Siemens CP 1515 - Manual (Page 188)

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188 Industrial Ethernet Wireless LAN RLM Manager, CP 1515 Manager
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life is much bigger than the transition, this can still be considered to be a
single key scheme, because most of the time only a single key is in use.
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Alternative schemes can be envisaged, which have main life periods in
which two or more keys are active. An example is given in
Table
6 Alternative Schemes