textfiles/hacking/psns.txt

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7/82: Explaination of PSNs
Name: Doc Holiday #14
Date: 12:34 am  Mon Jul 11, 1988



     Packet switching networks are
designed primarily for on-line
applications in which the data must be
delivered to its destination
immediately.  However, there can be a
store-and-forward capability which in
most cases is used only if a
destination terminal is inoperative.
     A packet of data is usually 128
bytes of data, part of which includes
the packet routing control information
required to get the packet to its 
destination.  A packet is sent through
a communications network as an 
individual transmission completely
independent of the rest of the
sentence or block of data.  The
complete message is normally assembled
only at its destination and not at a
store-and-forward network computer
node.  Thus the error detection and
correction function is limited to the
individual packet of data rather than
a block of data.
     The public packet switching
operators have selected an efficient 
communications protocol similar to
CCITT x.25 for transmision of data 
throughout the network, which includes
hundreds of cities around the US and 
around the world.  Unfortunately, most
of today's terminals do not operate 
under the x.25 protocol.  To resolve
this problem, the network vendors have 
devised a device called a PAD, which
acts as a combionation protocol 
converter, a packetizer/depacketizer,
and a multiplexer.  Also available are 
software protocol converter packages
for computers that use the network 
directly without a PAD.


     --Continued on next message--



Read:(1-82,7),? :


8/82: PSNs CONT
Name: Doc Holiday #14
Date: 12:51 am  Mon Jul 11, 1988



     The functions of a packet
switching network include all of those
of a message switching network. 
Today's public packet switching
networks are not looking for the
store-and-forward type of operation,
although they will provide a mailbox
arrangement in which one location
sends mail and receives mail as well
as the other location.
     Packet switching networks provide
users with a terminal compatibility 
enhancement feature.  Terminals that
are not otherwise able to communicate 
with each other can take advantage of
the protocol converter function so, for 
example, asynchronous terminals can
talk to synchronous terminal, and so
on..


     ADVANTAGES of packet switching
networks:
 
     The advantages of a packet
switching network include those of the
mesage switching network.  In
addition, however, there is the online
type of operation that affords the
excellent response time that is needed
for the brief transmissions between an
inquiry or data entry terminal and a
host computer.  One of the important
features of a packet network is the 
establishment of communications
between different types of terminals
that ordinarily cannot exchange data
without some kind of separate protocol 
converter.  Today's packet switching
networks are also noted for their
backup lines and alternate routing
capability plus a very efficient,
cost-effective use of the network.

     "I feel like I'm rambling.. so
I'll stop."
                  --Phantom Phreaker


 --Doc Holiday



Read:(1-82,8),? :


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