Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

Re: [802.3ae] Clarification of the 10GBASE-W ELTE function

On Fri, 18 Jan 2002, Gary Nicholl wrote:
[ If you connect a 10GBASE-W interface to a port on a SONET ADM] then,
> although the traffic will still pass error free (as described above
> and in the contribution),  you will get continuous pointer adjustments
> (PJCs) at a rate equal to the difference in frequency between the
> 10GBASE-W interface clock and the sonet network clock (likely to be
> Stratum 1). These PJCs will be detected and reported at every node in
> the network that the OC192c signal passes through. While the PJCs in
> themselves are not a major issue (i.e. do not cause any bit errors),
> they do represent an operational challenge for the network operator.
> In a synchronous sonet network the operator would not expect to see
> continuous PJCs reported from any node.  Continuous PJCs in such a
> network are usually an indication of a synchronization failure and
> that one of the nodes has lost traceability to the Startum 1 network
> clock. As such monitoring PJCs is one of the few tools available to the
> operator to detect and isolate synchronization failures in the network.

That is a very good point.

> A network operator would find it difficult (if not impossible) to
> distinguish between PJCs resulting from a true synchronization failure
> and those from a free-running 10GBASE-W interface connected to the
> network.  For these reasons I find it hard to believe that any operator
> would allow a free running 20ppm 10GBASE-W interface to be connected to
> the network. I think if 10GBASE-W is ever to be widely adopted then
> 802.3 will eventually have to give in and support loop (line) timing.

Is the situation really all that bleak?  The PJC-related performance
parameters defined in ANSI T1.231 include counts of pointer justifications
detected, pointer justifications generated, and the difference between
them.  I would think that the important statistic is the difference between
PJCs detected and PJCs generated, because that will be large at only at the
point where the timing sources at the input and output do not match ... in
other words at the points where a synchronization failure exists.  By
contrast, PJCs detected and PJCs generated will be tend to be large, but
differences will be small, at points downstream from the failure.  Now, in
the case of a 10GBASE-W interfaces connected to SONET ADMs, the PJC
difference would normally large only at ingress nodes where the 10GBASE-W
tributary interfaces exist.  It seems to me that it would not be all that
difficult to suppress PJC-difference alerts for those specific tributary
interfaces.  Wouldn't that adequately address the operational concerns?


Mike Heard