[MAMS] Precision Time

[Gary Abercrombie]

Another method for accurate time is now in use in cellular networks which
is Precision Time Protocol (*IEEE 1588-2008)*.  It is a packet based system
using a Grand Master clock (e.g. Symmetricon TP5000 GrandMaster clock).
The Grand Master can take input from a Quartz or Rubidium oscillator or GPS
to establish the master time.

Basically, the PTP master and slave(s) continually exchange a stream of
ethernet packets where the packets are timestamped just as they are sent
and again when they are received.  A double handshake is used which can
then determine propagation delays and maintain time within microseconds
between nodes.  By timestamping the packets just as the packets are
transmitted and received, the processing time by the transmitter and
receiver can be determined and removed from the equation.  A sophisticated
software PLL algorithm and feedback servo attempts to keep the time in sync.

This is how modern cell networks can eliminate the delays common to early
cell networks when handing off between towers.

The network time reported on your cell phone is a different issue however.
I have seen minutes not just seconds difference between WWV and time
reported at the application layer on a cell phone.

There is a simplified open-source implementation of PTP available which
does not require hardware timestamping at the physical layer.

BTW, I worked on PTP at Broadcom for used in cellular and automotive
industries.  I also developed TDMA / GSM / AMPS cellular phones for various
cell companies in a past life and time sync / control was a huge part of
implementing time-division protocols.

Since I no longer work in this area, I prefer the lyrics by Chicago:

"And I said
Does anybody really know what time it is
I don't
Does anybody really care?"

Gary, N8CQ