Computer Network Technology and Application

Chapter 9 Basic knowledge of local area network (1)
In Chapter 1, we introduced the geographical division, the smallest of which is the local area network (LAN).A LAN is an important way for local users to access the Internet.For example, campus networks, community networks, and hospital networks are common ways to access the Internet.

This chapter introduces the definition and equipment of local area network, types of local area network, establishment of local area network, Ethernet technology, wireless network, VLAN and so on.I hope that through the study of this chapter, I can master the types of LAN, related network protocols, principles and methods of conflict resolution, master the use of LAN equipment, and learn the techniques and methods of building LAN.

【Main content of this chapter】

LAN devices and components.

Create a local area network method.

Ethernet technology.

Example of LAN.

3.1 Definition and types of LAN

3.1.1 Definition and characteristics of LAN
A local area network refers to a computer network that interconnects communication devices (computers, switches, printers, etc.) within a limited geographical area.Its application range is very wide, and LANs are generally used in offices, laboratories, enterprises, hospitals, schools, residential areas, and homes.

The performance of the LAN is mainly determined by the topology of the network, the performance of the transmission medium, and the method of medium access control.

1. From the perspective of geographical scope
(1) The local area network covers a limited geographical area and is suitable for the networking needs of computers, terminals and various information processing equipment within a limited area such as campuses, institutions, companies, and factories.

(2) The data transmission rate of the local area network is high, and the transmission speed from 10Mbps to 10Gbps can be obtained.With the development of network technology, higher transmission speeds may be available in the future.

(3) The local area network has a high-quality transmission environment with a low bit error rate, and the data transmission delay time is short, but the connection capacity is limited.

2. From a functional point of view
(1) Only the lower layer [-] and application layer protocols of the network are used, and the lower layer [-] protocols are relatively simple, without a separate network layer.

(2) In the local area network, broadcast information is generally used. If full-duplex communication is not used, the data transmission shares the transmission medium and is prone to collisions. Therefore, media access control technologies must be used, such as CSMA/CD, Token Ring, etc. Ensure the efficiency of data transmission.

3.1.2 LAN type
Currently common LAN types include Ethernet (Ethernet), Fiber Distributed Data Interface (FDDI), Asynchronous Transfer Mode (ATM), and Token Ring.Among them, Ethernet is the most widely used, which is a broadcasting LAN. It is currently one of the fastest growing and most economical LANs, and it is highly scalable.

These types of LANs differ in topology, transmission medium, transmission rate, data format, and many other aspects.

Here is a brief introduction to Ethernet, optical fiber distributed data interface network and asynchronous transfer mode network.

1. Ethernet technology

Ethernet (Ethernet) is the most representative local area network and the most widely used network.The early Ethernet transmission rate was only 10Mbps, and the current Ethernet technology can increase the backbone network speed to 10Gbps or higher.

(1) Ethernet technical standards

最初由Xerox公司于1975年研制成功，1979年7月到1982年期间，DEC、Intel和Xerox三家公司制订了以太网的技术规范DIX，以此为基础形成的IEEE802.3以太网标准在1989年正式成为国际标准。

After more than 20 years of development, Ethernet technology has produced a variety of technical standards.The following briefly introduces various standards of Ethernet.

①10Base‐5标准是原始的以太网标准。它规定：使用直径10mm的50Ω粗同轴电缆作为数据传输介质，采用总线拓扑结构，站点网卡的接口为DB‐15连接器；每个网段允许有100个站点，每个网段最大允许距离为500m，网络直径为2500m，即可由5个500m长的网段和4个中继器组成；基带传输速率为10Mbps，并采用曼彻斯特编码传输数据。

②10Base‐2标准是为了降低10Base‐5的安装成本和复杂性而设计的标准。它使用廉价的R9‐58型50Ω细同轴电缆，总线拓扑结构，网卡通过T形接头连接到细同轴电缆上，末端连接50Ω端接器；每个网段允许30个站点，每个网段最大允许距离为185m，保持了10Base‐5的4个中继器和5个网段的结构，允许的最大网络直径为925m；基带传输速率为10Mbps，也采用曼彻斯特编码传输数据。

Compared with 10Base-5, 10Base-2 Ethernet is easier to install, easier to add new sites, and can significantly reduce costs.

The above two standards have basically become history and are no longer really applied to actual networks.

③10Base‐T是1990年通过的以太网物理层标准。该标准使用两对非屏蔽双绞线（UTP）：一对线发送数据，另一对线接收数据；使用RJ‐45作为端接器，采用星型拓扑结构；站点到中继器和中继器到中继器的最大距离为100m；保持了10Base‐5的4个中继器与5个网段的结构，使10Base‐T局域网的最大直径达到500m。

Twisted pair Ethernet is one of the main advances in Ethernet technology, and 10Base‐T has become popular because of its cheap price, flexible configuration and easy management, and has become one of the main forms of desktop access.

(2) Fast Ethernet

100Base‐T is the standard version of the Ethernet standard Fast Ethernet 100Mbps. It is a Fast Ethernet specification officially passed in May 1995, also known as the IEEE5u standard.It uses a star topology, contains 802.3 different physical layer specifications, and contains several new rules for network topology.

①100Base‐TX：此标准使用两对5类非屏蔽双绞线或1类屏蔽双绞线，其中一对线用于发送数据，另一对线用于接收数据，最大网段长度为100m；采用4B／5B编码，以125MHz的频率串行传输数据。IEEE802.3u规范采用三电平符号传输系统取代10Base‐T的两电平曼彻斯特编码。

4B／5B编码是将4位数据半字节转换为5位编码，目的是实现错误检测和增加控制码，例如数据流起始和终止定界符。将信号传输率提高到125Mbps，可补偿4B／5B内在的20%数据传输效率，但是这种带宽增加所产生的频谱会被曼彻斯特编码扩展到数百兆赫。

使用多电平传输3波形法，把信号频率降低到125MHz的1/3，即41.6MHz。

100Base‐TX is the most widely used physical layer specification in the 100Base‐T standard.

②100Base-FX: This standard uses multimode (62.5 or 125um) or single-mode fiber.It allows a maximum length of 412m for the switch-to-switch connection and the switch-to-network card connection of multimode fiber, and it can reach 2000m if a full-duplex link is used.It is mainly used in high-speed backbone networks or long-distance connections, or in environments with strong electrical interference, or in environments that require higher security and confidential links.

③自动协商模式：在100Base‐T被颁布之后，使用RJ‐45连接器可以发送或接收的信号在5种以上，包括10Base‐T、10Base‐T全双工、100Base‐TX、100Base‐TX全双工和100Base‐T4。

In order to simplify management, IEEE (International Institute of Electrical Engineers) launched Nway (IEEE auto-negotiation mode), which enables hubs and network cards to understand the possible speed at the other end of the line, and automatically adjust the speed to the highest speed that can be reached at both ends of the line.For example, the order of preference is 100Base-TX Full Duplex, 100Base-T4, 100Base-TX, 100Base-T Full Duplex, and 10Base-T.

In the technical parameters of the network card or other network devices, it can be found that the speed marked by the technical indicators is 10Mbps/100Mbps, which means that this component or device adopts the auto-negotiation mode.

This technique avoids possible network failures due to signal incompatibility.Devices of this nature also allow for manual selection of modes.

(3) Gigabit Ethernet

Gigabit Ethernet technology is a relatively new high-speed Ethernet technology, and it is usually used as an effective solution to improve the transmission capacity of the core network.Its biggest advantage is that it inherits the cheap advantages of traditional Ethernet technology. When the Ethernet is upgraded, it is not necessary to change the network application program, network management components and network operating system, which is convenient and economical.

Gigabit Ethernet technology is still Ethernet technology in essence, using the same frame format, frame structure, network protocol, full/half duplex working mode, flow control mode and wiring system as 10M Ethernet.This technology does not change the traditional Ethernet desktop applications and operating systems, so it can work well with 10M or 100M Ethernet and has good compatibility.

① Gigabit Ethernet technical standards: There are two, namely IEEE802.3z and IEEE802.3ab.The former is a standard for optical fiber and short-distance copper wire connection schemes, and the latter is a standard for longer-distance connection schemes on Category 5 twisted-pair wires.

IEEE802.3z：该标准定义了基于光纤和短距离铜缆的1000Base‐X标准，采用8B／10B编码技术，信道传输速度为1.25Gbps，实际可以实现1000Mbps的传输速度。

IEEE802.3z has the following Gigabit Ethernet standards.

1000Base‐SX标准：只支持多模光纤，可以采用直径为62.5μm或50μm的多模光纤，工作波长为770～860nm，传输距离为220～550m。

1000Base‐LX标准：1000Base‐LX是定义在IEEE802.3z中的针对光纤布线吉比特以太网的一个物理层规范。LX代表长波长，1000Base‐LX标准对应于802.11z标准，既可以使用单模光纤也可以使用多模光纤。1000Base‐LX标准所使用的光纤主要包括62.5μm多模光纤、50μm多模光纤和9μm单模光纤，其中多模光纤的最大传输距离为550m，单模光纤的最大传输距离为3km。1000Base‐LX标准采用8B／10B编码方式，使用的长波激光信号源，波长为1270～1355nm。针对多模光纤，1000Base‐LX可以采用直径为62.5μm或50μm的多模光纤，其工作波长范围为1270～1355nm，传输距离可达到550m。针对单模光纤，1000Base‐LX可以支持工作波长范围为1270～1355nm，传输距离可达到5km左右。

1000Base-CX standard: corresponding to the 802.11z standard, using copper cables as the transmission medium, using 9-core D-type connectors to connect the cables, and the maximum transmission distance is 25m. The 1000Base-CX standard adopts 8B/10B encoding method, which is suitable for the connection between switches, especially for the short-distance connection between the backbone switch and the main server.

IEEE802.3ab：该标准定义了基于5类UTP的1000Base‐T标准，速率为1000Mbps，传输距离为100m。1000Base‐T标准是1999年6月被IEEE标准化委员会批准的标准，这项技术使用现有的5类铜线，目前被最广泛安装在局域网上，提供1000Mbps速度；不支持8B／10B编码方式，但采用更加复杂的编码方式。1000Base‐T标准的优点是可以在原来100Base‐T的基础上进行平滑升级到1000Base‐T。1000Base‐T与10Base‐T、100Base‐T标准完全兼容。

② Features of Gigabit Ethernet.

Simple: Since Gigabit Ethernet inherits the simplicity of Ethernet and Fast Ethernet, its technical principle, installation and implementation, management and maintenance are all very simple.

Good scalability: Since Gigabit Ethernet adopts the basic technology of Ethernet and Fast Ethernet, it is very easy to upgrade from 10Base‐T and 100Base‐T to Gigabit Ethernet.

Reliable: Because Gigabit Ethernet maintains the installation and maintenance methods of Ethernet and Fast Ethernet, and adopts a star network structure, the network has high reliability.

Economical: Since Gigabit Ethernet inherits 10Base‐T and 100Base‐T, the research cost is relatively low. In addition, the upgrade to 10Base‐T and 100Base‐T is easier and more convenient.

Easy maintenance: Gigabit Ethernet adopts network management technologies based on Simple Network Management Protocol (SNMP) and Remote Network Monitoring (RMON). Many manufacturers have developed a large number of network management software, which makes centralized management and maintenance of Gigabit Ethernet very easy. easy.

③ Application of Gigabit Ethernet.

Because the bandwidth advantage of Gigabit Ethernet is more effective, it has a good development space and is mainly used in the following aspects.

构建主干网：千兆位以太网可提供超出快速以太网10倍以上的性能，并与现有的10／100Mbps以太网标准兼容。同时，由于为10／100／1000Mbps开发的虚拟网标准802.1Q以及优先级标准802.1p都被推广，千兆网已成为构成网络主干的主流技术。

Construction of metropolitan area network: With the advancement of optical fiber manufacturing and transmission technology, the transmission distance of Gigabit Ethernet can reach tens of kilometers or even hundreds of kilometers, which makes Gigabit Ethernet a technical choice for metropolitan area network.Because 10G Ethernet uses multimode fiber, it can support a transmission distance of 65-300m; using single-mode fiber, it can support a transmission distance of 10-40km, so 10G Ethernet technology can provide higher bandwidth at a lower cost. , and realize the connection of various networks.At present, 10G Ethernet has become the backbone network core technology of the metropolitan area network.It should be noted that the current 10G Ethernet (using IEEE802.3ae) technology is already very mature.

2. Fiber Published Data Interface (FDDI)

Fiber Distributed Data Interface (FDDI) is a high-speed, general-purpose ring network that uses optical fiber as a transmission medium. It is one of the mature LAN technologies at present and can be used in both metropolitan area networks and small-scale LANs.

(End of this chapter)