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TUCoPS :: Networks :: packetsw.txt

Ins and outs of packet switching

[]                                  []
[]         THE INS AND OUTS         []
[]                OF                []
[]         PACKET SWITCHING         []
[]                                  []
[]     by:       The Seker          []
[]         Tribunal of Knowledge!   []
[]    Written (c)  June 23, 1986    []

                         'TRIBUNAL COMMUNICATIONS LTD'
     Not many people are quite aware how complex packet switched networks are. 
In this file I hope to clear up all confusion and answer all questions 
concerning packet switching and making international datacalls via packet 
switched networks.

     Using normal phone lines, computers can only transmit data at speeds up to
1200 bps efficiently.  This is very slow compared to the inner workings of even
the slowest computer.  If computers could transmit across phone lines at higher
speeds, 9600 bps for example, there would still be the problem of using a 
compatible protocol.  Packet switched networks take care of these and other 
problems dealing with communications.
     The idea of developing a completely computerized network for computers was
first discussed in the mid 1960's..probably someplace like Bell Labs, MIT, or 
the like.  But it wasn't until a decade later that the theory was put into 
     The first packet network was a project of the Defense Department.  They 
labeled it ArpaNet.  It was and still is a boon for advanced hackers, as it is 
host to over 300 government related computers.  (See 'Hacking ArpaNet' written 
by the Wizard of ArpaNet for an indepth look at breaching this system.)
     Today there are over five commercial packet networks in the United States 
alone (Telenet, Tymnet, CompuServe, etc), and many more throughout the world.

     In essence, packet switching services operate at 4800 bps full duplex 
(both direction simulstaneously) and use a form of TDM (Time Division 
Multiplexing), a transmission which is basis for most of the world's voice 
communications.  Transmission streams are separated into convenient sized 
blocks or 'packets', each one of which contains a head and tail signifying the 
origination and destination of the data.  The packets are assembled by either 
the originating system or by a special facility supplied by the packet switch
system.  Packets in a single transmission may follow the same physical path 
(same cable) or may use an alternate route (ie. a detour cable) depending on 
the congestion of the system.  The packets from one 'conversation' are very 
likely to be interleaved with packets from other 'conversations'.  The 
originating and receiving computers see none of this mixing.  At the receiving 
end, the various packets are stripped of their routing information, and 
re-assembled in correct order before presentation to the computer terminal.
     All public networks that use packet switching have installed a standard 
protocall to try and be compatible with each other. (good luck)  The standard, 
which is called CCITT X.25 (Developed at the Geneva conferences.), is 
implemented on all international datacalls.  This is a complex system for 
interface between data terminal equipment and data circuit-terminating 

     Users (hackers) can access packet switching in a variety of ways.  Special
terminals called Packet Terminals, which are usually hard wired to the nearest 
PSS (Packet Switch Stream), that are able to create and arrange data into the 
correct format are often used.  This is very expensive, a reason why you will 
only be likely to see these type of terminals within large company office 
buildings.  The average person will probably access a packet network using an 
ordinary ascii terminal (computer and modem), and connect to a special PSS 
facility called a PAD (Packet Assembler/Disassembler) which will handle the 
formatting for them.

     To use a public packet network it is usually required for one to have a 
NUI (Network User Identity) which is registered at your local PSE (Packet 
Switch Exchange) for billing purposes...or a way around this. 
     Dial into your local PAD (often called port) and enter your NUI.  If a 
valid ID is not given, the port will usually throw you off. (There are a few 
exceptions which we will discuss later.)  Then one enters the NUA (Network User
Address) or call name of the computer he/she wants to access.  Each computer on
a network has one given to them.  This is usually in the form of numbers or 
somtimes letters. (As in Tymnet's case.)  After the correct information is 
entered, the network will connect you via its private sattelite system to the 
local phone system of your destination and then onto the computer you wish 
to access.

     Billing on networks is done to either the user or reversed and charged to 
the designated computer.  Charging is not done according to the distance of the
call or by the time passed, rather by how many packets exchanged and sometimes 
a small fee for CPU (Centeral Processing Unit) time.
     Many packet networks do not require you to have an NUI at all.  One of 
these that many of you probably have worked with is Telenet.  It is a leading 
public network throughout the continent.  Billing on there is a variation of 
the norm.  There is only a charge to a user when he/she wants to access a 
computer internationally or one which doesn't accept the charges of the 
datacall. (ie. REFUSE COLLECT CONNECT 00 AA)  Billing like this will probably 
disappear soon due to the greed of big business.

     If a person wishes to call a computer located on a foreign network, there 
is a little procedure which must be done.  As I said earlier, each computer on 
a network has its own address. (NUA)  Networks also have their own 'address', 
which is called a DNIC. (Data Network Identification Code)  This code is four 
numerical digits long.  The first three numbers in this code represent which 
country the network is located in.  The fourth digit is which service in that 
particuliar country, as some countries have more than one network. (For 
example, 5052 is Australia's Auspac DNIC.  505 is the country code.  2 is the 
service code.)  A list follows:

 COUNTRY                 NETWORK                  DNIC
 Australia               Auspac                   5052
 Australia               Midas                    5053
 Belgium                 Euronet                  2062/2063
 Canada                  Datapac                  3020
 Canada                  Globedat                 3025
 Canada                  Infoswitch               3029
 Denmark                 Euronet                  2383
 France                  Transpac                 2080
 France Antilles         Euronet                  3400
 Germany (West)          Datex P                  2624
 Germany (West)          Euronet                  2623
 Great Britain           IPSS                     2342
 Hong Kong               IDAS                     4542
 Irish Republic          Euronet                  2723
 Italy                   Euronet                  2223
 Japan                   DDX-P                    4401
 Japan                   Venux-P                  4408
 Luxembourg              Euronet                  2703
 Netherlands             Euronet                  2043
 Norway                  Norpak                   2422
 Singapore               Telepac                  5252
 South Africa            Saponet                  6550
 Spain                   TIDA                     2141
 Sweden                  Telepak                  2405
 Switzerland             Datalink                 2289
 Switzerland             Euronet                  2283
 USA                     Autonet                  3126
 USA                     CompuServe               3132
 USA                     ITT (UDTS)               3103
 USA                     RCA (LSDS)               3113
 USA                     Telenet                  3110
 USA                     Tymnet                   3106
 USA                     Uninet                   3125
 USA                     WUI (DBS)                3104

     As you can see, the the United States has many services.  But their DNIC 
doesn't follow the pattern I described earlier. (ie. first three digits 
represent country, last is service)  I am not quite sure why this is, but I 
think it may be because each of the US services listed are privately owned.
     As I was saying earlier, there is a little extra bit of information you 
must give the network when making an international call.  Instead of just 
emtering the NUA like on a domestic call, you have to enter the DNIC and append
the NUA or you will not complete you call and probably will get an error code. 
Here is what a call from Telenet to Cambridge University's port selector in 
England, which is located on Euronet (In Britain they call it IPSS.) would
look like:





@ c 234222339399

CONNECTED TO 234 222339399

     What I just did was connect to a Telenet port.  Enter my NUI.  Then enter 
the DNIC for IPSS in Britain (2342) and appended the NUA for Cambridge 
University. (22339399)  Then I was connected.

     For more detailed info on packet switching and its uses, etc, I recommend 
the following two books:

 'Data Communications: Facilities, Networks, and Systems Design'
  Doll, Dixon R., New York, Wiley, c1978

 'Packet Radio'
  Rouleau, Robert and Ian Hodgson, Blue Ridge Summit, Pa., Tab Books, c1981

     Much of the imformation within was provided by:

 Cyclone II
 Slave Driver

 NOTE: This document was written for informational purposes only.  Any 
application of what was provided within is responsibility of the user, not the 

        >>>>>>>>>>>>>>> (c) 1986 TRIBUNAL OF KNOWLEDGE! <<<<<<<<<<<<<<<

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