1. Objective

Local telephone addresses in the United States are represented by two bound groups of characters, each with a fixed number of characters:


def-ghij .

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 was restricted to either 0 or 1 but this limitation was recently rescinded.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.