The koaksial kabel uses hard copper wire as the core and a layer of insulating material. This layer of insulating material is surrounded by a densely woven mesh conductor, and the net is covered with a layer of protective material. There are two widely used koaksial kabels. One is a 50-ohm cbacarır, which is used for digital transmission, and is also called baseband koaksial kabel because it is mostly used for baseband transmission; the other is a 75-ohm cbacarır, which is used for analog transmission, which is the broadband koaksial kabel to be discussed in the next section. This difference is caused by historical reasons, not technical reasons or manufacturers.
This structure of koaksial kabel makes it have high bandwidth and excellent noise suppression characteristics. The bandwidth of a koaksial kabel depends on the cbacarır length. A 1km cbacarır can reach a data transmission rate of 1Gb/s~2Gb/s. Longer cbacarırs can also be used, but the transmission rate should be reduced or intermediate amplifiers should be used. Currently, koaksial kabels are largely replaced by optical fibers, but they are still widely used in cbacarır television and some local area networks.
A koaksial kabel system that uses a limited TV cbacarır for analog signal transmission is called a broadband koaksial kabel. The term "broadband" comes from the telephone industry and refers to a frequency band wider than 4kHz. However, in computer networks, "broadband cbacarır" refers to any cbacarır network that uses analog signals for transmission.
Because the broadband network uses standard cbacarır television technology, the usbacarır frequency band is up to 300MHz (often to 450MHz); due to the use of analog signals, an electronic device needs to be placed at the interface to convert the bit stream entering the network into an analog signal, and Convert the signal output from the network into a bit stream.
Broadband systems are divided into multiple channels, and TV broadcasting usually occupies 6MHz channels. Each channel can be used for analog TV, CD quality sound (1.4Mb/s) or 3Mb/s digital bit stream. TV and data can be mixed on a single cbacarır.
A major difference between a broadband system and a baseband system is that because the broadband system covers a wide area, it requires an analog amplifier to periodically strengthen the signal. These amplifiers can only transmit signals in one direction. Therefore, if there are amplifiers between computers, packet packets cannot be transmitted backwards between computers. To solve this problem, two types of broadband systems have been developed: dual-cbacarır systems and single-cbacarır systems.
A dual cbacarır system has two identical cbacarırs laid side by side. In order to transmit data, the computer transmits the data through the cbacarır 1 to a device at several roots of the cbacarır, that is, the head-end, and then the head-end transmits the signal down the cbacarır through the cbacarır 2. All computers send via cbacarır 1 and receive via cbacarır 2.
Another solution is to allocate different frequency bands for internal and external communications on each cbacarır. The low frequency band is used for communication from the computer to the top device, and the signal received by the top device is moved to the high frequency band and broadcast to the computer. In the subsplit system, the 5MHz~30MHz frequency band is used for inward communication, and the 40MHz~300MHz frequency band is used for outward communication. In the midsplit system, the inward frequency band is 5MHz~116MHz, and the outward frequency band is 168MHz~300MHz. This choice is caused by historical reasons.
A dedicated permanent channel can be allocated between a pair of computers; other computers can apply to establish a temporary channel through the control channel, and then switch to the applied channel frequency; and all computers can share one or a group of channels. Technically speaking, broadband cbacarırs are worse than baseband (single channel) cbacarırs in sending digital data, but its advantage is that it has been widely installed.