<Li> 2400--2483.5 MHz: worldwide use, up to sixteen channels (2003, 2006) </Li> <P> The original 2003 version of the standard specifies two physical layers based on direct sequence spread spectrum (DSSS) techniques: one working in the 868 / 915 MHz bands with transfer rates of 20 and 40 kbit / s, and one in the 2450 MHz band with a rate of 250 kbit / s . </P> <P> The 2006 revision improves the maximum data rates of the 868 / 915 MHz bands, bringing them up to support 100 and 250 kbit / s as well . Moreover, it goes on to define four physical layers depending on the modulation method used . Three of them preserve the DSSS approach: in the 868 / 915 MHz bands, using either binary or offset quadrature phase shift keying (the second of which is optional); in the 2450 MHz band, using the latter . An alternative, optional 868 / 915 MHz layer is defined using a combination of binary keying and amplitude shift keying (thus based on parallel, not sequential spread spectrum, PSSS). Dynamic switching between supported 868 / 915 MHz PHYs is possible . </P> <P> Beyond these three bands, the IEEE 802.15. 4c study group considered the newly opened 314--316 MHz, 430--434 MHz, and 779--787 MHz bands in China, while the IEEE 802.15 Task Group 4d defined an amendment to 802.15. 4 - 2006 to support the new 950--956 MHz band in Japan . First standard amendments by these groups were released in April 2009 . </P>

Which modulation scheme is followed by ieee 802.15.4 standard
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