Coaxial cable I. Overview The coaxial cable takes hard copper wire as the core and is covered with a layer of insulating material. This layer of insulating material is surrounded by a densely woven mesh conductor, and a layer of protective material is covered outside the mesh. There are two widely used coaxial cables. One is a 50-ohm cable for digital transmission. It is also called baseband coaxial cable because it is mostly used for baseband transmission. The other is a 75-ohm cable for analog transmission, which is the broadband coaxial cable that will be discussed in the next section. This difference is caused by historical reasons, not technical reasons or manufacturers. This structure of the coaxial cable makes it have high bandwidth and excellent noise suppression characteristics. The bandwidth of the coaxial cable depends on the cable length. The 1km cable can reach the data transmission rate of 1Gb / s ~ 2Gb / s. Longer cables can also be used, but the transmission rate is reduced or an intermediate amplifier is used. At present, coaxial cables are largely replaced by optical fibers, but they are still widely used in cable TV and some local area networks. 2. Broadband coaxial cable Coaxial cable systems that use limited television cables for analog signal transmission are called broadband coaxial cables. 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 TV 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 analog signals, and Convert the signal output by the network into a bit stream. The broadband system is divided into multiple channels, and TV broadcasting usually occupies a 6MHz channel. Each channel can be used for analog TV, CD quality sound (1.4Mb / s) or 3Mb / s digital bitstream. TV and data can be mixed and transmitted on one cable. One of the main differences between a broadband system and a baseband system is that the broadband system covers a wide area. Therefore, analog amplifiers are required to periodically strengthen the signal. These amplifiers can only transmit signals in one direction, so if there is an amplifier between the computers, the packet packets cannot be transmitted in the reverse direction between the computers. To solve this problem, people have developed two types of broadband systems: dual-cable systems and single-cable systems. 1) Double cable system The 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 the devices at the roots of the cable, that is, the head-end, and then the head-end device transmits the signal down the cable through the cable 2. All computers are sent via cable 1 and received via cable 2. 2) Single cable system 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. The signal received by the top device is moved to the high frequency band and broadcast to the computer. In a subsplit system, the 5MHz ~ 30MHz frequency band is used for inbound communication, and the 40MHz ~ 300MHz frequency band is used for outbound communication. In the midsplit system, the inward frequency band is 5MHz ~ 116MHz, while the outward frequency band is 168MHz ~ 300MHz. This choice is caused by historical reasons. 3) There are many ways to use broadband systems. A dedicated permanent channel can be allocated between a pair of computers; some computers can apply to establish a temporary channel through the control channel, and then switch to the channel frequency applied for; can also allow all computers to share a channel or a group of channels. Technically speaking, broadband cables are inferior to baseband (ie single channel) cables in transmitting digital data, but its advantage is that it has been widely installed. Coaxial cable networks can generally be divided into three categories: · Backbone network. The trunk line is different from other lines in terms of diameter and attenuation. The former is usually composed of a shielded cable. Coaxial cables cannot be twisted, and the parts are connected through low-loss connectors. The connector is physically matched to the cable. The middle connector and the coupler are wrapped with wire tubes to prevent accidental grounding. If you want to bury the cable in a place that is not exposed to light, it is best to bury the cable in the formation below the freezing point. If you don't want to bury the cable underground, it is best to use electric poles to erect. The coaxial cable has a mark every 100 meters to facilitate maintenance. Support the cable every 20 meters if necessary. When installing inside the building, it is necessary to consider the ease of maintenance and expansion, and the need to provide pipes and protection cables where necessary. Coaxial cables are generally installed between equipment. Each user location is equipped with a connector to provide an interface for the user. The installation method of the interface is as follows: (1) Cut the thin cable, install the BNC heads at both ends, and then connect them to the T-type connectors. Second, the parameter index (1) The characteristic impedance of the coaxial cable The average characteristic impedance of the coaxial cable is 50 ± 2Ω, the periodic change of the impedance along the single coaxial cable is a sine wave, the average value of the center is ± 3Ω, and its length is less than 2 meters. 2. Physical parameters of coaxial cable The coaxial cable is composed of a center conductor, a layer of insulating material, a shielding layer composed of a mesh fabric, and an external insulating material layer. Figure 1 Schematic diagram of coaxial cable structure The coaxial cable is flexible enough to support a bending radius of 254mm (10 inches). The center conductor is a solid copper wire with a diameter of 2.17 mm ± 0.013 mm. The insulation material must meet the electrical parameters of the coaxial cable. The shielding layer is composed of metal strips or sheets that meet the transmission impedance and ECM specifications. The inner diameter of the shielding layer is 6.15mm and the outer diameter is 8.28mm. The external insulation material is generally selected from polyvinyl chloride (such as PVC) or similar materials. 3. The main parameters for testing the cable are: (1) Open circuit of conductor or shielding layer. 3. Specifications Coaxial cable can be divided into two basic types, baseband coaxial cable and broadband coaxial cable. At present, the commonly used baseband cables have a shielded wire made of copper and a characteristic impedance of 50 (such as RG-8, RG-58, etc.); the shielding layer of cables commonly used for broadband coaxial cables is usually stamped with aluminum The characteristic impedance is 75 (such as RG-59, etc.). The thick coaxial cable and the thin coaxial cable refer to whether the diameter of the coaxial cable is large or small. Thick cable is suitable for relatively large local networks. It has long standard distance and high reliability. Since there is no need to cut off the cable during installation, the network access position of the computer can be flexibly adjusted as required. But the thick cable network must install transceivers and transceiver cables, installation is difficult, so the overall cost is high. On the contrary, the installation of the thin cable is relatively simple and the cost is low, but because the cable is cut during the installation process, the two ends must be equipped with a basic network connector (BNC), and then connected to the T-type connector at both ends, so when there are many connectors, it is easy to cause poor contact Hidden danger, this is one of the most common failures occurring in the Ethernet in operation. In order to maintain the correct electrical characteristics of the coaxial cable, the cable shield must be grounded. At the same time, there must be terminators at both ends to weaken the signal reflection. Both thick and thin cables are bus topologies, that is, multiple machines are connected to one cable. This topology is suitable for machine-dense environments. But when a contact breaks down, the fault will affect all the machines on the whole cable in series. The fault diagnosis and repair are very troublesome. Therefore, it will be gradually replaced by unshielded twisted pair or optical cable. The most commonly used coaxial cables are as follows: · RG-8 or RG-11 Computer network generally selects RG-8 Ethernet thick cable and RG-58 Ethernet thin cable. RG-59 is used in TV systems. RG-62 is used in ARCnet network and IBM3270 network. 4. Wiring structure In the computer network wiring system, there are three different construction methods for the thick and thin cables of the coaxial cable, namely the thin cable structure, the thick cable structure and the thick / thin cable mixed structure. 1. Thin cable structure The thin cable network structure is shown in Figure 2. Figure 2 Schematic diagram of thin cable network structure 1) Hardware configuration (1) Network interface adapter: Each node in the network requires an Ethernet card, a portable adapter or a PCMCIA card that provides a BNC interface. Thin cable (RG-58 A / U): a thin coaxial cable with a diameter of 5 mm and a characteristic impedance of 50 ohms. (4) Repeater: For Ethernet using thin cables, the length of each trunk section cannot exceed 185 meters. You can use a repeater to connect the two trunk sections to expand the length of the trunk cable. Up to four repeaters can be used in each Ethernet to connect five trunk line cables. 2) Technical parameters · Maximum trunk length: 185 meters. 3) Features · Easy to install. 2. Thick cable structure The thick cable Ethernet structure is shown in Figure 3. Figure 3 Schematic diagram of thick cable Ethernet structure 1) Hardware configuration Building a thick cable Ethernet requires a series of hardware devices, including: (1) Network interface adapter: Each node in the network needs an Ethernet card, portable adapter or PCMCIA card that provides an AUI interface. Thick cable (RG-11 A / U): a thick coaxial cable with a diameter of 10 mm and a characteristic impedance of 50 ohms, with a mark every 2.5 meters. (5) Repeater: For Ethernet using thick cables, the length of each trunk section does not exceed 500 meters, and two trunk sections can be connected with repeaters to expand the length of the trunk cable. Up to four repeaters can be used in each Ethernet to connect five-segment trunk cable. 2) Technical parameters · Maximum trunk length: 500 meters. 2500 meters. 3) Features · It has high reliability and strong anti-interference ability. 3. Mixed thick / thin cable structure When setting up a thick / thin hybrid cable Ethernet, in addition to using the same hardware as thick and thin cable Ethernet, the connection hardware between thick and thin cables must also be provided. Connected hardware includes: · N-series socket to BNC socket connector. 2) Technical parameters · Maximum trunk length: greater than 185 meters and less than 500 meters. In order to reduce the cost of the system, thin cables should be used as much as possible while ensuring the maximum length that a mixed trunk line can reach. You can use the following formula to calculate the maximum length of thin cable that can be used in a mixed trunk section t = (500-L) / 3.28, where: L is the length of the trunk section to be constructed, and t is the largest thin cable that can be used length. For example, to construct a 400-meter trunk section, the maximum length of thin cable that can be used is: (500-400) / 3.28 = 30 (meters). 3) Features · The construction cost is reasonable.
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1. Baseband coaxial cable
3. Coaxial cable network
· Secondary backbone network. The diameter of the secondary backbone cable is smaller than that of the backbone cable. When using sub-backbone cables at different building levels, high-gain distributed amplifiers should be used, and the interface between the cable and the user outlet should be considered.
· Cable.
(2) Thick cable Thick cable is generally installed with a tap device similar to a splint. It uses the guide pin on the tap to penetrate the insulation layer of the cable and is directly connected to the conductor. There are terminators at both ends of the cable to weaken the reflection of the signal.
1. Main electrical parameters
(2) The attenuation of the coaxial cable generally refers to the attenuation value of the cable section of 500 meters long. When measured with a 10MHz sine wave, its value does not exceed 8.5db (17db / km); while measured with a 5MHz sine wave, its value does not exceed 6.0db (12db / km).
(3) The minimum propagation speed required for the propagation speed of the coaxial cable is 0.77C (C is the speed of light).
(4) The sum of the resistance of the center conductor of the coaxial cable DC loop resistance cable and the resistance of the shield does not exceed 10 milliohms / meter (measured at 20 ° C). 
(2) Short circuit between conductor and shield.
(3) Conductor grounding.
(4) Short circuit between shielding connectors.
50Ω
· RG-58
50Ω
· RG-59
75Ω
· RG-62
93Ω
(2) BNC-T type connector: each node on the thin cable Ethernet is connected to the network through a T-type connector.Its two plugs in the horizontal direction are used to connect two thin cables, and the vertical socket is Connect to the BNC connector on the network interface adapter.
(3) Cable system: The cable system used to connect the thin cable Ethernet includes:
· BNC connector plugs: installed at both ends of the thin cable segment.
· BNC barrel connector: used to connect two thin cables.
· BNC terminal matchers: BNC 50 ohm terminal matchers are installed at both ends of the trunk section to prevent the reflection of electronic signals. The terminal matchers at both ends of the trunk cable must have a ground.
· Maximum network trunk cable length: 925 meters.
· Maximum number of nodes supported by each trunk segment: 30.
· Minimum distance between BNC-T connectors: 0.5 meters.
· Low cost.
· Strong anti-interference ability of the network.
· Network maintenance and expansion are more difficult.
· There are many breakpoints in the cable system, which affects the reliability of the network system.
(2) Transceiver: Each node on the thick cable Ethernet is connected to the network through an external transceiver installed on the trunk cable. When connecting a thick cable Ethernet, the user can choose any standard Ethernet (IEEE802.3) type external transceiver.
(3) Transceiver cable: used to connect the node and external transceiver, usually called AUI cable.
(4) Cable system: The cable system connecting the thick cable Ethernet includes:
· N-series connector plugs: installed at both ends of the thick cable section.
· N-series barrel connector: used to connect two thick cables.
· N-series terminal matchers: N-series 50 ohm terminal matchers are installed at both ends of the trunk cable section to prevent the reflection of electronic signals. The terminal matchers at both ends of the trunk cable section must have a ground.
· Maximum network trunk cable length:
· Maximum number of nodes supported by each trunk segment: 100.
· Minimum distance between transceivers: 2.5 meters.
· Maximum length of transceiver cable: 50 meters.
· With a large geographic coverage, the longest distance can reach 2500 meters.
· Network installation, maintenance and expansion are difficult.
· High cost.
1) Hardware configuration
· N-series plug to BNC socket connector.
· Maximum network trunk cable length: greater than 925 meters and less than 2500 meters.
· Strong anti-interference ability of the network.
· The system is complex.
· Network maintenance and expansion are more difficult.
· Increased the number of breakpoints in the cable system, affecting the reliability of the network.