In the early 1980s the International Organization for Standardization started work on a set of protocols designed to promote open network environments. These were essential to allow the multi-vendor computer systems to all talk to each other using internationally accepted communication protocols. These standards and protocols eventually developed into the OSI reference model.
The protocols defined in each layer of the model have different responsibilities but generally these fall into two specific categories.
- Communicating with the same level protocol layer on another computer.
- Providing services to one layer above it.
This peer level communication offers a method in which each layer can exchange messages or other forms of data. The model is the same whether you’re routing through a US IP address to Netflix to a secure communication link to an application server. For instance, the transport protocol is able to send a transmission requesting a pause to a peer computer in the sending computer. It’s able to do this not using a direct connection but by placing a message in the packet where it is managed by the layer below. All lower layers must provide this service to the layer above them taking messages and passing them down to the lowest level of the protocol stack. At this point they can be transmitted across the physical link to the remote destination.
It is important to remember that the OSI is merely a reference model in that it provides a general description of what services should be provided at which layer. The OSI model itself does not provide any specifics of the standard protocols. In fact you’ll often find the OSI model used to define all sorts of other protocols including TCP/IP for example. The Internet Protocol was often described as a network layer protocol, purely because it performs the same functions that are defined in the network layer of the OSI reference model.
The ISO did create some protocols that followed the OSI model, however these were never widely adopted and you’ll be lucky to find them in use anywhere. The main reason for this was the popularity of a rival communication suite – TCP/IP.
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