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Local Number Portability (LNP) is a bureaucrat’s dream and an engineer’s nightmare. While it may achieve the goal of leveling the playing field for competitive LECs, it is of debatable value for wireless consumers, and provides an enormous challenge to the integrity of telecom networks wherever it is implemented. Currently the wireless industry is challenging both the need for wireless number portability and, as a backup plan, is challenging also the deadlines for implementation, as unachievable.
Most wireless carriers will have to implement some support for number portability, even if their own numbers are never required to be portable, as they must be able to route to ported wireline phone numbers. An example would be a wireless carrier in New York City routing a call to a phone number that is today managed by NYNEX. That wireline customer could port their phone number to a competitive local exchange carrier (CLEC), which would require all nearby switches (including wireless) to be able to query a number portability database to obtain routing instructions, prior to completing a call to that number.
LNP impacts are limited to nearby switches because, as the name implies, portability is “Local”. A New York City subscriber can port their number to one of several carriers in New York City, but not to even a carrier in a different city. This reduces the impact of portability by requiring that switches only know about local blocks of numbers that have been ported.
The LNP capability for wireless switches is being provided in a TIA standard that is close to publication. Currently known by its project number (PN-3980A), it will soon become an important TIA interim standard. It provides a query message (NumberPortabilityRequest or NPREQ) that can be sent from an MSC (Mobile Switching Center) to a number portability database to obtain the LRN (routing number) associated with a ported directory number. If the number is not ported, no routing information is returned, resulting in use of the directory number as the routing number.
One of the concerns about Local Number Portability that is not often expressed is network load (and its impact on reliability). Processing a local call in an area that provides flat-rate local calling is relatively simple. Digit analysis confirms that a local number has been dialed, and the call is routed. Wireline switches have no need for database queries nor even for billing record generation. LNP changes may eventually force a database query on most local calls. This is a truly massive change in the processing requirements for wireline switches, and will effectively reduce their call processing capacity. Wireless carriers may be shedding no tears, because they are used to IS-41 (or similar) queries on a significant fraction of calls. While LNP will also increase the processing load on wireless switches, it will be to a significantly lesser extent than for wireline switches, further eroding the cost differential between the two services.
Another aspect of LNP that may significantly reduce network reliability is the administration of number changes. Whenever a subscriber ports, both the old and new carriers have to report the change, and this has to be reflected in all copies of the local database. An error in this process (either hardware or software) could result in database inconsistencies and possible looping of calls. Software errors in particular hold a real risk that could easily deny service to an entire local calling area.
Phase II of LNP requires wireless carriers to allow their customers to participate in porting. It is currently a battle ground, with the CTIA fighting for a delay and Bell Atlantic NYNEX Mobile fighting for elimination of the requirement entirely. Wireless carriers are upset because they feel that LNP will have minimal benefit for their customers (at considerable cost to the carriers and, ultimately, to consumers) and because they feel that the technical complexity was underestimated, resulting in at least the need for a delay. It is notable that most wireless customers do not widely circulate their phone numbers, and therefore would suffer little if the number had to change when changing carriers. There are, however, some wireless companies that feel that portability will be beneficial, including PCS carrier Omnipoint and various resellers. They obviously feel that the business benefits to their companies outweigh the technical challenges.
In this climate of uncertainty, the development of standards is forced to go ahead. TIA subcommittee TR-45.2 is developing PN-4186, which will provide a standard method for adherence to LNP Phase II requirements, if and when required.
One of the major impacts of LNP Phase II is to force the separation of MIN and Directory number. The directory number, by definition, has to be retained when a subscriber ports. If the MIN remains the same as the directory number, it would require a number portability database query every time a roamer appears in a system (even if they never make a call). Since the MIN is generally hidden from the subscriber, having to modify the MIN whenever a phone is ported is not believed to be a large hurdle.
The separation of MIN and MDN will have a negative impact on wireless carriers who are outside the LNP mandate (i.e. those in rural areas). When a 9-1-1 call is made, these carriers may not have access to the Mobile Directory Number (MDN). Yet, the FCC’s Phase I 9-1-1 mandate requires the ability to perform callback (which requires the MDN) while the FCC’s Phase II LNP mandate makes this difficult to achieve.
The separation of MIN and directory number will have many other impacts on wireless carriers. For one thing, employees will have to be retrained to recognize the distinction and many applications will have to be reprogrammed. Perhaps the biggest impact will be the need to separately allocate MIN and directory number resources. This will require a new administrator, who will initially have to determine all current allocations of MINs and educate the entire industry about the new procedures for obtaining blocks of MINs. This new organization (currently being developed by the CTIA) will have to be managed jointly with other countries in the North American Number Plan, most notably Canada. This is a significant undertaking, although the separation of MIN and MDN will have long-term benefits by reducing the impact of area code changes, and potentially facilitating international roaming.
Phase II of LNP also has a big impact on special features, such as Calling Name Presentation (CNAP), Short Message Service (SMS) and Calling Party Pays (CPP). It will also rapidly accelerate the movement from MF tone-based signaling to SS7 ISUP interconnection.
Calling Name Presentation is usually implemented via a database access to the home carrier (which may be wireless or wireline). However, if the number is ported, it is not so easy to identify the home carrier. If this information is not carried in an ISUP message from the home system, then a further number portability query will be required before the appropriate calling name database can even be identified.
Short Message Service will also be affected because of difficulties identifying the mobile to which to direct the message. Take, for example, a carrier that allows short messages to be sent via the internet using messages addressed as MDN@carrier.com. The internet can deliver the message to the carrier’s domain, but the MDN will not always identify the specific HLR for the mobile, resulting in a requirement for an additional number portability query. This problem is somewhat limited because there is no need to deliver short messages sent to MDN@carrier.com when the MDN has been ported to a different carrier (this is considered to be Service Portability, which is not covered by the LNP mandate).
Another SMS problem occurs when a mobile originates a short message. In this case, it may just specify the phone number of another mobile, which could legitimately belong to a different carrier. However, the MDN does not necessarily identify the carrier, resulting in yet another number portability query.
Calling Party Pays is a service that has grown slowly in North America, for a number of reasons. The Local Number Portability mandate will make most current CPP implementations obsolete, something that has already forced the CTIA to rethink the service. Current implementations rely on a recognizable block of numbers, something that is not permissible when a number may be changed from one carrier to another at will.
ISUP is generally agreed to be a better interconnection method than the older MF-tone based methods. However, the transition to ISUP has been slow because it is not so much better to justify the cost. LNP will change all that, because it will help eliminate unnecessary database queries by carrying a flag that identifies whether an LNP query has already been performed.
Local Number Portability is a big headache for wireless carriers. Currently, the emphasis is on just getting the appropriate functionality implemented to meet the deadlines, but long-term concerns must include cost reduction, the reliability and performance of the network, and the feasibility of advanced services that are adversely affected by this FCC mandate.
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