Synchronization in Manchester coding

Question

Lately I have been reading about Manchester encoding and I think I'm beginning to understand most of it now, but still I have got some whys that need addressing. Mainly 3 for the moment:

1) Most articles on Internet when introducing Manchester coding start by telling how bad NRZI really was and one of the disadvantages that gets mentioned is that synchronization becomes a problem when lengthy 1's or 0's get sent. Why is that a problem, since most places where NRZI is used have got separate clock and data lines. As long as the clock signal is there why should that ever be a problem?

2) Also, is Manchester supposed to work on a fixed frequency? Or can it work like I2C where clock frequency can be variable?

3) The good thing that gets mentioned about Manchester encoding is that it does not require separate clock line and that clock is embedded in the data and can be recovered by the receiver. Frequent transitions in Manchester help in synchronization and that the transitions happen in the middle and so clock can be recovered from transition. But my question is, if there are repeated 1's or 0's transition can happen in the middle and in the end as well (see attached waveform pic, look at the transitions when sending 111). So when a receiver sees a transition how does it figure out whether it is in the middle or at the end?

If I'm talking rubbish I would love to be corrected.

Answer 1

regarding your third question: I'm also brushing up on manchester and it appears that to recover a clock you need a differential signal:

Reference: "Data Communications, Computer Networks and Open Systems" by Fred Halsall, page 104, figure 3.8