The divided environment of data transmission Ethernet remains the main reason of charges of this technology in insufficient stability and reliability. For overcoming the described lacks firm Kalpana (subsequently bought Cisco) has offered technology of segments Ethernet switching. Thus, the divided environment (the domain of collisions) completely disappeared.

Work of the switch was based on simple, but at that time difficult the technological base - parallel processing of the acting staff on different ports. It is feature has allowed switches Kalpana to transfer the staff independently between each pair ports. It has allowed to realize idea of refusal of the divided environment in practice.

Switches are transparent for reports of a network level, routers "do not see" them. It has allowed to not change the basic scheme of networks among themselves.

In prompt distribution of switches the important role simplicity of their adjustment and installation has played. This self-trained device. The switch is not obligatory for configuring. Correctly enough to connect to it cable system. Further it can work without intervention of the manager of a network, and thus is effective to carry out a task in view.

Today it is possible to tell with full confidence, that switches are the most powerful, universal and convenient class of the equipment. In the elementary case is multi-ports bridge Ethernet. But development of technology has brought so many changes in their properties, that the main principle of work is heavy for seeing behind useful technical opportunities.

The technical basis of work of the switch is simple enough. A package, which gets on its input (source port), goes not on all active ports but only on destination port to which the device with the MAC-addresses conterminous with the address of the staff purpose is connected.

Accordingly, the first question which it is necessary to solve - conformity of ports of the switch to the connected devices (MAC-addresses). For work the special table (content-addressable memory) is used. The switch forms during "self-training" by a following principle: it is necessary to port to receive the answer from the device with physical X address as in CAM-table there is a corresponding line of conformity.

Staff with the source address (SA), available in the table, go on corresponding port. Thus the staff, intended to all units, or the having unknown person to the switch the destination address (DA), goes on all active ports. During work physical addresses of the connected equipment can vary. Thus in the table there is a new record. If in it there is no empty seat, the oldest record (a principle of replacement) is erased.

As speed of the necessary address sample directly depends on the tables size which have been not used in current of a long time interval of record automatically leave.

However such simplified algorithm rigidly (without changes) operates only in uncontrollable switches (dumb switshes). These are inexpensive, simple devices which successfully supersede hubs from a niche of the elementary networks. Usually they have a small amount of ports and not high characteristics. The opportunity of management is absent.

Following step of development became smart switches. In them, using port RS-232, usual Ethernet the manager can change many important configuration parameters. For example, thus it is possible to block the mechanism of "self-training" (to make the static table of ports conformity to MAC-addresses), to establish a filtration, virtual networks, to set speed and many other things.

But the greatest opportunities have intelligent switches. They have the interface to the high-grade processor (more precisely, to a computer as it has also the memory) which allows to supervise work and to change parameters of the device without reload. As there is an opportunity in real time to observe of passing packages, to consider passing traffic.

Base restrictions Ethernet (as divided environment of data transmission) not used to a network constructed with use of switches. Collisions are absent, there is no physical substantiation of concept of the maximal length of a line, and a maximum quantity of the connected devices.

For example, the fibre-optical lines, transferring staff Ethernet on hundreds kilometers can really be used, and local networks can unite hundreds workstations or servers.

For definition of port (or ports) purposes, it is necessary for processor of the switch to have for the analysis access to heading of staff Ethernet. Accordingly, these data it is necessary to accept in the buffer. Distinction of switches on a way of the staff promotion from here follows:

• cut-through;
• Store-and-Forward.

At switching "hurriedly", the switch can not place the coming staff in the buffer entirely. Their record entirely occurs only in case of when it is necessary to coordinate speeds of transfer, the trunk, or port of destination is borrowed. Thus, at great volume of the traffic the most part of data all the same will be buffered to some extent.

In other words, the switch only analyzes the address of purpose in heading of a package, and according to itself (time of a delay from 10-40 usec) directs the staff to corresponding port. The situation when the staff still entirely has not acted on entrance port is regular, and its heading is already transferred through target.

At a method of full buffering (Store-and-Forward) the staff enters the name entirely, and only then the processor of port makes a decision on transfer (or filtrations). Such way destruction of the spoiled staff, support of diverse networks has some lacks (big time of a delay), and essential advantages, for example. The Most part of modern switches supports such operating mode.

The most complex and dear models have an opportunity of automatic change of the switch work mechanism (adaptation). Depending on volume of the traffic, quantity of the spoiled staff, and some other parameters one of the described modes can be used.

Except for a way of promotion of the staff, switches can be divided into groups on internal logic architecture:

• A switching matrix;
• Multiport divided memory;
• The general trunk.

The fastest way which has been realized in the first industrial switch. After the analysis of heading of the entering staff the processor of port, according to the table of switching, in the beginning of the staff adds number of destination port. Then the staff (number of destination port) got in a two-dimensional matrix of logic switches, each of which coped certain beaten numbers of destination port.

The switching matrix tries to establish a way up to port of destination. If it probably, consistently passing through switches, the staff appears in necessary proceeding to port.

If the necessary proceeding port is borrowed (for example, connected to other entering port), the staff remains in the buffer of entrance port, and the processor expects an opportunity of formation by a switching matrix of the necessary way.

The important feature is that physical channels are switched. Thus, if some staff should pass on the same port, or through one "general" switch of a matrix make it they can only consistently. Besides it is possible to carry to lacks quickly accrueing with increase in number of ports complexity. As a matter of fact, it is possible to tell, that the decision is badly scaled, and now applied very seldom (though still there are variants of use of multistage switches).

In this case entrance and output blocks incorporate through the general memory which connection to which blocks the special manager of turns of target ports operates. It will organize in memory a little (usually on number of ports) turns of data.

Entrance blocks transfer the manager inquiries about data recording (a part of the staff) in turn of the necessary proceeding port.

Systems of such type are complex enough, demand dear high-speed memory, but do not possess thus serious advantages before more simple bus architecture. Therefore, wide practical application of system with divided memory have not found.

The name speaks for itself - for communication of port processors one trunk is used. For preservation of high efficiency its speed should be at least in C/2 (where C - the sum of speeds of all ports) times more, than speed of receipt of data in port of the switch.

Besides depends on a way of data transmission on the trunk much. It is clear, that the staff entirely to transfer it is undesirable, as at this time other ports will stand idle. What to bypass this restriction, usually apply a method strongly similar on ATM. Data are broken into small blocks (on some tens in byte), and transferred "almost in parallel at once between several ports.

Thus, this architecture realizes a method of time switching parts of the staff (it is possible to name them by analogy with ATM cells). The decision is easily scaled, simply enough, reliably, and at the moment certainly dominates over the market.