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Local Number Portability (LNP) is the third new mandate being imposed by the US government on wireless carriers, that I have discussed in my last four columns (the other two being enhanced emergency services and compliance with the CALEA lawfully authorized electronic surveillance legislation). LNP also must be implemented in a similar time frame, with wireless carriers being required to support some capabilities by the end of 1998 and full LNP by mid-1999. US wireless carriers must yearn for the days when the word “cellular” was taken for a medical term inside the beltway, and when “wireless” was still thought to be an old fashioned term for a radio.
Local Number Portability allows anyone to keep their phone number when they move from one carrier to another in the same area. It will even work between technologies (e.g. wireline to wireless, or cellular to PCS). LNP was initially designed to promote local phone competition, under the assumption that consumers would be reluctant to change carriers if they had to reprint business cards and other stationery and tell all their friends and colleagues about their new number. LNP requirements have been extended to wireless by the FCC, although the demand will not be as great, as many wireless users do not give out their cellphone number freely, and usually do not have it listed in any directories. The restriction on LNP is that it is ‘local’, so that a number may only be moved between carriers within the same local area. The area of interest for local number portability is the MSA (Metropolitan Statistical Area) which corresponds to cellular license boundaries, but n ot to PCS systems (which are based on MTA and BTA regions).
Wireline carriers will be using Intelligent Network capabilities to provide number portability, similar to the way 1-800 calls are handled. First a potentially ported number has to be identified. This is not as easy as for 1-800 calls, as any local number could potentially be ported. For each candidate number, a query has to be initiated to the Local Number Portability SCP, which will return a Local Routing Number if the number is in fact ported. Following that query, the call can be routed using ISUP, with a flag set to indicate that number portability translation has already occurred. Without this indicator, some nasty infinite loops could occur.
The CTIA has been encouraging study of the impact of LNP on wireless carriers, through a Request For Information issued in August, 1996 and a forum in October, 1996. They have recently issued their view of the requirements for standardization to TIA standards subcommittee TR-45.2. Two requirements stand out – the separation of the MIN from the directory number and the need to migrate to SS7-based interconnect.
One of the most interesting of the CTIA recommendations is to separate the MIN from the directory number. Ironically, after sex and food, this is my favorite topic! Currently, for the vast majority of phones, the MIN and the directory number are the same number. This equality is not necessary, and breaking it will simplify the implementation of LNP for wireless carriers.
What is the difference between a MIN and a directory number? The MIN is the 10-digit number that is used to identify a mobile over the air interface, in both directions. The directory number is just simply that, the number that would be published in a telephone directory. Keeping these numbers the same has been a significant convenience for US cellular carriers, because obtaining and managing separate lists of MINs is not required. However, there are several solid reasons to break this connection. Future area code changes will not require phone reprogramming of the MIN, international roaming will be simplified and the implementation of LNP will be easier.
To see why the separation of MIN and directory number is so important, imagine for a moment that the MIN and directory number were kept the same after a customer went from their original carrier to a competing carrier. Every time the mobile transmitted its MIN to register in a new system, the registration message would be sent to the old home system, which would then have to either do a number portability query, or keep track of all of its old subscribers to be able to reroute the message itself. If the MIN is reprogrammed to reflect the new home system, mobile originated messages (mostly registrations and originations) can be routed directly to this system.
One of the strange consequences of this separation of identifiers is that after a customer ports to a different system, their MIN could be reused. This will result in one customer having a MIN that is the same as the phone number of a customer in a different system, yet no ambiguity will result!
Another major recommendation of the CTIA is to use SS7 ISUP for call setup, at least for local calls that might be ported. As I noted above, this is to accommodate a flag to indicate whether translation has yet occurred. This kind of change is easier to make in SS7, which is a structured computer-to-computer communications protocol. Most cellular carriers today use MF-tone based signaling. It does the job, but is restricted in capacity and flexibility. It would be possible to upgrade MF protocols to carry an equivalent LNP indicator, but this would require the cooperation of the landline carriers, which might not be forthcoming. The upgrade to SS7 may become a major cost issue for existing wireless carriers.
The TIA will be developing standards for LNP by the time this article is published. They will have to choose between three different protocols (or a mixture) as their basis for work: the IN protocol used for landline LNP queries, the IS-41 protocol used for wireless intersystem communications, or the new Wireless Intelligent Network protocol, currently under development. They will also have to consider the implications of the CTIA requirements at a deep technical level, which may uncover some unexpected problems, possibly requiring a mid-course correction. All of this will be performed under heavy time pressure, although the TIA standards committees are getting used to this, with projects to support CALEA and enhanced 9-1-1 well under way. I cannot speak for the next millenium, but the waning years of this millenium are going to be busy ones for wireless standards committees.
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