Transmission Medium

Transmission Media is the communication channel used in signal transmission; it can be divided into two categories: wired transmission and wireless transmission.

The so-called wired transmission, as the name suggests, uses physical lines such as twisted pair, coaxial cable or optical fiber to transmit data, while wireless transmission uses radio waves to transmit network data.

Wired transmission has the advantages of low cost and simple installation, so most network users use wired methods to access the Internet.

Considering space constraints or mobile factors, wireless transmission has its comprehensiveness and convenience.

Twisted pair

Twisted Wire Pair

Twisted-pair wire uses copper wire as a transmission line. There are 4 pairs of wires twisted with each other, and the transmission medium is covered with insulating material.

The twisted pair structure can not only reduce the noise interference of other electronic devices, but also slow the attenuation of the transmission signal.

The effective transmission distance of twisted pair cable does not exceed 100 meters.

Twisted pair can be divided into shielded twisted pair (Shielded Twisted Pair, STP) and unshielded twisted pair (Unshielded Twisted Pair, UTP) according to whether there is a metal layer between the outer layer and the twisted wire. ). STP has better noise immunity, but the price is higher. Generally, the regional network is mainly UTP twisted pair.

Since there is always the subject of electricity generation and magnetism and magnetism generation in the transmission of electricity, it is also the main factor that interferes with signal transmission, which is the so-called electromagnetic interference (EMI). This is a phenomenon that exists in nature itself, as long as it is the transmission of electricity , We can only do everything possible to minimize this phenomenon, but it can’t completely disappear. The main purpose of twisted-pair twisting is to offset the magnetic field effect generated on the two wires and reduce the electromagnetic phenomenon’s impact on the transmission. The negative impact produced, and the higher the degree of twisting of the twisted pair, the higher the effectiveness of the electromagnetic effect, and the better the transmission effect, but the relative cost will also be higher.

Coaxial Cable

Coaxial cable can transmit analog and digital signals, with a maximum transmission rate of 10Mbps.

The cable installed in general cable television is a type of coaxial cable (RG-59), and the effective transmission distance is between 200 and 500 meters.

In the local area network, RG-58 coaxial cable is used.

Advantages and disadvantages of coaxial cable

advantage

Because of the double protection (metal copper mesh and insulating skin), it is less susceptible to external interference and has a longer life.

shortcoming

Compared with twisted pair cable, the price is more expensive, and it is also very heavy.

Optical Fiber

optical fiber

An optical fiber is a medium that transmits information from one end to the other. It is a glass or plastic fiber used as a transmission medium for information to pass through. It is roughly the same thickness as a human hair.

Optical fiber is similar to coaxial cable, except that there is no mesh shielding layer. The center is a glass core for light propagation. The core is surrounded by a glass envelope with a lower refractive index than the core to keep the optical fiber in the core. The outside is a glass core. A thin plastic jacket is used to protect the envelope. Optical fibers are usually bundled and protected by an outer shell. The fiber core is usually a double-layer concentric cylinder with a small cross-sectional area made of quartz glass. Its texture is brittle and easy Fracture, so a protective layer is needed.

Optical fiber can only transmit digital signals. The diameter of an optical fiber is only 125 ~ 140 microns (), which is about the thickness of a hair. The optical fiber is made of high-purity glass fiber (Core) and is covered by a material with a lower refractive index (Cladding). Live, plus an opaque protective layer.

Optical signals are transmitted in the central fiber, and the cladding provides reflection. Generally, dozens of optical fibers are wrapped together, which is an optical cable to facilitate the wiring and erection of the cable.

The transmission speed of optical fiber is about 100Mbps to 1Gps, which is currently the fastest and most expensive transmission medium.

Operation of fiber optic system

The process of any communication transmission includes: encoding → transmission → decoding. Of course, the transmission process of the optical fiber system is roughly the same. After the electronic signal is input, the signal is digitally encoded through the transmitter to become an optical signal, and the light is transmitted through the optical fiber as a medium. To the receiver at the other end, the receiver decodes the signal and restores it to the original electronic signal output.

Since optical fiber is a transmission medium, it can transmit data such as telephone calls or computer data like ordinary copper cables. The difference is that optical fiber transmits optical signals instead of electrical signals. Therefore, optical fiber has many unique advantages.

Advantages of optical fiber

Optical fiber has many advantages over copper wire and coaxial cable. Because it has good tensile strength, low quality, and is relatively small, it can minimize installation problems. This is why telephone companies like to use optical fiber. Many current Some cable ducts are completely filled, and it is impossible to add new cables. Replacing all cables with optical fibers can save a lot of space, and copper material manufacturers increase the price a lot when manufacturing copper wires. Optical fibers are much lighter than copper wires. This greatly reduces the need for expensive mechanical support systems that must be maintained. For new lines, because of the low installation cost, optical fiber is better than copper wire.

Because optical fibers are not charged, they are ideal for use in hazardous environments such as flammable and explosive. In this environment, if copper wire is used, sparks that burst from the gap when the copper wire breaks will cause an explosion. Moreover, if the optical fiber is damaged, there is no danger of electric shock to humans. In addition, unlike the traditional copper and aluminum wires used for structural wiring installation, the glass fiber is corrosion resistant. Material.

Especially in terms of signal transmission, optical fiber cables have unparalleled advantages over traditional cables. Because optical fibers are not affected by the interference of electromagnetic fields, optical fibers provide clearer signals than copper wires. Optical fibers will not be affected by engine rotation or power failure. In addition, the signal attenuation in the optical fiber is small, and a repeater is needed every 30km on a long line, and a repeater is needed every 5km on a copper wire, which makes the optical fiber save a lot of money. In addition, the optical fiber cable A length of 550 meters (1800 feet) can be used continuously.

Optical fiber technology provides unparalleled flexibility and scalability to meet future needs. Optical fiber can provide much higher bandwidth than copper wire, which makes it used in advanced networks. Optical fiber does not leak light and is difficult to splice, so optical fiber networks are difficult Being tapped, the safety factor is very high.

The reason why the optical fiber is better than the copper wire is due to the different internal physical structure. When the electrons move along the copper wire, they affect each other and are also affected by the electrons outside the copper wire. However, the photons in the optical fiber not only do not affect each other ( There is no charge and discharge), and it is not affected by photons outside the fiber.

Optical fiber has the advantages of light weight, small size, longer service life, no radiation, no electromagnetic interference, low attenuation, large transmission bandwidth, protection from lightning and high security of signal stealing, etc., and it has been widely used as main trunk lines. ) Transmission cable.

At present, the unfavorable factors for the promotion of optical fibers are the high price and the higher difficulty of laying.

However, the disadvantage of optical fiber is that because the transmission of light is one-way, two-way transmission requires two optical fibers or two frequency bands on one optical fiber. Finally, the price of optical fiber interfaces is more expensive than electronic interfaces. But in the future, all the distances beyond a few meters It is possible to use optical fiber for digital communication on fixed lines.