The Internet, part 2

One of the most confusing aspects here is that there are a very wide variety
of technologies and protocols.  But we'll start with how you connect from
your home.

The earliest home connections were not to computers (which were rather
primitive back then--there was also no Internet!) but to what were (and
still are) called "dumb terminals".  A dumb terminal is NOT a computer--
it has no CPU, no hard drive, no inherent processing power of its own.  It
was a monitor and keyboard, and the monitor had only 2 colors--gray and
white or green and white.  The screen held about 40 lines of text, each line
being about 70 characters wide--it was designed for displaying text, not
graphics, although some creative people managed to produce D&D games
for this environment.  Some of these games have evolved over the years
into low-footprint PC games--see nethack, for example.  Dumb terminals
connected to mainframes, and the mainframe provided the computing
power.  A typical session would have you editing a program (which is on
the mainframe's hard drives, not on the dumb terminal) and then running
the program (on the mainframe) and viewing the results.  Not very elegant,
but much better than having to go to a card punch and having your program
be a deck of punch cards which you had to carry around.

If you were at, say, UTK, you could get a reasonably fast connection (for that
time) between the mainframe and the dumb terminal.  If you wanted a
terminal for your home--the terminal cost several hundred dollars, and you
needed a modem to connect to the mainframe over your telephone line from
home.  Early modems were 300-baud (roughly bits per second) acoustic
couplers.  You dialed up the modem pool, such as UTK had, and hopefully
got a connection rather than a busy signal (not uncommon).  When you got
the connection, you placed the phone's headset into the acoustic coupler
and then you were off and running, galloping along at 300 baud--about
30 characters per second.  Remember--no internet, no web, just text! You
also worried about noise.  If someone else in the house picked up another
extension--you got disconnected.  If you coughed loudly or a door slammed,
you got disconnected--noise could creep into the acoustic coupler.  Later,
bliss! a 2400-baud modem with no acoustic coupler--you no longer had to
worry about coughing.  Of course, you were still using the phone line.  If
you wanted to get or place a call--too bad!  Modem speeds grew, and you
got PCs, got hard drives, and now you could connect your PC somewhere.

With standard dial-up modems, speeds ranged up to 56Kbaud--and you
could browse the web and download stuff.  When you were connecting
you could hear whistles and squeeks--your modem and the server were
talking to each other to determine things such as the communication
speed.  Well, you ask, why worry about speed?  It's a 56K modem.  But
it was rare to actually communicate at 56K--usually it was lower,
about 40-44K perhaps, and maybe much lower if you had an old noisy
phone line.  Also--it is important to realize that there is always an overhead
here.  It's nice to think that 56K means 7000 bytes per second, so that to
download  a 7 megabyte file (7,000,000 bytes) would take 1000 seconds--
about 18 minutes.  But with the overhead, at 56K you might be downloading
5600 bytes of file per second at 56K, and about 4000 bytes of file per second
if your modem is communicating at 40K. 

There are modems designed to connect to cell phones--I have one of these,
but the speed is 14,400--about a quarter the speed of a 56K modem.  Not
ideal for web browsing, but OK for emergencies.

The next stage up from 56K dial-up modems is ISDN (Integrated Services
Digital Network) which runs from about 64K to about a megabit.  You still
need an ISP (Internet service Provider).

Nowadays, the two most common high-speed home connections are DSL
(Digital Subscriber Line) and cable modems.  DSL you get through the phone
company, cable through the cable company.  Both can provide up to about
5 megabit speeds.  If you have cable in your home or neighborhood, you can
get a cable modem--this will split off from Comcast's (or whatever) cable
to your TV--you can watch cable TV and browse the web at the same time.
For DSL, you get special phone jacks.  These have two connections--one to
your phone, as usual, the other to your DSL modem.  The phone company
send your phone calls at the regular frequencies, and can also send the
data to and from your computer at other frequencies at the same time.
With the dial-up modems, once intitally established, the speed remains the
same.  With DSL, for example, things are more dynamic--speeds can vary
as you're communicating.   You may also be able to get different rate costs,
so that if you want 3mb service, you pay one rate, for 1mb service a lower
rate, etc.  There is always communication overhead--so downloading a
5 megabyte file with a 5mb DSL or cable modem--without the overhead
you might say that 5 megabits is 5/8ths of a megabyte per second,  so in 8
seconds you can download  5/8 * 8 = 5 megabytes of the 5 megabyte file.
Overhead slows this down.  Also--with a lot of these services, the upload
and download maximum speeds may be differeent.  Connections nowadays
to your PC are usually done through serial ports, USB ports, and Ethernet