Computer Network Technology and Application

Chapter 11 Basic knowledge of local area network (3)
3. Allow 10Mbps/100Mbps/1000Mbps coexistence
由于采用了10Mbps／100Mbps／1000Mbps自动侦测（Autosense）技术，交换机的端口支持10Mbps／100Mbps／1000Mbps三种速率，以及全双工／半双工两种工作方式。端口能自动测试出所连接的网卡的速率是10Mbps、100Mbps还是1000Mbps或更高，是全双工还是半双工方式。端口能自动识别并作出相应的调整，从而大大减轻了网络管理的负担。

4. Support virtual local area network service
Switched LAN is the basis of virtual local area network. Currently, Ethernet switches can basically support virtual local area network services.Through the virtual local area network, you can easily adjust the distribution of network load, improve bandwidth utilization, network manageability and security.

3.4 Virtual Local Area Network
If there are multiple second-level departments in an enterprise with a medium size or above, when interconnecting the independent networks of each department, it is necessary to carry out the management and security of different functional departments and the stable operation of the overall network. Independent and unified management, the best choice at this time is to use a virtual local area network (Virtual Local Area Network, VLAN).

VLAN is not only beneficial to network security and preventing network broadcast storms, but also can improve the efficiency of network operation.

3.4.1 Understanding virtual local area network
Virtual local area network can logically divide computer users into workgroups with relatively independent functions according to address, function, application, etc. regardless of the user's physical location; each computer is connected to a switch port that supports VLAN and belong to a VLAN.

Members in the same VLAN form a broadcast domain, and broadcast information between different VLANs is isolated from each other.Dividing the entire network into multiple different broadcast domains (VLANs) can achieve the goal that computers on a switch belong to different virtual networks.

The advantages of VLAN are: to avoid the expansion of broadcast information; to establish a secure network; to facilitate the addition and removal of wireless devices; to reduce the pressure of network management.

Description:

The division of the virtual local area network must have the support of the three-layer switching technology, which is embodied in the three-layer switch.

Layer [-] switching technology is Layer [-] switching technology plus Layer [-] forwarding technology.After the division of network segments in the LAN, the management of the subnets in the network segment used to rely on routers. After the appearance of the three-layer switch, the network bottleneck problem caused by the low speed and complexity of traditional routers was solved.

3.4.2 Methods of dividing VLANs

There are mainly two ways to divide VLANs.

(1) Statically divide VLAN, that is, port-based VLAN. This VLAN method needs to be set for each port in the switch.

(2) Dynamically divide VLANs into MAC address-based VLANs, network layer-based VLANs, and user-based VLANs. These three dynamic VLANs are characterized by strong flexibility.

1. Port-based VLAN
这种划分VLAN的方法是根据以太网交换机的端口来划分的。例如，可以将交换机的1～4端口设置为VLAN1；5～10为VLAN2；11～24为VLAN3。属于同一VLAN的端口可以是不连续的，由网络管理员决定。

The second-generation port VLAN technology allows multiple different ports across multiple switches to divide VLANs, and several ports on different switches can form the same virtual network.If there are multiple switches, you can specify that the ports on different switches are in the same VLAN.For example, ports 1-10 of switch A and ports 1-10 of switch B belong to the same VLAN, that is, the same VLAN can span several Ethernet switches.The method of dividing VLANs according to ports is currently the most widely used method for defining VLANs. IEEE802.1Q stipulates an international standard for dividing VLANs based on ports of Ethernet switches.

The advantage of this division method is that it is very simple to define VLAN members, as long as all the ports are defined.The disadvantage is that if a user of the VLAN leaves the original port and goes to a port of a new switch, the VLAN needs to be redefined.

2. Divide VLAN based on MAC address
This method of dividing VLANs is divided according to the MAC address of each host, and the host of each MAC address is configured with a group to which it belongs.

The biggest advantage of the method of dividing VLANs based on the MAC address is that when the user's physical location moves, that is, when changing from one switch to other switches, the VLAN does not need to be reconfigured.Therefore, it can be considered that this division method based on the MAC address is based on the user's VLAN.

The biggest shortcoming of this method is that during initialization, all users must be configured, and if the number of users is huge like this, configuration just needs to carry out manually, because all may have the member of a lot of VLAN groups at the port of each switch.Moreover, this division method cannot limit broadcast packets.Furthermore, for users who use laptops, their wireless network cards may need to be replaced frequently, so VLANs must be constantly configured.

Overall, this approach is not very popular.

3. Divide VLANs based on the network layer
This division method is based on each host's network IP address or protocol type (if multiple protocols are supported).

Although it checks the IP address of each data packet, because it is not a route, there is no routing protocol such as RIP and OSPF, but a bridge (equivalent to a switch) exchange.

The advantage of this method is that when the physical location of the user changes, the VLAN to which it belongs does not need to be reconfigured, and the VLAN can be divided according to the protocol type.This is very important for network administrators.In addition, this method does not require additional frame tags to identify VLANs, which can reduce network traffic.

The disadvantage of this method is that it is inefficient, because checking the network layer address of each data packet consumes processing time (compared to the previous two methods), requires higher technology, and is more time-consuming.In other words, the requirements for the switch are higher.

4. Divide VLAN based on IP multicast
IP multicast is a method of selectively broadcasting information to users, and is often used in video conferencing.This is actually a definition of VLAN, and a multicast group is considered to be a VLAN.

This method of division extends the VLAN to the wide area network, which has greater flexibility and is easy to expand through routers.But this method is not suitable for LAN, mainly because the efficiency is not high.

Among the above division methods, the first division method is more common.

3.5 Wireless LAN
3.5.1 Understanding WLAN
Wireless Local Area Networks (WLAN) is a very convenient local area network, which uses radio frequency (Radio Frequency, RF) technology to replace the local area network composed of twisted pair wires.

The wireless local area network uses wireless switching equipment and wireless network cards to realize communication functions, which saves a lot of wiring time and space, avoids a lot of cable installation, and makes the environment more tidy.

The wireless local area network uses IEEE802.11 series to standardize the MAC layer and the physical layer of the wireless local area network.

The data transfer rate of wireless local area network has reached 11Mbps, 22Mbps, 54Mbps and 108Mbps now, but the transmission distance is limited.

As a supplement and extension to the wired networking method, the wireless local area network makes the computers on the network mobile, and can quickly and conveniently solve the network connection problem that is difficult to achieve using the wired method.

3.5.2 Devices for WLAN

1. Wireless LAN card
The wireless local area network card is generally called a wireless network card (Wireless LANCard). The difference between it and the traditional Ethernet network card is that the data transmission of the former is through radio waves, while the latter needs to pass through ordinary network cables.

At present, the specifications of wireless network cards can be roughly divided into 11Mbps, 22Mbps, 54Mbps and 108Mbps, which can be applied to PCMCIA, ISA and PCI interface standards.

2. WLAN access point

Generally, a wireless LAN access point (Access Point, AP) is called a network bridge, which is used as a bridge between a traditional wired LAN and a wireless LAN, so any computer equipped with a wireless network card can pass AP to share wired LAN resources.

In addition, the AP itself also has the function of network management, which can do necessary management for the computer connected with the wireless network card.

3. Wireless router

Wireless router is an upgraded product of wireless access point. Taking D-LINK wireless router as an example, its DI624+A and DI724UP+A are two ultra-high-performance wireless routers, which can conveniently build an environment for multiple people to surf the Internet at the same time in small and medium-sized enterprises, and has a strong The firewall security features can be based on MAC address, IP address, URL and domain name and other data filtering to implement access security policies; at the same time, it supports IPSec, PPTP, L2TP and other VPN services for secure encrypted transmission of sensitive data.

4. Wireless Antenna
Different from the antennas of ordinary TVs and mobile phones, the frequency of the wireless antenna (also called antenna, Antenna) used by WLAN is 2.4GHz.

The function of the antenna is to transmit the signal of the source to a distant place through the characteristics of the antenna itself.As for how far it can be transmitted, in addition to considering the output strength of the source, another important factor is the dB value of the antenna itself. The higher the dB value, the farther the distance that can be conveyed.Generally, for every 8dB increase, the relative distance that can be communicated can be doubled to the original distance.

3.6 Example of LAN
According to the current demand and usage of LAN technology, give examples of switched, virtual LAN and wireless LAN.

3.6.1 Example of Switched LAN/Virtual LAN
This section uses a "400-node enterprise network design scheme" to illustrate the construction of commonly used LANs.

1. Enterprise needs

Suppose an enterprise has sales department, after-sales service department, design department, finance department, IT department and other departments, with 390 employees. According to the needs, it is hoped that 20 computers will be allocated to the sales department, 20 computers to the after-sales service department, and 20 computers to the finance department. 320 computers in the design department; 20 servers are required to provide network management and services.

2. Establish enterprise LAN
According to the actual situation above, the entire enterprise network can be divided into 6 VLANs; if the number of computers in the design department is large, the computers in this department can also be divided into two VLANs.

(End of this chapter)