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.

Tips on network, routers, and switches

1. Plan your network design carefully.

2. Use network bridge in network to reduce collisions within broadcast domains and to increase the number of collision domains in your network.

3. Using hubs can contribute to traffic congestion.

4. Routers break up collision domains as well as broadcast domains.

5. Although bridges/switches are used to segment networks, they WILL NOT isolate broadcast or multicast domains.

6. Even though you have a switched network, you still need a router to provide your inter-VLAN communication or internetworking.