Re: [Linuxptp-users] Master Clock Choice

[Vanderpool, C. <cly...@ax...>]

Just realized that the ping statistics I posted are reversed.  The correct
info is...

*2-A-->2-G-->Ground Gateway*
Average ping time - 1.203 micro sec
Stand Dev - .342 micro sec

*3-A-->3-G-->Ground Gateway*
Average ping time - 67.806 micro sec
Stand Dev - 6.333 micro sec

Sorry for the confusion.

Clyde

On Thu, Jun 23, 2016 at 11:57 AM, Vanderpool, Clyde <
cly...@ax...> wrote:

> PTP users,
>
> I have a question about how the choice of a parent clock is decided.  I
> have included a block diagram of the LAN that I have synchronized with
> ptp4l (software time stamping) in the attachment of this email.  Names of
> various 'links' are inside the green squares.
>
> The left side of the diagram is synchronized first, with the Ground
> Gateway assuming the Grand Master role.  Then the right side of the diagram
> is brought up.  On the right side link 1-A (which has no direct connection
> to the left side) and the Air Gateway will always pick link 3-A as it's
> parent boundary clock (despite the order in which they are synced to the
> LAN).  I find this strange due to the fact that the path to the Grand
> Master needs to go over a radio (3-A-->3-G-->Ground Gateway) instead of
> going through a direct Ethernet connection (2-A-->2-G-->Ground Gateway).
>
> I then tried to take link 3-G offline.  This caused links 1-A, 3-A, and
> the Air Gateway to pick 2-A as their parent boundary clock.  This makes
> sense, as 2-A offers the only connection to the Grand Master.  When I bring
> back link 3-G, link 3-A picks it as it's parent boundary clock.  This makes
> sense to me as it requires less steps to get to the Grand Master.  What
> does not make sense to me is that both link 1-A and the Air Gateway drop
> link 2-A as their parent and once again sync to link 3-A despite the fact
> that the steps removed from the Grand Master remains the same.
>
> I performed a ping test to try see how long it takes to send/receive
> information on both of the previous pathways to the Grand Master.  The
> results are as follows.
>
> *3-A-->3-G-->Ground Gateway *
>
> Average ping time - 1.203 micro sec
> Stand Dev - .342 micro sec
>
> *2-A-->2-G-->Ground Gateway*
>
> Average ping time - 67.806 micro sec
> Stand Dev - 6.333 micro sec
>
>
> I was just wondering why the two machines with no direct connection to the
> Grand Master would consistently choose the longer pathway (time wise) to
> the Grand Master.  Is there something about the Best Master Clock Algorithm
> that makes this so?
>
> I apologize if this is a bit wordy and hard to follow, but I cannot reason
> out how the choice of parent clocks is made.  Any information is greatly
> appreciated and if anyone needs any further clarification please feel free
> to ask.  Thank you in advance!
>
> Clyde
>
>
>
>