The OSI-model Post: 17863

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Special Forums IP Networking The OSI-model Post 17863 by thomas.jones on Wednesday 20th of March 2002 07:38:06 PM

03-20-2002

Registered User

You are partially correct! The data does traverse down the OSI model on the source node.......across the network medium.......to the destination node....and back up the OSI model.

BUT, transmissions do not have to actually start at the application level. Have you ever heard of two or three tier routers? They only use part of the OSI model.....hence two or three layers are transmitted....not all 7.

It goes like this:

Data to be sent in this datagram = XXXXXXXXXX

Layer 7 puts on application specific headers:

AAXXXXXXXXXXA

Then Layer 6 for presentation:

PPAAXXXXXXXXXXAP

Then Layer 5 does the same on a session basis:

SSPPAAXXXXXXXXXXAPS

Layer 4 for transport portion:

TTSSPPAAXXXXXXXXXXAPST

Layer 3 goes next:

NNTTSSPPAAXXXXXXXXXXAPSTN

Layer 2 puts on the Data Link layer stuff:

DDNNTTSSPPAAXXXXXXXXXXAPSTND

The finally the Physical layer:

PPDDNNTTSSPPAAXXXXXXXXXXAPSTNDP

This is then transmitted. At the destination...each layer strips the corresponding layer and sends it up to the next layer....etc.

This is very basic...it's more complex than this tutorial...each layer does not have the same size of headers...and such...but this gets the idea across.

thomas.jones

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THUBAN(1)						      General Commands Manual							 THUBAN(1)

NAME

       thuban - interactive geographic data viewer

SYNOPSIS

       thuban [thuban-session-file]

DESCRIPTION

       This  manual page documents briefly the thuban command.	This manual page was written for the Debian distribution because the original pro-
       gram does not have a manual page.

       thuban is an interactiv geographic data viewer. Thuban can read geographic data in the shapefile format. To control the	visual	appearance
       of a layer you have to select the layer in the session window of thuban.  Afterwards you can change the layers color with the Layer-menu.

       You  can  load  the layers table with the Layer-table-menu. Afterwards you can query the table for feature selection. You can also load new
       tables with the Table-menu and make a table join to the current layer table.

       Maps can be printed or exported to the PS-format by Map/print.

       All changes can be saved in a thuban session file. If a thuban session file has been indicated on command line it will be loaded by thuban.
       If no thuban session file has been given thuban starts with a new session.

SEE ALSO

       http://thuban.intevation.org/

AUTHOR

       Thuban was written by Intevation GmbH, <bh@intevation.de>

       This manual page was written by Silke Reimer <silke@intevation.de>, for the Debian GNU/Linux system (but may be used by others).

COPYRIGHT

       Thuban may be copied and modified under te terms of GNU General Public License.

								  July	28, 2003							 THUBAN(1)