ExamNotes for Network+ N10-007 sub-objective 1.2 Explain devices, applications, protocols and services at their appropriate OSI layers

The OSI layers have been a recurring feature of the Network+ exams from one version to the next for over a decade now. CompTIA is so serious about it that it dedicates this entire sub-objective to this one item.

CompTIA Exam Sub-objective 1.1: “Explain devices, applications, protocols and services at their appropriate OSI layers.”

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Mnemonic anyone?

One issue is, of course, to just remember all these layers in the right order on exam day. There are a few mnemonics out there listing the first letter of each layer from the bottom up i.e. from layer 7 to 1. Here are three examples of that:

– People Seem To Need Data Processing

– Please Do Not Throw Salami Pizza Away

– Please Do Not Tell Secret Passwords Anytime

Pick whichever one works best for you (or make up your own), and on exam date, jot it down immediately on the note pages made available to you at the testing center. That will be one less thing to remember (or mess up) as you go through the exam questions.

Layer 1 – Physical Layer

This is the Physical layer on the Network Interface Controller aka Network Interface Card or NIC. It receives the frame and places the actual transmission on the network.

Layer 2 – Data Link Layer

The packet is passed to the Link layer on the NIC, which encapsulates this data with its own header and trailer, creating what is referred to as a frame. The link layer’s frame includes the physical addresses used to find nodes on the local network.

Layer 3 – Network Layer

The Network layer adds a header to the segment. That header identifies the IP address of the destination host as well as the message aka a packet.

OSI Layer 4 – Transport Layer

The Transport layer protocol, typically TCP (for wide area networks or the internet) or UDP (mostly for local area networks) adds a header to the payload. The function of the header is to attach a port number that identifies the application on the destination host. From here on the message, depending on the protocol used, is either a segment or a datagram.

Layer 5 – Session Layer

The Session layer is responsible for establishing maintaining and terminating connections between computers. The connection is called a session. The session layer can also play a role in error handling and connection recovery.

Layer 6 – Presentation Layer

The Presentation layer handles the task of preparing the data and making it acceptable to the Application layer. This preparation includes encryption and decryption, compression and graphics handling.

Layer 7 – Application Layer

The Application layer is the only OSI layer that interacts with the user. As the user sends email or browses the internet the applications they use employ Application layer protocols such as SMTP or HTTP respectively to communicate with the lower OSI layers.

That’s all for objective 1.2! Good luck on the test!

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