🚀 go-pugleaf

In article <4a7frs$t64@news4.digex.net>, joe@access4.digex.net (Joe
Nardone) wrote:

>John McGing (jmcging@access.digex.net) wrote:
>: On 7 Dec 1995, Moo's Access Point wrote:
>: 
>: > In article <4a6lu4$eqp@access2.digex.net>,
>: > Chris Smolinski <cps@access2.digex.net> wrote:
>: > 
>: > >Did anyone from Digex examine the specifications of these slicks, to see 
>: > >if they were suitable for 28.8 operation? Sounds to me like Digex bought 
>: > >cheaper voice grade equipment, rather than the required data grade.
>: > 
>: > Bell Atlantic purchased, installed, and owns the AT&T SLC-96 <or whatever
>: > they're called> equipment. 


I have just a few points to make here...

1) a SLC-96 is basically a channel bank designed for use in the outside
plant.  Than means that it can tolerate high and low temp and high and low
humidity.
The A/D mechanism used is EXACTLY the same that is used within a digital
central office switch (8 bit Mu-Law companding).

2) The reason SLCs and their generic cousins DLCs (Digital Loop Carriers)
are used is to REDUCE the amount of copper being strung that is supporting
voice grade communications.  When the Bell System (yes, this stuff goes
back that far) first started deploying SLCs, they sliced up their whole
coverage area into Carrier Serving Areas (CSA).  These CSAs are about 2
miles on each side.  They are designed so that they can put a SLC in the
middle, and still reach any customer in the CSA with less than 12 Kft of
copper.

3) Back when I was a Digex customer, I remember discussions about Digex
getting T1 feeds on their prem for voice lines.  If this is indeed the
case, the amount of copper feeding them at voice grade is VERY short (i.e.
from the SLC bank in their building to the modem bank).

4) Remember that 28.8 is really pushing the envelope of what can be
transmitted in analog over a DS0 channel (running at 64 K).  A slight
degredation of S/N will drop the data rate.  That S/N rise could be either
a drop in signal level (and thus a corresponding rise in noise due to
quantization), or outside noise being picked up in the copper feed or in
the electronics.  Also, inter-office signaling is done in-band through bit
robbing.  Every sixth frame the lowest significant bit of the channel
transmits signalling state info.  This lowers the S/N on any call that
traverses more than one office.  In fact as you go through each hop
(switch), their is no superframe synchronization, so the decision on where
the sixth frame is is made independently; thus for a multihop call, more
than every sixth LSB is stepped on.

Nevertheless, a lot less fingerpointing would be appropriate.  Perhaps
Digex should ask BA for a face-to-face customer meeting with the
responsible engineer for their district.

Dave

-- 
Dave Gellerman                       dave_gellerman@newbridge.com
Newbridge Networks                       http://www.newbridge.com                  
Herndon VA USA                                       703 834-5313