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
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 complete.
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
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 K56Plus 56 000 bps (downloading)
33 600 bps (uploading)
ISDN single channel 64 000 bps (both ways)
ISDN two channels 128 000 bps (both ways)
SDSL 384 000 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
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 directions.
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?
Source: Essay UK - http://www.essay.uk.com/coursework/modems.php