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Telephony numbers are like endangered species. When they are first discovered, it appears that their numbers are infinite. Then comes the phase of wasteful utilization of the resource, and finally panic when it is discovered that it is going to be completely extinct in a few years. This has occurred with not only telephone numbers, but also MIN, SID, IMSI, and ESN codes.
Telecommunications systems seem to have an identifier for everything. In wireless alone, important identifiers are:
MDN – Mobile Directory Number. A dialable number assigned to a wireless phone. Also known as MSISDN.
ESN – Electronic Serial Number that identifies an AMPS, TDMA or CDMA phone.
IMEI – International Mobile Equipment Identifier to identify a GSM phone
MIN – To identify an AMPS, TDMA or CDMA subscription.
IMSI – To identify a GSM subscription and SIM card and, in future, TDMA or CDMA subscriptions as well. Also broadcast by GSM base stations to identify the license holder.
SID – System Identifier. Broadcast by a an AMPS, TDMA or CDMA base stations to identify the license holder.
If identifiers were assigned individually, the number assigned would exactly match the number required. This is not practical, unfortunately, because it would require a centralized assignment authority for individual numbers, and because the numbers would not directly identify the ‘owner’ of each block, a lookup in a massive centralized database before routing to the owner would be required.
For example, the first 6 digits of an MDN usually identifies the switch to which it is assigned, the first 8 or 14 bits of an ESN code identify the manufacturer of a phone, and the first 6 digits of a MIN or IMSI identify the home system. This information allows routing to the owning system without a database query being performed first. It is true that local number portability does require a database query as numbers can be removed from their assigned carrier, but this is only a local query, and even so has drastically increased the complexity and cost of telephone networks.
Since identifiers are assigned in blocks, there is a minimum number that can be assigned at one time, creating waste if the full amount are not required. For example, MDNs are usually assigned in blocks of 10,000, and ESN codes in blocks of 17 million. Finding the right tradeoff is critical. Assigning phone numbers in groups of 10,000 significantly increases the management overhead but, because of the small areas to which blocks are assigned in the US, assigning them in larger groups would significantly decrease the already low efficiency of assignment.
Almost every numbering or identifying resource used in wireless has assignment problems although, in fairness, if wireless hadn’t been such a darned successful technology, the original schemes would have been just fine.
Phone numbers (e.g. MDNs) are assigned to rate centers, small geographical areas that are the fundamental billing unit in the United States. Because rate centers are often too small, and there is political inertia preventing them all being increased in size, MDNs have to be assigned in small groups of 10,000. Even with these small groups, they are assigned inefficiently when there are many competitors. Wireless companies are actually much more efficient than most, simply because they do not have to respect rate center boundaries.
ESN codes have long been assigned using an 8-bit manufacturer code prefix. This assigns almost 17 million unique identifiers to each manufacturer. Although this is good for the large, it means that the small are forced to waste millions of codes. Now the resource is approaching exhaustion, yet the number of phones that need ESN codes is far less than the 4 billion available. The transition to a 14-bit manufacturer’s code will instantly multiply the remaining number of manufacturer codes by 64, allowing the resource to last considerably longer. Another solution is the re-use of manufacturer codes assigned more than a decade ago to manufacturers who are believed to have only produced a small number of phones.
SID codes suffered from over-enthusiastic assignment in the early days of cellular. After they were assigned in the US by the FCC, someone sat down with an atlas and, based on the population and political boundaries of the time, assigned a group of SID codes to every country in the world, leaving very few left over. Many of those countries have only GSM systems and do not even need one SID! Luckily, the belief that every licensed area would need a separate SID (as originally assigned by the FCC) has proved to be false, as many carriers have merged several systems into local calling areas using a single SID. Because of this, most of the assignments have proved to be adequate. However, the resource remains over 90% assigned, even though most codes are not utilized.
The MIN is a 10-digit number, and therefore, there should be 10 billion available. However, by tying the MIN to the phone number, US and Canadian carriers ensured the unavailability of most of the 8 billion MIN codes that match North American phone numbers. This forced international carriers to invent the International Roaming MIN (IRM), a MIN that begins with the digit 0 or 1 and that therefore cannot match a North American phone number (and, therefore, is safe from confusion with a North American MIN). The CTIA initiative to assign MIN codes separately from MDN will free up literally billions of MIN codes, and give this resource a new lease on life.
The IMSI is 15 digits long, and theoretically can provide 1 billion numbers to 1 million separate carriers. How could that possibly be exhausted? Well, GSM carriers have tried their best. By tying routing based on IMSI to phone numbers (the same mistake that US wireless carriers made with MIN), they have ensured that out of about 24,000 Mobile Network Codes (6-digit IMSI prefixes) assigned to North America, only 1,000 can be used. Plus, by implementing only a two-digit Mobile Network Code instead of three, the number available is reduced further to only 100. Luckily, with radical consolidation of carriers occurring, this may prove to be adequate.
Numbering is a strange and arcane field within telecommunications. It seems that it should be really easy, but it almost always ends up with extinction just being barely avoided. The problem is that when identifier formats and allocation schemes are first decided upon, future patterns of utilization cannot be predicted. So far, we have been able to rely on the ability of engineers to come up with a clever scheme that will conserve the resource before it goes the way of the Passenger Pigeon and Dodo bird.
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