Section 3.4 clarify the use of CCA

[Xavier Vilajosana]

16. Without having read the IEEE 802.15.4 docs recently to confirm, I'll suggest that almost all of section 3.4 is a restatement of the IEEE 802.15.4 spec. The last sentence is important, and isn't entirely clear. It would be clearer to write:

For the minimal configuration specified in this document, the use of CCA is OPTIONAL.

TW> Editorial. TW> @Editor, please make the following change: TW> OLD TW> As for a minimal configuration, CCA is OPTIONAL. TW> NEW TW> For the minimal configuration specified in this document, TW> the use of CCA is OPTIONAL.