A computer network is a union of different technical, primarily independent electronic systems (in particular computers, in addition, sensors, actuators, radio-technological components etc.), which makes communication possible of the individual systems among themselves.

Communication is made by different minutes, which can be structured by means of the ISO/OSI model. Although in practice no computer network illustrates the ISO/OSI model completely, it is of crucial importance for the understanding of computer networks, there here from small fundamental structures by linkage, larger and more complex structures to be formed. Higher (more complex) protocol layers access functionalities of simpler underlying protocol layers.

An important principle thereby is that one can hand data (Payload) in each case to most protocol layers over (using) to transport. The protocol layer adds further data on, which are important by the protocol layer for the completion of transport to these utilizable data (their contents of it to a large extent ignored) in front and partly in the back. However there are also of it exceptions, there some minutes not in addition meant is stranger utilizable data to be transported, but exclusively as independent information systems for certain tasks function.

The most well-known lattice structure is that Internet and most well-known minutes is the TCP and IP minutes, however a number of further minutes important roles and Internet do not play are a homogeneous net separate are developed from a multiplicity of partly quite differently conceived subnetworks also in the Internet, which have only the upper protocol layers together and which utilizable data communication on the lower protocol layers to handle partly very different.

Computer networks can be classified among other things on the basis the following criteria.

• Organizational cover
• Transmission path/transmission technology

Topologies

This is a characteristic which can be understood relatively easily for the basic understanding is important. By the topology one understands the kind like the different components involved (thus mostly computers) in the net by physical or logical routings is connected. Theoretically one could connect each computer directly with every other computer involved. This is however not practicable in practice. Therefore one forms nets in those it for connections and knots gives over which one if necessary over several intermediate points of each range of the net to every other range of the net to come can.

There is a set of basicstereotyped, which arise however rarely in such a way in this clear form in practice. With the star topology there is a central point of distribution, which can control if necessary everything, but functions without the nothing. With the tree topology one uses a similar beginning which one however hierarchically graduates. In the ring topology each computer has a position in a ring and is connected only with its neighbours. The meshed net is a practical form in each computer is connected with several neighbours and redundant ways exists, so that even in case of failure the net remains connected by another line for a line still. In a bus all computers involved access together and all medium used of, whereby it can come to collisions on it. The cell topology plays a special role with radio nets with its special access characteristics.

In practice combinations this stereotyped one arise and it nearly always give still another set of names for certain special forms.

Closer details for this see under the keyword topology of nets.

Organizational cover

This criterion it often used there appears less complicated than other characteristics of nets. In practice this distinction has however only limited meaning.

• of local area networks
  • Body AREA network (BAN)
  • Personal AREA network (PAN)
    • Wireless personnel AREA network (WPAN) as term
  • Local AREA network (LAN)
    • Wireless LAN (WLAN) as term
• non-local of nets
  • Metropolitan Area Network (MAN ONE)
  • Wide AREA network (WAN)
  • Global AREA network (GAN)
  • Virtual private network (VPN)

Transmission path

Wire-bound nets

Ethernet

The most common technology with wire-bound nets is the Ethernet, which meets one particularly in local firm nets and home nets. It is line and today in developments 10BaseT, 100-Base-Tx and 1000-Base-Tx is used. The number designates the theoretical maximum transmission rate of 10, 100 or 1000 Mbit per second and T in each case says out that it a gedrilltes copper cable concerns (twisted pair). Depending upon speed a cable of the appropriate quality is necessary, which is called CAT [number]. For 100 Mbit this is e.g. CAT5 to use with 1000 Mbit CAT5e, CAT5+ or CAT6.

In former times the Ethernetvariante 10Base2 with coaxial cables was common, which became extinct to a large extent however since the year 2000. The name agitates Ethernet from this time, which suggests nevertheless that one was over kind "“ether"” function background that many computers a common Ethernet strand divided and it could come so also to disturbances when simultaneous use (so-called collisions). The "“access procedure"” calls itself thereby CSMA/CD (carrier scythe multiple ACCESS/Collision Detection), whereby each computer listens only whether the line (carrier) is free and if, then simply sends. It has pitch gives it nevertheless a collision, because still another computer does the same. This collision recognized (Collsion Detection) and both to try it at a coincidental time later again.

Besides each computer, which used the Ethernetstrang, had knows when it was meant. In addition the so-called MAC (Media Access Control) was introduced layer, which regulated the simultaneous access on the together used medium. Each computer involved got its own clear MAC address and it gave oneself procedures like all computers with the occurrence of a collision to behavior had. In practice the Ethernet/MAC minutes has the advantage the fact that it with small data traffic very fast and simply functions, since everyone can loose-transmit immediately. However arise to more and more collisions in the together genutzen net section with higher data traffic, which can let the data traffic break sometime completely together. In practice such Ethernet of nets functioned therefore only with small data traffic well. With high data traffic large delays could develop and theoretically could it for a computer pass that it could coincidentally at all never send collision-free something.

With the arising of the cousin T variants and Switchen these MAC procedures became again actually unimportant, since now again each computer had its own exclusive network area, which was controlled by Switche, however remained the MAC layer. Nevertheless there are today with Switchen no more collision problems, whereby the lines can be used more efficiently, however can it still to back-up and overloading at the Switchen come.

Token ring

Another way of the access supervision went the token loop net, which is used 2005 particularly for nets with special quality requirements. The advantage from token loop nets is that each computer can surely send something after at the latest a certain time. In addition a so-called token (too German pledge coin) in form of a small information package is handed. Who has tokens may one while utilizable data send, hears then and that passes tokens on. The sequence in that is passed on it is exactly fixed and circular, whereby one gets tokens again and again. Token loop nets are developed often so that each computer is directly connected with its two neighbours in the ring in each case and either token passes these on or hands or passes themselves on information over either kept, depending on for whom it are intended. There is also a variant the token ring of over Ethernet calls itself. All computers in a together used Ethernet are connected, but continue to themselves give there in each case a token reihum (token Passing), whereby collisions are avoided and the line are better used. The complicated at this virtual ring is that firstonce must be clarified which computers to exist and which order those in the virtual ring to take. Besides one must recognize if new computers come or existing in the ring disappear.

Really importantly the characteristics of token loop nets in sensitive nets, in which it is important, are to be known precisely like for a long time it maximally last, until a message can be sent. This can be determined easily on the basis the number of computers, thus at the length of the ring. Such nets are used for example in the automotive engineering and financial industry for critical systems.

Some further net types are:

• ARCNET
• FDDI - Optical waveguide cable
• AT

Further access procedure:

• CSMA/CA

Wireless nets

Common techniques with wireless nets are mobile phones

Infrastructure nets

• Portable radio nets such as GSM or UMTS
• WLANs in the infrastructure mode, i.e. with interface to a wire-bound net by means of Wireless ACCESS POINTs. Common WLANs of the type 802.11 are furthest

spontaneous Adhoc nets (see MANET)

• WLANs of the type 802.11 in the Ad-hoc-mode. In this mode the devices of the net without additional infrastructure communicate.
• with very small range the devices in direct environment connect, so-called Wireless personnel AREA network (WPAN)
• the standard Bluetooth (see also Toothing)
• Lattice structures for sensor nets, current Forschungsgebiet

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