Cisco Term 1, Lesson 3

Lesson 3 Terms

Physical Layer - Layer 1 of the OSI Reference Model. The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

Encoding - Process by which bits are represented by voltages.

Twisted Pair - Transmission medium consisting of four or eight insulated wires arranged by pairs in a regular spiral pattern. The wires can be shielded or unshielded. Twisted pair is common in telephony applications and is most common in data networks.

Coaxial - Cable consisting of a hollow outer cylindrical conductor that surrounds a single inner wire conductor. Two types of coaxial cable are currently used in LANs; 50-ohm cable, which is used for digital signaling, and 75-ohm cable, which is used for analog signal and high-speed digital signaling.

Fiber-optic - Physical medium capable of conducting modulated light transmission. Compared with other transmission media, fiber-optic cable is more expensive, but is not susceptible to electromagnetic interference, and is capable of higher data rates. Sometimes called optical fiber.

Media - Plural of medium. The various physical environments through which transmission signals pass. Common networking media include twisted-pair coaxial and fiber optic cable, and the atmosphere (through which microwave, laser, and infrared transmission occurs). Sometimes called physical media.

Network Media Analogy:
To understand this concept more fully, think of two cities located several miles apart. Even though they are several miles apart, the cities are connected to each other by means of roads. Some of the roads may be very basic in terms of their size and the types of materials used to construct them. An example of this would be an unimproved one lane gravel road. Other roads may be larger in size and constructed of more sophisticated materials. An example of this would be a reinforced concrete four lane superhighway.
If you are interested in taking a leisurely Sunday afternoon sightseeing drive from City A to City B, you would most likely choose the unimproved one lane gravel road for your trip. On the other hand, if you were an ambulance driver in City A attempting to deliver a critical patient to a hospital in City B, it would be foolhardy to follow the same route. Because it is faster, smoother, and wider, the four lane superhighway would accommodate your needs as an ambulance driver, while the one lane country road would have difficulty in doing so.
In this analogy, the cities are like two computers trying to communicate and the roads are like the physical layer.

Introduction
As you learned in Lesson 2, the OSI reference model has become the primary model for network communications. Although other models have been created, today, most network vendors relate their network products to the OSI reference model when they want to educate users about their products. Thus, the model is the best tool available to people hoping to learn about network technology.

The OSI reference model is not something tangible. Rather it is a conceptual framework specifying the network functions that occur at each layer. In this lesson and the ones following it, you will learn the network functions that occur at each layer of the OSI model.

What is the first building block of the OSI Reference Model?
For our purposes, the terms we use to describe how a network functions will be linked to the OSI reference model. Just as a house must first have a foundation before it can be built, so, too, must a network have a foundation upon which to build. In the OSI model, this foundation is called the Physical layer. The function layer is the transmission of data. Typically such transmission is accomplished through the use of such things as wires, connectors, and voltages.

How is information stored in computers?
Information in computers is stored using the binary number system, in which the only possible symbols, or binary digits, or "bit", are 1 and 0. These bits - many of which are called data - are used to represent information, like text, pictures, and sounds. In the physical layer, a 1 bit is often represented by the presence of voltage (electrical pressure) on a copper conducting cable or light in an optical fiber.

To help you picture these bits, imagine measuring the voltage at one point on the cable as time goes on (for a fiber, imagine measuring the light intensity versus time). Your measurements would allow you to create a graph of voltage versus time (for a fiber, light intensity versus time). How the bits (1s and 0s) might be represented on the cable is shown in the graphic. There are many ways bits can be represented with voltages. This process is called encoding. Many of the LANs you will be studying about use "Manchester Encoding." In this type of encoding bits are represented by different voltage patterns that the ones shown in the graphic. (FIND THE DAMN GRAPHIC)

What types of materials might one see in the Physical layer of the OSI model?
In order for computers to communicate this encoded information with each other, they must be physically connected to each other. The materials used to connect computers vary. They include twisted pair, coaxial, and fiber-optic cable.

What are all the physical connecting materials referred to as?
All of these physical connecting materials are referred to as the Network Media.

What are some of the criteria for determining what type of media will be used?
The type of connecting material used by a network will determine things such as how much data and how fast the data can travel across the network. While these factors are important in determining what type of networking media to use, other factors such as expense and where the cable will be used are important as well

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