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Roaming is a technique that ensures uninterrupted mobile communication when the subscriber leaves a radio cell and enters the radio range of a neighboring radio cell. In order for a mobile

station to be able to move from radio cell to radio cell within a mobile radio system without



communication, there is a special handover procedure between the radio cells called roaming.

Roaming allows the subscriber to move within the entire coverage area of a mobile radio network without having to restrict his communication

. Even if the area in which the subscriber is located is not covered by his operator, he can use the network without restrictions. Roaming can also take place across borders, namely if the operator abroad has a corresponding billing agreement with the national operator. Thisproblem occurs in the same

waywith WLANs

as with mobile radio systems.

When a mobile station moves from one radio cell to the next, it changes from one LAN segment of the local network to another. To get from the transmitter to the mobile station, the data packets

now have to take a different path than before. Theautomatic and uninterrupted re-routing of the data packets into the new segment

isnot provided for in the protocols for wired networks

and must therefore be specially implemented in wireless local area networks

The techniques of roaming

The management of the locations of the individual stations by the access points of a multi-cell network is one way of regulating and handling the data traffic

between stations located in different radio cells. Each access point must periodically determine the stations within its radio cell and track them in a table. Data packets destined for stations that are not within that radio cell are not passed. If a station moves from one radio cell to another, it is received by two access points simultaneously at the cell boundary. In this case, the access points must negotiate and handle the handover procedure between themselves. Thefact that a special router

performs this function represents another possibility.

This router centrally performs the functions of the access points. This has the advantage that the access points require less intelligence and computing capacity and can therefore be produced more cheaply.

The WiFi Alliance has published the WISPr standard for WLAN roaming, which serves as the basis for many other specifications and standards.

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Englisch: roaming
Updated at: 12.04.2012
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