First, computer networks

1.1. Basic concepts

  • Computer network refers to a computer system that connects a number of independent computers and their external devices with different geographical locations through communication lines, and realizes resource sharing and information transfer under the management and coordination of network operating system, network management software and network communication protocol

1.2. Classification of computer networks

  • According to transmission distance:

    • Local area network (LAN) : generally distributed in a range of several meters to several kilometers
    • Metropolitan area network (MAN) : generally distributed in several kilometers to dozens of kilometers or an urban area
    • Wide area network (WAN) : generally distributed in tens of kilometers to thousands of kilometers

  • According to the working mode, it is divided into:

    • Peer-to-peer networks
    • Server-based networking

  • According to transmission media:

    • Wired network: twisted pair network, coaxial cable network, fiber network, fiber coaxial hybrid network, etc
    • Wireless network: radio, microwave, infrared, etc

1.3. Twisted-pair cable

  • Twisted pair is formed by placing two insulated copper wires side by side and twisting them together in a regular way. Stranding reduces electromagnetic interference to adjacent wires

    • Unshielded Twisted pair (UTP)
    • Shielded Twisted pair (STP)

  • Straight line and cross line

    • Straight-through cable: The cable with the same sequence at both ends (both ends are T568A, or both ends are T568B)(different devices)
    • Crossover cable: One end of a cable is connected according to T568A and the other end is connected according to T568B (for the same device).

1.4. Coaxial cable

  • It is composed of inner conductor copper core wire, insulation layer, mesh woven outer conductor shield layer and protective plastic outer layer, widely used for transmission of high speed data

1.5. Optical fiber

  • Short for photoconductive fiber, a fiber made of glass or plastic used as a means of conducting light. The transmission principle is total reflection of light

  • Optical fibers can be divided into:

    • Multimode fiber
    • Single-mode fiber

  • Multimode fiber: An optical fiber that can transmit simultaneously in more than one mode at a given operating wavelength

    • The light source of multi-mode fiber is light-emitting diode, and the visible light emitted is poorly oriented, and the light enters the fiber core at different angles. Multimode fiber is suitable for short distance transmission because there are many rays with different incident angles in one fiber

  • Single-mode fiber: Reduced in diameter to the length of one wavelength of light, allowing light to travel in straight lines without multiple reflections. Wide transmission band and large transmission capacity

    • The light source of single-mode fiber uses laser diode with good orientation
    • Single-mode fiber has low loss and long transmission distance

1.6. Radio

  • The frequency band of radio is limited. The upper limit is 300GHz, while the lower limit frequency is not uniform in various specifications. There are three common statements that are 3kHz respectively300GHz(international wire union regulation), 9kHz300 GHZ, 10 KHZ ~ 300 GHZ
    • Medium wave mainly propagates along the ground, has strong diffraction ability, and is suitable for broadcasting and maritime communication
    • Short wave has strong ionospheric reflection ability and is suitable for global communication
    • VHF and microwave have poor diffraction ability and can be used as line-of-sight or beyond line-of-sight relay communication

1.7. The microwave

  • Microwave refers to the frequency at 0.3GHzElectromagnetic waves in the range of 300GHz are mainly used at present 2GHz40GHz frequency range
  • The main purpose of the terrestrial microwave system is to complete long distance telecommunication services and to establish short distance point-to-point communication between buildings

1.8. The infrared

  • The main features of infrared
    • Can’t penetrate solid objects, better security against eavesdropping than radio systems
    • Almost no electrical, sky, human interference, strong anti-interference
    • Infrared communication machine is small in size, light in weight, simple in structure and low in price
    • Communication must be within direct range and transmission is affected by weather

1.9. The laser

  • The laser communication system consists of two parts: sending and receiving

  • Advantages of laser communication:

    • Large communication capacity
    • Confidentiality is strong
    • The structure is light and the equipment is economical

  • Disadvantages of laser communication:

    • The distance is limited to visual distance (several kilometers to tens of kilometers range), vulnerable to climate
    • Difficulty in aiming. The high directionality of laser beam makes it difficult to aim between transmitting and receiving point

Second, the composition of computer network

2.1. Composition

  • Computer networks consist of resource subnets and communication subnets
    • The server
    • The workstation
    • Communication equipment
    • Transmission medium