The hub hub (HUB) belongs to the infrastructure in the data communication system, which is the same as a twisted pair or the like, which is a hardware device that does not require any software support or only rarely manages software management. It is widely used in various occasions. The hub is operated in a local area network (LAN) environment, like a network card, applied to the OSI reference model first layer, so it is also referred to as a physical layer device. The interior of the hub uses an electrical interconnection. When maintaining the LAN is a logical bus or a ring structure, it is possible to establish a physical star or tree network structure with a hub. In this regard, the role of the hub is equivalent to a multi-port repeater. In fact, the hub is actually one of the repeater, and the difference is only that the hub can provide more port services, so the hub is also called a multi-cart. According to the IEEE 802.3 protocol, the hub function is randomly selects a port of a port and allows it to communicate all of the entire bandwidth, communicating with the uplink of the hub (switches, routers, or servers, etc.). It can be seen that the hub has the following two characteristics at work. The first is that the HUB is just a multi-port signal amplification device, and when a port receives the data signal, since the signal has attenuated from the source port to the HUB, the HUB is placed to be shaped, Regenerating (recovered) to the time of the time, followed by forwarding to all of the other ports. As can be seen from Hub's working mode, it only plays only signal amplification and retransmission in the network, which is the purpose of expanding the transmission range of the network without having signal orientation transmission capabilities, and is a standard shared device. So some people say that the hub is "silly HUB" or "dumb HUB". Second, the HUB is only communicating with its upper link (such as the upper HUB, switch or server). Communication is not directly communicated between the portions of the same level, but is broadcast on all ports by the upper and the upper end. It can be seen that even if communication is performed between the different two ports of the same HUB, it must be two steps: the first step is to upload the information to the uplink device; the second step is the Upline device and then the information Broadcast to all ports. The main function of the router router is to perform routing. When a host in a network wants to send a packet in another network, it first gives the packet to the router used for the network connection in the same network, the router selects the appropriate route according to the destination address information, and transmits the packet To the destination network is used in the network connected router, then the packet is finally submitted to the destination host by the routing protocol used inside the destination network. The concept of routers and bridges is similar, all receive protocol data unit PDUs, check the header field, and forwards according to a table of header information and content. In fact, the bridge only checks the frame header of the data link frame, and does not view and modify the network layer packet header carrying the frame; and the router checks the network layer group header, and determines the address information in it. When it passes the packet to the data link layer, it doesn't know if it doesn't care if it is transmitted through an Ethernet or a token ring network. Layer 3 / Layer 3 Exchange (L2 / L3 Exchange) General Ethernet Switch, Implementation OSI Layer 2 Exchange Frame does not make any modifications, only check the exchange table, forward. Router / Layer 3 switch (with a switch of routing), working on the network layer, similar to the Ethernet switch, just the exchangeable object is a packet, not a frame. The node of which subnet is determined by the IP address, and the frame may change, and it becomes a new frame after the router.
The three-layer switch makes full use of the three-layer function of the router, retains the flexible virtual local area network (VLAN) division of the Layer 2 switch and the advantages of high exchange speed, and solves the "broadcast storm" problem that cannot be processed by the Layer 2 network. The maximum difference between the router is to complete the high-speed routing and exchange of the third layer message through hardware, and keep the line speed in the case of introducing router billing and access control functions. With the development of exchange technology, many manufacturers' switches can support fourth floors. Switch Ethernet switches, also known as swap hubs, is a simplified (typical) bridge, typically used to interconnect the same type of LAN (eg, Ethernet / Ethernet interconnect). The switch and bridge difference are: there are many exchange ports; the data transfer efficiency of the switch is high. The Ethernet switch adopts the Store-Forward technology or the CUT-THROUGH technology to achieve the forwarding of the information frame. DC Switching: When the destination address (MAC address) of a frame is received, the forwarded destination port is declared, and forward, without waiting for receiving all the bytes of a frame, forward it. Relative storage forwarding techniques, the transmission delay is reduced, but the verification cannot be performed during the transmission, and the broadcast storm may be delivered. Storage forwarding exchange: From functional, the technique used by the bridge, etc. After receiving all data, it will be processed, including check, forwarding, etc. Relative to PTR, the transmission delay is large. Some switches can use both technologies simultaneously. When the network bitmap rate is low, it is powered by power, and storage forwarding techniques are adopted when the network bitmode rate is high. This switch is called an adaptive switch.
