- Transmission medium, also known as transmission medium or transmission medium, is the physical path between a transmitter and a receiver in a data transmission system .
- Transmission media can be divided into two categories, namely guided transmission media and unguided transmission media .
- In guided transmission media, electromagnetic waves are guided along a solid medium (copper wire or optical fiber) .
- Unguided transmission media refers to free space . In unguided transmission media, the transmission of electromagnetic waves is often referred to as wireless transmission .
2.3.1 Guided Transmission Media
twisted pair
- The most commonly used transmission media.
- Both analog transmission and digital transmission can use twisted pair, and the communication distance is generally several to ten kilometers.
- Shielded twisted pair STP (Shielded Twisted Pair): with metal shield
- Unshielded Twisted Pair UTP (Unshielded Twisted Pair)
twisted pair standard
- In 1991, the American Electronics Industry Association EIA and the Telecommunications Industry Association jointly issued a standard EIA/TIA-568 for unshielded twisted pair and shielded twisted pair cables for indoor data transmission .
- In 1995, the wiring standard was updated to EIA/TIA-568-A .
- This standard specifies 5 types of UTP standards (from Category 1 to Category 5).
- For transmitting data, the most commonly used UTP is Category 5 (Category 5 or CAT5) .
coaxial cable
- Coaxial cables have good anti-interference characteristics and are widely used to transmit higher-speed data.
- The bandwidth of a coaxial cable depends on the quality of the cable.
- 50 ohm coaxial cable - commonly used for LAN/digital transmission
- 75 ohm coaxial cable - commonly used for cable TV/analog transmission
fiber optic cable
- Optical fiber is the transmission medium for optical fiber communication.
- Since the frequency of visible light is very high, on the order of 108 MHz, the transmission bandwidth of an optical fiber communication system is far greater than the bandwidth of various other transmission media at present.
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 angle of incidence is large enough, total reflection occurs and the light travels down the fiber.
How Fiber Optics Work
As long as the incident angle of the light rays from the core to the surface of the core is greater than a certain critical angle, total reflection can occur.
Multimode Fiber vs Single Mode Fiber
- Multimode fiber: There can be multiple light rays incident at different angles to transmit in one fiber. Such fibers are called multimode fibers.
- Single-mode fiber: If the diameter of the fiber is reduced to only one wavelength of light , the fiber acts like a waveguide, allowing the light to travel all the way forward without multiple reflections. Such fibers are called single-mode fibers.
Bands of light waves used in fiber optic communications
- The centers of the three commonly used bands are located at 850 nm, 1300 nm and 1550 nm, respectively.
- All three bands have a bandwidth of 25,000 to 30,000 GHz, which shows that the communication capacity of optical fiber is very large.
Fiber Advantages
- Very large communication capacity.
- The transmission loss is small and the relay distance is long.
- Good anti-lightning and electromagnetic interference performance.
- No crosstalk interference, good confidentiality.
- Small size and light weight.
2.3.2 Unguided Transmission Media
- Free space is called "unguided transmission medium".
- Wireless transmission uses a wide range of frequency bands.
- Short-wave communication (ie high-frequency communication) mainly relies on the reflection of the ionosphere, but the communication quality of the short-wave channel is poor and the transmission rate is low.
- Microwaves propagate mainly in straight lines in space.
- There are two ways of traditional microwave communication: terrestrial microwave relay communication, satellite communication
ISM bands used by wireless LANs
- To use a certain segment of the radio spectrum for communication, a license from the relevant radio spectrum management agency of the country's government is usually required. However, there are also some radio frequency bands that are free to use. For example: ISM. ISM standards may vary slightly from country to country.
