Understanding the Open Systems (OSI) Reference Model

• Hubs work at Layer 1, switches work at Layer 2, routers work at Layer 3, and TCP works at Layer 4 of the OSI Reference Model.

• Starting with Layer 7, the OSI model layers are: application, presentation, session, transport, network, data link, and physical.

• The generic term for data encapsulated at each level is a Protocol Data Unit (PDU). The PDU at Layer 2 (such as from a switch) is a frame. The PDU at Layer 3 (such as from a router) is a datagram. The PDU at Layer 4 (such as from HTTP traffic) is a segment.

• The OSI model allows engineers to compartmentalize their designs so their work can easily integrate with the work of other engineers.

The Origin of Switching

• Most switches are flavors of Ethernet based on IEEE 802.3.

• Switches bridge data using ASICs instead of slower processors.

• Transparent bridging and switching are generally used interchangeably.

• Switches segment network traffic into separate collision domains.

Switching Standards and Features

• Switches vary from each other by their speed, management, size, performance, memory, multilayer switching abilities, port aggregation, VLAN abilities, and other special features.

• All switches must meet certain interoperability standards before they can claim IEEE 802.3 compatibility.

• In most cases, you can connect switches with CAT 5e cable up to 100 meters. Beyond that distance, you should use fiber.

• Manufacturers can add proprietary features to their switches and still achieve IEEE 802.3 compatibility.

Moving...