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This document explains the similarities and differences between the CC1100 and the CC1101 RF transceivers. The CC1101 is an upgrade of the existing CC1100 transceiver, and all performance parameters are equal or better on the CC1101. For most customers the CC1101 can be a drop-in replacement of the CC1100 transceiver, and this can be determined by reading the design note.
http://www.ti.com/lit/swra145