- 1. Objective
2. PRESENT ADDRESSING PROTOCOL
- Local telephone addresses in the United
States are represented by two bound groups of characters, each with a fixed
number of characters:
- The first group <def> identifies
the central office exchange (COX), and comprises three characters, the
first character <d> is restricted to values between 2 and 9. The
second group <ghij> called the line number (line number) identifies
a terminal device such as a telephone on a subscriber's line, and comprises
four characters. Together they comprise the telephone address. This fixed
grouping also serves mnemonic purposes: by separating the telephone address
into fixed COX and line number groups subscribers can more readily remember
their local telephone addresses.
- Telephone addresses comprising these
seven digits in two bound groups are administered by the Local Exchange
Carrier (LEC). The physical interconnections are made by a simple Public
Switching Telephone Network (PSTN), an analog arrangement which will probably
undergo extensive modifications in the near future (Horak, 1996).
- To implement direct long-distance dialing
between LECs the North American Numbering Plan (NANP) was
implemented. Those countries that joined the program were sectioned into
discrete Numbering Plan Areas (NPA). Each NPA is identified by a three
character area code <abc>. The first character of an area code: <a>
is restricted to values between 2 and 9. Originally the second character
<b> was restricted to either 0 or 1 but this limitation was recently
rescinded. Although connectivity within any NPA did not require the area
code, the full telephone address for each subscribed now took the form:
(abc) def-ghij .
- Care was taken however not to disturb
the accepted arrangement of two bound groups for telephone addresses of
the PSTN, each with a fixed number of characters. Because <d> cannot
equal 0,1 these two digits were available to be used as an access code
<n>, where n=0,1. According to the NANP if the first character dialed
is an <n> the telephone exchange will recognize that ten characters
will follow, rather than the expected seven characters for a PSTN local
call (Bell, 1977). Hence, the first digit dialed of a local seven-digit
address cannot be a <0> or a <1>.
n (abc) def-ghij
- With direct long-distance dialing came
PSTN electronic switching: telecommunications maven having earlier recognized
that conventional electromechanical switching was much too slow to accommodate
the switching requirements of the ever-increasing number of telephones
in service, and moreover electromechanical switching was far too prone
to mechanical failure. PSTN electronic switching was the logical next step.
There are several varieties of PSTN electronic switches available, with
electronic common control probably being the most widely utilized
(Brooks, 1976). Electronic switching was accomplished by assigning
an audible tone to each dial designation, with each telephone equipped
with a tone generator, with the PSTN switches recognizing specific tones
generated by telephone dialing. The dial itself could be replaced with
key switches arranged on a keypad as shown below.
- The dial tones heard on telephone dialing
however are not the routing tones, which are generated at the COX. Because
stray tones on the telephone lines could trigger random PSTN dialing errors,
the routing tone comprises two frequencies transmitted simultaneously and
denoted multi-frequency tones, of which there are sixteen. The eight frequencies
adopted were well spaced to eliminate misrecognition errors.
- This matrix arrangement permits 16 audible
frequency combinations, shown in hertz. With some prescience, this new
arrangement would allow an additional six switches to be added to telephone
keypads when future demands required. Two were designed <*>
and <#> keys and four were designated A, B, C and D keys: the latter
a very poor choice indeed considering that these letters already appeared
on the keys in conformity with the old dial arrangement. The expected confusion
mitigated against adopting these keys.
- Although the letters Q and Z were still
available for the four additional keys, other typographic symbols such
as @, &, % and $ have specific means that would again lead to confusion.
Nevertheless there are those considering them seriously, at least for area
codes (Rohde, 1996).
- Accordingly, the result of PSTN switching
over electromechanical switching is speed, leaving unsolved the perceived
problem of the diminishing availability of addresses. With the proliferation
of various telecommunication devices this problem has become acute. In
virtually all commercial establishments an additional facsimile line with
its own address is now required in normal business practice, and the need
for modems is now rapidly reaching this status.
- Nevertheless, as presently formulated
NANP provides some 6.4 billion possible addresses for public service, while
less than five percent of these are in actual service. Evidently the problem
is not inadequacy of available addresses but maldistribution of these addresses
under the NANP. To totally revamp NANP at this late date is both politically
inconceivable and technically impractical. Hence any proposed remedy must
adhere to the NANP.
OF PRESENT REMEDIES