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Beginners Hacking Guide (TI) #2
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[\] The Beginners Manual Chapter 2 [/]
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[\] Written by...... [/]
[/] The Judge March 29, 1990 [\]
[\] [/]
[/] Call Treasure Island for other Chapters..604-946-7445 [\]
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The Beginners number two here will deal public networks, packet switching,
packets and virtual curcuits.... Everyone should be able to understand
the stuff here its all laid out very plain and simple so that you
beginners can understand things easier than in some other philes out
their. So enjoy number 2 look forward for 3 and ect ect......
The Judge
An Overview of the Process
Regardless of what you plan to do once you get there, and regardless of
the kind of system you plan to call, the fundamental goal of going online
is always the same: You want to establish a connection that will transport
the characters and commands you type at your keyboard to a distant computer-
and transport that comptuer's respones back to your screen. A variety of
systems and components are needed to achieve this goal.
The process can best be explained by following a bit of electronic
information as it travles from your keyboard into your systems, and out
into the telephone stream. The various components of a personal computer-the
keyboard, disk drives, printer, memory, video hardware, and so on-communicate
with each other by transmitting electronic impulses that for the purpose
of this discussion we can think of as jelly beans. When you press your
<R> key, for example, your keyboard sends a hail of colored jelly beems
hurtling into the system to tell it that you want it to display the letter
R on the screen. Your system responds correctly because it recognizes the
pattern of colorede beans the keyboard has selected to as representing the
letter R.
The jelly beans, of course are "bits" (binary digits) of computer
information. And there are only two colors, red and blue. There are no
pinks, powder blues, purples, or other shades. There is red and blue, and
onle red and blue. For the convenience, programs and other binary bean
counters symbolize these two colors on paper as 1s and 0s. They could
just as easily use exclamation points and dollar signs or some other pair of
symbols. As long as each symbol is unique and as long as there are two and
only two of them, the actual symbols used do not matter.
As surprising as itmay seem, the simple concept of two symbols is at the
very core or personal and every other kind of computing, and we will be
encountering it again and again in computer communications. For exampmle,
in reality, the bits on a floppy disk are symbolized by areas of the disk track
that are either magentized or not magnetized. The disk drive heads see these
areas as a pattern that they use to generate a matching series of electronic
pulses that flow into the computer. The drive heads, in other words, make a
copy of the pattern on the disk by translating its elements into voltage
pulses. The pulses are of either a "high" voltage ("on") or a "low" voltage
("off"), with nothing in between.
[[-- Note: Alright enough of the technical talk.. we just had to say some
for some of you who are more into learning the techno of it,
and to piece in later... --]]
Public Data Netowrks and Packet Switching
All of this sounds great, doesn't it? Imagine being able to call any
communicating computer anywhere in the world and use it as if you were sitting
at its very own keyboard. Imagine the size of your phone bill when all of
these long distance charges are added up each month-calls to Californaia
Washington D.C.... Call to Great Britain or France. Obviously, if one had
to pay regular long-distance rates for personal computer communications, the
only systems most of us would call would be the bulletin boars in our own
neighborhoods.
Fortuantly, although most systems can be reached by direct dial, there's
no need to make your connection that way, thanks to the public data netowrks
(PDNs). The PDN use a technology called "packet switching" that is
specifically designed to allow one computer to talk to another and to do so
inexpensively. As long as bother locationsa re within the continental U.S.,
for example, the cost of using a PDN has historically been as low as $2 an
hour (a l ittle over three cents a minute) during non business, evening hours.
The two leading public dat networks in the U.S. are Telenet, owned by
U.S. Sprint Communications Corporation, which in turn is owned by GTE and
United Telecommunications; and Tymnet, owned by the McDonnell Douglas Corp..
In Canada, the leading network is Datapac, owned and operated by Telecom Canada.
Other companies like ComuServe, and General Electric, operate networks as well,
but all work in essentially the same was.
You don't need to know a great deael about the PDNs, but its improtant to have
a broad familiarity with them. (For more information on them check out the
upcoming chapters......) You will be using a PDN vitually every time you
connect up with a major commercial system-and you may be able to use on for
BBs in selected cites as well. And you will most certainly be using them for
international calling. Heh you use them for every fucken thing in the book
pretty much dudes...
Data Packets and the Virtual Circuit
A packet-switching network consists of hundreds of computers and thousands
of modems. The computers and modems are scattered all over the country and
connected to each other by high-speed (up to 56,000 bps, or 56 kpbs) data
lines. Each location on the network is called a "node," and if you live near
a medium to large city, the chances are there is at least one node within your
local calling area.
When you and your computer dial one of these nodes, one of the modems at
that location will answer the phone and you will connect up. At this point
you and your computer will be in direct contact with the node computer, and
after keying in some preliminary information, you will be free to tell the
network node which commercial system you wish to talk to. The node computer
will then patch you through to your target system using the most efficient
network route available at the time. In the targe system's computer room-lets
say it's Palo Alto, California where DIALOG's massive array of computers are
located-the phone will ring and one of the DIALOG's modems will answer to
complete the connection.
The word "efficient" is the key here, since the efficienceies made possible
by packet-switching technology are one of the main reasons using a PDN is
cheaper than placing a conventional long-distance call. When you make a
voice call, and actual physical circuit must be established between your
phone and the person you are calling. This requires lots of switches and a
lot of wires leading to a lot of different places. But the result is a
circuit that's as real and complete as any you ever assembled in
seventh-grade science class. And of course the circuit must remain in
existence for the duration of your call, typing up all of the physical
resources involved in making the connection.
Computer-to-computer calls are different. Because they communicate
digitally-using the 1s and 0s or "jelly beans" we've spoken of before-their
transmissions can easily be chopped up into discrete units or packets. Like
the counterman at a hardware store pulling lengths of rope off a spool and
cutting them to a uniform size, the node computer cuts your computer's data
stream into uniformly sized packets of bits. It then stamps them with the
address of your target system and a packet sequence number, and sends them
on their way.
PDN follow an international recommended standard called X.25 (pronounced
"X dot twenty-five"). The standard specifies, for example, that a PDN
packet always contain 128 bits. Padding characters are used to round out the
packet when necessary to bring it up to that level. When a packet comes into
the node computer from a remote system, the computer strips off any
irrelevant bits, checks the packet's address, and channels it to the correct
caller. This is why a network node computer is often referred to as a
"packet assembler/disassembler" or PAD.
A single PAD may have many incoming lines but only two or three high-
speed lines connecting it to the network. However, since each packet is
unique, the packets from several callers can be interleaved. What's more,
they can be shot out into the network in different directions as the PAD
strives for maximum efficiency in the face of ever-changing network load
conditions and traffic patterns. Other node computers at various locations
in the network receive the packets, check their address, and relay them to
another node or to the host system itself. Thanks to the address and sequence
number, however, the packets are received by the correct host in the correct
order, regardless of the route each packet took to reach its destination.
The result is what's called a "virtual circuit." In most cases, when you
use a paket-switching network, the connection between you and a host system
appears to be identical to the connection that would exist were you to dial
the remote system directly, even though an unbroken, continuous physical
connection between you never actually exists.
The Impact of the PDNs
So what effect does this have on you? The first and most importnat effect
is that a PDNs ability to interleave packets from several callers on the
same line, and its ability to constantly reconfigure connections in
response to varying load factors, makes for very efficient use of the
equipment. Greater efficiency means lower costs which mean lower prices.
Second, it helps explain at least some of the delays that can occur on
a PDN during peak times of the day. It explains other points as well.
Sometimes, for example, you may call your local node and the local node
computer will tell you that no outgoing lines are available. You may also
find that occasionally all incoming lines are in use, resulting in a
conventional busy signal when you call. The solutions are the same in
both cases: Keep trying, wait a while and try again, or simply call a
different node. There is also the potential effect of a PDN when you are
transfering files using an error-checking protocol like Xmodem, but thats
for later on....
---------For Chapter 3 and other philes call........
Treasure Island.........12/24baud.............604-946-7445
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Written By.........
The Judge.....
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