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In our single-digit years, for most of us, the difference between boys and girls was not that significant. Interesting, but not significant. But, only a few years later it becomes of paramount importance to most people. So it is with the MIN – commonly, but erroneously, thought to be your cellular phone number (or “directory” number). In the single-digit years of the young cellular industry, the difference between MIN and directory number was considered to be of only academic interest, similar to arguments over whether punctuation should go inside or outside of quotations marks. But, as cellular enters its teen years, the difference is becoming very significant due to new realities: international roaming, vertical market segments, number portability and area code changes.
Defining a mobile’s “directory” number (or MDN, as I shall acronymize it) is easy – it is the number that would be in the phone book, if cellular phone numbers were listed, which they generally are not. It is the number that people dial to make your phone ring. The MIN (or Mobile Identification Number) is a 10-digit number used by the phone to identify itself to the system, and used by the system to get the attention of a specific phone. At first, the relationship between the MIN and the MDN was simple – they were the same. Gradually that relationship has eroded, and over the next few years it will crumble into dust.
The first international cellular systems had a challenge deciding how to program their MINs. Many of these countries have less than a 10-digit dialing plan, and so had to fill some of the 10 MIN digits, often using their country code. To give them more choices, there are two sets of country codes defined: the E.164 set that we are most familiar with when dialing overseas, and a new E.212 set of “Mobile” country codes (country codes for mobile phones, not codes for mobile countries). Mexico, for example, started by programming the first two digits of MINs with their E.164 country code, “52” and the remaining 8 digits with their 8-digit national number. This worked very well until the US and Canada started to allocate area codes that started with 52. The first such area code, 520, was allocated to Arizona, near the Mexican border! Soon after this occurred, the Mexican government decided to move to a 9-digit numbering plan.
The problem for Mexican cellular carriers (and many South American countries are experiencing similar problems) was that just as they were starting to develop international roaming, numbering changes were starting to make it impossible to distinguish their mobiles from mobiles originating from the US and other countries. The Mexicans were one of the first group of carriers to recognize that the MIN and MDN for a cellular phone did not need to be the same, and they have initiated a plan to reprogram all cellular phones in Mexico with MINs that start with the digits “05”.
At the same time that many cellular systems were being built outside North America, some companies within the cellular heartland were starting to develop niche markets, requiring special MIN programming. Two examples are HighwayMaster, which provides cellular-based communications equipment to trucking companies, and Cantel, a nationwide carrier in Canada, which wanted to sell pre-programmed shrink-wrapped phones without committing directory numbers ahead of time. Both companies (and several others) started to program phones with MINs that started with either the digit “0” or “1”, and therefore could not conflict with a regular North American directory number (which can only start with the digits “2” through “9”). These companies proved that the relationship between MIN and MDN was artificial, but because of the specific market niches, their solution was not seen as generally applicable.
The new capability that is going to force the entire cellular industry to “bite the bullet” and separate MINs from MDNs, no matter how messy the operation, is Number Portability. The CTIA has recommended that when a cellular phone number is ported, the directory number stays with the subscriber (as required by FCC mandate), but the MIN stays with the carrier. The subscriber will then need a new MIN from their new carrier, programmed in their phone. This MIN will identify the new carrier, and will cut down the network signaling traffic that would otherwise be required.
One of the benefits that follows from separating the MIN from the directory number is that area code changes will be easier to swallow. If a bunch of mobile subscribers are caught up in an area code change, only their directory numbers need to change – their MIN can stay the same. The advantage of this is that the directory number can be changed in the switch while the MIN requires physical access to the mobile. Consequently, an area code change can be effected without recalling any mobiles for reprogramming.
There is a downside to the separation of MIN from MDN. One problem is with the roamer port, the number that you can dial (usually XXX-ROAM) to reach a roamer known to be in a specific system. With automatic call delivery being quite widespread and more convenient, this older method may just fade away, although it can save double-long-distance charges in some cases. Another annoyance will be that older cellular phones will display their MIN, but do not know their own directory number. After porting your phone number, when you dial RCL-# (or whatever code your cellphone requires), you may be viewing someone else’s phone number! Carriers have the expensive problem – some ancillary systems (e.g. subscriber entry and billing systems) assume the MIN and MDN are the same, and must be redesigned to accommodate two different numbers.
Another important problem is management of MINs as a pool of numbers completely separate from directory numbers. Currently, management is very simple; if you own a phone number, you own the corresponding MIN. This simple rule means the majority of potential MINs are being hogged by wireline carriers that will simply never use them – a wasted resource of mammoth size. The MIN management problem is not just limited to the US and Canada. It is a global problem, as MINs must be unique around the world if international roaming is to work. Whichever organization is appointed to take on this role will have a big job on their hands. Even just the first step is a big one, which is that of identifying all the MIN blocks currently used by cellular carriers around the world.
The rewards are great, because the MIN resource could, if not constrained, uniquely identify 10 billion different phones! A small part of the MIN management mess is being solved by the fledgling IFAST (International Forum on AMPS Standards Technology), which will be allocating all MINs that start with the digit “0” or “1” (whether in the US or elsewhere). However, they have not touched the much bigger, and politically more sensitive, issue of the remaining 80% of the MIN pool that mirrors dialable numbers.
The MIN management problem also affects PCS systems that are based on the AMPS family of standards (i.e. IS-95 CDMA systems and IS-136 TDMA systems). These also use the MIN as an identifier. GSM-based PCS systems have always used the alternate IMSI (International Mobile Station Identity), which does not have the problems of the MIN identifier. The “AMPS” cellular and PCS industry is moving towards IMSI, but slowly, although the transition will be eased by the independence that a standalone MIN will bring.
The teen years are a difficult transition for most people, and the MIN issue shows that they aren’t always easy for maturing industries either. Ironically, but probably not surprisingly, all of the problems that require separation of MIN and MDN resources have arisen from the spectacular growth of cellular since its birth.
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