Network Communication and OSI Model Overview

Study Notes

Network devices facilitate communication by forwarding traffic at different layers: switches operate at Layer 2, while routers function at Layer 3.
Forwarding mechanisms in routers and switches are essential to understanding how network traffic is managed.
The OSI model, a seven-layer architecture, standardizes networking functions to ensure interoperability among different vendors.

TCP/IP is the predominant networking protocol today, evolving from a diverse range of device-specific protocols.
Each OSI layer can be altered independently without affecting adjacent layers, promoting flexibility in network design.
Data originates from applications at Layer 7 and traverses down the OSI model, being encapsulated until it reaches Layer 1 for transmission.

At Layer 3, a device determines if data is destined for another application on the same device or needs to be sent to a different device.
On the receiving end, data ascends from Layer 1 to Layer 7, ultimately reaching the target application.

Early Layer 2 devices included bridges and switches, while Layer 3 devices were primarily routers.
Advances in hardware and physical media prompted the development of application-specific integrated circuits (ASICs).
Multilayer switches (MLSs) emerged, enabling hardware-based Layer 2 forwarding with added Layer 3 functionalities.

The data link layer (Layer 2) is responsible for directing communication between hosts utilizing unique addressing.
Layer 2 addressing typically employs MAC addresses in Ethernet networks, and different protocols have distinct addressing conventions.

A MAC address consists of 48 bits, divided into six octets, represented in hexadecimal format.
The first three octets identify the device manufacturer (Organizationally Unique Identifier or OUI), while the last three must be unique.
Devices listen for packets addressed to their MAC address for data processing at Layer 3, while broadcasts (addressed as FF:FF:FF:FF:FF:FF) are received by all devices within the same network segment.

Ethernet operates as a shared medium, where multiple devices may transmit simultaneously, leading to potential data collisions.
Carrier Sense Multiple Access/Collision Detect (CSMA/CD) is employed in Ethernet to manage data transmission, ensuring only one device transmits at a time within a collision domain.
A collision domain is defined as a network segment where devices can detect simultaneous transmissions, effectively requiring devices to defer transmission until the medium is clear.