Computers are essentially solitary beasts that prefer
to keep their own company. However, that’s not to say that PCs don’t
have a social side as well; you just have to work a bit to dig it out.
There are three ways to go about this:
Add network
interface cards to the computers, and then sling some cable around (or
use wireless devices) to set up a local
area network (LAN).
Use a special cable (or infrared)
to connect the serial ports of two computers, and then use a direct
cable connection to exchange files between them. I don’t discuss this in
the book, but it’s easy enough to establish the connection. Select
Start, All Programs, Accessories, Communications, New Connection Wizard. In the first New Connection Wizard dialog
box, click Next, activate Set Up an Advanced Connection, and click Next.
Then activate Connect Directly to Another Computer, click Next, and
follow the rest of the dialog boxes.
Attach a modem to your computer and use it to connect
to remote systems.
The third method is the
focus of this article. As an appetizer, this section presents a bit of
background info that serves to get you comfortable with the underlying
principles of modem communications. This knowledge will make it much
easier for you to set up and work with your modem, and it will be
invaluable when you need to troubleshoot the inevitable communications
problems.
Modems: The Inside Story
Modems are, by now, a
ubiquitous feature of the PC terrain, but they remain more mysterious
than the other peripherals. Perhaps it’s the alphabet soup of modem
standards, or the inherent complexities of modem-to-modem
communications, or just all those strange sounds modems make when they
converse with one another. To help you penetrate the mysteries of the
modem, this section examines the inner workings of these electronic
marvels.
The
Modulation/Demodulation Thing
When you speak into a telephone, a
diaphragm inside the mouthpiece vibrates. This vibration is converted
into an electromagnetic wave that mirrors the amplitude and frequency of
the original analog wave created by your voice. This wave travels along
the telephone lines, and at the destination, electromagnets in the
receiver vibrate another diaphragm that reproduces your voice.
Note that this process
is entirely analog, from the original sound wave of your voice, to the
electromagnetic wave that traverses the phone system, to the
reconstituted sound wave created by the receiver. Computers, of course,
are resolutely digital, so this analog state of affairs just won’t do.
For a computer to send data along a telephone line, the individual bits
that make up the data must be converted into some kind of analog wave.
This digital-to-analog process is called modulation. In essence, the 1s and 0s that compose
digital data are converted into signals (or symbols) that can be
represented as tones that fall within the frequency range of the human
voice (between 300Hz and 3,000Hz). These tones can then be sent along
regular telephone lines, where they’re converted back into their
original digital format. This reconversion process is called demodulation. The device that modulates the data, sends the
resulting tones, and demodulates the tones on the receiving end is a modulator/demodulator, or as it’s more
familiarly known, a modem. Now you know why modems make such a racket
while they’re communicating with each other: It’s all those tones
exchanged back and forth.
Note
Although most telephone systems are analog,
digital phone lines are cropping up with increasing frequency. These
lines work by sampling the voice, much like the way a sound card samples
analog audio. The samples are then sent across the lines as bits
without the need of modulation or demodulation, and so without the need
of a modem.
The Difference
Between Baud and Bits Per Second
The speed at which
modems transmit data is called the data
transfer rate, and it’s measured in bits
per second (bps). The current standards
for the data transfer rate are 14,400bps on the low end and 56,000bps on
the high end. Another measure of transmission speed does, however,
exist—the baud
rate—and the two terms are often confused.
The baud rate defines
the number of symbols (which might be variations in, say, voltage or
frequency, depending on the modulation standard being used) per second
that can be exchanged between two modems. Each of these symbols,
however, can incorporate multiple bits of data. For example, a
2,400-baud modem might be able to cram six bits of data into a symbol,
thus resulting in a data transfer rate of 14,400bps.
In the old days, modems
incorporated only a single bit per baud, so the bps and baud rates were
synonymous. Now, however, all modems support multibit baud rates, so the
only true measure of a modem’s transmission speed is bps.
Understanding
Modem Standards
For modems to
communicate with each other successfully, they must speak the same
language—language
in this sense meaning, among other things, the type of modulation used,
the data transfer rate, how errors are handled, and whether any data
compression is used.
At one time, there were
almost as many modem languages as there were modem manufacturers,
resulting in what I call the Tower of Babel problem in communications.
In other words, you could never be sure that the modem you were trying
to connect with would have the faintest idea what your modem was saying.
To solve this problem, the major players in the data communications
game put together a series of modem standards to help ensure
compatibility between devices from different manufacturers. These
standards cover three aspects of modem communications: modulation, error
correction, and data compression.
Note
You might still see
some modems described as Hayes compatible. This is a holdover from the days when Hayes
modems were the market leader, so other modems had to fall in line with
the Hayes standard to gain consumer acceptance. In this case, however,
the standard had nothing to do with modem communications. Instead, it
defined a command set used by applications to control the modem. For
example, the command ATDT (attention dial
tone) tells the modem to get a dial tone. By now, however, every modem
supports this command set (which is usually just called the AT
command set because most of the commands begin with AT), so being Hayes compatible is no longer a big deal.
Modulation
Standards
When a modem modulates digital data into a carrier
wave, the receiving modem must understand how this modulation was
performed in order to reverse the procedure during demodulation. This
is, for obvious reasons, the most crucial aspect of modem compatibility,
so having modulation standards is critical. These standards are set by a
United Nations umbrella group called the International
Telecommunication Union-Telecommunications Standardization Section
(ITU-TSS; it was formerly another mouthful: the Consultative Committee
on International Telephone and Telegraph, or CCITT). The ITU-TSS
consists of representatives from modem manufacturers, telephone
companies, and government agencies.
As modem technology
improved, new standards had to be hammered out, so numerous modulation
standards have been implemented over the years. Here’s a review of the
most common ones.
Note
When speaking the name
of any modulation standard, the V. part is pronounced as vee dot. So, for example, the standard V.90 is
pronounced vee dot ninety.
| V.22 | This
is a 1,200bps standard that was used mostly outside of the United
States and Canada. (The corresponding standard used in the United States
and Canada was called Bell 212A, which was a standard implemented by
Bell Labs.) |
| V.22bis | This is a 2,400bps
standard, and the first of the international standards. (The bis part is French for again or encore.) |
| V.29 | This is
the standard for half-duplex (that is, one-way) communication at
9,600bps. It’s used for Group III fax transmissions and so is the
standard facsimile implementation in fax/modems. |
| V.32 | This
is the standard for full-duplex (that is, two-way) communications at
9,600bps. This standard incorporates a technique called trellis coding
that enables on-the-fly error checking and reduces the effect of line
noise. |
| V.32bis | This
standard defines full-duplex transmission at 14,400bps. It’s basically
the same as V.32, except that the number of bits per signal change was
upped from four in V.32 to six in V.32bis (both standards operate at
2,400 baud). |
| V.32fast or V.FC | These
standards upped the V.32 and V.32bis transmissions to 28,800bps, but
they’ve been replaced by V.34. |
| V.34 | This is the standard for
full-duplex transmission at 33,600bps. |
| V.90 | This
is the standard for full-duplex transmission at 56,000bps for downloads (data coming
into your computer) and 33,600bps for uploads
(data going out of your computer). |
| V.92 | This
is the standard for full-duplex transmission at 56,000bps for downloads
and 48,000bps for uploads. V.92 also boasts a Quick Connect feature
that cuts
modem connect time in half, as well as a Modem
On Hook (MOH)
feature that enables you to initiate and receive voice phone calls
while a modem connection is active. V.92 represents the current state of
the art for analog modem communications. Despite their
latest-and-greatest status, V.92 modems are relatively inexpensive, so
you should shoot for V.92 if you’re in the market for a modem. |
Most experts used
to believe that V.34 represented the ceiling for analog transmission
speed. However, modem companies continued to push the transmission rate
envelope. U.S. Robotics and 3Com, for example, introduced x2 technology,
which allowed for 56Kbps rates over standard phone lines. A competing
technology called K56—also known as K56Flex—was also
available, and eventually the combined standard V.90 was established.
How was this speed increase achieved? V.34 treats the entire network as
though it were analog, so it’s limited to 33,600bps. However, V.90 takes
advantage of the fact that most of the network involved in modem
communications is digital. In particular, the connection between a
service provider and the switched telephone network is, in most cases,
entirely digital. This means that no download modulation is necessary.
In other words, the data that is downloaded to your modem doesn’t have
to go through the costly digital-to-analog conversion, so it can make
the most use out of the wider bandwidth on digital telephone networks.
So, with V.90, downloaded data can achieve theoretical rates of up to
56,000bps, while uploaded data still transfers at 33,600bps. V.92 gets
rid of most of the upload modulation as well, so uploaded data can
transfer at up to 48,000bps.
Does it work? Well, in
practice, you’re not likely to see true 56Kbps transmission rates due to
line noise and other factors. However, rates in the 35Kbps to 50Kbps
range are achievable, thus making V.92 a viable alternative.
Error Correction
Standards
One of the
problems with analog telephone lines is that they suffer from line noise
and other factors that can wreak havoc on the carefully crafted symbols
sent by modems. To ensure that data arrives intact, the ITU-TSS has set
up error correction standards. These standards enable the receiving modem to
check the integrity of incoming symbols and, if it finds a problem, ask
the originating modem to resend the data.
The current
standard for error correction is V.42, which incorporates two protocols:
Link Access Procedure for Modems (LAPM) and Microcom
Networking Protocol (MNP) 4. Both
protocols correct errors by asking that corrupted data be retransmitted.
The default protocol is LAPM because it’s a bit faster than MNP 4.
Compression
Standards
If you apply
compression to a folder, NTFS compresses the files by replacing
redundant character strings with tokens. Many modern modems can perform
the same process on your outgoing data. In other words, the modem first
uses a compression technique to reduce the size of the data and then
converts the compressed data into symbols. This means that less data is
sent, thus reducing upload times.
Note
To apply compression to a
folder, right-click the folder and then click Properties. In the
General tab of the folder’s property sheet, click Advanced and then
activate the Compress Contents to Save Disk Space check box. Note that
with Windows XP you can only compress folders on NTFS partitions
(although there are third-party utilities that enable you to compress
folders on any file system).
Of course, the
receiving modem must be able to decompress the data, so the ITU-TSS has
implemented compression standards. The current standard is V.44, which
can compress data up to 6:1. The old standard was V.42bis, which can
compress data up to 4:1. (Most modems also support another compression
scheme called MNP 5. However, this scheme provides a maximum compression
ratio of only 2:1.)
Caution
Data compression sounds
great, but it really works only on text transfers. Because binary files
contain few redundant character strings, they can’t be compressed all
that much, so you won’t see a significant increase in throughput. In
fact, if you’re dealing with files that have already been compressed
(such as ZIP files), data compression might lead to slower download
times because compressing an already-compressed file generally increases
the size of the file.
A Review of Modem
Types
Modems come in
various shapes and sizes, and most brand-name models provide similar
features. If you’re looking to purchase a modem, your main criterion
should be that the modem supports the ITU-TSS standards, especially one
of the modulation standards (such as V.90). Also, many modems come with
built-in fax capabilities so look for
V.29 compatibility as well.
After standards
compliance, your next criterion will be the type of modem (or fax/modem)
you need. Here’s a summary of the three main types:
| External | These modems are standalone boxes you connect to a
serial port with a special cable. (You can also get USB modems that plug
into a USB port.) Although external modems require a separate power
source and tend to be more expensive than an equivalent internal modem,
they have several advantages. For one, they can be transported between
machines fairly easily. For another, most external modems have a series
of LED indicators on their front panel that tell you the current state
of the modem. These lights can be invaluable during troubleshooting.
Here’s a summary of the LEDs that appear on most external modems and
what each light represents: |
| | LED | Description |
| | AA | Auto Answer—When lit, indicates that the modem will
answer incoming calls automatically. |
| | CD | Carrier
Detect—Lights up when the modem receives a valid data signal from a
remote modem. This indicates that data transmission is possible, and the
light remains on during the entire connection. |
| | CS | Clear
to Send—Lights up when the modem has determined that it’s okay for an
application to start sending data. |
| | MR | Modem Ready—Lights up when the modem’s power is
turned on. |
| | OH | Off
Hook—Lights up when the modem takes control of the phone line (which is
the modem equivalent of taking the telephone receiver off the hook). |
| | RD | Receive
Data—Lights up when the modem receives data. |
| | RS | Request
to Send—Lights up when your computer has asked the modem whether it’s
okay to start sending data. |
| | SD | Send Data—Lights up when the modem sends data. |
| | TR | Terminal
Ready—Lights up when the modem receives a DTRdata terminal ready) signal from the computer. This means that the current
communications program is ready to start sending data.
( |
| Internal | These modems are cards you insert into a slot on your
computer’s expansion bus. This type of modem is convenient because no
external power source is required, it’s one less device taking up
valuable desk space, and no external serial port is used up. Most modem
jockeys, however, dislike internal modems because of the lack of LED
indicators for troubleshooting. (As you’ll see later, though, Windows XP
does provide an icon during modem connections that shows you the state
of the RD and SD signals.) |
| PC Card | These are
modems that use the credit-card–size, PC Card (PCMCIA) format and plug
directly into a PC Card slot. If possible, look for PC Card modems that
accept an RJ-11 jack directly because these kinds are more reliable than
the “dongles” used by some PC Card modems. |
