Regarding the HRM, legacy heart rate straps that adhere to older versions of the ANT+ specifications use a single data page and only send heart rate, beat count and time. In order to maintain backwards compatibility, the page change toggle bit (msb of the data page number) is used to identify whether the background data pages containing manufacturer, product and battery info are present. An ANT+ receiver should implement all the defined data pages, and look at the page change toggle bit to determine if the transmitter is using any other pages. Background data pages 2 and 3 are required for all new sensor designs.
Most bike power sensors are required to send common data pages, however, crank torque frequency sensors are a special case and they do not broadcast any ANT+ common data pages. Specific requirements for each type of bike power sensor are outlined in the device profile.
