Siemens CP 1515 - page 188

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188 Industrial Ethernet Wireless LAN RLM Manager, CP 1515 Manager
C79000-G8976-C171-01
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
6A lternative Schemes