OSI Model

The seven-layer Open Systems Interconnection (OSI) model reduces complexity, ensures interoperability among devices, and simplifies learning. Although you will not be tested on the OSI model on the A+ exam, a general knowledge of these layers will help you remember information on which you will be tested. Table 21-1 describes the layers of the OSI model. It is important to memorize this table.

Table 21-1 OSI Model

Layer 1 devices cannot make decisions. They just retime, regenerate, and reproduce the signals (bits) they receive. Hubs, cables, and connectors function at Layer 1. When troubleshooting networks, it is often smart to start with Layer 1.

Layer 2 devices make up the majority of networking traffic. Layer 2 devices, such as switches and NICS, read and determine whether they recognize the MAC address in frames. If not, they send it to everyone to see whether the others have heard of the address. If they have heard of the destination MAC address, Layer 2 devices forward it down that port only. This is called switching. The name comes from switching locomotive trains and from old-school telephone operators who manually connected and disconnected, or switched, wires to make connections.