Guided Transmission Media in Computer Networks

Guided means are those that provide a conductor from one device to the other and include twisted-pair cables, coaxial cables, and fiber optic cables. A signal traveling by any of these means is directed and contained by the physical limits of the medium. Twisted pair and coaxial cable use metallic (copper) conductors that accept and carry electrical current signals. Fiber optic is a glass or plastic cable that accepts and transports signals in the form of light.

Twisted pair cable

The twisted-pair cable comes in two forms: unshielded and shielded.

Unshielded twisted pair cable (UTP)

The UTP (Unshielded Twisted Pair) cable is the most frequent type of communication medium currently used. Although it is the most familiar for its use in telephone systems, its frequency range is adequate to transmit both data and voice, which ranges from 100Hz to 5MHz. A twisted pair is usually made up of two copper conductors, each with colored plastic insulation. The plastic insulation has a color assigned to each band for identification see figure. Colors used both to identify the specific wires of a cable and to indicate which wires belong to a pair and how they relate to the other pairs of a bundle of wires.

The most common application of twisted pair is in the telephone system — almost all telephones connected to the telephone exchange by a twisted pair. Several kilometers of twisted pair can laid without the need for amplification, but repeaters needed for longer distances. When many interlaced pairs run substantial distances in parallel, such as the cables that run from an apartment building to the telephone exchange, they are tied in a bundle and lined with a protective cover. The pairs of these beams would interfere with each other if it were not for the entanglement. In some parts of the world where telephone lines hang from poles on the ground, it is common to see beams several centimeters in diameter.

The twisted pairs can use for both analog and digital transmission. The bandwidth depends on the thickness of the cable and the distance, but in many cases, you can achieve several megabits/sec for a few kilometers. Intertwined pairs widely used because of their proper performance and low cost, and it does not look like this change for a few years.

The advantages of UTP are its cost and its ease of use. UTP is cheap. Flexible and easy to install. In many LAN technologies, including Ethernet and annotated ring, high-end UTP is used.

The electronic industries association (EIA) has developed standards to graduate UTP cables according to their quality. The categories determined according to the quality of the cable, which varies from 1, for the lowest 5, to the highest. Each category of the EIA is suitable for certain types of uses and not for others:

Category 1: The basic twisted pair cable used in telephone systems. This level of quality is suitable for voice but unsuitable for anything other than low-speed data communications.

Category 2: The next highest grade, suitable for voice and data transmission up to 4 Mbps.

Category 3: Must have at least nine braids per meter and can be used for data transmission up to 10Mbps. It is currently the standard cable in most telephony telecommunications systems.

Category 5: Used for data transmission up to 100 Mbps.

UTP connectors: UTP cables usually connected to network devices through a type of connector and a type of plug like the one used in telephone plugs. The connectors can be male (the plug) or female (the receptacle). The male connectors enter the female connectors and have a movable tab (called a key) that blocks them when they located in a place. Each wire of a cable is attached to these plugs are the RJ45, which have eight conductors, one for each strand of four twisted pairs.

Shielded twisted pair cable (STP)

The STP cable has a metal sheath or an interlocking mesh coating that surrounds each pair of insulated conductors. See the figure. the metal housing prevents electromagnetic noise from penetrating. It also eliminates a phenomenon called interference, which is an unwanted effect of one circuit (or channel) on another circuit (or channel). It occurs when a line (acting as a receiving antenna) picks up some of the signals traveling on another line (acting as a transmitting antenna) This effect is experienced during telephone conversations when background conversations heard. By shielding each pair of twisted pair cable, most interference can eliminate.

The STP has the same quality considerations and uses the same connectors as the UTP, but it is necessary to connect the shield to ground. STP materials and manufacturing requirements are more expensive than those of UTP, but they result in cables less susceptible to noise.

Coaxial cable

The coaxial cable (or coax) carries signals with higher frequency ranges than the twisted pair cables ranging from 100KHz to 500MHz, in part because both media built quite differently. Instead of having two wires, the coaxial cable has a central conductor core formed by a solid or threaded wire (usually copper) covered by an insulator of dielectric material, which is, in turn, covered by an outer sheet of conductive metal, mesh or a combination of both (also usually copper). The outer metal cover serves as a shield against noise and as a second conductor, which completes the circuit. An insulating shield also covers this outer conductor, and a plastic cover protects the entire cable. See figure.

The different coaxial cable designs can be categorized according to their government radio (RG) ratings. Each RG number denotes a unique set of physical specifications, including the thickness of the internal conductor wire, the thickness and type of the inner insulation, the construction of the shield and the size and type of the outer shell.
Each cable defined by the RG classifications is adapted for a specialized function. The most frequent are:
RG-8, RG-9 and RG 11. Used in thick cable Ethernet
RG-58. Used in fine cable Ethernet
RG-59. used for TV

Coaxial cable connectors

Over the years, several connectors have been designed for use in coaxial cable, usually by manufacturers seeking specific solutions to specific product requirements. A few of the most widely used connectors have become standards. The most frequent of them all is called a barrel connector because of its shape. Of the barrel connectors, the most popular is the bayonet network connector (BNC, Bayonet Network Connector), which is pressed inward and locked in place with a half turn. Other types of barrel connectors are screwed together, which requires more installation effort, or they tighten without locking, making it less secure. Generally, a cable ends in a male connector that is plugged into or screwed into its corresponding female connector associated with the device. All coaxial connectors have a single pin that extends from the center of the male connector and enters an iron sleeve of the female connector. The coaxial connectors are very familiar due to the TV cables and the VCR plugs, which use both the pressure and the sliders.
Two other types of connectors that frequently used are the T connectors and the terminators. A T connector and terminators. A T-connector (used in the thin-cable ethernet) allows an auxiliary cable or other cables to route from the mainline. A cable that leaves a computer, for example, can be branched to connect to several terminals. Terminators are necessary for bus topologies where there is the leading cable that acts as a trunk with branches to several devices.

Optical fiber

Up to this moment, conductor (metal) cables have seen that transmit signals in the form of current. The fiber optic, on the other hand, is made of plastic or glass and transmits the signals in the form of light. To understand how optical fiber works, it is necessary first to explore several aspects of the nature of light.

The nature of light

Light is a form of electromagnetic energy that reaches its maximum speed in a vacuum: 300,000 kilometers/second (approximately 186,000 miles/second). The speed of light depends on the medium through which it propagates (the higher the density, the lower the speed).

Refraction: The light propagates in a straight line while moving through a single uniform substance. If a ray of light that propagates through one substance suddenly enters another (more or less dense), its velocity changes abruptly, causing the ray to change direction. This change is called refraction. A straw that protrudes from a glass of water seems to be twisted, or even broken because the light through which we see changes direction as the air moves to the water.

Fiber optic makes use of the properties of refraction to control the propagation of light through a fiber channel.

Reflection: When the angle of incidence becomes more significant than the critical angle, a phenomenon called reflection occurs (or, more accurately, full reflection, because some aspects of reflection always coexist with refraction). In this case, no light passes to the less dense medium, because the angle of incidence is always equal to the angle of reflection.

Fiber optics uses reflection to transmit light through a channel: a glass or plastic core surrounded by a less dense glass or plastic cover. The density difference of both materials must be such that the cover reflects the ray of light moving through the core instead of being refracted by it. The information encoded within a beam of light as a series of one-off flashes that represent bits one and zero.

Propagation modes

Current technology provides two modes of light propagation along optical channels, each of which requires fibers with different characteristics: multimode and single-mode. In turn, the multimode can be implemented in two ways: graduated index or gradual gradient index.

Multimode: The multimode is so named because there are multiple light rays from a light source that moves through the nucleus in different ways. How these rays move within the cable depends on the structure of the core.

In multimode multipath fiber, the density of the core remains constant from the center to the edges. A ray of light moves to this constant density in a straight line until it reaches the interface of the core and the cover. At the interface, there is an abrupt change to a lower density that alters the angle of motion of the beam. The term stepped index refers to the speed of this change.

The figure shows several beams (or rays) that propagate through a staggered index fiber. Some rays from the center travel in a straight line through the nucleus and reach the destination without being reflected or refracted. Some other rays hit the core interface and reflect at an angle smaller than the critical angle; These rays penetrate the deck and are lost. There are still others that hit the edge of the nucleus with angles more significant than the critical angle and reflect inside the nucleus to the other side, swinging back and forth along the channel until it reaches its destination.

The multimode fiber of gradual index, which decreases this distortion of the signal through the cable. The word index refers in this case to the index of refraction. The refractive index is related to density. Therefore, a fiber of gradual index has variable density. The density is higher in the center of the nucleus and gradually decreases to the edge. The figure shows the impact of this variable density on the propagation of light rays.

The signal introduced in the center of the nucleus. From this point, only the horizontal ray moves in a straight line through the central zone, of the constant density. Rays at other angles move through a series of densities that change constantly. Each density difference causes the beam to refract into a curve.

Singlemode The Singlemode uses staggered index fiber and a very focused light source that limits the rays to a minimal range of angles, all close to the horizontal. Monomode fiber is manufactured with a much smaller diameter than multimode fibers and with a density (refractive index) substantially lower density decrease results in a critical angle that is very close to 90 degrees to make the propagation of rays are almost horizontal. In this case, the propagation of the different rays is almost identical, and the delays are negligible. All rays reach the destination (together) and can recombine without distorting the signal.

Fiber size

Optical fibers define by the ratio between the diameter of their core and the diameter of their cover; both expressed in microns (micrometer).

Cable composition

The figure shows the composition of a typical fiber optic cable. The fiber is formed by a core surrounded by a cover. In most cases, the fiber covered by an intermediate level that protects it from communication. Finally, the entire cable enclosed by an outer casing.

Both the core and the cover can make of glass or plastic, but they must be of different densities. Also, the inner core must be ultra-pure and completely regular in shape and size. The chemical differences of the material and even small variations in the size and shape of the channel alter the angle of reflection and distort the signal. Some applications may admit some distortion, and their cables may be cheaper, but others depend on complete uniformity.

The outer covering (or sheath) can make with various materials, including a coating of Teflon, plastic, fibrous plastic, metal pipe, and metal mesh. Each of these materials serves a different purpose. Plastics are light and cheap but do not provide structural strength and can emit smoke when burned. Metal tubing provides greater strength but increases costs. Teflon is lightweight and can use outdoors, but it is expensive and does not increase the robustness of the cable. The choice of material depends on where the cable installed.

Various light sources for optical cables

As we have seen, the objective of the fiber optic cable is to contain and direct rays of light from the source to the destination.

For transmission, the emitting device must equip with a light source and the receiving device with a photosensitive cell (called a photodiode) capable of translating the received light into the current that can use in a computer. The light source can be either a Light Emitting Diode (LED) or a Laser Diode (ILD). LEDs are the cheapest source, but they provide an unfocused light that starts at the ends of the channel with uncontrolled angles and fades with distance. For this reason, the use of LEDs limited to short distances.

Connectors for fiber optics

The connectors for the fiber optic cable must be as accurate as of the cable itself. With metal means, the connections do not need to be as accurate as long as both conductors are in physical contact. On the other hand, with the optical fiber, any misalignment with either another segment of the core or with photodiode results in the signal reflected the emitter, and any difference in the size of the two connected channels results in a change in the angle of the signal. Also, the connections must complete even if the connected fibers not wholly joined. An interval between both cores results in a dissipated signal; A tightly pressed connection can compress both cores and alter the angle of reflection.

With these restrictions in mind, manufacturers have developed several connectors that are precise and easy to use. All popular connectors are barrel-shaped and connectors in male and female versions. The cable equipped with a male connector that is blocked or connected to a female connector associated with the device to be connected.

Advantages of fiber optics

The main advantage offered by the fiber optic cable over the twisted pairs and the coaxial cable is Immunity to noise, less attenuation of the signal and higher bandwidth.

Immunity to noise: Because transmissions use light instead of electricity, noise is not essential. The external light, the only possible interference, is blocked by the opaque coating of the channel.

Less attenuation of the signal: The transmission distance of the optical fiber is significantly higher than that achieved in other guided media. A signal can be transmitted over miles without the need for regeneration.

Higher bandwidth: The fiber optic cable can provide bandwidths (and thus data rates) substantially more significant than any twisted pair or coaxial cable. Currently, data rates and the use of bandwidth in fiber optic cables not limited by the medium, but the available technology of generation and reception of the signal.

The disadvantage of fiber optics

The main disadvantages of fiber optics are cost, installation, maintenance, and fragility.

Cost: The fiber optic cable is expensive. Because any impure or imperfection of the nucleus can interrupt the signal, the fabrication must be laboriously precise. Equally, getting a laser light source can cost thousands of dollars, compared to the hundreds of dollars needed for electric signal generators.

Installation / maintenance

Any crack or scratch of the core of a fiber optic cable diffuses the light and alters the signal. All brands must be polished and cast with precision. All connections must provide perfectly coupled joints but without excessive pressures. The connections of the metallic media, on the other hand, can be done with relatively few sophisticated cutting and pressing tools.

Fragility: Glass fiber breaks more easily than cable, which makes it less useful for applications where it is necessary to transport the hardware.

As manufacturing techniques have improved and costs have reduced, high data rates and noise immunity have made fiber optic an increasingly popular medium.