For binary PCM systems, what mechanism controls 'ones-density'.?

a bit is encoded by a polarity transition or the lack thereof. Encoding 000000 would then be easy - just keep the magnetic phase unchanged for a few micrometers. However, when decoding, uncertainty creeps in - how many micrometers did we read? Does this correspond to 6 zeroes or 5? To prevent this problem,How is data coded in such a way that these long stretches of no transitions do not occur?