At-a-Glance: OSI Model

Why should I Care About the OSI Model?

The OSI model serves as a guideline for creating and implementing network standards, devices, and internetworking schemes. Advantages of using the OSI model include the following:

• Breaks up interrelated aspects of network operation into less-complex elements.
• Enables companies and individual engineers to specialize design and development efforts on modular functions.
• Provides standard interfaces for plug-and-play compatibility and multivendor integration.
• Abstracts different layers of the network from each other to provide easier adoption of new technologies within a layer.

Develop mnemonics to help you memorize the layers and their order. Here's an example:

Please Do Not Throw Sausage Pizza Away

What Problems Need to Be Solved?

OSI layers can communicate only with the layer directly above or below it on the stack, and with its peer layer on another device. A process must be used so that information can be passed down the stack, across the network, and back up the stack on the peer device.

Important additional functions of the OSI reference model's upper layers

 

Application layer

• The application layer acts as an interface between the actual applications programs.
  • Example - Microsoft Word – actually does not reside at the application layer, but instead just interfaces at the application layer protocol, using data presented to it by the presentation layer.

Presentation layer

• The presentation layer presents data to the application layer up top.
• Responsible for data translation and code formatting.
• Acts as a translator that presents and provides coding and conversion functions.
  • Example – EBCDIC to ASCII
• Some presentation layer may also involves multimedia operations too.

Session Layer

• The session layer is responsible for setting up, managing, and then tearing down sessions between layer entities.
• Coordinates communication between systems and serves to organize their communication by offering THREE different modes:
  • Simplex
  • Half Duplex
  • Full Duplex
• Keeps different application data separate from other applications data.

A quick overview of the OSI layers and their functions

Applications

• file
• print
• message
• database
• application services

Presentations

• data encryption
• compression
• translation

Session

• dialog control

Transport

• end-to-end connection

Network

• routing

Data Link

• framing

Physical

• physical topology

Facts and notes on the OSI reference model bottom layers

The bottom layers of the OSI reference model include

1. Transport
2. Network
3. Data Link
4. Physical

Functions at each of the bottom layers

Transport
- provides reliable or unreliable delivery
- performs error correction before re-transmit

Network
- provides logical addressing, which routers use for path information

Data Link
- combines packets into bytes and bytes into frames
- provides access to media, using MAC address
- performs error detection, NOT correction

Physical
- moves bits between devices
- specifies voltage, wire speed, and pin-out of cables

The following operates at all seven layers of the OSI reference model

1. Network Management Stations (NMS)
2. Web and application servers
3. Gateways (not default gateways)
4. Network Hosts

Facts and notes on the OSI reference model

1. The OSI reference model has seven layers and is divided into two groups.
2. The top three layers define how the applications within the end stations will communicate with each other and with users.
3. The bottom four layers define how data is transmitted end to end.

The upper layers of the OSI reference model include

1. Application layer
2. Presentation layer
3. Session layer

*Note: Users interfaces with computer at the application layer.

*Note: The upper layers are responsible for applications communications between hosts.

Diagram

Advantages of using an OSI reference model

Advantages of using an OSI model include:

1. Prevents changes in one layer from affecting other layers.
2. Divides the network communication process into smaller and simpler components, thus aiding components development, design, and troubleshooting.
3. Allows multiple-vendor development through standardization of network components.
4. Encourages industry standardization by defining what functions occur at each layer of the model.
5. Allows various types of hardware and software to communicate

*Reference model: conceptual blueprint of how communications should take place.

*Binding: communication processes that are related to each other are bounded, or grouped together, at a particular layer.