Content

| Overview

The physical layer transmits bits from one computer to another and regulates the transmission of a stream of bits over the physical medium. This layer defines how the cable or antenna is attached to the network adapter and what transmission technique is used to send data over the medium.

Repeaters, hubs, multiplexers, and network interface cards (NICs) operate at the physical layer. A NIC is primarily a Layer 2 device, but also operates at the physical layer of the OSI model. A NIC performs Media Access Control (MAC) sub-layer functions, but it also encodes bits and sends them out on the medium. Repeaters, hubs, and multiplexers have no interest in higher layer function and simply regenerate and propagate received bit-streams to outgoing ports.

Problems that occur at Layer 1 are distinctly different from problems that occur at higher layers. The physical layer is the only layer with physically tangible properties such as wires, cards, and antennas. In this lesson, identifiable characteristics and commands will be used to isolate problems at the physical layer.

Failures and suboptimal conditions at the physical layer do not merely inconvenience users. Networks that experience these kinds of conditions usually come to a grinding halt. Since the upper layers of the OSI model depend on the physical layer in order to function, a network technician must have the ability to effectively isolate and correct problems at this layer.

Upon completing this module, the students will be able to:

- Identify the characteristics of a physical layer failure problem
- Identify the characteristics of a physical layer optimization problem
- Identify end-system commands and applications that gather physical layer component information
- Identify the Cisco commands and applications that gather physical layer component information
- Recognize possible causes of common physical layer problems
- Isolate a problem at the physical layer