gwilks98

I wasn't sure which forum would be appropriate for this thread...

But I'm confused on the OSI model of networking (the seven layers). Is there anyone here who could explain briefly how data is sent across them. Is data sent on only one layer? Or is it fragmented and different pieces are sent on different layers? Or are the 7 layers just different "magnifications" on the same data?

Any help would be appreciated.

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CluelessSi

the different layers are just a model that systems try to follow. Some layers are physical (hardware) some are purely software. umm... like programs such as this browser will have to contact other layers (programs such as OS) to do things like show you this message or to post another message. I am not sure what you want to know but there is not a concreate 7 layers. it is just a model and some programs might span several layers... haha i guess kinda like how MS wants browsers to be part of OS =P and screw up the model

http://www.isrv.com/~jhunter/html/PR...el.shtml#30757

shows a bit on how they communicate

http://www.netc.org/network_guide/c.html

shows a small desc about the model... there are many other sites and textbooks that explain it in more detail



[Edited by CluelessSi on 06-04-2001 at 11:41 AM]

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zenbooty

The OSI model is a methodology for designing and implementing network systems. In the OSI methodology, the network can be broken down into seven layers of abstraction, each layer relying on the one beneath it to service its needs, and each helping the one above it satisfy its needs. The actual specifics of how data is passed from layer to layer is dependent on the protocols designed which fit the methodology. THink of the model as a very loose set of guidelines for how to go about designing the components and protocols that make up your network. I think the best way to describe them is individually:

Layer one (bottom): Physical layer - this layer deals with the physical specifications of how bits get sent between two adjacent points on a network. Stuff like voltages of signals, how long each signalled bit should be, wiring specifications, etc. Note the OSI model doesn't specify what any of these things should be. The model just tells you that when you design your network system according to the OSI methodology, this is the bottom layer of functionality and will require consideration and design.

Layer two: Data Link Layer - Once we've established how to send bits between two points, the next level of abstraction is to be able to send packets of data between two points. This requires developing data protocols that give nodes a common language to understand one another. Error checking and receive acknowledgements must bew designed to ensure that any signals lost are resent and not duplicated, etc.
Ethernet, Token Ring, and FDDI(Fibre Ring) are the classic examples of layer two protocols.

Layer Three: Network Layer - Now that we can send packets of data from one point to the next, the next layer of abstraction is to be able to send data packets between any two nodes on the network, adjacent or not. This requires the development of protocols that handle addressing and determining the shortest path to send data between nodes to get to the endpoint. Other things to consider include how to gracefully degrade and recuperate when one or more nodes on the network go down (how will the paths get updates?).

Layer Four: Transport Layer - Now that any two nodes on our network can communicate data packets, how should we divide network messages into the bite-size data packets that the network and data-link layer can send effectively and efficiently? How will the receiver know what order these packets should be reassembled in order to get the correctly send message? How do we find errors and recover from them when not all the packets make it? This is the realm that the transport layer involves.

The above four levels are sort of the bottom half of the model, and represent the network's low-level functionality of passing data messages between nodes. The upper levels exist more on the application side of things, and what to do with these messages once we can pass them. The upper levels functionality is often a bit more nebulous, with the division between layers being less cut and dry.

Layer Five: Session Layer - Now that we can send messages from one computer to another, we need to present abstract constructs that programmers can use to access this functionality. Sockets and pipes are examples of layer five-based technologies.

Layer Six: Presentation Layer - The presentation layer handles data manipulations that occur just beneath the application layer of the model. Encryption of data and language-based data conversion are good examples of layer six components.

Layer Seven: Application Layer - finally at the top, this layer represents the actual applications that users can utilize to access the network. FTP, Telnet, and Netscape can be considered layer seven components.

Whew! Hope this helps clear things up.

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