Friday, May 3, 2013

Network+ OSI Topics

Network+ OSI Topics

If you're preparing for the Network+ exam, you can expect to see some questions related to Network+ OSI topics.  The Open Systems Interconnection (OSI) reference model includes seven layers and you're expected to have some basic knowledge about these layers including the specific layers where different devices and protocols operate. The CompTIA Network+ OSI Topics (A Kindle Short) (only $2.99) covers the OSI model in depth, but this blog summarizes the important topics.

The OSI model is fully documented in ISO/IEC 7498 and can be quite complex. However, you don't need to maser the OSI model completely when studying the Network+ OSI topics. As an example, here are two practice test questions:

 Which layer of the OSI model is responsible for transmitting individual bits?
 A. 1
 B. 3
 C. 4
 D. 7

Which layer of the OSI model transmits data as frames?
A. Data Link
B. Physical
C. Network
D. Transport
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Know the Layers

At the most basic layer, you need to know the layers including their names and numbers. The following figure shows them along with two common mnemonics used to memorize them.
Network+ OSI Layers
After memorizing the mnemonic, you also need to remember which layer is layer 1, and which layer is layer 7. This memory technique may help you remember that the Application layer is layer 7. You may have heard about a “layer 8 error” which is a cryptic way of saying the error is a user error. What does a user interact with? Applications. A user on the mythical layer 8 interacts with applications, which are on layer 7.

I don’t mean to belittle users or user errors – I’m a user and I make my fair share of errors. However, this memory trick has helped me and many other people remember that the Application layer is layer 7. Combined with one of the mnemonics, you can then list all of the layers. The following sections introduce each of these layers.

Know the Hardware

In addition to knowing the layers, you also need to know what devices operate on different layers. The following figure shows some of the common hardware devices you should know about.

Network  OSI hardware

Know the Protocols

You should also be aware of different protocols specified within the Network+ objectives and which layer they operate on. The following table identifies common protocols used on different layers that you should know:

 Network+ OSI Protocols
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Know the Data Names

Data is referred to with different names on different layers within the OSI model as follows:
  • Layer 1 Physical Bits (or a bit stream)
  • Layer 2 Data Link Frames
  • Layer 3 Network Packets
  • Layer 4 Transport Segments
  • Layer 5 Session protocol data unit (PDU)
  • Layer 6 Presentation PDU
  • Layer 7 Application PDU

 A Simple Exercise to Remember the OSI Model

When I teach Network+ classes, we often repeat the following exercise regularly. Students start with a blank sheet of paper and then simply write down the mnemonic that they remember (such as Please Do Not Throw Sausage Pizza Away or All People Seem To Need Data Processing) with each word on a separate line. Next, they write down the layer numbers and the layer names.

When doing this, I encourage them to include everything they can remember without looking. Afterwards, I ask them to check their work and fill in everything that they missed or they couldn’t remember. From a learning perspective, there’s a lot to be said for combining multiple senses and this forces them to engage different parts of their brain to remember and then write.

During a weeklong class, we do this once or twice a day and during a college class we do it for almost every class meeting. However, I have the students progressively add more and more information such as the data name at different layers, then the devices, and the protocols. By the end of the class, they have written this table out several times and when they take the live exam, OSI questions are a breeze.

 You can do the same exercise. Periodically write down everything you can remember from memory and then double check your work with the following two tables. Don’t worry if you don’t memorize them all at first, but make sure you still fill in the blanks. The more you do it, the more you’ll remember.
  Network+ OSI Devices   Network+ OSI Protocols

Match the OSI and TCP/IP Model Layers

Last, you need to know the names of the TCP/IP model layers and how they match to the OSI model. The following table shows how they match up.You can do the same exercise starting with a blank sheet of paper and listing the layers of the OSI and TCP/IP models.
  OSI dnd TCP/IP Models

Practice Test Question Answers

Which layer of the OSI model is responsible for transmitting individual bits?
A. 1
B. 3
C. 4
D. 7

A is correct. Bits are transmitted on layer 1 (the Physical layer) of the Open Systems Interconnection (OSI) model. Bits are not transmitted on layer 3 (the Network layer), layer 4 (the Transport layer), or layer 7 (the Application layer).
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Which layer of the OSI model transmits data as frames?
A. Transport
B. Physical
C. Network
D. Data Link

D is correct. The Data Link layer (layer 2) of the Open Systems Interconnection (OSI) model transmits data as frames. The Transport layer (layer 4) transmits data segments. The Physical layer (layer 1) transmits data as bits. The Network layer (layer 3) transmits data as packets.

More Network+ Resources

If you're studying for the Network+ exam and want to pass the first time you take it, check out these resources:


If you're preparing for the Network+ exam, you should know the Network+ OSI layers including their names, numbers, and the specific layers where different devices and protocols operate. You'll also need to know how the TCP/IP models map to the OSI layers.