Presents
The OSI Model
Presents
The OSI Model
| The OSI Model, or Open Systems
Interconnection Model, was first developed in the 70's by the US Department
of Defence. The name Open Systems Interconnection basically describes what
it is, a standard which allows disparate systems to run over a common networking
infrastructure. The primary objective of the OSI model is to simplify the
way that developers and hardware manufacturers integrate their products
into a networked environment. Without it, we would have proprietary methods
and protocols for delivering streams of data across our networks, just
like the bad old days.
It is important to realize that this is a reference model and in the real world it is not strictly adhered to. TCP/IP for example, uses a four layer model which is mapped to the OSI model as the diagram below shows. On the left are the seven layers of the OSI model and on the right are the corresponding TCP/IP layers. However, if you are studying anything to do with networking, you will need to learn the OSI model and remember all the layers and what they are responsible for, and how the data is encapsulated. The seven layers in the Model each plays its part in the way that data is communicated between networked devices. Also, because each layer is independent of the layers above and below itself, it thinks it is communicating directly with its peer at the other end. Lets take a look at the model and see what each layer does. |
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The application layer is also responsible for synchronizing with the application being used. This data is then sent to the next layer down, the Presentation layer. |
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Some presentation layer protocols
are:
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Examples of session layer
protocols are:
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In a Connection Oriented transmission, the receiving machine sends back an acknowledgement after receiving so many chunks of data, this lets the sending machine know that everything is working OK. The mechanism for doing this requires additional information to be added to the data segment in order for the 'hand shaking' to work, unfortunately this also makes it slower. Connectionless transmissions just send packets one after another and hope for the best! This may not sound like the most reliable way to communicate, but if the connection is sound then it is much faster than the Connection Oriented method because it doesn't have the overheads of a connection oriented protocol. Some transport layer protocols
are:
Encapsulation type at the Transport Layer: Segments |
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Some Network Layer Protocols
are:
Encapsulation type at the Network Layer: Packets |
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The Data Link Layer (DLL) consists of two sub-layers, the Logical Link Control (LLC) and the Media Access Control (MAC). The Logical Link Control uses Connection Oriented or Connectionless control (where have I heard that before?) and provides the interface to the Network Layer, "why do we need more flow control?" I hear you ask. Well we don't really, so its not used much these days, but it's nice to know that its there if we do need it. The MAC sub-layer adds the MAC address to the packet and transforms it miraculously into a frame! The MAC address is read from the Network Interface Card (NIC). All network cards are given a MAC address when they are manufactured, this address is unique to each card and so no two cards should ever have the same number. This is sometimes referred to as the 'burned in' address. Encapsulation type at the Data Link Layer: Frames |
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This layer takes the frame of data and puts it onto the wire at the back of our machine. The Physical layer includes all the copper and fibre cables as well as hubs and repeaters on our LAN. Encapsulation type at the Physical Layer: Bits |
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Down the Wire
| So our data is now on its
way, first across our Local Area Network using the MAC address to get to
the router where it is passed out onto the internet. The router doesn't
need to know about the MAC address so it strips it off and reads the logical
address (IP) that was added at the Network Layer. Now it can be routed
through the internet until it gets to the recipients LAN. Here the router
reads the IP address and adds the corresponding MAC address so it can be
delivered to the recipients machine via the LAN switches or hubs.
Once it get to this machine it goes back up through the seven layers of the model and eventually reaches the application layer. Here it is displayed on the recipients machine in the correct format for the particular application that it is associated with. Well that's it in a nutshell, and there are plenty of books and web pages that explain this process in more detail, however, if you are studying anything to do with networking you need a way of remembering the layers. |
How to remember the OSI Seven Layer Model
| OK you may have noticed
the images next to the descriptions for each layer, and maybe you've
worked out what each one represents, if not let me explain.
The use of images is an excellent way to memorize all sorts of things because the mind seems to store them more easily than words. An unusual connection between the image and the information to be memorized is used to commit the data to memory, in fact, it has been my experience that the more bizarre the connection is, the easier it is to learn. |
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This image for the top layer
should make you think: Apple = Application.
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The gift or 'present' should make you
think Presentation
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Meet Clive, he's a musician, a session musician and he likes to play Jive. Think 'session' musician, and Clive & Jive rhymes with Five, Layer 5. |
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Well,... trains are a type
of transport and train starts with the same letters as Transport.
It has 2 small wheels and 2 big wheels,
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OK the net is pretty obvious to represent the Network layer and the packet of cigarettes should remind you that the network layer converts the segments from the transport layer into 'Packets'. The reason I've put the word Datagram in the picture is mainly because we need 3 objects to represent Layer 3, but also because packets are sometimes referred to as datagrams. |
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So..... the 'date' in the image should remind you of 'Data Link' and the number 2 means Layer 2. The frame around it will remind you that the data link layer converts the packets from the network layer into 'Frames'. |
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Now we're getting really
tenuous!..... The runner is performing a 'physical' activity (Physical
Layer) and there is one piranha representing Layer one.
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Study each of the pictures (or make up your own), and you will find that it's much easier than any other way to remember the OSI Seven Layer Model. If you need to remember other aspects of the OSI model, such as the different protocols used, then simply visualize them as something obscure and add them to the picture. For example, one of the transport layer protocols is UDP, so this could be underpants .............What?........... oh, here we go again, one track mind!!! If you are not very good at drawing just use clip art like I have, there are plenty of sites on the web with "free clip art" although finding one which is actually free to use is quite difficult. One source that is very useful is Google, click on the 'Images' Tab and then type in your search keyword. Good luck, and please let me know if this tutorial-ette has been useful. |
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By the way, if
you find all of the above isn't working for you, try this image. Each Layers
name has its number in the word. Some of the numbers are upside down or
back to front but it still works.
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