The OSI (Open Systems Interconnect) model is just that – a model. It’s kind of like a reference in which vendors go by for communications purposes.
The OSI model should become your best friend because it is best used as a troubleshooting step. By memorizing the OSI model you’ll be able to quickly troubleshoot any networking issue which I’ll talk about in future posts.
OSI layers & what they do
Layer 7 – Application
This is basically the layer the end-user sees. All communications between software and underlying applications use this layer. The more familiar protocols seen are FTP, SSL, HTTP, Telnet, etc. Authentication is also done at this layer
Layer 6 – Presentation
Formatting is the core purpose of this layer. It will properly format data to and from the Application layer. Encryption is also done at this layer.
Layer 5 – Session
The session layer defines end-user application communications – start, control and end.
Layer 4 – Transport
An important layer to understand for core CCNA studies. This layer handles the data delivery of segments, delivering data, to the end host. It uses multiplexing to construct data packets that include information such as source & destination address. There are two types of connections under the Transport layer – TCP & UDP (connection-oriented or connection-less).
Layer 3 – Network
Another important layer to know is the Network layer. Defined in the Network layer is logical addressing, routing and path determination. A core part of the CCNA involves understanding addressing, where data is crossed and how it determines the next route.
Layer 2 – Data link
This layer defines the rules to transfer data between networks. Some of the protocols within this layer are Ethernet, PPP, Frame Relay and HDLC. The sub-layers defined here are Logical Link Control and Media Access Control.
Layer 1 – Physical
This is the physical medium in which all data is transported through. At this layer standards are defined such as the connectors, pins, encoding, etc.
Related posts: