Network Layers (the OSI model)

[devildell]

hello all, remember me. sorry i have not psted in a while, i've been studying!!!!!

i need your help.

in my college text book it is explaining the OSI model and the diffrent network layers. however it is a bit confusing to me.

1. Application
2. Presentation
3. session
4. Transport
5. Network
6. Data link
7. Physical

can someone please explain what each of these layers do and how they intract with each other in a way that i can understand.


thanks........ NOW BACK TO MY BOOKS!!!!

[KorpDeath]

First off so you don't get conmpletely confused alter on, the numbering goes the other way. 1 is the physical layer, 2 the data link layer, etc.etc.

For more info: http://webopedia.internet.com/quick_ref/OSI_Layers.asp

enjoy.

[|3lack|ce]

The International Standards Organization developed the OSI model in an attempt to standardize the way computers communicated between one another. Below is a link to a site that nicely describes what each layer does in plain english. Hope it helps!

http://www.tcpipguide.com/free/t_Und...lAnAnalogy.htm

BTW I googled for this site. Just a suggestion for future questions like this.

[Relyt]

Good Evening,


Sure, however you will also need to learn the very specific nomenclature in order to pass the exams and to continue on in the class. Because that verbage will come to haunt you later on as the class progresses, if’in you don’t. The links above will help a lot.

Well when I learned it we used a little phrase, “All People Seem To Need Data Processors”

Application Layer (Layer Seven): applications – email, word, etc. Closest to the user, provides network services to the user. However it does not provide services to any other layer of the model.

Presentation Layer (Layer Six): translates the data between multiple formats. Ensures the data sent by the Application Layer is readable by the application layer of another system.

Session Layer (Layer Five): opens and establishes a communication channel. Manages & terminates sessions. Provides services to the Presentation Layer.

Transport Layer (Layer Four): packages info, error management, segmentations, number, etc., into 1 kb blocks. Segments & reassembles data into data streams. Think of flow control and fault detection.

Network Layer (Layer Three): Provides connectivity and path selection between two hosts. Think of a house with several rooms being its own Host(network) and this layer provides the connectivity between different Hosts.

Data Link Layer (Layer Two): Mac (Physical) Address. Provides reliable transit of data across a physical link. Also think of line discipline and error notification.

Physical Layer (Layer One): actual cable etc.

Good luck

[CybertecOne]

IMO, asking alot of different people a question results in getting alot of different answers, some even more confusing than the college text book of yours.

sure , look around and learn some things but dont get caught up in what other people know, your there to learn from a book and college, which is unified in the way it teachers and expects you to learn...

i just dont want you to confuse yourself further

[hobbdebub]

Hey there,

Physical
The physical layer is pretty much the most straight forward because it consits of the phsical networking components such as the Cat5 cable or phone line.

Datalink
The datalink acts as the organizer of the system. It gives error control when frames are missing/damaged. It also regulates the speed of the data transmission.

Network
The network layer is responsible for the control of the subnet. It is the part that controls the route for the packet being sent. It also keeps up with how much data is sent.

Transport
Pretty much the main function for the transport layer is to recive data from the session layer and break it up into 'pieces' and pass it onto the network layer and make sure the 'pieces' arrive at the destination alright. It also is responsible for establishing the correct network connection as set forth by the session layer. A good example of a protocol that uses the transport layer is TCP.

Session
The session layer establishes a session between two computers. The main purpose of this layer is to enable syncronization so thatcollision doesn't happen through transmission.

Presentation
This layer is used to format information being sent or recieved by the two computers according to preferences. An example of this is an EBCDIC text file is converted to an ASCII file.

Application
This layer allows communication between two different apps on two different machines, this layer deals with file name conventions and protocols used on different systems and provides conversion between the two if needed.

I know this isn't very in-depth, but I hope it has helped you a bit.

<edit>Sorry Relyt! When I began my post only Korp had posted.</edit>

[phishphreek]

Originally posted here by Relyt

Presentation Layer (Layer Six): translates the data between multiple formats. Ensures the data sent by the Application Layer is readable by the application layer of another system.

Session Layer (Layer Five): opens and establishes a communication channel. Manages & terminates sessions. Provides services to the Presentation Layer.

Transport Layer (Layer Four): packages info, error management, segmentations, number, etc., into 1 kb blocks. Segments & reassembles data into data streams. Think of flow control and fault detection.

Network Layer (Layer Three): point to point. I.E. from one room in your house to another. Finds source and destination. Provides connectivity and path selection between two end systems.

Data Link Layer (Layer Two): Mac (Physical) Address. Provides reliable transit of data across a physical link. Also think of line discipline and error notification.

Physical Layer (Layer One): actual cable etc.

If I may elaborate a bit on your explanations of layers 2 and 3.

You give an example in layer 3 that may be more appropriate for layer 2.

point to point. I.E. from one room in your house to another

I would think of this as a switch... because not many people are going to have separate routers in each of their rooms. They'd most likely have a switch, which would be layer 2. (unmanaged... I realize that some switches operate at more than layer 2)

Layer 3 would be network to network... which requires routers to route from one network to another.

I agree with the rest of your post though! Just thought that it may be confusing. When I think of a house, I normally think of a small network which you would generally have one router and switches or hubs (hubs are layer 1). There is no real need for multiple routers unless you want to keep your boxes separate on different subnets... like I do. ex: One network for wired, one network for wireless, one network for bluetooth, etc. Not to mention plenty of access controls between the networks... but thats a whole other story.

[Relyt]

phishphreek80

Thanks much and I'll tweak it. Ole Devildell should have quite the study guide now!

cheers

[devildell]

thanks, i think i got a britty good grasp of it now, heres another question for you.

in the network layer we have the IP address
in the data link layer we have the mac address

what is the diffrence between how the two are used?


sorry to pick your brains so much, but i have savrial question mainly because the instructer of the class just basicly gave us the book and said the test on chapter one will be in one week, CLASS DISMISSED....

<EDIT>
i just answord my own question. i pulled out my non college textbook Using TCP/IP
by John Ray
</EDIT>

[Relyt]

devildell

Glad to help. Since you found MAC & IP addresses in your book, I won't go into them much. But here's some characteristics of both addresses which you might need to know:

A MAC address is 48 bits in length and is expressed as twelve hexidecimal digits. The first six ID the mfg and is the Organizational Unique Identifier (OUI).

IP address is considered a “Dotted-Decimal” number and is 32 bits in length. (ie 192.168.5.1)


Since we are talking about the OSI Model you might also encounter "Encapsulation"

The Encapsulation Process is: Data &gt; Segments &gt; Packets &gt; Frames &gt; Bits.

The Data from the Application, Presentation, and Session Layers (considered the upper layers 7, 6, & 5) is placed into Layer 4 (Transport Layer) Segments. The Segments are packaged into Layer 3 (Network Layer) Packets. The packets are placed into Layer 2 (Data Link Layer) Frames. And the frames are placed as a series of Bits on Layer 1 (the Physical Layer)

cheers