems, using a software-based communications
protocol can be counterproductive with many high-speed
modems, slowing the transfer rate to a crawl. Most makers of
modems using built-in error-checking will advise against
using such software protocols.
6. MODEM CONTROL
----------------
Besides its basic purpose of converting digital data into
modulated audio signals, the modem is often called upon to
handle other chores of convenience. For example, it may be
called upon to automatically dial or answer the phone or
report the condition of the telephone line. These features
of the modem must be able to be controlled by your computer,
and the modem must be able to signal your computer about
what it does and what it finds out.
6.1 Dual Modes
--------------
Most modems operate alternately in one of two modes. In
«command mode», the modem receives and carries out
instructions sent by your computer. In «communications
mode», it operates as transparently as a modem can, merely
converting data.
Changing modes is mostly a matter of sending control
characters to the modem. The characters can only be received
and processed in command mode. In communication mode, they
would be passed along down the telephone line.
6.2 Hayes Command Set
---------------------
Today, most modems use a standardized set of instructions
called the «Hayes command set», after Hayes the modem
manufacturer (which was, in turn, named after Dennis Hayes,
its founder). For the most part, the Hayes command set
comprises several dozen modem instructions that begin with a
two character sequence called «attention character». The
sequence is almost mnemonic — the letters AT, which must be
capitals. Other characters specifying the command follow
the attention character. Because the AT is part of nearly
every command, the Hayes command set is also termed the «AT
command set», most often by Hayes' competitors that don't
want to give the competition credit. A modem that
understands the Hayes command set (or the AT command set) is
said to be «Hayes-compatible». (The basic Hayes command set
is listed in Appendix A of this document).
Most AT commands follow the attention characters with
one letter that specifies the family of the command and
another character that indicates the nature of the command.
For example, H stands for Hook. H0 means put the phone «on
the hook» or hang up. H1 indicates that the modem should
take the phone off the hook, that is, make a connection to
the line.
Several commands and their modifiers can be combined on
a single line after an initial attention command. For
example, to command a Hayes or Hayes-compatible modem to
dial information on a tone-dialing line, the proper sequence
of commands would read: ATDT15511212. The AT is the
attention signal, D is the Dial command, the T tells the
modem to use tones for dialing, and the 15511212 is the
number of the telephone company information service.
All AT commands must be followed by a carriage return.
The modem waits for the carriage return as a signal that the
computer has sent the complete command and that the modem
should start processing it.
At first it would appear that shifting into
communications mode would be a one-way street for the modem,
particularly were it only able to receive instruction in
command mode. Fortunately, the Hayes command set allows the
modem to react to exactly one command in communications
mode, a command that instructs the modem to break off
communications and shift back to command mode.
The tricky part of designing such a command it that it
must be a character sequence that will never appear in the
data that the modem is supposed to be communicating across
the telephone line. Although it's impossible to guarantee
that any command sequence will never appear in the normal
progress of communications, the command in the Hayes set is
specifically designed to be statistically unlikely. This
command simply consists of a string of three «plus signs» -
that is, +++. To make the command stand out from data, the
Hayes command set also specifies that the three plus signs
be isolated from any other characters by at least one
second, before and after. Such a pause followed by three
specific characters, followed by a pause should never
occur (well, almost) except when the command is really
meant.
6.3 Extended Hayes Command Set
------------------------------
At the time the Hayes command set was developed, modems had
relatively few special features. As modems became more
sophisticated, they became more loaded with abilities and
features. The original Hayes command set had to be extended
to handle all the possibilities. Note that many
Hayes-compatible modems recognize only the original command
set. All of their features — if they have them — may not
work with software that expects the extended Hayes set.
6.4 S-Registers
---------------
The extensions to the original Hayes command set include
sufficient new functions that the command language would
become ungainly and confusing. After all, there are only 26
letters in the alphabet that might be used for one-letter
commands. Hayes added the facility of a special register or
memory area called the «S-register» inside its modems that
allows the settings of the modem's operating parameters. By
setting the value contained by the S-register, a variety of
modem functions can be controlled. (S-register settings are
shown in Appendix B of this document)
6.5 Response Codes
------------------
Commands sent to a Hayes-compatible modem by their very name
and nature are one-way. Absent some means of confirmation,
you would never know whether the modem actually received
your command, let alone acted upon it. Moreover, you also
need some means for the modem to tell you what it discovers
about your connection to the telephone line. For example,
the modem needs to signal you when it detects another modem
at the end of the line — and when that connections is broken.
Part of the Hayes command set is a series of «response
codes» which serve that feedback function. When the modem
needs to tell you something, it sends back — via the same
connection used to send data between your computer and modem
— code numbers or words to appraise you of the situation. In
the Hayes scheme of things, you can set the modem to send
simple «numeric» codes, consisting solely of codes (which you
can then look up in your modem manual, if you have one) or
«verbose» responses, which may be one or more words long in
something close to everyday English.
Typical responses include «OK» to signify that a command
has been received and acted upon, «CONNECT 1200» to indicate
that you've linked with a 1200 bit per second modem, and
«RING» to show that the phone at the other end of the
connection is ringing. (Hayes response codes are listed in
Table below).
Note that because the response codes flow from your
modem to your computer as part of the regular data stream,
you may accidentally confuse them with text being received
from the far end of your connection.
+---------+----------------+-------------------------------+
| Numeric | Verbose | |
| code | vcode | Definition |
+---------+----------------+-------------------------------+
| 0 | OK | Command executed without error|
| 1 | CONNECT | Connection established (300bps)|
| 2 | RING | Phone is ringing |
| 3 | NO CARRIER | Carrier lost or never detected|
| 4 | ERROR | Error in command line or line |
| | | too long |
| 5 | CONNECT 1200 | Connection established at 1200|
| 6 | NO DIALTONE | Dialtone not detected in |
| | | waiting period |
| 7 | BUSY | Modem detected a busy signal |
| 8 | NO ANSWER | No silenced detected while |
| | | waiting for a quiet answer |
| 10 | CONNECT 2400 | Connection established at 2400|
+---------+----------------+-------------------------------+
7. MODEM FEATURES
-----------------
The broad term «features» describes various subtle — and
some not-sosubtle — ways in which modems differ from one
another. For the most part, the features of a modem taken
together determine how easily and conveniently you can put
it to work. A no-frills modem, for example, may require that
you spin the dial of your phone with your index finger or
answer incoming calls before turning them over to your
computer system when you hear the carrier tone of the modem
at the other end of the line. Many people are willing to put
up with such petty inconveniences to save on the price of a
modem.
Although none of the tasks that features-deficient
modems foist upon you will tax your mind or constitution, a
no-frills modem short-changes the capabilities of your
computer. With a full-featured modem, your PC can dial the
phone faster and with fewer errors and can handle the chore
automatically when you're not around. Or with the latest
memory-resident communication software, your PC can dial
the full-featured modem and collect your messages while you
are in the midst of browbeating data into shape with another
program.
Actually, nearly every modem made today - including the
lowest budget models made in obscure foreign lands — has all
the standard features that you might normally want. Once you
start integrating features into circuit chips, adding a few
more features is not an arduous process. The only time
you're likely to run into a modem deficient in today's
convenience features is when you try to make do with one
manufactured to yesterday's standards - the modem you
inherit from some corporate higher-up, one that you find
lying face-down in the gutter and you nurse back to life,
one that you buy used from a shady-looking character in a
trench coat on a deserted street corner.
Of the various features of a state-of-the-art modem, the
ones you should expect in any new product that you buy
include:
7.1 Auto-Answer
---------------
An auto-answer modem is capable of detecting incoming
ringing voltage (the low-frequency, high-voltage signal that
makes the bell on a telephone ring) and seizing the
telephone line as if it had been answered by a person. Upon
seizing the phone line, the auto-answer modem sends a signal
to its host computer to the effect that it has answered the
phone. The computer then can interact with the caller.
An auto-answer modem allows you or other to call into
your computer system without anyone being present to answer
the telephone and make the connection to your computer.
7.2 Auto-Dial
-------------
An auto-dial modem is capable of generating pulse-dial or
DTMF (dual-tone modulated frequency or touch-tone) dialing
signals independent of a telephone set.
An auto-dial modem can dial the telephone under computer
command, for example, after hours when you're asleep and
phone rates are low. Without auto-dial, you would have to
dial the phone yourself, listen for the screech of the
far-end modem's answer, plug in your modem, and finally hang
up the phone.
7.3 Automatic Speed Sensing
---------------------------
Before a connection is made, you may have no way of knowing
at what speed a distant modem will be operating. Most of
today's modems can automatically adjust for the speed of the
distant modem — if it is within the speed range that can be
handled. High speed modems usually negotiate the highest
possible shared speed to operate at using proprietary
protocols.
Many modems also attempt to adjust to the speed at which
you send them data — again if is within the range of speeds
that the modem can handle. The attention code of the Hayes
command set conveys enough data that a modem can lock into
the data and appropriately match its operating speed to that
of the information flow.
7.4 CCITT Compatibility
-----------------------
A branch of the United Nations, the CCITT, which roughly
translated from the original French means «Cooperative
Committee for International Telephony and Telegraphy» has
created a number of communications standards that have won
great worldwide acceptance. Many of these standards apply to
modems. For example, many modems boast of «CCITT
compatibility» with the v.22 standard, which describes
operation at a data rate of 2400 bits per second. At higher
speeds other CCITT standards are gaining popularity, such
as v.32 for 9600 bit per second modems.
In theory, the adoption of the CCITT standard is good
news for people who want to communicate overseas (where the
Bell standards may be illegal). The principal value of the
CCITT standards is, however, that many manufacturers are
taking them to heart and designing products to match,
increasing compatibility and eliminating marketplace
confusion.
7.5 Acoustic Couplers
---------------------
Really vintage modems made no electrical contact with
telephone lines at all. That's because years ago hooking
your modem directly to the phone line was neither practical
or legal. It wasn't practical before the now-common modular
telephone plug-and-jack arrangement allowed anyone to plug
in telephone equipment without fear of embarrassment or
electrocution. It wasn't legal because telephone company
regulations dating long before the AT&T telephone monopoly
was split up did not permit individuals to directly connect
modems to their telephone lines.
Instead of electrical connections, vintage modems sent
their signals to telephones as sound waves. A device called
an «acoustic coupler» was used to convert the tone-like
analog signals made by the modem into sounds which are then
picked up the microphone in the telephone handset and
passed through the telephone network again as electrical
signals. To make the sound connection a two-way street, the
acoustic coupler also incorporated a microphone to pick up
the squawks emanating from the earpiece loudspeaker of the
telephone handset, convert them into electrical signals, and
supply them to the modem for demodulation.
Acoustic couplers can take many forms. In early
equipment, the acoustic coupler was integral to the modem -
a special cradle in which you lay the telephone handset.
Today you're more likely to see couplers made from two
rubber cups designed to engulf the mouthpiece and earpiece
of a telephone handset. This latter form of acoustic coupler
persists because it allows modems to be readily connected
and disconnected from non-modular telephones — those that
you cannot unplug to directly attach a modem. This
connectability is especially important for roving computers
that may be called upon to tie their internal modems into
nonmodular pay station and hotel room telephones.
7.6 Direct-Connect Modems
-------------------------
Modems that directly plug into the electrical wires of the
telephone system are called, quite logically,
«direct-connect» modems. Almost in tribute to the acceptance
of the modular telephone wiring system, nearly every modem
that you can buy today is direct-connect.
7.7 Asynchronous Modems
-----------------------
Almost any modem that you buy for normal use with your PC
will feature «asynchronous transmission». This odd-sounding
term describes a method of exchanging information between
two different computer systems that operate completely
independently and do not share any timing information.
Normally, the time at which a pulse occurs in relation
to the ticking of a computer's system clock determines the
meaning of a bit in a digital signal, and the pulses must be
synchronized to the clock for proper operation. In
asynchronous transmissions, however, the digital pulses are
not locked to the system clock of either computer. Instead,
the meaning of each bit of a digital word is defined by its
position in reference to the clearly (and unambiguously)
defined start bit. Because the timing is set within each
word in isolation, each word of asynchronous signal is self-
contained and essentially independent of any time relations
beyond its self-defined bounds.
The signals of modems that use the telephone system are
generally asynchronous because it is more expensive and
difficult to synchronize signals through the telephone
system through which signals may be rerouted at any time
without any warning.
7.8 Synchronous Modems
----------------------
Most dedicated-line modems use a special communication
technique often used among mainframes called «synchronous
transmission». In this method of transmitting data across
phone lines, the two ends of the channel share a common time
base and the communicating modems operate continuously at
substantially the same frequency and are continually
maintained in the connection and adjust for the circuit
conditions. Higher speed modem — 2400 bits per second and
beyond — often use synchronous transmissions.
In synchronous transmissions the timing of each bit
independently is vital, but framing bits (start and stop
bits) are unnecessary, which makes this form of
communication a bit — actually two or three bits - faster.
One problem in using it is that before information can be
exchanged, not just the two ends of the connection must be
synchronized, but also the link between the modem and
computer must be synchronized. Autodialing features usually
won't work in synchronous mode because without a connection
being made there's nothing to synchronize to - and the
connection cannot be made without dialing.
7.9 Autosynchronous Modems
--------------------------
Hayes solved the dialing problem for synchronous
communications by adding an «autosynchronous» feature to
their newest higher speed modems. This special mode allows
the connection between PC and modem to operate
asynchronously. The modem translates those signals into
synchronous mode before sending them down the telephone
line. It also works the other way and translates synchronous
signals from the far end of the line into asynchronous for
sharing with the host computer. The autosynchronous features
can help PCs talk to mainframe and other computers that use
synchronous communication as easily as they communicate with
other PCs.
7.10 Modem Packaging
--------------------
Perhaps the biggest choice you have in buying a new modem is
whether in installs inside your PC as an «internal modem» or
connects outside your PC through a cable as an «external
modem». Internal modems are like any other expansion cards
that plug into a vacant slot inside your PC. External modems
are additional boxes to find a place for on your desk.
In many cases When you have to choose between actual
products, physical appeal may be the best guiding factor
because exactly the same circuitry is often available in the
different packages.
There are a few practical reasons for preferring one
style of modem packaging over another. External modems offer
the advantage of portability. You can move your external
modem between different computer systems (even those that
are not IBM compatible) simply by pulling the plug. Moving
an internal modem requires popping the lid off your PC and
the recipient and all the folderol that follows.
Additionally, internal modems can restrict you to
certain computer systems. Some internal modems are built as
full-length expansion cards, which means you can only
install them in full-size PCs, XTs, ATs, and hardware
compatibles. You'll need a different modem for a
foreshortened computer (like the Tandy 1000). When you make
the move from PC to PS/2 architecture, you'll also have to
shell out the cash for a new internal modem. Most laptops
and the PCjr can use only internal modems that were
specifically designed for their proprietary expansion buses.
If you have a PC with its original, minimal 63.5-watt
power supply, adding an internal modem - an older,
full-length modem card in particular — may limit the number
of other expansion boards that you can plug into your
system. Such modems are notoriously power-hungry and may
leave few watts for other cards, such as hard disks and EMS
boards.
On the other hand, internal modems tend to be a few
dollars cheaper than external models because the internal
units don't need extra packaging or power supplies (although
they need some extra signal circuitry). You can also forego
the cost of a serial cable, which might cost you $30 or more
from a local dealer. With an internal modem, you don't have
to deal with a tangle of cords, plugs or transformers vying
for the few holes in your wall outlet, extra boxes on your
desk, or another thing to switch off when you put your
system to sleep at night.
7.11 Port Assignments
---------------------
Other than matters of power supply, the impact on your
system resources will be the same no matter the style of
modem you choose. While external modems require a serial
port and cable, internal modems also require the use of a
serial port address — which means you still lose the use of
that address by a serial port, COM1 or COM2 (or COM3 or COM4
in PS/2s and the latest DOS versions). If you use versions
of DOS before 3.3, you'll only be able to add one serial
port in addition to the port or address used by either an
external or internal modem. While some internal modems can
have their addresses to be set as COM3 or COM4, you must be
sure that the communications software that you choose can
control the ports beyond COM2.
If there is any general rule, it's that you should
choose an external modem for its flexibility and its ability
to move to new and different computer systems; choose an
internal modem for its neatness and lower overall cost.
8. OTHER MODEM CONSIDERATIONS
-----------------------------
Selecting one modem from the hundreds of products currently
available is no small task. However, it can be made more
tractable by making four separate judgments about each
particular modem's performance, compatibility, features, and
price.
Over a perfect telephone line, nearly all modems
function perfectly - without errors. However, perfect
telephone lines are impossible to find, and even getting an
acceptable one nowadays seems to require bribing an
operator. The performance differences between modems appear
as line quality goes down. Better modems are better able to
cope with bad connections. They work with worse circuits and
can pull data through with fewer errors.
One of the critical parameters of the telephone line is
the amount of noise it contains in relation to the strength
of the signal it carries. This relationship is usually
termed the signal-to-noise ratio of the line. The higher
this ratio is, the better the connection.
The signal-to-noise ratio is often expressed in decibels
(one tenth of a measurement unit called the Bel, which is
named after Alexander Graham Bell, by the way) which form a
logarithmic measurement scheme — that is, a signal-to-noise
ratio that is twice as good will only appear three decibels
better.
The compatibility of a modem refers to software and not
its hardware connection. Controlling all the features of an
advanced dial-up modem requires that commands be sent to it
from the computer that it is connected to. These commands
are usually sent invisibly by the communications program
that is actually in control. More compatible modems
recognize the commands of a wider range of software packages.
Obviously, to be useful communications software must
know the commands that the modem recognizes. Some
communications programs let you define the modem commands
yourself (usually during the installation process) and in
theory will work with any modem. However, most
communications programs are designed to accept a one or more
standard sets of commands.
In dial-up modems, no commands set has received official
sanction as the one and only standard. Most modems today,
however, follow a de facto standard, the Hayes or AT command
set. Originally developed by Hayes Microsystems and used in
the company's line of Smartmodems, the popularity of the
hardware led to many software companies incorporating the
commands in their products. Newer modems were adapted to
accept the existing software, resulting in an acceptance of
the standard. A modem that recognizes the Hayes command set
will therefore work with the widest variety of software.
Hayes-compatible modems are thus more versatile.
Note, however, that the Hayes standard is not immutable.
As new modem features and capabilities are developed, the
commands set becomes richer. Moreover, varying degrees of
Hayes compatibility exist. Some modems only recognize the
most rudimentary of commands, for example, using ATDT to
initial the dialing sequence. Other more elaborately mimic
the operation of Hayes products and incorporate the same
registers as used by Smartmodems, which permit, for example,
setting the number of rings required before the modem
answers.
Unless you have masochistic tendencies or software that
is specially designed for another modem command set, the
safe bet is selecting a modem that's as Hayes compatible as
possible.
The price difference between different manufacturers
dwarfs the difference between the internal an external
modems of any given manufacturer. Exactly how much you
should spend depends on what you're looking for and what
you're willing to settle for. As with any other PC product,
you should carefully consider every aspect of your modem
purchase before making your decision — select the one that
you're absolutely sure you want, then settle for the one you
can afford.
Unlike other common peripherals, modems often are not
plug-and-play devices, external modems particularly. Perhaps
that's to be expected because they plug into ornery serial
ports.
8.1 Modem Cabling
-----------------
The easiest part of installing a modem is hooking up the
cable. MOdems connected to the serial ports used by PCs and
PS/2s use «straight-through» cables. Only AT-style serial
ports with nine-pin connectors require adapters to match
them to most modems.
The problems begin with software. In the interactions
various communications packages make with modems, a number
of serial port control lines are brought into play. Some
communications programs, such as PC-Talk III make minimal
use of the indications modems provide. Others monitor every
connection. Consequently, the number of control lines that
must be connected — and thus the number of wires that must
be available in the cable that links your modem to your
computer — depends on the communications software you plan
to use. In some situations the minimal triad — pins 2,3 and
7 — will suffice. Other programs require the full complement
of ten connections. The moral to be drawn from this story is
that should you not know the type of cable required by your
modem, use a straight-through serial cable equipped with at
least ten conductors.
8.2 Modem Switch Settings
-------------------------
Modems themselves can be programmed to treat their various
connections in different ways to match the needs of
software. For example, some programs require that the modem
keep them abreast of the connection through the Carrier
Detect signal. Other programs couldn't care less about
carrier detect but carefully scrutinize Data Set Ready. To
accommodate the range of communications applications, most
modems have setup switches that determine the handling of
their control lines. In one position, a switch may force
Carrier Detect to stay on continually, for example. The
other setting might cause the status of Carrier Detect to
follow the state of the modem's conversations.
These switches take two forms, mechanical and
electrical. Mechanical switches are generally of the DIP
variety. In the prototypical modem, the original Hayes
Smartmodem 1200, these switches are hidden behind the front
panel of the modem. (To get to them, carefully pry up the
trailing ears of the sides of the bezel, first one side,
then the other of the black front panel of the modem. Then
pull it forward and off).
Most commercial modems that use DIP switches are
patterned after the Hayes Smartmodem 1200. (Its DIP switch
settings are shown in Appendix C of this document).
The other kind of switch is electrical, exemplified by
the Smartmodem 2400. Made from EEPROM memory, these switches
are set by sending commands to the modem from your computer.
Because of their EEPROM nature, they retain their settings
even when the modem is turned off or unplugged.
Other modems may follow this pattern exactly or may use
another memory technology — for example, battery backed-up
dynamic RAM. A few don't make any effort toward removing the
volatility. Such modems require you to reprogram their
settings every time you turn them on. While you can't do
much to make modem memory non-volatile when it's not, you
can make life easier using disk memory. Simply add the modem
settings you wish to enforce to the setup strings that many
communications software packages send to the modem before
they begin to make a connection.
(The setup commands for the industry-standard Smartmodem
2400 are shown in Appendix D of this document).
P.S: Appendices A-D will be available later in our «Player's
Dream» BBS station. Make your orders, gentlemen!