The Open Systems Interconnection (OSI) Seven Layer Model Part 2
Following on from part 1 of this tutorial, we will now take a look at some of the technologies that are used to realize the OSI conceptual model and illustrate how data is transmitted up an down the stack. The diagram below helps to show one of the the benefits of having a layered approach to data networking.
You can see that the only end to end layers are 4 to 7. Indeed connections are made and managed between end stations by the the session and transport layers at 5 and 4. Layers 3 to 1 are not end to end, but node to node. The layered approached lends itself to the concept of a routers at layer 3 and switches at layer 2. The router in the above example only need operate as high as layer 3 as it is only interested in network addresses to help get the data to the destination.
Data being transmitted from one host to another migrates down the stack from layer 7 and onto the wire at layer 1. On reaching the router it is passed up layer 3 so that the network layer information can be inspected and a routing decision made. It is then passed back down to layer 1 on the relevant router interface. At the destination host, the data is passed up the layer 7.
The TCP/IP Model Applied to the OSI Model
The diagram below shows how TCP/IP is applied to the OSI model.
IP operates at layer 3 as it is a routed protocol. TCP operates at layer 4 as its function is error correction and flow control between the sending and receiving hosts. The applications, of which there are many, operate at layers 5 to 7. A full list of applications running over TCP/IP can be found at this website.
It is also worth noting the terminology for data at the various layers of the stack. Data at layers 5 to 7 is simply called data. Data or a datagram at layer 4 is called a segment. A Datagram at layer 3 is a called a packet. A datagram at layer 2 is a called a frame. At layer 1 the data is simply a bit stream on the wire.
There TCP/IP model is built on RFC standards that are open to all. There are also some manufacturer specific implementations that are now largely redundant. Novells’ IPX/SPX model is shown below and is still in use today in some local area network environments.
There are others are I would encourage you to explore them. Appletalk, DECnet and Banyan Vines are but some. These are all largely part of history now, but elements of these can still be found in some data networks todays.
Finally, it is worth noting how data is transferred through the stack. The diagram below shows data from layer 7 traversing down the stack. At layer 4 the data is encapsulated in a segment header (H1). The data in not changed, it is simply encapsulated. At layer 3 a packet header (H2) is added to the data and the segment header so that too is encapsulated. At layer 2 a frame header (H3) is added to encapsulate the lot prior to transmission on the wire. At the receiving end, each header is stripped in turn as the datagram passes up the stack until the original data reaches layer 5 and above.