Transmission medium, also known as transmission medium, is the physical path between the sending device and the receiving device. Transmission media can be divided into guided transmission media and non-guided transmission media. In guided transmission media, electromagnetic waves are guided to propagate along a solid medium (copper wire or optical fiber), while non-guided transmission media can be air, vacuum, or seawater.
1. Twisted pair
Twisted pair is an ancient and commonly used transmission medium. It consists of two mutually insulated copper wires that are twisted side by side with certain rules. Twisting reduces electromagnetic interference from adjacent wires. In order to further improve the anti-electromagnetic interference ability, a shielding layer made of metal wires can be added to the outside of the twisted pair, which is the shielded twisted pair (STP), and the twisted pair without the shielding layer is called unshielded. Twisted Pair (UTP),
Twisted pair cables are inexpensive and one of the most commonly used transmission media, commonly used in local area networks and traditional telephone networks. The bandwidth of the twisted pair depends on the thickness of the copper wire and the distance traveled. Both analog transmission and data transmission can use twisted pair, and the communication distance is generally several kilometers to tens of kilometers. When the distance is too great, for analog transmission, an amplifier is used to amplify the weakened signal; for digital transmission, a repeater is used to shape the distorted signal.
2. Coaxial cable
Coaxial cables are composed of conductor copper core wires, insulating layers, network braided shielding layers and plastic outer layers. According to different characteristic impedance values, coaxial cables are usually divided into two categories: 50π coaxial cables and 75π coaxial cables. Among them, 50π coaxial cable is mainly used to transmit baseband digital information, also known as baseband coaxial cable, which is widely used in local area networks; 75π coaxial cable is mainly used to transmit broadband signals, also known as broadband coaxial cable. Mainly used in cable TV systems.
Due to the effect of the outer conductor shielding layer, the coaxial cable has good anti-interference characteristics and is widely used to transmit higher-speed data, and its transmission distance is longer, but the price is more expensive than twisted pair.
3. Optical fiber
Optical fiber communication is the use of optical fibers (optical fibers for short) to transmit light pulses for communication. The presence of light pulses represents 1, and the absence of light pulses represents 0. The frequency of visible light is about 10^8MHZ, so the broadband range of the optical fiber communication system is extremely large.
Optical fibers are mainly composed of a core and a cladding through which light waves are conducted, and the cladding has a lower refractive index than the core. When light is directed from a medium with a high index of refraction to a medium with a low index of refraction, the angle of refraction will be greater than the angle of incidence. Therefore, if the incident angle is large enough, total reflection will occur, that is, when the light hits the cladding, it will be refracted back to the core. This process is repeated, and the light will be transmitted along the fiber.
As long as the incident angle of light from the core to the surface of the core is greater than a certain critical angle, total reflection can occur. Therefore, there can be many light incidents from different angles transmitted in one optical fiber, which is called a multimode fiber, and the light source of the multimode fiber is a light-emitting diode. However, when the optical pulse is transmitted in the multimode fiber, it will gradually broaden and cause distortion, so the multimode fiber is suitable for short-distance transmission.
If the diameter of the fiber is reduced to only one wavelength of light, the fiber acts like a waveguide, which allows the fiber to propagate forward without multiple reflections. Such a fiber is a single-mode fiber. The core of single-mode fiber is very thin, only a few microns in size, and the manufacturing cost is high. At the same time, the light source of the single-mode fiber is a laser diode with good directivity, so the loss of the single-mode fiber is small, and it is suitable for long-distance transmission.