From: floyd@hayes.ims.alaska.edu (Floyd Davidson)
Newsgroups: comp.dcom.modems
Subject: Re: Need explanation of {{{{, clock slips??
Date: 6 Apr 92 16:45:18 GMT
Article-I.D.: raven.1992Apr6.164518.14839
Organization: University of Alaska Institute of Marine Science

In article <1992Apr3.030742.8070@eskimo.celestial.com> nanook@eskimo.celestial.com (Robert Dinse) writes:
>In article <1992Mar28.024406.11844@raven.alaska.edu>, floyd@hayes.ims.alaska.edu (Floyd Davidson) writes:
>> 
>> "Clock slips" (also called "frame slips") happen when the
>> receiving circuit of a digital carrier system is not in sync with
>> the sending end.  The reasons can be either line noise interfering
>> with the data on the digital span (and hence the clock recovery
>> circuit will wander), or the two ends have master clock signals
>> that are not synced to each other or are unstable.
>> 
>> The receiving end either gets too far ahead, or too far behind,
>> the sending end.  When that happens the receiver either totally
>> repeats or discards, as the case may be, one frame of data.  A
...

>Ah, there is more to the story...  Depends on the manufacturer,
>but with Western D4 banks at least, there is circuitry that
>momentarily mutes the receive circuitry when a clock slip occures.
>The purpose of this is to prevent the customer from hearing noise
>that might otherwise be audible.

You must be confusing loss of framing with frame slips.  How would
you "mute the receive" for the duration of a skipped frame?  There
is no time interval to mute.  (I don't have any "Western D4"
banks, so I can't check that.  They could just mute it for a set
number of frames...  but that is a cure worse than the disease.)

>> Clock slips can occur on *any* digital system.  They might be
>> from a digital cable carrier system between the CO and your
>> phone, or on the trunks between two CO switches, or even
>> between equipment within the CO.
>
>     Sort of true, they CAN occure, but are less frequent between digital
>offices because the digital offices themselves are all synchronized to a
>master clock.  They are far more likely to be a problem when one office is
>analog and the other digital and the D4 bank at the analog end has to time
>itself to the digital office.

I don't agree at all.  Perhaps your particular brand of D4 channel
bank is prone to sync problems.  Most aren't.  The most sensitive
T1 interface that I know of is the 6X50AA card in a DMS Digital
Trunk Controller.  It is sensitive no matter what is on the other
end.

Likewise the syncronizing of digital switches is NOT usually the
problem, once it has been done correctly.  The clock recovery
circuitry in a T1 interface is much more likely to cause problems,
as is the transmit circuitry.  The fact that the interface is in a
D4 channel bank, a T-Mux FDM-to-T1, in an M13, or in a DTC hasn't
much to do with how it works.  The differences have to do with
who designed it (and how long ago).  Some are better than others,
but the exact design is different from one to another...  and
hence figuring out if it is slipping or not is different from
one to another.

>> Tracking them down is a major problem because it is a fairly
>> sophisticated trouble that is different with each type of
>> equipment.  On some equipment it is very difficult to determine
>> that it is happening.  On others (e.g. digital switches) it
>> may be printed out as a trouble report every time it happens.

>It is not a difficult problem at all, just there aren't that many
>people that understand it.  There are dedicated test boxes that
>will tell you that you have slip and even give you a count of
>lost/duplicated frames.

If by "dedicated test boxes" you mean one built into the specific
piece of equipment, I'll agree in principle.  It isn't a test box.
It is just a counter that allows you to record how many slips and
indicates if the tx was faster or slower than the rx...  and as I
stated above, on some equipment it is easy, e.g.  digital
switches, because they have such facilites.  Other equipments
don't make it as nice (you may have to attach a black box readout
device!  and ask it.  The DMS switch however likes to tell you
every 15 minutes all about every carrier facility that suffered
from a frame slip, or any of a number of other maladies.

>  It's easy to check with a dual trace
>scope, put one probe on the incoming span, one on the outgoing,
>and trigger the scope off the incoming, slippage will appear as
>one waveform sliding past the other.

That will NOT measure, count, or in any way tell you about clock
slips.  It will tell you about differences in timing (which may or
may not cause slips).  For instance an intermitant momentary shift
of the transmit clock might show up in such a test...  but you
would have no way of telling if it was severe enough to cause a
frame slip or how many occured if any did.  You might be watching
a very acceptable amount of wander instead of a real problem.

The acid test for a clock slip is if the input buffer in the T1
interface was slipped.  There is no way to tell if the input was
slipped short of monitoring the input circuit frame pointers in
the hardware.  Any box attached just reads out what the internal
hardware counts.  You cannot attach anything to the data stream
and determine that a slip occured.

>> In any case there is no way to attach a black box and have
>> it tell you if it is happening.  Instead someone has to become

>Yes, there is a way to attach a box, maybe not black, green seems
>to be the preferred color, that will tell you it's happening and
>takes a whole of maybe 15 seconds to plug it in.

I'm not sure what your "green" box is.  If it attaches to a specific
piece of equipment (such as a particular brand of D4 channel bank),
then it isn't really a test set, but a display set.  

On the other hand if you are bridging it across a T1 span line,
or at a DSX jack, or doing a drop and insert...  you are mistaken
about what the test set is telling you.  It is counting how many
times *IT* slips a frame, not how many times some other equipment
slips a frame.  The two counts are NOT related unless you have
a gross timing error between the two ends.  (And actually most
T1 test sets, such as made by Tau Tron and Sage, have better
clock recovery circuits than most other T1 interfaces.  The test
set won't skip a beat, and the T1 interface gets hundreds of
clock slips per day.)

>> And it is a problem that you will not be able to discuss in
>> any intelligent way with the clerk who takes trouble reports,

>You can't discuss anything in any intelligent way with the clerks
>that take trouble reports because they have zero technical
>knowledge.

Could be.  But I know a couple that know enough to discuss it
with YOU.

>     I'm one of the folks that occasionally gets to chase down these
>problems WHEN they get past the repair clerks!

Sniff.  I miss all the funs.  I only get to chase 'em when they
get by other techs and still don't work...  which means not too
often around here.  (Of course we've been digital for an awful
long time...)

Floyd

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