Ethernet hub

Bus topology The logical topology of a stroke represents a bus

The stroke in the OSI model

As Hub (English hub, hub ' [ technically ], node ') are in telecommunications refers to devices that connect network nodes ( physical) star-shaped. Usually, the abbreviation stroke is used for repeating hubs. They are used to interconnect devices in a computer network, for example through an Ethernet to each other.

Also sometimes referred to simply as stroke Bridging hubs or switching hubs, the (not quite correctly ) are referred to as a switch, however, much more frequently. These look externally very similar, in fact, there are significant technical differences. The confusion provides, inter alia, feed that also devices that operate on the OSI / ISO layer two to four, are also sold under the name hub.

A repeating hub is working, as a repeater, only one level of the ISO / OSI reference model ( physical layer ) and is therefore also called a multiport repeater. The signal of a network station is not analyzed, but only amplifies on bit or symbol level. For collision detection, a stroke, however, carries at most. Unlike a switch - the searches for targeted ports of the receiver - bits / symbols to all other network nodes are forwarded (see broadcast). For this reason, one can at each port of a hub ( as opposed to those of a switch) also analyze or record the traffic between network nodes with network sniffers.

A hub has only connections ( also called ports ) at the same speed ( with the same MII, but quite different MDI). If a stroke, for example, a BNC and RJ45 connectors, so its speed is half- duplex 10 Mbps. For connection of further Repeating hubs or switches either a special uplink port (also X-Port or MDI -X) or a crossover cable is being used.

A special feature is dual-speed hubs. They consist internally of a 10 -Mbit and 100 -Mbit hub and a " store and forward bridge". Ports are connected when connecting a device depending on the auto-negotiation with one or the other hub.

When using a hub in the network has a star topology is realized by the wiring in the physical sense. The logical structure similar to a bus topology because each transmitted information reaches all participants. All participants in a network that are connected to a hub, located in the same collision domain. However, the reliability is increased compared to a physical bus network through a hub: The failure of a cable shall not here the whole network lame, but affected only a single participant who is then no longer accessible. In addition, the error is easier to locate.

Repeating hubs can not be cascaded to achieve greater network size on an Ethernet. A specific for each speed maximum round -trip delay time ( RTDT ) must not be exceeded. The RTDT is the time required for a data frame in order to get from one end of the network to the furthest end of the other network - and back again. If the network is too large, so the RTDT too high, collisions are frequent, unrecognized collisions possible and affects the overall network traffic. Such disturbances are difficult to isolate because transmission can function normally under low network load. As with repeaters so 5- 4-3 - rule must be followed in order for problems with high signal propagation times ( RTDT ) should be avoided. Partly because of this limit switches are used almost everywhere today. In the gigabit range, therefore, no hubs / repeaters were more common at higher speeds, their use is no longer possible.

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