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Re: T1 Circuit actual throughput 1290Kbps

  • From: Brett Frankenberger
  • Date: Sun Jul 12 23:55:52 1998

:: Robert E. Seastrom writes ::
> 
> uh, my wrong...  it's the AMI not the D4 that causes the big hit.  of
> course, as a matter of course, D4/AMI and ESF/B8ZS go together and you
> never see combinations like D4/B8ZS...  
 
My employer runs a few hundred T1s at ESF/AMI, because we like ESF
better, and can't run B8ZS because our old repeaters don't like it. 
(The repeaters don't even understand framing, so I can run D4, ESF, or
anything else I want.)

I also know of cases in carriers where D4/B8ZS is run because it's
their standard to do everything internally at B8ZS (line coding doesn't
traverse higher-order spans, so if the signal is going inside a DS3,
for example, you can have B8ZS on one end and AMI on the other), but
the customer ordered D4.  (Usuall D4 with AMI, but if the customer
wants D4 with B8ZS, they can have that also.  Telcos will also sell
ESF/AMI.)

> which, in answer to the fellow
> who asked, is why you can't just invert the data on the HDLC and run
> it down the line and get your 12%  back...  HDLC is layer 2, whilst
> framing is layer 1...

Know what one of my pet peeves is?  People telling me that I can't do
things that I have been doing in production for years.  Buy yourself a
couple of DSU/CSUs, strap them for AMI, 24x64, data inverted, connect
up a router, and try to make 'em lose sync.

If you're running AMI, the data you present to the T1 has to maintain
ones density some how.  It so happens that if you run HDLC, HDLC
guarantees 0's density.  So if you invert the HDLC, you get guaranteed
1's density (considerably higher density than required, even).  So, if
you present inverted HDLC to an AMI T1, you will have the required 1's
density.  The differing layers are irrelevant technobabble, but if you
insist: Inverted Data with AMI framing at layer 1 presents an interface
to layer 2 that requires a certain 0's density.  If layer 2 is HDLC,
that 0's density will be met.

It was pointed out in response to my original posting that running
inverted HDLC over D4 at 24x64 can cause spurious yellow alarms,
because anytime you get a stream of flags, there's a 75% change that
the result will be 0's in bit 2 for all DS0s for long enough to
trigger a yellow.  (24x56 avoids this, regardless of whether the HDLC
is inverted or not, because by having only 7 bits available per
channel, the flag patter is effectively shifted one bit for each
channel.  It's still possible to send a string of data that will
trigger a false yellow, but it's much, much less likely).  This isn't
an issue over ESF/AMI, of course, since the yelow alarm code isnt' send
as part of the normal paylod.  And in some cases of D4/AMI, you might
not care about the yellow.

          - Brett  (brettf@netcom.com)
 
------------------------------------------------------------------------------
                               ... Coming soon to a      | Brett Frankenberger
.sig near you ... a Humorous Quote ...                   | brettf@netcom.com
 




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