How do you apply/use the OSI MODEL?
 

Could somebody give me a basic network and list how the OSI model was used, I would appreciated it????

I've studied on my own, in theory i know what it is, but I am clueless as to how I am suppose to apply it. This is what I know...


Layer 1: Physical layer

The physical layer defines all the electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. Hubs and repeaters are physical-layer devices. The major functions and services performed by the physical layer are:

establishment and termination of a connection to a communications medium.
participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling -- copper and fibre optic, for example. SCSI operates at this level.

Layer 2: Data link layer

The data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. The addressing scheme is physical which means that the addresses (MAC address) are hard-coded into the network cards at the time of manufacture. The addressing scheme is flat. Note: The best known example of this is Ethernet. Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and LLC and Aloha for local area networks. This is the layer at which bridges and switches operate. Connectivity is provided only among locally attached network nodes.

Layer 3: Network layer

The network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing, flow control, segmentation/desegmentation, and error control functions. The best known example of a layer 3 protocol is the Internet Protocol. Routers operate at this layer -- sending data throughout the extended network and making the Internet possible (there also exist layer 3 (or IP) switches). This is a logical addressing scheme - values are chosen by the network engineer. The addressing scheme is hierarchical. This layer can be of least significance in case of Broadcasting Networking.

Layer 4: Transport layer

The transport layer provides transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer. The transport layer controls the reliability of a given link. Some protocols are stateful and connection oriented. This means that the transport layer can keep track of the packets and retransmit those that fail. The best known example of a layer 4 protocol is TCP.

Layer 5: Session layer

The session layer provides the mechanism for managing the dialogue between end-user application processes (By dialog we mean that whose turn it is to transmit). It provides for either duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures (keeping a track so as to restart from the very same point where they had left in case of a crash). This layer is responsible for setting up and tearing down TCP/IP sessions.

Layer 6: Presentation layer

The presentation layer relieves the Application layer of concern regarding syntactical differences in data representation within the end-user systems. MIME encoding, data compression, encryption, and similar manipulation of the presentation of data is done at this layer. An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file or serializing objects and other data structures into and out of XML.

Layer 7: Application layer

The application layer interfaces directly to and performs common application services for the application processes. The common application services provide semantic conversion between associated application processes. Examples of common application services include the virtual file, virtual terminal (for example, Telnet), transfer and Manipulation protocol" (JTM, standard ISO/IEC 8832).

Re: How do you apply/use the OSI MODEL?
 

i didn't read every word of your post - i'm shot & about to hit the hay - but it seems like you've got it all down pat. as far as how would you apply it, that sounds too much like an assignment of some type, so i'm not going to answer it directly. why don't you do this, ask yourself why is the scheme broken down into layers to begin with - what's the pro's & con's - and then, also, outline a transaction (data transmission) from the user of one pc, through the pc & it's local network, out to the "internet cloud/" through the target pc's local network, target pc & ultimately the interaction with the target user. From what you've listed here, and from what you say you know, you should be able to do that, and once you do you would have answered your own questions. Good Luck! :)

Re: How do you apply/use the OSI MODEL?
 

My post wasn't my words...but the more i read it the more i realized that the OSI model isn't really a set of rules is more more like a set of guidelines to form and understand a network. I guess my question now is how do I put (it all) each layer together to make a complete network...or maybe i'm still looking at it wrong!?!?!?

ultimately, im trying to learn how to build a LOCAL and WIDE area network from scratch!

Re: How do you apply/use the OSI MODEL?
 

WoW going to teach yourself huh? Bravo!

Here is a picture that might help you see how the OSI Model works. Foward then reverse from the host computer to the client computer

http://kurtkoenig.homeunix.net/datae..._files/osi.jpg

You may want to check out a few articles on here about Networking, How Stuff Works has a nice article for students to view on the basics.

But you can always just ask the experts around here if you get stumped on something. These guys know a ton, and can give you much more information that you can probably find in a few articles.

If you want some books http://course.com is where my school gets all of there technology books.

Re: How do you apply/use the OSI MODEL?
 

i'd recommend spending a few hours reading at a local public library - even an olderbook (within the last 5 or 6 years) will give you the basics that are still adhered to today (except it undoubtedly won't cover wireless issues to an appreciable degree if it's much more than four yrs old; depending on your needs that may not be an issue).

Re: How do you apply/use the OSI MODEL?
 

I use the OSI to remember what kinds of Pizza to keep (Sausage) and which ones get Thrown Away.

It may be quite old now but Jason Nash had a very nice description in his MCSE study guide (NT4) for Networking Essentials (IDG Press?) another great one to look at is the Cisco Press study guide for "Intro" by Odum which is still in press (ISBN 1587200945)

Re: How do you apply/use the OSI MODEL?
 

I'm going to gank Lightninghawk's pic... hope he doesn't mind:

http://kurtkoenig.homeunix.net/datae..._files/osi.jpg

Layer 1 consists of the "physical" stuff-- the things you could touch and manipulate, like wires, and the specifications of how the data is transferred, like +5v for 1, +0v for a 0.
Layer 2 deals with MAC addressing, mainly. ARP is a tool that translates IP addresses to MAC addresses, and RARP does the opposite.
Layer 3 is IP, mainly. That's where your IP address comes into play, and that's where routing functions take place at. Again, IP addresses mean nothing if they don't correlate back to a MAC address on Layer 2.
Layer 4 is TCP, or UDP, or ICMP, even. This specifies what "port" to listen on for a particular service. I equate this out to being in an Office building-- you've arrived at the building (the IP address), but you still need to specify what office you're going to (the TCP port number)
Layer 5 deals with entering, leaving, and staying in that office. "Handshaking" is a common term to see in this layer.
Layer 6 takes the data we've recieved from the lower layers, and formats it properly for the requesting application. This deals with things like character sets (ASCII, EBCIDIC, Unicode, etc)
Layer 7 is where all of the data is actually worked with. The previous layers were just there for the data to reach the final endpoint of the application. This can be anything from a web browser, a MUD client, or a database server application.

...That's how I've always understood it. That understanding was good enough to get me a degree in this stuff, at least.

Re: How do you apply/use the OSI MODEL?
 

THANKS!!!...see my response below in your quote.

Re: How do you apply/use the OSI MODEL?
 

This may help or hurt but here is a real basic example of how the layers work.

7-Person types words in a new email and clicks SEND
6-Outlook knows it needs to use HTLM formatting for message
5-PGP key is added to the outgoing message
4-A TCP connection to the port on the email server is created
3-The server does a destination lookup and finds the IP of the router
2-Since the router is on the same network as the server it uses the MAC address for all remaining packets
1-the copper wires are pulsed with voltage (those v's) to create 1's and 0's

At the other end the layers coordinate with the originating layer (roughly, not a 101 level course there)

The first chapter of Cisco CCNA INTRO is a MUCH better example if you can get your hands on it.

Re: How do you apply/use the OSI MODEL?
 

I'll respond in line to your questions:

when you're talking about signalling, you've got a base voltage, let's say it's 5 volts. (I forget offhand what ethernet uses). A 0 would equal 5 volts on the line, and a 1 would equal +5v, so it would equal 10volts. That way, if it pulses 5 volts, the NIC interprets that as 0, if it's 10 volts, it interprets that as a 1. The voltages can be arbitrary, but I think they were chosen for a specific reason. What reason? I'm not sure.
You are correct-- MAC addresses are burnt onto the hardware, but they can also be "spoofed", ie, changed, by software. Normally, though, it is burnt in. "Mapping" Just means "translating", or "look up"-- each host carries its own ARP table, and it either performs the lookup for host-> IP address there. If it's not located there, it broadcasts out, "WHO HAS THIS IP ADDRESS?" to the network, and the host with that IP responds with its MAC.

When you say it's a "broadcast" network, that's all you can say-- one host contacts the other essentially by saying, "I WANT TO TALK TO MAC ADDRESS XX:XX:XX:XX:XX:XX", and that host responds to the system.

Sort of. ARP kind of "spans" the layers-- it's a table that keeps up with the level 2 MAC address, and correlates that to the Layer 3 IP address. Routing is all done at Layer 3. Routing tables accomplish this, by saying, "I know this IP address is accessible through this other router 1.1.1.1" Usually, there's a "default gateway", that unknown IP addresses are routed to.
Thanks. The Cisco curriculum didn't make any sense to me. As a matter of fact, the concept of ports didn't make sense to me until I started working with multiple services on a system, and I started messing around with Port forwarding and IP Masquerading on routers. You'll run into that fun later, probably :)

Handshaking isn't really like a ping. A ping would look like this:

System 1: I'm up, system 2, are you there?
System 2: Yeah, I'm up.

Where a handshake basically looks like this:

System 1: I'm ready to send data to you... you ready?
System 2: Yeah, I'm ready-- you ready?
System 1: Oh, I'm SO ready. Here it comes!
kind of silly, but it gets the job done

Really, it could be software, or hardware-- it depends on where it's implemented in the system. (Kind of like how you could have an MPEG decoder card, or you could just have a software codec to do it). Usually, it's done in software.

Typically, this doesn't get mentioned much, because we usually work in ASCII. Look up Character sets to get more information on it-- ASCII uses a certain number of bits to represent a character, EBCIDIC uses another amount, and Unicode uses an entirely different amount of characters to represent that same character. This is where, as I understand it, that translation takes place.

There's always the chance that I'm slightly off. I think that a more precise positioning of the Application layer is at the very end of the process, when the TCP/IP stack hands the data to the actual requesting application. My instructor (keep in mind, this was like, 2-3 years ago) didn't really make too big of a distinction. Basically, I look at the Application layer as the "glue" or "pipe" that actually connects the application to the network.
Trust me, it makes a lot more sense once you start plugging in wires. Also, keep in mind that the OSI model is not the actual model used these days-- the TCP/IP stack is what's used for this. The TCP/IP stack is much more simple:

http://www.pku.edu.cn/academic/resea...ml/TC0102.html
There's only 4 layers:

• Application
• Transport
• Internetwork
• Network access layer

In that one, network access layer is the wires and MAC addressing type of stuff. Internetwork is IP addressing and routing. Transport is the TCP/UDP ports. The Application layer is essentially "everything else"-- where you actually "do stuff with the data". Much more simple, because you don't have to deal with the presentation layer, which is often done implicitly.