deej@bellcore.bellcore.com> (01/11/90)
Given the apparent resumption of the discussion of dialing plans to do
with interchangeable codes, I thought it might be a good idea to present
"the straight story", from Notes on the Network. I took the effort to
get the following excerpts from Notes typed in.
{However, this is an unauthorized reproduction. While I shouldn't get
into *too* much trouble...}
I would ask that:
(1) all distribution in electronic form of the following excerpts
reproduce the excerpts in their entirety, without change, deletion, or
modification of any parts;
(2) all distribution in electronic form of the following excerpts
include the full citation and copyright notices;
(3) no more than one hardcopy, for personal use only, be made by any
reader of the following excerpts.
[Moderator's Note: I appreciate your concern, and have prepended your
note to me to the start of this article. I ask that readers observe
your special requests under the circumstances. PT]
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The following is excerpted from _Notes on the BOC Intra-LATA Networks - 1986
Issue 1, April 1986
TR-NPL-000275
Copyright (c) American Telephone and Telegraph, Inc., 1980, 1983
Copyright (c) Bell Communications Research, Inc., 1986
All rights reserved.
3.0 Interchangeable Codes
3.01 The subject of interchangeable codes is frequently
misunderstood, perhaps in large part due to the terminology used. Some
definitions of interchangealbe codes that will be useful in the
following discussion are provided below:
* Interchangeable CO codes: This refers to anygiven NPA in
which codes of the format N 0/1 X (i.e., those codes traditionally used
only as NPA codes) are used for CO codes, in addition to the traditional
CO codes of the format NNX. Thus, interchangeable CO codes are of the
format NXX. Interchangeable CO codes are currently in use in NPA 201
(Northern New Jersey), NPAs 212/718 (New York City), 213/818 (Los
Angeles), 312 (Chicago), 214/817 (Dallas/Fort Worth), 905 (Mexico City),
and 706 (Northwest Mexico). {Note -- as of April, 1986. I presume 718
is also using interchangeable CO codes; some others may be as well.}
* Interchangeable NPA codes: This refers to codes of the format
NNX (i.e., those codes traditionally used only as CO codes) are used as
NPA codes in addition to the traditional NPA codes of the format N 0/1
X.
Thus, interchangeable NPA codes are also of the format NXX. No
interchangeable NPA codes are in use at this time. However, Bellcore
has recommended that the NANP service area implement interchangeable NPA
code capability by July 1, 1995.
3.02 The two interchangeable code situations together are often
referred to as "full code interchangeability" or "fully interchangeable
codes." The term "interchangeable codes" is frequently used to refer to
either situation. Unlike the ANC (All Number Calling) situation, there
are some significant effects on both caller-dialing procedures and the
switching equipment when interchangeable codes are implemented. These
effects are discussed in the remainder of part 3.0.
3.03 omitted
3.04 omitted
3.05 Interchangeable CO codes are implemented on an individual NPA
basis without any resulting effect on the dialing procedures or
switching system arrangements in any other NPAs. However, within the
NPA in which the conversion takes place, all switching systems must be
modified to accept codes of the format N 0/1 X as the first three digits
of a 7-digit number, as well as the first three digits of a 10-digit
number.
3.06 When interchangeable CO codes are introduced in an NPA, the
ability of the switching systems to distinguish between a 7- and a
10-digit address by examing the first three address digit is impaired.
Thus, some other method for making that distinction is required.
Several possibilities exist. One possibility is to have the callers
provide a positive indication of their intention by dialing "1" in front
of the area code on all 10-digit (and only 10-digit) calls. This is
referred to as the "1+" or "prefix" method. Another is to have the
switching machine wait a fixed period of time (i.e. approximately four
seconds) after seven digits have been received to see if additional
digits are received. If no additional digits are received within the
required period, the switching machine will time out and process the
call on a 7-digit basis. This is referred to as the "timing" method.
Yet a third possibility is a compromise between the prefix and timing
methods. This will be referred to as the "hybrid" method in this
document. The hybrid method is summarized below.
CALL TYPE DIALING PROCEDURE TIMING REQUIRED
Local 7D NXX-XXXX No
Toll 7D 1+NXX-XXXX Only if NXX is also
an assigned NPA code
All 10D 1+NPA-NXX-XXXX No
3.07 Thus, the hybrid method requires timing only in those cases
where a toll call is dialed on a 1+7D basis, and the dialed NXX code is
assigne as both a CO code within the HNPA {Note -- Home NPA a.k.a. home
area code} and an NPA code elsewhere in the North American network. The
successful application of the hybrid method depends upon a switching
system capabile of examining the first three digits received after a
leading "1" to determin whether they are an NPA code only, an office
code only, or an ambiguous code that is in use as both. Only in the
latter case will the timing option have to be applice, after the seventh
digit, to determine whether a 7- or 10-digit number is being received.
3.08 The 1+ or prefix method is the recommended standard dialing
procedure. As expected, there are advantages and disadvantages for both
the prefix and the timing methods. The decision to designate the prefix
method as the standard for the future include the following considerations:
* The prefix method has the disadvantage of requiring the dialing
of an extra digit. However, this is applicable only on 10-digit calls,
and over 90% of BOC main stations already use a 1+ when dialing 10-digit
calls.
* The prefix method avoids the need for imposing a four-second
post-dialing delay scheme on some, or all, local calls. Avoidance of
this extra delay on local calls is beneficial both to the telephone
companies in the form of reduced holding time for the switching system's
common control equipment, and to the caller in the form of reduced
call-setup delay.
* With the timing method, common control switching systems have
the ability to avoid the need for timing except where ambiguous codes
(i.e., those codes actually assigned for both NPA and CO use) actually
exist. The need for timing would be minimized initially. However, as
time passes and the quantity of ambiguous codes increases, the quantity
of time-out applications will also increase. Also, for those callers
with an ambiguous office code, all intra-NXX calls would be subjected to
the timing delay.
* With the advent of Common Channel Signaling (CCS), connections
can be completed within a few seconds after dialing. As the average
long-haul post-dialing delay continues to decrease, the existance of a 4
to 6 second delay on an increasing number of local calls would become
increasingly irritating to callers.
* Those areas with step-by-step switching equipment will have an
additional penalty under the prefix scheme. HNPA toll calls will have
to be dialed on a 1+10D basis instead of the 1+7D basis often used now.
As the common control systems replace the step-by-step equipment, all
HNPA calls will be dialable on the recommended 7D basis.
* The recommended dialing procedure for use with interchangeable
codes specifies 0+10D dialing for all operator-assisted calls, both HNPA
and Foreign NPA (FNPA). The HNPA 0+ call is the one case where, even
after step-by-step is replaced, customers will be asked to dial
additional digits (other than the 1+) beyond their present dialing
patterns (i.e., 0+10D instead of 0+7D for HNPA operator-assisted calls).
The additional three digits will avoid the nominal 4-second timing
period that occurs if only seven digits are dialed.
3.09 omitted
Effects of Implementing Interchangeable Numbering Plan Area (NPA) Codes
3.10 omitted
3.11 ... Dialing procedures also will have to be changed in many
areas. The same comments made in parts 3.03 through 3.09 for
interchangeable CO codes apply to interchangeable NPA codes with regard
to the requirement for 1+ dialing for 10-digit calls...
The Prefix or 1+ Method As USed with Interchangeable Codes
3.12 It should be noted that, in this document, references to the
prefix or 1+ method in connection with interchangeable codes mean that
1+ is used *only* for 10-digit calls and without regard to whether or
not the 10-digit call is a toll or local call. With the prefix method,
dialing 1+7D will result in a partial-dial situation once
interchangeable codes have been implemented in an NPA. Many callers
that are presently required to dial 1+10D are also required to dial 1+7D
for 7D toll calls. The latter procedure will have to be eliminated (or
the hybrid timing method implemented) prior to implementation of
interchangeable CO codes in an NPA. Similar action will be required in
all NPAs prior to implementation of interchangeable NPA codes in the
NANP. The temporary use of 1+HNPA+7D may be required in some cases.
TABLE D
Type of call Without With
Interchangeable CO Codes
Local Direct Dialed
HNPA NNX-XXXX R NXX-XXXX R*
1+NNX-XXXX NR 1+NXX-XXXX NR
NPA+NXX-XXXX NR NPA+NXX-XXXX NR
1+NPA+NXX-XXXX P 1+NPA+NXX-XXXX P
FNPA NNX-XXXX R NXX-XXXX R
(protected codes) 1+NNX-XXXX NR 1+NXX-XXXX NR
NPA+NXX-XXXX NR NPA+NXX-XXXX NR
1+NPA+NXX-XXXX P 1+NPA+NXX-XXXX P
FNPA NPA+NNX-XXXX R NPA+NXX-XXXX NR
(nonprotected codes) 1+NPA+NNX-XXXX P 1+NPA+NXX-XXXX R*
Toll Direct Dialed
HNPA NNX-XXXX R NXX-XXXX R*
1+NNX-XXXX NR 1+NXX-XXXX NR
NPA+NXX-XXXX NR NPA+NXX-XXXX NR
1+NPA+NXX-XXXX P 1+NPA+NXX-XXXX P
FNPA NPA+NNX-XXXX R NPA-NXX-XXXX NR
1+NPA+NNX-XXXX P 1+NPA+NXX-XXXX R*
R = Recommended
R* = Recommended, These are the recommended long-term procedures to be
applicable after step-by-step equipment and protected codes are
obsolete.
NR = Not Recommended
P = Permissive; may be permitted in addition to recommended procedure.
Note: Code protection is discussed in part 4.02, item 4:
4.02 CO code conservation forestalls the need for code relief and the
associated expenditures... The following is a list of CO
code-conservation measures recommended for CO code administration:
...
4) Code protection is an arrangement wherein a CO code assigned in one
NPA is excluded from assignment in an adjacent NPA, to permit 7-digit
dialing across the common boundary. This is a permissible arrangement
and has advantages where a community of interest bridges the boundary in
question, but is acceptable only as long as it can be contunued without
causing exhaustion of the CO code universe in the NPA protecting the
code...
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End of excerpt
David G Lewis ...!bellcore!nvuxr!deej
(@ Bellcore Navesink Research & Engineering Center)
"If this is paradise, I wish I had a lawnmower."
[Moderator's Note: Thank you very much for the work involved in getting
this prepared for the Digest. PT]