Osi

[geepod]

This is my first tutorial so i hope it is ok and i have tried to keep it simple until i get into the swing of the tutorial forum.

This is my interpretation of the OSI reference model which is the Open Systems Interconnect model designed by the ISO (International standards organisation) whose acronym also means equal in greek.

When you ask your computer to do something which involves some type of networking then the information will go through a layer which can visualised with the frame of reference provided by the ISO.

The OSI model allows us to see what gets appended to our packet and why, before it eventually becomes a serial bit stream of 1's and 0's on the network medium whether that be a bound or boundless cable or wireles etc) media.

The model is split up into 7 layers and at each layer operates different services,protocols and applications and devices.

Upon sending the information it is appended from the top down where as upon receipt the information is stripped of its OSI layer information until it becomes the original sent information again.

The layers can be remembered with various mnemonics but the one that i made up and seems to work well for my students (sorry ladies if this offends but it is all about remembering and i defy you to forget it with this one)


All
Pussy
Seems
To
Need
Deeper
Penetration

or what it should be

A pplication
P resentation
S ession
T ransport
N etwork
D ata link
P hysical

Upon receipt the information is stripped of its appendages from each layer by each layer until it returns back to the original information. .

Example (simple explanation) of printing from IE to a network printer.

Application Layer
IE and HTTP both operate here so they will be responsible for sending your print request and the information to be printed down to the next layer the presentation layer.

Presentation Layer
Here the print request and data information will be maybe compressed for smaller size transmission and also perhaps encrypted for secure traversal of the network along with redirector information such

as a service like the server service or workstation service (a service is a daemon in *nix) and is then sent further down to the session layer.

Session Layer
Here things like the RPC or remote procedure call will be added allowing the remote request to be executed, also synchronisation info or maybe the netbios name of the printer etc.

Transport
This layer is responsible for error free transmission of the now packet onto and through the network. TCP for example will establish the threee way handshake responsible for the alleged integrity of the

connection or session. Perhaps NWlink is being used so this info will be added such as frame type etc. It is this layer that takes our information and turns it into a packet or datagram which is then turned into a binary for transmission

Network
The packet needs to contain source address and destination address information so it here at the network layer where the senders IP address is added and the destination IP iaddress, it will also be responsible ofr routing information such as subnet mask etc and router or gateway addresses.

Data Link
Which is further sub layered into Logical Link Control and Media Access Control, here perhaps your network is token ring so that information needs to be added or perhaps it is ethernet so that will be added. Also the NIC driver info needs to be added so that the packet can be sent through the NIC onto the medium for transmision. it is now a parallel bit stream of 1's and 0's ready for the physical layer.

Physical Layer
Here the stream will be transmitted in a serail bit stream out onto the actual medium be that cable,wireless, blah blah.


now the information will tavel the network in a matter of milliseconds to the destination where the physical layer there will convert it back to a parallel form for the NIC and the upward travel through the

OSI layer where the appendages are then stripped until finally the required app or service in the application layer will handle the original data.



This is of course is a over simplified view of the process but hopefully will be of benefit to newbies maybe or indeed anyone else who can use it. There are also alot of other info added at each layer and

some services and protocols etc operate at more than one layer such as the gateway.

There is also the DOD or TCP/IP 4 layer model which was used befroe this OSI but the two do directly map to each other (sort of) but that is for another tutorial depending on the response to this one.

The attached file is a print out i made for my students as a handout so i hope it makes sense.

Enjoy and live well and love life and dont take life too seriously

[d00dz Attackin]

Thats a great tut there geepod. That is what it's all about . Greenies for you and keep up the great work.

Cya

[CS4Life]

Excellent tutorial! Kudos to your for the OSI layers acronym.

[Networker]

Good post, though just a remark

Originally posted here by geepod
[B]
Transport
This layer is responsible for error free transmission of the now packet onto and through the network.

This is not entirely true!
UDP for instance is a transport protocol that even does not rely on bidirectionnal flows.
UDP role is to link the packet this the good upper layer entity.

[geepod]

yes i understand what you are saying networker ! i wasnt stating that the transport layer was only responsible for that however it is responsible for error free transmission i just didnt state that UDP was connectionless etc. I did state it was a over simplified view and was not concise but i get your point.

i didnt mention alot of things or protocols or how they are handled by each layer i was just giving a overview but thanks anyway

[souleman]

TCP/IP follows the DOD standards. It doesn't follow the ISO/OSI model. You can get a basic mapping between the two, but the DOD model only has 4 layers instead of 7, so there are some differences.

[cyclops07]

, at last.. there is something new to the OSI layers acronym...

years b4... i only remembered it as "Please Do Not Touch Steve's Pet Alligator".. i referred it in some network books... maybe they should change it now..

[Ghost_25inf]

Heres a layout of the TCP/IP in referance to the OSI Model

------ OSI Model Layers -------

7. Application----Telnet -----FTP ------DHCP------TFTP

6. Presentation ------HTTP-------SMTP------DNS------SNMP

5. Session
Application Layer
_____________________________________________

4. Transport ------------TCP ---------------------- UDP

Transport Layer
_____________________________________________

3. Network ---- ICMP ----- ARP --------- RARP -------IP

Internet Layer

------------------------------------------------------------------------

2. Data Link Network Interface Layer


1. Physical


This is TCP/IP compared to the OSI model

[geepod]

TCP/IP follows the DOD standards. It doesn't follow the ISO/OSI model. You can get a basic mapping between the two, but the DOD model only has 4 layers instead of 7, so there are some differences.

by souleman

didnt i already say that ? in my original post ?

There is also the DOD or TCP/IP 4 layer model which was used befroe this OSI but the two do directly map to each other (sort of) but that is for another tutorial depending on the response to this one.

geepod

[mandraketux]

Wow, great site here, but I think you could make an entire message board of all the OSI model Tutorials. I think i saw at least 20 different posts explaining it. Not saying this ones bad, it's good, but wow there is alot of posts and tutorials on it. *sees word greenies* huh? *off to read FAQs*