phrack/issue24/10.txt

                                ==Phrack Inc.==

                     Volume Two, Issue 24, File 10 of 13

                    ()()()()()()()()()()()()()()()()()()()
                    ()                                  ()
                    ()        Network Progression       ()
                    ()                                  ()
                    ()         by Dedicated Link        ()
                    ()                                  ()
                    ()           January 1989           ()
                    ()                                  ()
                    ()()()()()()()()()()()()()()()()()()()


This file provides a general overview of how networks have progressed from
phone lines to T1 lines.

There are numerous reasons to share networking facilities.  The concept of
networking is to optimize all the aspects of voice and data transmission, and
to utilize all the amounts of space in the transmission lines.

Not long ago companies used AT&T's switching facilities for all local calls.
This means use of the Centrex, which is the switching of local calls by AT&T
(which is much more expensive than using your own switching facilities).  Then
the larger organizations started to put in PBXes (Private Branch Exchange) to
enable them to switch local calls (class 5 ESS) without having anything to do
with AT&T.  The process of using a PBX (or a Computerized Branch Exchange CBX)
is much more efficient if the phone traffic is high.  This is the beginning of
a Local Area Network (LAN).  Once an organization has it's own LAN it can lease
the extra transmission space to another company, because they are paying for it
anyway.  Another method of bypassing AT&T's service is to use a foreign
exchange (FX) line.  Which is a long distance dedicated point-to-point private
line, which is paid for on a flat rate basis.  A FX line can be purchased from
AT&T or many other vendors.  These private lines (PL) are used with voice and
data transmissions.  Data transmission must have a higher grade quality than
voice because any minor break in the transmission can cause major, expensive
errors in data information being processed.

One of the most optimum ways of transmitting data is a T1 line which transmits
data at 1.544 megabits per second. Microwave, Satellite, and Fiber Optic
systems are being used for data transmission.  These methods multiplex several
lines into one to create greater capacity of the transmission.  A multiplexed
line has 24 channels that can be divided into the appropriate space needed to
utilize each transmission (i.e. a simple voice transmission which has about
300-3000 Hz uses a small portion of the multiplexed line).  There are two types
of multiplexing; time-division and frequency.  Time-division multiplexing
divides the channels into separate time slots.  Frequency-division multiplexing
separates the different channels with the use of different bandwidths.
Typically, data is transmitted through digital systems rather than analog.
However, all the state-of-the-art equipment is now digital.

When the data is being processed from the computer to another computer there
must be a standard protocol for communicating the interexchange within the
network.  The protocol is the set of rules that the computer says are necessary
to have in order for the other computer to connect to it.  This is the standard
way of communicating (The American Standard Code for Interface Interexchange,
ASCII).  Also, there are encryption codes which are used for security reasons.
Encryption codes can be scrambled on a hourly, daily, weekly, or monthly basis,
depending on the level of security.

The information that is being sent is organized by packet switching.  The most
used packet switching is called X.25, and this is the interface that the CCITT
(Comittee Consultif Interaction Telephonique & Telegraphique) recommends to use
for connection between the Data Terminal Equipment (DTE) and the Data
Circuit-terminating Equipment (DCE).

Within this network it is crucial that there is software providing Automatic
Route Selection (ARS).  There must be an ARS (the least cost path length)
programmed within the transmission.  It is the job of the system analyst or
operator to assign the proper cost of each path where the transmission goes in
order for the packet to go through it's least cost route (LCR).

The packet travels through a path from it's source to it's final destination.
The system analyst or operator must have full knowledge of the exact path
length, the exact alternative path length, plus the exact third alternative
path length.  The path length is measured in hops, which equals to the number
of circuits between central nodes.  The system manager must set a maximum value
of hops at which the path can never exceed.  This is the actual circuit cost
which is assigned to each possible path.  It is important that the system
manager has knowledge of the circuit costs in order for the ARS to be
programmed effectively.

These are just some of the basics that are involved in transmitting information
over a network.  I hope it helped you lots!
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