Review Part I: The OSI Model

ThePistonDoctorThePistonDoctor Member Posts: 62 ■■□□□□□□□□
Hi all,

Since I am taking my Network+ test tomorrow I've decided to throw up a few posts throughout the evening/morning (working an to help reinforce some of the things I've read are most prominent on the Network+ exam. This will be done with no notes so I get a true idea of where my understanding lies. Here is Part I: The OSI Model...

The OSI model was developed by ISO (The Internation Organization for Standardization). It stands for Open Systems Interconnection, and defines a methodology for systems of different types to interact with each other. There are 7 main layers in the OSI model and two sub-layers. The layers, in order from top to bottom (or closest to user to furthest from user) are as follows:

7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Datalink
--MAC sub-layer
--LLC sub-layer
1. Physical

The functions of the layers are defined below as I understand them. These are somewhat simplified but get the point across:

7. Application - This layer takes user input from the actual terminal and converts it into data that can be used by the other layers of the model. It also provides file and print services to the different nodes on the network. HTTP, SMTP, FTP, POP3, SFTP, TFTP and HTTPS function here.

6. Presentation - The presentation layer is responsible for conversion, compression and encryption of data. It is what defines file types like GIF, JPG, PNG, etc. It converts data between different formats so that the other layers in the model can use it.

5. Session - Creates, manages and closes sessions between applications.

4. Transport - Creates, manages and closes sessions between devices. Also it is responsible for error checking and segmentation (breaking data from the upper layers into segments or "packets" which can be used by the network layer). TCP, UDP AND SPX function here.

3. Network - The network layer is responsible for deciding what the best path for data to take is. It utilizes protocols such as IP, RIP, OSPF, IPX and ICMP to accomplish this. Routers function at this level.

2. Data Link - Responsible for converting data into a format that can be used by the physical layer (i.e. 1s and 0s). The data link layer has two sub layers: the LLC sublayer (related to standard 802.2 by the IEEE but some clarification on how would be nice), and the MAC sub layer which is responsible for physical addressing. Switches, bridges, NICs and WAPs operate at the data link layer.

1. Physical - The physical layer is where the magic happens. NICs operate here (and somewhat at the data link layer), as well as hubs, and it is responsible for actually sending the data as 1s, 0s and electrical signals over the wires. The physical layer defines voltages, electrical signals, cable types, lengths, and speeds. Any device that amplifies or modifies the signal (repeaters, signal boosters, etc) operates at this level.

I think I understand this stuff pretty well. I'm primarily concerned with which protocols and devices operate at which layers, so please correct me if any of that stuff is wrong or add to it if it is missing something important. There will be three other parts to this review: Cabling and standards, port numbers, and one other thing, which I haven't decided yet. Basically, i'm going to spam all you guys with my knowledge to help myself icon_lol.gificon_lol.gif

Thanks for any input!!!


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