Classification
Coaxial cables can be divided into two basic types, baseband coaxial cables and broadband coaxial cables.
Baseband coaxial cable
The shielding layer of baseband coaxial cable is usually made of copper mesh, with a characteristic impedance of 50Ω. This type of cable is used to transmit digital signals, and the general models are: RG-8 (thick cable) and RG-58 (thin cable). The most intuitive difference between a thick cable and a thin cable is the cable diameter. Thick cable is suitable for medium and large local area networks. It has a long standard distance and high reliability. However, the thick cable network must be equipped with transceivers and transceiver cables, which is difficult to install and the overall cost is high. In contrast, the use of thin cables to connect the network is simple and low in cost.
Whether it is a network connected by thick or thin cables, when a contact fails, the fault will affect all stations on the entire cable in series, and the diagnosis and repair of the fault is very troublesome . Therefore, baseband coaxial cables have been replaced by twisted pairs or optical fibers.
Broadband coaxial cable
The shielding layer of broadband coaxial cable is usually stamped from aluminum, with a characteristic impedance of 75Ω. This type of cable is used to transmit analog signals, and the model is RG-59. It is the standard transmission cable in CATV. To access Internet through a cable TV cable, this requires an intermediate device. One end of the device is connected to the computer and the other end is connected to the cable TV network. It is responsible for converting the bit stream entering the network into an analog signal, and retransmitting the analog signal output from the network. Convert to bitstream.
Ethernet and baseband coaxial cable
Introduction to Ethernet
In 1975, Xerox PARC (Xerox Palo Alto Research Center, Xerox Palo Alto Research Center) in the United States In the past, the ether, which used to represent the propagation of electromagnetic waves, was named the baseband bus local area network, called Ethernet (Ethernet). PARC originally designed an Ethernet with a data rate of 2.94Mb/s. Ethernet adopts the CSMA/CD (Carrier Sense Multiple Access with Collision Detection) protocol. Under the control mechanism provided by this protocol, data packets caused by stations sending data randomly can be avoided as much as possible Conflict events, and can detect and handle conflicts that have occurred. CSMA/CD enables multiple sites to use common transmission channels in a simple, flexible and effective way. From 1980 to 1982, DEC, Intel and Xerox jointly formulated the Ethernet standard DIX Ethernet with a data rate of 10Mb/s. There were two versions DIX Ethernet VI and DIX Ethernet V2.
While DIX is carrying out Ethernet standardization work, IEEE established an organization specializing in the development of local area network and metropolitan area network standards in February 1980, called the IEEE 802 Committee. Although DIX has launched the Ethernet specification, it is not an internationally recognized standard. Therefore, the IEEE 802 committee decided to establish an 802.3 subcommittee to formulate an international standard based on the work of DIX. The 802.3 subcommittee formulated the IEEE Ethernet standard in 1983 on the basis of DIX Ethernet, that is, 802.3 LAN. There is very little technical difference between 802.3 LAN and DIX Ethemet V2. People are accustomed to refer to 802.3 LAN as Ethernet.
The IEEE 802 committee, forced by fierce commercial competition, did not unify the LAN standards, but developed a set of standards for different LANs. At that time, the IEEE 802.5 token-ring LAN standard was comparable to the IEEE 802.3 LAN (Ethernet) standard. However, the continuous improvement of Ethernet technology has met the growing needs of users, which made the Ethernet technology stand out and eventually become a LAN. Mainstream technology.
Using baseband coaxial cables to build traditional Ethernet
In the early days of the development of local area networks, people used thick or thin cables to build a bus-type local area network, namely Ethernet, with a transmission rate of 10Mb/s. Because of the differences in transmission rate and transmission media characteristics between the Ethernet at that time and today's Ethernet, people usually call the 10Mb/s Ethernet in the early years as "traditional Ethernet."
The earliest Ethernet standard introduced by the IEEE 802.3 committee is 10BASE-5, which uses thick cables as the network transmission medium. "10" means that the data rate is 10Mb/s, and "BASE" means that the signal on the transmission medium is For baseband signals (ie, digital signals are directly represented by two different voltages), "5" represents that the maximum distance of a thick cable trunk segment is 500 meters. Later, thin cables were more used for the construction of Ethernet due to their lower price and smarter network layout. Therefore, IEEE 802.3 introduced the 10BASE-2 standard. 2 means that the maximum distance of a thin cable trunk segment is nearly 200. Meters (actually 185 meters).
The following is a brief introduction to the process of using thick and thin cables to construct Ethernet.
1) Use thick cables to build an Ethernet network
Use thick cables to build an Ethernet network, you need the following components:
(1) Network adapters: each in the network The node needs an Ethernet card that provides an AUl interface.
(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 thick cable Ethernet, users can choose any standard Ethernet (IEEE802.3) type of external transceiver. The transceiver is composed of two parts: the MAU (Medium Attachment Unit) containing electronic components and the plug-in tap without electronic components. The latter is also called MDI (Medium Dependent Interface) a p>
(3) Transceiver cable: used to connect the node and external transceiver, usually called AUI cable.
(4) Cable system: including thick coaxial cable, cable connector and 50Ω terminal matching device. The terminal matcher is 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 section cable must have a ground.
(5) Thick cable repeater: For Ethernet with thick cable, the length of each trunk segment does not exceed 500 meters, and a repeater can be used to connect two trunk segments to expand the backbone cable The length. Up to four repeaters can be used in each Ethernet to connect five sections of cables.
A network card providing AUI interface must be installed on the computer. The network card and the transceiver are connected by a transceiver cable. The length of the transceiver cable cannot exceed 50 meters, and the minimum distance between adjacent transceivers is 2.5 meters. Terminal matchers are installed at both ends of the thick trunk line section, and one end needs to be grounded. The maximum length of the thick trunk line segment is 500 meters, and the number of nodes that can be connected to each trunk line segment is up to 100.
The network connected by thick cable has strong anti-interference ability and large geographic coverage (network span can reach 2500 meters). However, thick cables are cumbersome and expensive, and network installation, maintenance, and expansion are also difficult.
2) Use thin cables to build Ethernet
In the early years, thin cables were used to build LANs, mainly from the perspective of saving equipment components and cable costs. Thin cables are sometimes used when twisted-pair cables cannot be used to connect hubs.
Using thin cables to build Ethernet requires the following components:
(1) Network adapter: Each node in the network needs an Ethernet card that provides a BNC interface.
(2) BNC (Bayonet Nut Connector, bayonet nut connector) connector: BNC connector is installed at both ends of each thin cable segment. BNC cable connector includes three parts: BNC connector jacket, tail tube and contact pin.
(3) BNC T-connector: Each node on the thin cable Ethernet is connected to the network through a BNC T-connector, and its two horizontal plugs are used to connect two sections of thin cables , The vertical socket is connected with the BNC interface on the network interface adapter.
(4) BNC terminal matcher: BNC 50Ω terminal matcher is installed at both ends of the trunk section to prevent the reflection of electronic signals. There must be one and only one grounding for the terminal matching device at both ends of the trunk cable.
(5) Thin cable repeater: The length of each thin cable trunk segment is required to not exceed 185 meters. A repeater with a BNC interface can be used to connect two trunk segments to expand the backbone cable. The length.
Cut the thin cable at the joining site, and install BNC connectors at both ends of each section of the thin cable, and then connect the cut thin cable with the horizontal connector of the BNC T connector, and connect the vertical connector The BNC interface of the computer network card. The connection of the edge of the trunk is slightly different. The horizontal connector of the BNC T connector is connected to a cut thin cable at one end, and the other end is connected to a terminal matching device. The vertical connector is still connected to the BNC interface of the computer network card. The minimum separation distance between two adjacent BNC T-type connectors is 0.5 meters.
The maximum number of nodes supported by each thin cable trunk segment is 30. When a repeater is needed to extend the cable range, a maximum of 4 repeaters can be used to connect 5 trunk segments.
Compared with thick cables, the installation of thin cables is simpler, has better anti-interference ability, and has a lower cost. However, the installation of thin cables requires cutting off the cables, and there are a lot of mechanical joints on the bus. Breakpoints are prone to occur, and the resulting faults may affect all stations on the bus in series.