The coaxial cable 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 coaxial cables. One is a 50-ohm cable, which is used for digital transmission, and is also called baseband coaxial cable because it is mostly used for baseband transmission; the other is a 75-ohm cable, which is used for analog transmission, which is the broadband coaxial cable to be discussed in the next section. This difference is caused by historical reasons, not technical reasons or manufacturers.
This structure of coaxial cable makes it have high bandwidth and excellent noise suppression characteristics. The bandwidth of a coaxial cable depends on the cable length. A 1km cable can reach a data transmission rate of 1Gb/s~2Gb/s. Longer cables can also be used, but the transmission rate should be reduced or intermediate amplifiers should be used. Currently, coaxial cables are largely replaced by optical fibers, but they are still widely used in cable television and some local area networks.
A coaxial cable system that uses a limited TV cable for analog signal transmission is called a broadband coaxial cable. The term "broadband" comes from the telephone industry and refers to a frequency band wider than 4kHz. However, in computer networks, "broadband cable" refers to any cable network that uses analog signals for transmission.
Because the broadband network uses standard cable television technology, the usable 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 cable.
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-cable systems and single-cable systems.
A dual cable system has two identical cables laid side by side. In order to transmit data, the computer transmits the data through the cable 1 to a device at several roots of the cable, that is, the head-end, and then the head-end transmits the signal down the cable through the cable 2. All computers send via cable 1 and receive via cable 2.
Another solution is to allocate different frequency bands for internal and external communications on each cable. 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 cables are worse than baseband (single channel) cables in sending digital data, but its advantage is that it has been widely installed.