Network systems and network hardware from the operating system composed of two parts.PC, operating system, such as DOS, OS / 2, Windows, etc., are managed local resources to deal with application requests access to these local resources.Similarly, the network operating system focused on management of shared resources, and expand the PC, operating system, so that applications can easily access these shared resources.The network hardware, such as: server, workstation, connect the communication media, network interface adapters, bridges, repeaters, routers and other hardware devices, constitute the basic conditions of network topology.Transmission medium in which communication is an important part of the network system, is the second computer network basic elements; and integrated wiring technology is constructed network system, the key communications transmission technology.
1, computer network transmission media
A service that wants to be in a separate shared before, the computer must have a contact channel with the other computers.At present computers are based on current, radio waves, microwaves or electromagnetic spectrum is used to transfer the energy spectrum of the signal, transmission of these energy pathways is the second computer network basic elements - the transmission medium.Before the establishment of a network, the first question is to determine the most suitable transmission medium.Transmission medium can be divided into two kinds of wired and wireless.Cable is only using cable or optical fiber to act as a transmission conductor, and wireless is not.The capacity of each type of transmission media are defined by bandwidth, we often referred to as bandwidth, it is used to define the frequency range Hz.Bandwidth measurement is relative, because the media's capacity as the transmission distance and signal coding techniques used vary.
In the computer network, we are concerned about the main problems are:
backup bin conf config data eshow_sitemap.html generate.sh log maint sitemap.html svn tmp effective bandwidth in the media, you can have the number of binary data (0 or 1) can be reliably transfer - transfer rate;
backup bin conf config data eshow_sitemap.html generate.sh log maint sitemap.html svn tmp in the transmission process, the energy is weaken the trend or the extent of distortion - decay;
backup bin conf config data eshow_sitemap.html generate.sh log maint sitemap.html svn tmp difficult to restrain and guide the energy of the impact on the normal signals - interference.
1, twisted pair cable
Twisted pair (TP) is one of the most common transmission medium.Twisted pair is two insulated copper wires with the composition of the two insulated copper wires, twisted by a certain density with each other, can reduce the impact of crosstalk and signal the degree of radiation, every wire in the conductive transmission releaseThe radio will be another root offset by the wave line.
Twisted pair consists of two 22 to 26, the intertwining of insulated copper wire is made, and will be a pair or more pairs of twisted-pair placement in a set of Jue, which is forming a twisted pair cable.
Stranded cable is widely used in traditional communications.In the early stages of computer network communications, all point to point transmission using twisted pair cables.As technology advances, twisted-pair cable can support the communication speed improved.At present three types of twisted-pair cable can support 10Mbps100 meters, that 10BASE-T standard, five 100 m twisted pair to support 100Mbps speed, or even to support 155Mbps CDDI standard of the ATM rate.According to the latest research findings to support 600Mbps over twisted-pair rate.
a, unshielded twisted pair cable
Unshielded twisted-pair cable is more on the twisted pair and form a plastic skin.International Electrical Industry Association (EIA) for the twisted-pair cable defined five different quality levels.
Often used in computer networks is the third and fifth, and ultra-unshielded twisted pair cable.
The third category applies to most of the twisted pair local area network computer, and the fifth class of twisted pair to increase the winding density, high-quality must like material, has greatly improved the nature of transmission medium.
As the successor of the voice communication means telecommunications, computer network using unshielded twisted-pair cable installation is usually the same with most phone systems, using the same method, a user equipment, via RJ-45 (4 on the line) or RJ-11 (2 on the line) telephone connection port connected with unshielded twisted-pair cable.Currently, non-shielded twisted-pair can be 100 meters, making data transfer rate of 100Mbps (megabits per second).
b, shielded twisted-pair cable
Shielded twisted-pair cable and unshielded twisted pair cables inside, like a twisted-pair copper wire, outer wrapped with aluminum foil.Apple Computer and IBM, used in a variety of transmission media are required to use shielded twisted-pair cable.Shielded twisted pair relatively more expensive, but it is still cheaper than coaxial cable Culan and more.Its installation difficult than some of unshielded twisted-pair cable similar to coaxial cable.It must support the shielding with the special connectors and the corresponding installation technology.It has a high transfer rate, 100 meters up to 500Mbps, but usually no more than the use of the transmission rate of 155Mbps.Currently the most common rate is 16Mbps.The maximum UTP cable distance limitations within the 100 meters.
2, coaxial cable
Coax (Coaxial Cable) from the around the same axis composed of the two conductors.A typical coaxial cable central (axis) is a single core copper wire or copper wire surge, which consists of foam insulation to open the outer package with.This layer of insulation has been the second layer was meshed conductor (some with a conductive foil) wrap for shielding electromagnetic interference and radiation.Finally, the cable insulation surface by a hard plastic encapsulation.
The most common coaxial cable are the following:
RG-8 or RG-11, 50 ohms (Ω);
RG-58, 50 ohms (Ω);
RG-59, 75 ohms (Ω);
RG-62, 93 ohms (Ω).
The most common computer network is: RG-8 Ethernet Culan, RG-58 Ethernet thin cable.The RG-62 is the IBM 3270 ARCnet network and Internet use, RG-59 cable for the television system.These must not be confused, or you spend a lot of energy, the result is duplication of efforts.
Most of coaxial cable installed in the equipment and devices, in each user location is equipped with a connector provides the interface for users.Interface installation method: thin cable is to cut off, two BNC installed first, and then connected to the T-connectors at both ends; Culan plywood, using a similar device for installation Tap, Tap on it to guide needle use, wearthrough cable insulation, directly connected with the conductor.
In order to maintain the correct electrical characteristics of coaxial cable, cable must be grounded, while two have terminal device to weaken the signal reflection.Thin coaxial cable installation should be relatively simple, Culan to be slightly more complex.Generally done by Culan Link, then thin cable through the Tap, this could make it easier to connect devices.Coaxial cable transmission rate of the current generally 10Mbps.
3, fiber optic cable
Cable from the optical glass or plastic core composition.It is another layer of glass wrap, called the cladding, the outermost layer is the hard protective layer.Center for the optical path, from the multi-layer reflective glass cladding composition.It can be light refraction to the SMIC on.Each core and cladding or tight or loosely wrapped by shells.In the tight structure, the fiber was completely wrapped around the outer plastic case; to loose structure, the optical fiber with a layer of liquid between the protective shell, or other plastic materials.No matter what kind of structure, shell strength is playing the role of providing the necessary cables to prevent the optical fiber by the external temperature, bending, outside pull, break and other effects.
Fiber-optic cable can be constituted by a single shell, can also be tied together on multi-strand fiber optic cable center.Fiber is much smaller than the copper wire, much less so than with the size of a large cable to the copper conductor cable has more effect.The feature of fiber in space limited environments better.
Fiber can be divided into two kinds of single and multi-mode transmission.
Only a single-mode optical path;
Provide a number of multi-mode optical path.
Multimode fiber cladding by its different refractive index under the light of different mode to control the speed, so that each part of the optical fiber transmission signals arrive simultaneously, the receiver only feel a pulse.
Single-mode fiber has a larger capacity, but its production expensive than multimode fiber.
Optical fiber transmission is characterized by low loss, wide band, small crosstalk, due to fiber itself does not conduct electricity, communications by the external electromagnetic minimal.As the fiber optic cable has obvious advantages than cable, so long lines have gradually become an important transmission medium.
With the increased speed of the computer network itself, in particular the popularity of FDDI, fiber-optic cable will be computer network cabling system components in one of the most commonly encountered.
Fiber optic and twisted pair and coaxial cable as used to transmit analog signals can also be used to transmit digital signals.With the exception of TV images and so on still use analog signal transmission, the majority of applications, including data and phone are digital signals.
The type of fiber from the mold, material (glass style plastic fiber) and core and the outer size of the decision, the core of the size and purity determine the amount of light transmission, fiber optic cable commonly used are:
* 8.3-micron core / 125 micron outer layer, single-mode
* 62.5-micron core / 125 micron outer layer, multi-mode
* 50-micron core / 125 micron outer layer, multi-mode
* 100 μm core / 140 micron outer layer, multi-mode
Currently the most commonly used is 62.5/125 multimode fiber, followed by 8.3/125 single-mode optical fiber.
Computer network cable installation in general is starting from the user equipment.Since each fiber at any time only one-way transmission.Therefore, to implement two-way communication, it must be in pairs, one for input, one for output, at both ends of the optical fiber interface.
A fiber-optic cables to connect each need to carefully polished ends, through electrical or chemical Huan chlorine process barbecue Optical interface together with the entire installation process, we must ensure that the optical channel is not blocked, nor can Guangqian La tooor the formation of tight angle.
Most commonly used in computer networks multimode fiber glass 850mm wavelength LED, transfer rate 100Mbps, effective range is about 20 km.
Currently being developed high-speed optical fiber communication system, in addition to 2.4Gbps system has been commercialized outside, 5Gbps, 10 Gbps, 20Gbps system is in development stage.High-speed systems are the most important issue is the speed fiber optic and electronic devices rate Xian Zhi, therefore needed research to high-speed modulation methods and devices.
In increased capacity, using special high-frequency or microwave as a negative charge wave modulation and frequency division multiplexing formed with multiple modulation techniques.
Optical transmission distance is limited only by the wavelength of its decay rate is very low.Meanwhile, in order more effectively increase the transmission distance, usually using 1.55μm fiber zero dispersion wavelength, while use of erbium-doped fiber amplifier as a receiver or preamplifier circuit in the optical fiber as a repeater, enables the transmission distance of optical fiberseveral tens of kilometers, and even hundreds of kilometers.
As the fiber is used in spectroscopy, it does not leak signal phenomenon, but also from the impact of electromagnetic waves and high frequency distortion.These features make it more suitable for dangerous, high pressure or leakage of the signal, interference signal is very strong environment.
Second, the wireless medium
Wireless media do not use electrical or optical conductors for the transmission of electromagnetic signals work.In theory, the Earth's atmosphere as most of the wireless transmission to provide a physical data path.Due to a variety of electromagnetic waves can be used to carry signals, the electromagnetic wave was considered to be a medium.
1, radio frequency waves
10KHz to 1GHz electromagnetic spectrum between the radio frequency, which contains the broadcast channel is called:
Short-wave radio frequency band;
Very high frequency (VHF) television and FM radio frequency band;
Ultra High Frequency (UHF) radio and television frequency band.
Radio frequency controlled by the control bandwidth and the bandwidth of non-division:
Control bandwidth users must obtain permits from the radio management departments to use.Radio management (such as the U.S. FCC, Canada, CDC, etc.) the right to manage the frequency region, once the user license, you can guarantee that in this particular region are well within the transmission.
In the United States, FCC to 902MHz to 928MHz, 2.4GHz, 5.72GHz to 5.82GHz given to unlicensed person to use.2.4GHz international general not be regulated, and unregulated frequency, in the absence of being fully utilized.The frequency of non-competition control the rapid growth of the use of the most current 900MHz, while the 2.4GHz of the fastest growing.
Radio waves can be generated through a variety of transmission antennas or to launch all-round broadcast.Typical antenna tower, including the direction, winding the antenna, half-wave dipole antenna and the rod antenna.
Transmitter antenna determines the frequency and power of wireless signals.Sending and receiving stations for the system requirements to use the frequency range, the global system using short-wave, the local video system to spread the use of VHF signals.
⑴ low-power, single-frequency radio only a short distance, open environment.Despite the relatively long wavelength of low frequency in most of the material can be passed, but limited by the properties of low power of this system is only a short distance or a barrier-free access on the transmission.Low-power, low frequency can not maintain a high transfer rate, its standard transmission rate is 1Mbps.Single-frequency system can provide transmission rates of copper similar, but its decay rate of a larger, anti-electromagnetic interference capability is also very small, so its effective range of only tens of meters in general.
⑵ high-power, single-frequency radio can also use the radio frequency range, with low-power, single-frequency radio difference is mainly used for long-distance outdoor environment.Power flexibility to determine the signaling pathway, now it has become the ideal means of mobile transfer.Its transfer rate up to 10Mbps, but the cost is quite expensive.
⑶ spread spectrum radio transmission, also depends on the frequency, but it is also a way using a variety of frequencies.Currently AT & T's WaveLAN and Windata FreePort wireless networks are using this technology.