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Full Discussion: The OSI-model
Special Forums IP Networking The OSI-model Post 17863 by thomas.jones on Wednesday 20th of March 2002 07:38:06 PM
Old 03-20-2002
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.
 

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pods::SDLx::Layer(3pm)					User Contributed Perl Documentation				    pods::SDLx::Layer(3pm)

NAME
SDLx::Layer - Storage object for surface and position information CATEGORY
Extension SYNOPSIS
use SDLx::Layer; use SDLx::LayerManager; use SDL::Image; use SDL::Surface; use SDL::Video; # creating layers my $layer1 = SDLx::Layer->new( SDL::Image::load('image1.png'), {userdata => '7'} ); my $layer2 = SDLx::Layer->new( SDL::Image::load('image2.png'), 100, 200, {userdata => '42'} ); # creating the manager that holds the layers my $layermanager = SDLx::LayerManager->new(); $layermanager->add( $layer1 ); $layermanager->add( $layer2 ); my $display = # create your video surface here $layer1->foreground; printf( "%s ", $layer1->behind->[0]->data->{userdata} ); # prints 42 DESCRIPTION
A layer (see SDLx::Layer) is an SDL::Surface, the position of the surface on screen and some additional information, e.g. ingame states. METHODS
new my $layer = SDLx::Layer->new( $surface ); my $layer = SDLx::Layer->new( $surface, %data ); my $layer = SDLx::Layer->new( $surface, $pos_x, %data ); my $layer = SDLx::Layer->new( $surface, $pos_x, $pos_y, %data ); my $layer = SDLx::Layer->new( $surface, $pos_x, $pos_y, $clip_w, %data ); my $layer = SDLx::Layer->new( $surface, $pos_x, $pos_y, $clip_w, $clip_h, %data ); This constructs the layer object. See how you can omit the position and dimension of the layer. The hash %data is for your use only. The layer object just pass it through. index my $index = $layer->index; The method "index" represents the z-index of this layer within its layermanager. x my $x = $layer->x; This is a shortcut for $layer->pos->x. y my $y = $layer->y; This is a shortcut for $layer->pos->y. w my $w = $layer->w; This is a shortcut for $layer->clip->w. h my $h = $layer->h; This is a shortcut for $layer->pos->h. surface my $surface = $layer->surface; my $surface = $layer->surface( $new_surface ); Example: SDL::Video::blit_surface( $layer->surface, $layer->clip, $destination_surface, $layer->pos ); This method let you retrieve the current or set a new surface. pos my $rect = $layer->pos; The method "pos" returns an SDL::Rect object. The pos x and y are stored there. Example: SDL::Video::blit_surface( $layer->surface, $layer->clip, $destination_surface, $layer->pos ); clip my $rect = $layer->clip; The method "clip" returns an SDL::Rect object. The clip width and height are stored there. Example: SDL::Video::blit_surface( $layer->surface, $layer->clip, $destination_surface, $layer->pos ); data my %data = %{ $layer->data }; my %data = %{ $layer->data( %new_data) }; This method returns the hash %data. You can set %data by passing a hash. ahead my @layers = $layer->ahead; This method returns all layers that are ahead of the given layer. Ahead means that a layer has a higher z-index and is blitted over the given layer. Note: This method doesn't check for transparency. This will change in future versions. behind my @layers = $layer->behind; This method returns all layers that are behind of the given layer. Behind means that a layer has a lower z-index and is blitted over the given layer. Note: This method doesn't check for transparency. This will change in future versions. attach $layer->attach( $x, $y ); This function makes the given layer sticky to the mouse. If you move the mouse the layer will follow. The layermanager blits this layer at last, so they will appear on top of all layers. $x and $y should be set to the coords of the mouse, e.g. the coords of the mouse click. If you omit $x and $y the layer obtains them via SDL::Events::get_mouse_state. Note: The z-index is not changed for the given layer. detach_xy $layer->detach_xy( $x, $y ); "detach_xy" detaches the previously attached layer to the given coords. The upper left corner of this layer will be at $x and $y. foreground $layer->foreground; This method moves the given layer to the foreground so that it is blitted on top of the other layers. BUGS
Report at sdlperl.ath.cx SUPPORT
#sdl irc.perl.org AUTHORS
See "AUTHORS" in SDL. COPYRIGHT
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. The full text of the license can be found in the LICENSE file included with this module. SEE ALSO
perl(1), SDL(2). perl v5.14.2 2012-05-28 pods::SDLx::Layer(3pm)
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