Connecting
Anybody can "dial-up", but do you know why it works?
Modems
ADSL
Ethernet
Impact
How do Modems Work?
Almost everyone outside
of a university or business setting is connecting to the internet using
a modem. Everybody knows that they are used to connect to the internet,
but do you know how they do it? For instance, computers operate in the
digital world, but telephone lines require a different method of transmission.
The modem is what handles this conversion. Modem stands for modulator-demodulator.
The modem modulates the signal into a sine wave. This in turn can be broadcast
over a telephone line. The signal is tranmitted over the line until it
reaches the IP hub. At this point, the sine is demodulated into a digital
signal once more, and the connection with the internet is complete. Modems
can do this at a variety of speeds depending on the technology and availability
of access lines. The industry standard now is 56K which stands for 56,000
bits per second (bps), the maximum transmission speed of the modem. This
can also be read as 56K Baud, which stands for the amount of state changes
undergone by the transfer of information, i.e. the 0's and 1's in a digital
signal and the manipulations in a sine wave. Due to speed lost in transmission
and the quality of hardware and communication services, the maximum attainable
speed is usually around 53K.
Simple
connection
How fast modems transfer
information is based on how they convert it. Modems must change the sine
wave in order to emulate the digital signal. They do this based with the
commonly known Amplitude modulation (AM), Frequency modulation (FM) and
the less known Phase modulation (PM). As their names suggest, the sine
waves' amplitudes, frequencies and phases are manipulated to emulate the
digital signal.
Amplitude Modulation
Frequency Modulation
Phase Modulation
You can see the relationships
from the graphs above. For higher speed modems, such as anything above
9,600 bps, a combination method is required in order to transmit that much
information. High speed modems use Quadrature Amplitude Modulation (QAM)
which simultaneously changes both the phase and amplitude of the signal.
Here a 32 point "constellation" represents one of 32 signals, each encoded
with a 5 bit signal. This means that a 2,400 Baud signal can transmit at
12,000 bps (2400 signals at 5 bits a signal).
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How does ADSL work?
Asymmetric
Digital Subscriber Line (ADSL) uses a more complex monitoring system to
increase the speed of connections. The slowest part of the transmission
over phone lines was the connection between the phone and the local substation
for demodulating the signal. The telephone lines between your house and
the phone company are comprised of twisted copper wire which have maximum
bandwidth under 3 miles of 1.1 MHz. Along the wire the bandwidth varies
between 24 kHz and 1.1 MHz. This spectrum is divided into 249, 4 kHz channels.
A signal is assigned to one of these channels based on the bit rate and
quality of the signal. This way, many more signals can fit on at higher
speeds.
Division of Signals in ADSL
How does Ethernet work?
The ethernet, the wonderful
thing that allows for supremely fast MP3 downloads and the ability to be
on the computer without tying up your phone line. But do you know why you
can do that? The ethernet is the most commonly used Local Area Network
(LAN). A network is a group of computers, usually called a "user group",
that share information. These computers all connect to the internet and
to each other at a small substation. At this substation, there is an extremely
high speed transmission line with a large capacity for users. This line
is constantly connected to the internet, which accounts for the extremely
fast internet connection and reason you don't have to dial a number to
get on. These lines are commonly called T-base lines. T-1, T-2, T-3 are
the commonly known ones. These are all variants of the same concept, with
T-3 being the fastest. At the University of Delaware, we are currently
operating on a 10BaseT ethernet network with multiple T-1 lines as the
main out-going lines. All the T-1 lines added up give the University the
equivalent of a T-3 line, which runs at 45Mbps(45 Megabit), approximately
28 T-1 lines. The 10BaseT that the infomation is being transferred at 10Mbps
over a twisted wire line. A "hop" is when the signal has to run through
other systems to get to the internet. Unfortunately, the signal goes through
three "hops" before going to the internet, so it may not always connect
at 10Mbps. This is because ethernet runs at 10Mbps over shared network
lines, but not necessarily through T-base lines. In order to operate on
the ethernet, each PC must have a working ethernet network card. This card
must be configured to read the desired network. Ethernet works on CSMACD
protocol. This stands for Carrier Sense, Multiple Access, Collision Detect.
The "carrier sense" means that, before transmitting, the wire checks to
make sure no one else is transmitting. The "multiple access" means that
everyone is connected to one outgoing unit, i.e. the T-base line. When
two stations start transmitting at the same time, they collide nanoseconds
later. This is where the "collision detect" comes in. When a collision
occurs, both signals "back-off" and then wait a pre-programmed amount of
time before processing again. A repeater is used when one common type of
wire is not convenient to wire an entire building or campus. It's a small
transfer box which takes in the signal from one wire and then spits it
back out bit-for-bit to the other wire.
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Impact
The impact of
the modem is unparalleled in recent events. The ability to surf the internet
has led to an information explosion the world has not seen. The ability
to access trillions of bytes with a double click has led to a revolution,
an information revolution. Knowledge is no longer found only in books contained
only at the local library, it is found in the home. The improvements in
modem technology are improving at a phenomenal rate. It is possible to
access things in the home never before thought possible.
ADSL is still new, but
the promise is substantial. 10Mbps is on par with our ethernet connection
at the University. Once that kind of speed is available in the home, information
transfer will be even quicker. Applications too large to be handled by
a modem will become more commonplace. This means an improved internet which
will benefit everyone.
There is not a student
at the University who does not kiss the wires the network travels on. I
had heard stories about how great it was, but I had no idea until I came
here. It has opened up a whole new part of my life. I was not on the internet
a lot when I was in high school simply because it took a good seven minutes
from boot-up to log-in. Now, a day doesn't go by when I am not on
it for something. It has impacted every student at the University of Delaware
as well as hundreds of thousands of students nationwide. As for the private
sector, large scale businesses rely heavily on it for data tranference.
Some so much so that they have 100BaseT networks installed. That's 10 times
faster than the University's current network. Research is almost complete
on perfect the gigabit network becasue the institutional demand is there.
There is a percentage
of the population that is not on the internet and whose lives have not
been effected in some way by the information explosion, but they live in
the hills of Montana and shoot at targets all day, so they won't read this.
92 Million people in the US and Canada over the age of 16 use the internet.
An 18% increase from just 9 monthes ago. (survey dated April,
99. courtesy commerce.net)
The
bottom line is that without the connection and the improvements on the
connection, we would be lost in the limbo of ignorance, forever doomed
because of the inconvenience of obtaining knowledge. The internet has helped
us in ways we can not even understand and in a world where everything should
have been done yesterday, the faster the connection, the better.
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Comments, suggestions, or requests to 
Scen103xdogs@hotmail.com
attn: Art Bookout "connection.html"
Last updated May 14, 2000.
Copyright Group XDOGS, Student Project of the University of Delaware,
2000.
