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automatic fallback (AFB)

Fallback is a switching function in transmission links that is activated when the transmission quality falls below certain specified limit values. This can happen if the bandwidth provided is insufficient, the receive level falls below the limit value or if transmission parameters are impaired by interference.

In such cases, the fallback is triggered within a network component(modem) and reduces the transmission rate to a speed that ensures error-free reception. If the transmission quality improves, the fallback can be reversed and switched to a higher transmission rate. This is referred to as fallforward.

  1. If, for example, a modem operates with insufficient transmission quality, the fallback function switches the modem to a lower transmission speed. Some transmission methods allow fallback in small steps, like the Packetized Ensemble Protocol (PEP), others like V.32 only in very large steps. Some protocols perform fallback in up to 12 steps, others halve the baud rate each time, still others have only a few steps. A prerequisite for fallback operation is that both modems involved in the transmission support this mode of operation.

  2. The fallback function is also implemented in WLANs according to 802.11a, 802 .11b and 802.11g, whereby the resulting connection loss is prevented by automatically switching back the data rate if the reception field strength falls below a certain level. If the reception level of the client or access point (AP) drops, the data rate is automatically reduced.

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Englisch: automatic fallback - AFB
Updated at: 12.05.2008
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