Why does the RF detected output appear to be pulses?

I assume that each pulse in the RF output are modulated with spread spectrum. But why pulses? And why is the remote unit pulses in groups of four and then a group of two?

Are you pointing towards Red LED’s pattern from XBIB interface board? If yes, then each pattern has a message behind.

btw, 802.15.4 modules use QPSK modulation technique. See this article:

Thank you Mr. Killer for the link. I understood very little of it. I am a ham and know little of the digital aspects of communications. I also don’t know what a XBIB interface board is. My question concerned why the RF emission from the sending XBee is not continuous. It consists of a pattern of four or six RF pulses which repeats every 20 ms. The receiving XBee puts out a single burst every 20 ms. That is easy to explain. Or guess at. It is sending back a check sum. The separation between the RF pulses within a group from the sending XBee can’t possibly be the information. The information must be within each pulse in a manner which mere amplitude on the scope can’t resolve. It makes sense that what is unseen is a spread spectrum form of modulation, but if so why bother with the strange pattern of pulses.