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The Open Systems Interconnection (OSI) Seven Layer Model Part 1

If you are looking to get a start in data communications and networking, then this is a good place to begin. Arguably, the Open Standard Interconnection (OSI) seven layer model is the foundation of computer internetworking. The OSI model has been around since the 1970’s and it serves us well today as it did back then. Having said that, the OSI model is being challenged by some up and coning technologies, such as Software Defined Networking (SDN) and Network Functions Virtualization (NFV).

The purpose of the OSI model is the make the implementation of computer networks much simpler and to promote standardization. It represents the functions that are needed to deliver a computer network from the application layer to the physical transmission medium. The OSI model or OSI stack comprises seven functional layers. An underlying principle is that the interface between the layers is standardised, but a variety of technologies can be used to realise the layer itself. For example, Ethernet at Layer 2 will work over copper or fibre at layer 1. IP at layer 3 can be transported over Ethernet or PPP at layer 2. The diagram below illustrates the OSI seven layer model.

The OSI Seven Layer Model

Layer 1 - Physical

Layer 1 defines the physical characteristics of the transition medium. This can be fibre optic cable, Cat 5, twisted pair telephone cable, X.21 and many, many more. One key point is that layer 1 not agnostics to any higher layer and is not aware of any structure of the data frames transported over the medium.

Layer 2 - Datalink

The datalink layer provides for reliable communication between adjacent nodes. It is important to note that datalinks are only established between adjacent nodes and not end to end over a network.  This concept is better illustrated later on. The layer has the ability to correct errors over the link. The Point to Point Protocol (PPP), Ethernet, High Level Datalink Communications (HDLC) are all examples of layer 2 datalink protocols. There are actually two sublayers within layer 2. These are:

Layer 3 – Network

The function of the network layer is to move data from source to destination. Layer 3 uses the concept if addresses to identify nodes on a network.  Protocols developed to perform this function are referred to as routed protocols. In addition, protocols exist at layer 3 to route traffic between nodes. These are referred to as routing protocols.

The Internet Protocol (IP), Internetworking Packet Exchange (IPX), Appletalk are all examples of routed protocols. Open Shortest Path First (OSPF), Routing Information Protocol (RIP) are examples of routing protocols.

Layer 4 – Transport

The Transport layer is a host to host function. In other words communication is established between the transmitting and receiving nodes and not the intermediate network. The layer is responsible to quality of service functions, reliable data transfer and flow control.

The Transmission Control Protocol (TCP),

Layer 5 – Session

The session layer is all about control connection between source and destination nodes. It involve the establishment and tear down of connection. Modes of transmission such as full or half duplex will be taken care of in this layer. As network engineers, we tend not to get involved in these higher layers as they are embedded in the applications. This is illustrated in part 2 when we see some real implementations of the OSI seven layer model.

Layer 6 – Presentation

This is one of the most esoteric layers. In short, the presentation layer provides a translation function between the application layer and the session layer. It may be that the application layer data for network transmission needs to be reformatted. One potential application may be encryption. This would be a function of the presentation layer.

Layer 7 – Application

It’s important to note that the application layer is not the application itself. These software programs fall outside the scope if the OSI seven layer model. The application layer refers to the elements of a software application that control the communication over a network and it interacts with the application and the presentation layer. Examples of application layer functions are Hypertext Transfer Protocol (HTTP), Simple Mail Transfer Protocol (SMTP) and File Transfer Protocol (FTP).

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