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Roaming is a handover procedure that ensures uninterrupted mobile communications, namely when a subscriber leaves a radio cell and enters the radio range of a neighboring radio cell of another mobile network operator. The situation is different with handover, where the radio cells are supplied by the same mobile network operator.

In roaming, the subscriber can move within the entire coverage area of a mobile network without having to restrict his or her communications. Even if the area in which the subscriber is located is not supplied by his mobile network 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. This is known as international roaming.

This problem occurs in the same way with WLANs as with mobile communications systems. When a mobile station moves from one radio cell to the next, it switches 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. Automatic and uninterrupted re- routing of the data packets to the new segment is not provided for in the protocols for wired networks and must therefore be specially implemented in wireless local area networks.

Roaming techniques

Managing the locations of individual stations by the access points of a multi-cell network is a way of regulating and handling data traffic between stations located in different wireless 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 allowed to pass. If a station moves from one radio cell to another, it will be 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.

Another possibility is that a special router performs this function. This router performs the functions of the access points centrally. This has the advantage that the access points require less intelligence and computing capacity and can therefore be manufactured more cheaply.

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

Englisch: roaming
Updated at: 09.12.2021
#Words: 383
Links: handover (HO), procedure, mobile communications, radio cell, mobile network operator (MNO)
Translations: DE

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