Hub installation and connection
In the previous article, we introduced the basic knowledge of the hub. This article continues to introduce some other important aspects of the hub, that is, the installation and connection of the hub. We have learned from the previous article that the hub is a most basic network hub device, which mainly works on the data link layer of OSI. At the same time, we also know that it almost requires no software configuration, a purely plug-and-play hardware device. Nevertheless, because it has experienced a long historical development period, its technology has also been developed step by step, and there are many places in the application of the network that require special attention.
To use the hub correctly, the first thing to understand is the basic working principle of the hub. Only in this way can the hub be correctly selected, installed and connected.
1. The basic working principle of the hub
We know that there is only one physical signal transmission channel in the ring network, which is transmitted through a transmission medium. In this way, there is a contradiction between nodes competing for channels, and the transmission efficiency is low. After the introduction of the hub as a network hub device, each station is connected to the hub with its own dedicated transmission medium. There is no longer only one transmission channel between each node. The signals sent by each node are concentrated through the hub, and the hub then collects the signal. Shaped, amplified, and sent to all nodes, so that at least no collision occurs on the upstream channel. However, the hub-based network is still a shared-media local area network. The "sharing" here is actually the hub's internal bus, so there will still be signal collision when the upstream channel and the downstream channel send data at the same time. When the hub detects a collision from its internal port, it generates a collision enhancement signal (Jam) and transmits it to the target port to which the hub is connected. At this time, all data will not be sent successfully, forming a network "big traffic jam".
We can use an image to illustrate this network phenomenon, that is, there are two directions of the car on the single lane at the same time.
We know that on a single lane, only vehicles in one driving direction are usually allowed to pass, but in small cities and towns, limited conditions usually do not have such a rule, and single lanes are also likely to allow vehicles in two traveling directions, but must pass at different times . The same is true in the hub. Although the connection between each node and the hub has its own independent channel, there is only one common channel inside the hub. Both the upstream and downstream data must be sent and received through this shared channel. This may be like As with single lanes, traffic jams may occur when data is sent on both the upstream and downstream channels. Is it easy to understand?
Because of this shortcoming of the hub, it cannot be used alone in larger networks (usually to share a small part of the network communication load with switches and other equipment), just as there cannot be a single lane in the center of a large city, because The more the network, the greater the chance of network collisions. For this reason, the data transmission efficiency of the hub is relatively low, because it can only transmit data in one direction at a time, which is the so-called "simplex" method. If a hub is to be selected as a single hub device in the hub network, the network size should preferably be within 10 units, and the hub bandwidth should be 10 / 100Mbps or more.
In addition to the shortcomings of the shared bandwidth of the hub, there is another aspect that must be considered when selecting the hub, that is, its broadcast method. Because the hub belongs to the bottom layer of a pure hardware network, it basically does not have the "smart memory" ability, let alone the "learning" ability. It also does not have the MAC address table that the switch has, so it is not targeted when sending data, but it is sent by broadcast. In other words, when it wants to send data to a node, it does not send the data directly to the destination node, but sends the data packet to all nodes connected to the hub.
There are two deficiencies in this method of broadcasting data: (1) User data packets are sent to all nodes, which may bring unsafe factors for data communication. Some people with ulterior motives can easily intercept other people's data packets; ) Because all data packets are sent to all nodes at the same time, coupled with the shared bandwidth method described above, it is more likely to cause network traffic congestion and reduce network execution efficiency.
Second, the installation of the hub
After understanding the working principle of the hub, let's take a look at the installation and connection method of the hub in the LAN.
The most important thing for access equipment is its interface technology. Different interfaces are used in different application environments, and different applications correspond to corresponding interfaces. Not only the hub, but also the switches and routers that will be discussed later. The interface of the hub is relatively simple. In order to let everyone master the connection of various applications of the hub, we need to understand some of the main interfaces of the hub.
1. Hub common ports
Hubs usually provide three types of ports, namely RJ-45 ports, BNC ports and AUI ports, to be suitable for networks constructed by connecting different types of cables. Some high-end hubs also provide fiber ports and other types of ports.
(1) RJ-45 interface
The RJ-45 interface can be used to connect RJ-45 connectors. It is suitable for networks constructed with twisted pairs. This type of port is the most common. Generally, an Ethernet hub will provide this type of port. The number of hubs we usually talk about refers to the number of RJ-45 ports.
The RJ-45 port of the hub can be directly connected to terminal devices such as computers and network printers, and can also be connected to other hubs and routers such as switches and hubs. It should be noted that when connecting to different devices, the jumper method of the twisted pair cable used is different. For details, please refer to the content introduction of the previous network cable production.
(2) BNC port
The BNC port is an interface for connecting with a thin coaxial cable. It is generally connected through a BNC T-connector.
Most 10Mbit / s hubs have a BNC port. When the hub has both BNC and RJ-45 ports, since it can be connected to the twisted pair network through the RJ-45 port, and can also be connected to the thin cable network through the BNC interface, it can realize twisted pair and thin coaxial cable Connection between two networks using different communication transmission media. The characteristics of this dual interface can be used to be compatible with the original thin coaxial cable network (10Base-2), and can gradually achieve the transition to the mainstream twisted pair network (10Base-T), of course, can also achieve the same Connection between shaft cable networks (less than 185 meters).
Similarly, if the distance between the two networks is greater than 100 meters, and the connection between the two networks cannot be achieved using twisted pair, then the two input networks can also be connected by the thin coaxial cable transmission through the BNC port of the hub However, both networks can still use twisted pair, a cheap and common transmission medium, as shown in Figure 6. However, it should be noted that the distance between these two networks cannot be greater than 185m.
(3) AUI port
The AUI port can be used to connect the AUI connector of the thick coaxial cable, so this interface is used to connect to the thick coaxial cable network. Its schematic diagram is shown in Figure 7. At present, there are fewer hubs with this interface, mainly Only available in some backbone hubs.
Because the network using thick coaxial cables as the transmission medium is relatively expensive and the wiring is difficult, it is very rare in practice to use thick coaxial cables for wiring. However, because the transmission distance supported by the single-section thick coaxial cable (10Base-5) is up to 500 meters, the thick coaxial cable can be used as a communication cable for connecting between long-distance networks. Therefore, it can also be used as an inexpensive remote connection solution. The connection diagram is shown in FIG. 6 above, the difference is that the connection medium between the networks used here is a thick coaxial cable.
With the help of the transceiver, the AUI port can also be connected to the RJ-45 interface, BNC interface and even the fiber interface. From left to right shown in Figure 8 below are: AUI to RJ-45 transceiver (used to realize the connection between the AUI port and the RJ-45 interface), AUI to BNC transceiver (used to realize the AUI port and the BNC interface Connection), AUI to ST transceiver (used to realize the connection between the AUI port and the fiber interface). Of course, there are many types of transceivers, such as RJ-45 to RS-232, RJ-45 to BNC, etc. But do nâ€™t underestimate this little thing, guess its price. I think in most cases you will see it flat. I could nâ€™t figure out the price when I first bought it, but ask After many homes (which are really hard to find), I realized that I had to accept this price fact. Generally speaking, this kind of product is about 130 yuan for authentic products, is it beyond your expectations? The main brands of this interface transceiver are: D-Link, HP and so on. This product plays a role of interface type conversion (of course it is not as simple as cable connection, which needs to be completed by a certain circuit), so it is usually called an "adapter".
(4) Hub stacking port
This port is of course only available with stackable hubs, and its role is to connect two stackable hubs as its name suggests. Generally speaking, a stackable hub has two ports with similar appearance at the same time: one is marked as "UP", the other is marked as "DOWN". When connecting, a cable is connected from the "UP" port of one hub to the other. The "DOWN" ports of a stackable hub are all "female" heads, so the cable ends must be "male", but this type of cable is provided by the manufacturer when you buy a stackable hub. If you are damaged or lost, you can also make one directly in the computer city, as long as you explain the purpose to the business.
2. Hub installation
The installation of the hub is relatively simple, especially the fool hub, as long as it is fixed in the wiring cabinet and plugged in the power cord. As long as the twisted pairs need to be connected, just insert the RJ-45 of the twisted pairs into the hub port. Although the smart hub is fixed and can be used, however, if you want to achieve remote management, you must perform the necessary configuration to specify the IP address information for the hub. In addition, rack hubs are generally used in some large networks, which involves rack installation of the hub.
In terms of structure, there are two types of racks and desktops. The hubs used in general departments are desktops; enterprise computer rooms usually use racks. Rack-type hubs are easy to fix in a fixed place, and are generally installed in a cabinet with other hubs, switches, and some servers. This is convenient for network connection and management, and saves equipment. The space occupied. If you choose a rack-type hub when you buy it, you can choose a hub rack (generally provided by the manufacturer). Now let's take a look at the installation of rack hubs.
Rack-type hubs are generally installed in cabinets with other equipment. Of course, these cabinets have corresponding structural standards in the industry, especially in terms of size, there are strict regulations (such as width, 1U (unit) height, etc.) ), So that all devices can be conveniently and beautifully installed together, that â€™s why the hub is empty, but it â€™s not the same size. Of course, the large chassis also has another advantage, that is, it can dissipate heat .
The international standard cabinet can be roughly divided into three categories of 19 inches, 23 inches and 24 inches in width, which is mainly determined according to the requirements of the server cabinet. Depending on the number of installed devices, cabinets of different heights can also be selected. The height of the cabinet is usually "U" as the unit, "U" is actually the meaning of "Unit", the Chinese meaning is "one unit", 1U = 1.75 inches. The installation of such cabinets is generally carried out in the following steps.
Step 1: Fix the mounting bracket
Before installing the hub into the cabinet, the fixing bracket should be installed at the specified position of the hub (this should be done according to the operation manual). This is for the future installation of the hub on the rack. The mounting brackets of different hubs are quite different, but the installation principle is basically the same. Figure 10 shows a mounting bracket provided with the Cisco hub.
The size of Cisco's network equipment is mostly 19 inches (because 19 inches is the most popular cabinet standard in the world). When installing 19-inch network equipment to a 19-inch cabinet, the fixing method of the mounting bracket is shown in Figure 11. When the cabinet size is 23 or 24 inches, the network equipment needs to be installed in the 23 or 24 inch cabinet.
Step 2: Fix the equipment
After the mounting bracket is fixed, the next thing to do is to put the hub device with the bracket installed into the corresponding position of the cabinet and fix it in the cabinet. In fact, this installation method is very easy. In fact, it is only necessary to fix a few screws. For the installation method, see Figure 13.
Step 3: Fix the thread guide
After installing the hub in the cabinet, you must connect the network cables. In a cabinet, there are usually several network devices starting from the ground, so there are many network cables concentrated in this cabinet, if these network cables are not clear Network management will bring great inconvenience, for this we need to bundle and install the network cables. At this time, it is generally necessary to install a wire guide for the network cable, so that the bundled network cable becomes neat and beautiful, and easy to manage.
Above we introduced the method of installing rack hubs in cabinets, which is generally applicable to larger networks. For small offices, there is usually no cabinet, and the hub can only be installed on the desktop or wall.
For the installation of the hub on the desktop, you can first fix the mounting bracket on the desktop. This installation method should pay attention to two different installation directions: one is the horizontal installation method for the hub to be placed horizontally. ; The other is to let the hub be placed vertically.
There are also two ways to install the hub on the wall: one is to fix the hub horizontally on the wall, then the installation method shown in Figure 17 can be used; the other is to install the hub vertically on the wall.
Third, the connection of the hub
Although the connection of the hub is simple, basically no configuration is required, but through the understanding of its connection principle, the hub can be better used to meet the needs of small and medium-sized network applications. Before officially introducing the connection method of the hub, let's first understand the principle of the hub's signal forwarding.
1. The signal forwarding principle of the hub
The hub works at the physical layer of the OSI / RM reference model and the MAC (media access control) sublayer of the data link layer. The physical layer defines electrical signals, symbols, line status and clock requirements, connectors for data encoding and data transmission. Because the hub only shapes, amplifies and retransmits the signal, and does not encode it, it is a physical layer device. 10M hubs have 4 standard interfaces available at the physical layer, that is: 10BASE-5, 10BASE-2, 10BASE-T, 10BASE-F. The 10BASE-5 (AUI) port of the 10M hub is used to connect layer 1 and layer 2.
The hub uses the CSMA / CD (Carrier Frame Listening Multiple Access / Collision Detection) protocol. CSMA / CD is the MAC layer protocol, so the hub also contains the content of the data link layer.
As a special multi-port repeater, 10M hubs follow the 5-4-3 rule in the expansion of networked relays, that is: a network segment can only be divided into 5 sub-network segments; a network segment can only be There are 4 repeaters; a network segment can only have up to three sub-network segments containing PCs, as shown in Figure 19, sub-network segment 2 and sub-network segment 4 are used to extend the distance.
The working process of the hub is very simple. It can be simply described as follows: first, the node sends a signal to the line, the hub receives the signal, because the signal is attenuated in the cable transmission, the hub amplifies the attenuated signal after receiving the signal, and finally The hub forwards the amplified signal broadcast to all other ports.
2. Stacking of hubs
In order to make the hub meet the requirements of the number of ports in a large-scale network, the stacking or cascading method of the hub is generally adopted in larger networks. However, the main applications of these two methods are different. Let's first introduce the stacking method.
The stacking method refers to connecting several hubs with cables through the stacking ports to realize the expansion of the number of ports of a single hub. It should be noted that only stackable hubs have such ports. A stackable hub generally has "UP" at the same time. "And" DOWN stack ports.
The hub stack is directly connected from one "UP" stack port to the "DOWN" stack port of another hub through a dedicated connection cable provided by the manufacturer. All hubs in the stack can be regarded as a whole hub to manage, that is to say, all hubs in the stack can be regarded as a hub from the topology. Figure 20 shows the stack connection diagram of a 3Com SuperStack II PS Hub 40/50 stack hub, and Figure 21 shows the stack connection diagram of the Cisco FastHub 300/400 stack hub. The connection between such hubs usually does not occupy the original ordinary port on the hub, and has intelligent recognition performance in this stack port, so the hubs stacked together can be managed as a hub. Hub stacking technology uses a special management module and stack connection cable to establish a wide broadband link between hubs, so that each actually used user bandwidth may be wider (only if not all ports are Use case).
The expansion method of stacking hub ports is limited by the type and spacing distance of the hub. The first condition is that the stacking hub must be stackable; another such stack connection is generally very close to each other. The stack connection cable provided by the manufacturer is generally 1m), so this hub port expansion connection method is too limited by distance.
Cascading is another way of hub port expansion. It refers to the use of hub ordinary or specific ports to connect between hubs. The so-called common port is connected through a common port of the hub (such as RJ-45 port), and the so-called special port is a "cascade port" specially designed by the hub for cascading, generally marked with "UPLink". Because there are two cascading methods, in fact all hubs can be cascaded, at least through ordinary ports. Let's take a look at these two cascading methods separately.
(1) Use Uplink port cascade
Most hubs with "Uplink" cascade ports will come with it, as shown in Figure 22 is a hub with "Uplink" ports. When using the "Uplink port" provided by the hub specifically for upstream connection, you can usually use a twisted pair of straight-through jumpers to connect this port to any port other than the "Uplink port" on other hubs.
It should be noted here that between the two cascaded hubs, the port of the next hub connected by the cascaded twisted pair cable is no longer the "UPlink port" we imagined, but is connected to the ordinary port and connected The schematic diagram is shown in Figure 23. Another point to note is that some brands of hubs (such as 3Com) use a common port as an Uplink port, and use a switch (MDI / MDI-X switch) to switch between the two types. Figure 24 shows the 3Com SuperStack II MDI / MDI-X switch.
(2) Use common port cascade
In addition to using the dedicated cascade port (Uplink port) described above for cascading between hubs, you can also cascade through the common ports of the hub. , That is to say, the two ends of the twisted pair must be jumpered. The method of debounce is to swap the first 1-3 and 2-6 feet at one end.
From the above two hub port expansion methods ("stack" and "cascade"), it can be seen that the stack method is difficult to implement, the investment is large, and the distance between the hubs is also greatly limited. The cascading method is relatively easy to implement, and the investment is relatively cheap (hubs with cascading ports are everywhere, and they are not very expensive, and can also be cascaded through ordinary ports), also in terms of distance If there is much room for it, the maximum distance of a single-segment twisted-pair network segment can reach 100m, which is more flexible to implement. However, it must be noted that the stacking method is far superior to the cascading method in terms of performance, and the stacking method can realize the unified management of multiple hubs.
In addition to increasing the number of hub ports, cascading between hubs also has an important role to extend the range of the local area network (in fact, it also expands the number of hub ports). For a 10Base-T network, the maximum transmission distance allowed by unshielded twisted pair is 100 meters, that is, the network range is 100m around the hub, which is definitely for a larger network Is not enough. At this time, when the distance between the computer and the hub exceeds 100 meters, the distance can be extended by adding a hub in the middle of the line. Only the distance from the computer to the hub and the hub to the hub is less than 100 meters. problem. Although there are two dedicated "Uplink" port methods and "ordinary port" methods for the cascade connection of the hub, it is best to choose the "Uplink port method" from the perspective of the network connection distance, because this connection method can guarantee the maximum The bandwidth and signal strength of a hub, and the signal attenuation is severe if the common port is used for expansion, and the bandwidth is greatly affected by the network. This is more important in a network with multiple cascades.
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