[VoiceOps] Ambiguous dial plan pattern matching

Mary Lou Carey marylou at backuptelecom.com
Mon Dec 3 17:21:01 EST 2012

Routing is determined by the NPA type of that area AND by the local calling
guide established by the LEC. Everyone can set their own local calling areas
as far as rates are concerned, but you will be charged CABS according to the
way the LEC established their local calling area for each rate center. (Rate
centers are established by the NANP and can be a city, a portion of a city
or a group of cities. They are listed in the LERG, and on several other
websites such as localcallingguide.com, nationalpooling.com and nanpa.com). 

Seven digit dialing is only used in areas where there is one NPA for the
local area. Ten digit dialing is used in areas where there is an overlay
(more than one NPA per local area) and 11 digit (1 plus) is used in areas
where there was an area code split. The difference between a split and an
overlay is that a split required everyone (existing and new end users) to
change their NPA to the new area code. Overlays allow you to assign numbers
from the new NPA to new end users but allows the existing end users to keep
their old NPA. 

LECs typically break up their local and EAS calling areas by rate center and
while local calling areas are limited by rate center boundaries, local
calling areas can vary by rate center. For example, someone in Nashville TN
can make a local call to both Murfreesboro and Franklin, but a person in
Murfreesboro cannot make a local call to someone in Franklin. 

Every NPA-NXX is assigned to a specific rate center and a specific switch. A
switch can cover multiple rate centers and local calling areas, so the LECs
set up call scenarios that detail the dialing requirements and how the call
will be routed. They begin by adding every NPA-NXX-X associated for that
particular switch and map out which NPA-NXX combinations require 7 digit
dialing. Then they set up a scenario for each call type determined by route.
So if your end user calls another one of your end users in the same rate
center, it's going to use scenario A in which the switch will route the call
directly itself and only require 7 digit dialing. If your end user calls
someone on the B list (let's say that list contains all the NXXs within the
same NPA), then require 7 digit dialing and route the call to the local
tandem trunks. I could go on and on, but basically the concept is that after
you determine the dialing requirements for each rate center your switch
covers in a particular LATA and the available routes the call has to take,
you set up call scenarios in your switch to cover each dialing requirement /
route combination your switch will encounter.

Mary Lou Carey
BackUP Telecom Consulting
marylou at backuptelecom.com 
Office: 615-791-9969 x 2001

-----Original Message-----
From: voiceops-bounces at voiceops.org [mailto:voiceops-bounces at voiceops.org]
On Behalf Of Nathan Anderson
Sent: Monday, December 03, 2012 10:55 AM
To: 'voiceops at voiceops.org'
Subject: [VoiceOps] Ambiguous dial plan pattern matching

Sorry for what probably will end up sounding like a rather noob-ish
question, but I still consider myself a relative greenhorn...

We are a VoSP.  With the advent here in the U.S. of area code overlays,
10-digit dialing, and even carriers/vendors (VoIP and wireless, mostly)
continuing to blur the distinction between local and toll calls (at least
from the end-user's perspective), it is not an uncommon thing to run into
service platforms where both 7- and 10-digit dialing is supported, but the
7-digit support feels (again, from the customer's perspective) half-assed,
at least compared to the ILEC's implementation.  But if the customer still
lives in an area where 10-digit dialing is not mandatory, he/she expects it
to work, so we "have" to provide it.

It feels inferior because back when when local destinations were always
within the same NPA as the caller, the number pattern rules were simple, at
least for domestic calls: if it starts with a 1, it will be an 11-digit
number, and if it starts with 2-9, it will be a 7-digit number; but now, if
it starts with 2-9, it could be either 7 or 10 digits, and you won't know
for sure which one it is unless the caller has entered more than 7 digits.
And in these ambiguous scenarios where two destination patterns partially
overlap, it's the "did the caller stop dialing" part that's hard to measure,
right?  So now you have CPE gear generating dialtone (whether PBX or ATA)
which either feels to the caller like it is extremely slow at putting the
call through, or which requires the caller to remember to do something
special at the end of a 7-digit destination (like dial # or something to
signal they're done) if they don't want to wait.  (There's also been a
similar phenomenon for a whil!
 e with PBX systems that use outbound routing prefixes like '9' in order to
make it unambiguous whether you are dialing an internal extension or an
external destination.)

Now obviously the CO switch can't read minds either, so in areas without
mandatory 10-digit dialing, it seems inescapable that it would have to
determine whether the user is done dialing via some kind of timeout
mechanism as well.  But it sure seems like most Class 5 switches must have a
much more intelligent ambiguous number pattern matching algorithm than most
CPE gear does since, as a general rule, people that still have an analog
loop to the CO these days and regularly dial 7 digits don't experience
(e.g.) upwards of a 5-second delay between when they've finished dialing and
when they hear their first ringback.

The question is, what's the secret sauce, and why can't that same algorithm
be implemented in CPE gear?

Nathan Anderson
First Step Internet, LLC
nathana at fsr.com
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VoiceOps at voiceops.org

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