Understanding the OSI Model and Network Communication
Item 4: The Importance of Networking
First Isolated Equipment
Initially, isolated equipment led to several challenges:
- Duplication of devices, equipment, and resources.
- Inability to communicate effectively.
- Lack of control and network management.
The Need for Standards
In the 1970s, there was no unification of protocols. For a company to expand its network, it could only go to the original manufacturer. This created conflicts between business network systems from different manufacturers.
The OSI Reference Model
This model proposes a layered structure, where each layer provides a set of functions that use other layers. The OSI model has seven layers, each of which is based on the layer immediately below:
Physical Layer
You can directly use the physical transmission medium. As a service, it provides the transmission of bits. At this level, we define the following characteristics:
- The environment, i.e., connector types, cable diameter, etc.
- Electrical transmission, such as levels of transmission or the transmitted signal type.
- Functional environment, i.e., specifying what each wire or channel does.
Data Link Layer
Ensures that the physical link is safe and provides means to activate, maintain, and deactivate the link. Its most important functions are error detection and flow control. It is divided into two sublayers:
- MAC: Sends data blocks, holding frame synchronization, error control, and flow.
- LLC: Interfaces with the network layer.
Network Layer
Responsible for providing an end-to-end connection, i.e., the transmission of information between end systems through any communication network.
Transport Layer
Ensures error-free transmission from end to end, regardless of network type. Ensures that data arrives without errors, in order, without loss or duplicates.
Session Layer
Responsible for organizing and synchronizing the dialogue between the two endpoints. Provides mechanisms to manage dialogue between two endpoints by:
- Disciplines of dialogue, that is, who should issue at every moment.
- Grouping of data into logical units.
- Recovery, i.e., if there is a problem in communication, having a checkpoint from which to retransmit the data.
Presentation Layer
This layer defines the format of the data to be exchanged between applications and offers application programs a set of data processing services. If necessary, it also handles compression and encryption.
Application Layer
This last level is responsible for most common applications, such as HTTP, file transfer, and terminal access to remote computers. It also defines certain features that may be used by multiple applications.
Layer Protocol: Allows communication between different layers of the same system. This protocol of each layer exchanges information, called protocol data units.
Encapsulation of Information
The information received is called data or data packets. The process of placing data into the information packet is called encapsulation.
Frames and Packets
Networks must perform five conversion steps to encapsulate data:
- Create the data. When a user sends an email, its alphanumeric characters are converted into data that can traverse the internetwork.
- Package the data to be transported from end to end. The data is packaged for transport across the internetwork.
- Add the IP network address to the header. The data is placed in a packet containing a header with the logical addresses of the source and destination.
- Add the header and trailer of the data link layer. Each network device must put the packet into a frame.
- Convert to bits for transmission. The frame should become a pattern of ones and zeros for transmission through the medium.