Subject: [Diffserv] Followup on VW question

From: Dan Grossman <[email-masked]>
Date: Fri, 04 Aug 2000 17:45:35 -0400

I wanted to follow up on Jim Scott's question from Wednesday morning 
concerning the reference clock.

Is there an (unwritten) assumption that sender and receiver(s) will have 
reference clocks traceable to a Stratum 1 source or to the _same_ higher 
stratum (lower quality) source? 

If this is the case, doesn't this place a pretty strong requirement on the 
physical architecture of the DS-domain?  This kind of timing is delivered when 
the subnet is ATM over SONET or PPP over SONET.  It's not clear that it is 
when the subnet is Packet over Lambdas, and it surely is not when the subnet 
is Ethernet.  For that matter, if service is delivered over DS1, E1, DS3 or 
E3, there is no guarantee that timing is traceable.   

If not, what is the  relationship between variation in the phase difference 
between the respective reference clocks and either  failure of a an otherwise 
'on time' packet to meet a jitter bound or  buffer overflow?  Can this be 
quantified?

Dan

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From: Jon Crowcroft <[email-masked]>
Date: Mon, 07 Aug 2000 07:54:39 +0100

>>stratum (lower quality) source? 

good point - in the experiment i mentioned, our reference clocks were 
derived from a fairly low bitrate (OC12) synchronos network.....what accuracy
bounds you need to recover a clock is sort of well documented in the telco
literature tho...

 >>'on time' packet to meet a jitter bound or  buffer overflow?  Can this be 
 
hmmm - not qualified to answer this...thouhg note that we are talking about
recovering the clock on a packet boundary, so there;'s a LOT of slack, esp.
e.g. think of 20ms samples at 64kbps, or video frame times at 6Mbps - quite a
sloppy clock should do (there was a discussion on this on remconf ages ago...same
problem - )

 cheers

   jon