Читать книгу CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies - Ben Piper, David Higby Clinton - Страница 30

The primary function of the Data Link layer is to facilitate data transfer between two (and only two) nodes that are connected to a shared medium. Some physical media can support only two nodes, as is the case with a crossover cable or point-to-point serial link. Other media, such as wireless, can support more than two nodes.

When only two nodes share the same media, data transfer is easy. As long as both nodes are aware of the point-to-point nature of the link, one node can send the data, and the other node receives it, knowing that it's the intended recipient. The Point-to-Point Protocol (PPP) and High-level Data Link Control (HDLC) are two common layer 2 protocols used on T1 serial links.

But when multiple nodes can share a medium, as they did with early Ethernet, things get tricky. At this point you're rightly thinking that with the exception of wireless, nobody connects nodes to a shared medium anymore. Hubs went out of fashion long ago. Now we connect devices to switches (the marketing term for bridges). However, switches actually simulate the behavior of a shared medium. Time for a quick history lesson.