Pat - WirelessHART requires that CCA be supported, not enabled. In practice it isn't used unless required by local regulation. FYI 62591 v1.0 copies an error in the WirelessHART spec that mandates carrier sense for CCA. The latest WirelessHART spec (and the upcoming 62591 v2.0) corrects this to allow all CCA modes and defaults to energy detect.
Jonathan On Fri, Dec 18, 2015 at 11:38 AM, [email protected] < [email protected]> wrote: > Kris; > > It seems that your point is essentially that if the minimal network is the > only network within transmission range, then CCA is not necessary. > However, since this is an ISM band, ignoring other devices such as 802.11, > 802.15.4 CSMA, or even other minimal networks, will cause many devices to > retransmit due to the collisions, which would further congest the band. > > Moreover, you are correct that a device wishing to transmit would start > its CCA at T=1800 microseconds with CCA being 128 microseconds and Rx to Tx > being 192 microseconds, so a device would only detect another device if > that device started transmitting 320 microseconds “early” within the the > allowed margin of 1100 microseconds. I believe that the synchronization > errors for those devices minimally active will be much greater than more > active devices. > > Finally, I believe that a device should use CCA as is required in IEC > 62591(WHART) and IEC 62734 (ISA100.11a). > > Pat > >
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