When you create one of the first computer networks, has often been the problem of collisions on the link which effectively made use of the network. And to connect with each other devices used hubs (hubs), which operate the system in the first layer ISO / OSI, which is the physical layer. Were created using the network topology of a star. Transmit one bit after beating an input signal to the output hub, which at the time when there were no collisions, minimalizowało delays. This solution, however, generated other problems. Today hubs are sometimes connected to the router as a splitter.

Progress in the development of the network topology and has forced them to construct a newer generation of devices, namely the switch, which operates the system in the second layer ISO / OSI (data link). This is called. Intelligent hub. What is this intelligence?

First of all, hub, an input signal relay on all exits are not depending on whether the device is not in wpięte awaited this particular package of data or not. The switch case is quite different. Packets are sent frame by frame only to a specific device in the network, which does not generate unnecessary traffic on the link. To identify the recipient switch uses the physical address (MAC) contained in the header transmitted frame. It is not everything, a switch is able to remember a finite number of associations MAC addresses the recipient and sender in the future which allows faster transmission of the frame to the right place. These data are stored in the tables. That's why we say that the switches "learn to network topology."

Hub has made it possible to create a network topology stars, but only with the use of two segments. With the switch, we have no such restrictions.

Hubs do not have the duplex, which at the time the information could be sent only to one party, which generates collisions, and blocks the link. When the package was sent, the recipient only after you receive it in full could send information to the sender that picked up a pack and waiting for another. It looks as if some in the city trams captivated only one track, which would travel on the line of one warehouse. In switchu we have to deal with full duplex (transmute transmitter may be places with the receiver, the transmission and reception can take place at the same time without loss in the throughput), which can illustrate the current solution for the movement of trains or trams, which is two tracks , After which you can travel at the same time in different directions.

In addition, switches can be a part of the virtual network VLAN (this is a network inside artificially separated more physical network). The connection between the two virtual networks are carried out by a combination of special-called trunk (one port in switchu is configured to connect with other VLAN network, which allows you to connect online with one of several physical channel, which in turn are divided into logical channels). Each frame has sent so additional 4-byte field, in which the identifier is the virtual network, to which the transmission.

Switches used on a large scale in data centers (called Data Center) due to its versatility and bezkolizyjność. They allow you to obtain relatively small losses on the bandwidth so that you do not significantly inhibit the operation of servers.