TELECOM Digest OnLine - Sorted: NN0 Central Office Codes

NN0 Central Office Codes

A few issues back, there was a thread about NN0 codes as central
office codes.

AT&T's publication "Notes on Distance Dialing" (1975) [1] includes a
list of 63 NN0 codes that could be assigned either as area codes or as
central office codes. This list, identified as "Chart 5," includes
all NN0 codes in the range 220-990 (except for 950 which "is reserved
for a future network-wide service") in an arbitrary (non-numerical)
order. Each code is identified by a "sequence" number (#1 - #63). A
copy of the list is posted at http://tinyurl.com/8csz7 .

The accompanying text states:

2.03 Sometime after 1995, it is estimated that
the 21 NPA codes still unassigned (end of 1974)
will have been used and that it will be necessary
to start using NNX type codes as NPA codes. In
the interest of minimizing ambiguity, it is planned
to assign the NN0 codes first in accordance with
the sequence shown in Chart 5. (The NN0 codes have
been designated as the last to be assigned as CO
codes and a sequence that is the reverse of the of
that for NPA code assignment is recommended.)
Ultimately, it will become necessary to assign the
remaining NNX codes for NPA code purposes. [2]

Notes [3] on Chart 5 clarify the order of assignment:

Central office codes should be drawn from the
list in sequence-number order.

After all N0/1X area codes are exhausted, further
area codes should be drawn from the list in
reverse-sequence-number order.

If I understand this correctly, the idea was to assign NN0 codes as
central office codes *and* as area codes *before* the introduction of
interchangeable area codes, but to draw from opposite ends of the NN0
list in order to prevent ambiguity. Presumably, this would have
forestalled the need for interchangeable area codes until the list was
exhausted, theoretically somewhere in the middle of the list.

It didn't work out that way ...

Many NN0 codes were assigned as central office codes whenever and
wherever they were needed, without regard to their positions on Chart 5.

Examples that come to mind:

702-870 (#3 on the list) ca. 1989 Las Vegas
312-990 (#32 on the list) ca. 1988 Hinsdale
201-460 (#36 on the list) ca. 1982 Lyndhurst
414-730 (#52 on the list) ca. 1986 Appleton
214-680 (#54 on the list) ca. 1983 Dallas

I assume that one reason for selecting these combinations was an
attempt to maintain the look and feel of existing central office
codes. That was certainly the case in Appleton, where Wisconsin
Telephone was already using several other 73X combinations as central
office codes.

Curiously (as Mark Roberts noted in TD 24:482), 530 (#1 on the list)
was in service -- at least briefly -- in California in 1965, a decade
before Chart 5 was published. I wonder if this was just a
coincidence? Or had some early version of Chart 5 already been
published in 1965?

No NN0 area codes were assigned before 1/1/1995 (when interchangeable
area codes were introduced), but once the floodgates were open, many
NN0 codes appeared quickly. But they too were assigned as needed,
without regard to their positions on Chart 5. Seven of them were
assigned [4] during 1995:

360 (#6) Washington
630 (#15) Illinois
770 (#25) Georgia
540 (#29) Virginia
970 (#31) Colorado
860 (#34) Connecticut
520 (#61) Arizona

Note that Washington's 360 (#6 on the list) was actually #58 in the
reverse sequence, while Arizona's 520 (#61) was actually #3 in reverse
sequence.

And, of course, all N90 combinations were reserved for future use,
even though all eight of them appear on Chart 5. Apparently, Chart 5
had been abandoned before 1995.

I assume that a major factor in the selection of new area codes after
1/1/1995 was conflict-avoidance: avoiding conflict between an area
code and any central office code within the area code. This would
have been a further reason for abandoning Chart 5.

Even NPA 847 obeyed this constraint when it was first assigned; 847-847
appeared some time later.

-------- References -------------

[1] American Telephone and Telegraph Company, Engineering and Network
Services Department, Systems Planning Section. "Notes on Distance
Dialing," Section 2 ("Numbering Plan and Dialing Procedures"), 1975.

[2] Ibid, Section 2, p.2.

[3] Ibid, Section 2, p.17.

[4] Carl Moore: history.of.area.splits. November 2, 1995.
http://massis.lcs.mit.edu/archives/areacodes/history.area.splits.11-95

Neal McLain

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