Читать книгу CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies - Ben Piper, David Higby Clinton - Страница 29

The main function of the Physical layer is to convert bits to electromagnetic energy such as light, electrical current, or radio waves, and transmit them over some medium such as fiber-optic or copper cables or the airwaves. Whereas the functions of the other layers are performed in software, this particular function is performed by a node's physical network interface.

A challenge of using electromagnetic energy to send bits is that the physical media can carry only one bitstream at a time. In the early days of networking, two nodes would be connected via a pair of wires. If both simultaneously sent a signal, their signals would interfere with each other and create a collision. Hence, both nodes were in the same collision domain. To avoid this, the nodes had to use half-duplex communication wherein only one node could transmit at a time. Half-duplex wired communication may seem an irrelevant relic from the past, but as you'll learn in a moment, during its heyday half-duplex communication had an unfortunate impact on the Ethernet standards that still haunts us to this day. Broadcast storms and the infamous Spanning Tree Protocols (STPs) can be traced back to the early use of half-duplex communication.

Today, full-duplex communication is the norm in wired networks and something we take for granted. All that's needed for full-duplex communication is for the physical interface to separate the transmit and receive functions. Twisted-pair copper cabling, for example, does this by using two pairs of wires: one pair for transmitting and another pair for receiving. Likewise, fiber-optic cables have separate strands for transmitting and receiving. Wavelength-division multiplexing achieves full-duplex communication on a single fiber strand by using one light frequency for transmitting and another for receiving.