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

For the 3 question,

Whenever a signal is transmitted, initially a few redundant bits which contain info about clock are sent.

For example, 1111, now the receiver knows the real data will arrive next, and through those redundant bits clock signal is extracted as well as the “notification “ that a signal is going to come.

As for question 1, NRZ scheme can send lengthy 1’s and lengthy 0’s.... but here the problem is actually with lengthy 1’s, if you could check sending lengthy 1’s with some modulation scheme and a dipole antenna, you could observe that the power of carrier signal will start decaying exponentially. And the other reason would be the power needed to send that many lengthy 1’s, which is not favourable!

For question 2, yes it is possible to use it variable clock frequency but the condition is you should send redundant bits before you could change the clock frequency so that the receiver understands that the clock is changed from this point onwards.

Hope it’s clear now ;)

Answer 2

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