CCNA :) Be a Good Network Administrator

Wireless LAN organizations and standards 3.3.1 This page will introduce the regulations and standards that apply to wireless technology. These standards ensure that deployed networks are interoperable and in compliance. Just as in cabled networks, IEEE is the prime issuer of standards for wireless networks. The standards have been created within the framework of the regulations created by the Federal Communications Commission (FCC). A key technology contained within the 802.11 standard is Direct Sequence Spread Spectrum (DSSS). DSSS applies to wireless devices operating within a 1 to 2 Mbps range. A DSSS system may operate at up to 11 Mbps but will not be considered compliant above 2 Mbps. The next standard approved was 802.11b, which increased transmission capabilities to 11 Mbps. Even though DSSS WLANs were able to interoperate with the Frequency Hopping Spread Spectrum (FHSS) WLANs, problems developed prompting design changes by the manufacturers. In this case, IEEE’s task wa...

Signals and noise in optical fibers 3.2.9 This page explains some factors that reduce signal strength in optical media. Fiber-optic cable is not affected by the sources of external noise that cause problems on copper media because external light cannot enter the fiber except at the transmitter end. The cladding is covered by a buffer and an outer jacket that stops light from entering or leaving the cable. Furthermore, the transmission of light on one fiber in a cable does not generate interference that disturbs transmission on any other fiber. This means that fiber does not have the problem with crosstalk that copper media does. In fact, the quality of fiber-optic links is so good that the recent standards for gigabit and ten gigabit Ethernet specify transmission distances that far exceed the traditional two-kilometer reach of the original Ethernet. Fiber-optic transmission allows the Ethernet protocol to be used on metropolitan-area networks (MANs) and wide-area networks (WANs...

Multimode fiber 3.2.6 This page will introduce multimode fiber. The part of an optical fiber through which light rays travel is called the core of the fiber. Light rays can only enter the core if their angle is inside the numerical aperture of the fiber. Likewise, once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called "multimode" fiber. Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber. Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. One fiber carries transmitted data from device A to device B. The second fiber carries data from device B to device A. The fibers are similar to two one-way str...

Reflection 3.2.3 This page provides an overview of reflection. When a ray of light (the incident ray) strikes the shiny surface of a flat piece of glass, some of the light energy in the ray is reflected. The angle between the incident ray and a line perpendicular to the surface of the glass at the point where the incident ray strikes the glass is called the angle of incidence. The perpendicular line is called the normal. It is not a light ray but a tool to allow the measurement of angles. The angle between the reflected ray and the normal is called the angle of reflection. The Law of Reflection states that the angle of reflection of a light ray is equal to the angle of incidence. In other words, the angle at which a light ray strikes a reflective surface determines the angle that the ray will reflect off the surface. The next page describes refraction. Refraction 3.2.4 This page provides an overview of refraction. When a light strikes the interface between two transparent ...

Ray model of light 3.2.2 This page describes the properties of light rays. When electromagnetic waves travel out from a source, they travel in straight lines. These straight lines pointing out from the source are called rays. Think of light rays as narrow beams of light like those produced by lasers. In the vacuum of empty space, light travels continuously in a straight line at 300,000 kilometers per second. However, light travels at different, slower speeds through other materials like air, water, and glass. When a light ray called the incident ray, crosses the boundary from one material to another, some of the light energy in the ray will be reflected back. That is why you can see yourself in window glass. The light that is reflected back is called the reflected ray. The light energy in the incident ray that is not reflected will enter the glass. The entering ray will be bent at an angle from its original path. This ray is called the refracted ray. How much the incident lig...

The electromagnetic spectrum 3.2.1 This page introduces the electromagnetic spectrum. The light used in optical fiber networks is one type of electromagnetic energy. When an electric charge moves back and forth, or accelerates, a type of energy called electromagnetic energy is produced. This energy in the form of waves can travel through a vacuum, the air, and through some materials like glass. An important property of any energy wave is the wavelength. Radio, microwaves, radar, visible light, x-rays, and gamma rays seem to be very different things. However, they are all types of electromagnetic energy. If all the types of electromagnetic waves are arranged in order from the longest wavelength down to the shortest wavelength, a continuum called the electromagnetic spectrum is created. The wavelength of an electromagnetic wave is determined by how frequently the electric charge that generates the wave moves back and forth. If the charge moves back and forth slowly, the wavelen...

STP cable 3.1.8 STP cable combines the techniques of cancellation, shielded, and twisted wires. Each pair of wires is wrapped in metallic foil. The two pairs of wires are wrapped in an overall metallic braid or foil. It is usually 150-ohm cable. As specified for use in Token Ring network installations, STP reduces electrical noise within the cable such as pair to pair coupling and crosstalk. STP also reduces electronic noise from outside the cable such as electromagnetic interference (EMI) and radio frequency interference (RFI). STP cable shares many of the advantages and disadvantages of UTP cable. STP provides more protection from all types of external interference. However, STP is more expensive and difficult to install than UTP. A new hybrid of UTP is Screened UTP (ScTP), which is also known as foil screened twisted pair (FTP). ScTP is essentially UTP wrapped in a metallic foil shield, or screen. ScTP, like UTP, is also 100-ohm cable. Many cable installers and manufacturers m...