THE DEVICES CAN CONNECT TO THE NETWORK VIA RADIO OR BY WIRED MEANS.
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WIRELESS IEEE 802.11 STANDARD
Registered trademark of the Wi-Fi Alliance group dedicated to certifying that Wi-Fi products comply with IEEE 802.11 wireless standards. Each new standard improves performance, reliability and safety. The hardware must be manufactured specifically to support a standard.
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WIRED NETWORK STANDARD IEEE 802.3 (ETHERNET):
Speeds from 10 to 1000 Mbps on various physical media (coaxial, twisted pairs and optical fibers)
Comparative table of the most popular 802.3 standards:
The most commonly used are 100Base-TX (Fast Ethernet) which reached 100 Mbps with copper pairs and 1000BaseT (Gigabit ethernet over cat. 5 copper cables).
PHYSICAL MEANS
OVERVIEW OF TRANSMISSION MEANS
Doppino consists of a pair of twisted copper cables enclosed in a sheath. In a network cable there are normally 4 pairs of twisted pairs that can be shielded or not against interference, UTP (Unshielded Twisted Pair) if they are not shielded and STP (Shielded Twisted Pair) cable if shielded. The last frontier is the optical fiber which however presents the need to repeat the signal after a few KM introducing latency because a signal that theoretically travels at the speed of light is electronically repeated, the impossibility of making too pronounced curves that clearly in a medium that must reflect light would cause optical interference, however it is good that the theoretical speed of the transmission medium in the absence of repeaters would be that of the speed of light, therefore very high.
DEEPENING ON TRANSMISSION MEANS
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COAXIAL CABLE
The coaxial cable (usually abbreviated to coax,), in telecommunications, is an electrical cable used as a means of transmitting an electrical information signal, belonging to transmission lines and widely used in electrical communications. It is also commonly referred to as a video cable as it is often used in video signal transmission applications.
DESCRIPTION
It is composed of a single copper conductor placed in the center of the cable (core) and a dielectric (generally made of polyethylene or PTFE) which separates the central core from an external screen made up of intertwined metal wires (mesh) or a wrapped sheet spiral (braid), constantly ensuring insulation between the two conductors. The metal screen helps block out interference. The cable is then fitted with connectors at its connection ends. The signal travels in the form of an electromagnetic field between the core and the mesh at a speed v which is a fraction of that of light in a vacuum and equal to c/n with n refractive index of the interposed dielectric. The analysis of the propagation of the electromagnetic field in the coaxial cable is part of the theory of transmission lines, while the conveying effect is comparable to that of a metallic waveguide. In particular, the electromagnetic mode TEM propagates in it. Depending on the use, a bnc connector can be applied to the cable.
CHARACTERISTICS
Coaxial cables are produced of different types depending on the frequency of the signal to be carried and the power of the same. The impedance values are mainly two:
- 50 ohms, used for digital (such as early versions of Ethernet) or amateur radio transmissions, as well as for standard signals in the field of electronic measuring instruments;
- 75 ohm, used for analog video signal, SDI digital video, for television antennas (connection with terrestrial or satellite reception antenna) and for cable Internet connections.
The characteristic impedance of 50 ohms is preferred for transmitting or transceiver equipment, since the central conductor, with the same external diameter of the cable, has a larger diameter, therefore a lower resistance and better carries high currents; the characteristic impedance of 75 ohms, on the other hand, is preferred for only receiving devices since, with the same external diameter, it has a lower attenuation than a 50 ohms cable. There are also cables with characteristic impedance of 93 ohms and 105 ohms used for data connection networks. A particular type, characterized by extreme flexibility and good tear resistance, is used in oscilloscope probes. The coaxial cable, DC for analog transmissions and similar to the cable that carries radio and TV signals over long distances, was later adapted to digital data communication. Digital data is much more susceptible than analog data to noise and signal distortions that are introduced when signals travel over large distances.
RG-59 coaxial cable
A: plastic outer sheath
B: copper wire mesh or ground
C: internal dielectric insulator
D: copper core or hot pole
Therefore, networks that use coaxial cable as a transmission medium can only extend for limited distances unless signal repeaters are used that regenerate the signal periodically (repeater). Simple amplifiers would not be suitable, because these would also amplify the noise and distortion that the signal picks up as it travels through the medium. For a long time the coaxial cable was also the only economical choice to use in the cabling of high-speed local networks because compared to the classic twisted pair it guarantees a bandwidth capacity and therefore a higher transmission speed. The disadvantages of installing and maintaining a coaxial cable system include the fact that the cable is complex and expensive to manufacture, is difficult to use in confined spaces as it cannot be excessively bent around tight corners, and is prone to frequent mechanical breakage at connectors. However, the high resistance to signal interference should be noted. In these areas, the coaxial cable has been supplanted by optical fiber and related optical communications.
DOUBLE TWISTED
A twisted pair (also called simply twisted pair or two-wire pair) is a type of cabling (transmission line) consisting of a pair of insulated copper conductors used for the transmission of telephone and data communications by a service provider. It is an essential element of the telephone network in the section known as the access network.
DESCRIPTION
The twisted pair consists of a twisted pair of conductors by means of a doubling process. The doubling of the twisted pair has the purpose of equalizing on average the external electromagnetic fields of the two conductors. Using a differential transmission technique, it will be possible to eliminate further disturbances. The twisted pair can be single (a single pair, for example at the input of a single user) or braided by a multitude of pairs (for example in the cables of the so-called “last mile”). In this case, each pair has a different doubling pitch, to reduce as much as possible the phenomenon of crosstalk between the various contiguous pairs and, to allow identification, one conductor per pair is distinguished by a code printed on the insulation, different for color and length of the colored band. A typical problem of twisted pairs is delay skew (or propagation distortion), which is a variation in the propagation delay of the signal on the individual pairs, due to the different twisting pitch of the pairs in a multipair cable.
MULTI-PAIR CABLES IN LAN
The twisted pairs are also used, in a four-pair intertwining, to transmit data in a local network, through the ethernet protocol and RJ-45 plug. These twisted pairs can be shielded to reduce electromagnetic interference. Since the shield consists of metal tape that is coiled around the twisted pair, it should be grounded.
UNSHIELDED TWISTED PAIR
UTP stands for Unshielded Twisted Pair and identifies an unshielded cable commonly used for connection in ethernet networks. It is made up of eight copper wires twisted in pairs. Each pair is intertwined with a different step, and each pair is intertwined with the others. The intertwining of the wires has the purpose of reducing interference, disturbances and limiting crosstalk. The maximum length of a UTP cable in the ethernet standard is 100 m. UTP cables follow the standardized specifications in TIA/EIA which divide them into various categories based for example on the number of twists and the ability to carry signals. Currently category 5 and 5e are the most used, however there is also category 6 which allows you to reach higher speeds with the same maximum length. The categories range from 1 to 7. A UTP cable ends with 8P8C type connectors that plug directly into the device interface (network card, Hub, Switch, Router, etc). If two similar devices are to be connected (for example PC-PC or SWITCH-HUB), a crossed ethernet cable is used, while if different devices are to be connected (for example PC-SWITCH), a straight one is used. The straight cables have the 8 wires in the same order in both 2 connectors, while the cross ones have a different sequence, and are used to connect two ethernet hosts together. In the construction of the cable, or in the crimping of the connectors at its ends, two standards can be followed: TIA/EIA 568A and TIA/EIA 568B which have a different order of the 8 wires (identified by different colors).
SHIELDED TWISTED PAIR
Shielded Twisted Pair or STP identifies a shielded cable commonly used for connection in ethernet networks. It is made up of eight copper wires twisted in pairs (pair), in addition each pair is intertwined with the others. The intertwining of the wires has the purpose of reducing interference, disturbances and limiting crosstalk. The maximum length of an STP cable is 100m. Each pair is wrapped in sheets of conductive material that act as a screen for electromagnetic waves, outside the 4 pairs there is an additional sheet of conductive material. STP cables follow the standardized specifications in TIA/EIA which divide them into various categories based for example on the number of twists and the ability to carry signals. Currently categories 5 and 5e are the most used, however there are also categories 6 / 6a / 7 / 7a (TIA/EIA nomenclature), also defined as E / Ea / F / Fa (ISO/IEC nomenclature), which allow to reach higher speeds. An STP cable ends with RJ-45 type connectors (also shielded) that plug directly into the device interface (network card, hub, Switch, Router, etc). If two similar devices are to be connected (for example PC-PC or SWITCH-HUB), a “cross” type cable is used; while if you have to connect different devices (for example PC-SWITCH), one of the “straight” type. The straight cables have the 8 wires in the same order in both 2 connectors, while the cross ones have a different sequence. In the construction of the cable, or rather in crimping the connectors at its ends, two standards can be followed: TIA/ EIA 568A and TIA/EIA 568B which have a different order of the 8 wires (identified by different colors).
THE OPTICAL FIBER
Optical fiber, in the science and technology of materials, indicates a material consisting of glass or polymeric filaments, made in such a way as to be able to conduct light inside them, finding important applications in telecommunications, medical diagnostics and lighting technology: with a coating diameter ( mantle) of 125 micrometers (about the size of a hair) and very low weight, are available in the form of cables, flexible, immune to electrical disturbances and to the most extreme atmospheric conditions, and not very sensitive to temperature variations. Classified as dielectric waveguides based on the inhomogeneity of the medium, whose core is the seat of guided propagation of the electromagnetic field in the form of electromagnetic waves, in other words, they allow to convey and guide an electromagnetic field of sufficiently high frequency inside them ( generally near the infrared) with extremely limited losses in terms of attenuation. They are therefore commonly used in telecommunications as a means of transmission of optical signals even over large distances or on a transport network and in the supply of wired broadband network accesses (from 100 Mbit s to petabyte/s using WDM technology).
DESCRIPTION
Each single optical fiber is composed of two concentric layers of extremely pure transparent material: a central cylindrical core, or core, and a cladding around it. The core has a very small diameter of about 10 µm for singlemode and 50 µm for multimode, while the mantle has a diameter of about 125 µm. The two layers are made with materials with slightly different refractive index, the mantle must have a lower refractive index (typically 1.475) than the core (about 1.5). As a further feature, the mantle must have a thickness greater than the damping length of the evanescent wave, a characteristic of the transmitted light so as to capture the light that is not reflected in the core. The optical fiber works as a kind of tubular mirror. The light that enters the nucleus at a certain angle (limit angle) propagates through a series of reflections to the separation surface between the two materials of the nucleus and the mantle. Outside the fiber there is a protective polymeric sheath called jacket which serves to give resistance to physical stress and corrosion and to avoid contact between the fiber and the external environment. The different types of fibers are distinguished by core diameter, refractive indices, material characteristics, transition profile of the refractive index and doping (addition of small amounts of other materials to modify the optical characteristics). The core and the cladding of the optical fiber can be made of silica or plastic polymers.
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