[VoiceOps] AT&T Ground Start Loop Order

[Jay Hennigan]

On 10/13/10 1:26 PM, Eric Hiller wrote:

> We are trying to order the ground start loop I had posted about a few
> weeks back. This is for that same USOC in order to get the less expensive
> Ground Start loop ordered, any idea which of these NC codes is correct or
> if it even matters?

I missed your earlier post.  What problem are you trying to solve with
this circuit?

DID loops are quite different from ground start loops and used for
different things.

GROUND START:
Ground start analog loops are similar to conventional POTS lines in most
respects.  The difference is that the CO has no ground path on the tip
when idle.  For outbound calls, customer seizes the line with a ground
on the ring side.  CO acknowledges the seizure when ready for digits by
grounding the tip as well as providing dial tone and from that point
forward the call is handled as a conventional loop start trunk.  One
phone number per loop, no means to identify multiple inbound DIDs per
circuit.

Once the call is cleared, customer must ground the ring lead again to
initiate a new call.

On inbound calls to the customer, the tip is grounded at the CO along
with ringing voltage applied to the loop.  Answer is by a standard
closed-loop condition tip-to-ring from the customer.

This allows a PBX to use simple ground signaling to determine inbound
calls as well as the CO ready to receive dial pulse without having to
detect dial tone audibly.  It gives a positive indication of a cleared
call to return to idle.  It also helps to eliminate glare condition as
the tip ground from the CO is immediate on an incoming call where ring
voltage could be delayed for up to four seconds on a standard 2-on,
4-off ring cycle.

Ground-start trunks can usually both send and receive calls.

REVERSE BATTERY DID TRUNKS:
DID analog lines are a different animal.  Also originally for PBX but
now used mostly for fax server appliances and paging terminals where
multiple DIDs terminate on one or a group of pairs.

These kind of flip the role of who provides battery.  The customer
premise applies battery to ring, ground to tip when idle.  When the PSTN
has a call to deliver to the customer, the CO applies a short across tip
and ring to cause loop current to flow.

When the customer equipment is ready to receive digits, it sends a
"wink" to the CO.  This is typically a momentary reverse battery,
although it could be a momentary open circuit.

After the wink, the CO sends the last "n" digits of the dialed telephone
number where "n" is a number agreed on between the carrier and the
customer, usually 3 or 4 digits depending on the number of DIDs on the
trunk.

Customer equipment detects the digits which can be DTMF (most common
today), 10pps pulse, 20pps pulse, or MF signaling.  Customer equipment
then "rings the phone" or equivalent (sets up the fax, etc.)   Customer
plays ringback tone as audio, or a busy signal, intercept recording,  etc.

When the call is answered (or before the fax begins to send tone), the
customer equipment reverses the battery between tip and ring.  This lets
the CO know that the call is answered and billable, and the CO cuts
through forward audio.

On disconnect from the CO, the loop current is broken and the line goes
open, customer tears down call.

On disconnect from the customer, customer reverts battery to normal or
removes battery.  CO releases short from loop and we're back idle.

Reverse-battery DID trunks can generally only receive calls, although
there's supposedly a two-way DIOD flavor.  I've never seen one of these
on a two-wire trunk in the real world.

Note that both of these enforce an electrical signal from the side that
is receiving digits that it is ready (tip ground from CO on
ground-start, wink from customer on DID reverse battery.)  This is
because in the early days dial tone might not be immediate.  The
handshake ensured that the linefinder, marker, etc. were ready before
digits (pulse-dial originally) were sent.

The fact that I even know about this is somewhat scary.  This is really
old-school legacy PBX.  The people who know this stuff can tell you how
2600 magazine got its name.  What next?  E&M?  :-)



> This is all I have; the AT&T LSC ran the USOC through their internal
> software and this is what it referenced:
> 
>       Analog DID Reversed Battery LX- TXNU 02QC2.RVO(2W Reverse Battery-Term
> @ end user)
>       02QC2.RVT(2W Reverse Battery-Orig. @ end user) 02RV2.T
>       02RV2.O
> 
> "UNE LOOP W/O EQUIP,NO T"
> 
> The LSC says one NCI represents "with equipment" one represents "without".
> Verizon and Qwest describe it as "terminating" and "originating". It appears
> as though one needs a battery to power the line on the customer end, and one
> is provided power. That's at least my understanding of it.
> 
> This is for AT&T Midwest.
> 
> Thanks,
> Eric
> 
> 
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Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
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