18.1.3. Network Models

• OSI Model (7 layers)

• Physical, Data Link, Network, Transport, Session, Presentation, Application

The OSI Model, or Open Systems Interconnection Model, is a framework that divides network communication into seven distinct layers. Starting from the bottom up:

1. Physical Layer: This layer is concerned with the physical connection between devices. It includes hardware specifications such as cables, switches, and electrical signals.
2. Data Link Layer: This layer provides node-to-node data transfer, handling error correction from the physical layer. It is responsible for MAC addresses.
3. Network Layer: This layer is responsible for data packet routing and forwarding. It manages device addressing and location.
4. Transport Layer: This layer ensures complete data transfer and handles error recovery and flow control. It helps establish connections and can be connection-oriented (like TCP).
5. Session Layer: This layer manages sessions between applications, facilitating the exchange of information and maintaining communication.
6. Presentation Layer: It translates data formats between the application layer and the network layer, managing data encryption and compression.
7. Application Layer: This top layer is where applications access network services. It includes protocols like HTTP and FTP.

Think of the OSI Model like a postal service for sending letters.

• The Physical Layer would be the actual roads and vehicles used to transport letters (the physical infrastructure).
• The Data Link Layer would be similar to a local post office that ensures letters are delivered to the correct addresses (error handling).
• The Network Layer is like the postal routes that determine how letters get from one city to another (routing).
• The Transport Layer is the assurance that letters reach their destination (delivery confirmation).
• The Session Layer is akin to establishing a correspondence with someone (maintaining communication).
• The Presentation Layer is how the letters are formatted (encryption/decoding), and the Application Layer is the final recipient reading the letter.

• TCP/IP Model (4 layers)

• Network Access, Internet, Transport, Application

The TCP/IP Model, which stands for Transmission Control Protocol/Internet Protocol, simplifies networking into four layers.

1. Network Access Layer: This layer corresponds to the combination of the physical and data link layers of the OSI model. It determines how data is physically sent over the network, including hardware like routers and switches.
2. Internet Layer: This layer applies protocols such as IP (Internet Protocol) to handle the addressing and routing of data packets across different networks.
3. Transport Layer: Like the OSI’s equivalent, this layer manages the transmission of data between hosts with protocols like TCP and UDP. TCP is reliable and connection-oriented, while UDP is faster but connectionless.
4. Application Layer: It serves as an interface for applications to communicate over the network. This is where protocols like HTTP, FTP, and SMTP operate, allowing users to interact with the content they need.

Imagine you are hosting a party and need to prepare for guests.

• The Network Access Layer might involve setting up the venue (the actual hardware connections).
• The Internet Layer is like sending out invitations that include directions to your venue (addresses that help guests find their way).
• The Transport Layer ensures people can actually get in and out of the venue efficiently, whether through a RSVP system or just people arriving casually (data transfer).
• The Application Layer is where guests engage in activities, like enjoying food, music, or games (application-level protocols functioning for user experience).

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