Modems are used to connect two computers over a phone line. Modem is
short for Modulator Demodulator. It’s a device that converts data from digital
computer signals to analog signals that can be sent over a phone line. This is
called modulation. The analog signals are then converted back into digital data
by the receiving modem. This is called demodulation. A modem is fed digital
information, in the form of ones and zeros, from the CPU. The modem then
analyzes this information and converts it to analog signals, that can be sent
over a phone line. Another modem then receives these signals, converts them back
into digital data, and sends the data to the receiving CPU. At connection time,
modems send tones to each other to negotiate the fastest mutually supported
modulation method that will work over whatever quality line has been established
for that call. There are two main differences in the types of modems for PC,
internal and external modems.
Evolution of Modems
In the last 10 years, modem users have gone from data transfer rates of
300bps to 1,200 bps to 2,400 bps to 9,600 bps to 14.4Kbps to 28.8Kbps to, and to
33.6Kbps. Now new modem standards are emerging, reaching speeds of up to
56Kbps. Unlike the 33.6Kbps modems being sold today, 56Kbps is a significant
improvement over 28.8Kbps modems. Viewing complex graphics or downloading sound
files improves significantly with 56Kbps. The modem experts keep telling us that
we are about maxed out. For instance when the 28.8 modems where first introduced
they said that we’ve reached our maximum speed, and the same thing was said
about the 33.6 and now again for the 56K, but how true is this? The experts say
that the next major improvement will have to come from the telephone companies,
when they start laying down fibber-optic cables so we can have integrated
services digital network (ISDN) . The thing that makes digital modems better
than analog is because with analog modem transmission errors are very frequent
which results in your modem freezing or just freaking out. These errors are
caused mainly by some kind of noise on the line due to lightning storms,
sunspots, and other fascinating electromagnetic phenomena, noise occurs anywhere
on the line between your PC and the computer you’re communicating with 2,000
miles away. Even if line noise is minimal, most modems will automatically reduce
it’s speed to avoid introducing data errors.
Baud vs bps
While taking about modems, the transmission speed is the source of a lot
of confusion. The root of the problem is the fact that the terms “baud” and
“bits per second” are used interchangeably. This is a result of the fact that
it’s easier to say “baud” than “bits per second,” though misinformation has a
hand in it, too. A baud is “A change in signal from positive to negative or
vice-versa that is used as a measure of transmission speed” and bits per second
is a measure of the number of data bits (digital 0’s and 1’s) transmitted each
second in a communications channel. This is sometimes referred to as “bit rate.”
Individual characters (letters, numbers, spaces, etc.), also referred to as
bytes, are composed of 8 bits. Technically, baud is the number of times per
second that the carrier signal shifts value, for example a 1200 bit-per-second
modem actually runs at 300 baud, but it moves 4 bits per baud (4 x 300 = 1200
bits per second).
Synchronous vs. Asynchronous Data Transfer
Synchronous and Asynchronous data transfer are two methods of sending
data over a phone line. In synchronous data transmission, data is sent via a
bit-stream, which sends a group of characters in a single stream. In order to do
this, modems gather groups of characters into a buffer, where they are prepared
to be sent as such a stream. In order for the stream to be sent, synchronous
modems must be in perfect synchronization with each other. They accomplish this
by sending special characters, called synchronization, or syn, characters. When
the clocks of each modem are in synchronization, the data stream is sent.
In asynchronous transmission, data is coded into a series of pulses,
including a start bit and a stop bit. A start bit is sent by the sending modem
to inform the receiving modem that a character is to be sent. The character is
then sent, followed by a stop bit designating that the transfer of that bit is
A full page of English text is about 16,000 bits. And in order to view
full-motion full-screen video it would require roughly 10,000,000 bits-per-
-second, depending on data compression.
The Past 300 bps (both ways)
1 200 bps (both ways)
2 400 bps (both ways)
9 600 bps (both ways)
14 400 bps (both ways)
Current Speeds 28 000 bps (both ways)
33 600 bps (both ways)
X2 or K56Plu56 000 bps (downloading)
33 600 bps (uploading)
ISDN single channel64 000 bps (both ways)
ISDN two channel128 000 bps (both ways)
SDSL384 000 bps (both ways)
Satellite integrated modem 400 000 bps (downloading)
ADSL (T-1)1 544 000 bps (downloading)
128 000 bps (uploading)
Cable modem (T-1)1 600 000 bps (both ways)
Ethernet (T-2)10 000 000 bps (both ways)
Cable modem (T-2) 10 to 27 000 000 bps (both ways)
FDDI(T-3)100 000 000 bps (both ways)
In some cases, the modem-equipped PC with a 28.8Kbps modem would be
faster than a 33.6Kbps or even 56K modem, especially with sites that don’t have
a great deal of graphics. That’s because there are several factors that
determine how long it takes to reach and display a Web site. These include the
speed of your PC, your connection to your Internet service provider, your ISP’s
connection to the Internet itself, traffic on the Internet and the speed and
current traffic conditions on the site you’re visiting. A good example would be,
say you drive a fancy sports car and I drove along in my family minivan, you’ll
certainly beat me on an open stretch of road. But if we’re both stuck in a
traffic jam, you’ll move just as slowly as me. In short, any modem will
sometimes operate below its rated speed. According to the vice president of a
major 33.6Kbps modem company, you can expect a full 33.6Kbps connection about
one out of 10 tries.
X2 56K Modem
U.S. Robotics, Cardinal, Rockwell, and other manufacturers have
developed modems capable of 56K speeds over standard phone lines. U.S. Robotics
line of modems called X2, uses an “asymmetric” scheme. Basically, it lets you
download data at up to 56Kbps from any on-line service or Internet service
provider using matching U.S. Robotics modems. The company says AOL, Prodigy,
Netcom, and others are committed to deploying the X2 technology. The only catch
is the data you upload to the provider is still limited to 33.6Kbps or 28.8Kbps.
The main reason why everyone has not yet leap to 56Kbps is because there are no
set standards yet. Not all modem vendors are supporting the same 56Kbps
specification. That means your Rockwell-based modem won’t work with a U.S.
Robotics or Logicode model.
ISDN (Integrated Services Digital Network) is a way to move more data
over existing regular phone lines. ISDN cards are like modems, but approximately
5 times faster then regular 28.8 modems. They require special telephone lines,
which cost a little or a lot, depending on your phone company. It can provide
speeds of roughly 128,000 bits-per-second over regular phone lines. ISDN has a
couple of advantages. It uses the same pair wire found in regular phone lines,
so the phone company won’t necessarily have to run new wires into your house or
business. A single physical ISDN line offers two 64Kbps phone lines called
channels that can be used for voice and data. Unfortunately, ISDN isn’t cheap.
Installation fees can run a couple hundred dollars and setup can be confusing.
ISDN also requires a special digital adapter for your PC that costs around $200.
And though you could replace your old phone line with ISDN, I wouldn’t recommend
it. An ISDN line goes through a converter powered by AC current and if your
power fails, so does your phone line.
The access service to Internet by satellite is called DirecPC. It was
created by an American company of telecommunications called Hughes Network
Systems Inc. DirecPC offers speeds of up to 400 Kbps. That’s nearly 14 times
faster than a standard 28.8Kbps modem and four times faster than ISDN
(integrated system digital network). The draw back to this system is that it’s
too expensive, requires a relatively elaborate installation and configuration
and, in the end, doesn’t necessarily speed up your access to the World Wide Web.
The price for the 21″ dish, PC card and software is about $499 U.S.
retail. Then there is a $49.95 U.S. one-time activation fee. The monthly charges
start at $9.95 U.S., but that is for a limited account that also requires you to
pay to download data. The “Moon Surfer” account, which costs $39.95 U.S., gives
you unlimited access nights and weekends. If you want unlimited access during
the day, you’ll have to pay $129 U.S. a month for the “Sun Surfer” plan.
Customers pay between $149 and $199 U.S. for professional help, or $89 U.S. per
hour plus materials if custom installation is required. If you chose to install
the dish on ground level, Hughes Network Systems also has designed a hollow
fiber glass camouflage that looks like a huge rock which can be put over the
dish in order to prevent it from it being stolen.
In addition to these charges, you also need to be signed up with an
Internet service provider, or ISP, which approximately costs about $20 a month.
You can use any ISP other than on-line services such as Prodigy or America On-
line. The reason you need an ISP is because DirecPC is a one-way system. The
satellite sends data to your PC, but you need to use a standard modem and a
regular ISP to send data or commands to the DirecPC network. The data you send
flows at the speed of your modem, normally a 28.8 Kbps modem. The fact that the
satellite is only one-way isn’t as bad as it might seem. Most users send very
little data compared with what they receive. If you wish to view a Web site, for
example, you would send the Web address to the system via the modem, but the
site’s text and graphics would rush back to you via the satellite. Since the
address is typically only a few bytes, that takes almost no time at all, even if
you have a slow modem. The data from the site itself takes up far more time,
especially if it has a lot of graphics. Those who upload a lot of data,
including people who need to update their own Web sites, will get no advantage
from the satellite system while they are uploading.
In addition to the dish, you get a 16-bit card that plugs into an ISA
port of a desktop PC. The draw back to the system is that it eliminates Macs,
notebook PCs and any other machines that don’t have available slots.
You will find a noticeable difference when viewing sites with video
and lots of graphics. This could eventually be a big advantage as an increasing
number of information providers start using the Internet for full-motion video
and other multimedia presentations. But DirecPC for now doesn’t offer
spectacular advantages for normal Web surfing. And if you’re thinking about a
long-term investment, consider that in the future there will be other options
for high-speed Net access.
ADSL / SDSL
ADSL (Asymmetric Digital Subscriber Line) a method for moving data
over regular phone lines. An ADSL circuit is much faster than a regular phone
connection, and the wires coming into the subscriber’s home are the same copper
wires used for regular phone service. An ADSL circuit must be configured to
connect two specific locations. A commonly used configuration of ADSL is to
allow a subscriber to download data at speeds of up to 1.544 megabits per second,
and to upload data at speeds of 128 kilobits per second. ADSL is often used as
an alternative to ISDN, allowing higher speeds in cases where the connection is
always to the same place. SDSL (Symmetrical Digital Subscriber Line) is a
different configuration of ADSL capable of 384 Kilobits per second in both
Another type of modems are cable modems. It uses the same black coaxial
cable that connects millions of TVs nationwide and is also capable of carrying
computer data at the same time. It’s able to uploading and downloading
approximately 10 to 27 megabits per second. A 500K file that would take 1.5
minutes to download via ISDN but would take about one second over cable.
Classification Of Modems
A classification of modems that are capable of carrying data at
1,544,000 bits-per-second are called T-1. At maximum capacity, a T-1 line could
move a megabyte in less than 10 seconds. That is still not fast enough for full-
screen, full-motion video, for which you need at least 10,000,000 bits-per-
second. T-1 is the fastest speed commonly used to connect networks to the
Internet. Modems that are capable of carrying data at 3,152,000 bits-per-
second are refereed to as T-1C. Modems that are capable of carrying data at
6,312,000 bits-per-second are refereed to as T-2. And modems that are capable
of carrying data at 44,736,000 bits-per-second are refereed to as T-3. This is
more than enough to do full-screen, full-motion video. Modems that are capable
of carrying data at 274,176,000 bits-per-second are refereed to as T-4.
A very common method of networking computers in a LAN (local area
network) is called Ethernet. It will handle about 10,000,000 bits-per-second and
can be used with almost any kind of computer.
FDDI, (Fiber Distributed Data Interface) is a standard for transmitting
data on optical fiber cables at a rate of around 100,000,000 bits-per-second.
It’s 10 times as fast as Ethernet, and approximately twice as fast as T-3.
Most modems mentioned such as T-1, T-2, T-3, etc. are not intended for
home use. These high speed connections are use mainly for big businesses. But
even such speeds as T-4 and FDDI are use very little among big companies, but
more of the Army, NASA, the Government, etc. They’re highly priced which makes
them only available to larger corporations and organizations who need to send
huge amounts data from one place to another in little time or no time at all.
Apart the price factor when would you need to transfer data that is on a CD-ROM
disk holding it’s full capacity (650 Mb) across the world in 52 seconds?