Common Questions and Answers on the OSI Network Model

Students, networking professionals, corporate employees, and anyone else interested in the basic technology of computer networks can benefit from learning more about the OSI network model. The model is a good starting point for understanding the building blocks of computer networks such as switches, routers and network protocols.

While modern networks only loosely follow the conventions laid out by the OSI model, enough parallels exist to be useful.

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What Are Some Helpful Memory Aids for the OSI Model Layers?

Students learning networking often have difficulty memorizing the name of each layer of the OSI network model in correct order. OSI mnemonics are sentences in which each word starts with the same letter as the corresponding OSI model layer. For example, "All People Seem To Need Data Processing" is a common mnemonic when viewing the network model top-to-bottom, and "Please Do Not Throw Sausage Pizza Away" is also common in the other direction.

If the above doesn't help, try any of these other mnemonics to help you memorize the OSI model layers. From the bottom:

  • Programmers Dare Not Throw Salty Pretzels Away
  • People Don't Need To See Paula Abdul
  • People Desperately Need To See Pamela Anderson
  • Please Do Not Touch Superman’s Private Area
  • Please Do Not Touch my Samsung Phone Application
  • Please Do Not Tell Sales People Anything
  • Please Do Not Trust Sales Persons' Answers
  • Paula Did Networking Till She Passed Away

From the top:

  • A Perfectly Simple Technology Narrowed Down Physically
02
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What Is the Protocol Data Unit (PDU) Employed at Each Lower Layer?

The Transport layer packages data into segments for use by the Network layer.

The Network layer packages data into packets for use by the Data Link layer. (Internet Protocol, for example, functions with IP packets.)

The Data Link layer packages data into frames for use by the physical layer. This layer consists of two sublayers for Logical Link Control (LCC) and Media Access Control (MAC).

The Physical layer organizes data into bits, a bitstream for transmission over the physical network media.

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Which Layers Perform Error Detection and Recovery Functions?

The Data Link layer performs error detection on incoming packets. Networks often use cyclic redundancy check (CRC) algorithms to find corrupted data at this level.

The Transport layer handles error recovery. It ultimately ensures data are received in order and free of corruption.

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Are There Alternative Models to the OSI Network Model?

The OSI model failed to become a universal global standard due to the adoption of TCP/IP. Instead of following the OSI model directly, TCP/IP defined an alternative architecture based on four layers instead of seven. From bottom to top:

  • Network Access
  • Transport
  • Internetwork
  • Application

The TCP/IP model subsequently was refined to split the Network Access layer into separate Physical and Data Link layers, making a five layer model instead of four.

These Physical and Data Link layers roughly correspond to the same layers 1 and 2 of the OSI model. The Internetwork and Transport layers also correspond respectively to the Network (layer 3) and Transport (layer 4) portions of OSI model.

The Application layer of TCP/IP, however, deviates much more significantly from the OSI model. In TCP/IP, this one layer generally performs the functions of all three higher-level layers in OSI (Session, Presentation, and Application).

Because the TCP/IP model was focused on a smaller subset of protocols to support than OSI, the architecture is geared more specifically to its needs and its behaviors do not match exactly with the OSI even for layers of the same name.