The OSI model in a nutshell (for beginners)

Since I’m just getting things started on this blog, I figured I’d post some introductory material for those new to the industry, or for others who may want a quick refresher. Knowing the inner-workings of systems is important to provide interoperability and integration among so many diverse platforms. Without a foundational understanding in topics such as the OSI model, administrators will be less than effective.

So here’s a brief overview of each layer of the OSI model and their functions:

Layer 7 – Application: the interface between end-user applications and communications software. Protocols such as Telnet, HTTP, FTP, and SMTP communicate at this layer.

Layer 6 – Presentation: handles encryption, formatting, compression, and presentation of data formats (such as JPEG) to applications. SSL and TLS communicate at this layer.

Layer 5 – Session: responsible for the startup, control, and teardown of sessions for the presentation layer. NetBios and TCP sessions are examples of those controlled at this layer.

Layer 4 – Transport: handles all transport and data delivery issues to other systems (a focus on error recovery and controlling data flow). TCP and UDP protocols reside at this layer.

Layer 3 – Network: responsible for routing, addressing, and determining the best possible route. ICMP, IP, ARP, and IPSEC reside at this layer.

Layer 2 – Data Link: links the data from one host to another, while defining protocols when data is sent over a particular medium. Where the IP address is found at layer 3, MAC addresses are found at this layer. Ethernet, FDDI, ATM, and Token Ring reside at this layer.

Layer 1 – Physical: This layer provides the physical transportation of data. It focuses on connectors, currents, pins, light, and other specifications that define cabling standards. This layer focuses on binary transmission.

There are many mnemonic phrases used to help memorize each layer. Some include:

•    People don’t need to see Paula Abdul
•    Please do not take sausage pizza away
•    All people seem to need data processing

Whichever phrase you choose, the OSI layers are pretty easy to remember over time. Knowing how various systems work and what layer they reside on can help streamline troubleshooting. Click on the diagram below to see how communication takes place through the OSI model: