Hub (hub) concept

Hub, the English name also known as the Hub, in the OSI model belongs to the data link layer. Cheap is its biggest advantage, but because the hub is a shared device, resulting in the heavy network, the efficiency becomes very low, so we can not see the hub in the medium and large network. Today's hubs generally use Full-duplex mode, the common market on the hub transmission rate is generally 100Mbps. Let's take a look at some of the concepts of hubs:

Shared type

The most important feature of a hub is a shared mode, which means that when one port sends data to another port, the other port is in a "wait" state. Why "Wait"? For example, in a case where a packet is sent to B in a unit of time, A is sent to B, C, D three ports (this phenomenon is immediately described in the IP broadcast below), but only b received, the other ports in the first unit of time to determine not the data they need will not receive a sent data. Until a sends IP broadcasts again, the C,D is idle, or the data can be transferred between CDs, in the unit time before a sends IP broadcasts again. As shown in Figure 1, we can understand that there is only one channel inside the hub (that is, a public channel), and then all the ports are connected underneath the public channel.

IP Broadcast

The so-called IP broadcast (also called: Mass), refers to the hub when sending data to the underlying device, regardless of the origin of the data from where the data sent to each port, if there is a port need the source of data, will be in the receiving State, and the unwanted port is in a state of denial. For example, when client a sends a packet to client B in the net, the hub sends the data from a Baoqun to each port, at which point B is in the receiving State, and the other port is in the rejected state, and also outside the network, when client A is sending the domain name "www.163.com", through the hub, The IP address (202.108.36.172) is then sent back to the hub through DNS domain name resolution. At this point, the hub is sent to all incoming ports, the machine that needs this address is in the receiving State (client A is in the receiving state) and is not required to be rejected.

Unit time

This should be the simplest noun, can also be understood as a hub of the working frequency, such as the working frequency of 33MHz hub, then in the unit time hub can do? This is an example of how the shared type was explained, but one thing that needs to be explained is that, for example, we sometimes see a "at the same time" sending data to B, and C is transmitting data to D, which may seem contradictory and true, why does it look like 2 of them at the same time? Because a is sending data to B in the first unit of time, for broadcast reasons, B, C, D will be broadcast at the same time in the first unit, but C,d will refuse to receive the data from a from the 2nd unit of time, since C and D have judged that the data is not what they need. and C also sends a data broadcast at the 2nd unit of time, A,b,d accepts, but only D receives the data. These operations take only 2 to 3 units of time, but it is hard to perceive that it feels like "doing" at the same time.