Transmission media Physical Transmission Media-Twisted Pair A pair of mutually insulated metal wires are twisted to resist part of the external electromagnetic wave interference. In practice, multiple pairs (typically four pairs) of twisted pairs are wrapped together in an insulated cable sleeve. The thicker the line, the smaller the attenuation of the line. The high-speed network requires the thicker the diameter of the twisted pair. Advantages: easy wiring, cheap interface, suitable for short distance transmission. Disadvantages: The transmission rate is worse than the optical fiber, the transmission distance is short, and it is susceptible to interference. Physical transmission media-optical fiber Filaments drawn with pure quartz in a special process. The transmission rate is above several hundred "Tai" (T = 1000Gbps). Advantages: high transmission rate, light quality, free from electromagnetic interference, non-conductive, no grounding, and not easy to be eavesdropped. Disadvantages: Fiber optics are expensive and interfaces are expensive. Physical transmission media-coaxial cable It is composed of inner conductor core wire, insulation layer, outer conductor shielding layer and outer layer sleeve. The inner conductor core and the outer conductor shield are on the same axis and are called coaxial cables. 50 ohms for computer networks and 75 ohms for broadcast television. Physical transmission media-wireless Use the spectrum of microwave wavelengths or longer wavelengths. Such as 900 / 1800MHz cellular communication devices. Use the spectrum of the light wave range. Such as infrared communication. Physical transmission media-satellite communications Synchronous Earth Satellite: Located at an altitude of 36500 Km, it rotates around the earth at the same speed as the earth's rotation angular velocity, and the antenna direction is always aligned with the ground communication system. Ground satellites in low-Earth orbit: The flight orbit is low, hundreds of kilometers, and many satellite relays are required to make communication continuous. How to wear masks to protect against coronavirus ? Medical Surgical Mask,Disposable face mask,Masks n95 3m,Face shield Shenzhen Waweis Technology Co., Ltd. , https://www.waweis.com
Before putting on a mask, clean hands with alcohol-based hand rub or soap and water.
Cover mouth and nose with mask and make sure there are no gaps between your face and the mask.
Avoid touching the mask while using it; if you do, clean your hands with alcohol-based hand rub or soap and water.
Replace the mask with a new one as soon as it is damp and do not re-use single-use masks.
To remove the mask: remove it from behind (do not touch the front of mask); discard immediately in a closed bin; clean hands with alcohol-based hand rub or soap and water.