🚀 go-pugleaf

In article <1992Sep3.123226.14205@hubcap.clemson.edu> steve@hubcap.clemson.edu ("Steve" Stevenson) writes:
   [I saw this on sci.logic. I thought it was a nice problem for this group.
     steve]
   >From: dougs@tvnews.tv.tek.com (Doug Stevens)
   >Message-ID: <1992Sep1.234452.26917@tvnews.tv.tek.com>

   [....]

   A and B are two people at two ends of a very slow telephone connection. 
   They are both completely cut off from the world except for their connection 
   with each other.  In particular, they have no idea what time it is in the 
   real world.

   There is a delay AB in the transmission of what is being said by
   A to B, and a delay BA in the transmission of what is being said
   by B to A. The delay AB does not equal the delay BA. The delays, for the
   purpose of this problem, are on the order of minutes in each direction.

   Both A and B have watches. Before they begin to talk, the watches are not
   set to the correct time, nor are they set to the same time.

   The problem is to devise a procedure such that A and B can, via their 
   telephone connection only, accurately synchronize their watches (to
   within a few seconds). The watches do not have to reflect time in the
   real world, but must match each other. Both A and B will know the procedure 
   before the call begins, and each will know whether he is A or B.

   Thanks for any help.
----------------

OK I'll bite. It is trivial for A and B to figure out AB+BA. It is
also trivial for them to synchronize their watches assuming AB=BA (or
any preset assumtion on AB/BA). At this point their watches are
actually synchronized. You can not refute this with any communication
between A and B using this communication channel. My wild guess :) and
please tell me if this is wrong.

[I don't think that your assumptions are safe. Steve]


If you are using a third person "C" to check if their watches are
actually synchronized using delays AC,CA,BC and CB, then the system you
are solving is perturbed so the previous solution doesn't need to be
valid. But this is irrelevant because this is not the system  in the
problem. The solution is not also valid if you have another means of
communication between A and B. 

--
Sadun Anik, U of Illinois at Urbana-Champaign
Center for Reliable and High-performance Computing
e-mail: anik@crhc.uiuc.edu