Technical Issues
Complexity for number portability can come from many sources. Historically, numbers were assigned to various operators in blocks. The operators, who were often also service providers, then provided these numbers to the subscribers of telephone services. Numbers were also recycled in blocks. With number portability, it is envisioned that the size of these blocks may grow smaller or even to single numbers. Once this occurs the granularity of such operations will represent a greater workload for the telecommunications provider. With phone numbers assigned to various operators in blocks, the system worked quite well in a fixed line environment since everyone was attached to the same infrastructure. The situation becomes somewhat more complex in a wireless environment such as that created by cellular communications.
In number portability the “donor network” provides the number and the “recipient network” accepts the number. The operation of donating a number requires that a number is “snapped out” from a network and “snapped into” the receiving network. If the subscriber ceases to need the number then it is normal that the original donor receives the number back and “snaps back” the number to its network. The situation is slightly more complex if the user leaves the first operator for a second and then subsequently elects to use a third operator. In this case the second operator will return the number to the first and then it is assigned to the third.
In cellular communications the concept of a location registry exists to tie a “mobile station” (such as a cellular phone) to the number. If a number is dialed it is necessary to be able to determine where in the network the mobile station exists. Some mechanism for such forwarding must exist. (For an example of such a system, see the article on the GSM network.)
In the US, there are standards for portability defined by the FCC, the LNPA, NANPA and the ATIS which are agreed upon by all member providers to help make LNP as cost-efficient and expedient as possible while still retaining a healthy level security for all providers and in respect of the highest level of customer service. These rules, first defined in the 1st, 2nd and 3rd Reports and Orders by the FCC (publicly available at fcc.gov), are further detailed by the LNPA in order to ensure any provider can successfully port numbers to any other provider. NeuStar provides a national database called the NPAC (National Portability Administration Center) which contains the correct routing information for all ported and pooled numbers in the US and Canada. The NANC maintains detailed documentation of the procedure common among US carriers to port numbers as described here.
Providers use SS7 to route calls throughout the US/Canada network. SS7 accesses databases for various services such as CNAM, LIDB, Custom Local Area Signaling Services (CLASS) and LNP. Calls to ported numbers are completed when a customer who calls a ported number sends the dialed number to a provider's SSP (Service Switch Point), where it is identified either as a local call or not. If the call is local, the switch has the NPA-NXX in its routing table as portable, so it sends a routing request to the Signal Transfer Point (STP) which accesses a local database that is updated by an LSMS (Local Service Management System) which holds all routing for all ported numbers to which the carrier is responsible for completing calls. If routing information is found, a response is sent to the "query" containing the information necessary to properly route the call. If it is not a local number, the call is passed on to the STP and routed until it gets to a local carrier who will perform the "query" mentioned earlier and route the call accordingly.
The routing information necessary to complete these calls is known as a Local Routing Number (LRN). The LRN is no more than a simple 10-digit telephone number that resides in the switch of the service provider currently providing service for the ported telephone number.
When a provider receives a request to port a telephone number from a new customer, that provider sends an industry-standard Local Service Request (LSR) to the existing (or "old") provider. When the Old Provider receives this request, it sends back Firm Order Confirmation (FOC) and the process of porting the number(s) begins. Either provider can initiate the port using a Service Order Activator (SOA or LSOA) which directly edits the NPAC database mentioned before. Providers can also make these requests within the NPAC database directly. If the new provider initiates the port, it is called a "pull," and if the old provider initiates, it is a "push." Once the number is pulled or pushed, the providers must concur the request and the new provider must "activate" the number using the LRN of the switch serving the customer on the agreed due date. At the point this is completed, the number is ported.
Much of this process is duplicated in intermodal portability (porting between wireline and wireless providers). There are a few technical differences, however, in WLNP—Especially with concern to the time intervals allowed.
Read more about this topic: Local Number Portability
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