The OSI and TCP IP Model

Welcome to the ITFreeTraining video on the OSI Model. This video will look at the OSI model and in particular, how it relates to the internet protocol.

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OSI Model
0:13 – The OSI was first developed in the 1980s. It is a conceptual model aimed at standardizing network communication. The model was not implemented in its entirety, but it is still referenced today. The OSI model has 7 different layers. Each layer is directly linked to the layer above and below it, if a layer exists. As data travels down the OSI model, each layer adds additional information. This continues until it reaches the last layer. It is then transmitted across the network. Once the other side receives the data, the process is reversed. You can see how each layer requires the layer or layers adjacent to it. Also it is not possible to skip a layer. All communication either starts at the top layer or is received at the bottom layer.

At the top, is the application layer. This layer provides the application programming interface. The next layer, presentation, is responsible for data formatting. This includes encoding, encryption and compression. The next layer, session, is responsible for connecting and reconnection of the network if it is lost. This also includes authentication of the connection. The next layer, transport, manages packet size and the resending of data if it was lost or corrupted. The following layer, network, routes data between different nodes in the network. The second to last layer, data link, is responsible for flow and error control. For example, ensuring more data is not sent out then the local network can handle. The last layer, physical, communicates directly with the network and is responsible for transmitting data on the network. As stated, the OSI model is a conceptual model and not directly implemented in modern networking, but is referenced from time to time.

OSI Model & TCP/IP
2:20 – To demonstrate this, I will now compare it to the TCP/IP model. TCP stands for Transmission Control Protocol and IP as we know stands for Internet Protocol. A number of protocols are used to make internet communication possible. These protocols were grouped together under the name TCP/IP. Nowadays you don’t hear this name so much, generally you will hear just IPv4 or IPv6 but it is still valid. As you can see, TCP/IP has 4 different layers as opposed to the 7 layers of the OSI model. It is shown as an exact fit, however in reality it does not fit the OSI model exactly. The application layer maps to the first and third layers of the OSI model. Transport is a direct 1 to 1 mapping. The internet layer maps directly to the network layer and the last two layers of the OSI model map to the network access layer.

Protocols in TCP/IP
3:25 – Even though there are only 4 layers, each layer can be broken down further. As seen here, at the application layer there are a number of different protocols. You can see how software will generally use protocols to access services on the internet. The protocols themselves will access the transport layer. At this layer, there is a choice of TCP and UDP. TCP stands for Transmission Control Protocol. TCP gives reliable transmissions. If a packet is lost in transmission, the protocol will notice it has not arrived and request a resend. In contrast, UDP, which stands for User Datagram Protocol, is unreliable transmission. If a packet goes missing in transit, it is not resent. Think of UDP like mailing a letter. Once you mail the letter, you have no idea if it will arrive at its destination. The next layer determines if IPv4 or IPv6 is used. At the bottom is the device driver which communicates with the network. The most common type is Ethernet, however any other network devices are supported. You can see how TCP/IP is very modular. This makes it easy to add additional protocols and add additional hardware when they’re developed. I will now have a look at how the OSI model may get referenced today.

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References
“OSI model” https://en.wikipedia.org/wiki/OSI_model
“Internet protocol suite” https://en.wikipedia.org/wiki/Internet_protocol_suite

Credits
Trainer: Austin Mason http://ITFreeTraining.com
Voice Talent: HP Lewis http://hplewis.com
Companion Document: Phillip Guld http://philguld.com
Video Production: Kevin Luttman http://www.KevinLuttman.com