Introduction to common network equipment

hubs

Hub is the basic equipment in data communication system, it is a kind of hardware device that does not need any software support or only rarely manages software management, just like the transmission medium such as twisted-pair wire. It is widely used in various situations. Hubs work in a local area network (LAN) environment, which, like a network adapter, is applied to the first layer of the OSI Reference Model and is therefore called a physical layer device. The interior of the hub adopts the interconnection of electrical appliances, and when maintaining the LAN environment is a logical bus or ring-type structure, it is possible to establish a physical star or tree network structure with a hub. In this respect, the hub acts as a repeater for multiport ports. In fact, the hub is actually a repeater, the difference is only the hub can provide more port services, so the hub is also called the multiple mouth relay.

According to IEEE 802. 3 protocol, the hub function is to randomly select an end of the device, and let it exclusive full bandwidth, with the hub of the Allied devices (switches, routers or servers, etc.) to communicate. It can be seen that the hub in the work with the following two characteristics.

First, the hub is just a multiport signal amplification device, work when a port receives data signals, because the signal in the transmission from the source port to the hub has been attenuation, so the hub will be the signal to plastic amplification, so that the attenuation of the signal regeneration (recovery) to the status of the sending, It is then forwarded to all other ports on the working state. From the work of the hub can be seen, it only plays a role in the network signal amplification and retransmission, the purpose is to expand the network transmission range, and do not have the directional transmission of signals, is-a standard shared equipment. So some people call the hub "stupid hub" or "dumb hub".

The second is that the hub only communicates with its allied devices (such as the upper hub, switch, or server), and the ports in the same layer do not communicate directly to each other, but the information is broadcast to all ports through the Allied device. This shows that even in the same hub of different two ports to communicate between, must undergo two steps: The first step is to upload the information to the allied equipment; The second step is to connect the device and broadcast the information to all the ports.


Routers

The main function of a router is to make routing choices. When a host in a network sends a packet to a host in another network, it first sends the packet to the router in the same network for the connection between the network, the router chooses the appropriate route according to the destination address information, and passes the packet to the destination network for the routers connected to the network, The packet is then forwarded to the destination host through a routing protocol used internally within the destination network.

Routers and bridges are similar to the concept of receiving protocol data unit PDU, check the header field, and based on the header information and content of a table to forward. In fact, the bridge only checks the frame head of the data link frame and does not view and modify the packet head of the network layer carried by the frame; While the router checks the network layer packet head, and according to the address information to make a decision, when it passes the packet to the data link layer, it does not know and does not care whether it is through Ethernet or Token Ring network for transmission.

Two-layer/three-layer Exchange (L2/L3 Exchange) of the general Ethernet switch, the OSI two-layer exchange frames do not make any changes, just check the exchange table, for forwarding.

Routers/three-tier switches (switches with routing functions) that work at the network layer, similar to Ethernet switches, only Exchange objects that are grouped, not frames. By IP address to determine which subnet node, the frame may change, after the router into a new frame.

The three-tier switch leverages the router's three-tier capabilities, not only the advantages of flexible virtual local Area network (VLAN) and high switching speed of two-layer switch, but also the problem of "broadcast storm" which can not be handled by the two-layer network, the biggest difference between it and the traditional router is to finish the high speed Routing and exchange of the third Shong by hardware. And it can still keep the line speed while introducing the router billing and access control function. With the development of exchange technology, many manufacturers now have switches to support layer fourth exchange.


switch

Ethernet switches, also known as switched hubs, are simplified (typical) bridges that are commonly used to interconnect the same type of LAN (for example, Ethernet/Ethernet interconnect). The difference between switch and bridge is that the number of switch ports is more, and the data transmission efficiency of switch is higher. Ethernet switches use either storage-forwarding (Store-forward) or pass-through (Cut-through) technology to implement information-frame forwarding.

Pass-through switching: When the destination address (MAC address) of a frame is received, the destination port of the forwarding is determined and the forwarding is started, instead of waiting for the entire word after a frame to be forwarded. Relative to the storage and forwarding technology, the transmission delay is reduced, but the transmission process can not be verified, but also may pass the broadcast storm.

Storage and forwarding Exchange: From the function, is the network bridge used technology, wait until all the data received before processing, including verification, forwarding and so on. The transmission latency is higher relative to the pass-through technology.

Some switches can use both of these technologies at the same time. When the network error rate is low, the direct-pass technique is adopted, and the storage and forwarding technology is used when the network error rate is high. This switch is called an adaptive switch.