Computer Network Technology and Application

Chapter 8 Fundamentals of Network Technology (3)
The advantages of optical fiber communication are frequency bandwidth, large transmission capacity, light weight, small size, strong confidentiality, abundant raw materials, low production cost, strong resistance to electromagnetic interference, and low bit error rate.

Fiber is divided into two types: multimode and singlemode.

2.2.2 Wireless medium
A wireless medium means that the signal travels through the air, i.e. the signal is not confined within a physical conductor.

1. Radio waves
The ionosphere in the atmosphere is a conductive layer with ions and free electrons. Wireless communication is a long-distance communication method that uses radio waves emitted from the ground to reflect through the ionosphere, or the ionosphere and the ground are reflected multiple times to reach the receiving end.

The reason why radio waves are widely used in communications is that they can travel great distances and pass through buildings easily; moreover, radio waves propagate in all directions, so the radio wave transmitting and receiving devices do not have to require precise alignment.

The propagation characteristics of radio waves are related to frequency.At low frequencies, radio waves can easily bypass common obstacles, but their energy attenuates sharply as the propagation distance increases.At high frequencies, radio waves tend to travel in straight lines and are easily blocked by obstacles and absorbed by rain.

The downside of radio waves is that all frequencies are susceptible to various electromagnetic interference from other electronic devices.

2. Microwave communication
Microwave data communication systems come in two forms: terrestrial systems and satellite systems.

(1) Ground system

Because microwaves propagate in a straight line in space, if they propagate on the ground, because the surface of the earth is a curved surface, the propagation distance is limited, so the station using microwave transmission must be installed within the line of sight, and the transmission frequency is 4GHz ~ 6GHz and 21GHz ~ 23GHz , The transmission distance is generally only about 50km.In order to realize long-distance communication, several relay stations must be added between two terminals of a wireless communication channel, through which the information sent by the previous station is amplified and sent to the next station.

(2) Satellite system

Another kind of microwave communication uses satellite microwave. The satellite reflects signals between the sending station and the receiving station, and the transmission frequency is 11GHz to 14GHz.

3. Laser communication and infrared communication
The advantages of laser communication are higher bandwidth, good directionality, and good confidentiality performance. Laser communication is mostly used for short-distance transmission.

The disadvantage of laser communication is that its transmission efficiency is greatly affected by the weather.

Infrared communication is immune to electromagnetic interference and radio frequency interference.Infrared wireless transmission is based on infrared light, using light emitting diodes, laser diodes or photodiodes to exchange data between sites.

2.3 Network components and devices

2.3.1 Network transmission media connector

The function of the connector is to connect the transmission medium and the network equipment.Commonly used network transmission media connectors include shielded or unshielded twisted pair connectors and fiber optic connectors.

1. Shielded or unshielded twisted pair connector RJ-45
RJ‐45非屏蔽双绞线连接器有8根针脚，在10Base‐T标准中，使用4根针脚，即第1对双绞线使用第1针和第2针，第2对双绞线使用第3针和第6针（第3对和第4对作备用）。

2. Fiber optic connector

2.3.2 Network equipment
1. Hub

Hub (Hub) is a special repeater, as an intermediate node between network transmission media, it overcomes the defect of a single channel of the media.Commonly used hubs can be divided into passive (Passive) hubs, active (Active) hubs and intelligent (Intelligent) hubs.

2. Switch

The function and position of the switch in the network is very important.In the telephone network, it is used to transmit voice; in the local area network, it is widely used to connect different departmental networks or several computers.

The multiple ports of the switch can be connected to multiple subnets of the same type, and can also be connected to several computers; high-speed data forwarding can be realized simultaneously between multiple pairs of ports.At present, switches have become the best equipment for network interconnection and are widely used.

3. Router

A router is a core device connecting different networks and a data packet (packet) forwarding device working at the network layer of the OSI reference model.A router implements network interconnection by forwarding data packets. It usually connects two or more subnets or point-to-point logical ports, and has at least one physical port.

The main job of a router is to find an optimal transmission path for each data packet passing through the router, and effectively transmit the data packet to the destination site, so the strategy of selecting the best path, that is, the routing algorithm, is the key to the router.In order to complete this work, the routing table (Routing Table) of the relevant data of various transmission paths is saved in the router for use in routing selection.The routing table stores the subnet identification information, the number of routers, and the name of the next router.The routing table can be set to a fixed configuration by the system administrator, or it can be dynamically modified by the system.

The router determines the output port and the address of the next hop (forwarded to the next router) according to the network address in the received data packet and the router routing table.Routers usually maintain routing tables dynamically to reflect the current network topology.Routers maintain routing table contents by exchanging routing and link information with other routers on the network.

2.3.3 Network card
The network card, also known as the Network Interface Card (NIC), is inserted into the expansion slot of the computer motherboard and connected to the transceiver through the transceiver cable.In a local area network, one of the tasks of the network card is to send data from the workstation to the network, or receive data from other devices on the network and send it to the workstation.

2.3.4 Modem

A modem is a piece of computer hardware, commonly known as a "cat," that translates computer digital signals into pulsed signals that can be sent along ordinary telephone lines. These pulsed signals are received by another modem at the other end of the line and translated into A digital signal that can be read by a computer.This simple process completes the communication between the two computers.

【chapter summary】

This chapter focuses on data communication technology, network transmission media, network accessories and equipment, switching technology, and multiplexing technology.The purpose is to let readers understand basic content such as network technology and equipment, and lay the foundation for subsequent courses.

【Difficulties in this chapter】

(1) The relationship between phase modulation technology and speed.

(2) Different functions of various devices.

(3) The characteristics of the transmission medium.

Exercise 2
[-]. Multiple choice questions

1. The channel capacity represents the maximum data transmission rate of a (), the unit is b/s (bps).

A. Interface
B. Channel
c. service
D. Network
2. Multiplexing technology is a means and method to effectively improve the utilization rate of communication lines, and can use one communication channel to transmit () signals.

A. Multi-channel
B. No way
C. all the way

D. Network
3. The transmission medium that can transmit analog signals is ().

A. Coaxial and fiber optic

B. Twisted pair

C. Microwave and Optical Fiber

D. None of the above

4. Among the commonly used transmission media, () has the widest bandwidth, the smallest signal transmission attenuation, and the strongest anti-interference ability.

A. Twisted pair

B. coaxial cable
C. Optical fiber
D. microwave
5. The bit error rate is the error in the transmission of binary data bits (①), and it is an indicator to measure the transmission (②) of the data communication system under normal working conditions.

①A. Ratio B. Probability C. Number of bytes D. Number of digits
②A. Speed ​​B. Quality C. Bandwidth D. Latency
6. Commonly used data transmission rate units are kbps, Mbps, Gbps, and Tbps. 1Mbps is equal to ().

A.1×1013bps
B.1×1016bps
C.1×1012bps
D.1×1024kbps
7. Shannon's theorem describes the relationship between channel bandwidth and which parameters?
Ⅰ-maximum transmission rate, Ⅱ-signal power, Ⅲ-power noise ().

A. I, II and III
B. Only I and II
C. Only I and III 　
D. Only II and III
Two, short answer questions
1. Considering the performance and price of the transmission medium, which transmission medium is better to use indoors?Briefly explain why.

2. What is the basic principle of frequency modulation?
3. What type of data is in the computer?

4. In ADSL broadband, why use a Modem at the front end of the computer?

5. What is the purpose of using synchronization technology?
Experiment [-] to learn how to make twisted pair wires

【Purpose】

(1) Learn how to make twisted pair connectors.

(2) Learn the method of using twisted pair to connect the switch and the computer.

【Experimental content】

(1) Make a twisted pair connector (RJ-45).

(2) Connect the switch and computer.

(3) Test the connectivity of the network.

[class hours] 2
【Experimental Requirements】

(1) Master the production method of RJ-45 crystal head.

(2) Master the connection of equipment and the method of testing network connectivity.

【lab environment】

(1) Two computers and one switch.

(2) Twisted pair and crystal head.

(3) Wire strippers and crimping pliers.

【Experimental steps】

1. Make the crystal connector of the twisted pair
(1) The standard and connection method of the network cable

① There are two international standards for twisted pair wiring: EIA/TIA568A and EIA/TIA568B.

② There are two main connection methods for twisted pair cables: straight-through cable and crossover cable.

③ Both ends of the water lens of the straight-through cable follow the 568A or 568B standard. Each group of twisted-pair wires corresponds to each other at both ends, and the same color is kept consistent in the corresponding grooves of the crystal heads at both ends.It is mainly used to connect the Uplink port of the switch (or hub) to the ordinary port of the switch (or hub) or connect the ordinary port of the switch to the network card of the computer.

④交叉线缆的水晶头一端遵循568A标准，而另一端则采用568B标准，即A水晶头的1、2对应B水晶头的3、6，而A水晶头的3、6对应B水晶头的1、2，它主要用在交换机（或集线器）普通端口连接到交换机（或集线器）普通端口或网卡连接到网卡上。

The line sequence described by the T568A standard is as follows from left to right:

1白绿、2绿、3白橙、4蓝、5白蓝、6橙、7白棕、8棕。

The line sequence described by the T568B standard is as follows from left to right:

1白橙、2橙、3白绿、4蓝、5白蓝、6绿、7白棕、8棕。

In network construction, it is recommended to use the T568B standard.For general wiring system engineering, T568A is also applicable.

(2) production

① cut off.Use the cutting edge of the wire crimping pliers to cut an appropriate length of the network cable.

② Peeling.Use the cutting edge of the wire crimping pliers to cut the end of the wire evenly, and then put the end of the wire into the stripping knife, let the end of the wire touch the baffle, hold the crimping pliers slightly and rotate slowly, let the edge of the knife cut the protective rubber of the twisted pair, strip Lower the rubber.

【prompt】

The distance between the position of the network cable pliers and the stripping knife edge is usually exactly the length of the crystal head, which can effectively prevent the stripping from being too long or too short.If the cable is stripped too long, it is unattractive. On the other hand, it is easy to loosen because the network cable cannot be caught by the crystal head; It is in good contact with the core wire of the network cable.

③Sort.After peeling off the outer sheath, you can see 4 pairs of 8 core wires of the twisted pair network cable, and you can see that the colors of each pair are different.Each twisted pair of two core wires consists of one core wire dyed in the corresponding color plus a white core wire dyed only a little of the corresponding color.The colors of the four full-color core wires are: brown, orange, green, blue.

Each pair of wires is intertwined with each other. When making a network cable, the 4 thin wires of the 8 wire pairs must be disassembled, straightened out, and straightened one by one, and then arranged neatly according to the specified wire sequence.

Arrange the 8 pins of the crystal head:
Put the side of the crystal head with the plastic spring piece down, and the side with the pins up, so that the end with the pins points away from you, and the end with the square hole faces you. At this time, the leftmost pin is the first pin. The far right is pin 1, and the rest are arranged in sequence.

⑤Insert.Hold the crystal head with your thumb and middle finger with one hand, so that the side with the plastic shrapnel faces down, and the pin side faces away from you, and use your index finger to hold it; hold the rubber outside the twisted pair with the other hand, and slowly push the 8 wires together. The wires are inserted into the 8 wire grooves in the crystal head at the same time, and are inserted to the top of the wire grooves.

⑥ suppression.After confirming that all the wires are in place, and checking the order of the wires through the crystal head, you can use the crimping pliers to press the RJ-45 head, push the crystal head from the toothless side into the crimping pliers groove, and hold it firmly Wire pliers will do.

2. Connect the device
The main job of this step is to connect the switch to the computer.

(1) Insert the RJ-45 connector at one end of the prepared twisted pair into a port of the switch; insert the other end connector into the network card of the computer.

(2) Note: There are special ports in the interface of the switch, which cannot be used for ordinary network connections.It is recommended to read the instruction manual carefully before using the switch.

【Questions and Thoughts】

(1) How to make a twisted pair crystal head for connecting two computers?
(2) Is there a direct relationship between the number of ports of the switch and the scale of the network?

(End of this chapter)