Network Models - Part 1


Network Models - Part 1

Whilst explaining about the OSI and TCP/IP Models, I'm going to avoid discussing the Protocols that function within them. This is because I'm planning on writing separate posts about individual protocols.

There are two main types of Network model:
- OSI Model
- TCP/IP Model

Let's start by discussing the OSI Model. It is a seven layer representation of how data is transferred through a Network.
OSI stands for the Open Systems Interconnection Model and was developed by the International Organisation for Standardisation or ISO (This isn't a typo, 'ISO' is how it should be represented).

Going back to where we were originally, there are seven layers, and these are as follows:

- Application Layer - Layer 7
- Presentation Layer - Layer 6
- Session Layer - Layer 5
- Transport Layer - Layer 4
- Network Layer - Layer 3
- Data Link Layer - Layer 2
- Physical Layer - Layer 1

There are a number of different mnemonics for remembering the layout for the OSI model. A popular one for memorising Layer 7 down is: All People Seem to Need Data Processing.

- Application = All
- Presentation = People
- Session = Seem
- Transport = To
- Network = Need
- Data Link = Data
- Physical = Processing

One of the popular mnemonics I have seen for remembering the OSI Model from Layer 1 up, is:

- Physical = Please
- Data Link = Do
- Network = Not
- Transport = Throw
- Session = Sausage
- Presentation = Pizza
- Application = Away

However you choose to remember the Layers, ensure you are confident with naming between calling it either Layer 1 or Physical Layer, as you may need to troubleshoot based on that information alone.

The Application Layer is where the user begins the process, for example; Typing up an email and then hitting enter.
That is as much as the user needs to do. They will have logged onto the computer, ensured they had a Network connection, opened the Application and then completed the task.

The Presentation Layer is responsible for three acts:

- Encapsulate/De-encapsulate of Data
- Compression of Data ready to be transferred
- Translation of any Data that is received

It will encapsulate data ready to be sent, or De-encapsulate any data that is received ready to be used by the Application Layer.
It will compress any Data that is to be sent, this will allow for a high success rate with sending the Data. It is impossible to send the Data over the Network in once piece, as it would either time out or outright fail to send.
The Presentation Layer will also translate any Data that it receives.

The Session Layer establishes a connection between two Devices so that Data can be sent. The Session Layer also ensures that the connection remains open long enough for all Data to be sent or received.

The Transport Layer works with the Session Layer to ensure end to end communication. The Transport Layer receives any incoming Data and breaks it down into Segments so that it can be passed along to the Network Layer if necessary.
Once the Data has been transferred along the Network, the Transport Layer is also responsible for reassembling the Data.

The Network Layer continues the process by breaking down the segments into smaller pieces; these are known as 'Packets'. The Network Layer then chooses the best route and sends the Packets along to another Network.
If the Segments are being sent to a Device that is on the same Network, then the Network Layer isn't needed.

The Data Link Layer intercepts any incoming packets from the Network Layer and breaks them down into something smaller, known as a frame. The Data Link Layer is then responsible for transferring the 'Frames' between the different nodes on the Network.

The Physical Layer is concerned with the transferring of data between the cables and equipment. The data is broken down into a series of zeroes and ones and is transferred from device to device along the network.

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