Модель OSI | Курс "Компьютерные сети"

Introduction to Telecommunications Models

Overview of the Lecture

• The speaker introduces himself as Andrey Sazykhin and outlines the course topic: telecommunications networks and systems, focusing on the OSI model and VESOI model.

• The OSI (Open Systems Interconnection) model is defined as a standard established by the International Organization for Standardization (ISO) in 1983.

Definition of Open Systems

• In this context, an open system refers to one built according to open specifications that are publicly available, differing from the common understanding of open-source software.

• Windows is cited as an example of an open system because it operates based on these open specifications despite its closed source code.

Advantages of Open Systems

Benefits of Using Standards

• Open systems allow for interoperability among various components from different manufacturers, facilitating upgrades and integration without significant issues.

• The OSI model serves as a theoretical framework for network interaction but does not dictate specific protocols; it provides a common language for discussing network construction.

The Seven Layers of the OSI Model

Layer Structure

• The OSI model consists of seven layers arranged hierarchically, starting with the Physical layer at the bottom.

• The layers include:

• Physical

• Data Link (Канальный)

• Network

• Transport

• Session

• Presentation

• Application

Purpose of Each Layer

• The Physical layer transmits bits over communication channels without analyzing data content; its main task is signal representation.

Detailed Functions of Each Layer

Data Link Layer Responsibilities

• The Data Link layer manages message transmission rather than individual bits, ensuring error detection and correction in broadcast networks.

• It also handles physical addressing to direct messages to specific computers connected to a shared medium.

Network Layer Functions

• This layer facilitates large composite networks by reconciling differences between various technologies at the Data Link level through global addressing schemes.

Transport Layer Characteristics

Ensuring Reliable Communication

• The Transport layer guarantees reliable data transfer between processes on different computers, often providing error-free communication with guaranteed message order.

Session Layer Role

Managing Communication Sessions

Understanding the Session Layer and Its Functions

The Role of the Session Layer

• The session layer manages simultaneous network connections, ensuring that operations are executed sequentially to prevent loss of funds in banking transactions.

• It provides protection against network connection interruptions, allowing for continued operation after reconnection through synchronization functions.

Presentation Layer Responsibilities

• The presentation layer formats data so it is understandable by both sender and receiver, requiring agreement on both syntax (data format) and semantics (meaning).

• Different systems may use various character encodings or number storage formats (e.g., big endian vs. little endian), necessitating conversion during data transmission.

Example of Data Translation

• An illustrative example involves a phone call where speech in Russian is translated into an intermediate language (Chinese), then converted to English for the recipient in America.

• Although this automatic translation is not yet realized, encryption and decryption processes at the presentation layer are widely implemented.

Application Layer: The User-Facing Side of Networking

Applications Utilizing Network Infrastructure

• The application layer encompasses user-facing applications such as web pages, social networks, video/audio communication platforms like Skype, and email services.

Data Units Across OSI Model Layers

• Each level of the Open Systems Interconnection (OSI) model has specific data units: bits at the physical level, frames at the data link layer, packets at the network layer, segments at the transport layer, and messages at higher layers.

Equipment Functionality by OSI Levels

• Different hardware operates at various OSI levels: hubs function at the physical level; switches and access points operate at the data link level; routers work within the network layer.

Overview of OSI Model Standards

Significance of OSI Model

• The OSI model serves as a standard framework for organizing computer networks into seven distinct layers as recognized by international standards organizations.

Protocol Integration with OSI Model