Computer network

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A computer network , computer network or computer network is an amalgamation of various technical, primarily independent electronic systems (in particular computers , but also sensors , actuators , agents and other radio-technical components) that enables communication between the individual systems. The aim here is z. B. The sharing of resources such as network printers , servers , files and databases . The ability to centrally manage network devices , network users , their authorizations and data is also important . Direct communication between network users ( chat , VoIP telephony, etc.) is also of particular importance today .

Communication takes place using various protocols that can be structured using the ISO / OSI model . Although in practice no computer network completely maps the ISO / OSI model, it is of crucial importance for understanding computer networks, since larger and more complex structures are formed from small basic structures by linking them. Higher (more complex) protocol layers access the functionalities of simpler underlying protocol layers.

An important principle here is that so-called user data can be transferred to most of the protocol layers for transport. The protocol layer adds further data to this user data (the content of which it largely ignores) at the front and in some cases at the end, which is important for handling the transport through the protocol layer. However, there are exceptions to this, as some protocols are not intended to transport external user data, but only function as independent information systems for certain tasks.

The best-known network structure is the Internet , and the best-known protocols are TCP (Transmission Control Protocol) and IP (Internet Protocol) , but a number of other protocols also play important roles in the Internet. The Internet itself is not a homogeneous network, but consists of a large number of partly quite differently conceived subnetworks (subnets) that have only the upper protocol layers together and sometimes handle the transmission of user data to the lower protocol layers very different.

As a central computer or main computer within a computer network that computer - usually a mainframe  - referred to, the other connected computers (such as workstation computers or terminals ) or the programs operated on it, data , services , system programs , application programs , etc. Ä. provides.

Topologies

Diagram: network topology

The topology is understood as the way in which the various components involved (i.e. mostly computers) are connected in the network by physical or logical cable paths. In order to integrate several computers in a computer network, you need good planning, which is simplified by the division of the topology. In this way computer networks are formed in which there are connections and nodes via which one can reach any other area of ​​the network from any area of ​​the network via several intermediate points.

There are a number of basic stereotypes, but these seldom appear in this clear form in practice. With the star topology, there is a central distribution point that can control everything if necessary and without which nothing works. This topology is actually only used in very small networks, for example home networks or at LAN parties . A connection of several star topologies at their points of concentration is also referred to as an extended star topology. A similar approach is used with the tree topology, but it is hierarchically graded. The “top” computer is in control of all the others, the power decreases the further down you sit in the tree. In the ring topology, each computer has a position in a ring and is only connected to its neighbors. This has the consequence that the failure of a computer paralyzes the computer network. With the bus topology, all participating computers access a medium that is shared and used by all, which can lead to collisions on this. The meshed network is a form in which every computer is connected to several neighbors and in which redundant paths exist, so that even if one line fails, the network still remains connected via another line. The cell topology plays a special role in radio networks with its special access properties.

In practice there are almost always mixed forms of these stereotypes and there are also a number of names for certain special forms. The spontaneous, self-organized networking of any device is referred to as a smart network or smart grid .

Organizational coverage (network architecture)

This criterion is often used because it appears less complicated than other properties of networks. In practice, however, this distinction is of limited importance.

Local networks

Non-local networks

Transmission path

Wired networks

Ethernet

The most common technology in wired networks is Ethernet , which is used primarily in local company networks and home networks. Today it is created and used with copper cables in the versions 10BASE-T, 100BASE-TX and 1000BASE-T. The number denotes the theoretical maximum transmission speed ( channel capacity ) of 10, 100 or 1000 Mbit per second. The T says that it is a twisted copper cable ( twisted pair ). Depending on the speed, a cable of the appropriate quality is required, which is standardized according to categories . For 100 Mbit this is e.g. B. CAT5, with 1000 Mbit CAT5e, CAT5 + or CAT6 must be used.

There are also different standards for realizing Ethernet over fiber optic connections , e.g. B. 10BASE-FL, 100BASE-FX, 1000BASE-SX / -LX and various 10 Gigabit standards beginning with "10GBASE-".

The original access method for Ethernet is CSMA / CD (Carrier Sense Multiple Access / Collision Detection), whereby each computer first checks whether the line (carrier) is free and, if this is the case, sends. It is possible that another computer does the same and a collision occurs. As soon as this collision is detected (collision detection), both computers stop sending and both try again later at a random point in time. Addressing is done using the MAC address.

The form, which is now much more common, is that of a "switched" network, in which more intelligent concentrators (switches) are used, which allow collision-free full duplex operation and, in total, enable a significantly higher overall throughput.

Token ring

Another way of access control was the token ring network, which is mainly used for networks with special quality requirements. The advantage of token ring networks is that every computer can send something after a certain amount of time at the latest. For this purpose, a so-called token (in German deposit coin) is passed around in the form of a small information package. Whoever has the token is allowed to send useful data for a while, then stops and passes the token on. The order in which it is passed on is precisely defined and in the form of a ring, which means that you get the token again and again. Token-ring networks are often structured in such a way that each computer is directly connected to its two neighbors in the ring and either passes the token on to them or transfers information. There is also a variant called Token Ring over Ethernet . All computers are connected in a shared Ethernet, but each token is passed on in turn ( token passing ), which avoids collisions and makes better use of the line. The complicated thing about this virtual ring is that it must first be clarified which computers exist and which order they take in the virtual ring. You also have to recognize when new computers are added or existing ones disappear in the ring.

The properties of token ring networks are important in security-critical networks, where it is important to know precisely how long it will take at most before a message can be sent. This can easily be determined from the number of computers, i.e. the length of the ring. Such networks are used, for example, in automotive technology and in the financial sector for critical systems.

PowerLAN

The PowerLAN makes use of the existing power grid to set up a network. Special adapters establish the connection between the mains and a network device via the socket. The information to be transmitted is additionally modulated onto the line on the transmitting side and demodulated again on the receiving side. At least two PowerLAN adapters are required to set up a network. From a technical point of view, this wired network is a carrier frequency system .

Since the transmitted data is freely distributed in the power grid, similar to a radio network, security aspects also play an important role here. Therefore, the information is usually encrypted . Furthermore, interference must be taken into account, which on the one hand emanate from the PowerLAN as a carrier frequency system, but on the other hand also act on it from outside and influence the transmission.

Radio networks

Common technologies in radio networks are:

Infrastructure networks

Ad hoc networks (MANET)

  • WLANs of the type IEEE 802.11 in ad-hoc mode. In this mode, the devices in the network communicate without any additional infrastructure.
  • which connect devices in the immediate vicinity with a very short range, so-called Wireless Personal Area Networks (WPAN)
  • Bluetooth
  • Network structures for sensor networks , current research area

Physical components (hardware)

In addition to passive components (antennas, cables, glass fibers, connectors, junction boxes), active components are usually required for the physical and logical implementation of the network in order to guarantee functionality. Examples are gateways , routers , switches , access points , previously also hubs , repeaters and bridges . In some cases, such components can also be implemented as virtual (software) and not as physical hardware solutions.

Linguistic consideration of the net and network

In the German language, both the terms network (e.g. in electricity network , not electricity network; telephone network ) and network (e.g. in electrical engineering or in social networks ) are used. Computer network , however, is sometimes attributed to a wrong translation of the English word network , which corresponds to the German word Netz and found its way into the German vocabulary through computer technical language. However, the word found its way into the German dictionary as early as the 19th century .

The translation as network also produces words that enable a distinction, see the network card and various network cards or the network cable for the power supply and the network cable in the LAN .

The DIN ISO 2382-1 to -25 "terms of information technology" only define the term network, not the network.

See also

literature

Web links

Commons : Computer network  - album with pictures, videos and audio files
Wiktionary: Network  - explanations of meanings, word origins, synonyms, translations
Wikibooks: Network technology  - learning and teaching materials

Individual evidence

  1. ↑ Definition of social network ( memento of the original dated February 1, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , University of Hamburg @1@ 2Template: Webachiv / IABot / www.sign-lang.uni-hamburg.de
  2. ↑ Network / Network . In: Onion Fish Abc on Spiegel Online
  3. ↑ Network. In: Jacob Grimm , Wilhelm Grimm (Hrsg.): German dictionary . tape 13 : N, O, P, Q - (VII). S. Hirzel, Leipzig 1889 ( woerterbuchnetz.de ).
  4. ↑ Detail display for: ISO / IEC 2382-1: 1993-11  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Toter Link / www.beuth.de