 In order for this scheme to work, a data stream must transition frequently enough to correct for any drift in the PLL's oscillator.The limit for how long a clock-recovery unit can operate without a transition is known as its maximum consecutive identical digits (CID) specification.To ensure frequent transitions, some sort of self-clocking signal is used, often a run length limited encoding; 8b/10b encoding is very common, while Manchester encoding serves the same purpose in old revisions of 802.3 local area networks.