Why Can't Telephony Clocks Synch?

Factual Questions
1

I barely know what I ask, but I understand digital transmission of voice or data requires a periodic “dummy” pulse or code to keep the receiving “clock” in synch with the sending clock. But, why can’t the receiving clock keep synch after receiving, say, an initializing pulse or code to mark the starting point? Is it a case where relativity manifests itself because the transmission occurs at or near the speed of light?

2

Short version: No two clocks (oscillators really) run at precisely the same rate. Imagine we had a naive scheme to transmit data where we simply switch voltage on for 1 and off for zero at a rate of once per tick. So sending “1010011” would be on for one tick, off for one tick, on for one tick, off for two ticks, on for two ticks.

When the first on arrives, the receiveing system can say “OK, here comes a message” & over the course of 8 ticks the two oscillators will stay close enough to synced that the small drift won’t confuse the 8th bit.

Now imagine sending 1000 zeros. When does the receiving clock start? What if the relative drift is 1 part in 900? IOW, by the time we get to zero number 900, the receiver is counting 1 ahead or 1 behind the sender.

Bottom line: The dummy pulses serve as signposts. Every so often the sender says “are you still with me?” and the receiver can know “yes I am”, or “whoa, that wasn’t expected now; I must have gotten lost a couple ticks ago. Hey sender, let’s back up & try that again.” Signposts in and of themselves don’t provide retry, but they enable recognizing the situation where a retry is necessary.

3

There are serial data transmission protocols that are self-clocking. They are usually fairly arcane, since they need to balance the number of 1s and 0s so the clock recovery can work.

4

I’m not sure how arcane they are, but we use Manchester Encoding pretty often in my work. It is self-clocking (basically a variation of phase-shift keying).

But yes, the purpose of clock synchronization is to account for clock skew between the transmitter and receiver. This is often a non-trivial problem.

5

TAXI was the one I was trying to remember.